Centennial Edition

Wood Handbook

Wood as an Engineering Material

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical

Report

FPL–GTR–190
Centennial Edition

Wood Handbook

Wood as an Engineering Material

Forest Products Laboratory • United States Department of Agriculture Forest Service • Madison, Wisconsin
April 2010

Forest Products Laboratory. 2010. Wood handbook—Wood as an engineering material. General Technical Report FPL-GTR-190. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products Laboratory. 

508 p.

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726–2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin. 

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

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basis of race, color, national origin, age, disability, and where applicable, 

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and employer.

Abstract

Summarizes information on wood as an engineering material. Presents properties of wood and wood-based products 

of particular concern to the architect and engineer. Includes 

discussion of designing with wood and wood-based products along with some pertinent uses.

Keywords: wood structure, physical properties (wood), 

mechanical properties (wood), lumber, wood-based composites, plywood, panel products, design, fastenings, wood 

moisture, drying, gluing, fire resistance, finishing, decay, 

preservation, wood-based products, heat sterilization, sustainable use

Pesticide Precautionary Statement

This publication reports research involving pesticides. 

It does not contain recommendations for their use, nor 

does it imply that the uses discussed here have been 

registered. All uses of pesticides must be registered by 

appropriate State and/or Federal agencies before they 

can be recommended.

Caution: Pesticides can be injurious to humans, 

domestic animals, desirable plants, and fish or other 

wildlife, if they are not handled or applied properly. 

Use all pesticides selectively and carefully. Follow 

recommended practices for the disposal of surplus 

pesticides and pesticide containers.
iii

Chapter 6 Commercial Lumber, 

Round Timbers, and Ties

 Hardwood Lumber 6–2

 Softwood Lumber 6–6

 Purchase of Lumber 6–12

 Round Timbers and Ties 6–18

 Commonly Used Lumber, Round Timber, and Tie 

 Abbreviations 6–23

 References 6–25

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties

 Lumber 7–1

 Round Timbers and Ties 7–13

 Literature Cited 7–14

 Additional References 7–15

Chapter 8 Fastenings

 Nails 8–1

 Spikes 8–9

 Staples 8–9

 Drift Bolts 8–10

 Wood Screws 8–10

 Lag Screws 8–12

 Bolts 8–14

 Connector Joints 8–19

 Metal Plate Connectors 8–25

 Joist Hangers 8–25

 Fastener Head Embedment 8–26

 Literature Cited 8–27

 Additional References 8–27

Chapter 9 Structural Analysis Equations

 Deformation Equations 9–1

 Stress Equations 9–4

 Stability Equations 9–7

 Literature Cited 9–10

 Additional References 9–10

Chapter 10 Adhesives with Wood Materials 

Bond Formation and Performance

 Surface Properties of Wood for Bonding 10–2

 Physical Properties of Wood for Bonding 10–5

 Adhesives 10–8

 Bonding Process 10–14

 Bonded Joints 10–18

 Testing and Performance 10–19

 Standards 10–22

 References 10–23

CONTENTS

Chapter 1 Wood as a Sustainable 

Building Material

 Wood as a Green Building Material 1–1

 Forest Certification Programs 1–3

 Literature Cited 1–5

Chapter 2 Characteristics and Availability 

of Commercially Important Woods

 Timber Resources and Uses 2–2

 Species Descriptions 2–3

 Imported Woods 2–18

 Scientific Name Index 2–41

 Literature Cited 2–45

 Additional References 2–45

Chapter 3 Structure and Function of Wood

 Biological Structure of Wood at Decreasing Scales 3–2

 Microscopic Structure of Softwoods and Hardwoods 3–9

 Wood Technology 3–12

 Juvenile Wood and Reaction Wood 3–13

 Appearance of Wood as Sawn Lumber 3–13

 Wood Identification 3–16

 Literature Cited 3–16

Chapter 4 Moisture Relations and 

Physical Properties of Wood

 Wood–Moisture Relationships 4–1

 Thermal Properties 4–10

 Electrical Properties 4–15

 Friction Properties 4–17

 Nuclear Radiation Properties 4–17

 Literature Cited 4–18

 Additional Reference 4–19

Chapter 5 Mechanical Properties of Wood

 Orthotropic Nature of Wood 5–1

 Elastic Properties 5–2

 Strength Properties 5–3

 Vibration Properties 5–17

 Mechanical Properties of Clear Straight-Grained 

 Wood 5–21

 Natural Characteristics Affecting Mechanical 

 Properties 5–26

 Literature Cited 5–44

 Additional References 5–44
iv

Chapter 11 Wood-Based Composite Materials Panel Products, Glued-Laminated 

Timber, Structural Composite Lumber, and 

Wood–Nonwood Composite Materials

 Scope 11–2

 Conventional Wood-Based Composite Panels 11–2

 Glulam Timber 11–17

 Structural Composite Lumber 11–20

 Wood–Nonwood Composite Materials 11–22

 Literature Cited 11–26

Chapter 12 Mechanical Properties of 

Wood-Based Composite Materials

 Elastic Properties 12–2

 Strength Properties 12–3

 Panel Products 12–3

 Timber Elements/Structural Composite Lumber 12–5

 Wood–Nonwood Composites 12–7

 Testing Standards 12–9

 Literature Cited 12–10

Chapter 13 Drying and Control of Moisture 

Content and Dimensional Changes

 Determination of Moisture Content 13–1

 Recommended Moisture Content 13–3

 Drying of Wood 13–5

 Moisture Control during Transit and Storage 13–13

 Dimensional Changes in Wood 13–15

 Design Factors Affecting Dimensional Change 13–17

 Wood Care and Installation during Construction 13–18

 Literature Cited 13–19

Chapter 14 Biodeterioration of Wood

 Fungus Damage and Control 14–1

 Bacteria 14–9

 Insect Damage and Control 14–9

 Marine Borer Damage and Control 14–13

 References 14–15

Chapter 15 Wood Preservation

 Wood Preservatives 15–1

 Selecting Preservatives 15–13

 Evaluating New Preservatives 15–13

 Preservative Effectiveness 15–13

 Effect of Species on Penetration 15–15

 Preparation of Wood for Treatment 15–15

 Application of Preservatives 15–18

 Quality Assurance for Treated Wood 15–25

 Effects on the Environment 15–26

 Recycling and Disposal of Treated Wood 15–26

 References 15–27

Chapter 16 Finishing of Wood

 Factors Affecting Finish Performance 16–2

 Exterior Wood Finishes 16–16

 Application of Finishes, Special Uses 16–24

 Finish Failure or Discoloration 16–25

 Finishing Interior Wood 16–32

 Wood Cleaners and Brighteners 16–34

 Paint Strippers 16–35

 Lead-Based Paint 16–36

 Literature Cited 16–37

 Additional References 16–37

Chapter 17 Use of Wood in Buildings 

and Bridges

 Light-Frame Buildings 17–1

 Post-Frame and Pole Buildings 17–4

 Log Buildings 17–6

 Heavy Timber Buildings 17–6

 Considerations for Wood Buildings 17–9

 Literature Cited 17–12

Chapter 18 Fire Safety of Wood 

Construction

 Fire Safety Design and Evaluation 18–1

 Fire-Performance Characteristics of Wood 18–8

 Fire-Retardant-Treated Wood 18–15

 Literature Cited 18–18

 Additional References 18–18

Chapter 19 Specialty Treatments

 Plasticizing Wood 19–1

 Modified Woods 19–4

 Paper-Based Plastic Laminates 19–12

 References 19–14

Chapter 20 Heat Sterilization of Wood

 Heat Treatment Standards 20–1

 Factors Affecting Heating Times 20–2

 Methods for Estimating Heating Times 20–5

 American Lumber Standard Committee (ALSC)

 Enforcement Regulations 20–9

 Quality Mark 20–11

 Other Considerations 20–11

 Literature Cited 20–13

Glossary 

Index 
v

FOREWORD

We are proud to present this edition of the Wood 

Handbook—Wood as an Engineering Material, prepared and updated to include fascinating new developments in the field of wood utilization and released as 

part of the celebration of the Forest Products Laboratory’s first 100 years of service to the public. 

Efficient use of our nation’s timber is of critical importance. This handbook is intended to serve as a primary 

reference on the use of wood in a variety of applications—from general construction to use of wood for 

decorative purposes. It provides engineers, architects, 

and others who use wood with a source of information 

on the various properties of wood, its relationship with 

moisture, and characteristics of various wood-based 

materials. Continuing research holds promise for wider 

and more efficient utilization of wood in an increasing 

number of applications.

This handbook was prepared by the Forest Products 

Laboratory (FPL), a research unit within the USDA 

Forest Service. The FPL, first established in 1910 in 

Madison, Wisconsin, was the first institution in the 

world to conduct general research on wood and its 

utilization. The information that resulted from many 

of its scientific investigations of wood and wood products over the past century is the primary basis for this 

handbook. 

The Wood Handbook was first issued in 1935, and 

slightly revised in 1939, as an unnumbered publication. Further revisions in 1955, 1974, and 1987 were 

published by the U.S. Department of Agriculture as 

Agriculture Handbook No. 72. The 1999 revision was 

published by the FPL as General Technical Report 

FPL–GTR–113 and reprinted for broader distribution 

by the Forest Products Society.

The audience for the Wood Handbook is broad. Consequently, the coverage of each chapter is aimed at 

providing a general discussion of the topic, with references included for additional information. Thousands 

more publications are available on the FPL website 

(www.fpl.fs.fed.us). 

Wood resources continue to play an important role 

in the world, from packaging materials to buildings 

to transportation structures. Wood has been useful to 

human societies for thousands of years; archeological discoveries have shown wood was used by ancient 

civilizations as a construction material, as a substrate 

for ornate decorative objects, and for providing the 

final resting place for royalty. These discoveries 

highlight the unique, long-lasting performance characteristics of wood, as many of these artifacts have 

survived for thousands of years. FPL continues on its 

journey of discovery and public service; working with 

cooperators from around the world, we are discovering 

information that covers the entire spectrum of wood 

science—from the use of wood in ancient societies to 

developing new theories that describe the fundamental structure of wood based on the emerging field of 

nanoscience. If our forests are managed wisely, and if 

we continue to build our intellectual capacity to meet 

the challenges of evolving human needs and changing wood characteristics, this amazing material that is 

wood will serve the public well for years to come. 

Christopher D. Risbrudt, Director 

Michael A. Ritter, Assistant Director

Theodore H. Wegner, Assistant Director

USDA Forest Service

Forest Products Laboratory

vii

PREFACE

This edition of the Wood Handbook—Wood as an 

Engineering Material builds upon past editions, in 

particular the 1999 version, with some important additions and modifications:

•	 A chapter has been added that highlights the importance of wood as an environmentally responsible, sustainable material (Chapter 1). 

•	 Low-magnification micrographs of cross sections 

of commercial wood species have been added 

(Chapter 2).

•	 An extensive discussion on the microscopic structure of wood and its foundational elements are 

presented (Chapter 3).

•	 Reference to the most recent research on properties of the wood cell wall, at the nanoscale, has 

been included (Chapter 5). 

•	 To address the need to find uses for wood obtained 

from trees killed by invasive insect species as they 

propagate through various regions of the United 

States, a chapter has been added on heat-treating 

and sterilization procedures for wood products 

(Chapter 20).

•	 Important updates are included on wood–moisture interactions and wood preservation practices 

(Chapters 4 and 15). 

The Wood Handbook originally focused on construction practices that utilized solid-sawn wood. Since its 

first printing, the state-of-the-art in wood construction practices and the range of wood-based products 

available to the consumer have changed considerably. 

Excellent printed reference and websites have been 

developed by various trade associations and wood 

products manufacturers that document, in detail, current design information for the ever-changing range of 

products available. We have made a concerted effort 

to include the most current references, in addition to 

many historic ones, to help guide the reader to appropriate sources of information. 

This 2010 edition was reviewed by numerous individuals from industry, academia, and government. Several 

dozen industry, university, and government colleagues 

reviewed various sections and chapters of this edition 

during various stages of revision. We gratefully acknowledge their contributions. 

The following individuals provided in-depth technical 

reviews of this edition in its entirety: Donald Bender 

(Washington State University), David Green (USDA 

Forest Products Laboratory, retired), John Erickson 

(USDA Forest Products Laboratory, retired), Howard 

Rosen (USDA Forest Service, retired), World Nieh 

(USDA Forest Service), Robert White (USDA Forest 

Products Laboratory), and staff of the American Wood 

Council, American Forest & Paper Association. We 

gratefully acknowledge their contributions. 

Although listing every technical author and contributor 

to the Wood Handbook would be nearly impossible—

early editions did not even list individual contributors 

by name—we do acknowledge the authors of previous 

editions; they all made significant, noteworthy contributions. 

Finally, we thank our many research cooperators from 

industry, academia, and other government agencies. 

By working with you we are able to continue developing the technical base for using wood, wood-based 

materials, and wood structural systems in a technically 

sound manner. 

Robert J. Ross, Editor

USDA Forest Service

Forest Products Laboratory

CHAPTER 1

Wood as a Sustainable Building Material

Robert H. Falk, Research General Engineer

1–1

Few building materials possess the environmental benefits 

of wood. It is not only our most widely used building material but also one with characteristics that make it suitable 

for a wide range of applications. As described in the many 

chapters of this handbook, efficient, durable, and useful 

wood products produced from trees can range from a minimally processed log at a log-home building site to a highly 

processed and highly engineered wood composite manufactured in a large production facility. 

As with any resource, we want to ensure that our raw materials are produced and used in a sustainable fashion. One 

of the greatest attributes of wood is that it is a renewable 

resource. If sustainable forest management and harvesting 

practices are followed, our wood resource will be available 

indefinitely.

Wood as a Green Building Material

Over the past decade, the concept of green building1 has 

become more mainstream and the public is becoming aware 

of the potential environmental benefits of this alternative 

to conventional construction. Much of the focus of green 

building is on reducing a building’s energy consumption 

(such as better insulation, more efficient appliances and 

heating, ventilation, and air-conditioning (HVAC) systems) 

and reducing negative human health impacts (such as controlled ventilation and humidity to reduce mold growth). 

However, choosing building materials that exhibit positive 

environmental attributes is also a major area of focus. Wood 

has many positive characteristics, including low embodied 

energy, low carbon impact, and sustainability. These characteristics are important because in the United States, a little 

more than half the wood harvested in the forest ends up as 

building material used in construction.

Embodied Energy

Embodied energy refers to the quantity of energy required to 

harvest, mine, manufacture, and transport to the point of use 

a material or product. Wood, a material that requires a minimal amount of energy-based processing, has a low level 

1Green building is defined as the practice of increasing the efficiency with which buildings use resources while reducing building 

impacts on human health and the environment—through better 

siting, design, material selection, construction, operation, maintenance, and removal—over the complete building life cycle.

Contents 

Wood as a Green Building Material 1–1

 Embodied Energy 1–1

 Carbon Impact 1–2

 Sustainability 1–3

Forest Certification Programs 1–3

 Forest Stewardship Council (FSC) 1–4

 Sustainable Forestry Initiative (SFI) 1–4

 American Tree Farm System (ATFS) 1–4

 Canadian Standards Association (CSA) 1–5

 Programme for the Endorsement of Forest 

 Certification (PEFC) Schemes 1–5

 Additional Information 1–5

Literature Cited 1–5
1–2

Their calculations show that the energy consumed in the 

manufacture of building materials (mining iron and coal for 

steel or harvesting wood for lumber) and the construction 

of a steel-framed house in Minneapolis is 17% greater than 

for a wood-framed house (Lippke and others 2004). The 

difference is even more dramatic if one considers the use 

of bioenergy in the manufacture of wood products. By this 

comparison, the steel-framed house uses 281% more nonbioenergy than the wood-framed house (Perez-Garcia and 

others 2005). Global warming potential, air emission index, 

and water emission index are all higher for steel construction than for wood construction (Table 1–2).

These analyses indicate that the amount of energy necessary 

to produce wood products is much less than comparable 

products made from other materials. If wood is substituted 

for these other materials (assuming similar durability allows 

equal substitution), energy is saved and emissions avoided 

each time wood is used, giving it a distinct environmental 

advantage over these other materials (Bowyer and others 

2008). 

Carbon Impact

The role of carbon in global climate change and its projected 

negative impact on ecosystem sustainability and the general 

health of our planet have never been more elevated in the 

public’s consciousness. 

Forests play a major role in the Earth’s carbon cycle. The 

biomass contained in our forests and other green vegetation affects the carbon cycle by removing carbon from the 

atmosphere through the photosynthesis process. This process converts carbon dioxide and water into sugars for tree 

growth and releases oxygen into the atmosphere:

energy (sunlight) + 6H2O + 6CO2  C6H12O6 + 6O2

A substantial amount of carbon can be sequestered in forest 

trees, forest litter, and forest soils. Approximately 26 billion 

metric tonnes of carbon is sequestered within standing trees, 

forest litter, and other woody debris in domestic forests, and 

another 28.7 billion tonnes in forest soils (Birdsey and 

Lewis 2002). According to Negra and others (2008), between 1995 and 2005 the rate of carbon sequestration in 

U.S. forests was about 150 million tonnes annually (not including soils), a quantity of carbon equivalent to about 10% 

of total carbon emissions nationally.

Unfortunately, deforestation in tropical areas of the world is 

responsible for the release of stored carbon, and these forests are net contributors of carbon to the atmosphere. Tropical deforestation is responsible for an estimated 20% of total 

human-caused carbon dioxide emissions each year (Schimel 

and others 2001).

Carbon in wood remains stored until the wood deteriorates 

or is burned. A tree that remains in the forest and dies releases a portion of its carbon back into the atmosphere as the 

woody material decomposes. On the other hand, if the tree 

General Technical Report FPL–GTR–190

of embodied energy relative to many other materials used in 

construction (such as steel, concrete, aluminum, or plastic). 

The sun provides the energy to grow the trees from which 

we produce wood products; fossil fuels are the primary energy source in steel and concrete manufacture. Also, over 

half the energy consumed in manufacturing wood products 

in the United States is from biomass (or bioenergy) and is 

typically produced from tree bark, sawdust, and by-products 

of pulping in papermaking processes. The U.S. wood products industry is the nation’s leading producer and consumer 

of bioenergy, accounting for about 60% of its energy needs 

(Table 1–1) (Murray and others 2006, EPA 2007). Solidsawn wood products have the lowest level of embodied 

energy; wood products requiring more processing steps (for 

example, plywood, engineered wood products, flake-based 

products) require more energy to produce but still require 

significantly less energy than their non-wood counterparts.

In some plantation forest operations, added energy costs 

may be associated with the use of fertilizer, pesticides, and 

greenhouses to grow tree seedlings. During the harvesting 

operation, energy is used to power harvesting equipment and 

for transporting logs to the mill. Lumber milling processes 

that consume energy include log and lumber transport, sawing, planing, and wood drying. Kiln drying is the most 

energy-consumptive process of lumber manufacture; however, bioenergy from a mill’s waste wood is often used to 

heat the kilns. Unlike burning fossil fuels, using bioenergy 

for fuel is considered to be carbon neutral. Also, advances in 

kiln technologies over the past few decades have significantly reduced the amount of energy required in wood drying. 

Overall, the production of dry lumber requires about twice 

the energy of producing green (undried) lumber. 

The Consortium for Research on Renewable Industrial 

Materials (CORRIM) found that different methods of forest 

management affect the level of carbon sequestration in trees 

(Perez-Garcia and others 2005). They found that shorter rotation harvests can sequester more total carbon than longer 

rotation harvests. 

CORRIM also calculated differences in energy consumed 

and environmental impacts associated with resource extraction, materials production, transportation, and disposal of 

homes built using different materials and processes. 

Table 1–1. Wood products industry 

fuel sourcesa

Fuel source 

Proportion used 

(%)

Net electricity 19 

Natural gas 16 

Fuel oil 3 

Other (primarily biomass) 61 

a

EPA (2007). 
1–3

is used to produce a wood or paper product, these products 

store carbon while in use. For example, solid wood lumber, 

a common wood product used in building construction (the 

building industry is the largest user of sawn wood in the 

United States), sequesters carbon for the life of the building. 

At the end of a building’s life, wood can be recovered for 

re-use in another structure, chipped for use as fuel or mulch, 

or sent to a landfill (usual fate). If burned or mulched, stored 

carbon is released when the wood decomposes, essentially 

the reverse process of photosynthesis:

C6H12O6 + 6O2 → 6CO2 + 6H2O

Carbon contained in wood products currently in-use and as 

wood debris in landfills is estimated at 2.5 billion tonnes and 

accumulates at a rate of about 28 million tonnes per year 

(Skog 2008). Much of the carbon contained within wood 

products resides in the nation’s housing stock, estimated 

at 116 million units in 2000. Skog (2008) estimated that in 

2001, about 680 million tonnes of carbon was stored in the 

nation’s housing stock, nearly a third of the total carbon (2.5 

billion tonnes) cited above. 

As indicated in Table 1–3, carbon emitted to produce a 

tonne of concrete is about eight times that emitted to produce a tonne of framing lumber. A similar comparison for 

steel indicates that its production emits about 21 times as 

much carbon as an equal weight of framing lumber. Wood 

products also mitigate carbon emissions to the degree that 

they substitute for steel or concrete, which emit more greenhouse gases in their production.

Also, because wood products have this low level of embodied energy compared with other building products and 

because wood is one-half carbon by weight, wood products 

can actually be carbon negative (Bowyer and others 2008). 

Comparisons of the environmental impact of various wood 

products have also been made using life cycle analysis 

software (Calkins 2009). The more processing involved in 

the manufacture of wood products (such as flaking, veneer 

cutting, added heat for pressing, gluing, kiln drying), the 

more impact on energy use, solid waste production, pollution production, and global warming potential (carbon).

Sustainability

Unlike metals and fossil-fuel-based products (such as 

plastics), our forest resource is renewable and with proper 

management a flow of wood products can be maintained 

indefinitely. The importance of forest-based products to our 

economy and standard of living is hard to overemphasize—

half of all major industrial raw materials we use in the United States come from forests. However, the sustainability of 

this resource requires forestry and harvesting practices that 

ensure the long-term health and diversity of our forests. Unfortunately, sustainable practices have not always been applied in the past, nor are they universally applied around the 

world today. Architects, product designers, material specifiers, and homeowners are increasingly asking for building 

products that are certified to be from a sustainable source. 

For the forest products sector, the result of this demand has 

been the formation of forest certification programs. These 

programs not only ensure that the forest resource is harvested in a sustainable fashion but also that issues of biodiversity, habitat protection, and indigenous peoples’ rights are 

included in land management plans.

Forest Certification Programs

More than 50 different forest certification systems in the 

world today represent nearly 700 million acres of forestland 

and 15,000 companies involved in producing and 

Table 1–2. Environmental performance indices for above-grade wall 

designs in residential constructiona

Wood

frame

Steel 

frame Difference

Changeb

(%)

Minneapolis design 

 Embodied energy (GJ) 250 296 46 +18 

 Global warming potential (CO2 kg) 13,009 17,262 4,253 +33 

 Air emission index (index scale) 3,820 4,222 402 +11 

 Water emission index (index scale) 3 29 26 +867 

 Solid waste (total kg) 3,496 3,181 –315 –0.9 

Atlanta design 

 Embodied energy (GJ) 168 231 63 +38 

 Global warming potential (CO2 kg) 8,345 14,982 6,637 +80 

 Air emission index (index scale) 2,313 3,373 1,060 +46 

 Water emission index (index scale) 2 2 0 0 

 Solid waste (total kg) 2,325 6,152 3,827 +164 a

Lippke and others (2004). 

b

% change = [(Steel frame – Wood frame)/(Wood frame)] × 100.

Chapter 1 Wood as a Sustainable Building Material
productivity while allowing for conservation of resources 

(Howe and others 2004). The growing trends in green 

building are helping drive certification in the construction 

market in the United States.

Forest Stewardship Council (FSC)

FSC is an independent, non-governmental 

organization established to promote responsible management of the world’s forests 

and is probably the most well-known forest certification program worldwide. More 

than 280 million acres of forest worldwide are certified to 

FSC standards and are distributed over 79 countries. The 

FSC program includes two types of certifications. The Forest Management Certification applies FSC standards of 

responsible forestry to management of the forest land. A 

Chain-of-Custody (COC) certification ensures that forest 

products that carry the FSC label can be tracked back to 

the certified forest from which they came. More than 9,000 

COC certifications are in use by FSC members. The FSC 

has certified 18 certification bodies around the world. Four 

are located in the United States, including the non-profit 

Rainforest Alliance’s SmartWood program and the for-profit 

Scientific Certification Systems. Both organizations provide 

up-to-date lists of FSC-certified wood suppliers across the 

country. 

Sustainable Forestry Initiative (SFI) 

The SFI program was established 

in 1994 and currently certifies over 

152 million acres in the United 

States and Canada. This program 

has a strong wood industry focus 

and has been adopted by most of the major industrial forest 

landowners in the United States. It is based on the premise 

that responsible forest practices and sound business decisions can co-exist. The SFI program includes third-party 

certification, which verifies the requirements of the SFI 

2010–2014 Standard. Independent certification bodies evaluate planning, procedures, and processes in the forest and in 

wood processing operations. Annual surveillance audits are 

mandatory on all certified operations, and a full recertification audit is required for forest operations every 3 years.

American Tree Farm System (ATFS) 

The American Tree Farm System, a 

program of the American Forest Foundation’s Center for Family Forests, is the 

oldest of forest certification programs 

and was established in 1941. The ATFS 

focuses its program on private family 

forest landowners in the United States. Currently, ATFS 

has certified 24 million acres of privately owned forestland 

and more than 90,000 family forest owners. The ATFS forest certification standard requires forest owners to develop 

1–4

marketing certified products. These programs represent 

about 8% of the global forest area and 13% of managed 

forests. From 2007 to 2008, the world’s certified forest area 

grew by nearly 9%. North America has certified more than 

one-third of its forests and Europe more than 50% of its forests; however, Africa and Asia have certified less than 0.1%. 

Approximately 80% to 90% of the world’s certified forests 

are located in the northern hemisphere, where two thirds 

of the world’s roundwood is produced (UNECE 2008). In 

North America, five major certification systems are used:

•	 Forest Stewardship Council (FSC) 

•	 Sustainable Forestry Initiative (SFI) 

•	 American Tree Farm System (ATFS)

•	 Canadian Standards Association (CSA)

•	 Programme for the Endorsement of Forest 

 Certification (PEFC) schemes 

In terms of forest acreage under certification, the Forest 

Stewardship Council and the Sustainable Forestry Initiative dominate in the United States. These two systems 

evolved from different perspectives of sustainability. The 

FSC’s guidelines are geared more to preserve natural 

systems while allowing for careful harvest, whereas 

the SFI’s guidelines are aimed at encouraging fiber 

Table 1–3. Net carbon emissions in producing a 

tonne of various materials 

Material 

Net carbon 

emissions

(kg C/t)a,b

Near-term net carbon 

emissions including 

carbon storage 

within material 

(kg C/t)c,d

Framing lumber 33 –457 

Medium-density 

fiberboard (virgin fiber) 

60 –382 

Brick 88 88 

Glass 154 154 

Recycled steel 

(100% from scrap) 

220 220 

Concrete 265 265 

Concretee 291 291 

Recycled aluminum 

(100% recycled content) 

309 309 

Steel (virgin) 694 694 

Plastic 2,502 2,502 

Aluminum (virgin) 4,532 4,532 a

Values are based on life-cycle assessment and include gathering and 

processing of raw materials, primary and secondary processing, and 

transportation.

b

Source: EPA (2006). 

c

From Bowyer and others (2008); a carbon content of 49% is assumed 

for wood. 

d

The carbon stored within wood will eventually be emitted back to the 

atmosphere at the end of the useful life of the wood product. 

e

Derived based on EPA value for concrete and consideration of 

additional steps involved in making blocks.

General Technical Report FPL–GTR–190
a management plan based on strict environmental standards 

and pass an inspection by an ATFS inspecting forester. 

Third-party certification audits, conducted by firms accredited by the ANSI–ASQ National Accreditation Board 

(ANAB) or the Standards Council of Canada (SCC), are 

required for all certifications of the ATFS.

Canadian Standards Association (CSA)

The Canadian Standards Association is a 

non-profit organization and has developed 

over 2,000 different standards for a variety of industries. The CSA first published 

Canada’s National Standard for Sustainable Forest Management (SFM) CAN/CSA-Z809 in 1996. 

The SFM program has four components: the SFM Standard 

itself, a Chain-of-Custody program, product marking, and 

the CSA International Forest Products Group, which promotes the program. The CSA Standard has been adopted by 

the major industrial forestland managers in Canada. As 

of June 2007, about 60% (198 million acres) of Canadian 

forests were certified under the CAN/CSA-Z809 SFM 

Standard.

Programme for the Endorsement of 

Forest Certification (PEFC) Schemes 

The multitude of certification programs 

with competing standards and claims has 

made it difficult for land managers, members of the wood industry, and consumers 

to determine which certification program 

fits their needs (Fernholz and others 2004). 

The Programme for the Endorsement of Forest Certification schemes was developed to address this issue and serves 

as an umbrella endorsement system that provides international recognition for national forest certification programs. 

Founded in 1999, the PEFC represents most of the world’s 

certified forest programs and the production of millions of 

tons of certified timber. The FSC, SFI, and ATFS programs 

have received official PEFC endorsement.

Additional Information

Helpful online tools provide more information and data on 

forest certification, including the Forest Certification Resource Center (www.metafore.org), which identifies forests, 

manufacturers, distributors, importers, and retailers certified under FSC, SFI, and CSA programs. The database is 

searchable by product, location, and certification system. 

Another helpful resource is the Forest Products Annual 

Market Review (www.unece.org), which provides general 

and statistical information on forest products markets in 

the United Nations Economic Commission for Europe 

(UNECE) and covers the regions of Europe, North America, and the Commonwealth of Independent States.

Literature Cited 

Birdsey, R.; Lewis, G. 2002. Carbon in U.S. forests and 

wood products, 1987–1997: state by state estimates. Gen. 

Tech. Rep. GTR–NE–310. Washington, DC: U.S. Department of Agriculture, Forest Service.

Bowyer, J.; Bratkovich, S.; Lindberg, A.; Fernholz, K. 2008. 

Wood products and carbon protocols: carbon storage and 

low energy intensity should be considered. Minneapolis, 

MN: Dovetail Partners, Inc. www.dovetailinc.org. (28 April 

2008). 13 p.

Calkins, M. 2009. Materials for sustainable sites: a complete 

guide to the evaluation, selection, and use of sustainable 

construction materials. Hoboken, NJ: John Wiley & Sons. 

457 p.

EPA. 2006. Solid waste management and greenhouse gases—a life cycle assessment of emissions and sinks. 3rd Ed. 

Washington, DC: U.S. Environmental Protection Agency.

EPA. 2007. Energy trends in selected manufacturing sectors: 

opportunities and challenges for environmentally preferable 

energy outcomes. Washington, DC: U.S. Environmental 

Protection Agency, Office of Policy, Economics and Innovation. (March 2007).

Fernholz, K.; Howe, J.; Guillery, P.; Bowyer, J. 2004. Beginner’s guide to third-party forest certification: shining a 

light on the programme for the endorsement of forest certification schemes (PEFC). Minneapolis, MN: Dovetail Partners, Inc. www.dovetailinc.org.

Fernholz, K.; Howe, J.; Guillery, P.; Bowyer, J. 2005. Beginner’s guide to third-party forest certification: shining a 

light on the Canadian Standards Association (CSA). Minneapolis, MN: Dovetail Partners, Inc. www.dovetailinc.org. 

(18 January 2005). 10 p.

Howe, J.; Fernholz, K.; Guillery, P.; Bowyer, J. 2004. A land 

manager’s guide to FSC & SFI—to certify or not to certify: 

is that a question? Minneapolis, MN: Dovetail Partners, Inc. 

www.dovetailinc.org. (15 September 2004). 13 p.

Lippke, B.; Wilson, J.; Perez-Garcia, J.; Bowyer, J.; Meil, J. 

2004. CORRIM: life-cycle environmental performance of 

renewable building materials. Forest Products Journal.

54(6): 13. (June 2004).

Murray, B.; Nicholson, R.; Ross, M.; Holloway, T.; Patil, S. 

2006. Biomass energy consumption in the forest products 

industry. Washington, DC: U.S. Department of Energy; Research Triangle Park, NC: RTI International. 81 p.

Negra, C.; Sweedo, C.; Cavender-Bares, K.; O’Malley, R. 

2008. Indicators of carbon storage in U.S. ecosystems: baseline for terrestrial carbon accounting. Journal of Environmental Quality. 37: 1376–1382.

1–5

Chapter 1 Wood as a Sustainable Building Material
1–6

Perez-Garcia, J.; Lippke, B.; Briggs, D.; Wilson, J.; Bowyer, 

J.; Meil, J. 2005. The environmental performance of renewable building materials in the context of residential construction. Wood and Fiber Science. (37)12: 3–17.

Schimel, D.S.; House, J.I.; Hibbard, K.A.; Bousquet, P.; 

Ciais, P.; Peylin, P.; Braswell, B.H.; Apps, M.J.; Baker, 

D.; Bondeau, A.; Canadell, J.; Churkina, G.; Cramer, W.; 

Denning, A.S.; Field, C.B.; Friedlingstein, P.; Goodale, C.; 

Heimann, M.; Houghton, R.A.; Melillo, J.M.; Moore, B., 

III; Murdiyarso, D.; Noble, I.; Pacala, S.W.; Prentice, I.C.; 

Raupach, M.R.; Rayner, P.J.; Scholes, R.J.; Steffen, W.L.; 

Wirth, C. 2001. Recent patterns and mechanisms of carbon 

exchange by terrestrial ecosystems. Nature. 414: 169–172.

Skog, K. 2008. Sequestration of carbon in harvested wood 

products for the United States. Forest Products Journal. 

58(6): 56–72.

UNECE / FAO. 2008. Forest products annual market review 

2007–2008. Geneva, Switzerland: United Nations Economic 

Commission for Europe / Food and Agriculture Organization. www.unece.org/trade/timber. 

General Technical Report FPL–GTR–190
CHAPTER 2

Characteristics and Availability of Commercially 

Important Woods

Michael C. Wiemann, Research Botanist

2–1

Throughout history, the unique characteristics and abundance of wood have made it a natural material for homes 

and other structures, furniture, tools, vehicles, and decorative objects. Today, for the same reasons, wood is prized for 

a multitude of uses.

All wood is composed of cellulose, lignin, hemicelluloses, 

and minor amounts (usually less than 10%) of extraneous 

materials contained in a cellular structure. Variations in the 

characteristics and proportions of these components and 

differences in cellular structure make woods heavy or light, 

stiff or flexible, and hard or soft. The properties of a single 

species are relatively constant within limits; therefore, selection of wood by species alone may sometimes be adequate. 

However, to use wood to its best advantage and most effectively in engineering applications, specific characteristics or 

physical properties must be considered.

Historically, some species filled many purposes, whereas 

other less available or less desirable species served only 

one or two needs. For example, because white oak is tough, 

strong, and durable, it was highly prized for shipbuilding, 

bridges, cooperage, barn timbers, farm implements, railroad 

crossties, fence posts, and flooring. Woods such as black 

walnut and cherry were used primarily for furniture and 

cabinets. Hickory was manufactured into tough, hard, and 

resilient striking-tool handles, and black locust was prized 

for barn timbers. It was commonly accepted that wood from 

trees grown in certain locations under certain conditions was 

stronger, more durable, more easily worked with tools, or 

finer grained than wood from trees in other locations. Modern research on wood has substantiated that location and 

growth conditions do significantly affect wood properties.

This chapter presents brief descriptions of many species; 

current and, in many cases, historic uses are cited to illustrate the utility of the wood.

Gradual reductions in use of old-growth forests in the 

United States have reduced the supply of large clear logs for 

lumber and veneer. However, the importance of high-quality 

logs has diminished as new concepts of wood use have been 

introduced. Second-growth wood, the remaining old-growth 

forests, and imports continue to fill the needs for wood in 

the quality required. Wood is as valuable an engineering 

material as ever, and in many cases, technological advances 

have made it even more useful.

Inherent factors that keep wood in the forefront of raw 

materials are many and varied, but a chief attribute is its 

Contents 

Timber Resources and Uses 2–2

 Hardwoods and Softwoods 2–2

 Commercial Sources of Wood Products 2–2

 Use Classes and Trends 2–2

Species Descriptions 2–3

 U.S. Hardwoods 2–3

 U.S. Softwoods 2–11

Imported Woods 2–18 

 Imported Hardwoods 2–19

 Imported Softwoods 2–38

Scientific Name Index 2–41

 U.S. Wood Species—Hardwoods 2–41

 U.S. Wood Species—Softwoods 2–42

 Imported Woods—Hardwoods 2–43

 Imported Woods—Softwoods 2–44

Literature Cited 2–45

Additional References 2–45
2–2

Botanically, softwoods are gymnosperms or conifers; their 

seeds are not enclosed in the ovary of the flower. Anatomically, softwoods are nonporous (they do not contain 

vessels). Softwoods are usually cone-bearing plants with 

needle- or scale-like evergreen leaves. Some softwoods, 

such as larches and baldcypress, lose their needles during 

autumn or winter.

Major resources of softwood species are spread across the 

United States, except for the Great Plains, where only small 

areas are forested. The hardwood resource is concentrated in 

the eastern United States, with only a few commercial species found in Washington, Oregon, and California. Softwood 

and hardwood species of the continental United States are 

often loosely grouped in three general regions, as shown in 

Table 2–1.

Commercial Sources of Wood Products

Softwoods are available directly from sawmills, wholesale 

and retail yards, or lumber brokers. Softwood lumber and 

plywood are used in construction for forms, scaffolding, 

framing, sheathing, flooring, moulding, paneling, cabinets, 

poles and piles, and many other building components. 

Softwoods may also appear in the form of shingles, sashes, 

doors, and other millwork, in addition to some rough products such as timber and round posts.

Hardwoods are used in construction for flooring, architectural woodwork, interior woodwork, and paneling. These items 

are usually available from lumberyards and building supply 

dealers. Most hardwood lumber and dimension stock are 

remanufactured into furniture, flooring, pallets, containers, 

dunnage, and blocking. Hardwood lumber and dimension 

stock are available directly from manufacturers, through 

wholesalers and brokers, and from some retail yards. Both 

softwood and hardwood products are distributed throughout 

the United States. Local preferences and the availability of 

certain species may influence choice, but a wide selection 

of woods is generally available for building construction, 

industrial uses, remanufacturing, and home use.

Use Classes and Trends

Major wood-based industries include those that convert 

wood to thin slices (veneer), particles (chips, flakes), or fiber 

pulps and reassemble the elements to produce various types 

of engineered panels such as plywood, particleboard, oriented strandboard, laminated veneer lumber, paper, paperboard, 

and fiberboard products. Another newer wood industry is the 

production of laminated wood products. The lumber industry has also produced smaller amounts of railroad crossties, 

cooperage, shingles, and shakes.

General Technical Report FPL–GTR–190

availability in many species, sizes, shapes, and conditions to 

suit almost every demand. Wood has a high ratio of strength 

to weight and a remarkable record for durability and performance as a structural material. Dry wood has good insulating properties against heat, sound, and electricity. It tends 

to absorb and dissipate vibrations under some conditions 

of use, and yet it is an incomparable material for musical 

instruments. The grain patterns and colors of wood make 

it an esthetically pleasing material, and its appearance may 

be easily enhanced by stains, varnishes, lacquers, and other 

finishes. It is easily shaped with tools and fastened with adhesives, nails, screws, bolts, and dowels. Damaged wood is 

easily repaired, and wood structures are easily remodeled or 

altered. In addition, wood resists oxidation, acid, saltwater, 

and other corrosive agents, has high salvage value, has good 

shock resistance, can be treated with preservatives and fire 

retardants, and can be combined with almost any other material for both functional and aesthetic uses.

Timber Resources and Uses

In the United States, more than 100 wood species are available to the prospective user; about 60% of these are of major 

commercial importance. Another 30 species are commonly 

imported in the form of logs, cants, lumber, and veneer for 

industrial uses, the building trade, and crafts.

A continuing program of timber inventory is in effect in the 

United States through the cooperation of Federal and State 

agencies, and new information on wood resources is published in State and Federal reports. Two of the most valuable 

sourcebooks are An Analysis of the Timber Situation in the 

United States: 1952 to 2050 (Haynes 2003) and The 2005 

RPA Timber Assessment Update (Haynes and others 2007). 

Current information on wood consumption, production, imports, and supply and demand is published periodically by 

the Forest Products Laboratory (Howard 2007).

Hardwoods and Softwoods

Trees are divided into two broad classes, usually referred to 

as hardwoods and softwoods. These names can be confusing because some softwoods are actually harder than some 

hardwoods, and conversely some hardwoods are softer than 

some softwoods. For example, softwoods such as longleaf 

pine and Douglas-fir are typically harder than the hardwoods basswood and aspen. Botanically, hardwoods are 

angiosperms; their seeds are enclosed in the ovary of the 

flower. Anatomically, hardwoods are porous; that is, they 

contain vessel elements. A vessel element is a wood cell 

with open ends; when vessel elements are set one above another, they form a continuous tube (vessel), which serves as 

a conduit for transporting water or sap in the tree. Typically, 

hardwoods are plants with broad leaves that, with few exceptions in the temperate region, lose their leaves in autumn 

or winter. Most imported tropical woods are hardwoods. 
2–3

Species Descriptions

In this chapter, each species or group of species is described 

in terms of its principal location, characteristics, and uses. 

More detailed information on the properties of these and 

other species is given in various tables throughout this 

handbook. Information on historical and traditional uses is 

provided for some species to illustrate their utility. A lowmagnification micrograph of a representative cross-section 

of each species or species group accompanies each description. The slides for these micrographs are from the Forest 

Products Laboratory collection. The micrographs are printed 

at magnifications of approximately 15×. Their color is a 

consequence of the stains used to accentuate anatomical features and is not indicative of the actual wood color.

U.S. Hardwoods

Alder, Red

Red alder (Alnus rubra) 

grows along the Pacific 

coast between Alaska and 

California. It is the principal 

hardwood for commercial 

manufacture of wood 

products in Oregon and 

Washington and the most 

abundant commercial hardwood species in these two 

states.

The wood of red alder varies from almost white to pale 

pinkish brown, and there is no visible boundary between 

heartwood and sapwood. Red alder is moderately light in 

weight and intermediate in most strength properties but low 

in shock resistance. It has relatively low shrinkage.

The principal use of red alder is for furniture, but it is also 

used for sash and door panel stock and other millwork.

Ash (Black Ash Group)

The black ash group includes 

black ash (Fraxinus nigra) 

and pumpkin ash (F. profun‐

da). Black ash grows in the 

Northeast and Midwest, and 

pumpkin ash in the South.

The heartwood of black ash 

is a darker brown than that of 

American white ash; the sapwood is light-colored or nearly 

white. The wood of the black ash group is lighter in weight 

(basic specific gravity of 0.45 to 0.48) than that of the white 

ash group (basic specific gravity greater than 0.50). 

Principal uses for the black ash group are decorative veneer, 

cabinets, millwork, furniture, cooperage, and crates.

Ash (White Ash Group)

Important species of the 

white ash group are American white ash (Fraxinus 

americana), green ash 

(F. pennsylvanica), blue 

ash (F. quadrangulata), and 

Oregon ash (F. latifolia). 

The first three species grow 

in the eastern half of the United States. Oregon ash grows 

along the Pacific Coast.

The heartwood of the white ash group is brown, and the 

sapwood is light-colored or nearly white. Second-growth 

trees are particularly sought after because of the inherent 

qualities of the wood from these trees: it is heavy, strong, 

hard, and stiff, and it has high resistance to shock. Oregon 

ash has somewhat lower strength properties than American 

Table 2–1. Major resources of U.S. woods according to 

region 

Western 

Northern 

and Appalachian Southern

Hardwoods 

Alder, red Ash Ash 

Ash, Oregon Aspen Basswood 

Aspen Basswood Beech 

Birch, paper Beech Butternut 

Cottonwood Birch Cottonwood 

Maple, bigleaf Buckeye Elm 

Oak, California black Butternut Hackberry 

Oak, Oregon white Cherry Hickory 

Tanoak Cottonwood Honeylocust 

Elm Locust, black 

Hackberry Magnolia 

Hickory Maple, soft 

Honeylocust Oak, red and white 

Locust, black Sassafras 

Maple, hard Sweetgum 

Maple, soft Sycamore 

Oak, red and white Tupelo 

Sycamore Walnut 

Walnut Willow 

Yellow-poplar Yellow-poplar 

Softwoods 

Douglas-fir Cedar, northern white Baldcypress 

Fir, western Fir, balsam Cedar, Atlantic white

Hemlock, western 

 and mountain 

Hemlock, eastern 

Pine, eastern white 

Fir, Fraser 

Pine, southern 

Incense-cedar Pine, Jack Redcedar, eastern 

Larch, western Pine, red 

Pine, lodgepole Redcedar, eastern 

Pine, ponderosa Spruce, eastern 

Pine, sugar Tamarack 

Pine, western white 

Port-Orford-cedar 

Redcedar, western 

Redwood 

Spruce, Engelmann 

Spruce, Sitka 

Yellow-cedar 

Chapter 2 Characteristics and Availability of Commercially Important Woods
2–4

of either species. Basswood grows in the eastern half of 

North America from the Canadian provinces southward. 

Most basswood lumber comes from the Lake, Middle Atlantic, and Central States.

The heartwood of basswood is pale yellowish brown with 

occasional darker streaks. Basswood has wide, creamy 

white or pale brown sapwood that merges gradually into 

heartwood. When dry, the wood is without odor or taste. 

It is soft and light in weight, has fine, even texture, and is 

straight grained and easy to work with tools. Shrinkage in 

width and thickness during drying is rated as high; however, 

basswood seldom warps in use.

Basswood lumber is used mainly in venetian blinds, sashes 

and door frames, moulding, apiary supplies, wooden ware, 

and boxes. Some basswood is cut for veneer, cooperage, excelsior, and pulpwood, and it is a favorite of wood carvers.

Beech, American

Only one species of 

beech, American beech 

(Fagus grandifolia), is 

native to the United States. 

It grows in the eastern 

one-third of the United 

States and adjacent 

Canadian provinces. 

The greatest production 

of beech lumber is in the Central and Middle Atlantic 

States.

In some beech trees, color varies from nearly white sapwood to reddish-brown heartwood. Sometimes there is no 

clear line of demarcation between heartwood and sapwood. 

Sapwood may be roughly 7 to 13 cm (3 to 5 in.) wide. The 

wood has little figure and is of close, uniform texture. It has 

no characteristic taste or odor. The wood of beech is classed 

as heavy, hard, strong, high in resistance to shock, and highly suitable for steam bending. Beech shrinks substantially 

and therefore requires careful drying. It machines smoothly, 

is an excellent wood for turning, wears well, and is rather 

easily treated with preservatives.

Most beech is used for flooring, furniture, brush blocks, 

handles, veneer, woodenware, containers, and cooperage. 

When treated with preservative, beech is suitable for railway ties.

Birch

The three most important 

species are yellow birch 

(Betula alleghaniensis), 

sweet birch (B. lenta), and 

paper birch (B. papyrifera). 

These three species are the 

source of most birch lumber 

and veneer. Other birch 

species of some commercial 

white ash, but it is used for similar purposes on the West 

Coast.

American white ash (F. americana) and green ash (F. penn‐

sylvanica) that grow in southern river bottoms, especially 

in areas frequently flooded for long periods, produce buttresses that contain relatively lightweight and brash wood.

American white ash is used principally for nonstriking tool 

handles, oars, baseball bats, and other sporting and athletic 

goods. For handles of the best grade, some handle specifications call for not less than 2 nor more than 7 growth rings 

per centimeter (not less than 5 nor more than 17 growth 

rings per inch). The additional weight requirement of 

690 kg m–3 (43 lb ft–3) or more at 12% moisture content 

ensures high-quality material. Principal uses for the white 

ash group are decorative veneer, cabinets, furniture, flooring, millwork, and crates.

Aspen

Aspen is a generally 

recognized name that is 

applied to bigtooth 

(Populus grandidentata) 

and quaking (P. tremuloides) 

aspen. Aspen lumber 

is produced principally in 

the northeastern and Lake 

States, with some production 

 in the Rocky Mountain States.

The heartwood of aspen is grayish white to light grayish brown. The sapwood is lighter colored and generally 

merges gradually into the heartwood without being clearly 

marked. Aspen wood is usually straight grained with a 

fine, uniform texture. It is easily worked. Well-dried aspen 

lumber does not impart odor or flavor to foodstuffs. The 

wood of aspen is lightweight and soft. It is low in strength, 

moderately stiff, and moderately low in resistance to shock 

and has moderately high shrinkage.

Aspen is cut for lumber, pallets, boxes and crating, pulpwood, particleboard, strand panels, excelsior, matches, veneer, and miscellaneous turned articles. Today, aspen 

is one of the preferred species for use in oriented 

strandboard, a panel product that dominates the sheathing 

market.

Basswood

American basswood (Tilia 

americana) is the most 

important of the native 

basswood species; next in 

importance is white basswood (T. heterophylla), 

and no attempt is made 

to distinguish between 

these species in lumber form. In commercial usage, “white 

basswood” is used to specify the white wood or sapwood 

General Technical Report FPL–GTR–190
2–5

importance are river birch (B. nigra), and gray birch 

(B. populifolia). Paper birch is transcontinental, whereas 

yellow and sweet birch grow principally in the Northeast and the Lake States; yellow and sweet birch also 

grow along the Appalachian Mountains to northern 

Georgia.

Yellow birch has white sapwood and light reddish-brown 

heartwood. Sweet birch has light-colored sapwood and 

dark brown heartwood tinged with red. For both yellow and 

sweet birch, the wood is heavy, hard, and strong, and has 

good shock-resisting ability. The wood is fine and uniform 

in texture. Paper birch is lower in weight, softer, and lower 

in strength than yellow and sweet birch. Birch shrinks considerably during drying.

Yellow and sweet birch lumber is used primarily for the 

manufacture of furniture, boxes, baskets, crates, wooden 

ware, cooperage, interior woodwork, and doors; veneer 

plywood is used for doors, furniture, paneling, cabinets, 

aircraft, and other specialty uses. Paper birch is used for 

toothpicks, tongue depressors, ice cream sticks, and turned 

products, including spools, bobbins, small handles, and 

toys.

Buckeye

Buckeye consists of two species, yellow buckeye (Aes‐

culus octandra) and Ohio 

buckeye (A. glabra). These 

species range from the Appalachians of Pennsylvania, 

Virginia, and North Carolina 

westward to Kansas, Oklahoma, and Texas. Buckeye is 

not customarily separated from other species when manufactured into lumber and can be used for the same purposes 

as aspen (Populus), basswood (Tilia), and sapwood of 

yellow-poplar (Liriodendron tulipifera).

The white sapwood of buckeye merges gradually into the 

creamy or yellowish white heartwood. The wood is uniform 

in texture, generally straight grained, light in weight, soft, 

and low in shock resistance. It is rated low on machinability 

such as shaping, mortising, boring, and turning.

Buckeye is suitable for pulping for paper; in lumber form, 

it has been used principally for furniture, boxes and crates, 

food containers, wooden ware, novelties, and planing mill 

products.

Butternut

Also called white walnut, 

butternut (Juglans cinerea) 

grows from southern New 

Brunswick and Maine west 

to Minnesota. Its southern 

range extends into northeastern Arkansas and eastward 

to western North Carolina.

The narrow sapwood is nearly white and the heartwood is 

light brown, frequently modified by pinkish tones or darker 

brown streaks. The wood is moderately light in weight, 

rather coarse textured, moderately weak in bending and 

endwise compression, relatively low in stiffness, moderately soft, and moderately high in shock resistance. Butternut 

machines easily and finishes well. In many ways, butternut 

resembles black walnut, especially when stained, but it does 

not have the same strength or hardness.

Principal uses are for lumber and veneer, which are further 

manufactured into furniture, cabinets, paneling, interior 

woodwork, and miscellaneous rough items.

Cherry, Black

Black cherry (Prunus sero‐

tina) is sometimes known 

as cherry, wild black cherry, 

and wild cherry. It is the 

only native species of the 

genus Prunus that produces 

commercial lumber. Black 

cherry is found from southeastern Canada throughout the eastern half of the United 

States. Production is centered chiefly in the Middle Atlantic 

States.

The heartwood of black cherry varies from light to dark 

reddish brown and has a distinctive luster. The nearly white 

sapwood is narrow in old-growth trees and wider in secondgrowth trees. The wood has a fairly uniform texture and 

very good machining properties. It is moderately heavy, 

strong, stiff, and moderately hard, with high shock resistance. Although it has moderately high shrinkage, it is very 

dimensionally stable after drying.

Black cherry is used principally for furniture, fine veneer 

panels, and architectural woodwork. Other uses include 

burial caskets, wooden ware, novelties, patterns, and 

paneling.

Chestnut, American

American chestnut (Casta‐

nea dentata) is also known 

as sweet chestnut. Before 

this species was attacked by 

a blight in the 1920s, it grew 

in commercial quantities 

from New England to northern Georgia. Practically all 

standing chestnut has been killed by blight, and most supplies of the lumber come from salvaged timbers. Because 

of the species’ natural resistance to decay, standing dead 

trees in the Appalachian Mountains continued to provide 

substantial quantities of lumber for several decades after the 

blight, but this source is now exhausted.

The heartwood of chestnut is grayish brown or brown and 

darkens with age. The sapwood is very narrow and almost 

Chapter 2 Characteristics and Availability of Commercially Important Woods
2–6

September (U. serotina) elm. American elm is also known 

as white elm, slippery elm as red elm, rock elm as cork 

elm, and winged elm as wahoo. American elm is threatened 

by two diseases, Dutch Elm disease and phloem necrosis, 

which have killed hundreds of thousands of trees.

Sapwood of elm is nearly white and heartwood light brown, 

often tinged with red. Elm may be divided into two general 

classes, soft and hard, based on the weight and strength of 

the wood. Soft elm includes American and slippery elm. 

It is moderately heavy, has high shock resistance, and is 

moderately hard and stiff. Hard elm includes rock, winged, 

cedar, and September elm. These species are somewhat 

heavier than soft elm. Elm has excellent bending qualities.

Historically, elm lumber was used for boxes, baskets, 

crates, slack cooperage, furniture, agricultural supplies and 

implements, caskets and burial boxes, and wood components in vehicles. Today, elm lumber and veneer are used 

mostly for furniture and decorative panels. Hard elm is preferred for uses that require strength.

Hackberry

Hackberry (Celtis occi‐

dentalis) and sugarberry 

(C. laevigata) supply the 

lumber known in the trade 

as hackberry. Hackberry 

grows east of the Great 

Plains from Alabama, Georgia, Arkansas, and Oklahoma northward, except along the Canadian boundary. Sugarberry overlaps the southern part of the hackberry range and 

grows throughout the Southern and South Atlantic States.

Sapwood of both species varies from pale yellow to greenish or grayish yellow. The heartwood is commonly darker. 

The wood resembles elm in structure. Hackberry lumber is 

moderately heavy. It is moderately strong in bending, moderately weak in compression parallel to grain, moderately 

hard to very hard, and high in shock resistance, but low in 

stiffness. Hackberry has high shrinkage but keeps its shape 

well during drying.

Most hackberry is cut into lumber; small amounts are used 

for furniture parts, dimension stock, and veneer.

Hickory (Pecan Hickory Group)

Species of the pecan hickory group include bitternut 

hickory (Carya cordifor‐

mis), pecan hickory 

(C. illinoensis), water 

hickory (C. aquatica), 

and nutmeg hickory 

(C. myristiciformis). Bitternut hickory grows throughout the eastern half of the United States; pecan hickory, 

from central Texas and Louisiana to Missouri and Indiana; 

white. The wood is coarse in texture; growth rings are made 

conspicuous by several rows of large, distinct pores at the 

beginning of each year’s growth. Chestnut wood is moderately light in weight, moderately hard, moderately low in 

strength, moderately low in resistance to shock, and low in 

stiffness. It dries well and is easy to work with tools.

Chestnut was once used for flooring, poles, railroad crossties, furniture, caskets, boxes, shingles, crates, and corestock for veneer panels. At present, it appears most frequently as wormy chestnut for paneling, interior woodwork, 

and picture frames.

Cottonwood

Cottonwood includes several species of the genus 

Populus. Most important are 

eastern cottonwood (P. del‐

toides and its varieties), also 

known as Carolina poplar 

and whitewood; swamp cottonwood (P. heterophylla), 

also known as river cottonwood and swamp poplar; black 

cottonwood (P. trichocarpa); and balsam poplar (P. balsam‐

ifera). Eastern and swamp cottonwood grow throughout 

the eastern half of the United States. Greatest production 

of lumber is in the Southern and Central States. Black cottonwood grows on the West Coast and in western Montana, 

northern Idaho, and western Nevada. Balsam poplar grows 

from Alaska across Canada and in the northern Great Lakes 

States.

The heartwood of cottonwood is grayish white to light 

brown. The sapwood is whitish and merges gradually with 

the heartwood. The wood is comparatively uniform in texture and generally straight grained. It is odorless when well 

dried. Eastern cottonwood is moderately low in bending 

and compressive strength, moderately stiff, moderately soft, 

and moderately low in ability to resist shock. Most strength 

properties of black cottonwood are slightly lower than those 

of eastern cottonwood. Both eastern and black cottonwood 

have moderately high shrinkage. Some cottonwood is difficult to work with tools because of its fuzzy surface, which 

is mainly the result of tension wood.

Cottonwood is used principally for lumber, veneer, pulpwood, excelsior, and fuel. Lumber and veneer are used primarily for boxes, crates, baskets, and pallets.

Elm

Six species of elm grow 

in the eastern United 

States: American (Ulmus 

americana), slippery 

(U. rubra), rock (U. thoma‐

sii), winged (U. alata), 

cedar (U. crassifolia), and 

General Technical Report FPL–GTR–190
2–7

water hickory, from Texas to South Carolina; and nutmeg 

hickory, in Texas and Louisiana.

The sapwood of this group is white or nearly white and relatively wide. The heartwood is somewhat darker. The wood 

is heavy and sometimes has very high shrinkage.

Heavy pecan hickory is used for tool and implement 

handles and flooring. The lower grades are used for pallets. 

Many higher grade logs are sliced to provide veneer for furniture and decorative paneling.

Hickory (True Hickory Group)

True hickories are found 

throughout the eastern half 

of the United States. The 

species most important 

commercially are shagbark 

(Carya ovata), pignut 

(C. glabra), shellbark 

(C. laciniosa), and mockernut (C. tomentosa). The greatest commercial production 

of the true hickories for all uses is in the Middle Atlantic 

and Central States, with the Southern and South Atlantic 

States rapidly expanding to handle nearly half of all hickory 

lumber.

The sapwood of the true hickory group is white and usually quite wide, except in old, slow-growing trees. The 

heartwood is reddish. The wood is exceptionally tough, 

heavy, hard, and strong, and shrinks considerably in drying. 

For some purposes, both rings per centimeter (or inch) and 

weight are limiting factors where strength is important.

The major use for high quality hickory is for tool handles 

that require high shock resistance. It is also used for ladder rungs, athletic goods, agricultural implements, dowels, 

gymnasium apparatuses, poles, and furniture. Lower grade 

hickory is not suitable for the special uses of high quality 

hickory because of knottiness or other growth features and 

low density. However, the lower grade is useful for pallets 

and similar items. Hickory sawdust, chips, and some solid 

wood are used to flavor meat by smoking.

Honeylocust

The wood of honeylocust 

(Gleditsia triacanthos) has 

many desirable qualities, 

such as attractive figure 

and color, hardness, and 

strength, but it is little used 

because of its scarcity. This 

species is found most commonly in the eastern United States, except for New England 

and the South Atlantic and Gulf Coastal Plains.

Sapwood is generally wide and yellowish, in contrast to the 

light red to reddish-brown heartwood. The wood is very 

heavy, very hard, strong in bending, stiff, resistant to shock, 

and durable when in contact with the ground.

When available, honeylocust is primarily used locally for 

fence posts and general construction. It is occasionally used 

with other species in lumber for pallets and crating.

Locust, Black

Black locust (Robinia 

pseudoacacia) is sometimes called yellow locust. 

This species grows from 

Pennsylvania along the 

Appalachian Mountains 

to northern Georgia and 

Alabama. It is also native 

to western Arkansas and 

southern Missouri. The greatest production of black locust 

timber is in Tennessee, Kentucky, West Virginia, and 

Virginia.

Locust has narrow, creamy white sapwood. The heartwood, 

when freshly cut, varies from greenish yellow to dark 

brown. Black locust is very heavy, very hard, very resistant 

to shock, and very strong and stiff. It has moderately low 

shrinkage. The heartwood has high decay resistance.

Black locust is used for round, hewn, or split mine timbers 

as well as fence posts, poles, railroad crossties, stakes, and 

fuel. Other uses are for rough construction and crating. 

Historically, black locust was important for the manufacture 

of insulator pins and wooden pegs used in the construction 

of ships, for which the wood was well adapted because of 

its strength, decay resistance, and moderate shrinkage and 

swelling.

Magnolia

Commercial magnolia 

consists of three species: 

southern magnolia (Mag‐

nolia grandiflora), sweetbay (M. virginiana), 

and cucumbertree 

(M. acuminata). Other 

names for southern 

magnolia are evergreen 

magnolia, big laurel, and bull bay. Sweetbay is sometimes 

called swamp magnolia. The lumber produced by all three 

species is simply called magnolia. The natural range of 

sweetbay extends along the Atlantic and Gulf Coasts from 

Long Island to Texas, and that of southern magnolia extends from North Carolina to Texas. Cucumbertree grows 

from the Appalachians to the Ozarks northward to Ohio. 

Louisiana leads in the production of magnolia lumber.

Sapwood of southern magnolia is yellowish white, and 

heartwood is light to dark brown with a tinge of yellow or 

green. The wood, which has close, uniform texture and is 

Chapter 2 Characteristics and Availability of Commercially Important Woods
2–8

Heartwood and sapwood are similar in appearance to hard 

maple. Heartwood of soft maple is somewhat lighter in 

color than the sapwood and somewhat wider. The wood of 

soft maple, primarily silver and red maple, resembles that 

of hard maple but is not as heavy, hard, and strong.

Soft maple is used for railroad crossties, boxes, pallets, 

crates, furniture, veneer, wooden ware, and novelties.

Oak, Live

See Oak (Tropical)

Oak (Red Oak Group)

Most red oak comes 

from the Eastern States. 

The principal species are 

northern red (Quercus ru‐

bra), scarlet (Q. coccinea), 

Shumard (Q. shumardii), 

pin (Q. palustris), Nuttall 

(Q. nuttallii), black 

(Q. velutina), southern 

red (Q. falcata), cherrybark (Q. falcata var. pagodaefolia), 

water (Q. nigra), laurel (Q. laurifolia), and willow (Q. phel‐

los) oak.

The sapwood is nearly white and roughly 2 to 5 cm (1 to 

2 in.) wide. The heartwood is brown with a tinge of red. 

Sawn lumber of the red oak group cannot be separated by 

species on the basis of wood characteristics alone. Red oak 

lumber can be separated from white oak by the size and 

arrangement of pores in latewood and because it generally 

lacks tyloses in the pores. The open pores of red oak make 

this species group unsuitable for tight cooperage, unless the 

barrels are lined with sealer or plastic. Quartersawn lumber 

of the oaks is distinguished by its broad and conspicuous 

rays. Wood of the red oaks is heavy. Rapidly grown secondgrowth wood is generally harder and tougher than finer 

textured old-growth wood. The red oaks have fairly high 

shrinkage upon drying.

The red oaks are primarily cut into lumber, railroad crossties, mine timbers, fence posts, veneer, pulpwood, and 

fuelwood. Ties, mine timbers, and fence posts require preservative treatment for satisfactory service. Red oak lumber 

is remanufactured into flooring, furniture, general millwork, 

boxes, pallets and crates, agricultural implements, caskets, 

wooden ware, and handles. It is also used in railroad cars 

and boats.

Oak (White Oak Group)

White oak lumber comes 

chiefly from the South, 

South Atlantic, and Central States, including the 

southern Appalachian 

area. Principal species 

are white (Quercus alba), 

generally straight grained, closely resembles yellow-poplar 

(Liriodendron tulipifera). It is moderately heavy, moderately low in shrinkage, moderately low in bending and compressive strength, moderately hard and stiff, and moderately 

high in shock resistance. Sweetbay is much like southern 

magnolia. The wood of cucumbertree is similar to that of 

yellow-poplar (L. tulipifera). Cucumbertree that grows in 

the yellow-poplar range is not separated from that species 

on the market.

Magnolia lumber is used principally in the manufacture of 

furniture, boxes, pallets, venetian blinds, sashes, doors, veneer, and millwork.

Maple (Hard Maple Group)

Hard maple includes 

sugar maple (Acer sac‐

charum) and black maple 

(A. nigrum). Sugar maple 

is also known as rock 

maple, and black maple 

as black sugar maple. 

Maple lumber is manufactured principally in 

the Middle Atlantic and Great Lake States, which together 

account for about two-thirds of production.

The heartwood is usually light reddish brown but sometimes considerably darker. The sapwood is commonly white 

with a slight reddish-brown tinge. It is usually 8 to 12 cm 

(3 to 5 in.) wide. Hard maple has a fine, uniform texture. 

It is heavy, strong, stiff, hard, and resistant to shock and 

has high shrinkage. The grain of sugar maple is generally 

straight, but birdseye, curly, or fiddleback grain is often selected for furniture or novelty items.

Hard maple is used principally for lumber and veneer. A 

large proportion is manufactured into flooring, furniture, 

cabinets, cutting boards and blocks, pianos, billiard cues, 

handles, novelties, bowling alleys, dance and gymnasium 

floors, spools, and bobbins.

Maple (Soft Maple Group)

Soft maple includes silver 

maple (Acer sacchari‐

num), red maple 

(A. rubrum), boxelder 

(A. negundo), and bigleaf 

maple (A. macrophyl‐

lum). Silver maple is also 

known as white, river, 

water, and swamp maple; 

red maple as soft, water, scarlet, white, and swamp maple; 

boxelder as ash-leaved, three-leaved, and cut-leaved maple; 

and bigleaf maple as Oregon maple. Soft maple is found in 

the eastern United States except for bigleaf maple, which 

comes from the Pacific Coast.

General Technical Report FPL–GTR–190
2–9

chestnut (Q. prinus), post (Q. stellata), overcup (Q. lyrata), 

swamp chestnut (Q. michauxii), bur (Q. macrocarpa), 

chinkapin (Q. muehlenbergii), and swamp white 

(Q. bicolor). The most important western oak species, Oregon white oak (Q. garryana), is a member of this group.

The sapwood of the white oaks is nearly white and roughly 

2 to 5 cm (1 to 2 in.) wide. The heartwood is generally 

grayish brown. Heartwood pores are usually plugged with 

tyloses, which tend to make the wood impenetrable to liquids. Consequently, most white oaks are suitable for tight 

cooperage, although many heartwood pores of chestnut oak 

lack tyloses. The wood of white oak is somewhat heavier 

than the wood of red oak. Its heartwood has good decay 

resistance.

White oaks are usually cut into lumber, railroad crossties, 

cooperage, mine timbers, fence posts, veneer, fuelwood, 

and many other products. High-quality white oak is especially sought for tight cooperage. An important use of 

white oak is for planking and bent parts of ships and boats; 

heartwood is often specified because of its decay resistance. 

White oak is also used for furniture, flooring, pallets, agricultural implements, railroad cars, truck floors, furniture, 

doors, and millwork.

Sassafras

Sassafras (Sassafras albi‐

dum) ranges from southeastern Iowa and eastern 

Texas eastward. Sassafras 

is easily confused with 

black ash, which it resembles in color, grain, 

and texture. Sapwood is 

light yellow, and heartwood varies from dull grayish brown 

to dark brown, sometimes with a reddish tinge. Freshly cut 

surfaces have a characteristic odor. The wood is moderately 

heavy, moderately hard, moderately weak in bending and 

endwise compression, quite high in shock resistance, and 

resistant to decay.

Sassafras was highly prized by the native Americans for 

dugout canoes, and some sassafras lumber is still used for 

small boats. Locally, sassafras is used for fence posts and 

rails and for general millwork.

Sweetgum

Sweetgum (Liquidambar 

styraciflua) grows from 

southwestern Connecticut 

westward into Missouri 

and southward to the Gulf 

Coast. Almost all lumber 

is produced in the Southern and South Atlantic 

States.

The lumber from sweetgum is usually separated into sap 

gum (the light-colored sapwood) or redgum (the reddishbrown heartwood). Sweetgum often has a form of cross 

grain called interlocked grain, and it must be dried slowly. 

When quartersawn, interlocked grain produces a ribbontype stripe that is desirable for interior woodwork and 

furniture. The wood is moderately heavy and hard. It is 

moderately strong, moderately stiff, and moderately high 

in shock resistance.

Sweetgum is used principally for lumber, veneer, plywood, 

slack cooperage, railroad crossties, fuel, pulpwood, boxes 

and crates, furniture, interior moulding, and millwork.

Sycamore, American

American sycamore 

(Platanus occidentalis) is 

sometimes called buttonwood or buttonball‐tree. 

Sycamore grows from 

Maine to Nebraska, 

southward to Texas, and 

eastward to Florida.

The heartwood of sycamore is reddish brown; the sapwood 

is light in color and from 4 to 8 cm (2 to 3 in.) wide. The 

wood has a fine texture and interlocked grain. It has high 

shrinkage in drying. It is moderately heavy, moderately 

hard, moderately stiff, moderately strong, and it has good 

shock resistance.

Sycamore is used principally for lumber, veneer, railroad 

crossties, slack cooperage, fence posts, and fuel. The lumber is used for furniture, boxes (particularly small food 

containers), pallets, flooring, handles, and butcher blocks. 

Veneer is used for fruit and vegetable baskets and some 

decorative panels and door skins.

Tanoak

Tanoak (Lithocarpus 

densiflorus) is also 

known as tanbark-oak 

because high-grade tannin was once obtained 

in commercial quantities from its bark. This 

species is found from 

southwestern Oregon to 

southern California, mostly near the coast but also in the 

Sierra Nevadas.

Sapwood of tanoak is light reddish brown when first cut and 

turns darker with age to become almost indistinguishable 

from heartwood, which also ages to dark reddish brown. 

The wood is heavy and hard. Except for compression perpendicular to grain, the wood has roughly the same strength 

properties as those of eastern white oak. Tanoak has higher 

shrinkage during drying than does white oak, and it has a 

tendency to collapse during drying. Tanoak is quite susceptible to decay, but the sapwood takes preservatives easily. 

Chapter 2 Characteristics and Availability of Commercially Important Woods
2–10

The heartwood of black walnut varies from light to 

dark brown; the sapwood is nearly white and up to 8 cm 

(3 in.) wide in open-grown trees. Black walnut is normally 

straight grained, easily worked with tools, and stable in use. 

It is heavy, hard, strong, and stiff, and has good resistance 

to shock. Black walnut is well suited for natural finishes.

Because of its good properties and interesting grain pattern, 

black walnut is much valued for furniture, architectural 

woodwork, and decorative panels. Other important uses are 

gunstocks, cabinets, and interior woodwork.

Willow, Black

Black willow (Salix nig‐

ra) is the most 

important of the many 

willows that grow in 

the United States. It is 

the only willow marketed 

under its own name. 

Most black willow 

comes from the Mississippi Valley, from Louisiana to southern Missouri and 

Illinois.

The heartwood of black willow is grayish brown or light 

reddish brown and frequently contains darker streaks. The 

sapwood is whitish to creamy yellow. The wood is uniform 

in texture, with somewhat interlocked grain, and is light in 

weight. It has exceedingly low strength as a beam or post, is 

moderately soft, and is moderately high in shock resistance. 

It has moderately high shrinkage.

Black willow is principally cut into lumber, which is then 

remanufactured into boxes, pallets, crates, caskets, and furniture. Small amounts have been used for slack cooperage, 

veneer, excelsior, charcoal, pulpwood, artificial limbs, and 

fence posts.

Yellow-Poplar

Yellow-poplar (Liri‐

odendron tulipifera) is 

also known as poplar, 

tulip-poplar, and tulipwood. Sapwood from 

yellow-poplar is sometimes called white poplar 

or whitewood. Yellowpoplar grows from Connecticut and New York southward to Florida and westward 

to Missouri. The greatest commercial production of yellowpoplar lumber is in the South and Southeast.

Yellow-poplar sapwood is white and frequently several centimeters wide. The heartwood is yellowish brown, sometimes streaked with purple, green, black, blue, or red. These 

colorations do not affect the physical properties of the 

Tanoak has straight grain, machines and glues well, and 

takes stains readily.

Because of its hardness and abrasion resistance, tanoak is 

excellent for flooring in homes or commercial buildings. 

It is also suitable for industrial applications such as truck 

flooring. Tanoak treated with preservative has been used for 

railroad crossties. The wood has been manufactured into 

baseball bats with good results, and it is also suitable for 

veneer, both decorative and industrial, and for high quality 

furniture.

Tupelo

The tupelo group includes water (Nyssa 

aquatica), black (N. syl‐

vatica), swamp (N. syl‐

vatica var. biflora), and 

Ogeechee (N. ogeche) 

tupelo. Water tupelo is 

also known as tupelo 

gum, swamp tupelo, and 

sourgum; black tupelo, as blackgum and sourgum; swamp 

tupelo, as swamp blackgum, blackgum, and sourgum; 

and Ogeechee tupelo, as sour tupelo, gopher plum, and 

Ogeechee plum. All except black tupelo grow principally 

in the southeastern United States. Black tupelo grows in the 

eastern United States from Maine to Texas and Missouri. 

About two-thirds of the production of tupelo lumber is from 

Southern States.

Wood of the different tupelo species is quite similar in appearance and properties. The heartwood is light brownish 

gray and merges gradually into the lighter-colored sapwood, 

which is generally many centimeters wide. The wood has 

fine, uniform texture and interlocked grain. Tupelo wood is 

moderately heavy, moderately strong, moderately hard and 

stiff, and moderately high in shock resistance. Buttresses of 

trees growing in swamps or flooded areas contain wood that 

is much lighter in weight than that from upper portions of 

the same trees. Because of interlocked grain, tupelo lumber 

requires care in drying.

Tupelo is cut principally for lumber, veneer, pulpwood, and 

some railroad crossties and slack cooperage. Lumber goes 

into boxes, pallets, crates, baskets, and furniture.

Walnut, Black

Black walnut (Juglans 

nigra) ranges from Vermont to the Great Plains 

and southward into Louisiana and Texas. About 

three-quarters of walnut 

wood is grown in the 

Central States.

General Technical Report FPL–GTR–190
2–11

wood. The wood is generally straight grained and comparatively uniform in texture. Slow-grown wood is moderately 

light in weight and moderately low in bending strength, 

moderately soft, and moderately low in shock resistance. 

The wood has moderately high shrinkage when dried from 

a green condition, but it is not difficult to dry and is stable 

after drying.

The lumber is used primarily for furniture, interior moulding, siding, cabinets, musical instruments, and engineered 

wood composites. Boxes, pallets, and crates are made from 

lower-grade stock. Yellow-poplar is also made into plywood for paneling, furniture, piano cases, and various other 

special products.

U.S. Softwoods

Baldcypress

Baldcypress or cypress 

(Taxodium distichum) is 

also known as southerncypress, red-cypress, 

yellow-cypress, and 

white-cypress. Commercially, the terms tidewater 

red-cypress, gulfcypress, red-cypress 

(coast type), and yellow-cypress (inland type) are frequently used. About half of the cypress lumber comes from 

the Southern States and about a fourth from the South Atlantic States. Old-growth baldcypress is difficult to find, but 

second-growth wood is available.

Sapwood of baldcypress is narrow and nearly white. The 

color of heartwood varies widely, ranging from light yellowish brown to dark brownish red, brown, or chocolate. 

The wood is moderately heavy, moderately strong, and 

moderately hard. The heartwood of old-growth baldcypress is one of the most decay resistant of U.S. species, but 

second-growth wood is only moderately resistant to decay. 

Shrinkage is moderately low but somewhat higher than 

that of the cedars and lower than that of Southern Pine. The 

wood of certain baldcypress trees frequently contains pockets or localized areas that have been attacked by a fungus. 

Such wood is known as pecky cypress. The decay caused 

by this fungus is stopped when the wood is cut into lumber 

and dried. Pecky cypress is therefore durable and useful 

where water tightness is unnecessary, appearance is not 

important, or a novel effect is desired.

When old-growth wood was available, baldcypress was 

used principally for building construction, especially where 

resistance to decay was required. It was also used for 

caskets, sashes, doors, blinds, tanks, vats, ship and boat 

building, and cooling towers. Second-growth wood is used 

for siding and millwork, including interior woodwork and 

paneling. Pecky cypress is used for paneling in restaurants, 

stores, and other buildings.

Douglas-Fir

Douglas-fir (Pseu‐

dotsuga menziesii) is 

also known locally as 

red-fir, Douglas-spruce, 

and yellow-fir. Its range 

extends from the Rocky 

Mountains to the Pacific 

Coast and from Mexico 

to central British 

Columbia.

Sapwood of Douglas-fir is narrow in old-growth trees but 

may be as much as 7 cm (3 in.) wide in second-growth 

trees of commercial size. Young trees of moderate to rapid 

growth have reddish heartwood and are called red-fir. Very 

narrow-ringed heartwood of old-growth trees may be yellowish brown and is known on the market as yellow-fir. The 

wood of Douglas-fir varies widely in weight and strength.

Douglas-fir is used mostly for building and construction 

purposes in the form of lumber, marine fendering, piles, 

plywood, and engineered wood composites. Considerable 

quantities are used for railroad crossties, cooperage stock, 

mine timbers, poles, and fencing. Douglas-fir lumber is 

used in the manufacture of sashes, doors, laminated beams, 

general millwork, railroad-car construction, boxes, pallets, 

and crates. Small amounts are used for flooring, furniture, 

ship and boat construction, and tanks. 

Fir, True (Eastern Species)

Balsam fir (Abies bal‐

samea) grows principally 

in New England, New 

York, Pennsylvania, and 

the Great Lake States. 

Fraser fir (A. fraseri) 

grows in the Appalachian 

Mountains of Virginia, 

North Carolina, and 

Tennessee.

The wood of the eastern true firs is creamy white to pale 

brown. The heartwood and sapwood are generally indistinguishable. The similarity of wood structure in the true 

firs makes it impossible to distinguish the species by examination of the wood alone. Balsam and Fraser firs are 

lightweight, have low bending and compressive strength, 

are moderately low in stiffness, are soft, and have low resistance to shock.

The eastern firs are used mainly for pulpwood, although 

some lumber is produced for structural products, especially 

in New England and the Great Lake States.

Chapter 2 Characteristics and Availability of Commercially Important Woods
construction and in the manufacture of boxes, pallets, and 

crates.

Hemlock, Western and Mountain

Western hemlock (Tsuga 

heterophylla) is also 

known as West Coast 

hemlock, Pacific hemlock, British Columbia 

hemlock, hemlock–

spruce, and western 

hemlock–fir. It grows 

along the Pacific coast 

of Oregon and Washington and in the northern Rocky 

Mountains north to Canada and Alaska. A relative of western hemlock, mountain hemlock (T. mertensiana) grows in 

mountainous country from central California to Alaska. It is 

treated as a separate species in assigning lumber properties.

The heartwood and sapwood of western hemlock are almost white with a purplish tinge. The sapwood, which is 

sometimes lighter in color than the heartwood, is generally 

not more than 2.5 cm (1 in.) wide. The wood often contains 

small, sound, black knots that are usually tight and dimensionally stable. Dark streaks are often found in the lumber; 

these are caused by hemlock bark maggots and generally 

do not reduce strength. Western hemlock is moderately 

light in weight and moderate in strength. It is also moderate in hardness, stiffness, and shock resistance. Shrinkage 

of western hemlock is moderately high, about the same as 

that of Douglas-fir (Pseudotsuga menziesii). Green hemlock lumber contains considerably more water than does 

Douglas-fir and requires longer kiln-drying time. Mountain 

hemlock has approximately the same density as that of 

western hemlock but is somewhat lower in bending strength 

and stiffness.

Western hemlock and mountain hemlock are used principally for pulpwood, lumber, and plywood. The lumber 

is used primarily for building material, as well as in the 

manufacture of boxes, pallets, crates, flooring, furniture, 

and ladders.

Incense-Cedar

Incense-cedar (Caloce‐

drus decurrens) grows in 

California, southwestern 

Oregon, and extreme 

western Nevada. Most 

incense-cedar lumber 

comes from the northern 

half of California.

Sapwood of incense-cedar is white or cream colored, and 

heartwood is light brown, often tinged with red. The wood 

2–12

Fir, True (Western Species)

Six commercial species 

make up the western true 

firs: subalpine fir (Abies 

lasiocarpa), California 

red fir (A. magnifica), 

grand fir (A. grandis), 

noble fir (A. procera), 

Pacific silver fir (A. ama‐

bilis), and white fir 

(A. concolor). The western true firs are cut for lumber primarily in Washington, Oregon, California, western Montana, and northern Idaho, and they are marketed as white 

fir throughout the United States.

The wood of the western true firs is similar to that of the 

eastern true firs, and it is not possible to distinguish among 

the true fir species by examination of the wood alone. 

Western true firs are light in weight and, with the exception 

of subalpine fir, have somewhat higher strength properties 

than does balsam fir. Shrinkage of the wood is low to moderately high.

Lumber of the western true firs is primarily used for building construction, boxes and crates, planing-mill products, 

sashes, doors, and general millwork. Some western true fir 

lumber is manufactured into boxes and crates. High-grade 

lumber from noble fir is used mainly for interior woodwork, 

moulding, siding, and sash and door stock. Some of the 

highest quality material has been used for aircraft construction. Other special uses of noble fir are venetian blinds and 

ladder rails.

Hemlock, Eastern

Eastern hemlock (Tsuga 

canadensis) grows from 

New England to northern 

Alabama and Georgia, 

and in the Great Lake 

States. Other names are 

Canadian hemlock and 

hemlock‐spruce. The production of hemlock lumber is divided fairly evenly among the New England States, 

Middle Atlantic States, and Great Lake States.

The heartwood of eastern hemlock is pale brown with a reddish hue. The sapwood is not distinctly separated from the 

heartwood but may be lighter in color. The wood is coarse 

and uneven in texture, and old trees tend to have considerable shake. The wood is moderately lightweight, moderately hard, moderately low in strength, moderately stiff, 

and moderately low in shock resistance.

Eastern hemlock is used principally for lumber and 

pulpwood. The lumber is used primarily in building 

General Technical Report FPL–GTR–190
has a fine, uniform texture and a spicy odor. Incense-cedar 

is light in weight, moderately low in strength, soft, low in 

shock resistance, and low in stiffness. It has low shrinkage 

and is easy to dry, with little checking or warping.

Incense-cedar is used principally for lumber and fence 

posts. Nearly all the high-grade lumber is used for pencils 

and venetian blinds; some is used for chests and toys. Much 

incense-cedar wood is more or less pecky; that is, it contains pockets or areas of disintegrated wood caused by advanced stages of localized decay in the living tree. There is 

no further development of decay once the lumber is dried. 

Other uses are railroad crossties, poles, split shingles, pencils, and composite fireplace logs.

Larch, Western

Western larch (Larix 

occidentalis) grows in 

western Montana, northern Idaho, northeastern 

Oregon, and on the eastern slope of the Cascade 

Mountains in Washington. About two-thirds 

of the lumber of this 

species is produced in Idaho and Montana and one-third in 

Oregon and Washington.

The heartwood of western larch is yellowish brown and the 

sapwood is yellowish white. The sapwood is generally not 

more than 2.5 cm (1 in.) wide. The wood is stiff, moderately strong and hard, moderately high in shock resistance, and 

moderately heavy. It has moderately high shrinkage. The 

wood is usually straight grained, splits easily, and is subject 

to ring shake. Knots are common but generally small and 

tight.

Western larch is used mainly for rough dimension wood in 

building construction, small timbers, planks and boards, and 

railroad crossties and mine timbers. It is used also for piles, 

poles, and posts. Some high-grade material is manufactured 

into interior woodwork, flooring, sashes, doors, and ladder 

stock. The properties of western larch are similar to those of 

Douglas-fir (Pseudotsuga menziesii), and these species are 

sometimes sold mixed.

Pine, Eastern White

Eastern white pine (Pinus 

strobus) grows from 

Maine to northern Georgia and in the Great Lake 

States. It is also known 

as white pine, northern 

white pine, Weymouth 

pine, and soft pine. About 

one-half the production of eastern white pine lumber occurs 

in New England, about one-third in the Great Lake States, 

and most of the remainder in the middle Atlantic and south 

Atlantic States.

The heartwood of eastern white pine is light brown, often 

with a reddish tinge. It turns darker on exposure to air. The 

wood has comparatively uniform texture and is straight 

grained. It is easily kiln dried, has low shrinkage, and ranks 

high in stability. It is also easy to work and can be readily 

glued. Eastern white pine is lightweight, moderately soft, 

moderately low in strength, low in shock resistance, and 

low in stiffness.

Practically all eastern white pine is converted into lumber, 

which is used in a great variety of ways. A large proportion, mostly second-growth knotty wood or lower grades, 

is used for structural lumber. High-grade lumber is used for 

patterns. Other important uses are sashes, doors, furniture, 

interior woodwork, knotty paneling, caskets, shade and map 

rollers, and toys.

Pine, Jack

Jack pine (Pinus banksi‐

ana), sometimes known 

as scrub, gray, and black 

pine in the United States, 

grows naturally in the 

Great Lake States and in 

a few scattered areas in 

New England and northern New York. Sapwood 

of jack pine is nearly white; heartwood is light brown to 

orange. Sapwood may constitute one-half or more of the 

volume of a tree. The wood has a rather coarse texture and 

is somewhat resinous. It is moderately lightweight, moderately low in bending strength and compressive strength, 

moderately low in shock resistance, and low in stiffness. It 

also has moderately low shrinkage. Lumber from jack pine 

is generally knotty.

Jack pine is used for pulpwood, box lumber, and pallets. 

Less important uses include railroad crossties, mine timber, 

slack cooperage, poles, posts, and fuel.

Pine, Lodgepole

Lodgepole pine (Pinus 

contorta), also known 

as knotty, black, and 

spruce pine, grows in 

the Rocky Mountain and 

Pacific Coast regions as 

far northward as Alaska. 

Wood for lumber and 

other products is 

2–13

Chapter 2 Characteristics and Availability of Commercially Important Woods
The heartwood of ponderosa pine is light reddish brown, 

and the wide sapwood is nearly white to pale yellow. The 

wood of the outer portions of ponderosa pine of sawtimber 

size is generally moderately light in weight, moderately low 

in strength, moderately soft, moderately stiff, and moderately low in shock resistance. It is generally straight grained 

and has moderately low shrinkage. It is quite uniform in 

texture and has little tendency to warp and twist.

Ponderosa pine has been used mainly for lumber and to a 

lesser extent for piles, poles, posts, mine timbers, veneer, 

and railroad crossties. The clear wood is used for sashes, 

doors, blinds, moulding, paneling, interior woodwork, and 

built-in cases and cabinets. Low-grade lumber is used for 

boxes and crates. Knotty ponderosa pine is used for interior 

woodwork.

Pine, Red

Red pine (Pinus resi‐

nosa) is frequently called 

Norway pine. This species grows in New England, New York, Pennsylvania, and the Great 

Lake States.

The heartwood of red 

pine varies from pale red to reddish brown. The sapwood is 

nearly white with a yellowish tinge and is generally from 5 

to 10 cm (2 to 4 in.) wide. The wood resembles the lighter 

weight wood of the Southern Pine species group. Red pine 

is moderately heavy, moderately strong and stiff, moderately soft, and moderately high in shock resistance. It is generally straight grained, not as uniform in texture as eastern 

white pine (P. strobus), and somewhat resinous. The wood 

has moderately high shrinkage, but it is not difficult to dry 

and is dimensionally stable when dried.

Red pine is used principally for lumber, cabin logs, and 

pulpwood, and to a lesser extent for piles, poles, posts, and 

fuel. The lumber is used for many of the same purposes 

as for eastern white pine (P. strobus). Red pine lumber is 

used primarily for building construction, including treated 

lumber for decking, siding, flooring, sashes, doors, general 

millwork, and boxes, pallets, and crates.

Pine, Southern Group

A number of species are 

included in the group 

marketed as Southern 

Pine lumber. The four 

major Southern Pine 

species and their growth 

ranges are as follows: 

(a) longleaf pine (Pinus 

palustris), eastern North 

2–14

produced primarily in the central Rocky Mountain States; 

other producing regions are Idaho, Montana, Oregon, and 

Washington.

The heartwood of lodgepole pine varies from light yellow 

to light yellow-brown. The sapwood is yellow or nearly 

white. The wood is generally straight grained with narrow 

growth rings. The wood is moderately lightweight, is fairly 

easy to work, and has moderately high shrinkage. It is moderately low in strength, moderately soft, moderately stiff, 

and moderately low in shock resistance.

Lodgepole pine has been used for lumber, mine timbers, 

railroad crossties, and poles. Less important uses include 

posts and fuel. Lodgepole pine is being used increasingly 

for structural lumber, millwork, cabinet logs, and engineered wood composites.

Pine, Pitch and Pond

Pitch pine (Pinus 

rigida) grows from 

Maine along the mountains to eastern Tennessee and northern 

Georgia. A relative of 

pitch pine (considered 

by some to be a 

subspecies), pond 

pine (P. serotina) grows in the coastal region from 

New Jersey to Florida.

The heartwood is brownish red or dark orange and resinous; 

the sapwood is wide and light yellow. The wood is moderately heavy to heavy, moderately strong, stiff, and hard, and 

moderately high in shock resistance. Shrinkage ranges from 

moderately low to moderately high.

Pitch and pond pine are used for general construction, lumber, posts, poles, fuel, and pulpwood. 

Pine, Ponderosa

Ponderosa pine (Pinus 

ponderosa) is also 

known as western yellow, bull, and blackjack 

pine. Jeffrey pine 

(P. jeffreyi), which grows 

in close association 

with ponderosa pine in 

California and Oregon, 

is usually marketed with 

ponderosa pine and sold under that name. Major ponderosa 

pine producing areas are in Oregon, Washington, and California. Other important producing areas are in Idaho and 

Montana; lesser amounts come from the southern Rocky 

Mountain region, the Black Hills of South Dakota, and 

Wyoming.

General Technical Report FPL–GTR–190
Carolina southward into Florida and westward into eastern 

Texas; (b) shortleaf pine (P. echinata), southeastern New 

York southward to northern Florida and westward into 

eastern Texas and Oklahoma; (c) loblolly pine (P. taeda), 

Maryland southward through the Atlantic Coastal Plain and 

Piedmont Plateau into Florida and westward into eastern 

Texas; (d) slash pine (P. elliottii), Florida and southern 

South Carolina, Georgia, Alabama, Mississippi, and Louisiana east of the Mississippi River. Lumber from these 

four species is classified as Southern Pine by the grading 

standards of the industry. Southern Pine lumber is produced 

principally in the Southern and South Atlantic States. Georgia, Alabama, North Carolina, Arkansas, and Louisiana lead 

in Southern Pine lumber production.

The wood of these southern pines is quite similar in appearance. Sapwood is yellowish white and heartwood is reddish 

brown. The sapwood is usually wide in second-growth 

stands. The heartwood begins to form when the tree is about 

20 years old. In old, slow-growth trees, sapwood may be 

only 2 to 5 cm (1 to 2 in.) wide.

Longleaf and slash pine are classified as heavy, strong, stiff, 

hard, and moderately high in shock resistance. Shortleaf 

and loblolly pine are usually somewhat lighter in weight 

than is longleaf. All the southern pines have moderately 

high shrinkage but are dimensionally stable when properly 

dried. 

The denser and higher strength southern pines have been 

extensively used in the form of stringers in the construction 

of factories, warehouses, bridges, trestles, and docks, and 

also for roof trusses, beams, posts, joists, and piles. Southern Pine is also used for tight and slack cooperage. When 

used for railroad crossties, piles, poles, mine timbers, and 

exterior decking, it is usually treated with preservatives. 

The manufacture of engineered wood composites from 

Southern Pine is a major wood-using industry, as is the 

production of preservative-treated lumber.

Pine, Spruce

Spruce pine (Pinus gla‐

bra), is also known as 

cedar pine and Walter 

pine. Spruce pine grows 

most commonly on low 

moist lands of the coastal 

regions of southeastern 

South Carolina, Georgia, 

Alabama, Mississippi, 

and Louisiana, and northern and northwestern Florida.

The heartwood of spruce pine is light brown, and the wide 

sapwood is nearly white. Spruce pine wood is lower in most 

strength values than the wood of the major Southern Pine 

species group. Spruce pine compares favorably with the 

western true firs in important bending properties, crushing 

strength (perpendicular and parallel to grain), and hardness. It is similar to denser species such as coast Douglas-fir 

(Pseudotsuga menziesii) and loblolly pine (Pinus taeda) in 

shear parallel to grain.

In the past, spruce pine was principally used locally for 

lumber, pulpwood, and fuelwood. The lumber reportedly 

was used for sashes, doors, and interior woodwork because 

of its low specific gravity and similarity of earlywood and 

latewood. 

Pine, Sugar

Sugar pine (Pinus lam‐

bertiana), the world’s 

largest species of pine, is 

sometimes called California sugar pine. Most 

sugar pine lumber grows 

in California and southwestern Oregon.

The heartwood of sugar pine is buff or light brown, sometimes tinged with red. The sapwood is creamy white. The 

wood is straight grained, fairly uniform in texture, and 

easy to work with tools. It has very low shrinkage, is readily dried without warping or checking, and is dimensionally stable. Sugar pine is lightweight, moderately low in 

strength, moderately soft, low in shock resistance, and low 

in stiffness.

Sugar pine is used almost exclusively for lumber products. 

The largest volume is used for boxes and crates, sashes, 

doors, frames, blinds, general millwork, building construction, and foundry patterns. 

Pine, Virginia

Virginia pine (Pinus vir‐

giniana), also known as 

Jersey and scrub pine, 

grows from New Jersey 

and Virginia throughout 

the Appalachian region 

to Georgia and the Ohio 

Valley. It is classified as a 

minor species in the grading rules for the Southern Pine species group.

The heartwood is orange, and the sapwood is nearly white 

and relatively wide. The wood is moderately heavy, 

moderately strong, moderately hard, and moderately 

stiff and has moderately high shrinkage and high shock 

resistance.

2–15

Chapter 2 Characteristics and Availability of Commercially Important Woods
Redcedar, Eastern

Eastern redcedar (Juni‐

perus virginiana) grows 

throughout the eastern 

half of the United States, 

except in Maine, Florida, 

and a narrow strip along 

the Gulf Coast, and at the 

higher elevations in the 

Appalachian Mountains. 

Commercial production is principally in the southern Appalachian and Cumberland Mountain regions. Another species, southern redcedar (J. silicicola), grows over a limited 

area in the South Atlantic and Gulf Coastal Plains.

The heartwood of redcedar is bright or dull red, and the 

narrow sapwood is nearly white. The wood is moderately 

heavy, moderately low in strength, hard, and high in shock 

resistance, but low in stiffness. It has very low shrinkage 

and is dimensionally stable after drying. The texture is fine 

and uniform, and the wood commonly has numerous small 

knots. Eastern redcedar heartwood is very resistant to 

decay.

The greatest quantity of eastern and southern redcedar is 

used for fence posts. Lumber is manufactured into chests, 

wardrobes, and closet lining. Other uses include flooring, 

novelties, pencils, scientific instruments, and small boats.

Redcedar, Western

Western redcedar (Thuja 

plicata) grows in the 

Pacific Northwest and 

along the Pacific Coast to 

Alaska. It is also called 

canoe‐cedar, giant arborvitae, shinglewood, and 

Pacific redcedar. Western redcedar lumber is 

produced principally in Washington, followed by Oregon, 

Idaho, and Montana.

The heartwood of western redcedar is reddish or pinkish 

brown to dull brown, and the sapwood is nearly white. The 

sapwood is narrow, often not more than 3 cm (1 in.) wide. 

The wood is generally straight grained and has a uniform 

but rather coarse texture. It has very low shrinkage. This 

species is lightweight, moderately soft, low in strength 

when used as a beam or post, and low in shock resistance. 

The heartwood is very resistant to decay.

Western redcedar is used principally for shingles, lumber, 

poles, posts, and piles. The lumber is used for exterior 

siding, decking, interior woodwork, ship and boat building, 

boxes and crates, sashes, and doors.

2–16

Virginia pine is used for lumber, railroad crossties, mine 

timbers, and pulpwood.

Pine, Western White

Western white pine 

(Pinus monticola) is also 

known as Idaho white 

pine or white pine. 

The heartwood of western white pine is cream 

colored to light reddish 

brown and darkens on 

exposure to air. The sapwood is yellowish white and generally from 2 to 8 cm 

(1 to 3 in.) wide. The wood is straight grained, easy to 

work, easily kiln-dried, and stable after drying. This species 

is moderately lightweight, moderately low in strength, moderately soft, moderately stiff, and moderately low in shock 

resistance and has moderately high shrinkage.

Practically all western white pine is sawn into lumber, 

which is used mainly for millwork products, such as sashes 

and door frames. In building construction, lower-grade 

boards are used for knotty paneling. High-grade material 

is made into siding of various kinds, exterior and interior 

woodwork, and millwork. Western white pine has practically the same uses as eastern white pine (P. strobus) and 

sugar pine (P. lambertiana).

Port-Orford-Cedar

Port-Orford-cedar 

(Chamaecyparis law‐

soniana) is also known 

as Lawson-cypress or 

Oregon-cedar. It grows 

along the Pacific Coast 

from Coos Bay, Oregon, 

southward to California. It 

does not extend more than 

65 km (40 mi) inland.

The heartwood of Port-Orford-cedar is light yellow to pale 

brown. The sapwood is narrow and hard to distinguish from 

the heartwood. The wood has fine texture, generally straight 

grain, and a pleasant spicy odor. It is moderately lightweight, stiff, moderately strong and hard, and moderately 

resistant to shock. Port-Orford-cedar heartwood is highly 

resistant to decay. The wood shrinks moderately, has little 

tendency to warp, and is stable after drying.

Some high-grade Port-Orford-cedar was once used in the 

manufacture of storage battery separators, matchsticks, and 

specialty millwork. Today, other uses are archery supplies, 

sash and door construction, flooring, interior woodwork, 

furniture, and boats.

General Technical Report FPL–GTR–190
Redwood

Redwood (Sequoia sem‐

pervirens) grows on the 

coast of California and 

some trees are among 

the tallest in the world. 

A closely related species, 

giant sequoia (Sequoia‐

dendron giganteum), is 

volumetrically larger and 

grows in a limited area in the Sierra Nevadas of California, 

but its wood is used in very limited quantities. Other names 

for redwood are coast redwood, California redwood, and 

sequoia. Production of redwood lumber is limited to California, but the market is nationwide.

The heartwood of redwood varies from light “cherry” red 

to dark mahogany. The narrow sapwood is almost white. 

Typical old-growth redwood is moderately lightweight, 

moderately strong and stiff, and moderately hard. The wood 

is easy to work, generally straight grained, and shrinks 

and swells comparatively little. The heartwood from oldgrowth trees has high decay resistance; heartwood from 

second-growth trees generally has low to moderate decay 

resistance.

Most redwood lumber is used for building. It is remanufactured extensively into siding, sashes, doors, blinds, 

millwork, casket stock, and containers. Because of its durability, redwood is useful for cooling towers, decking, tanks, 

silos, wood-stave pipe, and outdoor furniture. It is used in 

agriculture for buildings and equipment. Its use as timbers 

and large dimension in bridges and trestles is relatively minor. Redwood splits readily and plays an important role in 

the manufacture of split products, such as posts and fence 

material. Some redwood veneer is produced for decorative 

plywood.

Spruce, Eastern

The term eastern spruce 

includes three species: red 

(Picea rubens), white (P. 

glauca), and black 

(P. mariana). White 

and black spruce grow 

principally in the 

Great Lake States and 

New England, and 

red spruce grows in New England and the Appalachian 

Mountains.

The wood is light in color, and there is little difference between heartwood and sapwood. All three species have about 

the same properties, and they are not distinguished from 

each other in commerce. The wood dries easily and is stable 

after drying, is moderately lightweight and easily worked, 

has moderate shrinkage, and is moderately strong, stiff, 

tough, and hard.

The greatest use of eastern spruce is for pulpwood. Eastern 

spruce lumber is used for framing material, general millwork, boxes and crates, and piano sounding boards.

Spruce, Engelmann

Engelmann spruce (Pi‐

cea engelmannii) grows 

at high elevations in the 

Rocky Mountain region 

of the United States. This 

species is also known as 

white spruce, mountain 

spruce, Arizona spruce, 

silver spruce, and balsam. 

About two-thirds of the lumber is produced in the southern 

Rocky Mountain States and most of the remainder in the 

northern Rocky Mountain States and Oregon.

The heartwood of Engelmann spruce is nearly white, with 

a slight tinge of red. The sapwood varies from 2 to 5 cm 

(1 to 2 in.) in width and is often difficult to distinguish from 

the heartwood. The wood has medium to fine texture and is 

without characteristic odor. Engelmann spruce is rated as 

lightweight, and it is low in strength as a beam or post. It is 

also soft and low in stiffness, shock resistance, and shrinkage. The lumber typically contains many small knots.

Engelmann spruce is used principally for lumber and for 

mine timbers, railroad crossties, and poles. It is used also 

in building construction in the form of dimension lumber, 

flooring, and sheathing. It has excellent properties for pulp 

and papermaking.

Spruce, Sitka

Sitka spruce (Picea 

sitchensis) is a large 

tree that grows along 

the northwestern coast 

of North America from 

California to Alaska. It 

is also known as yellow, 

tideland, western, silver, 

and west coast spruce. 

Much Sitka spruce timber is grown in Alaska, but most logs 

are sawn into cants for export to Pacific Rim countries. Material for U.S. consumption is produced primarily in Washington and Oregon.

The heartwood of Sitka spruce is a light pinkish brown. 

The sapwood is creamy white and shades gradually into the 

heartwood; the sapwood may be 7 to 15 cm (3 to 6 in.) wide 

or even wider in young trees. The wood has a comparatively fine, uniform texture, generally straight grain, and no 

2–17

Chapter 2 Characteristics and Availability of Commercially Important Woods
is easily worked and holds paint well, and the heartwood is 

highly resistant to decay. Because of its high durability it is 

used for poles, posts, cabin logs, railroad crossties, lumber, 

shingles, decorative fencing, boats, and water tanks.

White-Cedar, Northern

Northern white-cedar 

(Thuja occidentalis) is 

also known as eastern 

white‐cedar or arborvitae. It grows from Maine 

along the Appalachians 

and westward through the 

northern part of the Great 

Lake States. Production 

of northern white-cedar lumber is greatest in Maine and the 

Great Lake States.

The heartwood of Northern white-cedar is light brown, 

and the sapwood is nearly white and is usually narrow. The 

wood is lightweight, rather soft, low in strength and shock 

resistance, and with low shrinkage upon drying. It is easily 

worked and the heartwood is very decay resistant. Northern 

white-cedar is used for poles and posts, outdoor furniture, 

shingles, cabin logs, lumber, water tanks, boats and for 

wooden ware.

Yellow-Cedar

Yellow-cedar (Chamaecy‐

paris nootkatensis) grows 

in the Pacific Coast region of North America 

from southeastern Alaska 

southward through 

Washington to southern 

Oregon.

The heartwood of yellow-cedar is bright, clear yellow. The 

sapwood is narrow, white to yellowish, and hardly distinguishable from the heartwood. The wood is fine textured 

and generally straight grained. It is moderately heavy, 

moderately strong and stiff, moderately hard, and moderately high in shock resistance. Yellow-cedar shrinks little in 

drying and is stable after drying, and the heartwood is very 

resistant to decay. The wood has a mild, distinctive odor.

Yellow-cedar is used for interior woodwork, furniture, 

small boats, cabinetwork, and novelties.

Imported Woods

This section includes many of the species that at present 

are considered to be commercially important, but by no 

means can it be considered all-inclusive. The import timber 

market is constantly changing, with some species no longer 

available but with new species entering the market. The 

same species may be marketed in the United States under 

other common names. Because of the variation in common 

2–18

distinct taste or odor. It is moderately lightweight, moderately low in bending and compressive strength, moderately 

stiff, moderately soft, and moderately low in resistance to 

shock. It has moderately low shrinkage. On the basis of 

weight, Sitka spruce rates high in strength properties and 

can be obtained in long, clear, straight-grained pieces.

Sitka spruce is used principally for lumber, pulpwood, and 

cooperage. Boxes and crates account for a considerable 

amount of the remanufactured lumber. Other important uses 

are furniture, planing-mill products, sashes, doors, blinds, 

millwork, and boats. Sitka spruce has been by far the most 

important wood for aircraft construction. Other specialty 

uses are ladder rails and sounding boards for pianos.

Tamarack

Tamarack (Larix larici‐

na), also known as 

eastern larch and locally 

as hackmatack, is a 

small to medium tree 

with a straight, round, 

slightly tapered trunk. 

It grows from Maine to 

Minnesota, with the bulk of the stand in the Great Lake 

States.

The heartwood of tamarack is yellowish brown to russet 

brown. The sapwood is whitish, generally less than 3 cm 

(1 in.) wide. The wood is coarse in texture, without odor 

or taste, and the transition from earlywood to latewood is 

abrupt. The wood is intermediate in weight and in most 

mechanical properties.

Tamarack is used principally for pulpwood, lumber, railroad crossties, mine timbers, fuel, fence posts, and poles. 

Lumber is used for framing material, tank construction, and 

boxes, pallets, and crates. The production of tamarack lumber has declined in recent years.

White-Cedar, Atlantic

Atlantic white-cedar 

(Chamaecyparis thy‐

oides), also known as 

southern white-cedar, 

swamp-cedar and boatcedar, grows near the Atlantic Coast from Maine 

to northern Florida and 

westward along the Gulf 

Coast to Louisiana. It is strictly a swamp tree. Production 

of Atlantic white-cedar centers in North Carolina and along 

the Gulf Coast.

The heartwood of Atlantic white-cedar is light brown, and 

the sapwood is white or nearly so. The sapwood is usually 

narrow. The wood is lightweight, rather soft, and low in 

strength and shock resistance. It shrinks little in drying. It 

General Technical Report FPL–GTR–190
names, many cross-references are included. Text information is necessarily brief, but when used in conjunction with 

the shrinkage and strength data tables, a reasonably good 

picture may be obtained of a particular wood. The references at the end of this chapter contain information on many 

species not described in this section.

Imported Hardwoods

Afara 

(see Limba)

Afrormosia

Afrormosia or kokrodua 

(Pericopsis elata), a large 

West African tree, is 

sometimes used as a substitute for teak (Tectona 

grandis).

The heartwood is fine 

textured, with straight to 

interlocked grain. The wood is brownish yellow with darker 

streaks and moderately hard and heavy, weighing about 

700 kg m–3 (43 lb ft–3) at 15% moisture content. The wood 

strongly resembles teak in appearance but lacks its oily 

nature and has a different texture. The wood dries readily 

with little degrade and has good dimensional stability. It is 

somewhat heavier and stronger than teak. The heartwood 

is highly resistant to decay fungi and termite attack and is 

extremely durable under adverse conditions.

Afrormosia is often used for the same purposes as teak, 

such as boat construction, joinery, flooring, furniture, 

interior woodwork, and decorative veneer.

Albarco

Albarco, or jequitiba as it 

is known in Brazil, is the 

common name applied 

to species in the genus

Cariniana. The 10 species are distributed from 

eastern Peru and northern 

Bolivia through central 

Brazil to Venezuela and 

 Colombia.

The heartwood is reddish or purplish brown and sometimes 

has dark streaks. It is usually not sharply demarcated from 

the pale brown sapwood. The texture is medium and the 

grain straight to interlocked. Albarco can be worked satisfactorily with only slight blunting of tool cutting edges 

because of the presence of silica. Veneer can be cut without 

difficulty. The wood is rather strong and moderately heavy, 

weighing about 560 kg m–3 (35 lb ft–3) at 12% moisture 

content. In general, the wood has about the same strength 

as that of U.S. oaks (Quercus spp.). The heartwood is durable, particularly the deeply colored material. It has good 

resistance to dry-wood termite attack.

Albarco is primarily used for general construction and 

carpentry wood, but it can also be used for furniture components, shipbuilding, flooring, veneer for plywood, and 

turnery.

Amaranth 

(see Purpleheart)

Anani 

(see Manni)

Anaura 

(see Marishballi)

Andiroba

Because of the widespread distribution of 

andiroba (Carapa guia‐

nensis) in tropical America, the wood is known 

under a variety of names, 

including cedro macho, 

carapa, crabwood, and 

tangare. These names are 

also applied to the related species C. nicaraguensis, whose 

properties are generally inferior to those of C. guianensis.

The heartwood varies from medium to dark reddish brown. 

The texture is like that of true mahogany (Swietenia mac‐

rophylla), and andiroba is sometimes substituted for true 

mahogany. The grain is usually interlocked but is rated easy 

to work, paint, and glue. The wood is rated as durable to 

very durable with respect to decay and insects. Andiroba is 

heavier than true mahogany and accordingly is markedly 

superior in all static bending properties, compression parallel to grain, hardness, shear, and durability.

On the basis of its properties, andiroba appears to be suited 

for such uses as flooring, frame construction in the tropics, 

furniture and cabinetwork, millwork, utility and decorative 

veneer, and plywood.

Angelin 

(see Sucupira)

Angelique

Angelique (Dicorynia 

guianensis) comes from 

French Guiana and 

Suriname.

Because of the variability 

in heartwood color between different trees, two 

forms are commonly recognized by producers. The heartwood that is russet-colored 

when freshly cut and becomes superficially dull brown with 

a purplish cast is referred to as “gris.” The heartwood that is 

2–19

Chapter 2 Characteristics and Availability of Commercially Important Woods
Azobe (Ekki)

Azobe or ekki (Lophira 

alata) is found in West 

Africa and extends into 

the Congo basin.

The heartwood is dark 

red, chocolate–brown, or 

purple–brown with conspicuous white deposits 

in the pores (vessels). The texture is coarse, and the grain 

is usually interlocked. The wood is strong, and its density 

averages about 1,120 kg m–3 (70 lb ft–3) at 12% moisture 

content. It is very difficult to work with hand and machine 

tools, and tools are severely blunted if the wood is machined when dry. Azobe can be dressed to a smooth finish, 

and gluing properties are usually good. Drying is very difficult without excessive degrade, and the heartwood is extremely resistant to preservative treatment. The heartwood 

is rated as very durable against decay, resistant to teredo 

attack, but only moderately resistant to termites. Azobe is 

very resistant to acid and has good weathering properties.

Azobe is excellent for heavy construction work, harbor construction, heavy-duty flooring, and railroad crossties.

Bagtikan 

(see Seraya, White)

Balata

Balata or bulletwood 

(Manilkara bidentata) 

is widely distributed 

throughout the West 

Indies, Central America, 

and northern South 

America.

The heartwood of balata 

is light to dark reddish 

brown and not sharply demarcated from the pale brown 

sapwood. Texture is fine and uniform, and the grain is 

straight to occasionally wavy or interlocked. Balata is a 

strong and very heavy wood; density of air-dried wood is 

1,060 kg m–3 (66 lb ft–3). It is generally difficult to air dry, 

with a tendency to develop severe checking and warp. The 

wood is moderately easy to work despite its high density, 

and it is rated good to excellent in all machining operations. 

Balata is very resistant to attack by decay fungi and highly 

resistant to subterranean termites but only moderately resistant to dry-wood termites.

Balata is suitable for heavy construction, textile and pulpmill equipment, furniture parts, turnery, tool handles, flooring, boat frames and other bentwork, railroad crossties, 

violin bows, billiard cues, and other specialty uses.

2–20

more distinctly reddish and frequently shows wide purplish 

bands is called “angelique rouge.” The texture of the wood 

is somewhat coarser than that of black walnut (Juglans 

nigra), and the grain is generally straight or slightly interlocked. In strength, angelique is superior to teak (Tectona 

grandis) and white oak (Quercus alba), when green or air 

dry, in all properties except tension perpendicular to grain. 

Angelique is rated as highly resistant to decay and resistant 

to marine borer attack. Machining properties vary and may 

be due to differences in density, moisture content, and silica 

content. After the wood is thoroughly air or kiln dried, it 

can be worked effectively only with carbide-tipped tools.

The strength and durability of angelique make it especially suitable for heavy construction, harbor installations, 

bridges, heavy planking for pier and platform decking, and 

railroad bridge ties. The wood is also suitable for ship decking, planking, boat frames, industrial flooring, and parquet 

blocks and strips.

Apa 

(see Wallaba)

Apamate 

(see Roble)

Apitong 

(see Keruing)

Avodire

Avodire (Turraeanthus 

africanus) has a rather 

extensive range in Africa, 

from Sierra Leone westward to the Congo region 

and southward to Zaire 

and Angola. It is most 

common in the eastern 

region of the Ivory Coast 

and is scattered elsewhere. Avodire is a medium-size tree of 

the rainforest where it forms fairly dense but localized and 

discontinuous timber stands.

The wood is cream to pale yellow with high natural luster; it eventually darkens to a golden yellow. The grain is 

sometimes straight but more often wavy or irregularly interlocked, which produces an unusual and attractive mottled 

figure when sliced or cut on the quarter. Although avodire 

weighs less than northern red oak (Quercus rubra), it has 

almost identical strength properties except that it is lower 

in shock resistance and shear. The wood works fairly easily with hand and machine tools and finishes well in most 

operations.

Figured material is usually converted into veneer for use in 

decorative work, and it is this kind of material that is chiefly 

imported into the United States. Other uses include furniture, fine joinery, cabinetwork, and paneling.

General Technical Report FPL–GTR–190
Balau

Balau, red balau, and selangan batu constitute a 

group of species that 

are the heaviest of the 

200 Shorea species. About 

45 species of this group 

grow from Sri Lanka and 

southern India through 

southeast Asia to the 

Philippines.

The heartwood is light to deep red or purple–brown, and 

it is fairly distinct from the lighter and yellowish- to reddish- or purplish-brown sapwood. The texture is moderately 

fine to coarse, and the grain is often interlocked. The wood 

weighs more than 750 kg m–3 (47 lb ft–3) at 12% moisture 

content. Balau is a heavy, hard, and strong timber that dries 

slowly with moderate to severe end checks and splits. The 

heartwood is durable to moderately durable and very resistant to preservative treatments.

Balau is used for heavy construction, frames of boats, decking, flooring, and utility furniture.

Balau, Red 

(see Balau)

Balsa

Balsa (Ochroma pyrami‐

dale) is widely distributed 

throughout tropical America from southern Mexico 

to southern Brazil and 

Bolivia, but Ecuador has 

been the principal source 

of supply since the wood 

gained commercial importance. It is usually found at lower elevations, especially 

on bottom-land soils along streams and in clearings and 

cutover forests. Today, it is often cultivated in plantations.

Several characteristics make balsa suitable for a wide variety of uses. It is the lightest and softest of all woods on the 

market. The lumber selected for use in the United States 

weighs, on the average, about 180 kg m–3 (11 lb ft–3) when 

dry and often as little as 100 kg m–3 (6 lb ft–3). The wood 

is readily recognized by its light weight; nearly white or 

oatmeal color, often with a yellowish or pinkish hue; and 

unique velvety feel.

Because of its light weight and exceedingly porous composition, balsa is highly efficient in uses where buoyancy, 

insulation against heat or cold, or low propagation of sound 

and vibration are important. Principal uses are for lifesaving equipment, floats, rafts, corestock, insulation, 

cushioning, sound modifiers, models, and novelties.

Banak, Cuangare

Various species of banak 

(Virola) occur in tropical 

America, from Belize 

and Guatemala southward to Venezuela, the 

Guianas, the Amazon 

region of northern Brazil, and southern Brazil, 

and on the Pacific Coast 

to Peru and Bolivia. Most of the wood known as banak is 

V. koschnyi of Central America and V. surinamensis and 

V. sebifera of northern South America. Botanically, cuangare (Dialyanthera) is closely related to banak, and the 

woods are so similar that they are generally mixed in the 

trade. The main commercial supply of cuangare comes from 

Colombia and Ecuador. Banak and cuangare are common 

in swamp and marsh forests and may occur in almost pure 

stands in some areas.

The heartwood of both banak and cuangare is usually pinkish or grayish brown and is generally not differentiated 

from the sapwood. The wood is straight grained and is of 

a medium to coarse texture. The various species are nonresistant to decay and insect attack but can be readily treated 

with preservatives. Machining properties are very good, but 

when zones of tension wood are present, machining may 

result in surface fuzziness. The wood finishes readily and is 

easily glued. Strength properties of banak and cuangare are 

similar to those of yellow-poplar (Liriodendron tulipifera).

Banak is considered a general utility wood for lumber, veneer, and plywood. It is also used for moulding, millwork, 

and furniture components.

Benge, Ehie, Bubinga

Although benge (Gui‐

bourtia arnoldiana), ehie 

(or ovangkol) (Guibourtia 

ehie), and bubinga (Gui‐

bourtia spp.) belong to the 

same West African genus, 

they differ rather markedly in color and somewhat 

in texture.

The heartwood of benge is pale yellowish brown to medium 

brown with gray to almost black stripes. Ehie heartwood 

tends to be more golden brown to dark brown with gray to 

almost black stripes. Bubinga heartwood is pink, vivid red, 

or red–brown with purple streaks, and it becomes yellow or 

medium brown with a reddish tint upon exposure to air. The 

texture of ehie is moderately coarse, whereas that of benge 

and bubinga is fine to moderately fine. All three woods are 

moderately hard and heavy, but they can be worked well 

with hand and machine tools. They are listed as moderately 

2–21

Chapter 2 Characteristics and Availability of Commercially Important Woods
Ceiba

Ceiba (Ceiba pentandra) is 

a large tree that grows to 

66 m (200 ft) in height 

with a straight cylindrical 

bole 13 to 20 m (40 to 

60 ft) long. Trunk diameters of 2 m (6 ft) or more 

are common. Ceiba grows 

in West Africa, from the 

Ivory Coast and Sierra Leone to Liberia, Nigeria, and the 

Congo region. A related species is lupuna (C. samauma) 

from South America.

Sapwood and heartwood are not clearly demarcated. The 

wood is whitish, pale brown, or pinkish brown, often with 

yellowish or grayish streaks. The texture is coarse, and the 

grain is interlocked or occasionally irregular. Ceiba is very 

soft and light; density of air-dried wood is 320 kg m–3 

(20 lb ft–3). In strength, the wood is comparable with 

basswood (Tilia americana). Ceiba dries rapidly without 

marked deterioration. It is difficult to saw cleanly and dress 

smoothly because of the high percentage of tension wood. It 

provides good veneer and is easy to nail and glue. Ceiba is 

very susceptible to attack by decay fungi and insects. It requires rapid harvest and conversion to prevent deterioration. 

Treatability, however, is rated as good.

Ceiba is available in large sizes, and its low density combined with a rather high degree of dimensional stability 

make it ideal for pattern and corestock. Other uses include 

blockboard, boxes and crates, joinery, and furniture 

components.

Chewstick 

(see Manni)

Courbaril, Jatoba

The genus Hymenaea

consists of about 25 species that occur in the West 

Indies and from southern 

Mexico through Central 

America into the Amazon 

basin of South America. 

The best-known and most 

important species is 

H. courbaril, which occurs throughout the range of the 

genus. Courbaril is often called jatoba in Brazil.

Sapwood of courbaril is gray–white and usually quite wide. 

The heartwood, which is sharply differentiated from the 

sapwood, is salmon red to orange–brown when freshly 

cut and becomes russet or reddish brown when dried. The 

heartwood is often marked with dark streaks. The texture 

is medium to rather coarse, and the grain is mostly interlocked. The wood is hard and heavy (about 800 kg m–3 

2–22

durable and resistant to preservative treatment. Drying 

may be difficult, but with care, the wood dries well.

These woods are used in turnery, flooring, furniture 

components, cabinetwork, and decorative veneers.

Brown Silverballi 

(see Kaneelhart)

Bubinga 

(see Benge)

Bulletwood 

(see Balata)

Carapa 

(see Andiroba)

Cativo

Cativo (Prioria co‐

paifera) is one of the few 

tropical American species that occur in abundance and often in nearly 

pure stands. Commercial 

stands are found in Nicaragua, Costa Rica, 

Panama, and Colombia.

Sapwood may be very pale pink or distinctly reddish, and 

it is usually wide. In trees up to 76 cm (30 in.) in diameter, 

heartwood may be only 18 cm (7 in.) in diameter. The grain 

is straight and the texture of the wood is uniform, comparable with that of true mahogany (Swietenia macrophylla). 

On flat-sawn surfaces, the figure is rather subdued as a result of exposure of the narrow bands of parenchyma tissue. 

The wood can be dried rapidly and easily with very little 

degrade. Dimensional stability is very good—practically 

equal to that of true mahogany. Cativo is classified as a nondurable wood with respect to decay and insects. It may contain appreciable quantities of gum. In wood that has been 

properly dried, however, the aromatics in the gum 

are removed and there is no difficulty in finishing.

Considerable quantities of cativo are used for interior 

woodwork, and resin-stabilized veneer is an important pattern material. Cativo is widely used for furniture and cabinet parts, lumber core for plywood, picture frames, edge 

banding for doors, joinery, and millwork.

Cedro 

(see Spanish-Cedar)

Cedro Macho 

(see Andiroba)

Cedro-Rana 

(see Tornillo)

General Technical Report FPL–GTR–190
(50 lb ft–3) at 12% moisture content). The strength properties of courbaril are quite high and very similar to those of 

shagbark hickory (Carya ovata), a species of lower specific 

gravity. Courbaril is rated as moderately to very resistant 

to attack by decay fungi and dry-wood termites. The heartwood is not treatable, but the sapwood is treatable with 

preservatives. Courbaril is moderately difficult to saw and 

machine because of its high density, but it can be machined 

to a smooth surface. Turning, gluing, and finishing properties are satisfactory. Planing, however, is somewhat difficult because of the interlocked grain. Courbaril compares 

favorably with white oak (Quercus alba) in steam-bending 

behavior.

Courbaril is used for tool handles and other applications 

that require good shock resistance. It is also used for steambent parts, flooring, turnery, furniture and cabinetwork, 

veneer and plywood, railroad crossties, and other specialty 

items.

Crabwood 

(see Andiroba)

Cristobal 

(see Macawood)

Cuangare 

(see Banak)

Degame

Degame or lemonwood (Calycophyllum 

candidissimum) grows 

in Cuba and ranges 

from southern Mexico 

through Central 

America to Colombia 

and Venezuela. It 

may grow in pure 

stands and is common on shaded hillsides and along 

waterways.

The heartwood of degame ranges from light brown to 

oatmeal-colored and is sometimes grayish. The sapwood is 

lighter in color and merges gradually with the heartwood. 

The texture is fine and uniform. The grain is usually straight 

or infrequently shows shallow interlocking, which may 

produce a narrow and indistinct stripe on quartered faces. 

In strength, degame is above the average for woods of 

similar density; density of air-dried wood is 817 kg m–3 

(51 lb ft–3). Tests show degame superior to persimmon 

(Diospyros virginiana) in all respects but hardness. Natural 

durability is low when degame is used under conditions favorable to stain, decay, and insect attack. However, degame 

is reported to be highly resistant to marine borers. Degame 

is moderately difficult to machine because of its density and 

hardness, although it does not dull cutting tools to any extent. Machined surfaces are very smooth.

Degame is little used in the United States, but its characteristics have made it particularly adaptable for shuttles, picker 

sticks, and other textile industry items that require resilience 

and strength. Degame was once prized for the manufacture 

of archery bows and fishing rods. It is also suitable for tool 

handles and turnery.

Determa

Determa (Ocotea rubra) 

is native to the Guianas, 

Trinidad, and the lower 

Amazon region of Brazil.

The heartwood is light 

reddish brown with a 

golden sheen and distinct 

from the dull gray or pale 

yellowish brown sapwood. The texture is rather coarse, and 

the grain is interlocked to straight. Determa is a moderately 

strong and heavy wood (density of air-dried wood is 

640 to 720 kg m–3 (40 to 45 lb ft–3)); this wood is moderately difficult to air dry. It can be worked readily with hand 

and machine tools with little dulling effect. It can be glued 

readily and polished fairly well. The heartwood is durable 

to very durable in resistance to decay fungi and moderately 

resistant to dry-wood termites. Weathering characteristics 

are excellent, and the wood is highly resistant to moisture 

absorption.

Uses for determa include furniture, general construction, 

boat planking, tanks and cooperage, heavy marine construction, turnery, and parquet flooring.

Ehie 

(see Benge)

Ekki 

(see Azobe)

Ekop

Ekop or gola (Tetraber‐

linia tubmaniana) grows 

only in Liberia.

The heartwood is light 

reddish brown and is 

distinct from the lighter 

colored sapwood, which 

may be up to 5 cm (2 in.) 

wide. The wood is medium to coarse textured, and the grain 

is interlocked, with a narrow striped pattern on quartered 

surfaces. The wood weighs about 735 kg m–3 (46 lb ft–3) at 

12% moisture content. It dries fairly well but with a marked 

tendency to end and surface checks. Ekop works well 

with hand and machine tools and is an excellent wood for 

turnery. It also slices well into veneer and has good gluing 

properties. The heartwood is only moderately durable and 

is moderately resistant to impregnation with preservative 

treatments.

2–23

Chapter 2 Characteristics and Availability of Commercially Important Woods
resistant to decay fungi and termites. It is also very resistant 

to marine borers in temperate waters but much less so in 

warm tropical waters.

Greenheart is used principally where strength and resistance 

to wear are required. Uses include ship and dock building, 

lock gates, wharves, piers, jetties, vats, piling, planking, 

industrial flooring, bridges, and some specialty items (fishing rods and billiard cue butts).

Guatambu 

(see Pau Marfim)

Guayacan 

(see Ipe)

Hura

Hura (Hura crepitans) 

grows throughout the 

West Indies from Central 

America to northern Brazil and Bolivia.

It is a large tree, commonly reaching a height 

of 30 to 43 m (90 to 

130 ft), with clear boles of 12 to 23 m (40 to 75 ft). The 

diameter often reaches 1 to 1.5 m (3 to 5 ft) and occasionally to 3 m (9 ft).

The pale yellowish-brown or pale olive-gray heartwood is 

indistinct from the yellowish-white sapwood. The texture 

is fine to medium and the grain straight to interlocked. 

Hura is a low-strength and low-density wood (density of 

air-dried wood is 240 to 448 kg m–3 (15 to 28 lb ft–3)); the 

wood is moderately difficult to air dry. Warping is variable 

and sometimes severe. The wood usually machines easily, 

but green material is somewhat difficult to work because of 

tension wood, which results in a fuzzy surface. The wood 

finishes well and is easy to glue and nail. Hura is variable 

in resistance to attack by decay fungi, but it is highly susceptible to blue stain and very susceptible to wood termites. 

However, the wood is easy to treat with preservative.

Hura is often used in general carpentry, boxes and crates, 

and lower grade furniture. Other important uses are veneer 

and plywood, fiberboard, and particleboard.

Ilomba

Ilomba (Pycnanthus an‐

golensis) is a tree of the 

rainforest and ranges from 

Guinea and Sierra Leone 

through tropical West 

Africa to Uganda and Angola. Common names include pycnanthus, walele, 

and otie.

2–24

Ekop is a general utility wood that is used for veneer, 

plywood, and furniture components.

Encino 

(see Oak)

Gola 

(see Ekop)

Gonçalo Alves

Most imports of gonçalo 

alves (Astronium graveo‐

lens and A. fraxinifolium) 

have been from Brazil. 

These species range from 

southern Mexico through 

Central America into the 

Amazon basin.

Freshly cut heartwood is russet brown, orange–brown, or 

reddish brown to red with narrow to wide, irregular, medium to very dark brown stripes. After exposure to air, the 

heartwood becomes brown, red, or dark reddish brown 

with nearly black stripes. The sapwood is grayish white and 

sharply demarcated from the heartwood. The texture is fine 

to medium and uniform. The grain varies from straight to 

interlocked and wavy.

Gonçalo alves turns readily, finishes very smoothly, and 

takes a high natural polish. The heartwood is highly resistant to moisture absorption; pigmented areas may present 

some difficulties in gluing because of their high density. 

The heartwood is very durable and resistant to both whiteand brown-rot organisms. The high density (1,010 kg m–3 

(63 lb ft–3)) of the air-dried wood is accompanied by equally 

high strength values, which are considerably higher in most 

respects than those of any U.S. species. Despite its strength, 

however, gonçalo alves is imported primarily for its beauty.

In the United States, gonçalo alves has the greatest value 

for specialty items such as archery bows, billiard cue butts, 

brushbacks, and cutlery handles, and in turnery and carving 

applications.

Greenheart

Greenheart (Chlorocar‐

dium rodiei) is essentially 

a Guyana tree, although 

small stands also occur in 

Suriname.

The heartwood varies 

from light to dark olive 

green or nearly black. The 

texture is fine and uniform, and the grain is straight to wavy. 

Greenheart is stronger and stiffer than white oak (Quercus 

alba) and generally more difficult to work with tools because of its high density; density of air-dried wood is more 

than 960 kg m–3 (60 lb ft–3). The heartwood is rated as very 

General Technical Report FPL–GTR–190
The wood is grayish white to pinkish brown and, in some 

trees, a uniform light brown. There is generally no distinction between heartwood and sapwood. The texture is medium to coarse, and the grain is generally straight. This species is generally similar to banak (Virola) but has a coarser 

texture. Air-dry density is about 512 kg m–3 (31 lb ft–3), and 

the wood is about as strong as yellow-poplar (Liriodendron 

tulipifera). Ilomba dries rapidly but is prone to collapse, 

warp, and splits. It is easily sawn and can be worked well 

with hand and machine tools. It is excellent for veneer and 

has good gluing and nailing characteristics. Green wood is 

subject to insect and fungal attack. Logs require rapid extraction and conversion to avoid degrade. Both sapwood 

and heartwood are permeable and can be treated with 

preservatives.

In the United States, this species is used only in the form of 

plywood for general utility purposes. However, ilomba is 

definitely suited for furniture components, interior joinery, 

and general utility purposes.

Ipe

Ipe, the common name 

for the lapacho group 

of the genus Tabebuia, 

consists of about 20 species of trees and occurs 

in practically every Latin 

America country except 

Chile. Other commonly 

used names are guayacan 

and lapacho.

Sapwood is relatively wide, yellowish gray or gray–brown, 

and sharply differentiated from heartwood, which is light to 

dark olive brown. The texture is fine to medium. The grain 

is straight to very irregular and often narrowly interlocked. 

The wood is very heavy and averages about 1,025 kg m–3

(64 lb ft–3) at 12% moisture content. Thoroughly air-dried 

heartwood specimens generally sink in water. Because of 

its high density and hardness, ipe is moderately difficult to 

machine, but glassy smooth surfaces can be produced. Ipe 

is very strong; in the air-dried condition, it is comparable 

with greenheart (Chlorocardium rodiei). Hardness is two 

to three times that of white oak (Quercus alba) or keruing 

(Dipterocarpus). The wood is highly resistant to decay and 

insects, including both subterranean and dry-wood termites, 

but susceptible to marine borer attack. The heartwood is 

impermeable, but the sapwood can be readily treated with 

preservatives.

Ipe is used almost exclusively for heavy-duty and durable 

construction. Because of its hardness and good dimensional 

stability, it is particularly well suited for heavy-duty flooring in trucks and boxcars. It is also used for decks, railroad 

crossties, turnery, tool handles, decorative veneers, and 

some specialty items in textile mills.

Ipil

(see Merbau)

Iroko

Iroko consists of two 

species (Milicia excelsa

and M. regia). Milicia 

excelsa grows across the 

entire width of tropical 

Africa from the Ivory 

Coast southward to Angola and eastward to 

East Africa. M. regia, 

however, is limited to extreme West Africa from Gambia to 

Ghana; it is less resistant to drought than is M. excelsa.

The heartwood varies from a pale yellowish brown to dark 

chocolate brown with light markings occurring most conspicuously on flat-sawn surfaces; the sapwood is yellowish 

white. The texture is medium to coarse, and the grain is 

typically interlocked. Iroko can be worked easily with hand 

or machine tools but with some tearing of interlocked grain. 

Occasional deposits of calcium carbonate severely damage cutting edges. The wood dries rapidly with little or no 

degrade. The strength is similar to that of red maple (Acer 

rubrum), and the weight is about 688 kg m–3 (43 lb ft–3) at 

12% moisture content. The heartwood is very resistant to 

decay fungi and resistant to termite and marine borer attack.

Because of its color and durability, iroko has been suggested as a substitute for teak (Tectona grandis). Its durability makes it suitable for boat building, piles, other marine 

work, and railroad crossties. Other uses include joinery, 

flooring, furniture, veneer, and cabinetwork.

Jacaranda 

(see Rosewood, Brazilian)

Jarrah

Jarrah (Eucalyptus mar‐

ginata) is native to the 

coastal belt of southwestern Australia and is one 

of the principal species 

for that country’s sawmill 

industry.

The heartwood is a 

uniform pink to dark red, often turning to deep brownish 

red with age and exposure to air. The sapwood is pale and 

usually very narrow in old trees. The texture is even and 

moderately coarse, and the grain is frequently interlocked 

or wavy. The wood weighs about 865 kg m–3 (54 lb ft–3) 

at 12% moisture content. The common defects of jarrah 

include gum veins or pockets, which in extreme instances 

separate the log into concentric shells. Jarrah is a heavy, 

hard timber possessing correspondingly high strength properties. It is resistant to attack by termites and rated as very 

2–25

Chapter 2 Characteristics and Availability of Commercially Important Woods
medium, and the grain is straight to slightly interlocked. 

The wood has a fragrant odor, which is lost in drying. Kaneelhart is a very strong and very heavy wood (density of 

air-dried wood is 833 to 1,153 kg m–3 (52 to 72 lb ft–3)); 

the wood is difficult to work. It cuts smoothly and takes an 

excellent finish but requires care in gluing. Kaneelhart has 

excellent resistance to both brown- and white-rot fungi and 

is also rated very high in resistance to dry-wood termites.

Uses of kaneelhart include furniture, turnery, boat building, 

heavy construction, and parquet flooring.

Kapur

The genus Dryobalanops

consists of nine species 

distributed over parts of 

Malaysia and Indonesia. 

For the export trade, the 

species are combined 

under the name kapur.

The heartwood is reddish 

brown and clearly demarcated from the pale sapwood. The 

wood is fairly coarse textured but uniform. In general, the 

wood resembles keruing (Dipterocarpus), but on the 

whole, kapur is straighter grained and not quite as coarse 

in texture. Density of the wood averages about 720 to 

800 kg m–3 (45 to 50 lb ft–3) at 12% moisture content. 

Strength properties are similar to those of keruing at comparable specific gravity. The heartwood is rated resistant 

to attack by decay fungi; it is reported to be vulnerable to 

termites. Kapur is extremely resistant to preservative treatment. The wood works with moderate ease in most hand 

and machine operations, but blunting of cutters may be 

severe because of silica content, particularly when the dry 

wood is machined. A good surface can be obtained from 

various machining operations, but there is a tendency toward raised grain if dull cutters are used. Kapur takes nails 

and screws satisfactorily. The wood glues well with urea 

formaldehyde but not with phenolic adhesives.

Kapur provides good and very durable construction wood 

and is suitable for all purposes for which keruing (Diptero‐

carpus) is used in the United States. In addition, kapur is 

extensively used in plywood either alone or with species of

Shorea (lauan–meranti).

Karri

Karri (Eucalyptus diver‐

sicolor) is a very large 

tree limited to southwestern Australia.

Karri resembles jarrah 

(E. marginata) in structure and general appearance. It is usually paler 

in color and, on average, slightly heavier (913 kg m–3

(57 lb ft–3)) at 12% moisture content. Karri is a heavy 

2–26

durable with respect to decay. The wood is difficult to work 

with hand and machine tools because of its high density and 

irregular grain.

Jarrah is used for decking and underframing of piers, jetties, 

and bridges, as well as piles and fenders for docks and harbors. As flooring, jarrah has high resistance to wear, but it 

is inclined to splinter under heavy traffic. It is also used for 

railroad crossties and other heavy construction.

Jatoba 

(see Courbaril)

Jelutong

Jelutong (Dyera costu‐

lata) is an important species in Malaysia where it 

is best known for its latex 

production in the manufacture of chewing gum 

rather than for its wood.

The wood is white or 

straw colored, and there is no differentiation between 

heartwood and sapwood. The texture is moderately fine 

and even. The grain is straight, and luster is low. The wood 

weighs about 465 kg m–3 (28 lb ft–3) at 12% moisture content. The wood is very easy to dry with little tendency to 

split or warp, but staining may cause trouble. It is easy to 

work in all operations, finishes well, and glues satisfactorily. The wood is rated as nondurable but readily permeable 

to preservatives.

Because of its low density and ease of working, jelutong is 

well suited for sculpture and pattern making, wooden shoes, 

picture frames, and drawing boards.

Jequitiba 

(see Albarco)

Kakaralli 

(see Manbarklak)

Kaneelhart

Kaneelhart or brown 

silverballi are names 

applied to the genus 

Licaria. Species of this 

genus grow mostly in 

Guyana, French Guiana, 

and Suriname and are 

found in association 

with greenheart (Chlo‐

rocardium rodiei) on hilly terrain and wallaba (Eperua) in 

forests.

The orange or brownish yellow heartwood darkens to yellowish or coffee brown on exposure to air. The wood is 

sometimes tinged with red or violet. The texture is fine to 

General Technical Report FPL–GTR–190
hardwood with mechanical properties of a correspondingly 

high order, even somewhat higher than that of jarrah. The 

heartwood is rated as moderately durable, though less so 

than that of jarrah. It is extremely difficult to treat with preservatives. The wood is fairly hard to machine and difficult 

to cut with hand tools. It is generally more resistant to cutting than is jarrah and has a slightly more dulling effect on 

tool edges.

Karri is inferior to jarrah for underground use and waterworks. However, where flexural strength is required, such 

as in bridges, floors, rafters, and beams, karri is an excellent 

wood. Karri is popular in heavy construction because of its 

strength and availability in large sizes and long lengths that 

are free of defects.

Kauta 

(see Marishballi)

Kempas

Kempas (Koompassia 

malaccensis) is distributed 

throughout the lowland 

forest in rather swampy 

areas of Malaysia and Indonesia.

When exposed to air, the 

freshly cut brick-red heartwood darkens to an orange–red or red–brown with numerous yellow–brown streaks as a result of the soft tissue 

(axial parenchyma) associated with the pores. The texture 

is rather coarse, and the grain is typically interlocked. 

Kempas is a hard, heavy wood (density of air-dried wood is 

880 kg m–3 (55 lb ft–3)); the wood is difficult to work with 

hand and machine tools. The wood dries well, with some 

tendency to warp and check. The heartwood is resistant 

to attack by decay fungi but vulnerable to termite activity. 

However, it treats readily with preservative retention as 

high as 320 kg m–3 (20 lb ft–3).

Kempas is ideal for heavy construction work, railroad 

crossties, and flooring.

Keruing (Apitong)

Keruing or apitong (Dip‐

terocarpus) is widely scattered throughout the IndoMalaysian region. Most of 

the more than 70 species 

in this genus are marketed 

under the name keruing. 

Other important species are 

marketed as apitong in the Philippine Islands and yang in 

Thailand.

The heartwood varies from light to dark red–brown or 

brown to dark brown, sometimes with a purple tint; the 

heartwood is usually well defined from the gray or buffcolored sapwood. Similar to kapur (Dryobalanops), the 

texture of keruing is moderately coarse and the grain is 

straight or shallowly interlocked. The wood is strong, hard, 

and heavy (density of air-dried wood is 720 to 800 kg m–3

(45 to 50 lb ft–3)); this wood is characterized by the presence of resin ducts, which occur singly or in short arcs as 

seen on end-grain surfaces. This resinous condition and the 

presence of silica can present troublesome problems. Sapwood and heartwood are moderately resistant to preservative treatments. However, the wood should be treated with 

preservatives when it is used in contact with the ground. 

Durability varies with species, but the wood is generally 

classified as moderately durable. Keruing generally takes to 

sawing and machining, particularly when green, but saws 

and cutters dull easily as a result of high silica content in 

the wood. Resin adheres to machinery and tools and may 

be troublesome. Also, resin may cause gluing and finishing 

difficulties.

Keruing is used for general construction work, framework 

for boats, flooring, pallets, chemical processing equipment, 

veneer and plywood, railroad crossties (if treated), truck 

floors, and boardwalks.

Khaya 

(see Mahogany, African)

Kokrodua 

(see Afrormosia)

Korina 

(see Limba)

Krabak 

(see Mersawa)

Kwila

(see Merbau)

Lapacho

(see Ipe) 

Lapuna 

(see Ceiba)

Lauan 

(see Meranti Groups)

Lemonwood 

(see Degame)

Lignumvitae

For many years, the only 

species of lignumvitae 

used on a large scale was 

Guaiacum officinale, 

which is native to the 

West Indies, northern 

Venezuela, northern Colombia, and Panama. With 

2–27

Chapter 2 Characteristics and Availability of Commercially Important Woods
tropical America from southern Mexico to the Brazilian 

Amazon region and Trinidad.

The bright red to reddish or purplish brown heartwood is 

more or less striped. Darker specimens look waxy, and the 

sapwood is sharply demarcated from the heartwood. The 

texture is medium to fine, and the grain is straight to curly 

or striped. The wood is not very difficult to work, and it 

finishes smoothly and takes on a high polish. Generally, 

macawood air dries slowly with a slight tendency to warp 

and check. Strength is quite high, and density of air-dried 

wood ranges from 880 to 1,170 kg m–3 (55 to 73 lb ft–3). 

The heartwood is reported to be highly resistant to attack by 

decay fungi, insects, and dry-wood termites. Although the 

sapwood absorbs preservatives well, the heartwood is resistant to treatment.

Macawood is a fine furniture and cabinet wood. It is also 

used in decorative veneers, musical instruments, turnery, 

joinery, and specialty items such as violin bows and billiard 

cues.

Machinmango 

(see Manbarklak)

Mahogany

The name mahogany is presently applied to several distinct 

kinds of commercial wood. The original mahogany wood, 

produced by Swietenia mahagoni, came from the American 

West Indies. This was the premier wood for fine furniture 

cabinet work and shipbuilding in Europe as early as the 

1600s. Because the good reputation associated with the 

name mahogany is based on this wood, American mahogany is sometimes referred to as true mahogany. A related 

African wood, of the genus Khaya, has long been marketed 

as “African mahogany” and is used for much the same purposes as American mahogany because of its similar properties and overall appearance. A third kind of wood called 

mahogany, and the one most commonly encountered in the 

market, is “Philippine mahogany.” This name is applied to 

a group of Asian woods belonging to the genus Shorea. In 

this chapter, information on the “Philippine mahoganies” is 

given under lauan and meranti groups.

Mahogany, African

The bulk of “African 

mahogany” shipped from 

west-central Africa is 

Khaya ivorensis, the most 

widely distributed and 

plentiful species of the 

genus found in the coastal 

belt of the so-called high 

forest. The closely allied 

species K. anthotheca has a more restricted range and is 

found farther inland in regions of lower rainfall but well 

within the area now being used for the export trade.

2–28

the near exhaustion of G. officinale, harvesters turned to 

G. sanctum, which is now the principal commercial species. 

G. sanctum occupies the same range as G. officinale but 

is more extensive and includes the Pacific side of Central 

America as well as southern Mexico.

Lignumvitae is one of the heaviest and hardest woods on 

the market. The wood is characterized by its unique green 

color and oily or waxy feel. The wood has a fine uniform 

texture and closely interlocked grain. Its resin content may 

constitute up to one-fourth of the air-dried weight of the 

heartwood.

Lignumvitae wood is used chiefly for bearing or bushing 

blocks for ship propeller shafts. The great strength and 

tenacity of lignumvitae, combined with self-lubricating 

properties resulting from the high resin content, make it 

especially adaptable for underwater use. It is also used for 

such articles as mallets, pulley sheaves, caster wheels, stencil and chisel blocks, and turned products.

Limba

Limba (Terminalia su‐

perba), also referred to as 

afara, korina, or ofram, is 

widely distributed from 

Sierra Leone to Angola and 

Zaire in the rainforest and 

savanna forest. Limba is 

also favored as a plantation 

species in West Africa.

The heartwood varies from gray–white to creamy or yellow 

brown and may contain dark streaks that are nearly black, 

producing an attractive figure that is valued for decorative 

veneer. The light color of the wood is considered an important asset for the manufacture of blond furniture. The wood 

is generally straight grained and of uniform but coarse 

texture. The wood is easy to dry and shrinkage is reported 

to be rather low. Limba is not resistant to decay, insects, 

or termites. It is easy to work with all types of tools and is 

made into veneer without difficulty.

Principal uses include plywood, furniture, interior joinery, 

and sliced decorative veneer.

Macacauba 

(see Macawood)

Macawood

Macawood and trebol are 

common names applied 

to species in the genus 

Platymiscium. Other 

common names include 

cristobal and macacauba. 

This genus is distributed 

across continental 

General Technical Report FPL–GTR–190
The heartwood varies from pale pink to dark reddish brown. 

The grain is frequently interlocked, and the texture is 

medium to coarse, comparable with that of American mahogany (Swietenia macrophylla). The wood is easy to dry, 

but machining properties are rather variable. Nailing and 

gluing properties are good, and an excellent finish is readily 

obtained. The wood is easy to slice and peel. In decay resistance, African mahogany is generally rated as moderately 

durable, which is below the durability rating for American 

mahogany.

Principal uses for African mahogany include furniture and 

cabinetwork, interior woodwork, boat construction, and 

veneer.

Mahogany, American

True, American, or Honduras mahogany (Swiete‐

nia macrophylla) ranges 

from southern Mexico 

through Central America 

into South America as far 

south as Bolivia. Plantations have been established within its natural range and elsewhere throughout 

the tropics.

The heartwood varies from pale pink or salmon colored to 

dark reddish brown. The grain is generally straighter than 

that of African mahogany (Khaya ivorensis); however, a 

wide variety of grain patterns are obtained from American 

mahogany. The texture is rather fine to coarse. American 

mahogany is easily air or kiln dried without appreciable 

warp or checks, and it has excellent dimensional stability. 

It is rated as durable in resistance to decay fungi and moderately resistant to dry-wood termites. Both heartwood and 

sapwood are resistant to treatment with preservatives. The 

wood is very easy to work with hand and machine tools, 

and it slices and rotary cuts into fine veneer without difficulty. It also is easy to finish and takes an excellent polish. 

The air-dried strength of American mahogany is similar to 

that of American elm (Ulmus americana). Density of airdried wood varies from 480 to 833 kg m–3 (30 to 52 lb ft–3).

The principal uses for mahogany are fine furniture and cabinets, interior woodwork, pattern woodwork, boat construction, fancy veneers, musical instruments, precision instruments, paneling, turnery, carving, and many other uses that 

call for an attractive and dimensionally stable wood.

Mahogany, Philippine 

(see Meranti Groups)

Manbarklak

Manbarklak is a common name applied to species in the 

genus Eschweilera. Other names include kakaralli, machinmango, and mata-mata. About 80 species of this genus are 

distributed from eastern 

Brazil through the Amazon basin, to the Guianas, 

Trinidad, and Costa Rica.

The heartwood of most 

species is light, grayish, 

reddish brown, or brownish buff. The texture is 

fine and uniform, and the grain is typically straight. Manbarklak is a very hard and heavy wood (density of air-dried 

wood ranges from 768 to 1,185 kg m–3 (48 to 74 lb ft–3)) 

that is rated as fairly difficult to dry. Most species are difficult to work because of the high density and high silica 

content. Most species are highly resistant to attack by decay 

fungi. Also, most species have gained wide recognition for 

their high degree of resistance to marine borer attack. Resistance to dry-wood termite attack is variable depending on 

species.

Manbarklak is an ideal wood for marine and other heavy 

construction uses. It is also used for industrial flooring, mill 

equipment, railroad crossties, piles, and turnery.

Manni

Manni (Symphonia globu‐

lifera) is native to the 

West Indies, Mexico, and 

Central, North, and South 

America. It also occurs in 

tropical West Africa. Other 

names include ossol (Gabon), anani (Brazil), waika 

(Africa), and chewstick 

(Belize), a name acquired because of its use as a primitive 

toothbrush and flossing tool.

The heartwood is yellowish, grayish, or greenish brown and 

is distinct from the whitish sapwood. The texture is coarse 

and the grain straight to irregular. The wood is very easy to 

work with both hand and machine tools, but surfaces tend 

to roughen in planing and shaping. Manni air-dries rapidly 

with only moderate warp and checking. Its strength is similar to that of hickory (Carya), and the density of air-dried 

wood is 704 kg m–3 (44 lb ft–3). The heartwood is durable 

in ground contact but only moderately resistant to dry-wood 

and subterranean termites. The wood is rated as resistant to 

treatment with preservatives.

Manni is a general purpose wood that is used for railroad 

ties, general construction, cooperage, furniture components, 

flooring, and utility plywood.

Marishballi

Marishballi is the common name applied to species of the 

genus Licania. Other names include kauta and anaura. Species of Licania are widely distributed in tropical America 

2–29

Chapter 2 Characteristics and Availability of Commercially Important Woods
resinous like some keruing (Dipterocarpus) species that resemble meranti. All the meranti groups are machined easily 

except white meranti, which dulls cutters as a result of high 

silica content in the wood. The light red and white merantis 

dry easily without degrade, but dark red and yellow merantis dry more slowly with a tendency to warp. The strength 

and shrinkage properties of the meranti groups compare 

favorably with that of northern red oak (Quercus rubra). 

The light red, white, and yellow merantis are not durable 

in exposed conditions or in ground contact, whereas dark 

red meranti is moderately durable. Generally, heartwood is 

extremely resistant to moderately resistant to preservative 

treatments.

Species of meranti constitute a large percentage of the total 

hardwood plywood imported into the United States. Other 

uses include joinery, furniture and cabinetwork, moulding 

and millwork, flooring, and general construction. Some 

dark red meranti is used for decking.

Merbau

Merbau (Malaysia), ipil 

(Philippines), and kwila 

(New Guinea) are names 

applied to species of the 

genus Intsia, most commonly I. bijuga. Intsia is 

distributed throughout 

the Indo–Malaysian 

region, Indonesia, 

Philippines, and many western Pacific islands, as well 

as Australia.

Freshly cut yellowish to orange–brown heartwood turns 

brown or dark red–brown on exposure to air. The texture 

is rather coarse, and the grain is straight to interlocked or 

2–30

Table 2–2. Woods belonging to the genus Shorea

Name Color 

Density 

of air-dried wood 

Dark red meranti 

(also called 

tanguile and dark 

red seraya) 

Dark brown, medium

to deep red, sometimes 

with a purplish tinge 

640+ kg m–3

(40+ lb ft–3)

Light red 

meranti (also 

called red seraya) 

Variable—almost white 

to pale pink, dark red, 

pale brown, or deep 

brown

400–640 kg m–3,

averaging 512 kg m–3

(25–40 lb ft–3,

averaging 32 lb ft–3)

White meranti 

(also called 

melapi) 

Whitish when freshly 

cut, becoming light 

yellow–brown on 

exposure to air 

480–870 kg m–3

(30–54 lb ft–3)

Yellow meranti 

(also called 

yellow seraya) 

Light yellow or yellow– 

brown, sometimes with 

a greenish tinge; darkens 

on exposure to air 

480–640 kg m–3

(30–40 lb ft–3)

but most abundant in the 

Guianas and the lower 

Amazon region of Brazil.

The heartwood is generally yellowish to dark 

brown, sometimes with a 

reddish tinge. The texture 

is fine and close, and the 

grain is usually straight. Marishballi is strong and very 

heavy; density of air-dried wood is 833 to 1,153 kg m–3

(52 to 72 lb ft–3). The wood is rated as easy to moderately 

difficult to air dry. Because of its high density and silica 

content, marishballi is difficult to work. The use of hardened cutters is suggested to obtain smooth surfaces. Durability varies with species, but marishballi is generally considered to have low to moderately low resistance to attack 

by decay fungi. However, it is known for its high resistance 

to attack by marine borers. Permeability also varies, but the 

heartwood is generally moderately responsive to treatment.

Marishballi is ideal for underwater marine construction, 

heavy construction above ground, and railroad crossties 

(treated).

Mata-Mata 

(see Manbarklak)

Mayflower 

(see Roble)

Melapi 

(see Meranti Groups)

Meranti Groups

Meranti is a common 

name applied commercially to four groups of 

species of Shorea from 

southeast Asia, most commonly Malaysia, Indonesia, and the Philippines. 

There are thousands of 

common names for the various species of Shorea, but the 

names Philippine mahogany and lauan are often substituted 

for meranti. The four groups of meranti are separated on the 

basis of heartwood color and weight (Table 2–2). About 70 

species of Shorea belong to the light and dark red meranti 

groups, 22 species to the white meranti group, and 33 species to the yellow meranti group.

Meranti species as a whole have a coarser texture than that 

of mahogany (Swietenia macrophylla) and do not have 

dark-colored deposits in pores. The grain is usually interlocked. All merantis have axial resin ducts aligned in long, 

continuous, tangential lines as seen on the end surface of 

the wood. These ducts sometimes contain white deposits 

that are visible to the naked eye, but the wood is not 

General Technical Report FPL–GTR–190
wavy. The strength of air-dried merbau is comparable with 

that of hickory (Carya), but density is somewhat lower 

(800 kg m–3 (50 lb ft–3) at 12% moisture content). The 

wood dries well with little degrade but stains black in the 

presence of iron and moisture. Merbau is rather difficult to 

saw because it sticks to saw teeth and dulls cutting edges. 

However, the wood dresses smoothly in most operations 

and finishes well. Merbau has good durability and high resistance to termite attack. The heartwood resists treatment, 

but the sapwood can be treated with preservatives. Merbau 

is used in furniture, fine joinery, turnery, cabinets, flooring, 

musical instruments, and specialty items.

Mersawa

Mersawa is one of the 

common names applied 

to the genus Anisoptera, 

which has about 15 species 

distributed from the Philippine Islands and Malaysia 

to east Pakistan. Names 

applied to this wood vary 

with the source, and three names are generally used in the 

lumber trade: krabak (Thailand), mersawa (Malaysia), and 

palosapis (Philippines).

Mersawa wood is light in color and has a moderately coarse 

texture. Freshly sawn heartwood is pale yellow or yellowish brown and darkens on exposure to air. Some wood may 

show a pinkish cast or pink streaks, but these eventually 

disappear on exposure to air. The wood weighs between 

544 and 752 kg m–3 (34 and 47 lb ft–3) at 12% moisture 

content and about 945 kg m–3 (59 lb ft–3) when green. The 

sapwood is susceptible to attack by powderpost beetles, and 

the heartwood is not resistant to termites. The heartwood is 

rated as moderately resistant to fungal decay and should not 

be used under conditions that favor decay. The heartwood 

does not absorb preservative solutions readily. The wood 

machines easily, but because of the presence of silica, the 

wood severely dulls the cutting edges of ordinary tools and 

is very hard on saws.

The major volume of mersawa is used as plywood because 

conversion in this form presents considerably less difficulty 

than does the production of lumber.

Mora

Mora (Mora excelsa and 

M. gonggrijpii) is widely 

distributed in the Guianas 

and also occurs in the Orinoco Delta of Venezuela.

The yellowish red–brown, 

reddish brown, or dark 

red heartwood with pale 

streaks is distinct from the yellowish to pale brown sapwood. The texture is moderately fine to rather coarse, and 

the grain is straight to interlocked. Mora is a strong and 

heavy wood (density of air-dried wood is 945 to 1,040 kg 

m–3 (59 to 65 lb ft–3)); this wood is moderately difficult to 

work but yields smooth surfaces in sawing, planing, turning, and boring. The wood is generally rated as moderately 

difficult to dry. Mora is rated as durable to very durable in 

resistance to brown- and white-rot fungi. M. gonggrijpii is 

rated very resistant to dry-wood termites, but M. excelsa is 

considerably less resistant. The sapwood responds readily 

to preservative treatments, but the heartwood resists 

treatment.

Mora is used for industrial flooring, railroad crossties, 

shipbuilding, and heavy construction.

Oak (Tropical)

The oaks (Quercus) are 

abundantly represented in 

Mexico and Central America with about 150 species, 

which are nearly equally 

divided between the red 

and white oak groups. 

More than 100 species 

occur in Mexico and about 25 in Guatemala; the number 

diminishes southward to Colombia, which has two species. 

The usual Spanish name applied to the oaks is encino or 

roble, and both names are used interchangeably irrespective 

of species or use of the wood.

In heartwood color, texture, and grain characteristics, tropical oaks are similar to the diffuse porous oaks of the United 

States, especially live oak (Quercus virginiana). In most 

cases, tropical oaks are heavier (density of air-dried wood 

is 704 to 993 kg m–3 (44 to 62 lb ft–3)) than the U.S. species. Strength data are available for only four species, and 

the values fall between those of white oak (Q. alba) and 

live oak (Q. virginiana) or are equal to those of live oak. 

The heartwood is rated as very resistant to decay fungi 

and difficult to treat with preservatives.

Utilization of the tropical oaks is very limited at present because of difficulties encountered in the drying of the wood. 

The major volume is used in the form of charcoal, but the 

wood is used for flooring, railroad crossties, mine timbers, 

tight cooperage, boat and ship construction, and decorative 

veneers.

Obeche

Obeche (Triplochiton scleroxylon) trees of west-central Africa reach a height of 50 m (150 ft) or more and a diameter 

of up to 2 m (5 ft). The trunk is usually free of branches for 

a considerable height so that clear lumber of considerable 

size can be obtained.

2–31

Chapter 2 Characteristics and Availability of Commercially Important Woods
The orange or golden yellow heartwood darkens on exposure to air and is clearly defined from the whitish or pale 

yellow sapwood. The texture is rather coarse, and the grain 

is usually interlocked or irregular. The density of air-dried 

wood (752 kg m–3 (47 lb ft–3)) is about the same as that of 

true hickory (Carya), but strength properties are somewhat 

lower. Quartersawn stock dries rapidly with little checking 

or warp, but flat-sawn lumber may develop considerable 

degrade. The wood works moderately well with hand and 

machine tools. It also glues and finishes satisfactorily. The 

heartwood is rated as very resistant to decay and moderately resistant to termite attacks. The sapwood is permeable 

to preservatives, but the heartwood is moderately resistant 

to preservative treatment.

Opepe is a general construction wood that is used in dock 

and marine work, boat building, railroad crossties, flooring, 

and furniture.

Ossol 

(see Manni)

Otie 

(see Ilomba)

Ovangkol 

(see Benge)

Palosapis 

(see Mersawa)

Para-Angelim 

(see Sucupira)

Pau Marfim

The range of pau marfim 

(Balfourodendron riede‐

lianum) is rather limited, 

extending from the State 

of Sao Paulo, Brazil, into 

Paraguay and the provinces 

of Corrientes and Missiones of northern Argentina. 

In Brazil, it is generally 

known as pau marfim and in Argentina and Paraguay, as 

guatambu.

In color and general appearance, pau marfim wood is very 

similar to birch (Betula) or sugar maple (Acer saccharum) 

sapwood. Although growth rings are present, they do not 

show as distinctly as those in birch and maple. There is no 

apparent difference in color between heartwood and sapwood. The wood is straight grained and easy to work and 

finish, but it is not considered resistant to decay. Average 

density of air-dried wood is about 802 kg m–3 (50 lb ft–3). 

2–32

The wood is creamy white 

to pale yellow with little 

or no difference between 

sapwood and heartwood. 

The wood is fairly soft, of 

uniform medium to coarse 

texture, and the grain is 

usually interlocked but 

sometimes straight. Airdry wood weighs about 385 kg m–3 (24 lb ft–3). Obeche 

dries readily with little degrade. It is not resistant to decay, 

and green sapwood is subject to blue stain. The wood is 

easy to work and machine, veneers and glues well, and 

takes nails and screws without splitting.

The characteristics of obeche make it especially suitable for 

veneer and corestock. Other uses include furniture, components, millwork, blockboard, boxes and crates, particleboard and fiberboard, patterns, and artificial limbs.

Ofram 

(see Limba)

Okoume

The natural distribution of 

okoume (Aucoumea klai‐

neana) is rather restricted; 

the species is found only 

in west-central Africa 

and Guinea. However, 

okoume is extensively 

planted throughout its 

natural range.

The heartwood is salmon-pink in color, and the narrow 

sapwood is whitish or pale gray. The wood has a high luster 

and uniform texture. The texture is slightly coarser than that 

of birch (Betula). The nondurable heartwood dries readily 

with little degrade. Sawn lumber is somewhat difficult to 

machine because of the silica content, but the wood glues, 

nails, and peels into veneer easily. Okoume offers unusual 

flexibility in finishing because the color, which is of medium intensity, permits toning to either lighter or darker 

shades.

In the United States, okoume is generally used for decorative plywood paneling, general utility plywood, and doors. 

Other uses include furniture components, joinery, and light 

construction.

Opepe

Opepe (Nauclea dider‐

richii) is widely distributed in Africa from Sierra 

Leone to the Congo region 

and eastward to Uganda. 

It is often found in pure 

stands.

General Technical Report FPL–GTR–190
In its areas of growth, pau marfim is used for much the 

same purposes as are sugar maple and birch in the United 

States. Introduced to the U.S. market in the late 1960s, pau 

marfim has been very well received and is especially esteemed for turnery.

Peroba, White 

(see Peroba de Campos)

Peroba de Campos

Peroba de campos (Pa‐

ratecoma peroba), also 

referred to as white peroba, 

grows in the coastal forests 

of eastern Brazil, ranging 

from Bahia to Rio de Janeiro. It is the only species 

in the genus Paratecoma.

The heartwood varies in color but is generally shades of 

brown with tendencies toward olive and red. The sapwood 

is a yellowish gray and is clearly defined from the heartwood. The texture is relatively fine and approximates that 

of birch (Betula). The grain is commonly interlocked, with 

a narrow stripe or wavy figure. The wood machines easily; 

however, particular care must be taken in planing to prevent 

excessive grain tearing of quartered surfaces. There is some 

evidence that the fine dust from machining operations may 

produce allergic responses in certain individuals. Density 

of air-dried wood averages about 738 kg m–3 (46 lb ft–3). 

Peroba de campos is heavier than teak (Tectona grandis) or 

white oak (Quercus alba), and it is proportionately stronger 

than either of these species. The heartwood of peroba de 

campos is rated as very durable with respect to decay and 

difficult to treat with preservatives.

In Brazil, peroba de campos is used in the manufacture of 

fine furniture, flooring, and decorative paneling. The principal use in the United States is shipbuilding, where peroba 

de campos serves as substitute for white oak (Quercus alba) 

for all purposes except bent members.

Peroba Rosa

Peroba rosa is the common 

name applied to a number of similar species in 

the genus Aspidosperma. 

These species occur in 

southeastern Brazil and 

parts of Argentina.

The heartwood is a distinctive rose-red to yellowish, often variegated or streaked 

with purple or brown, and becomes brownish yellow to 

dark brown upon exposure to air; the heartwood is often 

not demarcated from the yellowish sapwood. The texture 

is fine and uniform, and the grain is straight to irregular. 

The wood is moderately heavy; weight of air-dried wood is 

752 kg m–3 (47 lb ft–3). Strength properties are comparable 

with those of U.S. oak (Quercus). The wood dries with little 

checking or splitting. It works with moderate ease, and it 

glues and finishes satisfactorily. The heartwood is resistant 

to decay fungi but susceptible to dry-wood termite attack. 

Although the sapwood takes preservative treatment moderately well, the heartwood resists treatment.

Peroba is suited for general construction work and is favored for fine furniture and cabinetwork and decorative 

veneers. Other uses include flooring, interior woodwork, 

sashes and doors, and turnery.

Pilon

The two main species 

of pilon are Hieronyma 

alchorneoides and H.

laxiflora, also referred 

to as suradan. These species range from southern 

Mexico to southern Brazil 

including the Guianas, Peru, 

and Colombia. Pilon species are also found throughout the 

West Indies.

The heartwood is a light reddish brown to chocolate brown 

or sometimes dark red; the sapwood is pinkish white. The 

texture is moderately coarse and the grain interlocked. The 

wood air-dries rapidly with only a moderate amount of 

warp and checking. It has good working properties in all 

operations except planing, which is rated poor as a result of 

the characteristic interlocked grain. The strength of pilon 

is comparable with that of true hickory (Carya), and the 

density of air-dried wood ranges from 736 to 849 kg m–3

(46 to 53 lb ft–3). Pilon is rated moderately to very durable 

in ground contact and resistant to moderately resistant to 

subterranean and dry-wood termites. Both heartwood and 

sapwood are reported to be treatable with preservatives by 

both open tank and pressure vacuum processes.

Pilon is especially suited for heavy construction, railway 

crossties, marinework, and flooring. It is also used for furniture, cabinetwork, decorative veneers, turnery, and joinery.

Piquia

Piquia is the common name 

generally applied to species in the genus Caryocar. 

This genus is distributed 

from Costa Rica southward 

into northern Colombia and 

from the upland forest of 

the Amazon valley to eastern Brazil and the Guianas.

2–33

Chapter 2 Characteristics and Availability of Commercially Important Woods
all species is from Mexico through Central America and 

southward to southern Brazil.

Freshly cut heartwood is brown. It turns a deep purple upon 

exposure to air and eventually dark brown upon exposure to 

light. The texture is medium to fine, and the grain is usually 

straight. This strong and heavy wood (density of air-dried 

wood is 800 to 1,057 kg m–3 (50 to 66 lb ft–3)) is rated as 

easy to moderately difficult to air dry. It is moderately difficult to work with using either hand or machine tools, and 

it dulls cutters rather quickly. Gummy resin exudes when 

the wood is heated by dull tools. A slow feed rate and specially hardened cutters are suggested for optimal cutting. 

The wood turns easily, is easy to glue, and takes finishes 

well. The heartwood is rated as highly resistant to attack by 

decay fungi and very resistant to dry-wood termites. It is 

extremely resistant to treatment with preservatives.

The unusual and unique color of purpleheart makes this 

wood desirable for turnery, marquetry, cabinets, fine furniture, parquet flooring, and many specialty items, such as 

billiard cue butts and carvings. Other uses include heavy 

construction, shipbuilding, and chemical vats.

Pycnanthus 

(see Ilomba)

Ramin

Ramin (Gonystylus banca‐

nus) is native to southeast 

Asia from the Malaysian 

Peninsula to Sumatra and 

Borneo.

Both the heartwood and 

sapwood are the color 

of pale straw, yellow, or 

whitish. The grain is straight or shallowly interlocked. 

The texture is even, moderately fine, and similar to that of 

American mahogany (Swietenia macrophylla). The wood 

is without figure or luster. Ramin is moderately hard and 

heavy, weighing about 672 kg m–3 (42 lb ft–3) in the airdried condition. The wood is easy to work, finishes well, 

and glues satisfactorily. Ramin is rated as not resistant to 

decay but permeable with respect to preservative treatment.

Ramin is used for plywood, interior woodwork, furniture, 

turnery, joinery, moulding, flooring, dowels, and handles of 

nonstriking tools (brooms), and as a general utility wood.

Roble

Roble, a species in the 

roble group of Tabebuia

(generally T. rosea), ranges 

from southern Mexico 

through Central America 

to Venezuela and Ecuador. 

The name roble comes 

2–34

The yellowish to light grayish brown heartwood is hardly 

distinguishable from the sapwood. The texture is medium 

to rather coarse, and the grain is generally interlocked. The 

wood dries at a slow rate; warping and checking may develop, but only to a minor extent. Piquia is reported to be 

easy to moderately difficult to saw; cutting edges dull rapidly. The heartwood is very durable and resistant to decay 

fungi and dry-wood termites but only moderately resistant 

to marine borers.

Piquia is recommended for general and marine construction, heavy flooring, railway crossties, boat parts, and furniture components. It is especially suitable where hardness 

and high wear resistance are needed.

Primavera

The natural distribution of 

primavera (Tabebuia don‐

nell–smithii) is restricted 

to southwestern Mexico, 

the Pacific coast of Guatemala and El Salvador, and 

north-central Honduras. 

Primavera is regarded as 

one of the primary light-colored woods, but its use has been 

limited because of its rather restricted range and relative 

scarcity of naturally grown trees. Recent plantations have 

increased the availability of this species and have provided a more constant source of supply. The quality of the 

plantation-grown wood is equal in all respects to the wood 

obtained from naturally grown trees.

The heartwood is whitish to straw-yellow, and in some logs 

it may be tinted with pale brown or pinkish streaks. The 

texture is medium to rather coarse, and the grain is straight 

to wavy, which produces a wide variety of figure patterns. 

The wood also has a very high luster. Shrinkage is rather 

low, and the wood shows a high degree of dimensional 

stability. Despite considerable grain variation, primavera 

machines remarkably well. The density of air-dried wood 

is 465 kg m–3 (29 lb ft–3), and the wood is comparable in 

strength with water tupelo (Nyssa aquatica). Resistance to 

both brown- and white-rot fungi varies. Weathering characteristics are good.

The dimensional stability, ease of working, and pleasing appearance make primavera a suitable choice for solid furniture, paneling, interior woodwork, and special exterior uses.

Purpleheart

Purpleheart, also referred 

to as amaranth, is the name 

applied to species in the 

genus Peltogyne. The center of distribution is in the 

north-central part of the 

Brazilian Amazon region, 

but the combined range of 

General Technical Report FPL–GTR–190
from the Spanish word for oak (Quercus). In addition, 

T. rosea is called roble because the wood superficially resembles U.S. oak. Other names for T. rosea are mayflower 

and apamate.

The sapwood becomes a pale brown upon exposure to air. 

The heartwood varies from golden brown to dark brown, 

and it has no distinctive odor or taste. The texture is medium and the grain narrowly interlocked. The wood weighs 

about 642 kg m–3 (40 lb ft–3) at 12% moisture content. 

Roble has excellent working properties in all machine operations. It finishes attractively in natural color and takes 

finishes with good results. It weighs less than the average of U.S. white oaks (Quercus) but is comparable with 

respect to bending and compression parallel to grain. The 

heartwood of roble is generally rated as moderately to very 

durable with respect to decay; the darker and heavier wood 

is regarded as more decay resistant than the lighter-colored 

woods.

Roble is used extensively for furniture, interior woodwork, 

doors, flooring, boat building, ax handles, and general 

construction. The wood veneers well and produces attractive paneling. For some applications, roble is suggested as 

a substitute for American white ash (Fraxinus americana) 

and oak (Quercus).

Rosewood, Brazilian

Brazilian rosewood (Dal‐

bergia nigra), also referred 

to as jacaranda, occurs in 

eastern Brazilian forests 

from the State of Bahia to 

Rio de Janeiro. Because it 

was exploited for a long 

time, Brazilian rosewood 

is no longer abundant. 

The heartwood varies with respect to color, through shades 

of brown, red, and violet, and it is irregularly and conspicuously streaked with black. It is sharply demarcated from the 

white sapwood. Many kinds of rosewood are distinguished 

locally on the basis of prevailing color. The texture is 

coarse, and the grain is generally straight. The heartwood 

has an oily or waxy appearance and feel, and its odor is 

fragrant and distinctive. The wood is hard and heavy 

(weight of air-dried wood is 752 to 897 kg m–3 (47 to 

56 lb ft–3)); thoroughly air-dried wood will barely float in 

water. Strength properties of Brazilian rosewood are high 

and are more than adequate for the purposes for which this 

wood is used. For example, Brazilian rosewood is harder 

than any U.S. native hardwood species used for furniture 

and veneer. The wood machines and veneers well. It can 

be glued satisfactorily, provided the necessary precautions 

are taken to ensure good glue bonds, with respect to oily 

wood. Brazilian rosewood has an excellent reputation for 

durability with respect to fungal and insect attack, including 

termites, although the wood is not used for purposes where 

durability is necessary.

Brazilian rosewood is used primarily in the form of veneer 

for decorative plywood. Limited quantities are used in the 

solid form for specialty items such as cutlery handles, brush 

backs, billiard cue butts, and fancy turnery.

Rosewood, Indian

Indian rosewood (Dalber‐

gia latifolia) is native to 

most provinces of India 

except in the northwest.

The heartwood varies in 

color from golden brown 

to dark purplish brown 

with denser blackish 

streaks at the end of growth zones, giving rise to an attractive figure on flat-sawn surfaces. The narrow sapwood 

is yellowish. The average weight is about 849 kg m–3

(53 lb ft–3) at 12% moisture content. The texture is uniform 

and moderately coarse. Indian rosewood is quite similar in 

appearance to Brazilian (Dalbergia nigra) and Honduran 

(Dalbergia stevensonii) rosewood. The wood is reported to 

kiln-dry well though slowly, and the color improves during 

drying. Indian rosewood is a heavy wood with high strength 

properties; after drying, it is particularly hard for its weight. 

The wood is moderately hard to work with hand tools and 

offers a fair resistance in machine operations. Lumber with 

calcareous deposits tends to dull tools rapidly. The wood 

turns well and has high screw-holding properties. If a very 

smooth surface is required for certain purposes, pores (vessels) may need to be filled.

Indian rosewood is essentially a decorative wood for highquality furniture and cabinetwork. In the United States, it is 

used primarily in the form of veneer.

Sande

Practically all commercially available sande (mostly 

Brosimum utile) comes 

from Pacific Ecuador and 

Colombia. However, the 

group of species ranges 

from the Atlantic Coast in 

Costa Rica southward to 

Colombia and Ecuador.

The sapwood and heartwood show no distinction; the wood 

is uniformly yellowish white to yellowish or light brown. 

The texture is medium to moderately coarse and even, and 

the grain can be widely and narrowly interlocked. The density of air-dried wood ranges from 384 to 608 kg m–3 (24 

to 38 lb ft–3), and the strength is comparable with that of 

2–35

Chapter 2 Characteristics and Availability of Commercially Important Woods
may be up to 10 cm (4 in.) wide. The texture is rather fine. 

The grain is interlocked and produces narrow and uniform 

striping on quarter-sawn surfaces. The wood averages about 

674 kg m–3 (42 lb ft–3) at 12% moisture content, and its mechanical properties are in general higher than those of white 

oak (Quercus alba). The wood works fairly easily with machine tools, although the interlocked grain makes it difficult 

to plane. Sapele finishes and glues well. The heartwood is 

rated as moderately durable and is resistant to preservative 

treatment. 

As lumber, sapele is used for furniture and cabinetwork, 

joinery, and flooring. As veneer, it is used for decorative 

plywood.

Selangan Batu 

(see Balau)

Sepetir

The name sepetir applies 

to species in the genus Sin‐

dora and to Pseudosindora 

palustris. These species are 

distributed throughout Malaysia, Indochina, and the 

Philippines.

The heartwood is brown 

with a pink or golden tinge that darkens on exposure to air. 

Dark brown or black streaks are sometimes present. The 

sapwood is light gray, brown, or straw-colored. The texture 

is moderately fine and even, and the grain is narrowly interlocked. The strength of sepetir is similar to that of shellbark 

hickory (C. laciniosa), and the density of the air-dried wood 

is also similar (640 to 720 kg m–3 (40 to 45 lb ft–3)). The 

wood dries well but rather slowly, with a tendency to endsplit. The wood is difficult to work with hand tools and has 

a rather rapid dulling effect on cutters. Gums from the wood 

tend to accumulate on saw teeth, which causes additional 

problems. Sepetir is rated as nondurable in ground contact 

under Malaysian exposure. The heartwood is extremely 

resistant to preservative treatment; however, the sapwood is 

only moderately resistant.

Sepetir is a general carpentry wood that is also used for furniture and cabinetwork, joinery, flooring (especially truck 

flooring), plywood, and decorative veneers.

Seraya, Red and Dark Red

(see Meranti Groups)

Seraya, White

White seraya or bagtikan, 

as it is called in the Philippines, is a name applied to 

the 14 species of Parasho‐

rea, which grow in Sabah 

and the Philippines.

2–36

U.S. oak (Quercus). The lumber air dries rapidly with little 

or no degrade. However, material containing tension wood 

is subject to warp, and the tension wood may cause fuzzy 

grain as well as overheating of saws as a result of pinching. 

The wood is not durable with respect to stain, decay, and 

insect attack, and care must be exercised to prevent degrade 

from these agents. The wood stains and finishes easily and 

presents no gluing problems.

Sande is used for plywood, particleboard, fiberboard, 

carpentry, light construction, furniture components, and 

moulding.

Santa Maria

Santa Maria (Calophyllum 

brasiliense) ranges from 

the West Indies to southern Mexico and southward 

through Central America 

into northern South

America.

The heartwood is pinkish 

to brick red or rich reddish brown and marked by fine and 

slightly darker striping on flat-sawn surfaces. The sapwood 

is lighter in color and generally distinct from the heartwood. 

The texture is medium and fairly uniform, and the grain is 

generally interlocked. The heartwood is rather similar in appearance to dark red meranti (Shorea). The wood is moderately easy to work, and good surfaces can be obtained when 

attention is paid to machining operations. The wood averages about 608 kg m–3 (38 lb ft–3) at 12% moisture content. 

Santa Maria is in the density class of sugar maple (Acer

saccharum), and its strength properties are generally similar; the hardness of sugar maple is superior to that of Santa 

Maria. The heartwood is generally rated as moderately durable to durable in contact with the ground, but it apparently 

has no resistance against termites and marine borers.

The inherent natural durability, color, and figure on the 

quarter-sawn face suggest that Santa Maria could be used 

as veneer for plywood in boat construction. Other uses are 

flooring, furniture, cabinetwork, millwork, and decorative 

plywood.

Sapele

Sapele (Entandrophragma 

cylindricum) is a large African tree that occurs from 

Sierra Leone to Angola 

and eastward through the 

Congo to Uganda.

The heartwood ranges in 

color from that of American mahogany (Swietenia macrophylla) to a dark reddish or 

purplish brown. The lighter-colored and distinct sapwood 

General Technical Report FPL–GTR–190
The heartwood is light brown or straw-colored, sometimes 

with a pinkish tint. The texture is moderately coarse and the 

grain interlocked. White seraya is very similar in appearance and strength properties to light red meranti, and sometimes the two are mixed in the market. White seraya dries 

easily with little degrade, and works fairly well with hand 

and machine tools. The heartwood is not durable to moderately durable in ground contact, and it is extremely resistant 

to preservative treatments.

White seraya is used for joinery, light construction, moulding and millwork, flooring, plywood, furniture, and cabinet 

work.

Seraya, Yellow 

(see Meranti Groups)

Silverballi, Brown 

(see Kaneelhart)

Spanish-Cedar

Spanish-cedar or cedro 

consists of a group of about 

seven species in the genus 

Cedrela that are widely distributed in tropical America 

from southern Mexico to 

northern Argentina.

The heartwood of spanish cedar varies from light to dark reddish brown, and the 

sapwood is pinkish to white. The texture is rather fine and 

uniform to coarse and uneven. The grain is not interlocked. 

The heartwood is characterized by a distinctive odor. The 

wood dries easily. Although Spanish-cedar is not high 

in strength, most other properties are similar to those of 

American mahogany (Swietenia macrophylla), except for 

hardness and compression perpendicular to the grain, where 

mahogany is definitely superior. Spanish-cedar is considered decay resistant; it works and glues well.

Spanish-cedar is used locally for all purposes that require 

an easily worked, light but straight grained, and durable 

wood. In the United States, the wood is favored for millwork, cabinets, fine furniture, boat building, cigar wrappers 

and boxes, humidores, and decorative and utility plywood.

Sucupira (Angelin, Para-Angelim)

Sucupira, angelin, and 

para-angelim apply to 

species in four genera 

of legumes from South 

America. Sucupira applies 

to Bowdichia nitida from 

northern Brazil, B. virgili‐

oides from Venezuela, the 

Guianas, and Brazil, and 

Diplotropis purpurea from the Guianas and southern Brazil. 

Angelin (Andira inermis) is a widespread species that occurs throughout the West Indies and from southern Mexico 

through Central America to northern South America and 

Brazil. Para-angelim (Hymenolobium excelsum) is generally 

restricted to Brazil.

The heartwood of sucupira is chocolate-brown, red–brown, 

or light brown (especially in Diplotropis purpurea). Angelin heartwood is yellowish brown to dark reddish brown; 

para-angelim heartwood turns pale brown upon exposure 

to air. The sapwood is generally yellowish to whitish and 

is sharply demarcated from the heartwood. The texture of 

all three woods is coarse and uneven, and the grain can be 

interlocked. The density of air-dried wood of these species 

ranges from 720 to 960 kg m–3 (45 to 60 lb ft–3), which 

makes them generally heavier than true hickory (Carya). 

Their strength properties are also higher than those of true 

hickory. The heartwood is rated very durable to durable 

in resistance to decay fungi but only moderately resistant 

to attack by dry-wood termites. Angelin is reported to be 

difficult to treat with preservatives, but para-angelim and 

sucupira treat adequately. Angelin can be sawn and worked 

fairly well, except that it is difficult to plane to a smooth 

surface because of alternating hard (fibers) and soft (parenchyma) tissue. Para-angelim works well in all operations. 

Sucupira is difficult to moderately difficult to work because 

of its high density, irregular grain, and coarse texture.

Sucupira, angelin, and para-angelim are ideal for heavy 

construction, railroad crossties, and other uses that do not 

require much fabrication. Other suggested uses include 

flooring, boat building, furniture, turnery, tool handles, 

and decorative veneer.

Suradan 

(see Pilon)

Tangare 

(see Andiroba)

Tanguile 

(see Meranti Groups)

Teak

Teak (Tectona grandis) 

occurs in commercial 

quantities in India, Burma, 

Thailand, Laos, Cambodia, 

Vietnam, and the East Indies. Numerous plantations 

have been developed within 

its natural range and in tropical areas of Latin America and Africa, and many of these 

are now producing teakwood. The heartwood varies from 

yellow–brown to dark golden brown and eventually turns 

a rich brown upon exposure to air. Teakwood has a coarse, 

2–37

Chapter 2 Characteristics and Availability of Commercially Important Woods
Wallaba

Wallaba is a common 

name applied to the species 

in the genus Eperua. Other 

names include wapa and 

apa. The center of distribution is in the Guianas, but 

the species extends into 

Venezuela and the Amazon 

region of northern Brazil. Wallaba generally occurs in pure 

stands or as the dominant tree in the forest.

The heartwood ranges from light to dark red to reddish or 

purplish brown with characteristically dark, gummy streaks. 

The texture is rather coarse and the grain typically straight. 

Wallaba is a hard, heavy wood; density of air-dried wood 

is 928 kg m–3 (58 lb ft–3). Its strength is higher than that 

of shagbark hickory (Carya ovata). The wood dries very 

slowly with a marked tendency to check, split, and warp. 

Although the wood has high density, it is easy to work with 

hand and machine tools. However, the high gum content 

clogs sawteeth and cutters. Once the wood has been kiln 

dried, gum exudates are not a serious problem in machining. The heartwood is reported to be very durable and 

resistant to subterranean termites and fairly resistant to 

dry-wood termites.

Wallaba is well suited for heavy construction, railroad 

crossties, poles, industrial flooring, and tank staves. It is 

also highly favored for charcoal.

Wapa 

(see Wallaba)

Yang 

(see Keruing)

Imported Softwoods

Cypress, Mexican

Native to Mexico and Guatemala, Mexican cypress 

(Cupressus lusitanica) is 

now widely planted at high 

elevations throughout the 

tropical world.

The heartwood is yellowish, pale brown, or pinkish, 

with occasional streaking or variegation. The texture is fine 

and uniform, and the grain is usually straight. The wood 

is fragrantly scented. The density of air-dried wood is 

512 kg m–3 (32 lb ft–3), and the strength is comparable 

with that of yellow-cedar (Chamaecyparis nootkatensis) or 

western hemlock (Tsuga heterophylla). The wood is easy to 

work with hand and machine tools, and it nails, stains, and 

polishes well. Mexican cypress air dries very rapidly with 

little or no end- or surface-checking. Reports on durability 

2–38

uneven texture (ring porous), is usually straight grained, 

and has a distinctly oily feel. The heartwood has excellent 

dimensional stability and a very high degree of natural durability. Although teak is not generally used in the United 

States where strength is of prime importance, its properties 

are generally on par with those of U.S. oaks (Quercus). 

Teak is generally worked with moderate ease with hand 

and machine tools. However, the presence of silica often 

dulls tools. Finishing and gluing are satisfactory, although 

pretreatment may be necessary to ensure good bonding of 

finishes and glues.

Teak is one of the most valuable woods, but its use is limited by scarcity and high cost. Because teak does not cause 

rust or corrosion when in contact with metal, it is extremely 

useful in the shipbuilding industry, for tanks and vats, and 

for fixtures that require high acid resistance. Teak is currently used in the construction of boats, furniture, flooring, 

decorative objects, and decorative veneer.

Tornillo

Tornillo (Cedrelinga 

cateniformis), also referred 

to as cedro-rana, grows 

in the Loreton Huanuco 

provinces of Peru and in 

the humid terra firma of 

the Brazilian Amazon region. Tornillo can grow up 

to 52.5 m (160 ft) tall, with trunk diameters of 1.5 to 3 m 

(5 to 9 ft). Trees in Peru are often smaller in diameter, with 

merchantable heights of 15 m (45 ft) or more.

The heartwood is pale brown with a golden luster and 

prominently marked with red vessel lines; the heartwood 

gradually merges into the lighter-colored sapwood. The texture is coarse. The density of air-dried material collected in 

Brazil averages 640 kg m–3 (40 lb ft–3); for Peruvian stock, 

average density is about 480 kg m–3 (30 lb ft–3). The wood 

is comparable in strength with American elm (Ulmus ameri‐

cana). Tornillo cuts easily and can be finished smoothly, but 

areas of tension wood may result in woolly surfaces. The 

heartwood is fairly durable and reported to have good resistance to weathering.

Tornillo is a general construction wood that can be used for 

furniture components in lower-grade furniture.

Trebol 

(see Macawood)

Virola 

(see Banak)

Waika 

(see Manni)

Walele

(see Ilomba)

General Technical Report FPL–GTR–190
2–39

are conflicting. The heartwood is not treatable by the open 

tank process and seems to have an irregular response to 

pressure–vacuum systems.

Mexican cypress is used mainly for posts and poles, furniture components, and general construction.

Parana Pine

The wood commonly 

called parana pine (Ar‐

aucaria angustifolia) is 

a softwood but not a true 

pine. It grows in southeastern Brazil and adjacent 

areas of Paraguay and 

Argentina.

Parana pine has many desirable characteristics. It is available in large-size clear boards with uniform texture. The 

small pinhead knots (leaf traces) that appear on flat-sawn 

surfaces and the light or reddish-brown heartwood provide 

a desirable figure for matching in paneling and interior 

woodwork. Growth rings are fairly distinct and similar to 

those of eastern white pine (Pinus strobus). The grain is not 

interlocked, and the wood takes paint well, glues easily, and 

is free from resin ducts, pitch pockets, and pitch streaks. 

Density of air-dried wood averages 545 kg m–3 (34 lb ft–3). 

The strength of parana pine compares favorably with that of 

U.S. softwood species of similar density and, in some cases, 

approaches that of species with higher density. Parana 

pine is especially strong in shear strength, hardness, and 

nail-holding ability, but it is notably deficient in strength 

in compression across the grain. The tendency of the kilndried wood to split and warp is caused by the presence of 

compression wood, an abnormal type of wood with intrinsically large shrinkage along the grain. Boards containing 

compression wood should be excluded from exacting uses.

The principal uses of parana pine include framing lumber, 

interior woodwork, sashes and door stock, furniture case 

goods, and veneer.

Pine, Caribbean

Caribbean pine (Pinus 

caribaea) occurs along 

the Caribbean side of 

Central America from 

Belize to northeastern 

Nicaragua. It is also 

native to the Bahamas 

and Cuba. This low‐

elevation tree is widely 

introduced as a plantation species throughout the world 

tropics.

The heartwood is golden- to red-brown and distinct from 

the sapwood, which is light yellow and roughly 2 to 5 cm 

(1 to 2 in.) wide. This softwood species has a strong 

resinous odor and a greasy feel. The weight varies 

considerably and may range from 416 to 817 kg m–3

(26 to 51 lb ft–3) at 12% moisture content. Caribbean pine 

may be appreciably heavier than slash pine (P. elliottii), but 

the mechanical properties of these two species are rather 

similar. The lumber can be kiln dried satisfactorily. Caribbean pine is easy to work in all machining operations, but 

its high resin content may cause resin to accumulate on the 

equipment. Durability and resistance to insect attack vary 

with resin content; in general, the heartwood is rated as 

moderately durable. The sapwood is highly permeable and 

is easily treated by open tank or pressure–vacuum systems. 

The heartwood is rated as moderately resistant to preservative treatment, depending on resin content.

Caribbean pine is used for the same purposes as are the 

southern pines (Pinus spp.).

Pine, Ocote

Ocote pine (Pinus oocar‐

pa) is a high-elevation 

species that occurs from 

northwestern Mexico 

southward through Guatemala into Nicaragua. The 

largest and most extensive 

stands occur in Guatemala, 

Nicaragua, and Honduras.

The sapwood is a pale yellowish brown and generally up 

to 7 cm (3 in.) wide. The heartwood is a light reddish 

brown. The grain is not interlocked. The wood has a resinous odor, and it weighs about 656 kg m–3 (41 lb ft–3) at 

12% moisture content. The strength properties of ocote pine 

are comparable in most respects with those of longleaf pine 

(P. palustris). Decay resistance studies have shown ocote 

pine heartwood to be very durable with respect to white-rot 

fungal attack and moderately durable with respect to brown 

rot.

Ocote pine is comparable with the southern pines (Pinus

spp.) in workability and machining characteristics. It is a 

general construction wood suited for the same uses as are 

the southern pines.

Pine, Radiata

Radiata pine (Pinus ra‐

diata), also known as 

Monterey pine, is rare in 

its native range on the 

coast of central California 

and Guadalupe Island, 

Mexico, but is planted extensively in the southern 

hemisphere, mainly in Chile, New Zealand, Australia, and 

South Africa. Plantation-grown trees may reach a height of 

26 to 30 m (80 to 90 ft) in 20 years.

Chapter 2 Characteristics and Availability of Commercially Important Woods
2–40

The heartwood from plantation-grown trees is light brown 

to pinkish brown and is distinct from the paler creamcolored sapwood. Growth rings are primarily wide and distinct. False rings may be common. The texture is moderately 

even and fine, and the grain is not interlocked. Plantationgrown radiata pine averages about 480 kg m–3 (30 lb ft–3) at 

12% moisture content. Its strength is comparable with that 

of red pine (P. resinosa), although location and growth rate 

may cause considerable variation in strength properties. The 

wood air- or kiln-dries rapidly with little degrade. The wood 

machines easily, although the grain tends to tear around 

large knots. Radiata pine nails and glues easily, and it takes 

paint and finishes well. The sapwood is prone to attack by 

stain fungi and vulnerable to boring insects. However, 

plantation-grown stock is mostly sapwood, which treats 

readily with preservatives. The heartwood is rated as durable above ground and is moderately resistant to preservative 

treatment.

Radiata pine can be used for the same purposes as other 

pines grown in the United States. These uses include veneer, 

plywood, pulp, fiberboard, construction, boxes, and millwork.

General Technical Report FPL–GTR–190
2–41

Scientific Name Index

U.S. Wood Species—Hardwoods

Acer macrophyllum Pursh Maple, Bigleaf (Soft Maple Group)

Acer negundo L. Boxelder (Soft Maple Group)

Acer nigrum Michx. f. Maple, Black (Hard Maple Group)

Acer rubrum L. Maple, Red (Soft Maple Group)

Acer saccharinum L. Maple, Silver (Soft Maple Group)

Acer saccharum Marsh. Maple, Sugar (Hard Maple Group)

Aesculus glabra Willd. Buckeye, Ohio

Aesculus octandra Marsh. Buckeye, Yellow

Alnus rubra Bong. Alder, Red

Betula alleghaniensis Britton Birch, Yellow

Betula lenta L. Birch, Sweet

Betula nigra L. Birch, River

Betula papyrifera Marsh. Birch, Paper

Betula populifolia Marsh. Birch, Gray

Carya aquatica (Michx. f.) Nutt. Hickory, Water (Pecan Hickory Group)

Carya cordiformis (Wangenh.) K. Koch Hickory, Bitternut (Pecan Hickory Group)

Carya glabra (Mill.) Sweet Hickory, Pignut (True Hickory Group)

Carya illinoensis (Wangenh.) K. Koch Hickory, Pecan (Pecan Hickory Group)

Carya laciniosa (Michx. f.) Loud. Hickory, Shellbark (True Hickory Group)

Carya myristiciformis (Michx. f.) Nutt. Hickory, Nutmeg (Pecan Hickory Group)

Carya ovata (Mill.) K. Koch Hickory, Shagbark (True Hickory Group)

Carya tomentosa (Poir.) Nutt. Hickory, Mockernut (True Hickory Group)

Castanea dentata (Marsh.) Borkh. Chestnut, American

Celtis laevigata Willd. Sugarberry (Hackberry Group)

Celtis occidentalis L. Hackberry

Fagus grandifolia Ehrh. Beech, American

Fraxinus americana L. Ash, American White (White Ash Group)

Fraxinus latifolia Benth. Ash, Oregon (White Ash Group)

Fraxinus nigra Marsh. Ash, Black (Black Ash Group)

Fraxinus pennsylvanica Marsh. Ash, Green (White Ash Group)

Fraxinus profunda (Bush) Bush Ash, Pumpkin (Black Ash Group)

Fraxinus quadrangulata Michx. Ash, Blue (White Ash Group)

Gleditsia triacanthos L. Honeylocust

Juglans cinerea L. Butternut

Juglans nigra L. Walnut, Black

Liquidambar styraciflua L. Sweetgum

Liriodendron tulipifera L. Yellow-Poplar

Lithocarpus densiflorus (Hook. & Arn.) Rehd. Tanoak

Magnolia acuminata L. Cucumbertree (Magnolia Group)

Magnolia grandiflora L. Magnolia, Southern

Magnolia virginiana L. Sweetbay (Magnolia Group)

Nyssa aquatica L. Tupelo, Water

Nyssa ogeche Bartr. ex Marsh. Tupelo, Ogeechee

Nyssa sylvatica Marsh. Tupelo, Black

Nyssa sylvatica var. biflora (Walt.) Sarg. Tupelo, Swamp

Platanus occidentalis L. Sycamore, American

Populus balsamifera L. Balsam poplar (Cottonwood Group)

Populus deltoides Bartr. ex Marsh. Cottonwood, Eastern

Populus grandidentata Michx. Aspen, Bigtooth

Populus heterophylla L. Cottonwood, Swamp

Populus tremuloides Michx. Aspen, Quaking

Populus trichocarpa Torr. & Gray Cottonwood, Black

Chapter 2 Characteristics and Availability of Commercially Important Woods
2–42

Prunus serotina Ehrh. Cherry, Black

Quercus alba L. Oak, White (White Oak Group)

Quercus bicolor Willd. Oak, Swamp White (White Oak Group)

Quercus coccinea Muenchh. Oak, Scarlet (Red Oak Group)

Quercus falcata Michx. Oak, Southern Red (Red Oak Group)

Quercus falcata var. pagodaefolia Ell. Oak, Cherrybark (Red Oak Group)

Quercus garryana Dougl. Oak, Oregon White (White Oak Group)

Quercus kelloggii Newb. Oak, California Black (Red Oak Group)

Quercus laurifolia Michx. Oak, Laurel (Red Oak Group)

Quercus lyrata Walt. Oak, Overcup (White Oak Group)

Quercus macrocarpa Michx. Oak, Bur (White Oak Group)

Quercus michauxii Nutt. Oak, Swamp Chestnut (White Oak Group)

Quercus muehlenbergii Engelm. Oak, Chinkapin (White Oak Group)

Quercus nigra L. Oak, Water (Red Oak Group)

Quercus nuttallii Palmer Oak, Nuttall (Red Oak Group)

Quercus palustris Muenchh. Oak, Pin (Red Oak Group)

Quercus phellos L. Oak, Willow (Red Oak Group)

Quercus prinus L. Oak, Chestnut (White Oak Group)

Quercus rubra L. Oak, Northern Red (Red Oak Group)

Quercus shumardii Buckl. Oak, Shumard (Red Oak Group)

Quercus stellata Wangenh. Oak, Post (White Oak Group)

Quercus velutina Lam. Oak, Black (Red Oak Group)

Quercus virginiana Mill. Oak, Live (Tropical Oak Group)

Robinia pseudoacacia L. Locust, Black

Salix nigra Marsh. Willow, Black

Sassafras albidum (Nutt.) Nees Sassafras

Tilia americana L. Basswood, American

Tilia heterophylla Vent. Basswood, White

Ulmus alata Michx. Elm, Winged

Ulmus americana L. Elm, American

Ulmus crassifolia Nutt. Elm, Cedar

Ulmus rubra Muhl. Elm, Slippery

Ulmus serotina Sarg. Elm, September

Ulmus thomasii Sarg. Elm, Rock

U.S. Wood Species—Softwoods

Abies amabilis Dougl. ex Forbes Fir, Pacific Silver (Fir, True; Western Species)

Abies balsamea (L.) Mill. Fir, Balsam (Fir, True; Eastern Species)

Abies concolor (Gord. & Glend.) Lindl. ex Hildebr. Fir, White (Fir, True; Western Species)

Abies fraseri (Pursh) Poir. Fir, Fraser (Fir, True; Eastern Species)

Abies grandis (Dougl. ex D. Don) Lindl. Fir, Grand (Fir, True; Western Species)

Abies lasiocarpa (Hook.) Nutt. Fir, Subalpine (Fir, True; Western Species)

Abies magnifica A. Murr. Fir, California Red (Fir, True; Western Species)

Abies procera Rehd. Fir, Noble (Fir, True; Western Species)

Calocedrus decurrens (Torrey) Florin Incense-Cedar

Chamaecyparis lawsoniana (A. Murr.) Parl. Port-Orford-Cedar

Chamaecyparis nootkatensis (D. Don) Spach Yellow-Cedar

Chamaecyparis thyoides (L.) B.S.P. White-Cedar, Atlantic

Juniperus silicicola (Small) Bailey Redcedar, Southern (Redcedar, Eastern Group)

Juniperus virginiana L. Redcedar, Eastern

Larix laricina (Du Roi) K. Koch Tamarack

Larix occidentalis Nutt. Larch, Western

Picea engelmannii Parry ex Engelm. Spruce, Engelmann

Picea glauca (Moench) Voss Spruce, White (Spruce, Eastern Group)

Picea mariana (Mill.) B.S.P. Spruce, Black (Spruce, Eastern Group)

General Technical Report FPL–GTR–190
2–43

Picea rubens Sarg. Spruce, Red (Spruce, Eastern Group)

Picea sitchensis (Bong.) Carr. Spruce, Sitka

Pinus banksiana Lamb. Pine, Jack

Pinus contorta Dougl. ex Loud. Pine, Lodgepole

Pinus echinata Mill. Pine, Shortleaf (Pine, Southern Group)

Pinus elliottii Engelm. Pine, Slash (Pine, Southern Group)

Pinus glabra Walt. Pine, Spruce

Pinus jeffreyi Grev. & Balf. Pine, Jeffrey (see Pine, Ponderosa)

Pinus lambertiana Dougl. Pine, Sugar

Pinus monticola Dougl. ex D. Don Pine, Western White

Pinus palustris Mill. Pine, Longleaf (Pine, Southern Group)

Pinus ponderosa Dougl. ex Laws. Pine, Ponderosa

Pinus resinosa Ait. Pine, Red

Pinus rigida Mill. Pine, Pitch

Pinus serotina Michx. Pine, Pond

Pinus strobus L. Pine, Eastern White

Pinus taeda L. Pine, Loblolly (Pine, Southern Group)

Pinus virginiana Mill. Pine, Virginia

Pseudotsuga menziesii (Mirb.) Franco Douglas-Fir

Sequoia sempervirens (D. Don) Endl. Redwood

Sequoiadendron giganteum (Lindl.) Buchholz Sequoia, Giant

Taxodium distichum (L.) Rich. Baldcypress

Thuja occidentalis L. White-Cedar, Northern

Thuja plicata Donn ex D. Don Redcedar, Western

Tsuga canadensis (L.) Carr. Hemlock, Eastern

Tsuga heterophylla (Raf.) Sarg. Hemlock, Western

Tsuga mertensiana (Bong.) Carr. Hemlock, Mountain

Imported Woods—Hardwoods

Andira inermis (W. Wright) H.B.K. Angelin (see Sucupira)

Anisoptera spp. Mersawa

Aspidosperma spp. Peroba Rosa

Astronium spp. Gonçalo Alves

Aucoumea klaineana Pierre Okoume

Balfourodendron riedelianum (Engl.) Engl. Pau Marfim

Bowdichia spp. Sucupira

Brosimum utile (H.B.K.) Pittier Sande

Calophyllum brasiliense Cambess. Santa Maria

Calycophyllum candidissimum (Vahl) DC. Degame

Carapa spp. Andiroba

Cariniana spp. Albarco

Caryocar spp. Piquia

Cedrela spp. Spanish‐Cedar

Cedrelinga cateniformis (Ducke) Ducke Tornillo

Ceiba pentandra (L.) Gaertn. Ceiba

Ceiba samauma K. Schum. Lupuna (see Ceiba)

Chlorocardium rodiei (Schomb.) Rohwer, Richter & van der 

 Werff

Greenheart

Dalbergia latifolia Roxb. ex DC. Rosewood, Indian

Dalbergia nigra (Vell.) Allem. ex Benth. Rosewood, Brazilian

Dalbergia stevensonii Standl. Rosewood, Honduran

Dialyanthera spp. Cuangare (see Banak)

Dicorynia guianensis Amsch. Angelique

Diplotropis purpurea (Rich.) Amshoff Sucupira

Dipterocarpus spp. Keruing

Chapter 2 Characteristics and Availability of Commercially Important Woods
2–44

Dryobalanops spp. Kapur

Dyera costulata (Miq.) Hook. f. Jelutong

Entandrophragma cylindricum (Sprague) Sprague Sapele

Eperua spp. Wallaba

Eschweilera spp. Manbarklak

Eucalyptus diversicolor F. Muell. Karri

Eucalyptus marginata Donn ex Smith Jarrah

Gonystylus bancanus (Miq.) Baill. Ramin

Guaiacum spp. Lignumvitae

Guibourtia spp. Benge, Ehie, Bubinga

Hieronyma spp. Pilon

Hura crepitans L. Hura

Hymenaea spp. Courbaril, Jatoba

Hymenolobium excelsum Ducke Para‐Angelim (see Sucupira)

Intsia spp. Merbau

Khaya spp. Mahogany, African

Koompassia malaccensis Maing. ex Benth. Kempas

Licania spp. Marishballi

Licaria spp. Kaneelhart

Lophira alata Banks ex Gaertn. f. Azobe

Manilkara bidentata (A. DC.) A. Chev. Balata

Milicia spp. Iroko

Mora spp. Mora

Nauclea diderichii (De Wild.) Merrill Opepe

Ochroma pyramidale (Cav. ex Lam.) Urban Balsa

Ocotea rubra Mez Determa

Parashorea spp. Seraya, White

Paratecoma peroba (Record & Mell) Kuhlm. Peroba de Campos

Peltogyne spp. Purpleheart

Pericopsis elata (Harms) v. Meeuwen Afrormosia

Platymiscium spp. Macawood

Prioria copaifera Griseb. Cativo

Pseudosindora palustris Sym. Sepetir

Pycnanthus angolensis (Welw.) Warb. Ilomba

Quercus spp. Oak (Tropical)

Shorea spp. Balau

Shorea spp. Meranti

Sindora spp. Sepetir

Swietenia macrophylla King Mahogany, American

Symphonia globulifera L. f. Manni

Tabebuia donnell‐smithii Rose Primavera

Tabebuia rosea (Bertol.) DC. Roble

Tabebuia spp. Ipe

Tectona grandis L. f. Teak

Terminalia superba Engl. & Diels Limba

Tetraberlinia tubmaniana J. Leonard Ekop

Triplochiton scleroxylon K. Schum. Obeche

Turraeanthus africanus (Welw. ex DC.) Pellegr. Avodire

Virola spp. Banak

Imported Woods—Softwoods

Araucaria angustifolia (Bertol.) Kuntze Parana Pine

Cupressus lusitanica Mill. Cypress, Mexican

Pinus caribaea Morelet Pine, Caribbean

Pinus oocarpa Schiede Pine, Ocote

Pinus radiata D. Don Pine, Radiata

General Technical Report FPL–GTR–190
Literature Cited

Haynes, R.W. tech. coord. 2003. An analysis of the timber 

situation in the United States: 1952 to 2050. Gen. Tech. Rep. 

PNW–GTR–560. Portland, OR. U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 

253 p.

Haynes, R.W.; Adams, D.M.; Alig, R.J.; Ince, P.J.; Mills, 

J.R.; Zhou, X. 2007. The 2005 RPA timber assessment 

update. Gen. Tech. Rep. PNW–GTR–699. Portland, OR: 

U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station. 212 p.

Howard, J.L. 2007. U.S. timber production, trade, consumption, and price statistics, 1965–2005. Res. Pap. FPL–RP–

637. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 91 p.

Additional References

Alden, H.A. 1995. Hardwoods of North America. Gen. 

Tech. Rep. FPL–GTR–83. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 

136 p.

Alden, H.A. 1997. Softwoods of North America. Gen. Tech. 

Rep. FPL–GTR–102. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 151 p.

Berni, C.A.; Bolza, E.; Christensen, F.J. 1979. South 

American timbers—characteristics, properties, and uses of 

190 species. Melbourne, Australia: Commonwealth Scientific and Industrial Research Organization, Division of 

Building Research. 229 p.

Bolza, E.; Keating, W.G. 1972. African timbers—the properties, uses, and characteristics of 700 species. Melbourne, 

Australia: Commonwealth Scientific and Industrial Research 

Organization, Division of Building Research.

Chudnoff, M. 1984. Tropical timbers of the world. Agric. 

Handb. 607. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 464 p.

Farmer, R.H. 1972. Handbook of hardwoods. 2nd ed. Building Research Establishment, Princes Risborough Laboratory; London: Her Majesty’s Stationery Office. 243 p.

Hardwood Market Report [Current edition]. Memphis, 

TN: Lumber News Letter. 

Henderson, F.Y. 1977. A handbook of softwoods. Building 

Research Establishment, Princes Risborough Laboratory; 

London: Her Majesty’s Stationery Office. 63 p.

Keating, W.G.; Bolza, E. 1982. Characteristics, properties, 

and uses of timbers: Vol. 1. Southeast Asia, Northern Australia, and the Pacific. Melbourne, Australia: Inkata Press. 

362 p.

Kukachka, B.F. 1970. Properties of imported tropical 

woods. Res. Pap. FPL 125. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 

67 p.

Markwardt, L.J. 1930. Comparative strength properties of 

woods grown in the United States. Technical Bulletin 158. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 39 p.

Markwardt, L.J.; Wilson, T.R.C. 1935. Strength and related 

properties of woods grown in the United States. Tech. Bull. 

479. Washington, DC: U.S. Department of Agriculture. 99 p.

Panshin, A.J.; deZeeuw, C. 1980. Textbook of wood technology. 4th ed. New York: McGraw-Hill. 722 p.

Record, S.J.; Hess, R.W. 1949. Timbers of the new world. 

New Haven, CT: Yale University Press. 640 p.

Ulrich, A.H. 1981. U.S. timber production, trade, consumption, and price statistics, 1950–1980. Misc. Pub. 1408. 

Washington, DC: U.S. Department of Agriculture.

2–45

Chapter 2 Characteristics and Availability of Commercially Important Woods

CHAPTER 3

Structure and Function of Wood

Alex Wiedenhoeft, Botanist

3–1

Wood is a complex biological structure, a composite of 

many chemistries and cell types acting together to serve the 

needs of a living plant. Attempting to understand wood in 

the context of wood technology, we have often overlooked 

the key and basic fact that wood evolved over the course of 

millions of years to serve three main functions in plants― 

conduction of water from the roots to the leaves, mechanical support of the plant body, and storage of biochemicals. 

There is no property of wood—physical, mechanical, chemical, biological, or technological—that is not fundamentally 

derived from the fact that wood is formed to meet the needs 

of the living tree. To accomplish any of these functions, 

wood must have cells that are designed and interconnected 

in ways sufficient to perform these functions. These three 

functions have influenced the evolution of approximately 

20,000 different species of woody plants, each with unique 

properties, uses, and capabilities, in both plant and human 

contexts. Understanding the basic requirements dictated by 

these three functions and identifying the structures in wood 

that perform them allow insight to the realm of wood as an 

engineering material (Hoadley 2000). A scientist who understands the interrelationships between form and function 

can predict the utility of a specific wood in a new context. 

The objective of this chapter is to review the basic biological structure of wood and provide a basis for interpreting its 

properties in an engineering context. By understanding the 

function of wood in the living tree, we can better understand 

the strengths and limitations it presents as a material.

The component parts of wood must be defined and delimited at a variety of scales. The wood anatomical expertise 

necessary for a researcher who is using a solid wood beam 

is different from that necessary for an engineer designing 

a glued-laminated beam, which in turn is different from 

that required for making a wood–resin composite with 

wood flour. Differences in the kinds of knowledge required 

in these three cases are related to the scale at which one 

intends to interact with wood, and in all three cases the 

properties of these materials are derived from the biological 

needs of the living tree. For this reason, this chapter explains 

the structure of wood at decreasing scales and in ways that 

demonstrate the biological rationale for a plant to produce 

wood with such features. This background will permit the 

reader to understand the biological bases for the properties 

presented in subsequent chapters.

Although shrubs and many vines form wood, the remainder 

of this chapter will focus on wood from trees, which are the 

Contents 

Biological Structure of Wood at Decreasing 

Scales 3–2

 The Tree 3–2

 Softwoods and Hardwoods 3–2

 Sapwood and Heartwood 3–2

 Axial and Radial Systems 3–3

 Planes of Section 3–3

 Vascular Cambium 3–4

 Growth Rings 3–4

 Cells in Wood 3–6

 Cell Walls 3–7

 Pits 3–8

Microscopic Structure of Softwoods and 

Hardwoods 3–9

 Softwoods 3–9

 Hardwoods 3–10

Wood Technology 3–12

 Moisture Relations 3–12

 Density 3–12

Juvenile Wood and Reaction Wood 3–13

Appearance of Wood as Sawn Lumber 3–13

 Color and Luster 3–13

 Grain and Texture 3–13

 Plainsawn and Quartersawn 3–14

 Slope of Grain: Straight, Diagonal, 

 Spiral, and Interlocked Grain 3–14

 Knots 3–15

 Decorative Features 3–15

Wood Identification 3–16

Literature Cited 3–16
3–2

predominant source of wood for commercial and engineering applications and provide examples of virtually all features that merit discussion.

Biological Structure of Wood 

at Decreasing Scales

The Tree

A living, growing tree has two main domains, the shoot and 

the roots. Roots are the subterranean structures responsible 

for water and mineral nutrient uptake, mechanical anchoring of the shoot, and storage of biochemicals. The shoot is 

made up of the trunk or bole, branches, and leaves (Raven 

and others 1999). The remainder of the chapter will be concerned with the trunk of the tree.

If one cuts down a tree and looks at the stump, several 

gross observations can be made. The trunk is composed of 

various materials present in concentric bands. From 

the outside of the tree to the inside are outer bark, inner 

bark, vascular cambium, sapwood, heartwood, and the pith 

(Fig. 3–1). Outer bark provides mechanical protection to the 

softer inner bark and also helps to limit evaporative water 

loss. Inner bark is the tissue through which sugars (food) 

produced by photosynthesis are translocated from the leaves 

to the roots or growing portions of the tree. The vascular 

cambium is the layer between the bark and the wood that 

produces both these tissues each year. The sapwood is the 

active, “living” wood that conducts the water (or sap) from 

the roots to the leaves. It has not yet accumulated the oftencolored chemicals that set apart the non-conductive heartwood found as a core of darker-colored wood in the middle 

of most trees. The pith at the very center of the trunk is the 

remnant of the early growth of the trunk, before wood was 

formed.

Softwoods and Hardwoods

Despite what one might think based on the names, not all 

softwoods have soft, lightweight wood, nor do all hardwoods have hard, heavy wood. To define them botanically, 

softwoods are those woods that come from gymnosperms 

(mostly conifers), and hardwoods are woods that come 

from angiosperms (flowering plants). In the temperate portion of the northern hemisphere, softwoods are generally 

needle-leaved evergreen trees such as pine (Pinus) and 

spruce (Picea), whereas hardwoods are typically broadleaf, 

deciduous trees such as maple (Acer), birch (Betula), and 

oak (Quercus). Softwoods and hardwoods not only differ in 

terms of the types of trees from which they are derived, but 

they also differ in terms of their component cells. Softwoods 

have a simpler basic structure than do hardwoods because 

they have only two cell types and relatively little variation 

in structure within these cell types. Hardwoods have greater 

structural complexity because they have both a greater number of basic cell types and a far greater degree of variability 

within the cell types. The single most important distinction 

between the two general kinds of wood is that hardwoods 

have a characteristic type of cell called a vessel element (or 

pore) whereas softwoods lack these (Fig. 3–2). An important 

cellular similarity between softwoods and hardwoods is that 

in both kinds of wood, most of the cells are dead at maturity, 

even in the sapwood. The cells that are alive at maturity are 

known as parenchyma cells and can be found in both softwoods and hardwoods. 

Sapwood and Heartwood

In both softwoods and hardwoods, the wood in the trunk of 

the tree is typically divided into two zones, each of which 

serves an important function distinct from the other. The actively conducting portion of the stem in which parenchyma 

cells are still alive and metabolically active is referred to as 

sapwood. A looser, more broadly applied definition is that 

sapwood is the band of lighter colored wood adjacent to the 

bark. Heartwood is the darker colored wood found to the 

interior of the sapwood (Fig. 3–1).

In the living tree, sapwood is responsible not only for 

conduction of sap but also for storage and synthesis of biochemicals. An important storage function is the long-term 

storage of photosynthate. Carbon that must be expended to 

form a new flush of leaves or needles must be stored somewhere in the tree, and parenchyma cells of the sapwood 

are often where this material is stored. The primary storage 

forms of photosynthate are starch and lipids. Starch grains 

are stored in the parenchyma cells and can be easily seen 

with a microscope. The starch content of sapwood can have 

important ramifications in the wood industry. For example, 

in the tropical tree ceiba (Ceiba pentandra), an abundance 

Figure 3–1. Macroscopic view of a transverse section 

of a Quercus alba trunk. Beginning at the outside of 

the tree is the outer bark (ob). Next is the inner bark 

(ib) and then the vascular cambium (vc), which is too 

narrow to see at this magnification. Interior toward the 

vascular cambium is the sapwood, which is easily differentiated from the heartwood that lies toward the interior. At the center of the trunk is the pith (p), which is 

barely discernible in the center of the heartwood.

General Technical Report FPL–GTR–190
3–3

Chapter 3 Structure and Function of Wood

of starch can lead to growth of anaerobic bacteria that produce ill-smelling compounds that can make the wood commercially unusable (Chudnoff 1984). In southern yellow 

pines of the United States, a high starch content encourages 

growth of sap-stain fungi that, though they do not affect the 

strength of the wood, can nonetheless decrease the lumber 

value for aesthetic reasons (Simpson 1991).

Living cells of the sapwood are also the agents of heartwood 

formation. Biochemicals must be actively synthesized and 

translocated by living cells. For this reason, living cells at 

the border between heartwood and sapwood are responsible 

for the formation and deposition of heartwood chemicals, 

one important step leading to heartwood formation 

(Hillis 1996). Heartwood functions in long-term storage of 

biochemicals of many varieties depending on the species in 

question. These chemicals are known collectively as extractives. In the past, heartwood was thought to be a disposal 

site for harmful byproducts of cellular metabolism, the 

so-called secondary metabolites. This led to the concept of 

the heartwood as a dumping ground for chemicals that, to a 

greater or lesser degree, would harm living cells if not sequestered in a safe place. We now know that extractives are 

a normal part of the plant’s system of protecting its wood. 

Extractives are formed by parenchyma cells at the heartwood–sapwood boundary and are then exuded through pits 

into adjacent cells (Hillis 1996). In this way, dead cells can 

become occluded or infiltrated with extractives despite the 

fact that these cells lack the ability to synthesize or accumulate these compounds on their own.

Extractives are responsible for imparting several larger-scale 

characteristics to wood. For example, extractives provide 

natural durability to timbers that have a resistance to decay 

fungi. In the case of a wood like teak (Tectona grandis), 

known for its stability and water resistance, these properties 

are conferred in large part by the waxes and oils formed and 

deposited in the heartwood. Many woods valued for their 

colors, such as mahogany (Swietenia mahagoni), African 

blackwood (Diospyros melanoxylon), Brazilian rosewood 

(Dalbergia nigra), and others, owe their value to the type 

and quantity of extractives in the heartwood. For these 

species, the sapwood has little or no value, because the desirable properties are imparted by heartwood extractives. 

Gharu wood, or eagle wood (Aquilaria malaccensis), has 

been driven to endangered status due to human harvest of 

the wood to extract valuable resins used in perfume making (Lagenheim 2003). Sandalwood (Santalum spicatum), a 

wood famed for its use in incenses and perfumes, is valuable 

only if the heartwood is rich with the desired scented extractives. The utility of a wood for a technological application 

can be directly affected by extractives. For example, if a 

wood like western redcedar, high in hydrophilic extractives, 

is finished with a water-based paint without a stain blocker, 

extractives may bleed through the paint, ruining the product 

(Chap. 16).

Axial and Radial Systems

The distinction between sapwood and heartwood, though 

important, is a gross feature that is often fairly easily observed. More detailed inquiry into the structure of wood 

shows that wood is composed of discrete cells connected 

and interconnected in an intricate and predictable fashion 

to form an integrated system that is continuous from root 

to twig. The cells of wood are typically many times longer 

than wide and are specifically oriented in two separate systems of cells: the axial system and the radial system. Cells 

of the axial system have their long axes running parallel to 

the long axis of the organ (up and down the trunk). Cells of 

the radial system are elongated perpendicularly to the long 

axis of the organ and are oriented like radii in a circle or 

spokes in a bicycle wheel, from the pith to the bark. In the 

trunk of a tree, the axial system runs up and down, functions 

in long-distance water movement, and provides the bulk 

of the mechanical strength of the tree. The radial system 

runs in a pith to bark direction, provides lateral transport 

for biochemicals, and in many cases performs a large fraction of the storage function in wood. These two systems are 

interpenetrating and interconnected, and their presence is a 

defining characteristic of wood as a tissue.

Planes of Section

Although wood can be cut in any direction for examination, 

the organization and interrelationship between the axial and 

radial systems give rise to three main perspectives from 

which they can be viewed to glean the most information. 

These three perspectives are the transverse plane of section (the cross section), the radial plane of section, and the 

Figure 3–2. A, the general form of a generic softwood tree. 

B, the general form of a generic hardwood tree. C, transverse section of Pseudotsuga mensiezii, a typical softwood; the thirteen round white spaces are resin canals. 

D, transverse section of Betula allegheniensis, a typical 

hardwood; the many large, round white structures are 

vessels or pores, the characteristic feature of a hardwood. 

Scale bars = 780 μm.
3–4

tangential plane of section. Radial and tangential sections 

are referred to as longitudinal sections because they extend 

parallel to the axial system (along the grain).

The transverse plane of section is the face that is exposed 

when a tree is cut down. Looking down at the stump one 

sees the transverse section (as in Fig. 3–3H); cutting a board 

across the grain exposes the transverse section. The transverse plane of section provides information about features 

that vary both in the pith to bark direction (called the radial 

direction) and also those that vary in the circumferential direction (called the tangential direction). It does not provide 

information about variations up and down the trunk.

The radial plane of section runs in a pith-to-bark direction 

(Fig. 3–3A, top), and it is parallel to the axial system, so it 

provides information about longitudinal changes in the stem 

and from pith to bark along the radial system. To describe 

it geometrically, it is parallel to the radius of a cylinder, and 

extending up and down the length of the cylinder. In a practical sense, it is the face or plane that is exposed when a log 

is split exactly from pith to bark. It does not provide any information about features that vary in a tangential direction.

The tangential plane is at a right angle to the radial plane 

(Fig. 3–3A, top). Geometrically, it is parallel to any tangent 

line that would touch the cylinder, and it extends along the 

length of the cylinder. One way in which the tangential 

plane would be exposed is if the bark were peeled from a 

log; the exposed face is the tangential plane. The tangential 

plane of section does not provide any information about 

features that vary in the radial direction, but it does provide 

information about the tangential dimensions of features.

All three planes of section are important to the proper observation of wood, and only by looking at each can a holistic 

and accurate understanding of the three-dimensional structure of wood be gleaned. The three planes of section are 

determined by the structure of wood and the way in which 

the cells in wood are arrayed. The topology of wood and the 

distribution of the cells are accomplished by a specific part 

of the tree stem.

Vascular Cambium

The axial and radial systems and their component cells are 

derived from a part of the tree called the vascular cambium. 

The vascular cambium is a thin layer of cells that exists 

between the inner bark and the wood (Figs. 3–1, 3–4) that 

produces, by means of many cell divisions, wood (or secondary xylem) to the inside and bark (or secondary phloem) 

to the outside, both of which are vascular conducting tissues (Larson 1994). As the vascular cambium adds cells to 

the layers of wood and bark around a tree, the girth of the 

tree increases, and thus the total surface area of the vascular 

cambium itself must increase, and this is accomplished by 

cell division as well.

The axial and radial systems are generated in the vascular 

cambium by two component cells: fusiform initials and 

ray initials. Fusiform initials, named to describe their long, 

slender shape, give rise to cells of the axial system, and ray 

initials give rise to the radial system. For this reason, there 

is a direct and continuous link between the most recently 

formed wood, the vascular cambium, and the inner bark. 

In most cases, the radial system in the wood is continuous 

into the inner bark, through the vascular cambium. In this 

way wood, the water-conducting tissue, stays connected to 

the inner bark, the photosynthate-conducting tissue. They 

are interdependent tissues because the living cells in wood 

require photosynthate for respiration and cell growth and the 

inner bark requires water in which to dissolve and transport 

the photosynthate. The vascular cambium is an integral feature that not only gives rise to these tissue systems but also 

links them so that they may function in the living tree.

Growth Rings

Wood is produced by the vascular cambium one layer of cell 

divisions at a time, but we know from general experience 

that in many woods large groups of cells are produced more 

or less together in time, and these groups act together to 

serve the tree. These collections of cells produced together 

over a discrete time interval are known as growth increments or growth rings. Cells formed at the beginning of 

the growth increment are called earlywood cells, and cells 

formed in the latter portion of the growth increment are 

called latewood cells (Fig. 3–3D,E). Springwood and summerwood were terms formerly used to refer to earlywood 

and latewood, respectively, but their use is no longer recommended (IAWA 1989).

In temperate portions of the world and anywhere else with 

distinct, regular seasonality, trees form their wood in annual 

growth increments; that is, all the wood produced in one 

growing season is organized together into a recognizable, 

functional entity that many sources refer to as annual rings. 

Such terminology reflects this temperate bias, so a preferred 

term is growth increment, or growth ring (IAWA 1989). In 

many woods in the tropics, growth rings are not evident. 

However, continuing research in this area has uncovered 

several characteristics whereby growth rings can be correlated with seasonality changes in some tropical species 

(Worbes 1995, 1999; Callado and others 2001).

Woods that form distinct growth rings, and this includes 

most woods that are likely to be used as engineering 

materials in North America, show three fundamental patterns within a growth ring: no change in cell pattern across 

the ring; a gradual reduction of the inner diameter of conducting elements from the earlywood to the latewood; and 

a sudden and distinct change in the inner diameter of the 

conducting elements across the ring (Fig. 3–5). These patterns appear in both softwoods and hardwoods but differ in 

General Technical Report FPL–GTR–190
3–5

Chapter 3 Structure and Function of Wood

Figure 3–3. A, illustration of a cut-away tree at various magnifications, corresponding roughly with the images to its right; at the top, at an approximate magnification 

of 100×, a softwood cell and several hardwood cells are illustrated, to give a sense 

of scale between the two; one tier lower, at an approximate magnification of 50×, is 

a single growth ring of a softwood (left) and a hardwood (right), and an indication of 

the radial and tangential planes; the next tier, at approximately 5× magnification, illustrates many growth rings together and how one might produce a straight-grained 

rather than a diagonal-grained board; the lowest tier includes an illustration of the 

relative position of juvenile and mature wood in the tree, at 1× magnification. B,C, light 

microscopic views of the lumina (L) and cell walls (arrowheads) of a softwood (B) and 

a hardwood (C). D,E, hand-lens views of growth rings, each composed of earlywood 

(ew) and latewood (lw), in a softwood (D) and a hardwood (E). F, a straight-grained 

board; note that the line along the edge of the board is parallel to the line along the 

grain of the board. G, a diagonal-grained board; note that the two lines are markedly 

not parallel; this board has a slope of grain of about 1 in 7. H, the gross anatomy of a 

tree trunk, showing bark, sapwood, and heartwood.
each because of the distinct anatomical differences between 

the two.

Non-porous woods (or softwoods, woods without vessels) 

can exhibit any of these three general patterns. Some 

softwoods such as Western redcedar (Thuja plicata), northern white-cedar (Thuja occidentalis), and species of spruce 

(Picea) and true fir (Abies) have growth increments that 

undergo a gradual transition from the thin-walled widelumined earlywood cells to the thicker-walled, narrowerlumined latewood cells (Fig. 3–5B). Other woods undergo 

an abrupt transition from earlywood to latewood, such as 

southern yellow pine (Pinus), larch (Larix), Douglas-fir 

(Pseudotsuga menziesii), baldcypress (Taxodium disticum), 

and redwood (Sequoia sempervirens) (Fig. 3–5C). Because 

most softwoods are native to the north temperate regions, 

growth rings are clearly evident. Only in species such as araucaria (Araucaria) and some podocarps (Podocarpus) does 

one find no transition within the growth ring (Fig. 3–5A). 

Some authors report this state as growth rings being absent 

or only barely evident (Phillips 1948, Kukachka 1960).

Porous woods (or hardwoods, woods with vessels) have two 

main types of growth rings and one intermediate form. In 

diffuse-porous woods, vessels either do not markedly differ 

in size and distribution from the earlywood to the latewood, 

or the change in size and distribution is gradual and no 

clear distinction between earlywood and latewood can be 

found (Fig. 3–5D). Maple (Acer), birch (Betula), aspen/cottonwood (Populus), and yellow-poplar (Liriodendron tulipifera) are examples of diffuse porous species.

This pattern is in contrast to ring-porous woods wherein the 

transition from earlywood to latewood is abrupt, with vessel diameters decreasing substantially (often by an order or 

magnitude or more); this change in vessel size is often accompanied by a change in the pattern of vessel distribution 

as well. This creates a ring pattern of large earlywood vessels around the inner portion of the growth increment, and 

then denser, more fibrous tissue in the latewood, as is found 

in hackberry (Celtis occidentalis), white ash (Fraxinus 

americana), shagbark hickory (Carya ovata), and northern 

red oak (Quercus rubra) (Fig. 3–5F).

Sometimes the vessel size and distribution pattern falls more 

or less between these two definitions, and this condition is 

referred to as semi-ring-porous (Fig. 3–5E). Black walnut 

(Juglans nigra) is a temperate-zone semi-ring-porous wood. 

Most tropical hardwoods are diffuse-porous; the bestknown commercial exceptions to this are the Spanish-cedars 

(Cedrela spp.) and teak (Tectona grandis), which are generally semi-ring-porous and ring-porous, respectively.

Few distinctly ring-porous species grow in the tropics and 

comparatively few grow in the southern hemisphere. In genera that span temperate and tropical zones, it is common to 

have ring-porous species in the temperate zone and diffuseporous species in the tropics. The oaks (Quercus), ashes 

(Fraxinus), and hackberries (Celtis) native to the tropics 

are diffuse-porous, whereas their temperate congeners are 

ring-porous. Numerous detailed texts provide more information on growth increments in wood, a few of which are of 

particular note (Panshin and deZeeuw 1980, Dickison 2000, 

Carlquist 2001).

Cells in Wood

Understanding a growth ring in greater detail requires some 

familiarity with the structure, function, and variability of 

3–6

Figure 3–4. Light microscopic view of the vascular 

cambium. Transverse section showing vascular cambium (vc) and bark (b) in Croton macrobothrys. The 

tissue above the vascular cambium is wood. Scale 

bar = 390 μm.

Figure 3–5. Transverse sections of woods showing types 

of growth rings. Arrows delimit growth rings, when present. A–C, softwoods. A, no transition within the growth 

ring (growth ring absent) in Podocarpus imbricata.

B, gradual transition from earlywood to latewood in Picea 

glauca. C, abrupt transition from earlywood to latewood 

in Pseudotsuga mensiezii. D–F, hardwoods. D, diffuseporous wood (no transition) in Acer saccharum. E, semiring-porous wood (gradual transition) in Diospyros virginiana. F, ring-porous wood (abrupt transition) in Fraxinus 

americana. Scale bars = 300 μm.

General Technical Report FPL–GTR–190
cells that make up the ring. A living plant cell consists of 

two primary domains: the protoplast and the cell wall. The 

protoplast is the sum of the living contents that are bounded 

by the cell membrane. The cell wall is a non-living, largely 

carbohydrate matrix extruded by the protoplast to the exterior of the cell membrane. The plant cell wall protects the 

protoplast from osmotic lysis and often provides mechanical 

support to the plant at large (Esau 1977, Raven and others 

1999, Dickison 2000).

For cells in wood, the situation is somewhat more complicated than this highly generalized case. In many cases in 

wood, the ultimate function of the cell is borne solely by the 

cell wall. This means that many mature wood cells not only 

do not require their protoplasts, but indeed must completely 

remove their protoplasts prior to achieving functional maturity. For this reason, a common convention in wood literature is to refer to a cell wall without a protoplast as a cell. 

Although this is technically incorrect from a cell biological 

standpoint, this convention is common in the literature and 

will be observed throughout the remainder of the chapter.

In the case of a mature cell in wood in which there is no 

protoplast, the open portion of the cell where the protoplast 

would have existed is known as the lumen (plural: lumina). 

Thus, in most cells in wood there are two domains; the cell 

wall and the lumen (Fig. 3–3B,C). The lumen is a critical 

component of many cells, whether in the context of the 

amount of space available for water conduction or in the 

context of a ratio between the width of the lumen and the 

thickness of the cell wall. The lumen has no structure per se, 

as it is the void space in the interior of the cell. Thus, wood 

is a substance that has two basic domains; air space (mostly 

in the lumina of the cells) and the cell walls of the component cells.

Cell Walls

Cell walls in wood give wood the majority of its properties 

discussed in later chapters. Unlike the lumen, which is a 

void space, the cell wall itself is a highly regular structure, 

from one cell type to another, between species, and even 

when comparing softwoods and hardwoods. The cell wall 

consists of three main regions: the middle lamella, the 

primary wall, and the secondary wall (Fig. 3–6). In each 

region, the cell wall has three major components: cellulose 

microfibrils (with characteristic distributions and organization), hemicelluloses, and a matrix or encrusting material, 

typically pectin in primary walls and lignin in secondary 

walls (Panshin and deZeeuw 1980). In a general sense, cellulose can be understood as a long string-like molecule with 

high tensile strength; microfibrils are collections of cellulose 

molecules into even longer, stronger thread-like macromolecules. Lignin is a brittle matrix material. The hemicelluloses 

are smaller, branched molecules thought to help link the 

lignin and cellulose into a unified whole in each layer of the 

cell wall.

To understand these wall layers and their interrelationships, 

it is necessary to remember that plant cells generally do not 

exist singly in nature; instead they are adjacent to many 

other cells, and this association of thousands of cells, 

taken together, forms an organ, such as a leaf. Each of the 

individual cells must adhere to one another in a coherent 

way to ensure that the cells can act as a unified whole. This 

means they must be interconnected to permit the movement 

of biochemicals (such as photosynthate, hormones, cellsignaling agents) and water. This adhesion is provided by 

the middle lamella, the layer of cell wall material between 

two or more cells, a part of which is contributed by each of 

the individual cells (Fig. 3–6). This layer is the outermost 

layer of the cell wall continuum and in a non-woody organ 

is pectin rich. In the case of wood, the middle lamella is 

lignified.

The next layer formed by the protoplast just interior to the 

middle lamella is the primary wall (Fig. 3–6). The primary 

wall is characterized by a largely random orientation of 

cellulose microfibrils; like thin threads wound round and 

round a balloon in no particular order, where any microfibril 

angle from 0° to 90° relative to the long axis of the cell may 

3–7

Chapter 3 Structure and Function of Wood

Figure 3–6. Cut-away drawing of the cell wall, including 

the structural details of a bordered pit. The various layers of the cell wall are detailed at the top of the drawing, 

beginning with the middle lamella (ML). The next layer is 

the primary wall (P), and on the surface of this layer the 

random orientation of the cellulose microfibrils is detailed. Interior to the primary wall is the secondary wall 

in its three layers: S1, S2, and S3. The microfibril angle 

of each layer is illustrated, as well as the relative thickness of the layers. The lower portion of the illustration 

shows bordered pits in both sectional and face view. 
be present. In cells in wood, the primary wall is thin and is 

generally indistinguishable from the middle lamella. For this 

reason, the term compound middle lamella is used to denote 

the primary cell wall of a cell, the middle lamella, and the 

primary cell wall of the adjacent cell. Even when viewed 

with transmission electron microscopy, the compound middle lamella often cannot be separated unequivocally into its 

component layers. 

The remaining cell wall domain, found in virtually all cells 

in wood (and in many cells in non-woody plants or plant 

parts), is the secondary cell wall. The secondary cell wall is 

composed of three layers (Fig. 3–6). As the protoplast lays 

down the cell wall layers, it progressively reduces the lumen 

volume. The first-formed secondary cell wall layer is the S1

(Fig. 3–6), which is adjacent to compound middle lamella 

(or technically, the primary wall). This layer is a thin layer 

and is characterized by a large microfibril angle. That is to 

say, the cellulose microfibrils are laid down in a helical fashion, and the angle between the mean microfibril direction 

and the long axis of the cell is large (50° to 70°).

The next wall layer is arguably the most important cell wall 

layer in determining the properties of the cell and, thus, 

the wood properties at a macroscopic level (Panshin and 

deZeeuw 1980). This layer, formed interior to the S1 layer, 

is the S2 layer (Fig. 3–6). This is the thickest secondary cell 

wall layer and it makes the greatest contribution to the overall properties of the cell wall. It is characterized by a lower 

lignin percentage and a low microfibril angle (5° to 30°). 

The microfibril angle of the S2 layer of the wall has a strong 

but not fully understood relationship with wood properties 

at a macroscopic level (Kretschmann and others 1998), and 

this is an area of active research.

Interior to the S2 layer is the S3 layer, a relatively thin wall 

layer (Fig. 3–6). The microfibril angle of this layer is relatively high and similar to the S1 (>70°). This layer has the 

lowest percentage of lignin of any of the secondary wall 

layers. The explanation of this phenomenon is related 

directly to the physiology of the living tree. In brief, for water to move up the plant (transpiration), there must be adhesion between the water molecules and the cell walls of the 

water conduits. Lignin is a hydrophobic macromolecule, so 

it must be in low concentration in the S3 to permit adhesion 

of water to the cell wall and thus facilitate transpiration. For 

more detail on these wall components and information on 

transpiration and the role of the cell wall, see any collegelevel plant physiology textbook (for example, Kozlowski 

and Pallardy 1997, Taiz and Zeiger 1991).

Pits

Any discussion of cell walls in wood must be accompanied 

by a discussion of the ways in which cell walls are modified 

to allow communication and transport between the cells in 

the living plant. These wall modifications, called pit-pairs 

(or more commonly just pits), are thin areas in the cell walls 

between two cells and are a critical aspect of wood structure 

too often overlooked in wood technological treatments. Pits 

have three domains: the pit membrane, the pit aperture, and 

the pit chamber. The pit membrane (Fig. 3–6) is the thin 

semi-porous remnant of the primary wall; it is a carbohydrate and not a phospholipid membrane. The pit aperture 

is the opening or hole leading into the open area of the pit, 

which is called the pit chamber (Fig. 3–6). The type, number, size, and relative proportion of pits can be characteristic 

of certain types of wood and furthermore can directly affect 

how wood behaves in a variety of situations, such as how 

wood interacts with surface coatings (DeMeijer and others 

1998, Rijkaert and others 2001).

Pits of predictable types occur between different types of 

cells. In the cell walls of two adjacent cells, pits will form in 

the wall of each cell separately but in a coordinated location 

so that the pitting of one cell will match up with the pitting 

of the adjacent cell (thus a pit-pair). When this coordination 

is lacking and a pit is formed only in one of the two cells, it 

is called a blind pit. Blind pits are fairly rare in wood. Understanding the type of pit can permit one to determine what 

type of cell is being examined in the absence of other information. It can also allow one to make a prediction about 

how the cell might behave, particularly in contexts that involve fluid flow. Pits occur in three varieties: bordered, simple, and half-bordered (Esau 1977, Raven and others 1999).

Bordered pits are thus named because the secondary wall 

overarches the pit chamber and the aperture is generally 

smaller or differently shaped than the pit chamber, or both. 

The portion of the cell wall that is overarching the pit chamber is called the border (Figs. 3–6, 3–7A,D). When seen in 

face view, bordered pits often are round in appearance and 

look somewhat like a doughnut (Fig. 3–6). When seen in 

sectional view, the pit often looks like a pair of V’s with 

the open ends of the V’s facing each other (Fig. 3–7A,D). 

In this case, the long stems of the V represent the borders, 

the secondary walls that are overarching the pit chamber. 

Bordered pits always occur between two conducting cells, 

and sometimes between other cells, typically those with 

thick cell walls. The structure and function of bordered pits, 

particularly those in softwoods (see following section), are 

much-studied and considered to be well-suited to the safe 

and efficient conduction of sap. The status of the bordered 

pit (whether it is open or closed) has great importance in the 

field of wood preservation and can affect wood finishing and 

adhesive bonding.

Simple pits lack any sort of border (Fig. 3–7C,F). The pit 

chamber is straight-walled, and the pits are uniform in size 

and shape in each of the partner cells. Simple pits are typical 

between parenchyma cells and in face view merely look like 

clear areas in the walls. 

3–8

General Technical Report FPL–GTR–190
Half-bordered pits occur between a conducting cell and a 

parenchyma cell. In this case, each cell forms the kind of pit 

that would be typical of its type (bordered in the case of a 

conducting cell and simple in the case of a parenchyma cell) 

and thus half of the pit pair is simple and half is bordered 

(Fig. 3–7B,E). In the living tree, these pits are of great importance because they represent the communication between 

conducting cells and biochemically active parenchyma cells.

Microscopic Structure of 

Softwoods and Hardwoods

As discussed previously, the fundamental differences between woods are founded on the types, sizes, proportions, 

pits, and arrangements of different cells that comprise the 

wood. These fine details of structure can affect the use of a 

wood.

Softwoods

The structure of a typical softwood is relatively simple. 

The axial or vertical system is composed mostly of axial 

tracheids, and the radial or horizontal system is the rays, 

which are composed mostly of ray parenchyma cells.

Tracheids

Tracheids are long cells that are often more than 100 times 

longer (1 to 10 mm) than wide and they are the major component of softwoods, making up over 90% of the volume of 

the wood. They serve both the conductive and mechanical 

needs of softwoods. On the transverse view or section 

(Fig. 3–8A), tracheids appear as square or slightly rectangular cells in radial rows. Within one growth ring they are 

typically thin-walled in the earlywood and thicker-walled in 

the latewood. For water to flow between tracheids, it must 

pass through circular bordered pits that are concentrated in 

the long, tapered ends of the cells. Tracheids overlap with 

adjacent cells across both the top and bottom 20% to 30% of 

their length. Water flow thus must take a slightly zigzag path 

as it goes from one cell to the next through the pits. Because 

the pits have a pit membrane, resistance to flow is substantial. The resistance of the pit membrane coupled with the 

narrow diameter of the lumina makes tracheids relatively 

inefficient conduits compared with the conducting cells of 

hardwoods. Detailed treatments of the structure of wood in 

relation to its conductive functions can be found in the literature (Zimmermann 1983, Kozlowski and Pallardy 1997).

Axial Parenchyma and Resin Canal Complexes

Another cell type that is sometimes present in softwoods is 

axial parenchyma. Axial parenchyma cells are similar in size 

and shape to ray parenchyma cells, but they are vertically 

oriented and stacked one on top of the other to form a parenchyma strand. In transverse section they often look like 

axial tracheids but can be differentiated when they contain 

dark colored organic substances in the lumina of the cells. 

In the radial or tangential section they appear as long strands 

of cells generally containing dark-colored substances. Axial 

parenchyma is most common in redwood, juniper, cypress, 

baldcypress, and some species of Podocarpus but never 

makes up even 1% of the volume of a block of wood. Axial 

parenchyma is generally absent in pine, spruce, larch, hemlock, and species of Araucaria and Agathis.

In species of pine, spruce, Douglas-fir, and larch, structures 

commonly called resin ducts or resin canals are present axially (Fig. 3–9) and radially (Fig. 3–9C). These structures are 

voids or spaces in the wood and are not cells. Specialized 

parenchyma cells that function in resin production surround 

resin canals. When referring to the resin canal and all the 

associated parenchyma cells, the correct term is axial or 

radial resin canal complex (Wiedenhoeft and Miller 2002). 

In pine, resin canal complexes are often visible on the transverse section to the naked eye, but they are much smaller 

in spruce, larch, and Douglas-fir, and a hand lens is needed 

to see them. Radial resin canal complexes are embedded in 

specialized rays called fusiform rays (Figs. 3–8C, 3–9C). 

These rays are typically taller and wider than normal rays. 

Resin canal complexes are absent in the normal wood of 

other softwoods, but some species can form large tangential 

clusters of traumatic axial resin canals in response to substantial injury.

Rays

The other cells in Figure 3–8A are ray parenchyma cells 

that are barely visible and appear as dark lines running in 

a top-to-bottom direction. Ray parenchyma cells are 

3–9

Chapter 3 Structure and Function of Wood

Figure 3–7. Light micrographs and sketches of the three 

types of pits. A,D, longitudinal section of bordered pits in 

Xanthocyparis vietnamensis; the pits look like a vertical 

stack of thick-walled letter Vs. B,E, half-bordered pits in 

Pseudotsuga mensiezii; the arrow shows one halfbordered pit. C,F, simple pits on an end-wall in Pseudotsuga mensiezii; the arrow indicates one of five simple 

pits on the end wall. Scale bars = 20 μm.
rectangular prisms or brick-shaped cells. Typically they 

are approximately 15 μm high by 10 μm wide by 150 

to 250 μm long in the radial or horizontal direction 

(Fig. 3–8B). These brick-like cells form the rays, which 

function primarily in synthesis, storage, and lateral transport 

of biochemicals and, to a lesser degree, water. In radial view 

or section (Fig. 3–8B), the rays look like brick walls and the 

ray parenchyma cells are sometimes filled with dark-colored 

substances. In tangential section (Fig. 3–8C), the rays are 

stacks of ray parenchyma cells one on top of the other forming a ray that is only one cell in width, called a uniseriate 

ray.

When ray parenchyma cells intersect with axial tracheids, 

specialized pits are formed to connect the axial and radial 

systems. The area of contact between the tracheid wall and 

the wall of the ray parenchyma cells is called a cross-field. 

The type, shape, and size and number of pits in the 

cross-field are generally consistent within a species and can 

be diagnostic for wood identification.

Species that have resin canal complexes also have ray tracheids, which are specialized horizontal tracheids that normally are situated at the margins of the rays. These ray tracheids have bordered pits like axial tracheids but are much 

shorter and narrower. Ray tracheids also occur in a few 

species that do not have resin canals. Alaska yellow-cedar, 

(Chamaecyparis nootkatensis), hemlock (Tsuga), and 

rarely some species of true fir (Abies) have ray tracheids. 

Additional detail regarding the microscopic structure of 

softwoods can be found in the literature (Phillips 1948, 

Kukachka 1960, Panshin and deZeeuw 1980, IAWA 2004).

Hardwoods

The structure of a typical hardwood is much more complicated than that of a softwood. The axial system is composed 

of fibrous elements of various kinds, vessel elements in various sizes and arrangements, and axial parenchyma in various patterns and abundance. As in softwoods, rays comprise 

the radial system and are composed of ray parenchyma cells, 

but hardwoods show greater variety in cell sizes and shapes.

Vessels

Vessel elements are the specialized water-conducting cells 

of hardwoods. They are stacked one on top of the other to 

form vessels. Where the ends of the vessel elements come 

in contact with one another, a hole is formed called a 

3–10

Figure 3–8. Microscopic structure of Picea glauca, a typical 

softwood. A, transverse section, scale bar = 390 μm; the 

bulk of the wood is made of tracheids, the small rectangles 

of various thicknesses; the three large, round structures 

are resin canals and their associated cells; the dark lines 

running from the top to the bottom of the photo are the 

ray cells of the rays. B, radial section showing two rays 

(arrows) running from left to right; each cell in the ray is a 

ray cell, and they are low, rectangular cells; the rays begin 

on the right in the earlywood (thin-walled tracheids) and 

continue into and through the latewood (thick-walled tracheids) and into the earlywood of the next growth ring, on 

the left side of the photo; scale bar = 195 μm. C, tangential 

section; rays seen in end-view, mostly only one cell wide; 

two rays are fusiform rays; there are radial resin canals 

embedded in the rays, causing them to bulge; scale bar = 

195 μm.

Figure 3–9. Resin canal complexes in Pseudotsuga mensiezii. A, transverse section showing a single axial resin 

canal complex. In this view the tangential and radial diameters of the canal can be measured accurately. Scale bar

= 100 μm. B, radial section showing an axial resin canal 

complex embedded in the latewood. It is crossed by a ray 

that also extends into the earlywood on either side of the 

latewood. Scale bar = 195 μm. C, tangential section showing the anastomosis between an axial and a radial resin 

canal complex. The fusiform ray bearing the radial resin 

canal complex is in contact with the axial resin canal complex. Scale bar = 195 μm.

General Technical Report FPL–GTR–190
perforation plate. Thus hardwoods have perforated tracheary 

elements (vessels elements) for water conduction, whereas 

softwoods have imperforate tracheary elements (tracheids). 

On the transverse section, vessels appear as large openings 

and are often referred to as pores (Fig. 3–2D).

Vessel diameters may be small (<30 μm) or quite large 

(>300 μm), but typically range from 50 to 200 μm. They 

are much shorter than tracheids and range from 100 to 

1,200 μm, or 0.1 to 1.2 mm. Vessels can be arranged in various patterns. If all the vessels are the same size and more or 

less scattered throughout the growth ring, the wood is diffuse-porous (Fig. 3–5D). If the earlywood vessels are much 

larger than the latewood vessels, the wood is ring-porous 

(Fig. 3–5F). Vessels can also be arranged in a tangential or 

oblique arrangement in a radial arrangement, in clusters, or 

in many combinations of these types (IAWA 1989). In addition, individual vessels may occur alone (solitary arrangement) or in pairs or radial multiples of up to five or more 

vessels in a row. At the end of the vessel element is a hole 

or perforation plate. If there are no obstructions across the 

perforation plate, it is called a simple perforation plate. If 

bars are present, the perforation plate is called a scalariform 

perforation plate.

Where vessel elements come in contact with each other 

tangentially, intervessel or intervascular bordered pits are 

formed. These pits range in size from 2 to >16 μm in height 

and are arranged on the vessel walls in three basic ways. 

The most common arrangement is alternate, where the pits 

are offset by half the diameter of a pit from one row to the 

next. In the opposite arrangement, the pits are in files with 

their apertures aligned vertically and horizontally. In the 

scalariform arrangement, the pits are much wider than high. 

Combinations of these arrangements can also be observed in 

some species. Where vessel elements come in contact with 

ray cells, often half-bordered pits are formed called vessel–

ray pits. These pits can be the same size and shape as the 

intervessel pits or much larger.

Fibers

Fibers in hardwoods function almost exclusively as mechanical supporting cells. They are shorter than softwood 

tracheids (0.2 to 1.2 mm), average about half the width of 

softwood tracheids, but are usually two to ten times longer 

than vessel elements (Fig. 3–10B). The thickness of the fiber 

cell wall is the major factor governing density and mechanical strength of hardwood timbers. Species with thin-walled 

fibers, such as cottonwood (Populus deltoides), basswood 

(Tilia americana), ceiba, and balsa (Ochroma pyramidale), 

have low density and strength; species with thick-walled 

fibers, such as hard maple, black locust (Robinia pseudoacacia), ipe (Tabebuia serratifolia), and bulletwood (Manilkara 

bidentata), have high density and strength. Pits between fibers are generally inconspicuous and may be simple or bordered. In some woods such as oak (Quercus) and meranti/

lauan (Shorea), vascular or vasicentric tracheids are present, 

especially near or surrounding the vessels. These specialized 

fibrous elements in hardwoods typically have bordered pits, 

are thin-walled, and are shorter than the fibers of the species; they should not be confused with the tracheids in softwoods, which are much longer than hardwood fibers.

Axial Parenchyma

Axial parenchyma in softwoods is absent or only occasionally present as scattered cells, but hardwoods have a wide 

variety of axial parenchyma patterns (Fig. 3–11). The axial 

parenchyma cells in hardwoods and softwoods are roughly 

the same size and shape, and they also function in the same 

manner. The difference comes in the abundance and specific 

patterns in hardwoods. Two major types of axial parenchyma are found in hardwoods. Paratracheal parenchyma is associated with the vessels, and apotracheal parenchyma is not 

associated with the vessels. Paratracheal parenchyma is further divided into vasicentric (surrounding the vessels, Fig. 

3–11A), aliform (surrounding the vessel and with wing-like 

extensions, Fig. 3–11C), and confluent (several connecting 

patches of paratracheal parenchyma sometimes forming a 

band, Fig. 3–11E). Apotracheal parenchyma is divided into 

diffuse (scattered), diffuse-in-aggregate (short bands, Fig. 

3–11B), and banded, whether at the beginning or end of the 

growth ring (marginal, Fig. 3–11F) or within a growth ring 

(Fig. 3–11D). Each species has a particular pattern of 

axial parenchyma, which is more or less consistent from 

specimen to specimen, and these cell patterns are important 

in wood identification.

Rays

The rays in hardwoods are structurally more diverse than 

those found in softwoods. In some species such as willow 

(Salix), cottonwood, and koa (Acacia koa), the rays are 

exclusively uniseriate and are much like softwood rays. In 

hardwoods, most species have rays that are more than one 

cell wide. In oak and hard maple, the rays are two-sized, 

uniseriate and more than eight cells wide and in oak several 

3–11

Chapter 3 Structure and Function of Wood

Figure 3–10. Fibers in Quercus rubra. A, transverse section showing thick-walled, narrow-lumined fibers; three 

rays are passing vertically through the photo, and there 

are a number of axial parenchyma cells, the thin-walled, 

wide-lumined cells, in the photo; scale bar = 50 μm. B, 

macerated wood; there are several fibers (f), two of which 

are marked; also easily observed are parenchyma cells 

(arrows), both individually and in small groups; note the 

thin walls and small rectangular shape compared to the 

fibers; scale bar = 300 μm.
centimeters high (Fig. 3–12A). In most species the rays 

are one to five cells wide and <1 mm high (Fig. 3–12B). 

Rays in hardwoods are composed of ray parenchyma cells 

that are either procumbent or upright. As the name implies, 

procumbent ray cells are horizontal and are similar in shape 

and size to the softwood ray parenchyma cells (Fig. 3–12C). 

Upright ray cells have their long axis oriented axially 

(Fig. 3–12D). Upright ray cells are generally shorter than 

procumbent cells are long, and sometimes they are nearly 

square. Rays that have only one type of ray cell, typically 

only procumbent cells, are called homocellular rays. Those 

that have procumbent and upright cells are called heterocellular rays. The number of rows of upright ray cells, when 

present, varies from one to many and can be diagnostic in 

wood identification.

The great diversity of hardwood anatomy is treated in many 

sources throughout the literature (Metcalfe and Chalk 1950, 

1979, 1987; Panshin and deZeeuw 1980; IAWA 1989; 

Gregory 1994; Cutler and Gregory 1998; Dickison 2000; 

Carlquist 2001).

Wood Technology

Though briefly discussing each kind of cell in isolation is 

necessary, the beauty and complexity of wood are found in 

the interrelationship between many cells at a much larger 

scale. The macroscopic properties of wood such as density, 

hardness, bending strength, and others are properties derived 

from the cells that make up the wood. Such larger-scale 

properties are based on chemical and anatomical details 

of wood structure (Panshin and deZeeuw 1980).

Moisture Relations

The cell wall is largely made up of cellulose and hemicellulose, and the hydroxyl groups on these chemicals make 

the cell wall hygroscopic. Lignin, the agent cementing cells 

together, is a comparatively hydrophobic molecule. This 

means that the cell walls in wood have a great affinity for 

water, but the ability of the walls to take up water is limited 

in part by the presence of lignin. Water in wood has a strong 

effect on wood properties, and wood–water relations greatly 

affect the industrial use of wood in wood products. Additional information regarding dimensional changes of wood 

with changing moisture content can be found in Chapters 4 

and 13.

Density

Density (or specific gravity) is one of the most important 

physical properties of wood (Desch and Dinwoodie 1996, 

Bowyer and others 2003). Density is the weight or mass 

of wood divided by the volume of the specimen at a given 

moisture content. Thus, units for density are typically expressed as pounds per cubic foot (lb ft–3) or kilograms per 

cubic meter (kg m–3). When density values are reported in 

the literature, the moisture content of the wood must also 

be given. Specific gravity is the density of the sample normalized to the density of water. (This topic is addressed in 

greater detail in Chap. 4, including a detailed explanation of 

wood specific gravity.)

Wood structure determines wood density; in softwoods 

where latewood is abundant (Fig. 3–3D) in proportion to 

earlywood, density is higher (for example, 0.59 specific 

gravity in longleaf pine, Pinus palustris). The reverse is true 

when there is much more earlywood than latewood (Fig. 

3–5B) (for example, 0.35 specific gravity in eastern white 

pine, Pinus strobus). To say it another way, density increases 

as the proportion of cells with thick cell walls increases. 

In hardwoods, density is dependent not only on fiber wall 

thickness, but also on the amount of void space occupied by 

vessels and parenchyma. In balsa, vessels are large (typically >250 μm in tangential diameter) and there is an abundance of axial and ray parenchyma. Fibers that are present 

are thin walled, and the specific gravity may be <0.20. In 

dense woods, the fibers are thick walled, lumina are virtually absent, and fibers are abundant in relation to vessels and 

3–12

Figure 3–11. Transverse sections of various woods showing a range of hardwood axial parenchyma patterns. A, C, 

and E are woods with paratracheal types of parenchyma. 

A, vasicentric parenchyma in Enterolobium maximum;

note that two vessels in the middle of the view are connected by parenchyma, which is the feature also shown 

in E; the other vessels in the image present vasicentric 

parenchyma only. C, aliform parenchyma in Afzelia africana; the parenchyma cells are the light-colored, thinwalled cells, and are easily visible. E, confluent parenchyma in Afzelia cuazensis. B, D, and F are woods with 

apotracheal types of parenchyma. B, diffuse-in-aggregate 

parenchyma in Dalbergia stevensonii. D, banded parenchyma in Micropholis guyanensis. F, marginal parenchyma in Juglans nigra; in this case, the parenchyma 

cells are darker in color, and they delimit the growth rings 

(arrows). Scale bars = 780 μm.

General Technical Report FPL–GTR–190
parenchyma. Some tropical hardwoods have specific gravities >1.0. In all woods, density is related to the proportion of 

the volume of cell wall material to the volume of lumina of 

those cells in a given bulk volume.

Juvenile Wood and Reaction Wood

Two key examples of the biology of the tree affecting the 

quality of wood can be seen in the formation of juvenile 

wood and reaction wood. They are grouped together because they share several common cellular, chemical, and 

tree physiological characteristics, and each may or may not 

be present in a certain piece of wood.

Juvenile wood is the first-formed wood of the young tree—

the rings closest to the pith (Fig. 3–3A, bottom). Juvenile 

wood in softwoods is in part characterized by the production 

of axial tracheids that have a higher microfibril angle in the 

S2 wall layer (Larson and others 2001). A higher microfibril 

angle in the S2 is correlated with drastic longitudinal shrinkage of the cells when the wood is dried for human use, resulting in a piece of wood that has a tendency to warp, cup, 

and check. The morphology of the cells themselves is often 

altered so that the cells, instead of being long and straight, 

are often shorter and angled, twisted, or bent. The precise 

functions of juvenile wood in the living tree are not fully 

understood but are thought to confer little-understood mechanical advantages.

Reaction wood is similar to juvenile wood in several respects but is formed by the tree for different reasons. Most 

any tree of any age will form reaction wood when the 

woody organ (whether a twig, branch, or the trunk) is deflected from the vertical by more than one or two degrees. 

This means that all non-vertical branches form considerable quantities of reaction wood. The type of reaction wood 

formed by a tree differs in softwoods and hardwoods. In 

softwoods, the reaction wood is formed on the underside 

of the leaning organ and is called compression wood 

(Fig. 3–13A) (Timmel 1986). In hardwoods, the reaction 

wood forms on the top side of the leaning organ and is 

called tension wood (Fig. 3–13B) (Desch and Dinwoodie 

1996, Bowyer and others 2003). As mentioned above, the 

various features of juvenile wood and reaction wood are 

similar. In compression wood, the tracheids are shorter, misshapen cells with a large S2 microfibril angle, a high degree 

of longitudinal shrinkage, and high lignin content (Timmel 

1986). They also take on a distinctly rounded outline 

(Fig. 3–13C). In tension wood, the fibers fail to form a proper secondary wall and instead form a highly cellulosic wall 

layer called the G layer, or gelatinous layer (Fig. 3–13D).

Appearance of Wood as Sawn 

Lumber

Color and Luster

As mentioned previously when discussing heartwood and 

sapwood, the sapwood color of most species is in the white 

range. The color of heartwood depends on the presence, 

characteristics, and concentrations of extractives in the 

wood. The heartwood color of a given species can vary 

greatly, depending on growth history and health of the tree, 

genetic differences between trees, and other factors. Heartwood formation, particularly as it relates to final timber 

color, is not fully understood. Description of color in wood 

is highly dependent on the particular author; assertions that 

a particular wood is exactly one color are spurious.

Luster is a somewhat subjective characteristic of some 

woods and refers to the way in which light reflecting from 

the wood appears to penetrate into and then shine from the 

surface of the board. Genuine mahogany (Swietenia sp.) is 

one of the better-known woods with distinct luster.

Grain and Texture

The terms grain and texture are commonly used rather 

loosely in connection with wood. Grain is often used in 

reference to the relative sizes and distributions of cells, as 

in fine grain and coarse grain; this use of grain is roughly 

synonymous with texture (below). Grain is also used to indicate the orientation of the cells of the axial system (“fiber 

direction”), as in “along the grain,” straight grain, spiral 

grain, and interlocked grain, and this use of the term is preferred. Grain, as a synonym for fiber direction, is discussed 

in detail relative to mechanical properties in Chapter 5. 

3–13

Chapter 3 Structure and Function of Wood

Figure 3–12. Rays in longitudinal sections. A and B show 

tangential sections, scale bars = 300 μm. A, Quercus falcata showing a wide multiseriate ray (arrow) and many 

uniseriate rays. B, Swietenia macrophylla showing numerous rays ranging from 1 to 4 cells wide; note that in this 

wood the rays are arranged roughly in rows from side to 

side. C and D show radial sections, scale bars = 200 μm. 

C, homocellular ray in Tilia americana; all cells in the ray 

are procumbent cells; they are longer radially than they 

are tall. D, two heterocellular rays in Khaya ivorensis; the 

central portion of the ray is composed of procumbent 

cells, but the margins of the ray, both top and bottom, 

have two rows of upright cells (arrows), which are as tall 

as or taller than they are wide.
Wood finishers refer to wood as open grained and close (or 

closed) grained, which are terms reflecting the relative size 

of the cells and the need for fillers prior to finishing. Texture 

is another word used to describe a macroscopic summary 

of the relative sizes of cells in wood. Fine-textured woods 

have uniform structure with typically small cells. Coarsetextured woods generally have structure with concentrations 

of large diameter cells (such as the earlywood in ring porous 

hardwoods, Fig. 3–5F) that produce areas of clearly different appearance to the naked eye. Even-textured woods may 

have uniformly large or small cells, but their distribution 

is not concentrated in particular areas, such as in diffuse 

porous hardwoods. Even if terms used for describing the 

appearance of wood were universally agreed upon (and they 

are not), variations in wood structure within a tree, between 

trees of the same species, and between two or more species 

would defy complete characterization using these terms. 

For this reason, when discussing wood, reference should be 

made to specific properties when possible. At a minimum, it 

is desirable to ensure that the same operating definitions of 

terms like “open grained” or “coarse textured” are used by 

all parties.

Plainsawn and Quartersawn

When boards are cut from logs, a sawyer makes decisions 

about how to orient to the log with respect to the saw blade 

and in this way produces boards with different cuts. Specific 

nomenclature for these angles of cutting exists but is not 

precisely the same for hardwood lumber and softwood lumber; this unfortunate fact results in a parallel set of terms for 

hardwoods and softwoods. The sawyer can cut boards from 

a log in two distinct ways: (a) tangential to the growth rings, 

producing flatsawn or plainsawn lumber in hardwoods 

and flatsawn or slash-grained lumber in softwoods, and 

(b) radially from the pith or parallel to the rays, producing 

quartersawn lumber in hardwoods and edge-grained or vertical-grained lumber in softwoods (Fig. 3–14). In plainsawn 

boards, the surfaces next to the edges are often far from tangential to the rings, and quartersawn lumber is not usually 

cut strictly parallel with the rays. In commercial practice, 

lumber with rings at angles of 0° to 45° to the wide surface 

is called plainsawn and lumber with rings at angles of 45° 

to 90° to the wide surface is called quartersawn. Hardwood 

lumber in which annual rings form angles of 30° to 60° to 

the wide face is sometimes called bastard sawn. For many 

purposes, either plainsawn or quartersawn lumber is satisfactory, but each type has certain advantages that can be important for a particular use. Some advantages of plainsawn 

and quartersawn lumber are given in Table 3–1.

Slope of Grain: Straight, Diagonal, 

Spiral, and Interlocked Grain

The slope of grain of a board is determined by the way in 

which the sawyer cuts the board and the basic biological 

characteristics of the log from which the board is cut, but it 

is distinct from the type of cut (plainsawn or quartersawn). 

In an idealized saw log, the cells of the axial system in the 

wood are parallel to the length of the log; they run straight 

up and down the trunk. When this is the case, the grain angle of a board cut from the log is wholly a function of how 

the sawyer cuts the board. It is assumed that when a board is 

cut from the log, the long edge of the board will be parallel 

(or nearly so) with the cells of the axial system, or parallel 

with the grain (middle of Fig. 3–3A, 3–3F). Boards prepared 

in this way are straight-grained boards. When the long edge 

of the board is not parallel with the grain, the board has 

what is called diagonal grain (middle of Fig. 3–3A, 3–3F). 

Boards with diagonal grain will show atypical shrinking and 

swelling with changes in moisture content (Chap. 4), and 

altered mechanical properties (Chap. 5) depending on the 

slope of grain. The degree to which the long edge of a board 

is not parallel to the grain is referred to as slope of grain and 

is addressed in Chapter 5.

Not all logs have grain that runs perfectly straight up and 

down the length of the log. In some logs, the grain runs in a 

helical manner up the trunk, like the stripes on a barber pole 

or the lines on a candy cane. Such logs produce boards with 

spiral grain, and there is no way to cut long boards from 

3–14

Figure 3–13. Macroscopic and microscopic views of reaction wood in a softwood and a hardwood. A, compression 

wood in Pinus sp.; note that the pith is not in the center 

of the trunk, and the growth rings are much wider in the 

compression wood zone. B, tension wood in Juglans nigra; the pith is nearly centered in the trunk, but the growth 

rings are wider in the tension wood zone. C, transverse 

section of compression wood in Picea engelmannii; the 

tracheids are thick-walled and round in outline, giving 

rise to prominent intercellular spaces in the cell corners 

(arrow). D, tension wood fibers showing prominent gelatinous layers in Croton gossypiifolius; the gelatinous layers in the fibers are most pronounced across the top of 

the image on either side of and just below the vessel; the 

fibers in the lower half of the image show thinner gelatinous layers. Scale bars = 50 μm.

General Technical Report FPL–GTR–190
such a log to produce straight-grained lumber. In other logs, 

the angle of helical growth of the wood cells will change 

over time, such that the grain may curve in a right-handed 

helix (e.g., 5°) for a few years and then over the course 

of a few years change to a left-handed 5° helix, and so on 

over the life of the tree. This growth produces wood with 

interlocked grain. There is no way to saw a board from 

such a log to produce uniformly straight grain. Therefore, 

a straight-grained board can be cut only from a straightgrained log; a log with spiral or interlocked grain can never 

produce truly straight-grained lumber, regardless of the skill 

of the sawyer.

Knots

Knots are remnants of branches in the tree appearing in a 

board. In a flatwsawn board, knots appear as round and typically brown pieces of wood perpendicular to the grain of 

the board. In a quartersawn board, knots can be cut along 

their length and are referred to as spike knots. Independent 

of the cut of the board, knots occur in two basic varieties: 

intergrown knots and encased knots. These terms refer to 

the continuity, or lack thereof, of stem wood with wood 

of the branch. If the branch was alive at the time when the 

growth rings making up the board were formed, the wood 

of the trunk of the tree and that branch is continuous; the 

growth rings continue uninterrupted out along the branch, 

forming an intergrown knot. If the branch was dead at the 

time when growth rings of the board were formed, the stem 

wood curves around the branch without continuing up the 

branch, giving rise to a knot that is not continuous with the 

stem wood; this produces an encased knot. With intergrown 

knots, the grain angle of the trunk wood in the vicinity of 

the knot is typically more disturbed than in encased knots, 

and this influences wood properties (Chap. 5). Encased 

knots generally disturb the grain angle less than intergrown 

knots.

Decorative Features

The decorative value of wood depends upon its color, figure, luster, and the way in which it bleaches or takes fillers, 

stains, and transparent finishes. In addition to quantifiable 

or explicable characteristics, decorative value is also determined by the individual preferences of the end user.

The structure of a given wood, in conjunction with how 

the final wood product was cut from a log, gives rise to the 

majority of the patterns seen in wood. A general term for the 

pattern of wood is figure, which can refer to mundane features, such as the appearance of growth rings in a plainsawn 

board or the appearance of ray fleck on a quartersawn board, 

3–15

Chapter 3 Structure and Function of Wood

Table 3–1. Some advantages of plainsawn and quartersawn lumber 

Plainsawn Quartersawn 

Shrinks and swells less in thickness 

Surface appearance less affected by round or oval knots 

compared to effect of spike knots in quartersawn boards; 

boards with round or oval knots not as weak as boards 

with spike knots 

Shakes and pitch pockets, when present, extend through 

fewer boards 

Figure patterns resulting from annual rings and some 

other types of figure brought out more conspicuously 

Is less susceptible to collapse in drying 

Costs less because it is easy to obtain 

Shrinks and swells less in width 

Cups, surface-checks, and splits less in seasoning and in use 

Raised grain caused by separation in annual rings does not 

become as pronounced 

Figure patterns resulting from pronounced rays, interlocked 

grain, and wavy grain are brought out more conspicuously 

Does not allow liquids to pass through readily in some 

species 

Holds paint better in some species 

Sapwood appears in boards at edges and its width is limited 

by the width of the log 

Figure 3–14. Quartersawn (A) and plainsawn (B) boards 

cut from a log. 
or more exotic patterns determined by anomalous growth, 

such as birdseye, wavy grain, or wood burls.

Wood Identification

The identification of wood can be of critical importance to 

the primary and secondary wood using industry, government agencies, museums, law enforcement, and scientists 

in the fields of botany, ecology, anthropology, forestry, and 

wood technology. Wood identification is the recognition of 

characteristic cell patterns and wood features and is generally accurate only to the generic level. Because woods of 

different species from the same genus often have different 

properties and perform differently under various conditions, 

serious problems can develop if species or genera are mixed 

during the manufacturing process and in use. Because foreign woods are imported to the U.S. market, both buyers 

and sellers must have access to correct identifications and 

information about properties and uses.

Lumber graders, furniture workers, those working in the industry, and hobbyists often identify wood without laboratory 

tools. Features often used are color, odor, grain patterns, 

density, and hardness. With experience, these features can 

be used to identify many different woods, but the accuracy 

of the identification is dependent on the experience of the 

person and the quality of the unknown wood. If the unknown wood specimen is atypical, decayed, or small, often 

the identification is incorrect. Examining woods, especially 

hardwoods, with a 10× to 20× hand lens, greatly improves 

the accuracy of the identification (Panshin and deZeeuw 

1980, Hoadley 1990, Brunner and others 1994). Some 

foresters and wood technologists armed with a hand lens 

and sharp knife can accurately identify lumber in the field. 

They make a cut on the transverse surface and examine all 

patterns to make an identification.

Scientifically rigorous, accurate identifications require that 

the wood be sectioned and examined with a light microscope. With the light microscope, even with only a 10× objective, many more features are available for use in making 

a determination. Equally important as the light microscope 

in wood identification is the reference collection of correctly 

identified specimens to which unknown samples can be 

compared (Wheeler and Baas 1998). If a reference collection is not available, books of photomicrographs or books 

or journal articles with anatomical descriptions and dichotomous keys can be used (Miles 1978, Schweingruber 1978, 

Core and others 1979, Gregory 1980, Ilic 1991, Miller and 

Détienne 2001). In addition to these resources, several computer-assisted wood identification packages are available 

and are suitable for people with a robust wood anatomical 

background, such as the on-line searchable resource InsideWood (http://insidewood.lib.ncsu.edu/).

Wood identification by means of molecular biological techniques is a field that is still in its infancy. Substantial population-biological effects limit the statistical likelihood of a 

robust and certain identification for routine work (Canadian 

Forest Service 1999). In highly limited cases of great financial or criminal import and a narrowly defined context, the 

cost and labor associated with rigorous evaluation of DNA 

from wood can be warranted (Hipkins 2001). For example, 

if the question were “Did this piece of wood come from 

this individual tree?” or “Of the 15 species present in this 

limited geographical area, which one produced this root?” 

it is feasible to analyze the specimens with molecular techniques (Brunner and others 2001). If, however, the question 

were “What kind of wood is this, and from which forest did 

it come?” it would not be feasible at this time to analyze 

the specimen. Workers have shown that specific identification can be accomplished using DNA among six species 

of Japanese white oak (Ohyama and others 2001), but the 

routine application of their methods is not likely for some 

time. As technological advances improve the quality, quantity, and speed with which molecular data can be collected, 

the difficulty and cost of molecular wood identification 

will decrease. We can reasonably expect that at some point 

in the future molecular tools will be employed in routine 

identification of wood and that such techniques will greatly 

increase the specificity and accuracy of identification. For 

now, routine scientific wood identification is based on microscopic evaluation of wood anatomical features.

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Netherlands: The Tropenbos Foundation. 218 p.

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3–17

Chapter 3 Structure and Function of Wood
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3–18

General Technical Report FPL–GTR–190
CHAPTER 4

Moisture Relations and Physical Properties 

of Wood

Samuel V. Glass, Research Physical Scientist 

Samuel L. Zelinka, Materials Research Engineer

4–1

Wood, like many natural materials, is hygroscopic; it takes 

on moisture from the surrounding environment. Moisture 

exchange between wood and air depends on the relative humidity and temperature of the air and the current amount of 

water in the wood. This moisture relationship has an important influence on wood properties and performance. Many 

of the challenges of using wood as an engineering material 

arise from changes in moisture content or an abundance of 

moisture within the wood.

This chapter discusses the macroscopic physical properties of wood with emphasis given to their relationship with 

moisture content. Some properties are species-dependent; 

in such cases, data from the literature are tabulated according to species. The chapter begins with a broad overview 

of wood–water relations, defining key concepts needed to 

understand the physical properties of wood.

Wood–Moisture Relationships

Moisture Content and Green Wood

Many physical and mechanical properties of wood depend 

upon the moisture content of wood. Moisture content (MC) 

is usually expressed as a percentage and can be calculated 

from

(4–1)

where mwater is the mass of water in wood and mwood is 

the mass of the ovendry wood. Operationally, the moisture 

content of a given piece of wood can be calculated by

(4–2)

where mwet is the mass of the specimen at a given moisture 

content and mdry is the mass of the ovendry specimen.

Green wood is often defined as freshly sawn wood in which 

the cell walls are completely saturated with water and additional water may reside in the lumina. The moisture content of green wood can range from about 30% to more than 

200%. In green softwoods, the moisture content of sapwood 

is usually greater than that of heartwood. In green hardwoods, the difference in moisture content between heartwood and sapwood depends on the species. The average 

moisture content of green heartwood and green sapwood of 

some domestic species is given in Table 4–1. These values 

Contents 

Wood–Moisture Relationships 4–1

 Moisture Content and Green Wood 4–1

 Fiber Saturation and Maximum Moisture 

 Content 4–2

 Water Vapor Sorption 4–3

 Liquid Water Absorption 4–4

 Dimensional Stability 4–5

 Density and Specific Gravity 4–7

Thermal Properties 4–10

 Thermal Conductivity 4–10

 Heat Capacity 4–11

 Thermal Diffusivity 4–12

 Coefficient of Thermal Expansion 4–14

Electrical Properties 4–15

 DC Electrical Properties 4–15

 AC Electrical Properties 4–16

Friction Properties 4–17

Nuclear Radiation Properties 4–17

Literature Cited 4–18

Additional References 4–19
4–2

are considered typical, but variation within and between 

trees is considerable. Variability of green moisture content 

exists even within individual boards cut from the same tree. 

Additional information on moisture in green lumber is given 

in Chapter 13.

Fiber Saturation and Maximum Moisture 

Content

Moisture can exist in wood as free water (liquid water or 

water vapor in cell lumina and cavities) or as bound water 

(held by intermolecular attraction within cell walls). The 

moisture content at which only the cell walls are completely 

saturated (all bound water) but no water exists in cell lumina 

is called the fiber saturation point, MCfs. Operationally, the 

fiber saturation point is considered as that moisture content 

above which the physical and mechanical properties of 

wood do not change as a function of moisture content. The 

fiber saturation point of wood averages about 30% moisture 

content, but in individual species and individual pieces of 

wood it can vary by several percentage points from that 

value.

Conceptually, fiber saturation distinguishes between the two 

ways water is held in wood. However, in actuality, a more 

gradual transition occurs between bound and free water 

near the fiber saturation point. Within a piece of wood, in 

Table 4–1. Average moisture content of green wood, by species 

Moisture content 

(%)

Moisture content 

(%)

Species Heartwood Sapwood Species Heartwood Sapwood 

Hardwoods Softwoods 

Alder, red — 97 Baldcypress 121 171 

Apple 81 74 Cedar, eastern red 33 — 

Ash, black 95 — Cedar, incense 40 213 

Ash, green — 58 Cedar, Port-Orford 50 98 

Ash, white 46 44 Cedar, western red 58 249 

Aspen 95 113 Cedar, yellow 32 166 

Basswood, American 81 133 Douglas-fir, coast type 37 115 

Beech, American 55 72 Fir, balsam 88 173 

Birch, paper 89 72 Fir, grand 91 136 

Birch, sweet 75 70 Fir, noble 34 115 

Birch, yellow 74 72 Fir, Pacific silver 55 164 

Cherry, black 58 — Fir, white 98 160 

Chestnut, American 120 — Hemlock, eastern 97 119 

Cottonwood 162 146 Hemlock, western 85 170 

Elm, American 95 92 Larch, western 54 119 

Elm, cedar 66 61 Pine, loblolly 33 110 

Elm, rock 44 57 Pine, lodgepole 41 120 

Hackberry 61 65 Pine, longleaf 31 106 

Hickory, bitternut 80 54 Pine, ponderosa 40 148 

Hickory, mockernut 70 52 Pine, red 32 134 

Hickory, pignut 71 49 Pine, shortleaf 32 122 

Hickory, red 69 52 Pine, sugar 98 219 

Hickory, sand 68 50 Pine, western white 62 148 

Hickory, water 97 62 Redwood, old growth 86 210 

Magnolia 80 104 Spruce, black 52 113 

Maple, silver 58 97 Spruce, Engelmann 51 173 

Maple, sugar 65 72 Spruce, Sitka 41 142 

Oak, California black 76 75 Tamarack 49 — 

Oak, northern red 80 69 

Oak, southern red 83 75 

Oak, water 81 81 

Oak, white 64 78 

Oak, willow 82 74 

Sweetgum 79 137 

Sycamore, American 114 130 

Tupelo, black 87 115 

Tupelo, swamp 101 108 

Tupelo, water 150 116 

Walnut, black 90 73 

Yellow-poplar 83 106 

General Technical Report FPL–GTR–190
4–3

Chapter 4 Moisture Relations and Physical Properties of Wood

one portion all cell lumina may be empty and the cell walls 

partially dried, while in another part of the same piece, cell 

walls may be saturated and lumina partially or completely 

filled with water. Even within a single cell, the cell wall may 

begin to dry before all water has left the lumen of that same 

cell. 

The moisture content at which both cell lumina and cell 

walls are completely saturated with water is the maximum 

possible moisture content. Basic specific gravity Gb (based 

on ovendry mass and green volume—see section on Density 

and Specific Gravity) is the major determinant of maximum 

moisture content. As basic specific gravity increases, the 

volume of the lumina must decrease because the specific 

gravity of wood cell walls is constant among species. This 

decreases the maximum moisture content because less room 

is available for free water. Maximum moisture content 

MCmax for any basic specific gravity can be estimated from

(4–3)

where the specific gravity of wood cell walls is taken as 

1.54. Maximum possible moisture content varies from 267% 

at Gb = 0.30 to 44% at Gb = 0.90. Maximum possible moisture content is seldom attained in living trees. The moisture 

content at which wood will sink in water can be calculated 

by

(4–4)

Water Vapor Sorption

When wood is protected from contact with liquid water and 

shaded from sunlight, its moisture content below the fiber 

saturation point is a function of both relative humidity 

(RH) and temperature of the surrounding air. Wood in 

service is exposed to both long-term (seasonal) and shortterm (daily) changes in relative humidity and temperature of 

the surrounding air, which induce changes in wood moisture 

content. These changes usually are gradual, and short-term 

fluctuations tend to influence only the wood surface. Moisture content changes can be retarded, but not prevented, 

by protective coatings such as varnish, lacquer, or paint 

(Chap. 16). The objective of wood drying is to bring the 

moisture content close to the expected value that a finished 

product will have in service (Chap. 13).

Equilibrium Moisture Content

Equilibrium moisture content (EMC) is defined as that moisture content at which the wood is neither gaining nor losing 

moisture. The relationship between EMC, relative humidity, 

and temperature is shown in Figure 4–1 and Table 4–2. For 

most practical purposes, the values in Table 4–2 may be applied to wood of any species. These values have been calculated from the following equation:

(4–5)

where h is relative humidity (decimal) and the parameters 

W, K, K1, and K2 depend on temperature:

For temperature T in °C, 

 W = 349 + 1.29T + 0.0135T2

 K = 0.805 + 0.000736T - 0.00000273T2

K1 = 6.27 - 0.00938T - 0.000303T2

K2 = 1.91 + 0.0407T - 0.000293T2

For temperature T in °F,

 W = 330 + 0.452T + 0.00415T 2

 K = 0.791 + 0.000463T - 0.000000844T2

K1 = 6.34 + 0.000775T - 0.0000935T2

K2 = 1.09 + 0.0284T - 0.0000904T2

Simpson (1973) showed that this equation provides a good 

fit to EMC–RH–temperature data. 

Sorption Hysteresis

The relationship between EMC and relative humidity at 

constant temperature is referred to as a sorption isotherm. 

The history of a wood specimen also affects its EMC; this 

is called sorption hysteresis and is shown in Figure 4–2. A 

desorption isotherm is measured by bringing wood that was 

initially wet to equilibrium with successively lower values 

of relative humidity. A resorption, or adsorption, isotherm 

is measured in the opposite direction (from the dry state 

to successively higher RH values). As wood is dried from 

the initial green condition below the fiber saturation point 

(initial desorption), the EMC is greater than in subsequent 

desorption isotherms (Spalt 1958). Furthermore, the EMC 

10 20 30 40 50 60 70 80 90

10

20

30

40

50

60

70

80

90

100

Relative humidity (%)

Temperature (oC) 

2 4 6 8 10 12 14 16 20 24

Figure 4–1. Equilibrium moisture content of wood (labeled contours) as a function of relative humidity and 

temperature.
4–4

for resorption (adsorption) is lower than for desorption. The 

ratio of adsorption EMC to desorption EMC varies with 

species, RH, and temperature, with a mean value of about 

0.8 near room temperature (Stamm 1964, Skaar 1988). EMC 

values in Table 4–2 were derived primarily for Sitka spruce 

under conditions described as oscillating vapor pressure 

desorption (Stamm and Loughborough 1935), which was 

shown to represent a condition midway between adsorption 

and desorption. The tabulated EMC values thus provide a 

suitable and practical compromise for use when the direction of sorption is not always known.

Liquid Water Absorption

Wood products in service may be exposed to liquid water 

through a variety of mechanisms. Contact with liquid water 

can induce rapid changes in the moisture content of wood, 

in contrast to the slow changes that occur due to water vapor sorption. In addition, liquid water absorption can bring 

the moisture content of wood above fiber saturation (water 

vapor sorption alone cannot). As wood absorbs water above 

its fiber saturation point, air in the cell lumina is replaced 

by water. Absorption of liquid water may continue until the 

maximum moisture content is reached.

The mechanism of water absorption is called capillary action or wicking. Water interacts strongly with the wood cell 

wall and forms a concave meniscus (curved surface) within 

the lumen. This interaction combined with the water–air 

surface tension creates a pressure that draws water up the 

lumina.

The rate of liquid water absorption in wood depends on 

several factors. The rate of absorption is most rapid in the 

longitudinal direction (that is, when the transverse section or 

end grain is exposed to water). The rate at which air can escape from wood affects water absorption, as water displaces 

air in the lumina. Chapter 16 discusses the ability of surface 

finishes such as water repellents to inhibit water absorption.

International Standard ISO 15148 (ISO 2002) describes a 

method for measuring the rate of water absorption. One surface of a specimen is partially immersed in water. To limit 

absorption to this one surface and restrict moisture transport 

to one dimension, the sides of the specimen are coated with 

a water- and vapor-tight sealant. The specimen is periodically removed, surfaces are blotted, and the specimen is 

weighed and again partially immersed in the water. The 

mass of water absorbed per unit area of specimen surface is 

plotted against the square root of time. The initial part of the 

curve is usually linear, and the slope of this linear portion 

is the water absorption coefficient Aw (kg m–2 s–1/2). 

Measured values of Aw for softwoods are in the range 

10–16 g m–2 s–1/2 in the longitudinal direction and 

1–7 g m–2 s–1/2 in the transverse directions (IEA 1991; 

Kumaran 1999, 2002).

The liquid water diffusivity Dw (m2 s–1) is a measure of 

the rate of moisture flow (kg m–2 s–1) through a material 

Table 4–2. Moisture content of wood in equilibrium with stated temperature and relative humidity

Temperature Moisture content (%) at various relative humidity values 

(°C (°F)) 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95%

1.1 (30) 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.4 13.5 14.9 16.5 18.5 21.0 24.3

4.4 (40) 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.3 13.5 14.9 16.5 18.5 21.0 24.3

10.0 (50) 1.4 2.6 3.6 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.3 11.2 12.3 13.4 14.8 16.4 18.4 20.9 24.3

15.6 (60) 1.3 2.5 3.6 4.6 5.4 6.2 7.0 7.8 8.6 9.4 10.2 11.1 12.1 13.3 14.6 16.2 18.2 20.7 24.1

21.1 (70) 1.3 2.5 3.5 4.5 5.4 6.2 6.9 7.7 8.5 9.2 10.1 11.0 12.0 13.1 14.4 16.0 17.9 20.5 23.9

26.7 (80) 1.3 2.4 3.5 4.4 5.3 6.1 6.8 7.6 8.3 9.1 9.9 10.8 11.7 12.9 14.2 15.7 17.7 20.2 23.6

32.2 (90) 1.2 2.3 3.4 4.3 5.1 5.9 6.7 7.4 8.1 8.9 9.7 10.5 11.5 12.6 13.9 15.4 17.3 19.8 23.3

37.8 (100) 1.2 2.3 3.3 4.2 5.0 5.8 6.5 7.2 7.9 8.7 9.5 10.3 11.2 12.3 13.6 15.1 17.0 19.5 22.9

43.3 (110) 1.1 2.2 3.2 4.0 4.9 5.6 6.3 7.0 7.7 8.4 9.2 10.0 11.0 12.0 13.2 14.7 16.6 19.1 22.4

48.9 (120) 1.1 2.1 3.0 3.9 4.7 5.4 6.1 6.8 7.5 8.2 8.9 9.7 10.6 11.7 12.9 14.4 16.2 18.6 22.0

54.4 (130) 1.0 2.0 2.9 3.7 4.5 5.2 5.9 6.6 7.2 7.9 8.7 9.4 10.3 11.3 12.5 14.0 15.8 18.2 21.5

60.0 (140) 0.9 1.9 2.8 3.6 4.3 5.0 5.7 6.3 7.0 7.7 8.4 9.1 10.0 11.0 12.1 13.6 15.3 17.7 21.0

65.6 (150) 0.9 1.8 2.6 3.4 4.1 4.8 5.5 6.1 6.7 7.4 8.1 8.8 9.7 10.6 11.8 13.1 14.9 17.2 20.4

71.1 (160) 0.8 1.6 2.4 3.2 3.9 4.6 5.2 5.8 6.4 7.1 7.8 8.5 9.3 10.3 11.4 12.7 14.4 16.7 19.9

76.7 (170) 0.7 1.5 2.3 3.0 3.7 4.3 4.9 5.6 6.2 6.8 7.4 8.2 9.0 9.9 11.0 12.3 14.0 16.2 19.3

82.2 (180) 0.7 1.4 2.1 2.8 3.5 4.1 4.7 5.3 5.9 6.5 7.1 7.8 8.6 9.5 10.5 11.8 13.5 15.7 18.7

87.8 (190) 0.6 1.3 1.9 2.6 3.2 3.8 4.4 5.0 5.5 6.1 6.8 7.5 8.2 9.1 10.1 11.4 13.0 15.1 18.1

93.3 (200) 0.5 1.1 1.7 2.4 3.0 3.5 4.1 4.6 5.2 5.8 6.4 7.1 7.8 8.7 9.7 10.9 12.5 14.6 17.5

98.9 (210) 0.5 1.0 1.6 2.1 2.7 3.2 3.8 4.3 4.9 5.4 6.0 6.7 7.4 8.3 9.2 10.4 12.0 14.0 16.9

104.4 (220) 0.4 0.9 1.4 1.9 2.4 2.9 3.4 3.9 4.5 5.0 5.6 6.3 7.0 7.8 8.8 9.9 

110.0 (230) 0.3 0.8 1.2 1.6 2.1 2.6 3.1 3.6 4.2 4.7 5.3 6.0 6.7 

115.6 (240) 0.3 0.6 0.9 1.3 1.7 2.1 2.6 3.1 3.5 4.1 4.6 

121.1 (250) 0.2 0.4 0.7 1.0 1.3 1.7 2.1 2.5 2.9 

126.7 (260) 0.2 0.3 0.5 0.7 0.9 1.1 1.4 

132.2 (270) 0.1 0.1 0.2 0.3 0.4 0.4 

General Technical Report FPL–GTR–190
4–5

subjected to unit difference in moisture concentration 

(kg m–3) across unit thickness (m). An order-of-magnitude 

estimate of Dw can be made using the value of Aw as

(4–6)

where csat is the moisture concentration (kg m–3) in watersaturated wood (Kumaran 1999).

Dimensional Stability

Wood is dimensionally stable when moisture content is 

greater than the fiber saturation point. Below MCfs wood 

changes dimension as it gains moisture (swells) or loses 

moisture (shrinks), because volume of the cell wall depends 

on the amount of bound water. This shrinking and swelling 

can result in warping, checking, and splitting of the wood, 

which in turn can lead to decreased utility of wood products, 

such as loosening of tool handles, gaps in flooring, or other 

performance problems. Therefore, it is important that the 

dimensional stability be understood and considered when a 

wood product will be exposed to large moisture fluctuations 

in service.

With respect to dimensional stability, wood is an anisotropic 

material. It shrinks (swells) most in the direction of the annual growth rings (tangentially), about half as much across 

the rings (radially), and only slightly along the grain (longitudinally). The combined effects of radial and tangential 

shrinkage can distort the shape of wood pieces because 

of the difference in shrinkage and the curvature of annual 

rings. The major types of distortion as a result of these effects are illustrated in Figure 4–3.

Transverse and Volumetric Shrinkage

Data have been collected to represent the average radial, 

tangential, and volumetric shrinkage of numerous domestic 

species by methods described in American Society for Testing and Materials (ASTM) D 143—Standard Test Methods for Small Clear Specimens of Timber (ASTM 2007). 

Shrinkage values, expressed as a percentage of the green 

dimension, are listed in Table 4–3. Shrinkage values collected from the world literature for selected imported species 

are listed in Table 4–4.

The shrinkage of wood is affected by a number of variables. 

In general, greater shrinkage is associated with greater 

density. The size and shape of a piece of wood can affect 

shrinkage, and the rate of drying can affect shrinkage for 

some species. Transverse and volumetric shrinkage variability can be expressed by a coefficient of variation of approximately 15% (Markwardt and Wilson 1935).

Longitudinal Shrinkage

Longitudinal shrinkage of wood (shrinkage parallel to the 

grain) is generally quite small. Average values for shrinkage 

from green to ovendry are between 0.1% and 0.2% for most 

species of wood. However, certain types of wood exhibit excessive longitudinal shrinkage, and these should be avoided 

in uses where longitudinal stability is important. Reaction 

wood, whether compression wood in softwoods or tension 

wood in hardwoods, tends to shrink excessively parallel 

to the grain. Wood from near the center of trees (juvenile 

wood) of some species also shrinks excessively lengthwise. 

Reaction wood and juvenile wood can shrink 2% from green 

Chapter 4 Moisture Relations and Physical Properties of Wood

0

4

8

12

16

20

24

28

32

0 10 20 30 40 50 60 70 80 90 100

Initial desorption

Desorption

Adsorption

Oscillating vapor pressure desorption

Moisture content (%)

Relative humidity (%)

Figure 4–2. Moisture content–relative humidity relationship for wood under adsorption and various desorption 

conditions.

Figure 4–3. Characteristic shrinkage and distortion of 

flat, square, and round pieces as affected by direction 

of growth rings. Tangential shrinkage is about twice 

as great as radial.
4–6

General Technical Report FPL–GTR–190

Table 4–3. Shrinkage values of domestic woods

Shrinkagea

 (%) from green 

to ovendry moisture content 

Shrinkagea

 (%) from green 

to ovendry moisture content 

Species Radial Tangential Volumetric Species Radial Tangential Volumetric

Hardwoods Oak, white—con. 

Alder, red 4.4 7.3 12.6 Chestnut 5.3 10.8 16.4 

Ash Live 6.6 9.5 14.7 

Black 5.0 7.8 15.2 Overcup 5.3 12.7 16.0 

Blue 3.9 6.5 11.7 Post 5.4 9.8 16.2 

Green 4.6 7.1 12.5 Swamp, chestnut 5.2 10.8 16.4 

Oregon 4.1 8.1 13.2 White 5.6 10.5 16.3 

Pumpkin 3.7 6.3 12.0 Persimmon, common 7.9 11.2 19.1 

White 4.9 7.8 13.3 Sassafras 4.0 6.2 10.3 

Aspen Sweetgum 5.3 10.2 15.8 

Bigtooth 3.3 7.9 11.8 Sycamore, American 5.0 8.4 14.1 

Quaking 3.5 6.7 11.5 Tanoak 4.9 11.7 17.3 

Basswood, American 6.6 9.3 15.8 Tupelo 

Beech, American 5.5 11.9 17.2 Black 5.1 8.7 14.4 

Birch Water 4.2 7.6 12.5 

Alaska paper 6.5 9.9 16.7 Walnut, black 5.5 7.8 12.8 

Gray 5.2 — 14.7 Willow, black 3.3 8.7 13.9 

Paper 6.3 8.6 16.2 Yellow-poplar 4.6 8.2 12.7 

River 4.7 9.2 13.5 Softwoods

Sweet 6.5 9.0 15.6 Cedar 

Yellow 7.3 9.5 16.8 Yellow 2.8 6.0 9.2 

Buckeye, yellow 3.6 8.1 12.5 Atlantic white 2.9 5.4 8.8 

Butternut 3.4 6.4 10.6 Eastern redcedar 3.1 4.7 7.8 

Cherry, black 3.7 7.1 11.5 Incense 3.3 5.2 7.7 

Chestnut, American 3.4 6.7 11.6 Northern white 2.2 4.9 7.2 

Cottonwood Port-Orford 4.6 6.9 10.1 

Balsam poplar 3.0 7.1 10.5 Western redcedar 2.4 5.0 6.8 

Black 3.6 8.6 12.4 Douglas-fir, 

Eastern 3.9 9.2 13.9 Coastb 4.8 7.6 12.4 

Elm Interior northb

 3.8 6.9 10.7 

American 4.2 9.5 14.6 Interior westb 4.8 7.5 11.8 

Cedar 4.7 10.2 15.4 Fir 

Rock 4.8 8.1 14.9 Balsam 2.9 6.9 11.2 

Slippery 4.9 8.9 13.8 California red 4.5 7.9 11.4 

Winged 5.3 11.6 17.7 Grand 3.4 7.5 11.0 

Hackberry 4.8 8.9 13.8 Noble 4.3 8.3 12.4 

Hickory, pecan 4.9 8.9 13.6 Pacific silver 4.4 9.2 13.0 

Hickory, true Subalpine 2.6 7.4 9.4 

Mockernut 7.7 11.0 17.8 White 3.3 7.0 9.8 

Pignut 7.2 11.5 17.9 Hemlock 

Shagbark 7.0 10.5 16.7 Eastern 3.0 6.8 9.7 

Shellbark 7.6 12.6 19.2 Mountain 4.4 7.1 11.1 

Holly, American 4.8 9.9 16.9 Western 4.2 7.8 12.4 

Honeylocust 4.2 6.6 10.8 Larch, western 4.5 9.1 14.0 

Locust, black 4.6 7.2 10.2 Pine 

Madrone, Pacific 5.6 12.4 18.1 Eastern white 2.1 6.1 8.2 

Magnolia Jack 3.7 6.6 10.3 

Cucumbertree 5.2 8.8 13.6 Loblolly 4.8 7.4 12.3 

Southern 5.4 6.6 12.3 Lodgepole 4.3 6.7 11.1 

Sweetbay 4.7 8.3 12.9 Longleaf 5.1 7.5 12.2 

Maple Pitch 4.0 7.1 10.9 

Bigleaf 3.7 7.1 11.6 Pond 5.1 7.1 11.2 

Black 4.8 9.3 14.0 Ponderosa 3.9 6.2 9.7 

Red 4.0 8.2 12.6 Red 3.8 7.2 11.3 

Silver 3.0 7.2 12.0 Shortleaf 4.6 7.7 12.3 

Striped 3.2 8.6 12.3 Slash 5.4 7.6 12.1 

Sugar 4.8 9.9 14.7 Sugar 2.9 5.6 7.9 

Oak, red Virginia 4.2 7.2 11.9 

Black 4.4 11.1 15.1 Western white 4.1 7.4 11.8 

Laurel 4.0 9.9 19.0 Redwood 

Northern red 4.0 8.6 13.7 Old growth 2.6 4.4 6.8 

Pin 4.3 9.5 14.5 Young growth 2.2 4.9 7.0 

Scarlet 4.4 10.8 14.7 Spruce 

Southern red 4.7 11.3 16.1 Black 4.1 6.8 11.3 

Water 4.4 9.8 16.1 Engelmann 3.8 7.1 11.0 

Willow 5.0 9.6 18.9 Red 3.8 7.8 11.8 

Oak, white Sitka 4.3 7.5 11.5 

Bur 4.4 8.8 12.7 Tamarack 3.7 7.4 13.6 a

Expressed as a percentage of the green dimension. 

b

Coast type Douglas-fir is defined as Douglas-fir growing in the States of Oregon and Washington west of the summit of the Cascade

Mountains. Interior West includes the State of California and all counties in Oregon and Washington east of but adjacent to the Cascade 

summit. Interior North includes the remainder of Oregon and Washington and the States of Idaho, Montana, and Wyoming. 
to ovendry. Wood with cross grain exhibits increased shrinkage along the longitudinal axis of the piece.

Reaction wood exhibiting excessive longitudinal shrinkage 

can occur in the same board with normal wood. The 

presence of this type of wood, as well as cross grain, can 

cause serious warping, such as bow, crook, or twist, and 

cross breaks can develop in the zones of high shrinkage.

Relationship between Moisture Content and Shrinkage

For a sufficiently small piece of wood without moisture 

gradients, shrinkage normally begins at about the fiber saturation point and continues in a fairly linear manner until the 

wood is completely dry. However, in the normal drying of 

lumber or other large pieces, the surface of the wood dries 

first, causing a moisture gradient. When the surface MC 

drops below the fiber saturation point, it begins to shrink 

even though the interior can still be quite wet and not 

shrink. Because of moisture gradients, shrinkage of lumber 

can occur even when the average moisture content of the entire piece of lumber is above fiber saturation. With moisture 

gradients, the moisture content–shrinkage relationship is not 

linear but rather looks similar to the one in Figure 4–4. The 

exact form of the shrinkage curve with moisture gradients 

depends on several variables, principally size and shape of 

the piece, species of wood, and drying conditions used.

Considerable variation in shrinkage occurs for any species. 

Tangential shrinkage data for Douglas-fir boards, 22 by 

140 mm (7/8 by 5-1/2 in.) in cross section, are given in 

Figure 4–5 (Comstock 1965). The material was grown in 

one locality and dried under mild conditions from green to 

near equilibrium at 32 °C (90 °F) and two different 

humidity conditions: (1) 60–65% RH and (2) 30% RH. The 

figure shows that accurately predicting the shrinkage of an 

individual piece of wood is impossible; however, the average shrinkage of a quantity of pieces can be predicted 

accurately.

Average shrinkage data in Tables 4–3 and 4–4 can be used 

to estimate shrinkage for a particular species if a great deal 

of accuracy is not required. The following assumptions 

are made: (1) shrinkage begins at the fiber saturation point 

MCfs, and (2) dimensions decrease linearly with decreasing 

moisture content. The percent shrinkage Sx from the green 

condition to final moisture content x can be calculated from

(4–7)

where S0 is percent shrinkage from the green condition to 

ovendry (radial, tangential, or volumetric) from Table 4–3 

or 4–4. If MCfs is not known, 30% MC can be used as an 

approximation. Tangential values for S0 should be used for 

estimating width shrinkage of plainsawn material and radial 

values for quartersawn material. For mixed or unknown ring 

orientations, tangential values are suggested. Shrinkage values for individual pieces will vary from predicted shrinkage 

values. As noted previously, shrinkage variability is characterized by a coefficient of variation of approximately 15%. 

This applies to pure tangential or radial ring orientation and 

is probably somewhat greater in commercial lumber, where 

ring orientation is seldom aligned perfectly parallel or perpendicular to board faces. Chapter 13 contains additional 

discussion of shrinkage–moisture content relationships, 

including a method to estimate shrinkage for the relatively 

small moisture content changes of wood in service. Shrinkage assumptions for commercial lumber, which typically 

is not perfectly plainsawn or quartersawn, are discussed in 

Chapter 7.

Density and Specific Gravity

The density ρ of a substance is defined as the ratio of its 

mass to its volume and is expressed in the international 

system (SI) in units of kilograms per cubic meter (kg m–3), 

in the inch–pound system (I–P) in units of pounds per cubic foot (lb ft–3), or in the centimeter–gram–second system 

(CGS) in units of grams per cubic centimeter (g cm–3). The 

4–7

Chapter 4 Moisture Relations and Physical Properties of Wood

Figure 4–4. Typical moisture content–shrinkage 

curves.

Figure 4–5. Variation in individual tangential shrinkage 

values of several Douglas-fir boards from one locality, 

dried from green condition.
4–8

Table 4–4. Shrinkage values of some woods imported into the United Statesa

Shrinkageb

 from 

green to ovendry 

moisture content (%) 

Shrinkageb

 from 

green to ovendry 

moisture content (%) 

Species Radial

TangentialVolumetricLocationc Species Radial

TangentialVolumetricLocationc

Afrormosia (Pericopsis elata) 3.0 6.4 10.7 AF Lauan, white (Pentacme contorta) 4.0 7.7 11.7 AS 

Albarco (Cariniana spp.) 2.8 5.4 9.0 AM Limba (Terminalia superba) 4.5 6.2 10.8 AF 

Andiroba (Carapa guianensis) 3.1 7.6 10.4 AM Macawood (Platymiscium spp.) 2.7 3.5 6.5 AM 

Angelin (Andira inermis) 4.6 9.8 12.5 AM Mahogany, African (Khaya spp.) 2.5 4.5 8.8 AF 

Angelique (Dicorynia guianensis) 5.2 8.8 14.0 AM Mahogany, true (Swietenia macrophylla) 3.0 4.1 7.8 AM 

Apitong (Dipterocarpus spp.) 5.2 10.9 16.1 AS Manbarklak (Eschweilera spp.) 5.8 10.3 15.9 AM 

Avodire (Turreanthus africanus) 4.6 6.7 12.0 AF Manni (Symphonia globulifera) 5.7 9.7 15.6 AM 

Azobe (Lophira alata) 8.4 11.0 17.0 AM Marishballi (Licania spp.) 7.5 11.7 17.2 AM 

Balata (Manilkara bidentata) 6.3 9.4 16.9 AM Meranti, white (Shorea spp.) 3.0 6.6 7.7 AS 

Balsa (Ochroma pyramidale) 3.0 7.6 10.8 AM Meranti, yellow (Shorea spp.) 3.4 8.0 10.4 AS 

Banak (Virola spp.) 4.6 8.8 13.7 AM Merbau (Intsia bijuga and I. palembanica) 2.7 4.6 7.8 AS 

Benge (Guibourtia arnoldiana) 5.2 8.6 13.8 AF Mersawa (Anisoptera spp.) 4.0 9.0 14.6 AS 

Bubinga (Guibourtia spp.) 5.8 8.4 14.2 AF Mora (Mora spp.) 6.9 9.8 18.8 AM 

Bulletwood (Manilkara bidentata) 6.3 9.4 16.9 AM Obeche (Triplochiton scleroxylon) 3.0 5.4 9.2 AF 

Caribbean pine (Pinus caribaea) 6.3 7.8 12.9 AM Ocota pine (Pinus oocarpa) 4.6 7.5 12.3 AM 

Cativo (Prioria copaifera) 2.4 5.3 8.9 AM Okoume (Aucoumea klaineana) 4.1 6.1 11.3 AF 

Ceiba (Ceiba pentandra) 2.1 4.1 10.4 AM Opepe (Nauclea spp.) 4.5 8.4 12.6 AF 

Cocobolo (Dalbergia retusa) 2.7 4.3 7.0 AM Ovangkol (Guibourta ehie) 4.5 8.2 12 AF 

Courbaril (Hymenaea courbaril) 4.5 8.5 12.7 AM Para-angelium (Hymenolobium excelsum) 4.4 7.1 10.2 AM 

Cuangare (Dialyanthera spp.) 4.2 9.4 12.0 AM Parana pine (Araucaria angustifolia) 4.0 7.9 11.6 AS 

Degame (Calycophyllum 

 candidissimum)

4.8 8.6 13.2 AM Pau Marfim (Balfourodendron 

 riedelianum)

4.6 8.8 13.4 AM 

Determa (Ocotea rubra) 3.7 7.6 10.4 AM Peroba de campos (Paratecoma peroba) 3.8 6.6 10.5 AM 

Ebony, East Indian (Diospyros spp.) 5.4 8.8 14.2 AS Peroba Rosa (Aspidosperma spp.) 3.8 6.4 11.6 AM 

Ebony, African (Diospyros spp.) 9.2 10.8 20.0 AF Piquia (Caryocar spp.) 5.0 8.0 13.0 AM 

Ekop (Tetraberlinia tubmaniana) 5.6 10.2 15.8 AF Pilon (Hyeronima spp.) 5.4 11.7 17.0 AM 

Gmelina (Gmelina arborea) 2.4 4.9 8.8 AS Primavera (Cybistax donnell-smithii) 3.1 5.1 9.1 AM 

Goncalo alves (Astronium graveolens) 4.0 7.6 10.0 AM Purpleheart (Peltogyne spp.) 3.2 6.1 9.9 AM 

Greenheart (Ocotea rodiaei) 8.8 9.6 17.1 AM Ramin (Gonystylus spp.) 4.3 8.7 13.4 AS 

Hura (Hura crepitans) 2.7 4.5 7.3 AM Roble (Quercus spp.) 6.4 11.7 18.5 AM 

Ilomba (Pycnanthus angolensis) 4.6 8.4 12.8 AF Roble (Tabebuia spp. Roble group) 3.6 6.1 9.5 AM 

Imbuia (Phoebe porosa) 2.7 6.0 9.0 AM Rosewood, Brazilian (Dalbergia nigra) 2.9 4.6 8.5 AM 

Ipe (Tabebuia spp.) 6.6 8.0 13.2 AM Rosewood, Indian (Dalbergia latifolia) 2.7 5.8 8.5 AS 

Iroko (Chlorophora excelsa and C. regia) 2.8 3.8 8.8 AF Rubberwood (Hevea brasiliensis) 2.3 5.1 7.4 AM 

Jarrah (Eucalyptus marginata) 7.7 11.0 18.7 AS Sande (Brosimum spp. Utile group) 4.6 8.0 13.6 AM 

Jelutong (Dyera costulata) 2.3 5.5 7.8 AS Sapele (Entandrophragma cylindricum) 4.6 7.4 14.0 AF 

Kaneelhart (Licaria spp.) 5.4 7.9 12.5 AM Sepetir (Pseudosindora spp. and Sindora spp.) 3.7 7.0 10.5 AS 

Kapur (Dryobalanops spp.) 4.6 10.2 14.8 AS Spanish-cedar (Cedrela spp.) 4.2 6.3 10.3 AM 

Karri (Eucalyptus diversicolor) 7.8 12.4 20.2 AS Sucupira (Diplotropis purpurea) 4.6 7.0 11.8 AM 

Kempas (Koompassia malaccensis) 6.0 7.4 14.5 AS Teak (Tectona grandis) 2.5 5.8 7.0 AS 

Keruing (Dipterocarpus spp.) 5.2 10.9 16.1 AS Wallaba (Eperua spp.) 3.6 6.9 10.0 AM 

Lauan, light red and red (Shorea spp.) 4.6 8.5 14.3 AS 

Lauan, dark red (Shorea spp.) 3.8 7.9 13.1 AS a

Shrinkage values were obtained from world literature and may not represent a true species average. b

Expressed as a percentage of the green dimension. 

c

AF is Africa; AM is Tropical America; AS is Asia and Oceania. 

General Technical Report FPL–GTR–190
CGS system is convenient because of its relationship to 

specific gravity (also known as relative density). Specific 

gravity G is defined as the ratio of the density of a substance 

to the density of water ρw at a specified reference temperature, typically 4 °C (39 °F), where ρw is 1.000 g cm–3

(1,000 kg m–3 or 62.43 lb ft–3). Therefore, a material with a 

density of 5 g cm–3 has a specific gravity of 5.

At constant temperature, the density of materials that do 

not adsorb moisture is constant. For example, at room temperature the densities of steel, aluminum, and lead are 7.8, 

2.7, and 11.3 g cm–3, respectively. For materials that adsorb 

moisture but do not change volume, such as stone and brick, 

the density depends upon moisture content. For these materials, the density can be calculated at any moisture content 

as the ratio of mass to volume, and the relationship between 

density and moisture content is linear. Specific gravity has 

only one definition for these materials (because volume is 

constant): the ratio of ovendry density to density of water.

In contrast to these materials, for wood, both mass and 

volume depend on moisture content. The remainder of this 

section explains the relationships between moisture content, 

volumetric shrinkage, specific gravity, and density.

The density of ovendry wood ρ0 varies significantly between 

species. Although the ovendry density of most species falls 

between about 320 and 720 kg m–3 (20 and 45 lb ft–3), the 

range actually extends from about 160 kg m–3 (10 lb ft–3) for 

balsa to more than 1,040 kg m–3 (65 lb ft–3) for some other 

imported woods. Within a given species, ρ0 varies because 

of anatomical characteristics such as the ratio of earlywood 

to latewood and heartwood to sapwood. For a limited number of species, minerals and extractable substances may also 

affect density. A coefficient of variation of about 10% is 

considered suitable for describing the variability of ovendry 

density within common domestic species.

Wood is used in a wide range of conditions and thus has 

a wide range of moisture content values in service. Determining the density of wood (including water) at a given 

moisture content, ρx, is often necessary for applications such 

as estimating structural loads or shipping weights. Several 

methods can be used for determining ρx, as discussed in the 

following sections. The resulting value should be considered 

an approximation because of the inherent variability in the 

properties used in calculating ρx.

To make comparisons between species or products, a standard reference basis is desirable. Several valid choices are 

possible for wood, including ovendry density ρ0 and specific 

gravity G referenced to a particular volume basis. As shown 

in Table 4–5, the specific gravity of wood may be referenced 

to its volume at any moisture content, but in all cases G is 

based on ovendry mass. Commonly used bases for volume 

are (a) ovendry, (b) green, and (c) 12% moisture content. 

The combination of ovendry mass and ovendry volume is 

used in design specifications for wood, such as contained 

in the National Design Specification for Wood Construction (AF&PA 2005). The combination of ovendry mass and 

green volume is referred to as basic specific gravity Gb. 

Some specific gravity data are reported in Tables 5–3, 5–4, 

and 5–5 (Chap. 5) on both the green (basic) and 12% MC 

volume basis.

Converting between Different Specific Gravity Bases

In general, we use the symbol Gx to denote specific gravity 

based on the volume at a given moisture content x. If the 

value of Gx is known for a particular moisture content, the 

value at any other moisture content can be approximated 

using expressions for volumetric shrinkage. Explicitly, if the 

specific gravity is known at moisture content x', the value at 

x'' is

(4–8)

where Sx is the percent volumetric shrinkage from the green 

condition to moisture content x. In the case where basic specific gravity Gb is known, the value at any moisture content 

x below the fiber saturation point is

(4–9)

The shrinkage–moisture content relationship can be reasonably approximated using Table 4–3 or 4–4 and Equation 

(4–7). However, if the total volumetric shrinkage S0 is not 

known for the species of interest, it can be estimated from 

the basic specific gravity (Stamm 1964):

 

 (4–10)

Using this relation, Equation (4–9) then becomes

(4–11)

4–9

Chapter 4 Moisture Relations and Physical Properties of Wood

Table 4–5. Expressions for specific gravity and 

density of wooda

Symbol Mass basis Volume basis 

G0 Ovendry Ovendry 

Gb (basic specific gravity) Ovendry Green 

G12 Ovendry 12% MC 

Gx Ovendry x% MC 

ρ0 Ovendry Ovendry 

ρ12 12% MC 12% MC 

ρx x% MC x% MC 

a x is any chosen moisture content.

'' ''

' '
Methods for Calculating Density

The density of wood (including water) at a given moisture 

content, ρx, may be determined by any of three methods:

Method 1—Equations Using Basic Specific Gravity

The specific gravity Gx based on volume at the moisture 

content of interest may be calculated from Equation (4–9) 

or (4–11) with basic specific gravity taken from Table 5–3, 

5–4, or 5–5 (Chap. 5). Density is then calculated by

(4–12)

Method 2—Equations Using Ovendry Density

Density is given by

( ) 















-

- = +

x x S

S

x

100

100 1 /100 0 ρ ρ0 (4–13)

where Sx is calculated using Equation (4–7) and S0 is taken 

from Table 4–3 or 4–4. If S0 is not known for the particular 

species of interest, it can be estimated using the same relation as in Equation (4–10), which in terms of ovendry 

density is

(4–14)

Method 3—Using Figure 4–6 and Table 4–6 

Figure 4–6 depicts the relationship between specific gravity Gx and moisture content for different values of basic 

specific gravity. This figure adjusts for average dimensional 

changes that occur below the fiber saturation point (assumed to be 30% MC) and incorporates the assumptions in 

Equations (4–7), (4–10), and (4–11). The specific gravity of 

wood does not change at moisture content values above approximately 30% because the volume does not change. To 

use Figure 4–6, locate the inclined line corresponding to the 

known basic specific gravity (volume when green). From 

this point, move left parallel to the inclined lines until vertically above the target moisture content. Then read the specific gravity Gx corresponding to this point at the left-hand 

side of the graph.

For example, to estimate the density of white ash at 12% 

moisture content, consult Table 5–3a in Chapter 5. The average basic specific gravity Gb for this species is 0.55 (volume 

when green). Using Figure 4–6, the dashed curve for 

Gb = 0.55 is found to intersect with the vertical 12% 

moisture content dashed line at a point corresponding to 

G12 = 0.605. The density of wood (including water) at this 

moisture content can then be obtained from Table 4–6 (these 

values are based on Eq. (4–12)). By interpolation, the specific gravity of 0.605 corresponds to a density at 12% MC 

of 678 kg m–3 (42.2 lb ft–3).

Thermal Properties

Four important thermal properties of wood are thermal conductivity, heat capacity, thermal diffusivity, and coefficient 

of thermal expansion.

Thermal Conductivity

Thermal conductivity k is a measure of the rate of heat flow 

(W m–2 or Btu h–1 ft–2) through a material subjected to unit 

temperature difference (K or °F) across unit thickness 

(m or in.). The thermal conductivity of common structural 

woods is much less than the conductivity of metals with 

which wood often is mated in construction. It is about two 

to four times that of common insulating materials. For example, the conductivity of structural softwood lumber 

at 12% moisture content is in the range of 0.10 to 

0.14 W m–1 K–1 (0.7 to 1.0 Btu in. h–1 ft–2 °F–1) compared 

with 216 (1,500) for aluminum, 45 (310) for steel, 0.9 (6) 

for concrete, 1 (7) for glass, 0.7 (5) for plaster, and 0.036 

(0.25) for mineral wool. Thermal resistivity is simply the 

reciprocal of the thermal conductivity. Insulating materials 

of a given thickness are commonly compared by their “Rvalue,” or thermal resistance, which is simply the thermal 

resistivity times the thickness.

The thermal conductivity of wood is affected by a number 

of basic factors: density, moisture content, extractive content, grain direction, structural irregularities such as checks 

4–10

0.18

0.22

0.26

0.30

0.34

0.38

0.42

0.46

0.50

0.54

0.58

0.62

0.66

0.70

0.74

0.78

0.82

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0.20

0.24

0.28

0.32

0.36

0.40

0.44

0.48

0.52

0.56

0.60

0.64

(volume at current moisture content) Specific gravity Gx

Moisture content (%)

Basic specific gravity Gb

(volume when green)

Figure 4–6. Relationship of specific gravity and 

moisture content.

General Technical Report FPL–GTR–190
and knots, fibril angle, and temperature. Thermal conductivity increases as density, moisture content, temperature, 

or extractive content of the wood increases. Thermal conductivity is nearly the same in the radial and tangential 

directions. However, conductivity along the grain has been 

reported as greater than conductivity across the grain by a 

factor of 1.5 to 2.8, with an average of about 1.8.

For moisture contents below 25%, approximate thermal 

conductivity k across the grain can be calculated with a linear equation of the form

 k = Gx(B + Cx) + A (4–15)

where Gx is specific gravity based on ovendry mass and volume at moisture content x (%) and A, B, and C are constants. 

For Gx > 0.3, temperatures around 24 °C (75 °F), and 

x < 25% MC, the values of the constants are as follows:

A = 0.01864, B = 0.1941, C = 0.004064 (k in W m–1 K–1)

A = 0.129, B = 1.34, C = 0.028 (k in Btu in. h–1 ft–2 °F–1)

Equation (4–15) was derived from measurements made by 

several researchers on a variety of species. Table 4–7 provides average approximate conductivity values for selected 

wood species, based on Equation (4–15). However, actual 

conductivity may vary as much as 20% from the tabulated 

values.

Although thermal conductivity measurements have been 

made at moisture content values above 25%, measurements 

have been few in number and generally lacking in accuracy. 

Therefore, we do not provide values for moisture content 

values above 25%.

The effect of temperature on thermal conductivity is relatively minor: conductivity increases about 2% to 3% per 

10 °C (1% to 2% per 10 °F).

Heat Capacity

Heat capacity is defined as the amount of energy needed to 

increase one unit of mass (kg or lb) one unit in temperature 

(K or °F). The heat capacity of wood depends on the temperature and moisture content of the wood but is practically 

independent of density or species. Heat capacity of dry 

wood cp0 (kJ kg–1 K–1, Btu lb–1 °F–1) is approximately related to temperature T (K, °F) by

 cp0 = 0.1031 + 0.003867T (SI) (4–16a)

Chapter 4 Moisture Relations and Physical Properties of Wood

4–11

Table 4–6a. Density of wood as a function of specific gravity and moisture content (SI)

Moisture

content 

of wood 

(%) 

Density (kg m–3) when the specific gravity Gx is 

0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66 0.68 0.70

0 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700

4 312 333 354 374 395 416 437 458 478 499 520 541 562 582 603 624 645 666 686 707 728

8 324 346 367 389 410 432 454 475 497 518 540 562 583 605 626 648 670 691 713 734 756

12 336 358 381 403 426 448 470 493 515 538 560 582 605 627 650 672 694 717 739 762 784

16 348 371 394 418 441 464 487 510 534 557 580 603 626 650 673 696 719 742 766 789 812

20 360 384 408 432 456 480 504 528 552 576 600 624 648 672 696 720 744 768 792 816 840

24 372 397 422 446 471 496 521 546 570 595 620 645 670 694 719 744 769 794 818 843 868

28 384 410 435 461 486 512 538 563 589 614 640 666 691 717 742 768 794 819 845 870 896

32 396 422 449 475 502 528 554 581 607 634 660 686 713 739 766 792 818 845 871 898 924

36 408 435 462 490 517 544 571 598 626 653 680 707 734 762 789 816 843 870 898 925 952

40 420 448 476 504 532 560 588 616 644 672 700 728 756 784 812 840 868 896 924 952 980

44 432 461 490 518 547 576 605 634 662 691 720 749 778 806 835 864 893 922 950 979 1,008

48 444 474 503 533 562 592 622 651 681 710 740 770 799 829 858 888 918 947 977 1,006 1,036

52 456 486 517 547 578 608 638 669 699 730 760 790 821 851 882 912 942 973 1,003 1,034 1,064

56 468 499 530 562 593 624 655 686 718 749 780 811 842 874 905 936 967 998 1,030 1,061 1,092

60 480 512 544 576 608 640 672 704 736 768 800 832 864 896 928 960 992 1,024 1,056 1,088 1,120

64 492 525 558 590 623 656 689 722 754 787 820 853 886 918 951 984 1,017 1,050 1,082 1,115 1,148

68 504 538 571 605 638 672 706 739 773 806 840 874 907 941 974 1,008 1,042 1,075 1,109 1,142 1,176

72 516 550 585 619 654 688 722 757 791 826 860 894 929 963 998 1,032 1,066 1,101 1,135 1,170 1,204

76 528 563 598 634 669 704 739 774 810 845 880 915 950 986 1,021 1,056 1,091 1,126 1,162 1,197

80 540 576 612 648 684 720 756 792 828 864 900 936 972 1,008 1,044 1,080 1,116 1,152 1,188

84 552 589 626 662 699 736 773 810 846 883 920 957 994 1,030 1,067 1,104 1,141 1,178

88 564 602 639 677 714 752 790 827 865 902 940 978 1,015 1,053 1,090 1,128 1,166 

92 576 614 653 691 730 768 806 845 883 922 960 998 1,037 1,075 1,114 1,152 1,190 

96 588 627 666 706 745 784 823 862 902 941 980 1,019 1,058 1,098 1,137 1,176 

100 600 640 680 720 760 800 840 880 920 960 l,000 1,040 1,080 1,120 1,160 1,200 

110 630 672 714 756 798 840 832 924 966 1,008 1,050 1,092 1,134 1,176 1,218 

120 660 704 748 792 836 880 924 968 1,012 1,056 1,100 1,144 1,188 1,232

130 690 736 782 828 874 920 966 1,012 1,058 1,104 1,150 1,196 1,242 1,288 

140 720 768 816 864 912 960 1,008 1,056 1,104 1,152 1,200 1,248 1,296 

150 750 800 850 900 950 1,000 1,050 1,100 1,150 1,200 1,250 1,300 1,350 
 cp0 = 0.2605 + 0.0005132T (I–P) (4–16b)

The heat capacity of wood that contains water is greater 

than that of dry wood. Below fiber saturation, it is the sum 

of the heat capacity of the dry wood and that of water (cpw) 

and an additional adjustment factor Ac that accounts for the 

additional energy in the wood–water bond:

 cp,x = (cp0 + cpw x/100)/(1 + x/100) + Ac (4–17)

where x is moisture content (%). The heat capacity of water 

is about 4.18 kJ kg–1 K–1 (1.00 Btu lb–1 °F–1). The adjustment factor can be calculated from

 Ac = x(b1 + b2T + b3x) (4–18)

with

b1 = -0.06191, b2 = 2.36 × 10-4, b3 = -1.33 × 10-4 (T in K)

b1 = -4.23 × 10-4, b2 = 3.12 × 10-5, b3 = -3.17 × 10-5 (T in °F)

These formulas are valid for wood below fiber saturation 

at temperatures between 280 K (44 °F) and 420 K 

(296 °F). Representative values for heat capacity can be 

found in Table 4–8. The moisture content above fiber saturation contributes to heat capacity according to the simple rule 

of mixtures.

Thermal Diffusivity

Thermal diffusivity is a measure of how quickly a material 

can absorb heat from its surroundings. It is defined as the 

ratio of thermal conductivity to the product of density and 

heat capacity. Therefore, conclusions regarding its variation 

with temperature and density are often based on calculating 

the effect of these variables on heat capacity and thermal 

conductivity. Because of the low thermal conductivity and 

moderate density and heat capacity of wood, the thermal diffusivity of wood is much lower than that of other structural 

materials, such as metal, brick, and stone. A typical 

value for wood is 1.6 × 10–7 m2 s–1 (0.00025 in2 s–1), 

compared with 1 × 10-5 m2 s–1 (0.02 in2 s–1) for steel and 

1× 10-6 m2 s–1 (0.002 in2 s–1) for stone and mineral wool. 

For this reason, wood does not feel extremely hot or cold to 

the touch as do some other materials.

4–12

Table 4–6b. Density of wood as a function of specific gravity and moisture content (I–P)

Moisture

content 

of wood 

(%)

Density (lb ft–3) when the specific gravity Gx is 

0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66 0.68 0.70

0 18.7 20.0 21.2 22.5 23.7 25.0 26.2 27.5 28.7 30.0 31.2 32.4 33.7 34.9 36.2 37.4 38.7 39.9 41.2 42.4 43.7

4 19.5 20.8 22.1 23.4 24.7 26.0 27.2 28.6 29.8 31.2 32.4 33.7 35.0 36.6 37.6 38.9 40.2 41.5 42.8 44.1 45.4

8 20.2 21.6 22.9 24.3 25.6 27.0 28.3 29.6 31.0 32.3 33.7 35.0 36.4 37.7 39.1 40.4 41.8 43.1 44.5 45.8 47.2

12 21.0 22.4 23.8 25.2 26.6 28.0 29.4 30.8 32.2 33.5 34.9 36.3 37.7 39.1 40.5 41.9 43.3 44.7 46.1 47.5 48.9

16 21.7 23.2 24.6 26.0 27.5 29.0 30.4 31.8 33.3 34.7 36.2 37.6 39.1 40.5 42.0 43.4 44.9 46.3 47.8 49.2 50.7

20 22.5 24.0 25.5 27.0 28.4 30.0 31.4 32.9 34.4 35.9 37.4 38.9 40.4 41.9 43.4 44.9 46.4 47.9 49.4 50.9 52.4

24 23.2 24.8 26.3 27.8 29.4 31.0 32.5 34.0 35.6 37.1 38.7 40.2 41.8 43.3 44.9 46.4 48.0 49.5 51.1 52.6 54.2

28 24.0 25.6 27.2 28.8 30.4 31.9 33.5 35.1 36.7 38.3 39.9 41.5 43.1 44.7 46.3 47.9 49.5 51.1 52.7 54.3 55.9

32 24.7 26.4 28.0 29.7 31.3 32.9 34.6 36.2 37.9 39.5 41.2 42.8 44.5 46.1 47.8 49.4 51.1 52.7 54.4 56.0 57.7

36 25.5 27.2 28.9 30.6 32.2 33.9 35.6 37.3 39.0 40.7 42.4 44.1 45.8 47.5 49.2 50.9 52.6 54.3 56.0 57.7 59.4

40 26.2 28.0 29.7 31.4 33.2 34.9 36.7 38.4 40.2 41.9 43.7 45.4 47.2 48.9 50.7 52.4 54.2 55.9 57.7 59.4 61.2

44 27.0 28.8 30.6 32.3 34.1 35.9 37.7 39.5 41.3 43.1 44.9 46.7 48.5 50.3 52.1 53.9 55.7 57.5 59.3 61.1 62.9

48 27.7 29.6 31.4 33.2 35.1 36.9 38.8 40.6 42.5 44.3 46.2 48.0 49.9 51.7 53.6 55.4 57.3 59.1 61.0 62.8 64.6

52 28.5 30.4 32.2 34.1 36.0 37.9 39.8 41.7 43.6 45.5 47.4 49.3 51.2 53.1 55.0 56.9 58.8 60.7 62.6 64.5 66.4

56 29.2 31.2 33.1 35.0 37.0 38.9 40.9 42.8 44.8 46.7 48.7 50.6 52.6 54.5 56.5 58.4 60.4 62.3 64.2 66.2 68.1

60 30.0 31.9 33.9 35.9 37.9 39.9 41.9 43.9 45.9 47.9 49.9 51.9 53.9 55.9 57.9 59.9 61.9 63.9 65.9 67.9 69.9

64 30.7 32.7 34.8 36.8 38.9 40.9 43.0 45.0 47.1 49.1 51.2 53.2 55.3 57.3 59.4 61.4 63.4 65.5 67.5 69.6 71.6

68 31.4 33.5 35.6 37.7 39.8 41.9 44.0 46.1 48.2 50.3 52.4 54.5 56.6 58.7 60.8 62.9 65.0 67.1 69.2 71.3 73.4

72 32.2 34.3 36.5 38.6 40.8 42.9 45.1 47.2 49.4 51.5 53.7 55.8 58.0 60.1 62.3 64.4 66.5 68.7 70.8 73.0 75.1

76 32.9 35.1 37.3 39.5 41.7 43.9 46.1 48.3 50.5 52.7 54.9 57.1 59.3 61.5 63.7 65.9 68.1 70.3 72.5 

80 33.7 35.9 38.2 40.4 42.7 44.9 47.2 49.4 51.7 53.9 56.2 58.4 60.7 62.9 65.1 67.4 69.6 71.9 74.1 

84 34.4 36.7 39.0 41.3 43.6 45.9 48.2 50.5 52.8 55.1 57.4 59.7 62.0 64.3 66.6 68.9 71.2 73.5 

88 35.2 37.5 39.9 42.2 44.6 46.9 49.3 51.6 54.0 56.3 58.7 61.0 63.3 65.7 68.0 70.4 72.7 

92 35.9 38.3 40.7 43.1 45.5 47.9 50.3 52.7 55.1 57.5 59.9 62.3 64.7 67.1 69.5 71.9 74.3 

96 36.7 39.1 41.6 44.0 46.5 48.9 51.4 53.8 56.3 58.7 61.2 63.6 66.0 68.5 70.9 73.4 

100 37.4 39.9 42.4 44.9 47.4 49.9 52.4 54.9 57.4 59.9 62.4 64.9 67.4 69.9 72.4 74.9 

110 39.3 41.9 44.6 47.2 49.8 52.4 55.0 57.7 60.3 62.9 65.5 68.1 70.8 73.4 76.0 

120 41.2 43.9 46.7 49.4 52.2 54.9 57.7 60.4 63.1 65.9 68.6 71.4 74.1 76.9 

130 43.1 45.9 48.8 51.7 54.5 57.4 60.3 63.1 66.0 68.9 71.8 74.6 77.5 80.4 

140 44.9 47.9 50.9 53.9 56.9 59.9 62.9 65.9 68.9 71.9 74.9 77.9 80.9 

150 46.8 49.9 53.0 56.2 59.3 62.4 65.5 68.6 71.8 74.9 78.0 81.1 84.2 

General Technical Report FPL–GTR–190
4–13

Chapter 4 Moisture Relations and Physical Properties of Wood

Table 4–7. Thermal conductivity of selected hardwoods and softwoodsa

Specific 

gravity

Conductivity 

(W m–1 K–1 (Btu in. h–1 ft–2 °F–1))

Resistivity 

(K m W–1 (h ft2 °F Btu–1 in. –1)) 

Species Ovendry 12% MC Ovendry 12% MC 

Hardwoods 

Ash 

Black 0.53 0.12 (0.84) 0.15 (1.0) 8.2 (1.2) 6.8 (0.98) 

White 0.63 0.14 (0.98) 0.17 (1.2) 7.1 (1.0) 5.8 (0.84) 

Aspen 

Big tooth 0.41 0.10 (0.68) 0.12 (0.82) 10 (1.5) 8.5 (1.2) 

Quaking 0.40 0.10 (0.67) 0.12 (0.80) 10 (1.5) 8.6 (1.2) 

Basswood, American 0.38 0.092 (0.64) 0.11 (0.77) 11 (1.6) 9.0 (1.3) 

Beech, American 0.68 0.15 (1.0) 0.18 (1.3) 6.6 (0.96) 5.4 (0.78) 

Birch 

Sweet 0.71 0.16 (1.1) 0.19 (1.3) 6.4 (0.92) 5.2 (0.76) 

Yellow 0.66 0.15 (1.0) 0.18 (1.2) 6.8 (0.98) 5.6 (0.81) 

Cherry, black 0.53 0.12 (0.84) 0.15 (1.0) 8.2 (1.2) 6.8 (0.98) 

Chestnut, American 0.45 0.11 (0.73) 0.13 (0.89) 9.4 (1.4) 7.8 (1.1) 

Cottonwood 

Black 0.35 0.087 (0.60) 0.10 (0.72) 12 (1.7) 9.6 (1.4) 

Eastern 0.43 0.10 (0.71) 0.12 (0.85) 9.8 (1.4) 8.1 (1.2) 

Elm 

American 0.54 0.12 (0.86) 0.15 (1.0) 8.1 (1.2) 6.7 (0.96) 

Rock 0.67 0.15 (1.0) 0.18 (1.3) 6.7 (0.97) 5.5 (0.80) 

Slippery 0.56 0.13 (0.88) 0.15 (1.1) 7.9 (1.1) 6.5 (0.93) 

Hackberry 0.57 0.13 (0.90) 0.16 (1.1) 7.7 (1.1) 6.4 (0.92) 

Hickory, pecan 0.69 0.15 (1.1) 0.19 (1.3) 6.6 (0.95) 5.4 (0.77) 

Hickory, true 

Mockernut 0.78 0.17 (1.2) 0.21 (1.4) 5.9 (0.85) 4.8 (0.69) 

Shagbark 0.77 0.17 (1.2) 0.21 (1.4) 5.9 (0.86) 4.9 (0.70) 

Magnolia, southern 0.52 0.12 (0.83) 0.14 (1.0) 8.4 (1.2) 6.9 (1.0) 

Maple 

Black 0.60 0.14 (0.94) 0.16 (1.1) 7.4 (1.1) 6.1 (0.88) 

Red 0.56 0.13 (0.88) 0.15 (1.1) 7.9 (1.1) 6.5 (0.93) 

Silver 0.50 0.12 (0.80) 0.14 (0.97) 8.6 (1.2) 7.1 (1.0) 

Sugar 0.66 0.15 (1.0) 0.18 (1.2) 6.8 (0.98) 5.6 (0.81) 

Oak, red 

Black 0.66 0.15 (1.0) 0.18 (1.2) 6.8 (0.98) 5.6 (0.81) 

Northern red 0.65 0.14 (1.0) 0.18 (1.2) 6.9 (1.0) 5.7 (0.82) 

Southern red 0.62 0.14 (0.96) 0.17 (1.2) 7.2 (1.0) 5.9 (0.85) 

Oak, white 

Bur 0.66 0.15 (1.0) 0.18 (1.2) 6.8 (0.98) 5.6 (0.81) 

White 0.72 0.16 (1.1) 0.19 (1.3) 6.3 (0.91) 5.2 (0.75) 

Sweetgum 0.55 0.13 (0.87 0.15 (1.1) 8.0 (1.2) 6.6 (0.95) 

Sycamore, American 0.54 0.12 (0.86) 0.15 (1.0) 8.1 (1.2) 6.7 (0.96) 

Tupelo 

Black 0.54 0.12 (0.86) 0.15 (1.0) 8.1 (1.2) 6.7 (0.96) 

Water 0.53 0.12 (0.84) 0.15 (1.0) 8.2 (1.2) 6.8 (0.98) 

Yellow-poplar 0.46 0.11 (0.75) 0.13 (0.90) 9.3 (1.3) 7.7 (1.1) 
4–14

Table 4–7. Thermal conductivity of selected hardwoods and softwoodsa—con. 

Specific 

gravity

Conductivity 

(W m–1 K–1 (Btu in. h–1 ft–2 °F–1))

Resistivity 

(K m W–1 (h ft2 °F Btu–1 in. –1)) 

Species Ovendry 12% MC Ovendry 12% MC 

Softwoods 

Baldcypress 0.47 0.11 (0.76) 0.13 (0.92) 9.1 (1.3) 7.5 (1.1) 

Cedar 

Atlantic white 0.34 0.085 (0.59) 0.10 (0.70) 12 (1.7) 9.9 (1.4) 

Eastern red 0.48 0.11 (0.77) 0.14 (0.94) 8.9 (1.3) 7.4 (1.1) 

Northern white 0.31 0.079 (0.55) 0.094 (0.65) 13 (1.8) 11 (1.5) 

Port-Orford 0.43 0.10 (0.71) 0.12 (0.85) 9.8 (1.4) 8.1 (1.2) 

Western red 0.33 0.083 (0.57) 0.10 (0.68) 12 (1.7) 10 (1.5) 

Yellow 0.46 0.11 (0.75) 0.13 (0.90) 9.3 (1.3) 7.7 (1.1) 

Douglas-fir 

Coast 0.51 0.12 (0.82) 0.14 (0.99) 8.5 (1.2) 7.0 (1.0) 

Interior north 0.50 0.12 (0.80) 0.14 (0.97) 8.6 (1.2) 7.1 (1.0) 

Interior west 0.52 0.12 (0.83) 0.14 (1.0) 8.4 (1.2) 6.9 1.0) 

Fir 

Balsam 0.37 0.090 (0.63) 0.11 (0.75) 11 (1.6) 9.2 (1.3) 

White 0.41 0.10 (0.68) 0.12 (0.82) 10 (1.5) 8.5 (1.2) 

Hemlock 

Eastern 0.42 0.10 (0.69) 0.12 (0.84) 10 (1.4) 8.3 (1.2) 

Western 0.48 0.11 (0.77) 0.14 (0.94) 8.9 (1.3) 7.4 (1.1) 

Larch, western 0.56 0.13 (0.88) 0.15 (1.1) 7.9 (1.1) 6.5 (0.93) 

Pine 

Eastern white 0.37 0.090 (0.63) 0.11 (0.75) 11 (1.6) 9.2 (1.3) 

Jack 0.45 0.11 (0.73) 0.13 (0.89) 9.4 (1.4) 7.8 (1.1) 

Loblolly 0.54 0.12 (0.86) 0.15 (1.0) 8.1 (1.2) 6.7 (0.96) 

Lodgepole 0.43 0.10 (0.71) 0.12 (0.85) 9.8 (1.4) 8.1 (1.2) 

Longleaf 0.62 0.14 (0.96) 0.17 (1.2) 7.2 (1.0) 5.9 (0.85) 

Pitch 0.53 0.12 (0.84) 0.15 (1.0) 8.2 (1.2) 6.8 (0.98) 

Ponderosa 0.42 0.10 (0.69) 0.12 (0.84) 10 (1.4) 8.3 (1.2) 

Red 0.46 0.11 (0.75) 0.13 (0.90) 9.3 (1.3) 7.7 (1.1) 

Shortleaf 0.54 0.12 (0.86) 0.15 (1.0) 8.1 (1.2) 6.7 (0.96) 

Slash 0.61 0.14 (0.95) 0.17 (1.2) 7.3 (1.1) 6.0 (0.86) 

Sugar 0.37 0.090 (0.63) 0.11 (0.75) 11 (1.6) 9.2 (1.3) 

Western white 0.40 0.10 (0.67) 0.12 (0.80) 10 (1.5) 8.6 (1.2) 

Redwood 

Old growth 0.41 0.10 (0.68) 0.12 (0.82) 10 (1.5) 8.5 (1.2) 

Young growth 0.37 0.090 (0.63) 0.11 (0.75) 11 (1.6) 9.2 (1.3) 

Spruce 

Black 0.43 0.10 (0.71) 0.12 (0.85) 9.8 (1.4) 8.1 (1.2) 

Engelmann 0.37 0.090 (0.63) 0.11 (0.75) 11 (1.6) 9.2 (1.3) 

Red 0.42 0.10 (0.69) 0.12 (0.84) 10 (1.4) 8.3 (1.2) 

Sitka 0.42 0.10 (0.69) 0.12 (0.84) 10 (1.4) 8.3 (1.2) 

White 0.37 0.090 (0.63) 0.11 (0.75) 11 (1.6) 9.2 (1.3) a

Values in this table are approximate and should be used with caution; actual conductivities may vary by as 

much as 20%. The specific gravities also do not represent species averages. 

Coefficient of Thermal Expansion

The coefficient of thermal expansion is a measure of the 

relative change of dimension caused by temperature change. 

The thermal expansion coefficients of completely dry wood 

are positive in all directions; that is, wood expands on heating and contracts on cooling. Limited research has been carried out to explore the influence of wood property variability 

on thermal expansion. The thermal expansion coefficient of 

ovendry wood parallel to the grain appears to be independent of specific gravity and species. In tests of both 

hardwoods and softwoods, the parallel-to-grain values 

have ranged from about 3.1 to 4.5 × 10-6 K–1

(1.7 to 2.5 × 10-6 °F–1).

Thermal expansion coefficients across the grain (radial and 

tangential) are proportional to specific gravity. These coefficients range from about 5 to more than 10 times greater than 

the parallel-to-grain coefficients and are of more practical 

interest. The radial and tangential thermal expansion coefficients for ovendry wood, ar

 and at, can be approximated 

General Technical Report FPL–GTR–190
by the following equations, over an ovendry specific gravity 

range of about 0.1 to 0.8:

 ar = (32.4G0 + 9.9)10-6 K–1 (4–19a)

 ar = (18G0 + 5.5)10-6 °F–1 (4–19b)

 at = (32.4G0 + 18.4)10-6 K–1 (4–20a)

 at = (18G0 + 10.2)10-6 °F–1 (4–20b)

Thermal expansion coefficients can be considered independent of temperature over the temperature range of -51 to 

54 °C (-60 to 130 °F).

Wood that contains moisture reacts differently to varying 

temperature than does nearly ovendry wood. When moist 

wood is heated, it tends to expand because of normal thermal expansion and to shrink because of loss in moisture 

content. Unless the wood is very dry initially (perhaps 3% 

or 4% moisture content or less), shrinkage caused by moisture loss on heating will be greater than thermal expansion, 

so the net dimensional change on heating will be negative. 

Wood at intermediate moisture levels (about 8% to 20%) 

will expand when first heated, and then gradually shrink to 

a volume smaller than the initial volume as the wood gradually loses water while in the heated condition.

Even in the longitudinal (grain) direction, where dimensional change caused by moisture change is very small, such 

changes will still predominate over corresponding dimensional changes as a result of thermal expansion unless the 

wood is very dry initially. For wood at usual moisture levels, net dimensional changes will generally be negative after 

prolonged heating.

Electrical Properties

The electrical properties of wood depend strongly on moisture content, exhibiting changes that span almost 10 orders 

of magnitude over the range of possible moisture contents. 

Because electrical properties of wood undergo large changes 

with relatively small changes in moisture content below 

fiber saturation, electrical measurements have been used to 

accurately predict the moisture content of wood.

4–15

Chapter 4 Moisture Relations and Physical Properties of Wood

The literature on electrical properties of wood has been divided into measurements of either dielectric constant or resistivity. In general, dielectric constant data were measured 

with alternating current (AC), whereas resistivity measurements used direct current (DC). In a way, this is a false 

dichotomy because the dielectric constant can be measured 

using DC signals for some materials, and the complex resistivity, which is related to impedance, can be measured from 

AC signals. Furthermore, given the AC dielectric constant, 

one can calculate the AC resistivity. The remainder of this 

section will review AC and DC measurements of the electrical properties of wood, with emphasis on clarifying the 

nomenclature that is often used in the wood literature.

DC Electrical Properties 

Resistivity

When an electric potential or voltage V is applied between 

two points on a conducting solid, the amount of current I

that will flow between those points depends on the resistance R of the material. This measured resistance depends 

on the geometry of the specimen:

 A

L R = ρ (4–21)

where L is the distance the current travels, A is the crosssectional area through which the current travels, and ρ is 

a materials parameter, the resistivity with units of Ω m. In 

some situations, it is more convenient to talk about the conductivity σ, which is the reciprocal of the resistivity 

(σ ≡ 1 ρ ).

The resistivity of wood is a strong function of moisture content. For example, Figure 4–7 illustrates this dependence for 

slash pine (Pinus elliottii) in the longitudinal direction between 8% MC and 180% MC (Stamm 1929, 1964). As the 

moisture content of wood increases from near zero to fiber 

saturation, resistivity can decrease by a factor of over 1010

(in comparison, the circumference of the earth at the equator is 4 × 1010 mm). Resistivity is about 1015–1016 Ω m for 

ovendry wood and 103–104 Ω m for wood at fiber saturation 

(Stamm 1964). As the moisture content increases from fiber 

saturation to complete saturation of the wood structure, the 

further decrease in resistivity is smaller, generally amounting to less than a hundredfold.

The conductivity of wood also depends on temperature, 

grain angle, and the amount of water-soluble salts. Unlike 

conductivity of metals, the conductivity of wood increases 

with increasing temperature. Conductivity is greater along 

the grain than across the grain and slightly greater in the 

radial direction than in the tangential direction. Relative 

conductivity values in the longitudinal, radial, and tangential directions are related by the approximate ratio of 

1.0:0.55:0.50. When wood contains abnormal quantities of 

water-soluble salts or other electrolytic substances, such as 

preservative or fire-retardant treatment, or is in prolonged 

Table 4–8. Heat capacity of solid wood at selected 

temperatures and moisture contents 

Temperature Heat capacity (kJ kg–1 K–1 (Btu lb–1 °F–1))

(K) (°C (°F)) Ovendry 5% MC 12% MC 20% MC 

280 7 (44) 1.2 (0.28) 1.3 (0.32) 1.5 (0.37) 1.7 (0.41)

290 17 (62) 1.2 (0.29) 1.4 (0.33) 1.6 (0.38) 1.8 (0.43)

300 27 (80) 1.3 (0.30) 1.4 (0.34) 1.7 (0.40) 1.9 (0.45)

320 47 (116) 1.3 (0.32) 1.5 (0.37) 1.8 (0.43) 2.0 (0.49)

340 67 (152) 1.4 (0.34) 1.6 (0.39) 1.9 (0.46) 2.2 (0.52)

360 87 (188) 1.5 (0.36) 1.7 (0.41) 2.0 (0.49) 2.3 (0.56)
4–16

contact with seawater, electrical conductivity can be substantially increased.

DC Dielectric Constant

When an electric potential or voltage V is applied to a perfectly insulating material (σ ≡ 0 ) between two parallel 

plates, no current will flow and instead charge will build up 

on the plates. The amount of charge per unit voltage that 

these plates can store is called the capacitance C and 

is given by

 (4–22)

where A and L have the same meanings as in 

Equation (4–21), ε is a unitless materials parameter, 

the DC dielectric constant, and ε0 is a universal constant, the 

permittivity of a vacuum, and is 8.854 × 10–12 F m–1. The 

DC dielectric constant is the ratio of the dielectric permittivity of the material to ε0; it is essentially a measure of the 

potential energy per unit volume stored in the material in the 

form of electric polarization when the material is in a given 

electric field. As measured by practical tests, the dielectric 

constant of a material is the ratio of the capacitance of a capacitor using the material as the dielectric to the capacitance 

of the same capacitor using free space as the dielectric.

Because wood is not a perfect insulator (σ ≠ 0 at any moisture content), the DC dielectric constant of wood is not well 

defined and theoretically cannot be measured with DC techniques. Nevertheless, researchers have tried to measure this 

quantity and have found that it is difficult to measure and 

depends on experimental technique (Skaar 1988).

AC Electrical Properties

AC Dielectric Constant and Related Properties

When an alternating current is applied, the dielectric 

constant can no longer be represented by a scalar, because 

response will be out of phase with the original signal. The 

AC dielectric constant is a complex number ε = ε '+ jε"

with real component ε ' , imaginary component ε" , and

j ≡ -1 Instead of presenting the real and imaginary components of the dielectric constant, it is customary in the 

wood literature to present the real component of the dielectric constant ε ' and the loss tangent, tan(δ ) , defined by

 '

" tan( )

ε

ε δ = (4–23)

It is also customary in the wood literature to refer to the real 

component of the dielectric constant ε ' as simply “the dielectric constant” and to represent this with ε . This notation 

should not be encouraged, because it is ambiguous and also 

implies that the dielectric constant is not a complex number.

Both ε ' and tan(δ ) depend non-linearly on the frequency at 

which they are measured. The frequency dependence is related to the mechanism of conduction in wood, and this relationship between the frequency dependence and mechanism 

has been explored in the literature (James 1975, Zelinka and 

others 2007).

At a given frequency, ε ' increases with temperature and 

moisture content. At 20 Hz, ε ' may range from about 4 

for dry wood to near 1 × 106 for wet wood; at 1 kHz, from 

about 4 when dry to about 5,000 when wet; and at 1 MHz, 

from about 3 when dry to about 100 when wet. ε ' is larger 

for polarization parallel to the grain than across the grain.

Another parameter, the dielectric power factor pf given by

( ) ( ) 2 2 p

' "

" sin( )

ε ε

ε δ

+

f = = (4–24)

is used in dielectric moisture meters (James 1988). The 

power factor of wood is large compared with that of inert 

plastic insulating materials, but some materials, for example 

some formulations of rubber, have equally large power factors. The power factor of wood varies from about 0.01 for 

dry, low-density woods to as large as 0.95 for dense woods 

at high moisture levels. The power factor is usually, but not 

always, greater for electric fields along the grain than across 

the grain.

Because the power factor of wood is derived from ε ' and ε"

it is also affected by frequency, moisture content, and temperature. These factors interact in such a way to cause pf

to have maximum and minimum values at various 

combinations of these factors.

Impedance

Just as the AC dielectric constant was represented by a 

complex number to account for both magnitude and phase, 

the “resistance” of an AC circuit is also represented by a 

complex number called impedance, Z = Z'+ jZ " with real 

Figure 4–7. Resistivity of slash pine (Pinus elliottii) 

as a function of moisture content.

General Technical Report FPL–GTR–190

,

jZ

.
4–17

Chapter 4 Moisture Relations and Physical Properties of Wood

component Z' and imaginary component Z" . Impedance is 

related to the AC dielectric constant through

(4–25)

where ω is the angular frequency and Cc is a geometrical 

factor needed for unit analysis and represents the capacitance of an empty cell (that is, Cc = ε oA L ) (MacDonald 

and Johnson 1987). In short, this transforms the real component of the dielectric constant to the imaginary component 

of the impedance, and vice versa.

Recently, measurements of the impedance of wood have 

been used to determine moisture gradients (Tiitta and Olkkonen 2002), better understand the mechanism of electrical 

conduction in wood (Zelinka and others 2007), and quantify 

the corrosion of metals embedded in wood (Zelinka and 

Rammer 2005).

Friction Properties

Figure 4–8 depicts the forces acting on an object. The 

weight of the object FW (the gravitational force acting 

downward) is opposed by the normal force FN exerted by 

the surface supporting it. The applied horizontal force F

is opposed by the friction force Ff parallel to the surface. 

In the case in which the object is not moving but is on the 

verge of sliding across the surface, the coefficient of static 

friction μs is defined as

 

N

f s

(max)

F

F

μ = (4–26)

where Ff(max) is the magnitude of the maximum friction 

force and FN is the magnitude of the normal force. In the 

case in which the object is sliding across the surface at constant speed, the coefficient of kinetic friction μk is defined 

as

 

N

f

k F

F

μ = (4–27)

These coefficients depend on the moisture content of the 

wood, the roughness of the wood surface, and the characteristics of the opposing surface. They vary little with species 

except for woods that contain abundant oily or waxy extractives, such as lignumvitae (see Chap. 2). The coefficients of 

friction are an important safety consideration in applications 

such as wood decks, stairs, and sloped surfaces such as roof 

sheathing.

On most materials, the coefficients of friction for wood increase continuously as the moisture content of the wood increases from ovendry to fiber saturation, then remain about 

constant as the moisture content increases further until considerable free water is present. When the surface is flooded 

with water, the coefficients of friction decrease.

Coefficients of static friction are generally greater than those 

of kinetic friction, and the latter depend somewhat on the 

speed of sliding. Coefficients of kinetic friction vary only 

slightly with speed when the wood moisture content is less 

than about 20%; at high moisture content, the coefficient of 

kinetic friction decreases substantially as speed increases.

Coefficients of kinetic friction for smooth, dry wood against 

hard, smooth surfaces commonly range from 0.3 to 0.5; 

at intermediate moisture content, 0.5 to 0.7; and near fiber 

saturation, 0.7 to 0.9.

Nuclear Radiation Properties

Several techniques using high-energy radiation can be used 

to measure density and moisture content of wood. Radiation 

passing through matter is reduced in intensity according to 

the relationship

exp( ) 0 I = I -μz (4–28)

where I is the reduced intensity of the beam at depth z in the 

material, I0 is the incident intensity of a beam of radiation, 

and μ, the linear absorption coefficient of the material, is the 

fraction of energy removed from the beam per unit depth 

traversed. When density is a factor of interest in energy absorption, the linear absorption coefficient is divided by the 

density of the material to derive the mass absorption coefficient. The absorption coefficient of a material varies with 

the type and energy of radiation.

The linear absorption coefficient of wood for g radiation is 

known to vary directly with moisture content and density 

and inversely with the g ray energy. As an example, the irradiation of ovendry yellow-poplar with 0.047-MeV g rays 

yields linear absorption coefficients ranging from about 

0.065 to about 0.11 cm–1 over the ovendry specific gravity 

range of about 0.33 to 0.62. An increase in the linear absorption coefficient of about 0.01 cm–1 occurs with an increase 

in moisture content from ovendry to fiber saturation. Absorption of g rays in wood is of practical interest, in part for 

measuring the density of wood.

The interaction of wood with b radiation is similar in 

character to that with g radiation, except that the absorption 

coefficients are larger. The linear absorption coefficient of 

FW

FN

Ff F

Figure 4–8. Diagram depicting the forces acting on an 

object in contact with a surface.
4–18

wood with a specific gravity of 0.5 for a 0.5-MeV b ray is 

about 3.0 cm-1. The result of the larger coefficient is that 

even very thin wood products are virtually opaque to b rays.

The interaction of neutrons with wood is of interest because 

wood and the water it contains are compounds of hydrogen, 

and hydrogen has a relatively large probability of interaction 

with neutrons. Higher energy neutrons lose energy much 

more quickly through interaction with hydrogen than with 

other elements found in wood. Lower energy neutrons that 

result from this interaction are thus a measure of the hydrogen density of the specimen. Measurement of the lower 

energy level neutrons can be related to the moisture content 

of the wood.

When neutrons interact with wood, an additional result is 

the production of radioactive isotopes of the elements present in the wood. The radioisotopes produced can be identified by the type, energy, and half-life of their emissions, and 

the specific activity of each indicates the amount of isotope 

present. This procedure, called neutron activation analysis, 

provides a sensitive nondestructive method of analysis for 

trace elements.

Discussions in this section assume moderate radiation levels 

that leave the wood physically unchanged. However, very 

large doses of g rays or neutrons can cause substantial degradation of wood. The effect of large radiation doses on 

mechanical properties of wood is discussed in Chapter 5.

Literature Cited

AF&PA. 2005. National design specification for wood 

construction. ANSI/AF&PA NDS-2005. Washington, DC: 

American Forest & Paper Association, American Wood 

Council. 174 p.

ASTM. 2007. Standard test methods for small clear specimens of timber. ASTM Standard D143, 1994 (2007). West 

Conshohocken, PA: ASTM International.

Comstock, G.L. 1965. Shrinkage of coast-type Douglas-fir 

and old-growth redwood boards. Res. Pap. FPL 30. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory. 20 p.

IEA. 1991. Catalogue of material properties. In: International Energy Agency, Annex XIV: Condensation and Energy. 

Leuven, Belgium: Katholieke Universiteit Leuven, Laboratory for Building Physics. Report Annex XIV, Vol. 3.

ISO. 2002. Hygrothermal performance of building materials and products—determination of water absorption coefficient by partial immersion. International Standard ISO 

15148:2002. Geneva, Switzerland: International Organization for Standardization.

James, W.L. 1975. Dielectric properties of wood and hardboard: variation with temperature, frequency, moisture 

content, and grain orientation. Res. Pap. FPL–RP–245. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 32 p.

James, W.L. 1988. Electric moisture meters for wood. Gen. 

Tech. Rep. FPL–GTR–6. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

17 p.

Kumaran, M.K. 1999. Moisture diffusivity of building materials from water absorption measurements. Journal of Thermal Envelope and Building Science. 22: 349–355.

Kumaran, M.K. 2002. A thermal and moisture transport 

property database for common building and insulating materials. Final Report, ASHRAE Research Project 1018-RP. 

Atlanta, GA: American Society of Heating, Refrigerating 

and Air-Conditioning Engineers, Inc.

MacDonald, J.R.; Johnson, W.B. 1987. Fundamentals of impedance spectroscopy. In: MacDonald, J.R., ed. Impedance 

spectroscopy emphasizing solid materials and systems. New 

York, NY: John Wiley & Sons: 1–26.

Markwardt, L.J.; Wilson, T.R.C. 1935. Strength and related 

properties of woods grown in the United States. Tech. Bull. 

No. 479. Washington, DC: U.S. Department of Agriculture.

Simpson, W.T. 1973. Predicting equilibrium moisture content of wood by mathematical models. Wood and Fiber. 

5(1): 41–49.

Skaar, C. 1988. Wood–water relations. New York: SpringerVerlag. 283 p.

Spalt, H.A. 1958. The fundamentals of water vapor sorption 

by wood. Forest Products Journal. 8(10): 288–295.

Stamm, A.J. 1929. The fiber-saturation point of wood as obtained from electrical conductivity measurements. Industrial 

and Engineering Chemistry, Analytical Edition. 1(2): 94–97.

Stamm, A.J. 1964. Wood and Cellulose Science. New York: 

Ronald Press. 549 p.

Stamm, A.J.; Loughborough, W.K. 1935. Thermodynamics of the swelling of wood. Journal of Physical Chemistry. 

39(1): 121–132.

Tiitta, M.; Olkkonen, H. 2002. Electrical impedance spectroscopy device for measurement of moisture gradients in 

wood. Review of Scientific Instruments. 73(8): 3093–3100.

Zelinka, S.L.; Rammer, D.R. 2005. The use of electrochemical impedance spectroscopy (EIS) to measure the corrosion 

of metals in contact with wood. TMS Letters. 2(1): 15–16.

Zelinka, S.L.; Stone, D.S.; Rammer, D.R. 2007. Equivalent 

circuit modeling of wood at 12% moisture content. Wood 

Fiber Science. 39(4): 556–565.

General Technical Report FPL–GTR–190
4–19

Chapter 4 Moisture Relations and Physical Properties of Wood

Additional References

ASHRAE. 2009. 2009 ASHRAE Handbook—fundamentals. 

Atlanta, GA: American Society of Heating, Refrigerating 

and Air-Conditioning Engineers, Inc.

Beall, F.C. 1968. Specific heat of wood—further 

research required to obtain meaningful data. Res. Note 

FPL–RN–0184. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 8 p.

Kleuters, W. 1964. Determining local density of wood by 

beta ray method. Forest Products Journal. 14(9): 414.

Kollman, F.F.P.; Côté, W.A., Jr. 1968. Principles of wood 

science and technology I—solid wood. New York: SpringerVerlag. 592 p.

Kubler, H.; Liang, L.; Chang, L.S. 1973. Thermal expansion 

of moist wood. Wood and Fiber. 5(3): 257–267.

Kukachka, B.F. 1970. Properties of imported tropical 

woods. Res. Pap. FPL–RP–125. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 67 p.

Lin, R.T. 1967. Review of dielectric properties of wood and 

cellulose. Forest Products Journal. 17(7): 61.

McKenzie, W.M.; Karpovich, H. 1968. Frictional behavior 

of wood. Munich: Wood Science Technology. 2(2): 138.

Murase, Y. 1980. Frictional properties of wood at high sliding speed. Journal of the Japanese Wood Research Society. 

26(2): 61–65.

Panshin, A.J.; deZeeuw, C. 1980. Textbook of wood technology. New York: McGraw-Hill. Vol. 1, 4th ed.

Simpson, W.T., ed. 1991. Dry kiln operator’s manual. Agric. 

Handb. AH-188. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 274 p.

Simpson, W.T. 1993. Specific gravity, moisture content, 

and density relationships for wood. Gen. Tech. Rep. 

FPL–GTR–76. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 13 p.

Steinhagen, H.P. 1977. Thermal conductive properties of 

wood, green or dry, from -40° to +100°C: a literature review. Gen. Tech. Rep. FPL–GTR–9. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 10 p.

TenWolde, A.; McNatt, J.D.; Krahn, L. 1988. Thermal properties of wood panel products for use in buildings. ORNL/

Sub/87–21697/1. Oak Ridge, TN: Oak Ridge National 

Laboratory. 38 p.

Weatherwax, R.C.; Stamm, A.J. 1947. The coefficients of 

thermal expansion of wood and wood products. Transactions of American Society of Mechanical Engineers. 

69(44): 421–432.

CHAPTER 5

Mechanical Properties of Wood

David E. Kretschmann, Research General Engineer

5–1

The mechanical properties presented in this chapter were 

obtained from tests of pieces of wood termed “clear” and 

“straight grained” because they did not contain characteristics such as knots, cross grain, checks, and splits. These test 

pieces did have anatomical characteristics such as growth 

rings that occurred in consistent patterns within each piece. 

Clear wood specimens are usually considered “homogeneous” in wood mechanics.

Many of the mechanical properties of wood tabulated in this 

chapter were derived from extensive sampling and analysis 

procedures. These properties are represented as the average mechanical properties of the species. Some properties, 

such as tension parallel to the grain, and all properties for 

some imported species are based on a more limited number 

of specimens that were not subjected to the same sampling 

and analysis procedures. The appropriateness of these latter 

properties to represent the average properties of a species 

is uncertain; nevertheless, the properties represent the best 

information available.

Variability, or variation in properties, is common to all 

materials. Because wood is a natural material and the tree 

is subject to many constantly changing influences (such as 

moisture, soil conditions, and growing space), wood properties vary considerably, even in clear material. This chapter 

provides information, where possible, on the nature and 

magnitude of variability in properties.

This chapter also includes a discussion of the effect of 

growth features, such as knots and slope of grain, on clear 

wood properties. The effects of manufacturing and service 

environments on mechanical properties are discussed, and 

their effects on clear wood and material containing growth 

features are compared. Chapter 7 discusses how these 

research results have been implemented in engineering 

standards.

Orthotropic Nature of Wood

Wood may be described as an orthotropic material; that is, 

it has unique and independent mechanical properties in the 

directions of three mutually perpendicular axes: longitudinal, radial, and tangential. The longitudinal axis L is parallel 

to the fiber (grain); the radial axis R is normal to the growth 

rings (perpendicular to the grain in the radial direction); and 

the tangential axis T is perpendicular to the grain but tangent 

to the growth rings. These axes are shown in Figure 5–1.

Contents 

Orthotropic Nature of Wood 5–1

Elastic Properties 5–2

 Modulus of Elasticity 5–2

 Poisson’s Ratio 5–2

 Modulus of Rigidity 5–3

Strength Properties 5–3

 Common Properties 5–3 

 Less Common Properties 5–15

Vibration Properties 5–17

 Speed of Sound 5–17

 Internal Friction 5–17

Mechanical Properties of Clear Straight-Grained 

Wood 5–21 

Natural Characteristics Affecting Mechanical 

Properties 5–26

 Specific Gravity 5–26

 Knots 5–26

 Slope of Grain 5–28

 Annual Ring Orientation 5–31

 Reaction Wood 5–31

 Juvenile Wood 5–32

 Compression Failures 5–33

 Pitch Pockets 5–33

 Bird Peck 5–33

 Extractives 5–34

 Properties of Timber from Dead Trees 5–34

 Effects of Manufacturing and Service 

 Environments 5–34

 Temperature 5–35

 Time Under Load 5–38

 Aging 5–41

 Exposure to Chemicals 5–41

 Chemical Treatment 5–41

 Nuclear Radiation 5–43

 Mold and Stain Fungi 5–43

 Decay 5–43

 Insect Damage 5–44

Literature Cited 5–44

Additional References 5–44
5–2

Elastic Properties

Twelve constants (nine are independent) are needed to describe the elastic behavior of wood: three moduli of elasticity E, three moduli of rigidity G, and six Poisson’s ratios μ. 

The moduli of elasticity and Poisson’s ratios are related by 

expressions of the form

(5–1)

General relations between stress and strain for a homogeneous orthotropic material can be found in texts on 

anisotropic elasticity.

Modulus of Elasticity

Elasticity implies that deformations produced by low stress 

are completely recoverable after loads are removed. When 

loaded to higher stress levels, plastic deformation or failure 

occurs. The three moduli of elasticity, which are denoted 

by EL, ER, and ET, respectively, are the elastic moduli along 

the longitudinal, radial, and tangential axes of wood. These 

moduli are usually obtained from compression tests; however, data for ER and ET are not extensive. Average values 

of ER and ET for samples from a few species are presented in 

Table 5–1 as ratios with EL; the Poisson’s ratios are shown 

in Table 5–2. The elastic ratios, and the elastic constants 

themselves, vary within and between species and with moisture content and specific gravity.

The modulus of elasticity determined from bending, EL, 

rather than from an axial test, may be the only modulus of 

elasticity available for a species. Average EL values obtained 

from bending tests are given in Tables 5–3 to 5–5. Representative coefficients of variation of EL determined with 

bending tests for clear wood are reported in Table 5–6. As 

tabulated, EL includes an effect of shear deflection; EL from 

bending can be increased by 10% to remove this effect approximately. This adjusted bending EL can be used to determine ER and ET based on the ratios in Table 5–1.

Poisson’s Ratio

When a member is loaded axially, the deformation perpendicular to the direction of the load is proportional to the 

deformation parallel to the direction of the load. The ratio 

of the transverse to axial strain is called Poisson’s ratio. 

The Poisson’s ratios are denoted by μLR, μRL, μLT, μTL, μRT, 

and μTR. The first letter of the subscript refers to direction 

of applied stress and the second letter to direction of lateral 

deformation. For example, μLR is the Poisson’s ratio for 

deformation along the radial axis caused by stress along 

the longitudinal axis. Average values of experimentally determined Poisson’s ratios for samples of a few species are 

given in Table 5–2. The ideal relationship between Poisson’s 

ratio and the moduli of elasticity given in Equation (5–1) are 

not always closely met. Two of the Poisson’s ratios, μRL and 

μTL, are very small and are less precisely determined than 

are those for other Poisson’s ratios. Poisson’s ratios vary 

within and between species and are affected by moisture 

content and specific gravity.

Figure 5–1. Three principal axes of wood with respect to 

grain direction and growth rings.

Table 5–1. Elastic ratios for various species at 

approximately 12% moisture contenta

Species ET/EL ER/EL GLR/EL GLT/EL GRT/EL

Hardwoods

Ash, white 0.080 0.125 0.109 0.077 — 

Balsa 0.015 0.046 0.054 0.037 0.005

Basswood 0.027 0.066 0.056 0.046 — 

Birch, yellow 0.050 0.078 0.074 0.068 0.017

Cherry, black 0.086 0.197 0.147 0.097 — 

Cottonwood, eastern 0.047 0.083 0.076 0.052 — 

Mahogany, African 0.050 0.111 0.088 0.059 0.021

Mahogany, Honduras 0.064 0.107 0.066 0.086 0.028

Maple, sugar 0.065 0.132 0.111 0.063 — 

Maple, red 0.067 0.140 0.133 0.074 — 

Oak, red 0.082 0.154 0.089 0.081 — 

Oak, white 0.072 0.163 0.086 — — 

Sweetgum 0.050 0.115 0.089 0.061 0.021

Walnut, black 0.056 0.106 0.085 0.062 0.021

Yellow-poplar 0.043 0.092 0.075 0.069 0.011

Softwoods

Baldcypress 0.039 0.084 0.063 0.054 0.007

Cedar, northern white 0.081 0.183 0.210 0.187 0.015

Cedar, western red 0.055 0.081 0.087 0.086 0.005

Douglas-fir 0.050 0.068 0.064 0.078 0.007

Fir, subalpine 0.039 0.102 0.070 0.058 0.006

Hemlock, western 0.031 0.058 0.038 0.032 0.003

Larch, western 0.065 0.079 0.063 0.069 0.007

Pine

Loblolly 0.078 0.113 0.082 0.081 0.013

Lodgepole 0.068 0.102 0.049 0.046 0.005

Longleaf 0.055 0.102 0.071 0.060 0.012

Pond 0.041 0.071 0.050 0.045 0.009

Ponderosa 0.083 0.122 0.138 0.115 0.017

Red 0.044 0.088 0.096 0.081 0.011

Slash 0.045 0.074 0.055 0.053 0.010

Sugar 0.087 0.131 0.124 0.113 0.019

Western white 0.038 0.078 0.052 0.048 0.005

Redwood 0.089 0.087 0.066 0.077 0.011

Spruce, Sitka 0.043 0.078 0.064 0.061 0.003

Spruce, Engelmann 0.059 0.128 0.124 0.120 0.010

a

EL may be approximated by increasing modulus of elasticity values in 

Table 5–3 by 10%. 

General Technical Report FPL–GTR–190
5–3

Chapter 5 Mechanical Properties of Wood

Modulus of Rigidity

The modulus of rigidity, also called shear modulus, indicates the resistance to deflection of a member caused by 

shear stresses. The three moduli of rigidity denoted by GLR, 

GLT, and GRT are the elastic constants in the LR, LT, and 

RT planes, respectively. For example, GLR is the modulus 

of rigidity based on shear strain in the LR plane and shear 

stresses in the LT and RT planes. Average values of shear 

moduli for samples of a few species expressed as ratios with 

EL are given in Table 5–1. As with moduli of elasticity, the 

moduli of rigidity vary within and between species and 

with moisture content and specific gravity.

Strength Properties

Common Properties

Mechanical properties most commonly measured and represented as “strength properties” for design include modulus 

of rupture in bending, maximum stress in compression parallel to grain, compressive stress perpendicular to grain, and 

shear strength parallel to grain. Additional measurements 

are often made to evaluate work to maximum load in bending, impact bending strength, tensile strength perpendicular 

to grain, and hardness. These properties, grouped according 

to the broad forest tree categories of hardwood and softwood (not correlated with hardness or softness), are given in 

Tables 5–3 to 5–5 for many of the commercially important 

species. Average coefficients of variation for these properties from a limited sampling of specimens are reported in 

Table 5–6.

Modulus of rupture—Reflects the maximum load-carrying 

capacity of a member in bending and is proportional to maximum moment borne by the specimen. Modulus of rupture 

is an accepted criterion of strength, although it is not a true 

stress because the formula by which it is computed is valid 

only to the elastic limit.

Work to maximum load in bending—Ability to absorb 

shock with some permanent deformation and more or less 

injury to a specimen. Work to maximum load is a measure 

of the combined strength and toughness of wood under 

bending stresses.

Compressive strength parallel to grain—Maximum stress 

sustained by a compression parallel-to-grain specimen having a ratio of length to least dimension of less than 11.

Compressive stress perpendicular to grain—Reported as 

stress at proportional limit. There is no clearly defined ultimate stress for this property.

Shear strength parallel to grain—Ability to resist internal 

slipping of one part upon another along the grain. Values 

presented are average strength in radial and tangential shear 

planes.

Impact bending—In the impact bending test, a hammer of 

given weight is dropped upon a beam from successively increased heights until rupture occurs or the beam deflects 

152 mm (6 in.) or more. The height of the maximum drop, 

or the drop that causes failure, is a comparative value that 

represents the ability of wood to absorb shocks that cause 

stresses beyond the proportional limit.

Tensile strength perpendicular to grain—Resistance of 

wood to forces acting across the grain that tend to split a 

member. Values presented are the average of radial and 

tangential observations.

Hardness—Generally defined as resistance to indentation 

using a modified Janka hardness test, measured by the load 

required to embed a 11.28-mm (0.444-in.) ball to one-half 

its diameter. Values presented are the average of radial and 

tangential penetrations.

Tensile strength parallel to grain—Maximum tensile 

stress sustained in direction parallel to grain. Relatively few 

data are available on the tensile strength of various species 

Table 5–2. Poisson’s ratios for various species at 

approximately 12% moisture content

Species μLR μLT μRT μTR μRL μTL

Hardwoods

Ash, white 0.371 0.440 0.684 0.360 0.059 0.051

Aspen, quaking 0.489 0.374 — 0.496 0.054 0.022

Balsa 0.229 0.488 0.665 0.231 0.018 0.009

Basswood 0.364 0.406 0.912 0.346 0.034 0.022

Birch, yellow 0.426 0.451 0.697 0.426 0.043 0.024

Cherry, black 0.392 0.428 0.695 0.282 0.086 0.048

Cottonwood, eastern 0.344 0.420 0.875 0.292 0.043 0.018

Mahogany, African 0.297 0.641 0.604 0.264 0.033 0.032

Mahogany, Honduras 0.314 0.533 0.600 0.326 0.033 0.034

Maple, sugar 0.424 0.476 0.774 0.349 0.065 0.037

Maple, red 0.434 0.509 0.762 0.354 0.063 0.044

Oak, red 0.350 0.448 0.560 0.292 0.064 0.033

Oak, white 0.369 0.428 0.618 0.300 0.074 0.036

Sweetgum 0.325 0.403 0.682 0.309 0.044 0.023

Walnut, black 0.495 0.632 0.718 0.367 0.052 0.036

Yellow-poplar 0.318 0.392 0.703 0.329 0.030 0.019

Softwoods

Baldcypress 0.338 0.326 0.411 0.356 — — 

Cedar, northern white 0.337 0.340 0.458 0.345 — — 

Cedar, western red 0.378 0.296 0.484 0.403 — — 

Douglas-fir 0.292 0.449 0.390 0.374 0.036 0.029

Fir, subalpine 0.341 0.332 0.437 0.336 — — 

Hemlock, western 0.485 0.423 0.442 0.382 — — 

Larch, western 0.355 0.276 0.389 0.352 — — 

Pine

Loblolly 0.328 0.292 0.382 0.362 — — 

Lodgepole 0.316 0.347 0.469 0.381 — — 

Longleaf 0.332 0.365 0.384 0.342 — — 

Pond 0.280 0.364 0.389 0.320 — — 

Ponderosa 0.337 0.400 0.426 0.359 — — 

Red 0.347 0.315 0.408 0.308 — — 

Slash 0.392 0.444 0.447 0.387 — — 

Sugar 0.356 0.349 0.428 0.358 — — 

Western white 0.329 0.344 0.410 0.334 — — 

Redwood 0.360 0.346 0.373 0.400 — — 

Spruce, Sitka 0.372 0.467 0.435 0.245 0.040 0.025

Spruce, Engelmann 0.422 0.462 0.530 0.255 0.083 0.058
5–4

Table 5–3a. Strength properties of some commercially important woods grown in the United States (metric)a

Static bending 

Impact

bending

(mm) 

Compression

parallel

to grain

(kPa)

Compression

perpendicular 

to grain 

(kPa)

Shear 

parallel

to

grain 

(kPa)

Tension

perpendicular

to grain

(kPa)

Side

hardness

(N)

Modulus

of

rupture

(kPa)

Modulus

of

elasticityc

(MPa)

Work to

maximum 

load 

(kJ m–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Hardwoods

Alder, red Green 0.37 45,000 8,100 55 560 20,400 1,700 5,300 2,700 2,000

12% 0.41 68,000 9,500 58 510 40,100 3,000 7,400 2,900 2,600

Ash 

Black Green 0.45 41,000 7,200 83 840 15,900 2,400 5,900 3,400 2,300

12% 0.49 87,000 11,000 103 890 41,200 5,200 10,800 4,800 3,800

Blue Green 0.53 66,000 8,500 101 — 28,800 5,600 10,600 — — 

12% 0.58 95,000 9,700 99 — 48,100 9,800 14,000 — — 

Green Green 0.53 66,000 9,700 81 890 29,000 5,000 8,700 4,100 3,900

12% 0.56 97,000 11,400 92 810 48,800 9,000 13,200 4,800 5,300

Oregon Green 0.50 52,000 7,800 84 990 24,200 3,700 8,200 4,100 3,500

12% 0.55 88,000 9,400 99 840 41,600 8,600 12,300 5,000 5,200

White Green 0.55 66,000 9,900 108 970 27,500 4,600 9,300 4,100 4,300

12% 0.60 103,000 12,000 115 1,090 51,100 8,000 13,200 6,500 5,900

Aspen

Bigtooth Green 0.36 37,000 7,700 39 — 17,200 1,400 5,000 — — 

12% 0.39 63,000 9,900 53 — 36,500 3,100 7,400 — — 

Quaking Green 0.35 35,000 5,900 44 560 14,800 1,200 4,600 1,600 1,300

12% 0.38 58,000 8,100 52 530 29,300 2,600 5,900 1,800 1,600

Basswood, American Green 0.32 34,000 7,200 37 410 15,300 1,200 4,100 1,900 1,100

12% 0.37 60,000 10,100 50 410 32,600 2,600 6,800 2,400 1,800

Beech, American Green 0.56 59,000 9,500 82 1,090 24,500 3,700 8,900 5,000 3,800

12% 0.64 103,000 11,900 104 1,040 50,300 7,000 13,900 7,000 5,800

Birch 

Paper Green 0.48 44,000 8,100 112 1,240 16,300 1,900 5,800 2,600 2,500

12% 0.55 85,000 11,000 110 860 39,200 4,100 8,300 — 4,000

Sweet Green 0.60 65,000 11,400 108 1,220 25,800 3,200 8,500 3,000 4,300

12% 0.65 117,000 15,000 124 1,190 58,900 7,400 15,400 6,600 6,500

Yellow Green 0.55 57,000 10,300 111 1,220 23,300 3,000 7,700 3,000 3,600

12% 0.62 114,000 13,900 143 1,400 56,300 6,700 13,000 6,300 5,600

Butternut Green 0.36 37,000 6,700 57 610 16,700 1,500 5,200 3,000 1,700

12% 0.38 56,000 8,100 57 610 36,200 3,200 8,100 3,000 2,200

Cherry, black Green 0.47 55,000 9,000 88 840 24,400 2,500 7,800 3,900 2,900

12% 0.50 85,000 10,300 79 740 49,000 4,800 11,700 3,900 4,200

Chestnut, American Green 0.40 39,000 6,400 48 610 17,000 2,100 5,500 3,000 1,900

12% 0.43 59,000 8,500 45 480 36,700 4,300 7,400 3,200 2,400

Cottonwood

Balsam poplar Green 0.31 27,000 5,200 29 — 11,700 1,000 3,400 — — 

12% 0.34 47,000 7,600 34 — 27,700 2,100 5,400 — — 

Black Green 0.31 34,000 7,400 34 510 15,200 1,100 4,200 1,900 1,100

12% 0.35 59,000 8,800 46 560 31,000 2,100 7,200 2,300 1,600

Eastern Green 0.37 37,000 7,000 50 530 15,700 1,400 4,700 2,800 1,500

12% 0.40 59,000 9,400 51 510 33,900 2,600 6,400 4,000 1,900

Elm 

American Green 0.46 50,000 7,700 81 970 20,100 2,500 6,900 4,100 2,800

12% 0.50 81,000 9,200 90 990 38,100 4,800 10,400 4,600 3,700

Rock Green 0.57 66,000 8,200 137 1,370 26,100 4,200 8,800 — — 

12% 0.63 102,000 10,600 132 1,420 48,600 8,500 13,200 — — 

Slippery Green 0.48 55,000 8,500 106 1,190 22,900 2,900 7,700 4,400 2,900

12% 0.53 90,000 10,300 117 1,140 43,900 5,700 11,200 3,700 3,800

Hackberry Green 0.49 45,000 6,600 100 1,220 18,300 2,800 7,400 4,300 3,100

12% 0.53 76,000 8,200 88 1,090 37,500 6,100 11,000 4,000 3,900

General Technical Report FPL–GTR–190
5–5

Chapter 5 Mechanical Properties of Wood

Table 5–3a. Strength properties of some commercially important woods grown in the United States (metric)a—con.

Static bending 

Impact

bending

(mm) 

Compression

parallel

to grain

(kPa)

Compression

perpendicular 

to grain 

(kPa)

Shear 

parallel

to

grain 

(kPa)

Tension

perpendicular to

grain

(kPa)

Side

hardness

(N)

Modulus

of

rupture

(kPa)

Modulus

of

elasticityc

(MPa)

Work to

maximum

load 

(kJ m–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Hickory, pecan 

Bitternut Green 0.60 71,000 9,700 138 1,680 31,500 5,500 8,500 — — 

12% 0.66 118,000 12,300 125 1,680 62,300 11,600 — — — 

Nutmeg Green 0.56 63,000 8,900 157 1,370 27,400 5,200 7,100 — — 

12% 0.60 114,000 11,700 173 — 47,600 10,800 — — — 

Pecan Green 0.60 68,000 9,400 101 1,350 27,500 5,400 10,200 4,700 5,800 

12% 0.66 94,000 11,900 95 1,120 54,100 11,900 14,300 — 8,100 

Water Green 0.61 74,000 10,800 130 1,420 32,100 6,100 9,900 — — 

12% 0.62 123,000 13,900 133 1,350 59,300 10,700 — — — 

Hickory, trued

Mockernut Green 0.64 77,000 10,800 180 2,240 30,900 5,600 8,800 — 6,400 

12% 0.72 132,000 15,300 156 1,960 61,600 11,900 12,000 — 8,800 

Pignut Green 0.66 81,000 11,400 219 2,260 33,200 6,300 9,400 — 6,800 

12% 0.75 139,000 15,600 210 1,880 63,400 13,700 14,800 — 9,500 

Shagbark Green 0.64 76,000 10,800 163 1,880 31,600 5,800 10,500 — 6,500 

12% 0.72 139,000 14,900 178 1,700 63,500 12,100 16,800 — 8,400 

Shellbark Green 0.62 72,000 9,200 206 2,640 27,000 5,600 8,200 — 7,400 

12% 0.69 125,000 13,000 163 2,240 55,200 12,400 14,500 — 8,100 

Honeylocust Green 0.60 70,000 8,900 87 1,190 30,500 7,900 11,400 6,400 6,200 

12% — 101,000 11,200 92 1,190 51,700 12,700 15,500 6,200 7,000 

Locust, black Green 0.66 95,000 12,800 106 1,120 46,900 8,000 12,100 5,300 7,000 

12% 0.69 134,000 14,100 127 1,450 70,200 12,600 17,100 4,400 7,600 

Magnolia

Cucumbertree Green 0.44 51,000 10,800 69 760 21,600 2,300 6,800 3,000 2,300 

12% 0.48 85,000 12,500 84 890 43,500 3,900 9,200 4,600 3,100 

Southern Green 0.46 47,000 7,700 106 1,370 18,600 3,200 7,200 4,200 3,300 

12% 0.50 77,000 9,700 88 740 37,600 5,900 10,500 5,100 4,500 

Maple 

Bigleaf Green 0.44 51,000 7,600 60 580 22,300 3,100 7,700 4,100 2,800 

12% 0.48 74,000 10,000 54 710 41,000 5,200 11,900 3,700 3,800 

Black Green 0.52 54,000 9,200 88 1,220 22,500 4,100 7,800 5,000 3,700 

12% 0.57 92,000 11,200 86 1,020 46,100 7,000 12,500 4,600 5,200 

Red Green 0.49 53,000 9,600 79 810 22,600 2,800 7,900 — 3,100 

12% 0.54 92,000 11,300 86 810 45,100 6,900 12,800 — 4,200 

Silver Green 0.44 40,000 6,500 76 740 17,200 2,600 7,200 3,900 2,600 

12% 0.47 61,000 7,900 57 640 36,000 5,100 10,200 3,400 3,100 

Sugar Green 0.56 65,000 10,700 92 1,020 27,700 4,400 10,100 — 4,300 

12% 0.63 109,000 12,600 114 990 54,000 10,100 16,100 — 6,400 

Oak, red 

Black Green 0.56 57,000 8,100 84 1,020 23,900 4,900 8,400 — 4,700 

12% 0.61 96,000 11,300 94 1,040 45,000 6,400 13,200 — 5,400 

Cherrybark Green 0.61 74,000 12,300 101 1,370 31,900 5,200 9,100 5,500 5,500 

12% 0.68 125,000 15,700 126 1,240 60,300 8,600 13,800 5,800 6,600 

Laurel Green 0.56 54,000 9,600 77 990 21,900 3,900 8,100 5,300 4,400 

12% 0.63 87,000 11,700 81 990 48,100 7,300 12,600 5,400 5,400 

Northern red Green 0.56 57,000 9,300 91 1,120 23,700 4,200 8,300 5,200 4,400 

12% 0.63 99,000 12,500 100 1,090 46,600 7,000 12,300 5,500 5,700 

Pin Green 0.58 57,000 9,100 97 1,220 25,400 5,000 8,900 5,500 4,800 

12% 0.63 97,000 11,900 102 1,140 47,000 7,000 14,300 7,200 6,700 

Scarlet Green 0.60 72,000 10,200 103 1,370 28,200 5,700 9,700 4,800 5,300 

12% 0.67 120,000 13,200 141 1,350 57,400 7,700 13,000 6,000 6,200 

Southern red Green 0.52 48,000 7,900 55 740 20,900 3,800 6,400 3,300 3,800 

12% 0.59 75,000 10,300 65 660 42,000 6,000 9,600 3,500 4,700 

Water Green 0.56 61,000 10,700 77 990 25,800 4,300 8,500 5,700 4,500 

12% 0.63 106,000 13,900 148 1,120 46,700 7,000 13,900 6,300 5,300 
5–6

Table 5–3a. Strength properties of some commercially important woods grown in the United States (metric)a—con.

Static bending 

Impact

bending

(mm) 

Compression

parallel

to grain

(kPa)

Compression

perpendicular 

to grain 

(kPa)

Tension

perpendicular

to grain

(kPa)

Side

hardness 

(N)

Modulus

of

rupture

(kPa)

Modulus

of

elasticityc

(MPa)

Work to

maximum

load 

(kJ m–3)

Shear 

parallel

to

grain 

(kPa)

Common species 

names

Moisture

content 

Specific 

gravityb

Oak, red—con. 

Willow Green 0.56 51,000 8,900 61 890 20,700 4,200 8,100 5,200 4,400 

12% 0.69 100,000 13,100 101 1,070 48,500 7,800 11,400 — 6,500 

Oak, white 

Bur Green 0.58 50,000 6,100 74 1,120 22,700 4,700 9,300 5,500 4,900 

12% 0.64 71,000 7,100 68 740 41,800 8,300 12,500 4,700 6,100 

Chestnut Green 0.57 55,000 9,400 65 890 24,300 3,700 8,300 4,800 4,000 

12% 0.66 92,000 11,000 76 1,020 47,100 5,800 10,300 — 5,000 

Live Green 0.80 82,000 10,900 85 — 37,400 14,100 15,200 — — 

12% 0.88 127,000 13,700 130 — 61,400 19,600 18,300 — — 

Overcup Green 0.57 55,000 7,900 87 1,120 23,200 3,700 9,100 5,000 4,300 

12% 0.63 87,000 9,800 108 970 42,700 5,600 13,800 6,500 5,300 

Post Green 0.60 56,000 7,500 76 1,120 24,000 5,900 8,800 5,400 5,000 

12% 0.67 91,000 10,400 91 1,170 45,300 9,900 12,700 5,400 6,000 

Swamp chestnut Green 0.60 59,000 9,300 88 1,140 24,400 3,900 8,700 4,600 4,900 

12% 0.67 96,000 12,200 83 1,040 50,100 7,700 13,700 4,800 5,500 

Swamp white Green 0.64 68,000 11,000 100 1,270 30,100 5,200 9,000 5,900 5,200 

12% 0.72 122,000 14,100 132 1,240 59,300 8,200 13,800 5,700 7,200 

White Green 0.60 57,000 8,600 80 1,070 24,500 4,600 8,600 5,300 4,700 

12% 0.68 105,000 12,300 102 940 51,300 7,400 13,800 5,500 6,000 

Sassafras Green 0.42 41,000 6,300 49 — 18,800 2,600 6,600 — — 

12% 0.46 62,000 7,700 60 — 32,800 5,900 8,500 — — 

Sweetgum Green 0.46 49,000 8,300 70 910 21,000 2,600 6,800 3,700 2,700 

12% 0.52 86,000 11,300 82 810 43,600 4,300 11,000 5,200 3,800 

Sycamore, American Green 0.46 45,000 7,300 52 660 20,100 2,500 6,900 4,300 2,700 

12% 0.49 69,000 9,800 59 660 37,100 4,800 10,100 5,000 3,400 

Tanoak Green 0.58 72,000 10,700 92 — 32,100 — — — — 

12% — — — — — — — — — — 

Tupelo 

Black Green 0.46 48,000 7,100 55 760 21,000 3,300 7,600 3,900 2,800 

12% 0.50 66,000 8,300 43 560 38,100 6,400 9,200 3,400 3,600 

Water Green 0.46 50,000 7,200 57 760 23,200 3,300 8,200 4,100 3,200 

12% 0.50 66,000 8,700 48 580 40,800 6,000 11,000 4,800 3,900 

Walnut, black Green 0.51 66,000 9,800 101 940 29,600 3,400 8,400 3,900 4,000 

12% 0.55 101,000 11,600 74 860 52,300 7,000 9,400 4,800 4,500 

Willow, black Green 0.36 33,000 5,400 76 — 14,100 1,200 4,700 — — 

12% 0.39 54,000 7,000 61 — 28,300 3,000 8,600 — — 

Yellow-poplar Green 0.40 41,000 8,400 52 660 18,300 1,900 5,400 3,500 2,000 

12% 0.42 70,000 10,900 61 610 38,200 3,400 8,200 3,700 2,400 

Softwoods

Baldcypress Green 0.42 46,000 8,100 46 640 24,700 2,800 5,600 2,100 1,700 

12% 0.46 73,000 9,900 57 610 43,900 5,000 6,900 1,900 2,300 

Cedar 

Atlantic white Green 0.31 32,000 5,200 41 460 16,500 1,700 4,800 1,200 1,300 

12% 0.32 47,000 6,400 28 330 32,400 2,800 5,500 1,500 1,600 

Eastern redcedar Green 0.44 48,000 4,500 103 890 24,600 4,800 7,000 2,300 2,900 

12% 0.47 61,000 6,100 57 560 41,500 6,300 — — 4,000 

Incense Green 0.35 43,000 5,800 44 430 21,700 2,600 5,700 1,900 1,700 

12% 0.37 55,000 7,200 37 430 35,900 4,100 6,100 1,900 2,100 

Northern white Green 0.29 29,000 4,400 39 380 13,700 1,600 4,300 1,700 1,000 

12% 0.31 45,000 5,500 33 300 27,300 2,100 5,900 1,700 1,400 

General Technical Report FPL–GTR–190
Chapter 5 Mechanical Properties of Wood

5–7

Table 5–3a. Strength properties of some commercially important woods grown in the United States (metric)a—con.

Static bending 

Impact

bending

(mm) 

Compression

parallel

to grain

(kPa)

Compression

perpendicular 

to grain 

(kPa)

Shear 

parallel

to

grain 

(kPa)

Tension

perpendicular

to grain

(kPa)

Side

hardness

(N)

Modulus

of

rupture

(kPa)

Modulus

of

elasticityc

(MPa)

Work to

maximum 

load 

(kJ m–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Cedar—con. 

Port-Orford Green 0.39 45,000 9,000 51 530 21,600 2,100 5,800 1,200 1,700 

12% 0.43 88,000 11,700 63 710 43,100 5,000 9,400 2,800 2,800 

Western redcedar Green 0.31 35,900 6,500 34 430 19,100 1,700 5,300 1,600 1,200 

12% 0.32 51,700 7,700 40 430 31,400 3,200 6,800 1,500 1,600 

Yellow Green 0.42 44,000 7,900 63 690 21,000 2,400 5,800 2,300 2,000 

12% 0.44 77,000 9,800 72 740 43,500 4,300 7,800 2,500 2,600 

Douglas-fire

Coast Green 0.45 53,000 10,800 52 660 26,100 2,600 6,200 2,100 2,200 

12% 0.48 85,000 13,400 68 790 49,900 5,500 7,800 2,300 3,200 

Interior West Green 0.46 53,000 10,400 50 660 26,700 2,900 6,500 2,000 2,300 

12% 0.50 87,000 12,600 73 810 51,200 5,200 8,900 2,400 2,900 

Interior North Green 0.45 51,000 9,700 56 560 23,900 2,500 6,600 2,300 1,900 

12% 0.48 90,000 12,300 72 660 47,600 5,300 9,700 2,700 2,700 

Interior South Green 0.43 47,000 8,000 55 380 21,400 2,300 6,600 1,700 1,600 

12% 0.46 82,000 10,300 62 510 43,000 5,100 10,400 2,300 2,300 

Fir

Balsam Green 0.33 38,000 8,600 32 410 18,100 1,300 4,600 1,200 1,300 

12% 0.35 63,000 10,000 35 510 36,400 2,800 6,500 1,200 1,700 

California red Green 0.36 40,000 8,100 44 530 19,000 2,300 5,300 2,600 1,600 

12% 0.38 72,400 10,300 61 610 37,600 4,200 7,200 2,700 2,200 

Grand Green 0.35 40,000 8,600 39 560 20,300 1,900 5,100 1,700 1,600 

12% 0.37 61,400 10,800 52 710 36,500 3,400 6,200 1,700 2,200 

Noble Green 0.37 43,000 9,500 41 480 20,800 1,900 5,500 1,600 1,300 

12% 0.39 74,000 11,900 61 580 42,100 3,600 7,200 1,500 1,800 

Pacific silver Green 0.40 44,000 9,800 41 530 21,600 1,500 5,200 1,700 1,400 

12% 0.43 75,800 12,100 64 610 44,200 3,100 8,400 — 1,900 

Subalpine Green 0.31 34,000 7,200 — — 15,900 1,300 4,800 — 1,200 

12% 0.32 59,000 8,900 — — 33,500 2,700 7,400 — 1,600 

White Green 0.37 41,000 8,000 39 560 20,000 1,900 5,200 2,100 1,500 

12% 0.39 68,000 10,300 50 510 40,000 3,700 7,600 2,100 2,100 

Hemlock

Eastern Green 0.38 44,000 7,400 46 530 21,200 2,500 5,900 1,600 1,800 

12% 0.40 61,000 8,300 47 530 37,300 4,500 7,300 — 2,200 

Mountain Green 0.42 43,000 7,200 76 810 19,900 2,600 6,400 2,300 2,100 

12% 0.45 79,000 9,200 72 810 44,400 5,900 10,600 — 3,000 

Western Green 0.42 46,000 9,000 48 560 23,200 1,900 5,900 2,000 1,800 

12% 0.45 78,000 11,300 57 580 49,000 3,800 8,600 2,300 2,400 

Larch, western Green 0.48 53,000 10,100 71 740 25,900 2,800 6,000 2,300 2,300 

12% 0.52 90,000 12,900 87 890 52,500 6,400 9,400 3,000 3,700 

Pine

Eastern white Green 0.34 34,000 6,800 36 430 16,800 1,500 4,700 1,700 1,300 

12% 0.35 59,000 8,500 47 460 33,100 3,000 6,200 2,100 1,700 

Jack Green 0.40 41,000 7,400 50 660 20,300 2,100 5,200 2,500 1,800 

12% 0.43 68,000 9,300 57 690 39,000 4,000 8,100 2,900 2,500 

Loblolly Green 0.47 50,000 9,700 57 760 24,200 2,700 5,900 1,800 2,000 

12% 0.51 88,000 12,300 72 760 49,200 5,400 9,600 3,200 3,100 

Lodgepole Green 0.38 38,000 7,400 39 510 18,000 1,700 4,700 1,500 1,500 

12% 0.41 65,000 9,200 47 510 37,000 4,200 6,100 2,000 2,100 

Longleaf Green 0.54 59,000 11,000 61 890 29,800 3,300 7,200 2,300 2,600 

12% 0.59 100,000 13,700 81 860 58,400 6,600 10,400 3,200 3,900 

Pitch Green 0.47 47,000 8,300 63 — 20,300 2,500 5,900 — — 

12% 0.52 74,000 9,900 63 — 41,000 5,600 9,400 — — 
5–8

Table 5–3a. Strength properties of some commercially important woods grown in the United States (metric)a—con.

Static bending 

Impact

bending

(mm) 

Compression

parallel

to grain

(kPa)

Compression

perpenicular 

to grain 

(kPa)

Tension

perpendicular

to grain

(kPa)

Side

hardness

(N)

Modulus

of

rupture

(kPa)

Modulus

of

elasticityc

(MPa)

Work to 

maximum 

load 

(kJ m–3)

Shear 

parallel

to

grain 

(kPa)

Common species 

names

Moisture

content 

Specific 

gravityb

Pine—con.

Pond Green 0.51 51,000 8,800 52 — 25,200 3,000 6,500 — — 

12% 0.56 80,000 12,100 59 — 52,000 6,300 9,500 — — 

Ponderosa Green 0.38 35,000 6,900 36 530 16,900 1,900 4,800 2,100 1,400 

12% 0.40 65,000 8,900 49 480 36,700 4,000 7,800 2,900 2,000 

Red Green 0.41 40,000 8,800 42 660 18,800 1,800 4,800 2,100 1,500 

12% 0.46 76,000 11,200 68 660 41,900 4,100 8,400 3,200 2,500 

Sand Green 0.46 52,000 7,000 66 — 23,700 3,100 7,900 — — 

12% 0.48 80,000 9,700 66 — 47,700 5,800 — — — 

Shortleaf Green 0.47 51,000 9,600 57 760 24,300 2,400 6,300 2,200 2,000 

12% 0.51 90,000 12,100 76 840 50,100 5,700 9,600 3,200 3,100 

Slash Green 0.54 60,000 10,500 66 — 26,300 3,700 6,600 — — 

12% 0.59 112,000 13,700 91 — 56,100 7,000 11,600 — — 

Spruce Green 0.41 41,000 6,900 — — 19,600 1,900 6,200 — 2,000 

12% 0.44 72,000 8,500 — — 39,000 5,000 10,300 — 2,900 

Sugar Green 0.34 34,000 7,100 37 430 17,000 1,400 5,000 1,900 1,200 

12% 0.36 57,000 8,200 38 460 30,800 3,400 7,800 2,400 1,700 

Virginia Green 0.45 50,000 8,400 75 860 23,600 2,700 6,100 2,800 2,400 

12% 0.48 90,000 10,500 94 810 46,300 6,300 9,300 2,600 3,300 

Western white Green 0.36 32,000 8,200 34 480 16,800 1,300 4,700 1,800 1,200 

12% 0.35 67,000 10,100 61 580 34,700 3,200 7,200 — 1,900 

Redwood

Old-growth Green 0.38 52,000 8,100 51 530 29,000 2,900 5,500 1,800 1,800 

12% 0.40 69,000 9,200 48 480 42,400 4,800 6,500 1,700 2,100 

Young-growth Green 0.34 41,000 6,600 39 410 21,400 1,900 6,100 2,100 1,600 

12% 0.35 54,000 7,600 36 380 36,000 3,600 7,600 1,700 1,900 

Spruce 

Black Green 0.38 42,000 9,500 51 610 19,600 1,700 5,100 700 1,500 

12% 0.42 74,000 11,100 72 580 41,100 3,800 8,500 — 2,400 

Engelmann Green 0.33 32,000 7,100 35 410 15,000 1,400 4,400 1,700 1,150 

12% 0.35 64,000 8,900 44 460 30,900 2,800 8,300 2,400 1,750 

Red Green 0.37 41,000 9,200 48 460 18,800 1,800 5,200 1,500 1,600 

12% 0.40 74,000 11,400 58 640 38,200 3,800 8,900 2,400 2,200 

Sitka Green 0.37 39,000 8,500 43 610 18,400 1,900 5,200 1,700 1,600 

12% 0.40 70,000 10,800 65 640 38,700 4,000 7,900 2,600 2,300 

White Green 0.33 34,000 7,900 41 560 16,200 1,400 4,400 1,500 1,200 

12% 0.36 65,000 9,600 53 510 35,700 3,000 6,700 2,500 1,800 

Tamarack Green 0.49 50,000 8,500 50 710 24,000 2,700 5,900 1,800 1,700 

12% 0.53 80,000 11,300 49 580 49,400 5,500 8,800 2,800 2,600 a

Results of tests on clear specimens in the green and air-dried conditions, converted to metric units directly from Table 5–3b. Definition of properties: 

impact bending is height of drop that causes complete failure, using 0.71-kg (50-lb) hammer; compression parallel to grain is also called maximum 

crushing strength; compression perpendicular to grain is fiber stress at proportional limit; shear is maximum shearing strength; tension is maximum tensile 

strength; and side hardness is hardness measured when load is perpendicular to grain. b

Specific gravity is based on weight when ovendry and volume when green or at 12% moisture content. c

Modulus of elasticity measured from a simply supported, center-loaded beam, on a span depth ratio of 14/1. To correct for shear deflection, the modulus 

can be increased by 10%. 

d

Values for side hardness of the true hickories are from Bendtsen and Ethington (1975). e

Coast Douglas-fir is defined as Douglas-fir growing in Oregon and Washington State west of the Cascade Mountains summit. Interior West includes 

California and all counties in Oregon and Washington east of, but adjacent to, the Cascade summit; Interior North, the remainder of Oregon and 

Washington plus Idaho, Montana, and Wyoming; and Interior South, Utah, Colorado, Arizona, and New Mexico. 

General Technical Report FPL–GTR–190
Chapter 5 Mechanical Properties of Wood

5–9

Table 5–3b. Strength properties of some commercially important woods grown in the United States (inch–pound)a

Static bending 

Impact

bending

(in.) 

Compression

parallel

to grain

(lbf in–2)

Compression

perpendicular 

to grain 

(lbf in–2)

Side

hardness

(lbf)

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticityc

(106 lbf in–2)

Work 

to

maximum 

load 

 (in-lbf in–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Shear

parallel

to

grain

(lbf in–2)

Tension

perpendicular

to grain

(lbf in–2)

Hardwoods

Alder, red Green 0.37 6,500 1.17 8.0 22 2,960 250 770 390 440 

12% 0.41 9,800 1.38 8.4 20 5,820 440 1,080 420 590 

Ash 

Black Green 0.45 6,000 1.04 12.1 33 2,300 350 860 490 520 

12% 0.49 12,600 1.60 14.9 35 5,970 760 1,570 700 850 

Blue Green 0.53 9,600 1.24 14.7 — 4,180 810 1,540 — — 

12% 0.58 13,800 1.40 14.4 — 6,980 1,420 2,030 — — 

Green Green 0.53 9,500 1.40 11.8 35 4,200 730 1,260 590 870 

12% 0.56 14,100 1.66 13.4 32 7,080 1,310 1,910 700 1,200 

Oregon Green 0.50 7,600 1.13 12.2 39 3,510 530 1,190 590 790 

12% 0.55 12,700 1.36 14.4 33 6,040 1,250 1,790 720 1,160 

White Green 0.55 9,500 1.44 15.7 38 3,990 670 1,350 590 960 

12% 0.60 15,000 1.74 16.6 43 7,410 1,160 1,910 940 1,320 

Aspen

Bigtooth Green 0.36 5,400 1.12 5.7 — 2,500 210 730 — — 

12% 0.39 9,100 1.43 7.7 — 5,300 450 1,080 — — 

Quaking Green 0.35 5,100 0.86 6.4 22 2,140 180 660 230 300 

12% 0.38 8,400 1.18 7.6 21 4,250 370 850 260 350 

Basswood, American Green 0.32 5,000 1.04 5.3 16 2,220 170 600 280 250 

12% 0.37 8,700 1.46 7.2 16 4,730 370 990 350 410 

Beech, American Green 0.56 8,600 1.38 11.9 43 3,550 540 1,290 720 850 

12% 0.64 14,900 1.72 15.1 41 7,300 1,010 2,010 1,010 1,300 

Birch 

Paper Green 0.48 6,400 1.17 16.2 49 2,360 270 840 380 560 

12% 0.55 12,300 1.59 16.0 34 5,690 600 1,210 — 910 

Sweet Green 0.60 9,400 1.65 15.7 48 3,740 470 1,240 430 970 

12% 0.65 16,900 2.17 18.0 47 8,540 1,080 2,240 950 1,470 

Yellow Green 0.55 8,300 1.50 16.1 48 3,380 430 1,110 430 780 

12% 0.62 16,600 2.01 20.8 55 8,170 970 1,880 920 1,260 

Butternut Green 0.36 5,400 0.97 8.2 24 2,420 220 760 430 390 

12% 0.38 8,100 1.18 8.2 24 5,110 460 1,170 440 490 

Cherry, black Green 0.47 8,000 1.31 12.8 33 3,540 360 1,130 570 660 

12% 0.50 12,300 1.49 11.4 29 7,110 690 1,700 560 950 

Chestnut, American Green 0.40 5,600 0.93 7.0 24 2,470 310 800 440 420 

12% 0.43 8,600 1.23 6.5 19 5,320 620 1,080 460 540 

Cottonwood

Balsam, poplar Green 0.31 3,900 0.75 4.2 — 1,690 140 500 — — 

12% 0.34 6,800 1.10 5.0 — 4,020 300 790 — — 

Black Green 0.31 4,900 1.08 5.0 20 2,200 160 610 270 250

12% 0.35 8,500 1.27 6.7 22 4,500 300 1,040 330 350

Eastern Green 0.37 5,300 1.01 7.3 21 2,280 200 680 410 340

12% 0.40 8,500 1.37 7.4 20 4,910 380 930 580 430

Elm 

American Green 0.46 7,200 1.11 11.8 38 2,910 360 1,000 590 620

12% 0.50 11,800 1.34 13.0 39 5,520 690 1,510 660 830

Rock Green 0.57 9,500 1.19 19.8 54 3,780 610 1,270 — 940

12% 0.63 14,800 1.54 19.2 56 7,050 1,230 1,920 — 1,320

Slippery Green 0.48 8,000 1.23 15.4 47 3,320 420 1,110 640 660

12% 0.53 13,000 1.49 16.9 45 6,360 820 1,630 530 860

Hackberry Green 0.49 6,500 0.95 14.5 48 2,650 400 1,070 630 700

12% 0.53 11,000 1.19 12.8 43 5,440 890 1,590 580 880
5–10

Table 5–3b. Strength properties of some commercially important woods grown in the United States (inch–pound)a—con.

Static bending 

Impact

bending

(in.) 

Compression

parallel

to grain

(lbf in–2)

Compression

perpendicular 

to grain 

(lbf in–2)

Shear

parallel

to

grain

(lbf in–2)

Tension

perpendicular

to grain

(lbf in–2)

Side

hardness

(lbf)

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticityc

(106 lbf in–2)

Work 

to

maximum 

load 

(in-lbf in–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Hickory, pecan 

Bitternut Green 0.60 10,300 1.40 20.0 66 4,570 800 1,240 — — 

12% 0.66 17,100 1.79 18.2 66 9,040 1,680 — — — 

Nutmeg Green 0.56 9,100 1.29 22.8 54 3,980 760 1,030 — — 

12% 0.60 16,600 1.70 25.1 — 6,910 1,570 — — — 

Pecan Green 0.60 9,800 1.37 14.6 53 3,990 780 1,480 680 1,310

12% 0.66 13,700 1.73 13.8 44 7,850 1,720 2,080 — 1,820

Water Green 0.61 10,700 1.56 18.8 56 4,660 880 1,440 — — 

12% 0.62 17,800 2.02 19.3 53 8,600 1,550 — — — 

Hickory, trued

Mockernut Green 0.64 11,100 1.57 26.1 88 4,480 810 1,280 — 1,440

12% 0.72 19,200 2.22 22.6 77 8,940 1,730 1,740 — 1,970

Pignut Green 0.66 11,700 1.65 31.7 89 4,810 920 1,370 — 1,520

12% 0.75 20,100 2.26 30.4 74 9,190 1,980 2,150 — 2,140

Shagbark Green 0.64 11,000 1.57 23.7 74 4,580 840 1,520 — 1,460

12% 0.72 20,200 2.16 25.8 67 9,210 1,760 2,430 — 1,880

Shellbark Green 0.62 10,500 1.34 29.9 104 3,920 810 1,190 — 1,670

12% 0.69 18,100 1.89 23.6 88 8,000 1,800 2,110 — 1,810

Honeylocust Green 0.60 10,200 1.29 12.6 47 4,420 1,150 1,660 930 1,390

12% — 14,700 1.63 13.3 47 7,500 1,840 2,250 900 1,580

Locust, black Green 0.66 13,800 1.85 15.4 44 6,800 1,160 1,760 770 1,570

12% 0.69 19,400 2.05 18.4 57 10,180 1,830 2,480 640 1,700

Magnolia

Cucumbertree Green 0.44 7,400 1.56 10.0 30 3,140 330 990 440 520

12% 0.48 12,300 1.82 12.2 35 6,310 570 1,340 660 700

Southern Green 0.46 6,800 1.11 15.4 54 2,700 460 1,040 610 740

12% 0.50 11,200 1.40 12.8 29 5,460 860 1,530 740 1,020

Maple 

Bigleaf Green 0.44 7,400 1.10 8.7 23 3,240 450 1,110 600 620

12% 0.48 10,700 1.45 7.8 28 5,950 750 1,730 540 850

Black Green 0.52 7,900 1.33 12.8 48 3,270 600 1,130 720 840

12% 0.57 13,300 1.62 12.5 40 6,680 1,020 1,820 670 1,180

Red Green 0.49 7,700 1.39 11.4 32 3,280 400 1,150 — 700

12% 0.54 13,400 1.64 12.5 32 6,540 1,000 1,850 — 950

Silver Green 0.44 5,800 0.94 11.0 29 2,490 370 1,050 560 590

12% 0.47 8,900 1.14 8.3 25 5,220 740 1,480 500 700

Sugar Green 0.56 9,400 1.55 13.3 40 4,020 640 1,460 — 970

12% 0.63 15,800 1.83 16.5 39 7,830 1,470 2,330 — 1,450

Oak, red 

Black Green 0.56 8,200 1.18 12.2 40 3,470 710 1,220 — 1,060

12% 0.61 13,900 1.64 13.7 41 6,520 930 1,910 — 1,210

Cherrybark Green 0.61 10,800 1.79 14.7 54 4,620 760 1,320 800 1,240

12% 0.68 18,100 2.28 18.3 49 8,740 1,250 2,000 840 1,480

Laurel Green 0.56 7,900 1.39 11.2 39 3,170 570 1,180 770 1,000

12% 0.63 12,600 1.69 11.8 39 6,980 1,060 1,830 790 1,210

Northern red Green 0.56 8,300 1.35 13.2 44 3,440 610 1,210 750 1,000

12% 0.63 14,300 1.82 14.5 43 6,760 1,010 1,780 800 1,290

Pin Green 0.58 8,300 1.32 14.0 48 3,680 720 1,290 800 1,070

12% 0.63 14,000 1.73 14.8 45 6,820 1,020 2,080 1,050 1,510

Scarlet Green 0.60 10,400 1.48 15.0 54 4,090 830 1,410 700 1,200

12% 0.67 17,400 1.91 20.5 53 8,330 1,120 1,890 870 1,400

Southern red Green 0.52 6,900 1.14 8.0 29 3,030 550 930 480 860

12% 0.59 10,900 1.49 9.4 26 6,090 870 1,390 510 1,060

General Technical Report FPL–GTR–190
Chapter 5 Mechanical Properties of Wood

5–11

Table 5–3b. Strength properties of some commercially important woods grown in the United States (inch–pound)a—con.

Static bending 

Impact

bending

(in.) 

Compression

parallel

to grain

(lbf in–2)

Compression

perpendicular 

to grain 

(lbf in–2)

Shear

parallel

to

grain

(lbf in–2)

Tension

perpendicular

to grain

(lbf in–2)

Side

hardness

(lbf)

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticityc

(106 lbf in–2)

Work 

to

maximum 

load 

(in-lbf in–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Oak, red—con. 

Water Green 0.56 8,900 1.55 11.1 39 3,740 620 1,240 820 1,010 

12% 0.63 15,400 2.02 21.5 44 6,770 1,020 2,020 920 1,190 

Willow Green 0.56 7,400 1.29 8.8 35 3,000 610 1,180 760 980 

12% 0.69 14,500 1.90 14.6 42 7,040 1,130 1,650 — 1,460 

Oak, white 

Bur Green 0.58 7,200 0.88 10.7 44 3,290 680 1,350 800 1,110 

12% 0.64 10,300 1.03 9.8 29 6,060 1,200 1,820 680 1,370 

Chestnut Green 0.57 8,000 1.37 9.4 35 3,520 530 1,210 690 890 

12% 0.66 13,300 1.59 11.0 40 6,830 840 1,490 — 1,130 

Live Green 0.80 11,900 1.58 12.3 — 5,430 2,040 2,210 — — 

12% 0.88 18,400 1.98 18.9 — 8,900 2,840 2,660 — — 

Overcup Green 0.57 8,000 1.15 12.6 44 3,370 540 1,320 730 960 

12% 0.63 12,600 1.42 15.7 38 6,200 810 2,000 940 1,190 

Post Green 0.60 8,100 1.09 11.0 44 3,480 860 1,280 790 1,130 

12% 0.67 13,200 1.51 13.2 46 6,600 1,430 1,840 780 1,360 

Swamp chestnut Green 0.60 8,500 1.35 12.8 45 3,540 570 1,260 670 1,110 

12% 0.67 13,900 1.77 12.0 41 7,270 1,110 1,990 690 1,240 

Swamp white Green 0.64 9,900 1.59 14.5 50 4,360 760 1,300 860 1,160 

12% 0.72 17,700 2.05 19.2 49 8,600 1,190 2,000 830 1,620 

White Green 0.60 8,300 1.25 11.6 42 3,560 670 1,250 770 1,060 

12% 0.68 15,200 1.78 14.8 37 7,440 1,070 2,000 800 1,360 

Sassafras Green 0.42 6,000 0.91 7.1 — 2,730 370 950 — — 

12% 0.46 9,000 1.12 8.7 — 4,760 850 1,240 — — 

Sweetgum Green 0.46 7,100 1.20 10.1 36 3,040 370 990 540 600 

12% 0.52 12,500 1.64 11.9 32 6,320 620 1,600 760 850 

Sycamore, American Green 0.46 6,500 1.06 7.5 26 2,920 360 1,000 630 610 

12% 0.49 10,000 1.42 8.5 26 5,380 700 1,470 720 770 

Tanoak Green 0.58 10,500 1.55 13.4 — 4,650 — — — — 

12% — — — — — — — — — — 

Tupelo 

Black Green 0.46 7,000 1.03 8.0 30 3,040 480 1,100 570 640 

12% 0.50 9,600 1.20 6.2 22 5,520 930 1,340 500 810 

Water Green 0.46 7,300 1.05 8.3 30 3,370 480 1,190 600 710 

12% 0.50 9,600 1.26 6.9 23 5,920 870 1,590 700 880 

Walnut, Black Green 0.51 9,500 1.42 14.6 37 4,300 490 1,220 570 900 

12% 0.55 14,600 1.68 10.7 34 7,580 1,010 1,370 690 1,010 

Willow, Black Green 0.36 4,800 0.79 11.0 — 2,040 180 680 — — 

12% 0.39 7,800 1.01 8.8 — 4,100 430 1,250 — — 

Yellow-poplar Green 0.40 6,000 1.22 7.5 26 2,660 270 790 510 440 

12% 0.42 10,100 1.58 8.8 24 5,540 500 1,190 540 540 

Softwoods

Baldcypress Green 0.42 6,600 1.18 6.6 25 3,580 400 810 300 390 

12% 0.46 10,600 1.44 8.2 24 6,360 730 1,000 270 510 

Cedar 

Atlantic white Green 0.31 4,700 0.75 5.9 18 2,390 240 690 180 290 

12% 0.32 6,800 0.93 4.1 13 4,700 410 800 220 350 

Eastern redcedar Green 0.44 7,000 0.65 15.0 35 3,570 700 1,010 330 650 

12% 0.47 8,800 0.88 8.3 22 6,020 920 — — — 

Incense Green 0.35 6,200 0.84 6.4 17 3,150 370 830 280 390 

12% 0.37 8,000 1.04 5.4 17 5,200 590 880 270 470 

Northern White Green 0.29 4,200 0.64 5.7 15 1,990 230 620 240 230 

12% 0.31 6,500 0.80 4.8 12 3,960 310 850 240 320 
5–12

Table 5–3b. Strength properties of some commercially important woods grown in the United States (inch–pound)a—con.

Static bending 

Impact

bending

(in.) 

Compression

parallel

to grain

(lbf in–2)

Compression

perpendicular 

to grain 

(lbf in–2)

Shear

parallel

to

grain

(lbf in–2)

Tension

perpendicular

to grain

(lbf in–2)

Side

hardness

(lbf)

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticityc

(106 lbf in–2)

Work 

to

maximum 

load 

(in-lbf in–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Cedar—con. 

Port-Orford Green 0.39 6,600 1.30 7.4 21 3,140 300 840 180 380 

12% 0.43 12,700 1.70 9.1 28 6,250 720 1,370 400 630 

Western redcedar Green 0.31 5,200 0.94 5.0 17 2,770 240 770 230 260 

12% 0.32 7,500 1.11 5.8 17 4,560 460 990 220 350 

Yellow Green 0.42 6,400 1.14 9.2 27 3,050 350 840 330 440 

12% 0.44 11,100 1.42 10.4 29 6,310 620 1,130 360 580 

Douglas-fire

Coast Green 0.45 7,700 1.56 7.6 26 3,780 380 900 300 500 

12% 0.48 12,400 1.95 9.9 31 7,230 800 1,130 340 710 

Interior West Green 0.46 7,700 1.51 7.2 26 3,870 420 940 290 510 

12% 0.50 12,600 1.83 10.6 32 7,430 760 1,290 350 660 

Interior North Green 0.45 7,400 1.41 8.1 22 3,470 360 950 340 420 

12% 0.48 13,100 1.79 10.5 26 6,900 770 1,400 390 600 

Interior South Green 0.43 6,800 1.16 8.0 15 3,110 340 950 250 360 

12% 0.46 11,900 1.49 9.0 20 6,230 740 1,510 330 510 

Fir

Balsam Green 0.33 5,500 1.25 4.7 16 2,630 190 660 180 290 

12% 0.35 9,200 1.45 5.1 20 5,280 400 940 180 380 

California red Green 0.36 5,800 1.17 6.4 21 2,760 330 770 380 360 

12% 0.38 10,500 1.50 8.9 24 5,460 610 1,040 390 500 

Grand Green 0.35 5,800 1.25 5.6 22 2,940 270 740 240 360 

12% 0.37 8,900 1.57 7.5 28 5,290 500 900 240 490 

Noble Green 0.37 6,200 1.38 6.0 19 3,010 270 800 230 290 

12% 0.39 10,700 1.72 8.8 23 6,100 520 1,050 220 410 

Pacific silver Green 0.40 6,400 1.42 6.0 21 3,140 220 750 240 310 

12% 0.43 11,000 1.76 9.3 24 6,410 450 1,220 — 430 

Subalpine Green 0.31 4,900 1.05 — — 2,300 190 700 — 260 

12% 0.32 8,600 1.29 — — 4,860 390 1,070 — 350 

White Green 0.37 5,900 1.16 5.6 22 2,900 280 760 300 340 

12% 0.39 9,800 1.50 7.2 20 5,800 530 1,100 300 480 

Hemlock

Eastern Green 0.38 6,400 1.07 6.7 21 3,080 360 850 230 400 

12% 0.40 8,900 1.20 6.8 21 5,410 650 1,060 — 500 

Mountain Green 0.42 6,300 1.04 11.0 32 2,880 370 930 330 470 

12% 0.45 11,500 1.33 10.4 32 6,440 860 1,540 — 680 

Western Green 0.42 6,600 1.31 6.9 22 3,360 280 860 290 410 

12% 0.45 11,300 1.63 8.3 23 7,200 550 1,290 340 540 

Larch, western Green 0.48 7,700 1.46 10.3 29 3,760 400 870 330 510 

12% 0.52 13,000 1.87 12.6 35 7,620 930 1,360 430 830 

Pine

Eastern white Green 0.34 4,900 0.99 5.2 17 2,440 220 680 250 290 

12% 0.35 8,600 1.24 6.8 18 4,800 440 900 310 380 

Jack Green 0.40 6,000 1.07 7.2 26 2,950 300 750 360 400 

12% 0.43 9,900 1.35 8.3 27 5,660 580 1,170 420 570 

Loblolly Green 0.47 7,300 1.40 8.2 30 3,510 390 860 260 450 

12% 0.51 12,800 1.79 10.4 30 7,130 790 1,390 470 690 

Lodgepole Green 0.38 5,500 1.08 5.6 20 2,610 250 680 220 330 

12% 0.41 9,400 1.34 6.8 20 5,370 610 880 290 480 

Longleaf Green 0.54 8,500 1.59 8.9 35 4,320 480 1,040 330 590 

12% 0.59 14,500 1.98 11.8 34 8,470 960 1,510 470 870 

Pitch Green 0.47 6,800 1.20 9.2 — 2,950 360 860 — — 

12% 0.52 10,800 1.43 9.2 — 5,940 820 1,360 — — 

General Technical Report FPL–GTR–190
Chapter 5 Mechanical Properties of Wood

5–13

Table 5–3b. Strength properties of some commercially important woods grown in the United States (inch–pound)a—con.

Static bending 

Impact

bending

(in.) 

Compression

parallel

to grain

(lbf in–2)

Compression

perpendicular 

to grain 

(lbf in–2)

Shear

parallel

to

grain 

(lbf in–2)

Tension

perpendicular

to grain

(lbf in–2)

Side

hardness

(lbf)

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticityc

(106 lbf in–2)

Work 

to

maximum 

load 

(in-lbf in–3)

Common species 

names

Moisture

content 

Specific 

gravityb

Pine—con.

Pond Green 0.51 7,400 1.28 7.5 — 3,660 440 940 — — 

12% 0.56 11,600 1.75 8.6 — 7,540 910 1,380 — — 

Ponderosa Green 0.38 5,100 1.00 5.2 21 2,450 280 700 310 320 

12% 0.40 9,400 1.29 7.1 19 5,320 580 1,130 420 460 

Red Green 0.41 5,800 1.28 6.1 26 2,730 260 690 300 340 

12% 0.46 11,000 1.63 9.9 26 6,070 600 1,210 460 560 

Sand Green 0.46 7,500 1.02 9.6 — 3,440 450 1,140 — — 

12% 0.48 11,600 1.41 9.6 — 6,920 836 — — — 

Shortleaf Green 0.47 7,400 1.39 8.2 30 3,530 350 910 320 440 

12% 0.51 13,100 1.75 11.0 33 7,270 820 1,390 470 690 

Slash Green 0.54 8,700 1.53 9.6 — 3,820 530 960 — — 

12% 0.59 16,300 1.98 13.2 — 8,140 1,020 1,680 — — 

Spruce Green 0.41 6,000 1.00 — — 2,840 280 900 — 450 

12% 0.44 10,400 1.23 — — 5,650 730 1,490 — 660 

Sugar Green 0.34 4,900 1.03 5.4 17 2,460 210 720 270 270 

12% 0.36 8,200 1.19 5.5 18 4,460 500 1,130 350 380 

Virginia Green 0.45 7,300 1.22 10.9 34 3,420 390 890 400 540 

12% 0.48 13,000 1.52 13.7 32 6,710 910 1,350 380 740 

Western white Green 0.35 4,700 1.19 5.0 19 2,430 190 680 260 260 

12% 0.38 9,700 1.46 8.8 23 5,040 470 1,040 — 420 

Redwood

Old-growth Green 0.38 7,500 1.18 7.4 21 4,200 420 800 260 410 

12% 0.40 10,000 1.34 6.9 19 6,150 700 940 240 480 

Young-growth Green 0.34 5,900 0.96 5.7 16 3,110 270 890 300 350 

12% 0.35 7,900 1.10 5.2 15 5,220 520 1,110 250 420 

Spruce 

Black Green 0.38 6,100 1.38 7.4 24 2,840 240 740 100 340 

12% 0.42 10,800 1.61 10.5 23 5,960 550 1,230 — 530 

Engelmann Green 0.33 4,700 1.03 5.1 16 2,180 200 640 240 260 

12% 0.35 9,300 1.30 6.4 18 4,480 410 1,200 350 390 

Red Green 0.37 6,000 1.33 6.9 18 2,720 260 750 220 340 

12% 0.40 10,800 1.66 8.4 25 5,540 550 1,290 350 530 

Sitka Green 0.37 5,700 1.23 6.3 24 2,670 280 760 250 350 

12% 0.40 10,200 1.57 9.4 25 5,610 580 1,150 370 510 

White Green 0.33 5,000 1.14 6.0 22 2,350 210 640 220 270 

12% 0.36 9,400 1.43 7.7 20 5,180 430 970 360 410 

Tamarack Green 0.49 7,200 1.24 7.2 28 3,480 390 860 260 380 

12% 0.53 11,600 1.64 7.1 23 7,160 800 1,280 400 590 a

Results of tests on clear specimens in the green and air-dried conditions. Definition of properties: impact bending is height of drop that causes complete failure, 

using 0.71-kg (50-lb) hammer; compression parallel to grain is also called maximum crushing strength; compression perpendicular to grain is fiber stress at 

proportional limit; shear is maximum shearing strength; tension is maximum tensile strength; and side hardness is hardness measured when load is perpendicular 

to grain. 

b

Specific gravity is based on weight when ovendry and volume when green or at 12% moisture content. c

Modulus of elasticity measured from a simply supported, center-loaded beam, on a span depth ratio of 14/1. To correct for shear deflection, the modulus can be 

increased by 10%. 

d

Values for side hardness of the true hickories are from Bendtsen and Ethington (1975). e

Coast Douglas-fir is defined as Douglas-fir growing in Oregon and Washington State west of the Cascade Mountains summit. Interior West includes California 

and all counties in Oregon and Washington east of, but adjacent to, the Cascade summit; Interior North, the remainder of Oregon and Washington plus Idaho, 

Montana, and Wyoming; and Interior South, Utah, Colorado, Arizona, and New Mexico. 
5–14

Table 5–4a. Mechanical properties of some commercially important woods grown in Canada and imported 

into the United States (metric)a

Static bending Compression

parallel

to grain 

(kPa)

Compression 

perpendicular 

to grain 

(kPa)

Shear 

parallel

to grain

(kPa)

Common species 

names

Moisture

content 

Specific 

gravity

Modulus of

rupture

(kPa)

Modulus of 

elasticity 

(MPa)

Hardwoods 

Aspen

Quaking Green 0.37 38,000 9,000 16,200 1,400 5,000 

12% 68,000 11,200 36,300 3,500 6,800 

Big-toothed Green 0.39 36,000 7,400 16,500 1,400 5,400 

12% 66,000 8,700 32,800 3,200 7,600 

Cottonwood

Balsam, poplar Green 0.37 34,000 7,900 14,600 1,200 4,600 

12% 70,000 11,500 34,600 2,900 6,100 

Black Green 0.30 28,000 6,700 12,800 700 3,900 

12% 49,000 8,800 27,700 1,800 5,900 

Eastern Green 0.35 32,000 6,000 13,600 1,400 5,300 

12% 52,000 7,800 26,500 3,200 8,000 

Softwoods 

Cedar 

Northern white Green 0.30 27,000 3,600 13,000 1,400 4,600 

12% 42,000 4,300 24,800 2,700 6,900 

Western redcedar Green 0.31 36,000 7,200 19,200 1,900 4,800 

12% 54,000 8,200 29,600 3,400 5,600 

Yellow Green 0.42 46,000 9,200 22,300 2,400 6,100 

12% 80,000 11,000 45,800 4,800 9,200 

Douglas-fir Green 0.45 52,000 11,100 24,900 3,200 6,300 

12% 88,000 13,600 50,000 6,000 9,500 

Fir

Subalpine Green 0.33 36,000 8,700 17,200 1,800 4,700 

12% 56,000 10,200 36,400 3,700 6,800 

Pacific silver Green 0.36 38,000 9,300 19,100 1,600 4,900 

12% 69,000 11,300 40,900 3,600 7,500 

Balsam Green 0.34 36,000 7,800 16,800 1,600 4,700 

12% 59,000 9,600 34,300 3,200 6,300 

Hemlock

Eastern Green 0.40 47,000 8,800 23,600 2,800 6,300 

12% 67,000 9,700 41,200 4,300 8,700 

Western Green 0.41 48,000 10,200 24,700 2,600 5,200 

12% 81,000 12,300 46,700 4,600 6,500 

Larch, western Green 0.55 60,000 11,400 30,500 3,600 6,300 

12% 107,000 14,300 61,000 7,300 9,200 

Pine

Eastern white Green 0.36 35,000 8,100 17,900 1,600 4,400 

12% 66,000 9,400 36,000 3,400 6,100 

Jack Green 0.42 43,000 8,100 20,300 2,300 5,600 

12% 78,000 10,200 40,500 5,700 8,200 

Lodgepole Green 0.40 39,000 8,800 19,700 1,900 5,000 

12% 76,000 10,900 43,200 3,600 8,500 

Ponderosa Green 0.44 39,000 7,800 19,600 2,400 5,000 

12% 73,000 9,500 42,300 5,200 7,000 

Red Green 0.39 34,000 7,400 16,300 1,900 4,900 

12% 70,000 9,500 37,900 5,200 7,500 

Western white Green 0.36 33,000 8,200 17,400 1,600 4,500 

12% 64,100 10,100 36,100 3,200 6,300 

Spruce 

Black Green 0.41 41,000 9,100 19,000 2,100 5,500 

12% 79,000 10,500 41,600 4,300 8,600 

 

 

 

 

 

 

General Technical Report FPL–GTR–190
of clear wood parallel to grain. Table 5–7 lists average tensile strength values for a limited number of specimens of a 

few species. In the absence of sufficient tension test data, 

modulus of rupture values are sometimes substituted for 

tensile strength of small, clear, straight-grained pieces of 

wood. The modulus of rupture is considered to be a low or 

conservative estimate of tensile strength for clear specimens 

(this is not true for lumber).

Less Common Properties

Strength properties less commonly measured in clear wood 

include torsion, toughness, rolling shear, and fracture 

toughness. Other properties involving time under load 

include creep, creep rupture or duration of load, and fatigue 

strength.

Torsion strength—Resistance to twisting about a longitudinal axis. For solid wood members, torsional shear strength 

may be taken as shear strength parallel to grain. Two-thirds 

of the value for torsional shear strength may be used as an 

estimate of the torsional shear stress at the proportional 

limit.

Toughness—Energy required to cause rapid complete failure in a centrally loaded bending specimen. Tables 5–8 and 

5–9 give average toughness values for samples of a few 

hardwood and softwood species. Average coefficients of 

variation for toughness as determined from approximately 

50 species are shown in Table 5–6.

Creep and duration of load—Time-dependent deformation 

of wood under load. If the load is sufficiently high and the 

duration of load is long, failure (creep–rupture) will eventually occur. The time required to reach rupture is commonly 

called duration of load. Duration of load is an important factor in setting design values for wood. Creep and duration of 

load are described in later sections of this chapter.

Fatigue—Resistance to failure under specific combinations of cyclic loading conditions: frequency and number 

of cycles, maximum stress, ratio of maximum to minimum 

stress, and other less-important factors. The main factors 

affecting fatigue in wood are discussed later in this chapter. 

The discussion also includes interpretation of fatigue data 

and information on fatigue as a function of the service 

environment.

Rolling shear strength—Shear strength of wood where 

shearing force is in a longitudinal plane and is acting perpendicular to the grain. Few test values of rolling shear in 

solid wood have been reported. In limited tests, rolling shear 

strength averaged 18% to 28% of parallel-to-grain shear 

values. Rolling shear strength is about the same in the longitudinal–radial and longitudinal–tangential planes.

Nanoindentation hardness—This type of hardness measurement is conducted at the nanometer scale (the scale of 

the cell wall). Nanoindentation uses an extremely small 

indenter of a hard material and specified shape (usually a 

pyramid) to press into the surface with sufficient force that 

the wood deforms. The load and deformation history is used 

to develop mechanical property information. Nanoindention hardness provides a method for describing a material’s 

response to various applied loading conditions at a scale that 

may explain differences in wood cell structures and help 

predict material performance after chemical treatments have 

been applied (Moon and others 2006).

Fracture toughness—Ability of wood to withstand flaws 

that initiate failure. Measurement of fracture toughness 

helps identify the length of critical flaws that initiate failure 

in materials. 

To date, there is no standard test method for determining 

fracture toughness in wood. Three types of stress fields, and 

Chapter 5 Mechanical Properties of Wood

5–15

Table 5–4a. Mechanical properties of some commercially important woods grown in Canada and imported 

 into the United States (metric)a

—con.

Common species 

names

Moisture

content 

Specific 

gravity

Static bending Compression

parallel

to grain 

(kPa)

Compression 

perpendicular 

to grain 

(kPa)

Shear 

parallel

to grain

(kPa)

Modulus of

rupture

(kPa)

Modulus of 

elasticity 

(MPa)

Engelmann Green 0.38 39,000 8,600 19,400 1,900 4,800 

12% 70,000 10,700 42,400 3,700 7,600 

Red Green 0.38 41,000 9,100 19,400 1,900 5,600 

12% 71,000 11,000 38,500 3,800 9,200 

Sitka Green 0.35 37,000 9,400 17,600 2,000 4,300 

12% 70,000 11,200 37,800 4,100 6,800 

White Green 0.35 35,000 7,900 17,000 1,600 4,600 

12% 63,000 10,000 37,000 3,400 6,800 

Tamarack Green 0.48 47,000 8,600 21,600 2,800 6,300 

12% 76,000 9,400 44,900 6,200 9,000 a

Results of tests on clear, straight-grained specimens. Property values based on ASTM Standard D 2555–88. Information on additional

properties can be obtained from Department of Forestry, Canada, Publication No. 1104. For each species, values in the first line are from tests 

of green material; those in the second line are adjusted from the green condition to 12% moisture content using dry to green clear wood 

property ratios as reported in ASTM D 2555–88. Specific gravity is based on weight when ovendry and volume when green. 
5–16

Table 5–4b. Mechanical properties of some commercially important woods grown in Canada and imported 

into the United States (inch–pound)a

Static bending Compression

parallel to 

grain 

(lbf in–2)

Compression 

perpendicular 

to grain 

(lbf in–2)

Shear 

parallel to

grain 

(lbf in–2)

Common species 

names

Moisture

content 

Specific 

gravity

Modulus of 

rupture

(lbf in–2)

Modulus of 

elasticity 

(106 lbf in–2)

Hardwoods 

Aspen

Quaking Green 0.37 5,500 1.31 2,350 200 720 

12% 9,800 1.63 5,260 510 980 

Bigtooth Green 0.39 5,300 1.08 2,390 210 790 

12% 9,500 1.26 4,760 470 1,100 

Cottonwood

Balsam, poplar Green 0.37 5,000 1.15 2,110 180 670 

12% 10,100 1.67 5,020 420 890 

Black Green 0.30 4,100 0.97 1,860 100 560 

12% 7,100 1.28 4,020 260 860 

Eastern Green 0.35 4,700 0.87 1,970 210 770 

12% 7,500 1.13 3,840 470 1,160 

Softwoods 

Cedar 

Northern white Green 0.30 3,900 0.52 1,890 200 660 

12% 6,100 0.63 3,590 390 1,000 

Western redcedar Green 0.31 5,300 1.05 2,780 280 700 

12% 7,800 1.19 4,290 500 810 

Yellow Green 0.42 6,600 1.34 3,240 350 880 

12% 11,600 1.59 6,640 690 1,340 

Douglas-fir Green 0.45 7,500 1.61 3,610 460 920 

12% 12,800 1.97 7,260 870 1,380 

Fir

Balsam Green 0.34 5,300 1.13 2,440 240 680 

12% 8,500 1.40 4,980 460 910 

Pacific silver Green 0.36 5,500 1.35 2,770 230 710 

12% 10,000 1.64 5,930 520 1,190 

Subalpine Green 0.33 5,200 1.26 2,500 260 680 

12% 8,200 1.48 5,280 540 980 

Hemlock

Eastern Green 0.40 6,800 1.27 3,430 400 910 

12% 9,700 1.41 5,970 630 1,260 

Western Green 0.41 7,000 1.48 3,580 370 750 

12% 11,800 1.79 6,770 660 940 

Larch, western Green 0.55 8,700 1.65 4,420 520 920 

12% 15,500 2.08 8,840 1,060 1,340 

Pine

Eastern white Green 0.36 5,100 1.18 2,590 240 640 

12% 9,500 1.36 5,230 490 880 

Jack Green 0.42 6,300 1.17 2,950 340 820 

12% 11,300 1.48 5,870 830 1,190 

Lodgepole Green 0.40 5,600 1.27 2,860 280 720 

12% 11,000 1.58 6,260 530 1,240 

Ponderosa Green 0.44 5,700 1.13 2,840 350 720 

12% 10,600 1.38 6,130 760 1,020 

Red Green 0.39 5,000 1.07 2,370 280 710 

12% 10,100 1.38 5,500 720 1,090 

Western white Green 0.36 4,800 1.19 2,520 240 650 

12% 9,300 1.46 5,240 470 920 

Spruce 

Black Green 0.41 5,900 1.32 2,760 300 800 

12% 11,400 1.52 6,040 620 1,250 

Engelmann Green 0.38 5,700 1.25 2,810 270 700 

12% 10,100 1.55 6,150 540 1,100 

Red Green 0.38 5,900 1.32 2,810 270 810 

12% 10,300 1.60 5,590 550 1,330 

 

 

 

 

General Technical Report FPL–GTR–190
associated stress intensity factors, can be defined at a crack 

tip: opening mode (I), forward shear mode (II), and transverse shear mode (III) (Fig. 5–2a). A crack may lie in one of 

these three planes and may propagate in one of two directions in each plane. This gives rise to six crack-propagation 

systems (RL, TL, LR, TR, LT, and RT) (Fig. 5–2b). Of these 

crack-propagation systems, four systems are of practical 

importance: RL, TL, TR, and RT. Each of these four systems 

allow for propagation of a crack along the lower strength 

path parallel to the grain. The RL and TL orientations in 

wood (where R or T is perpendicular to the crack plane 

and L is the direction in which the crack propagates) will 

predominate as a result of the low strength and stiffness of 

wood perpendicular to the grain. It is therefore one of these 

two orientations that is most often tested. Values for mode I 

fracture toughness range from 220 to 550 kPa m1/2 (200 to 

500 lbf in–2 in1/2) and for mode II range from 1,650 to 

2,400 kPa m1/2 (1,500 to 2,200 lbf in–2 in1/2). Table 5–10 

summarizes selected mode I and mode II test results at 10% 

to 12% moisture content available in the literature. The 

limited information available on moisture content effects on 

fracture toughness suggests that fracture toughness is either 

insensitive to moisture content or increases as the material 

dries, reaching a maximum between 6% and 15% moisture 

content; fracture toughness then decreases with further 

drying.

Vibration Properties

The vibration properties of primary interest in structural 

materials are speed of sound and internal friction (damping 

capacity).

Speed of Sound

The speed of sound in a structural material is a function of 

the modulus of elasticity and density. In wood, the speed of 

sound also varies with grain direction because the transverse 

modulus of elasticity is much less than the longitudinal 

value (as little as 1/20); the speed of sound across the grain 

is about one-fifth to one-third of the longitudinal value. 

For example, a piece of wood with a longitudinal modulus 

of elasticity of 12.4 GPa (1.8 × 106 lbf in–2) and density of 

480 kg m–3 (30 lb ft–3) would have a speed of sound in the 

longitudinal direction of about 3,800 m s–1 (12,500 ft s–1). 

In the transverse direction, modulus of elasticity would be 

about 690 MPa (100 × 103 lbf in–2) and the speed of sound 

approximately 890 m s–1 (2,900 ft s–1).

The speed of sound decreases with increasing temperature 

or moisture content in proportion to the influence of these 

variables on modulus of elasticity and density. The speed of 

sound decreases slightly with increasing frequency and amplitude of vibration, although for most common applications 

this effect is too small to be significant. There is no recognized independent effect of species on the speed of sound. 

Variability in the speed of sound in wood is directly related 

to the variability of modulus of elasticity and density.

Internal Friction

When solid material is strained, some mechanical energy 

is dissipated as heat. Internal friction is the term used to 

denote the mechanism that causes this energy dissipation. 

Chapter 5 Mechanical Properties of Wood

5–17

Table 5–4b. Mechanical properties of some commercially important woods grown in Canada and imported 

 into the United States (inch–pound)a

—con.

Common species 

names

Moisture

content Specific 

gravity

Static bending Compression

parallel to 

grain 

(lbf in–2)

Compression 

perpendicular 

to grain 

(lbf in–2)

Shear 

parallel to

grain 

(lbf in–2)

Modulus of 

rupture

(lbf in–2)

Modulus of 

elasticity 

(106 lbf in–2)

Sitka Green 0.35 5,400 1.37 2,560 290 630 

12% 10,100 1.63 5,480 590 980 

White Green 0.35 5,100 1.15 2,470 240 670 

12% 9,100 1.45 5,360 500 980 

Tamarack Green 0.48 6,800 1.24 3,130 410 920 

12% 11,000 1.36 6,510 900 1,300 a

Results of tests on clear, straight-grained specimens. Property values based on ASTM Standard D 2555–88. Information on additional properties 

can be obtained from Department of Forestry, Canada, Publication No. 1104. For each species, values in the first line are from tests of green 

material; those in the second line are adjusted from the green condition to 12% moisture content using dry to green clear wood property ratios as 

reported in ASTM D 2555–88. Specific gravity is based on weight when ovendry and volume when green. 

Figure 5–2. Possible crack propagation systems for 

wood.
5–18

Table 5–5a. Mechanical properties of some woods imported into the United States other than Canadian imports 

(metric)a

Static bending Compression

parallel

to grain

(kPa)

Shear 

parallel

to grain 

(kPa)

Side

hardness 

(N)

Sample

originb

Common and 

botanical names 

of species 

Moisture

content 

Specific

gravity

Modulus

of

rupture

(kPa)

Modulus

of

elasticity

(MPa)

Work to 

maximum

load 

(kJ m–3)

Afrormosia (Pericopsis elata) Green 0.61 102,000 12,200 135 51,600 11,500 7,100 AF 

12% 126,900 13,400 127 68,500 14,400 6,900 

Albarco (Cariniana spp.) Green 0.48 — — — — — — AM 

12% 100,000 10,300 95 47,000 15,900 4,500 

Andiroba (Carapa guianensis) Green 0.54 71,000 11,700 68 33,000 8,400 3,900 AM 

12% — 106,900 13,800 97 56,000 10,400 5,000 

Angelin (Andira inermis) Green 0.65 — — — — — — AF 

12% 124,100 17,200 — 63,400 12,700 7,800 

Angelique (Dicorynia

 guianensis)

Green 0.6 78,600 12,700 83 38,500 9,200 4,900 AM 

12% — 120,000 15,100 105 60,500 11,400 5,700 

Avodire (Turraeanthus

 africanus)

Green 0.48 — — — — — — AF 

12% 87,600 10,300 65 49,300 14,000 4,800 

Azobe (Lophira alata) Green 0.87 116,500 14,900 83 65,600 14,100 12,900 AF 

12% 168,900 17,000 — 86,900 20,400 14,900 

Balsa (Ochroma pyramidale) Green 0.16 — — — — — — AM 

12% 21,600 3,400 14 14,900 2,100 — 

Banak (Virola spp.) Green 0.42 38,600 11,300 28 16,500 5,000 1,400 AM 

12% — 75,200 14,100 69 35,400 6,800 2,300 

Benge (Guibourtia arnoldiana) Green 0.65 — — — — — — AF 

12% 147,500 14,100 — 78,600 14,400 7,800 

Bubinga (Guibourtia spp.) Green 0.71 — — — — — — AF 

12% 155,800 17,100 — 72,400 21,400 12,000 

Bulletwood (Manilkara Green 0.85 119,300 18,600 94 59,900 13,100 9,900 AM 

bidentata) 12% 188,200 23,800 197 80,300 17,200 14,200 

Cativo (Prioria copaifera) Green 0.4 40,700 6,500 37 17,000 5,900 2,000 AM 

12% — 59,300 7,700 50 29,600 7,300 2,800 

Ceiba (Ceiba pentandra) Green 0.25 15,200 2,800 8 7,300 2,400 1,000 AM 

12% 29,600 3,700 19 16,400 3,800 1,100 

Courbaril (Hymenaea Green 0.71 88,900 12,700 101 40,000 12,200 8,800 AM 

courbaril) 12% — 133,800 14,900 121 65,600 17,000 10,500 

Cuangare (Dialyanthera spp.) Green 0.31 27,600 7,000 — 14,300 4,100 1,000 AM 

12% 50,300 10,500 — 32,800 5,700 1,700 

Cypress, Mexican (Cupressus Green 0.39 42,700 6,300 — 19,900 6,600 1,500 AF 

lustianica) 12% 71,000 7,000 — 37,100 10,900 2,000 

Degame (Calycophyllum Green 0.67 98,600 13,300 128 42,700 11,400 7,300 AM 

candidissimum) 12% 153,800 15,700 186 66,700 14,600 8,600 

Determa (Ocotea rubra) Green 0.52 53,800 10,100 33 25,900 5,900 2,300 AM 

12% 72,400 12,500 44 40,000 6,800 2,900 

Ekop (Tetraberlinia Green 0.6 — — — — — — AF 

tubmaniana) 12% 115,100 15,200 — 62,100 — — 

Goncalo alves (Astronium Green 0.84 83,400 13,400 46 45,400 12,100 8,500 AM 

graveolens) 12% — 114,500 15,400 72 71,200 13,500 9,600 

Greenheart (Chlorocardium Green 0.8 133,100 17,000 72 64,700 13,300 8,400 AM 

rodiei) 12% 171,700 22,400 175 86,300 18,100 10,500 

Hura (Hura crepitans) Green 0.38 43,400 7,200 41 19,200 5,700 2,000 AM 

12% 60,000 8,100 46 33,100 7,400 2,400 

General Technical Report FPL–GTR–190
Chapter 5 Mechanical Properties of Wood

5–19

Table 5–5a. Mechanical properties of some woods imported into the United States other than Canadian imports 

(metric)a

—con. 

Static bending Compression

parallel

to grain 

(kPa)

Shear 

parallel

to grain 

(kPa)

Side

hardness 

(N)

Sample

originb

Common and botanical 

names of species 

Moisture

content 

Specific

gravity

Modulus

of

rupture

(kPa)

Modulus

of

elasticity

(MPa)

Work to

maximum

load 

(kJ m–3)

Llomba (Pycnanthus Green 0.40 37,900 7,900 — 20,000 5,800 2,100 AF 

angolensis) 12% 68,300 11,000 — 38,300 8,900 2,700 

Ipe (Tabebuia spp., Green 0.92 155,800 20,100 190 71,400 14,600 13,600 AM 

lapacho group) 12% 175,100 21,600 152 89,700 14,200 16,400 

Iroko (Chlorophora spp.) Green 0.54 70,300 8,900 72 33,900 9,000 4,800 AF 

12% 85,500 10,100 62 52,300 12,400 5,600 

Jarrah (Eucalyptus marginata) Green 0.67 68,300 10,200 — 35,800 9,100 5,700 AS 

12% — 111,700 13,000 — 61,200 14,700 8,500 

Jelutong (Dyera costulata) Green 0.36 38,600 8,000 39 21,000 5,200 1,500 AS 

15% 50,300 8,100 44 27,000 5,800 1,700 

Kaneelhart (Licaria spp.) Green 0.96 153,800 26,300 94 92,300 11,600 9,800 AM 

12% 206,200 28,000 121 120,000 13,600 12,900 

Kapur (Dryobalanops spp.) Green 0.64 88,300 11,000 108 42,900 8,100 4,400 AS 

12% 126,200 13,000 130 69,600 13,700 5,500 

Karri (Eucalyptus diversicolor) Green 0.82 77,200 13,400 80 37,600 10,400 6,000 AS 

12% 139,000 17,900 175 74,500 16,700 9,100 

Kempas (Koompassia Green 0.71 100,000 16,600 84 54,700 10,100 6,600 AS 

malaccensis) 12% 122,000 18,500 106 65,600 12,300 7,600 

Keruing (Dipterocarpus spp.) Green 0.69 82,000 11,800 96 39,200 8,100 4,700 AS 

12% 137,200 14,300 162 72,400 14,300 5,600 

Lignumvitae (Guaiacum spp.) Green 1.05 — — — — — — AM 

12% — — — — 78,600 — 20,000 

Limba (Terminalia superba) Green 0.38 41,400 5,300 53 19,200 6,100 1,800 AF 

12% 60,700 7,000 61 32,600 9,700 2,200 

Macawood (Platymiscium spp.) Green 0.94 153,800 20,800 — 72,700 12,700 14,800 AM 

12% 190,300 22,100 — 111,000 17,500 14,000 

Mahogany, African Green 0.42 51,000 7,900 49 25,700 6,400 2,800 AF 

(Khaya spp.) 12% 73,800 9,700 57 44,500 10,300 3,700 

Mahogany, true Green 0.45 62,100 9,200 63 29,900 8,500 3,300 AM 

 (Swietenia macrophylla) 12% — 79,300 10,300 52 46,700 8,500 3,600 

Manbarklak (Eschweilera spp.) Green 0.87 117,900 18,600 120 50,600 11,200 10,100 AM 

12% 182,700 21,600 230 77,300 14,300 15,500 

Manni (Symphonia globulifera) Green 0.58 77,200 13,500 77 35,600 7,900 4,200 AM 

12% 116,500 17,000 114 60,800 9,800 5,000 

Marishballi (Lincania spp.) Green 0.88 117,900 20,200 92 52,300 11,200 10,000 AM 

12% 191,000 23,000 98 92,300 12,100 15,900 

Merbau (Intsia spp.) Green 0.64 88,900 13,900 88 46,700 10,800 6,100 AS 

15% — 115,800 15,400 102 58,200 12,500 6,700 

Mersawa (Anisoptera spp.) Green 0.52 55,200 12,200 — 27,300 5,100 3,900 AS 

12% 95,100 15,700 — 50,800 6,100 5,700 

Mora (Mora spp.) Green 0.78 86,900 16,100 93 44,100 9,700 6,400 AM 

12% 152,400 20,400 128 81,600 13,100 10,200 

Oak (Quercus spp.) Green 0.76 — — — — — — AM 

12% 158,600 20,800 114 — — 11,100 

Obeche (Triplochiton Green 0.3 35,200 5,000 43 17,700 4,600 1,900 AF 

scleroxylon) 12% 51,000 5,900 48 27,100 6,800 1,900 
5–20

Table 5–5a. Mechanical properties of some woods imported into the United States other than Canadian imports 

(metric)a

—con.

Static bending Compression

parallel

to grain 

(kPa)

Shear 

parallel

to grain 

(kPa)

Side

hardness 

(N)

Sample

originb

Common and botanical 

names of species 

Moisture

content 

Specific

gravity

Modulus

of

rupture

(kPa)

Modulus of

elasticity

(MPa)

Work to 

maximum

load 

(kJ m–3)

Okoume (Aucoumea Green 0.33 — — — — — — AF 

klaineana) 12% 51,000 7,900 — 27,400 6,700 1,700 

Opepe (Nauclea diderrichii) Green 0.63 93,800 11,900 84 51,600 13,100 6,800 AF 

12% 120,000 13,400 99 71,700 17,100 7,300 

Ovangkol (Guibourtia ehie) Green 0.67 — — — — — — AF 

12% 116,500 17,700 — 57,200 — — 

Para-angelim (Hymenolobium Green 0.63 100,700 13,400 88 51,400 11,000 7,700 AM 

excelsum) 12% 121,300 14,100 110 62,000 13,900 7,700 

Parana-pine (Araucaria Green 0.46 49,600 9,300 67 27,600 6,700 2,500 AM 

augustifolia) 12% — 93,100 11,100 84 52,800 11,900 3,500 

Pau marfim (Balfourodendron Green 0.73 99,300 11,400 — 41,900 — — AM 

riedelianum) 15% 130,300 — — 56,500 — — 

Peroba de campos Green 0.62 — — — — — — AM 

(Paratecoma peroba) 12% 106,200 12,200 70 61,200 14,700 7,100 

Peroba rosa (Aspidosperma Green 0.66 75,200 8,900 72 38,200 13,000 7,000 AM 

spp., peroba group) 12% 83,400 10,500 63 54,600 17,200 7,700 

Pilon (Hyeronima spp.) Green 0.65 73,800 13,000 57 34,200 8,300 5,400 AM 

12% 125,500 15,700 83 66,300 11,900 7,600 

Pine, Caribbean (Pinus Green 0.68 77,200 13,000 74 33,800 8,100 4,400 AM 

 caribaea) 12% — 115,100 15,400 119 58,900 14,400 5,500 

Pine, ocote (Pinus oocarpa) Green 0.55 55,200 12,000 48 25,400 7,200 2,600 AM 

12% — 102,700 15,500 75 53,000 11,900 4,000 

Pine, radiata (Pinus radiata) Green 0.42 42,100 8,100 — 19,200 5,200 2,100 AS 

12% — 80,700 10,200 — 41,900 11,000 3,300 

Piquia (Caryocar spp.) Green 0.72 85,500 12,500 58 43,400 11,300 7,700 AM 

12% 117,200 14,900 109 58,000 13,700 7,700 

Primavera (Tabebuia Green 0.4 49,600 6,800 50 24,200 7,100 3,100 AM 

 donnell–smithii) 12% 65,500 7,200 44 38,600 9,600 2,900 

Purpleheart (Peltogyne spp.) Green 0.67 94,000 13,800 102 48,400 11,300 8,100 AM 

12% 132,400 15,700 121 71,200 15,300 8,300 

Ramin (Gonystylus bancanus) Green 0.52 67,600 10,800 62 37,200 6,800 2,800 AS 

12% — 127,600 15,000 117 69,500 10,500 5,800 

Robe (Tabebuia spp., Green 0.52 74,500 10,000 81 33,900 8,600 4,000 AM 

roble group) 12% 95,100 11,000 86 50,600 10,000 4,300 

Rosewood, Brazilian Green 0.8 97,200 12,700 91 38,000 16,300 10,900 AM 

(Dalbergia nigra) 12% — 131,000 13,000 — 66,200 14,500 12,100 

Rosewood, Indian (Dalbergia Green 0.75 63,400 8,200 80 31,200 9,700 6,900 AS 

latifolia) 12% 116,500 12,300 90 63,600 14,400 14,100 

Sande (Brosimum spp., Green 0.49 58,600 13,400 — 31,000 7,200 2,700 AM 

utile group) 12% 98,600 16,500 — 56,700 8,900 4,000 

Santa Maria (Calophyllum Green 0.52 72,400 11,000 88 31,400 8,700 4,000 AM 

brasiliense) 12% — 100,700 12,600 111 47,600 14,300 5,100 

Sapele (Entandrophragma Green 0.55 70,300 10,300 72 34,500 8,600 4,500 AF 

cylindricum) 12% — 105,500 12,500 108 56,300 15,600 6,700 

Sepetir (Pseudosindora Green 0.56 77,200 10,800 92 37,600 9,000 4,200 AS 

palustris) 12% 118,600 13,600 92 61,200 14,000 6,300 

General Technical Report FPL–GTR–190
The internal friction mechanism in wood is a complex function of temperature and moisture content. In general, there 

is a value of moisture content at which internal friction is 

minimum. On either side of this minimum, internal friction 

increases as moisture content varies down to zero or up to 

the fiber saturation point. The moisture content at which 

minimum internal friction occurs varies with temperature. 

At room temperature (23 °C (73 °F)), the minimum occurs 

at about 6% moisture content; at -20 °C (-4 °F), it occurs at 

about 14% moisture content, and at 70 °C (158 °F), at about 

4%. At 90 °C (194 °F), the minimum is not well defined and 

occurs near zero moisture content.

Similarly, there are temperatures at which internal friction 

is minimum, and the temperatures of minimum internal 

friction vary with moisture content. The temperatures of 

minimum internal friction increase as moisture content decreases. For temperatures above 0 °C (32 °F) and moisture 

content greater than about 10%, internal friction increases 

strongly as temperature increases, with a strong positive 

interaction with moisture content. For very dry wood, there 

is a general tendency for internal friction to decrease as the 

temperature increases.

The value of internal friction, expressed by logarithmic decrement, ranges from about 0.1 for hot, moist wood to less 

than 0.02 for hot, dry wood. Cool wood, regardless of moisture content, would have an intermediate value.

Mechanical Properties of Clear 

Straight-Grained Wood

The mechanical properties listed in Table 5–1 to Table 5–9 

are based on a variety of sampling methods. Generally, the 

most extensive sampling is represented in Tables 5–3 and 

5–4. Values in Table 5–3 are averages derived for a number 

of species grown in the United States. The tabulated value 

is an estimate of the average clear wood property of the species. Many values were obtained from test specimens taken 

at a height of 2.4 to 5 m (8 to 16 ft) above the stump of the 

tree. Values reported in Table 5–4 represent estimates of the 

average clear wood properties of species grown in Canada 

and commonly imported into the United States.

Methods of data collection and analysis changed over the 

years during which the data in Tables 5–3 and 5–4 were 

collected. In addition, the character of some forests has 

changed with time. Because not all the species were 

Chapter 5 Mechanical Properties of Wood

5–21

Table 5–5a. Mechanical properties of some woods imported into the United States other than Canadian imports 

(metric)a

—con.

Static bending Compression

parallel

to grain

(kPa)

Shear 

parallel

to grain 

(kPa)

Side

hardness

(N)

Sample

originb

Common and botanical 

names of species 

Moisture

content 

Specific

gravity

Modulus

of

rupture

(kPa)

Modulus

of

elasticity

(MPa)

Work to

maximum

load 

(kJ m–3)

Shorea (Shorea spp., Green 0.68 80,700 14,500 — 37,100 9,900 6,000 AS 

baulau group) 12% 129,600 18,000 — 70,200 15,100 7,900 

Shorea, lauan–meranti group 

Dark red meranti Green 0.46 64,800 10,300 59 32,500 7,700 3,100 AS 

12% 87,600 12,200 95 50,700 10,000 3,500 

Light red meranti Green 0.34 45,500 7,200 43 23,000 4,900 2,000 AS 

12% 65,500 8,500 59 40,800 6,700 2,000 

White meranti Green 0.55 67,600 9,000 57 37,900 9,100 4,400 AS 

15% 85,500 10,300 79 43,800 10,600 5,100 

Yellow meranti Green 0.46 55,200 9,000 56 26,800 7,100 3,300 AS 

12% 78,600 10,700 70 40,700 10,500 3,400 

Spanish-cedar (Cedrela spp.) Green 0.41 51,700 9,000 49 23,200 6,800 2,400 AM 

12% — 79,300 9,900 65 42,800 7,600 2,700 

Sucupira (Bowdichia spp.) Green 0.74 118,600 15,700 — 67,100 — — AM 

15% 133,800 — — 76,500 — — 

Sucupira (Diplotropis purpurea) Green 0.78 120,000 18,500 90 55,300 12,400 8,800 AM 

12% 142,000 19,800 102 83,700 13,500 9,500 

Teak (Tectona grandis) Green 0.55 80,000 9,400 92 41,100 8,900 4,100 AS 

12% 100,700 10,700 83 58,000 13,000 4,400 

Tornillo (Cedrelinga Green 0.45 57,900 — — 28,300 8,100 3,900 AM 

cateniformis) 12% — — — — — — — 

Wallaba (Eperua spp.) Green 0.78 98,600 16,100 — 55,400 — 6,900 AM 

12% — 131,700 15,700 — 74,200 — 9,100 a

Results of tests on clear, straight-grained specimens. Property values were taken from world literature (not obtained from experiments conducted at the 

Forest Products Laboratory). Other species may be reported in the world literature, as well as additional data on many of these species. Some property 

values have been adjusted to 12% moisture content. 

b

AF is Africa; AM, America; AS, Asia. 
5–22

Table 5–5b. Mechanical properties of some woods imported into the United States other than Canadian imports 

(inch–pound)a

Static bending Compression

parallel

to grain 

(lbf in–2)

Shear 

parallel

to grain 

(lbf in–2)

Side

hardness

(lbf)

Sample

originb

Common and botanical 

names of species 

Moisture

content 

Specific

gravity

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticity 

(106

 lbf in–2)

Work to

maximum

load 

(in-lbf in–3)

Afrormosia (Pericopsis elata) Green 0.61 14,800 1.77 19.5 7,490 1,670 1,600 AF 

12% 18,400 1.94 18.4 9,940 2,090 1,560 

Albarco (Cariniana spp.) Green 0.48 — — — — — — AM 

12% 14,500 1.5 13.8 6,820 2,310 1,020 

Andiroba (Carapa guianensis) Green 0.54 10,300 1.69 9.8 4,780 1,220 880 AM 

12% — 15,500 2 14 8,120 1,510 1,130 

Angelin (Andira inermis) Green 0.65 — — — — — — AF 

12% 18,000 2.49 — 9,200 1,840 1,750 

Angelique (Dicorynia Green 0.6 11,400 1.84 12 5,590 1,340 1,100 AM 

guianensis) 12% — 17,400 2.19 15.2 8,770 1,660 1,290 

Avodire (Turraeanthus Green 0.48 — — — — — — AF 

africanus) 12% 12,700 1.49 9.4 7,150 2,030 1,080 

Azobe (Lophira alata) Green 0.87 16,900 2.16 12 9,520 2,040 2,890 AF 

12% 24,500 2.47 — 12,600 2,960 3,350 

Balsa (Ochroma pyramidale) Green 0.16 — — — — — — AM 

12% 3,140 0.49 2.1 2,160 300 — 

Banak (Virola spp.) Green 0.42 5,600 1.64 4.1 2,390 720 320 AM 

12% — 10,900 2.04 10 5,140 980 510 

Benge (Guibourtia arnoldiana) Green 0.65 — — — — — — AF 

12% 21,400 2.04 — 11,400 2,090 1,750 

Bubinga (Guibourtia spp.) Green 0.71 — — — — — — AF 

12% 22,600 2.48 — 10,500 3,110 2,690 

Bulletwood (Manilkara Green 0.85 17,300 2.7 13.6 8,690 1,900 2,230 AM 

bidentata) 12% 27,300 3.45 28.5 11,640 2,500 3,190 

Cativo (Prioria copaifera) Green 0.4 5,900 0.94 5.4 2,460 860 440 AM 

12% — 8,600 1.11 7.2 4,290 1,060 630 

Ceiba (Ceiba pentandra) Green 0.25 2,200 0.41 1.2 1,060 350 220 AM 

12% 4,300 0.54 2.8 2,380 550 240 

Courbaril (Hymenaea Green 0.71 12,900 1.84 14.6 5,800 1,770 1,970 AM 

courbaril) 12% — 19,400 2.16 17.6 9,510 2,470 2,350 

Cuangare (Dialyanthera spp.) Green 0.31 4,000 1.01 — 2,080 590 230 AM 

12% 7,300 1.52 — 4,760 830 380 

Cypress, Mexican (Cupressus Green 0.39 6,200 0.92 — 2,880 950 340 AF 

lustianica) 12% 10,300 1.02 — 5,380 1,580 460 

Degame (Calycophyllum Green 0.67 14,300 1.93 18.6 6,200 1,660 1,630 AM 

candidissimum) 12% 22,300 2.27 27 9,670 2,120 1,940 

Determa (Ocotea rubra) Green 0.52 7,800 1.46 4.8 3,760 860 520 AM 

12% 10,500 1.82 6.4 5,800 980 660 

Ekop (Tetraberlinia Green 0.6 — — — — — — AF 

tubmaniana) 12% 16,700 2.21 — 9,010 — — 

Goncalo alves (Astronium Green 0.84 12,100 1.94 6.7 6,580 1,760 1,910 AM 

graveolens) 12% — 16,600 2.23 10.4 10,320 1,960 2,160 

Greenheart (Chlorocardium rodiei) Green 0.8 19,300 2.47 10.5 9,380 1,930 1,880 AM 

12% 24,900 3.25 25.3 12,510 2,620 2,350 

Hura (Hura crepitans) Green 0.38 6,300 1.04 5.9 2,790 830 440 AM 

12% 8,700 1.17 6.7 4,800 1,080 550 

General Technical Report FPL–GTR–190
Chapter 5 Mechanical Properties of Wood

5–23

Table 5–5b. Mechanical properties of some woods imported into the United States other than Canadian imports 

(inch–pound)a

—con.

Static bending Compression

parallel

to grain

(lbf in–2)

Shear 

parallel

to grain 

(lbf in–2)

Side

hardness

(lbf)

Sample

originb

Common and botanical 

names of species 

Moisture

content 

Specific 

gravity

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticity 

(106

 lbf in–2)

Work to

maximum

load 

(in-lbf in–3)

Ilomba (Pycnanthus Green 0.4 5,500 1.14 — 2,900 840 470 AF 

angolensis) 12% 9,900 1.59 — 5,550 1,290 610 

Ipe (Tabebuia spp., Green 0.92 22,600 2.92 27.6 10,350 2,120 3,060 AM 

lapacho group) 12% 25,400 3.14 22 13,010 2,060 3,680 

Iroko (Chlorophora spp.) Green 0.54 10,200 1.29 10.5 4,910 1,310 1,080 AF 

12% 12,400 1.46 9 7,590 1,800 1,260 

Jarrah (Eucalyptus marginata) Green 0.67 9,900 1.48 — 5,190 1,320 1,290 AS 

12% — 16,200 1.88 — 8,870 2,130 1,910 

Jelutong (Dyera costulata) Green 0.36 5,600 1.16 5.6 3,050 760 330 AS 

15% 7,300 1.18 6.4 3,920 840 390 

Kaneelhart (Licaria spp.) Green 0.96 22,300 3.82 13.6 13,390 1,680 2,210 AM 

12% 29,900 4.06 17.5 17,400 1,970 2,900 

Kapur (Dryobalanops spp.) Green 0.64 12,800 1.6 15.7 6,220 1,170 980 AS 

12% 18,300 1.88 18.8 10,090 1,990 1,230 

Karri (Eucalyptus diversicolor) Green 0.82 11,200 1.94 11.6 5,450 1,510 1,360 AS 

12% 20,160 2.6 25.4 10,800 2,420 2,040 

Kempas (Koompassia Green 0.71 14,500 2.41 12.2 7,930 1,460 1,480 AS 

malaccensis) 12% 17,700 2.69 15.3 9,520 1,790 1,710 

Keruing (Dipterocarpus spp.) Green 0.69 11,900 1.71 13.9 5,680 1,170 1,060 AS 

12% 19,900 2.07 23.5 10,500 2,070 1,270 

Lignumvitae (Guaiacum spp.) Green 1.05 — — — — — — AM 

12% — — — — 11,400 — 4,500 

Limba (Terminalia superba) Green 0.38 6,000 0.77 7.7 2,780 880 400 AF 

12% 8,800 1.01 8.9 4,730 1,410 490 

Macawood (Platymiscium spp.) Green 0.94 22,300 3.02 — 10,540 1,840 3,320 AM 

12% 27,600 3.2 — 16,100 2,540 3,150 

Mahogany, African (Khaya spp.) Green 0.42 7,400 1.15 7.1 3,730 931 640 AF 

12% 10,700 1.4 8.3 6,460 1,500 830 

Mahogany, true Green 0.45 9,000 1.34 9.1 4,340 1,240 740 AM 

(Swietenia macrophylla) 12% — 11,500 1.5 7.5 6,780 1,230 800 

Manbarklak (Eschweilera spp.) Green 0.87 17,100 2.7 17.4 7,340 1,630 2,280 AM 

12% 26,500 3.14 33.3 11,210 2,070 3,480 

Manni (Symphonia globulifera) Green 0.58 11,200 1.96 11.2 5,160 1,140 940 AM 

12% 16,900 2.46 16.5 8,820 1,420 1,120 

Marishballi (Lincania spp.) Green 0.88 17,100 2.93 13.4 7,580 1,620 2,250 AM 

12% 27,700 3.34 14.2 13,390 1,750 3,570 

Merbau (Intsia spp.) Green 0.64 12,900 2.02 12.8 6,770 1,560 1,380 AS 

15% — 16,800 2.23 14.8 8,440 1,810 1,500 

Mersawa (Anisoptera spp.) Green 0.52 8,000 1.77 — 3,960 740 880 AS 

12% 13,800 2.28 — 7,370 890 1,290 

Mora (Mora spp.) Green 0.78 12,600 2.33 13.5 6,400 1,400 1,450 AM 

12% 22,100 2.96 18.5 11,840 1,900 2,300 

Oak (Quercus spp.) Green 0.76 — — — — — — AM 

12% 23,000 3.02 16.5 — — 2,500 

Obeche (Triplochiton Green 0.3 5,100 0.72 6.2 2,570 660 420 AF 

scleroxylon) 12% 7,400 0.86 6.9 3,930 990 430 
Table 5–5b. Mechanical properties of some woods imported into the United States other than Canadian imports 

(inch–pound)a

—con.

Static bending Compression

parallel

to grain 

(lbf in–2)

Shear 

parallel

to grain 

(lbf in–2)

Side

hardness

(lbf)

Sample

originb

Common and botanical 

names of species 

Moisture

content 

Specific

gravity

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticity 

(106

 lbf in–2)

Work to 

maximum

load 

(in-lbf in–3)

Okoume (Aucoumea Green 0.33 — — — — — — AF 

klaineana) 12% 7,400 1.14 — 3,970 970 380 

Opepe (Nauclea diderrichii) Green 0.63 13,600 1.73 12.2 7,480 1,900 1,520 AF 

12% 17,400 1.94 14.4 10,400 2,480 1,630 

Ovangkol (Guibourtia ehie) Green 0.67 — — — — — — AF 

12% 16,900 2.56 — 8,300 — — 

Para-angelim (Hymenolobium Green 0.63 14,600 1.95 12.8 7,460 1,600 1,720 AM 

excelsum) 12% 17,600 2.05 15.9 8,990 2,010 1,720 

Parana-pine (Araucaria Green 0.46 7,200 1.35 9.7 4,010 970 560 AM 

augustifolia) 12% — 13,500 1.61 12.2 7,660 1,730 780 

Pau marfim (Balfourodendron Green 0.73 14,400 1.66 — 6,070 — — AM 

riedelianum) 15% 18,900 — — 8,190 — — 

Peroba de campos Green 0.62 — — — — — — AM 

(Paratecoma peroba) 12% 15,400 1.77 10.1 8,880 2,130 1,600 

Peroba rosa (Aspidosperma Green 0.66 10,900 1.29 10.5 5,540 1,880 1,580 AM 

spp., peroba group) 12% 12,100 1.53 9.2 7,920 2,490 1,730 

Pilon (Hyeronima spp.) Green 0.65 10,700 1.88 8.3 4,960 1,200 1,220 AM 

12% 18,200 2.27 12.1 9,620 1,720 1,700 

Pine, Caribbean (Pinus caribaea) Green 0.68 11,200 1.88 10.7 4,900 1,170 980 AM 

12% — 16,700 2.24 17.3 8,540 2,090 1,240 

Pine, ocote (Pinus oocarpa) Green 0.55 8,000 1.74 6.9 3,690 1,040 580 AM 

12% — 14,900 2.25 10.9 7,680 1,720 910 

Pine, radiata (Pinus radiata) Green 0.42 6,100 1.18 — 2,790 750 480 AS 

12% — 11,700 1.48 — 6,080 1,600 750 

Piquia (Caryocar spp.) Green 0.72 12,400 1.82 8.4 6,290 1,640 1,720 AM 

12% 17,000 2.16 15.8 8,410 1,990 1,720 

Primavera (Tabebuia Green 0.4 7,200 0.99 7.2 3,510 1,030 700 AM 

donnell–smithii) 12% 9,500 1.04 6.4 5,600 1,390 660 

Purpleheart (Peltogyne spp.) Green 0.67 13,700 2 14.8 7,020 1,640 1,810 AM 

12% 19,200 2.27 17.6 10,320 2,220 1,860 

Ramin (Gonystylus bancanus) Green 0.52 9,800 1.57 9 5,390 990 640 AS 

12% — 18,500 2.17 17 10,080 1,520 1,300 

Robe (Tabebuia spp., Green 0.52 10,800 1.45 11.7 4,910 1,250 910 AM 

roble group) 12% 13,800 1.6 12.5 7,340 1,450 960 

Rosewood, Brazilian Green 0.8 14,100 1.84 13.2 5,510 2,360 2,440 AM 

(Dalbergia nigra) 12% — 19,000 1.88 — 9,600 2,110 2,720 

Rosewood, Indian (Dalbergia Green 0.75 9,200 1.19 11.6 4,530 1,400 1,560 AS 

latifolia) 12% 16,900 1.78 13.1 9,220 2,090 3,170 

Sande (Brosimum spp., Green 0.49 8,500 1.94 — 4,490 1,040 600 AM 

utile group) 12% 14,300 2.39 — 8,220 1,290 900 

Santa Maria (Calophyllum Green 0.52 10,500 1.59 12.7 4,560 1,260 890 AM 

brasiliense) 12% — 14,600 1.83 16.1 6,910 2,080 1,150 

Sapele (Entandrophragma Green 0.55 10,200 1.49 10.5 5,010 1,250 1,020 AF 

cylindricum) 12% — 15,300 1.82 15.7 8,160 2,260 1,510 

Sepetir (Pseudosindora palustris) Green 0.56 11,200 1.57 13.3 5,460 1,310 950 AS 

12% 17,200 1.97 13.3 8,880 2,030 1,410 

5–24

General Technical Report FPL–GTR–190
reevaluated to reflect these changes, the appropriateness of 

the data should be reviewed when used for critical applications such as stress grades of lumber.

Values reported in Table 5–5 were collected from the world 

literature; thus, the appropriateness of these properties to 

represent a species is not known. The properties reported 

in Tables 5–1, 5–2, 5–5, and 5–7 to 5–10 may not necessarily represent average species characteristics because of 

inadequate sampling; however, they do suggest the relative 

influence of species and other specimen parameters on the 

mechanical behavior recorded.

Variability in properties can be important in both production 

and consumption of wood products. The fact that a piece 

may be stronger, harder, or stiffer than the average is often 

of less concern to the user than if the piece is weaker; however, this may not be true if lightweight material is selected 

for a specific purpose or if harder or tougher material is 

difficult to work. Some indication of the spread of property 

values is therefore desirable. Average coefficients of variation for many mechanical properties are presented in 

Table 5–6.

The mechanical properties reported in the tables are significantly affected by specimen moisture content at time of 

test. Some tables include properties that were evaluated at 

different moisture levels; these moisture levels are reported. 

As indicated in the tables, many of the dry test data were 

adjusted to a common moisture content base of 12%.

Specific gravity is reported in many tables because this 

property is used as an index of clear wood mechanical 

properties. The specific gravity values given in Tables 5–3 

and 5–4 represent the estimated average clear wood specific 

gravity of the species. In the other tables, specific gravity 

values represent only the specimens tested. The variability 

of specific gravity, represented by the coefficient of variation derived from tests on 50 species, is included in Table 

5–6.

Mechanical and physical properties as measured and reported often reflect not only the characteristics of the wood 

but also the influence of the shape and size of the test specimen and the test mode. The test methods used to establish 

properties in Tables 5–3, 5–4, and 5–7 to 5–9 are based on 

standard procedures (ASTM D 143). Test methods for 

Table 5–5b. Mechanical properties of some woods imported into the United States other than Canadian imports 

(inch–pound)a

—con.

Static bending Compression

parallel

to grain 

(lbf in–2)

Sample

originb

Common and botanical 

names of species 

Moisture

content 

Specific

gravity

Modulus

of

rupture

(lbf in–2)

Modulus

of

elasticity 

(106

 lbf in–2)

Work to 

maximum

load 

(in-lbf in–3)

Shear 

parallel

to grain 

(lbf in–2)

Side

hardness

(lbf)

Shorea (Shorea spp., Green 0.68 11,700 2.1 — 5,380 1,440 1,350 AS 

bullau group) 12% 18,800 2.61 — 10,180 2,190 1,780 

Shorea, lauan–meranti group 

Dark red meranti Green 0.46 9,400 1.5 8.6 4,720 1,110 700 AS 

12% 12,700 1.77 13.8 7,360 1,450 780 

Light red meranti Green 0.34 6,600 1.04 6.2 3,330 710 440 AS 

12% 9,500 1.23 8.6 5,920 970 460 

White meranti Green 0.55 9,800 1.3 8.3 5,490 1,320 1,000 AS 

15% 12,400 1.49 11.4 6,350 1,540 1,140 

Yellow meranti Green 0.46 8,000 1.3 8.1 3,880 1,030 750 AS 

12% 11,400 1.55 10.1 5,900 1,520 770 

Spanish-cedar (Cedrela spp.) Green 0.41 7,500 1.31 7.1 3,370 990 550 AM 

12% — 11,500 1.44 9.4 6,210 1,100 600 

Sucupira (Bowdichia spp.) Green 0.74 17,200 2.27 — 9,730 — — AM 

15% 19,400 — — 11,100 — — 

Sucupira (Diplotropis purpurea) Green 0.78 17,400 2.68 13 8,020 1,800 1,980 AM 

12% 20,600 2.87 14.8 12,140 1,960 2,140 

Teak (Tectona grandis) Green 0.55 11,600 1.37 13.4 5,960 1,290 930 AS 

12% 14,600 1.55 12 8,410 1,890 1,000 

Tornillo (Cedrelinga Green 0.45 8,400 — — 4,100 1,170 870 AM 

cateniformis) 12% — — — — — — — 

Wallaba (Eperua spp.) Green 0.78 14,300 2.33 — 8,040 — 1,540 AM 

12% — 19,100 2.28 — 10,760 — 2,040 a

Results of tests on clear, straight-grained specimens. Property values were taken from world literature (not obtained from experiments conducted at the 

Forest Products Laboratory). Other species may be reported in the world literature, as well as additional data on many of these species. Some property 

values have been adjusted to 12% moisture content. 

b

AF is Africa; AM, America; AS, Asia. 

Chapter 5 Mechanical Properties of Wood

5–25
properties presented in other tables are referenced in the 

selected bibliography at the end of this chapter.

Common names of species listed in the tables conform to 

standard nomenclature of the U.S. Forest Service. Other 

names may be used locally for a species. Also, one common 

name may be applied to groups of species for marketing.

Natural Characteristics Affecting 

Mechanical Properties

Clear straight-grained wood is used for determining fundamental mechanical properties; however, because of natural 

growth characteristics of trees, wood products vary in specific gravity, may contain cross grain, or may have knots 

and localized slope of grain. Natural defects such as pitch 

pockets may occur as a result of biological or climatic elements influencing the living tree. These wood characteristics 

must be taken into account in assessing actual properties or 

estimating actual performance of wood products.

Specific Gravity

The substance of which wood is composed is actually heavier than water; its specific gravity is about 1.5 regardless of 

wood species. In spite of this, dry wood of most species 

floats in water, and it is thus evident that part of the volume 

of a piece of wood is occupied by cell cavities and pores. 

Variations in the size of these openings and in the thickness 

of the cell walls cause some species to have more wood 

substance per unit volume than other species and therefore 

higher specific gravity. Thus, specific gravity is an excellent 

index of the amount of wood substance contained in a piece 

of wood; it is a good index of mechanical properties as long 

as the wood is clear, straight grained, and free from defects. 

However, specific gravity values also reflect the presence of 

gums, resins, and extractives, which contribute little to mechanical properties.

Approximate relationships between various mechanical 

properties and specific gravity for clear straight-grained 

wood of hardwoods and softwoods are given in Table 5–11 

as power functions. Those relationships are based on average values for the 43 softwood and 66 hardwood species 

presented in Table 5–3. The average data vary around the 

relationships, so that the relationships do not accurately 

predict individual average species values or an individual 

specimen value. In fact, mechanical properties within a species tend to be linearly, rather than curvilinearly, related to 

specific gravity; where data are available for individual species, linear analysis is suggested.

Knots

A knot is that portion of a branch that has become incorporated in the bole of a tree. The influence of a knot on the mechanical properties of a wood member is due to the interruption of continuity and change in the direction of wood fibers 

Table 5–6. Average coefficients of variation for some 

mechanical properties of clear wood

Property 

Coefficient

of variationa

(%)

Static bending 

 Modulus of rupture 16 

 Modulus of elasticity 22 

Work to maximum load 34 

Impact bending 25 

Compression parallel to grain 18 

Compression perpendicular to grain 28 

Shear parallel to grain, maximum shearing strength 14 

Tension parallel to grain 25 

Side hardness 20 

Toughness 34 

Specific gravity 10 a

Values based on results of tests of green wood from approximately 

50 species. Values for wood adjusted to 12% moisture content may be 

assumed to be approximately of the same magnitude. 

Table 5–7. Average parallel-to-grain tensile 

strength of some wood speciesa

Species 

Tensile strength 

(kPa (lb in–2)) 

Hardwoods

Beech, American 86,200 (12,500) 

Elm, cedar 120,700 (17,500) 

Maple, sugar 108,200 (15,700) 

Oak

Overcup 77,900 (11,300) 

Pin 112,400 (16,300) 

Poplar, balsam 51,000 (7,400) 

Sweetgum 93,800 (13,600) 

Willow, black 73,100 (10,600) 

Yellow-poplar 109,600 (15,900) 

Softwoods

Baldcypress 58,600 (8,500) 

Cedar 

Port-Orford 78,600 (11,400) 

Western redcedar 45,500 (6,600) 

Douglas-fir, interior north 107,600 (15,600) 

Fir

California red 77,900 (11,300) 

Pacific silver 95,100 (13,800) 

Hemlock, western 89,600 (13,000) 

Larch, western 111,700 (16,200) 

Pine

Eastern white 73,100 (10,600) 

Loblolly 80,000 (11,600) 

Ponderosa 57,900 (8,400) 

Virginia 94,500 (13,700) 

Redwood

Virgin 64,800 (9,400) 

Young growth 62,700 (9,100) 

Spruce 

Engelmann 84,800 (12,300) 

Sitka 59,300 (8,600) a

Results of tests on clear, straight-grained specimens tested green. 

For hardwood species, strength of specimens tested at 12% moisture 

content averages about 32% higher; for softwoods, about 13% 

higher. 

5–26

General Technical Report FPL–GTR–190
associated with the knot. The influence of knots depends on 

their size, location, shape, and soundness; attendant local 

slope of grain; and type of stress to which the wood member 

is subjected.

The shape (form) of a knot on a sawn surface depends upon 

the direction of the exposing cut. A nearly round knot is produced when lumber is sawn from a log and a branch is sawn 

through at right angles to its length (as in a flatsawn board). 

An oval knot is produced if the saw cut is diagonal to the 

branch length (as in a bastard-sawn board) and a “spiked” 

knot when the cut is lengthwise to the branch (as in a quartersawn board).

Knots are further classified as intergrown or encased 

(Fig. 5–3). As long as a limb remains alive, there is continuous growth at the junction of the limb and the bole of the 

tree, and the resulting knot is called intergrown. After the 

branch has died, additional growth on the trunk encloses 

the dead limb, resulting in an encased knot; bole fibers are 

not continuous with the fibers of the encased knot. Encased 

knots and knotholes tend to be accompanied by less crossgrain than are intergrown knots and are therefore generally 

less problematic with regard to most mechanical properties.

Most mechanical properties are lower in sections containing 

knots than in clear straight-grained wood because (a) the 

clear wood is displaced by the knot, (b) the fibers around the 

knot are distorted, resulting in cross grain, (c) the discontinuity of wood fiber leads to stress concentrations, and (d) 

checking often occurs around the knots during drying. Hardness and strength in compression perpendicular to the grain 

Table 5–8. Average toughness values for a few hardwood 

speciesa

Toughnessb

Species 

Moisture

content 

Specific

gravityc

Radial

(J (in-lbf)) 

Tangential 

(J (in-lbf)) 

Birch, yellow 12% 0.65 8,100 (500) 10,100 (620) 

Hickory (mocker- Green 0.64 11,400 (700) 11,700 (720) 

 nut, pignut, sand) 12% 0.71 10,100 (620) 10,700 (660) 

Maple, sugar 14% 0.64 6,000 (370) 5,900 (360) 

Oak, red 

 Pin 12% 0.64 7,000 (430) 7,000 (430) 

 Scarlet 11% 0.66 8,300 (510) 7,200 (440) 

Oak, white 

 Overcup Green 0.56 11,900 (730) 11,100 (680) 

13% 0.62 5,500 (340) 5,000 (310) 

Sweetgum Green 0.48 5,500 (340) 5,400 (330) 

13% 0.51 4,200 (260) 4,200 (260) 

Willow, black Green 0.38 5,000 (310) 5,900 (360) 

11% 0.4 3,400 (210) 3,700 (230) 

Yellow-poplar Green 0.43 5,200 (320) 4,900 (300) 

12% 0.45 3,600 (220) 3,400 (210) a

Results of tests on clear, straight-grained specimens. b

Properties based on specimen size of 2 cm square by 28 cm long; radial indicates 

load applied to radial face and tangential indicates load applied to tangential face of 

specimens. 

c

Based on ovendry weight and volume at moisture content of test.

Figure 5–3. Types of knots. A, encased knot; B, 

intergrown knot.

Chapter 5 Mechanical Properties of Wood

5–27
are exceptions, where knots may be objectionable only in 

that they cause nonuniform wear or nonuniform stress distributions at contact surfaces.

Knots have a much greater effect on strength in axial tension 

than in axial short-column compression, and the effects on 

bending are somewhat less than those in axial tension. For 

this reason, in a simply supported beam, a knot on the lower 

side (subjected to tensile stresses) has a greater effect on the 

load the beam will support than does a knot on the upper 

side (subjected to compressive stresses).

In long columns, knots are important because they affect 

stiffness. In short or intermediate columns, the reduction in 

strength caused by knots is approximately proportional to 

their size; however, large knots have a somewhat greater 

relative effect than do small knots.

Knots in round timbers, such as poles and piles, have less 

effect on strength than do knots in sawn timbers. Although 

the grain is irregular around knots in both forms of timber, 

the angle of the grain to the surface is smaller in naturally 

round timber than in sawn timber. Furthermore, in round 

timbers there is no discontinuity in wood fibers, which results from sawing through both local and general slope of 

grain.

The effects of knots in structural lumber are discussed in 

Chapter 7.

Slope of Grain

In some wood product applications, the directions of important stresses may not coincide with the natural axes of 

fiber orientation in the wood. This may occur by choice in 

Table 5–9. Average toughness values for a few softwood 

speciesa

Toughnessb

Species 

Moisture

content 

Specific

gravityc

Radial

(J (in-lbf)) 

Tangential

(J (in-lbf)) 

Cedar 

Western red 9% 0.33 1,500 (90) 2,100 (130)

Yellow 10% 0.48 3,400 (210) 3,700 (230)

Douglas-fir 

Coast Green 0.44 3,400 (210) 5,900 (360)

12% 0.47 3,300 (200) 5,900 (360)

Interior west Green 0.48 3,300 (200) 4,900 (300)

13% 0.51 3,400 (210) 5,500 (340)

Interior north Green 0.43 2,800 (170) 3,900 (240)

14% 0.46 2,600 (160) 4,100 (250)

Interior south Green 0.38 2,100 (130) 2,900 (180)

14% 0.4 2,000 (120) 2,900 (180)

Fir

California red Green 0.36 2,100 (130) 2,900 (180)

12% 0.39 2,000 (120) 2,800 (170)

Noble Green 0.36 — — 3,900 (240)

12% 0.39 — — 3,600 (220)

Pacific silver Green 0.37 2,400 (150) 3,700 (230)

13% 0.4 2,800 (170) 4,200 (260)

White Green 0.36 2,300 (140) 3,600 (220)

13% 0.38 2,100 (130) 3,300 (200)

Hemlock

Mountain Green 0.41 4,100 (250) 4,600 (280) 

14% 0.44 2,300 (140) 2,800 (170)

Western Green 0.38 2,400 (150) 2,800 (170)

12% 0.41 2,300 (140) 3,400 (210)

Larch, western Green 0.51 4,400 (270) 6,500 (400)

12% 0.55 3,400 (210) 5,500 (340)

Pine

Eastern white Green 0.33 2,000 (120) 2,600 (160)

12% 0.34 1,800 (110) 2,000 (120)

Jack Green 0.41 3,300 (200) 6,200 (380)

12% 0.42 2,300 (140) 3,900 (240)

Loblolly Green 0.48 5,000 (310) 6,200 (380)

12% 0.51 2,600 (160) 4,200 (260)

Lodgepole Green 0.38 2,600 (160) 3,400 (210)

Ponderosa Green 0.38 3,100 (190) 4,400 (270)

11% 0.43 2,400 (150) 3,100 (190)

Red Green 0.4 3,400 (210) 5,700 (350)

12% 0.43 2,600 (160) 4,700 (290)

Shortleaf Green 0.47 4,700 (290) 6,500 (400)

13% 0.5 2,400 (150) 3,700 (230)

Slash Green 0.55 5,700 (350) 7,300 (450)

12% 0.59 3,400 (210) 5,200 (320)

Virginia Green 0.45 5,500 (340) 7,600 (470)

12% 0.49 2,800 (170) 4,100 (250)

Redwood

Old-growth Green 0.39 1,800 (110) 3,300 (200)

11% 0.39 1,500 (90) 2,300 (140)

Young-growth Green 0.33 1,800 (110) 2,300 (140)

12% 0.34 1,500 (90) 1,800 (110)

Spruce, Green 0.34 2,400 (150) 3,100 (190)

Engelmann 12% 0.35 1,800 (110) 2,900 (180) a

Results of tests on clear, straight-grained specimens. b

Properties based on specimen size of 2 cm square by 28 cm long; radial 

indicates load applied to radial face and tangential indicates load applied to 

tangential face of specimens. 

c

Based on ovendry weight and volume at moisture content of test.

Table 5–10. Summary of selected fracture 

toughness results

Fracture toughness 

(kPa m1/2 (lbf in–2 in1/2)) 

Mode I Mode II 

Species TL RL TL RL

Douglas-fir 320 (290) 360 

(330)

 2,230 

(2,030) 

Western hemlock 375 (340) 2,240 

(2,040)

Pine

Western white 250 (225) 260 

(240)

Scots 440 (400) 500 

(455)

2,050

(1,860)

Southern 375 (340) 2,070 

(1,880)

Ponderosa 290 (265) 

Red spruce 420 (380) 2,190 

(1,990)

1,665

(1,510) 

Northern red oak 410 (370) 

Sugar maple 480 (430) 

Yellow-poplar 517 (470) 

5–28

General Technical Report FPL–GTR–190
design, from the way the wood was removed from the log, 

or because of grain irregularities that occurred while the tree 

was growing.

Elastic properties in directions other than along the natural 

axes can be obtained from elastic theory. Strength properties 

in directions ranging from parallel to perpendicular to the 

fibers can be approximated using a Hankinson-type formula 

(Bodig and Jayne 1982):

(5–2)

where N is strength at angle q from fiber direction, Q

strength perpendicular to grain, P strength parallel to grain, 

and n an empirically determined constant.

This formula has been used for modulus of elasticity as well 

as strength properties. Values of n and associated ratios of 

Q/P tabulated from available literature are as follows:

Property n Q/P

Tensile strength 1.5–2 0.04–0.07

Compression strength 2–2.5 0.03–0.40

Bending strength 1.5–2 0.04–0.10

Modulus of elasticity 2 0.04–0.12

Toughness 1.5–2 0.06–0.10

The Hankinson-type formula can be graphically depicted as 

a function of Q/P and n. Figure 5–4 shows the strength in 

any direction expressed as a fraction of the strength parallel 

Table 5–11a. Functions relating mechanical properties to specific gravity of clear, 

straight-grained wood (metric)

Specific gravity–strength relationship 

Green wood 

Wood at 12% 

moisture content 

Propertya Softwoods Hardwoods Softwoods Hardwoods 

Static bending 

MOR (kPa) 109,600 G1.01 118,700 G1.16 170,700 G1.01 171,300 G1.13

MOE (MPa) 16,100 G0.76 13,900 G0.72 20,500 G0.84 16,500 G0.7

WML (kJ m–3) 147 G1.21 229 G1.51 179 G1.34 219 G1.54

Impact bending (N) 353 G1.35 422 G1.39 346 G1.39 423 G1.65

Compression parallel (kPa) 49,700 G0.94 49,000 G1.11 93,700 G0.97 76,000 G0.89

Compression perpendicular (kPa) 8,800 G1.53 18,500 G2.48 16,500 G1.57 21,600 G2.09

Shear parallel (kPa) 11,000 G0.73 17,800 G1.24 16,600 G0.85 21,900 G1.13

Tension perpendicular (kPa) 3,800 G0.78 10,500 G1.37 6,000 G1.11 10,100 G1.3

Side hardness (N) 6,230 G1.41 16,550 G2.31 8,590 G1.49 15,300 G2.09

a

Compression parallel to grain is maximum crushing strength; compression perpendicular to grain is fiber stress 

at proportional limit. MOR is modulus of rupture; MOE, modulus of elasticity; and WML, work to maximum 

load. For green wood, use specific gravity based on ovendry weight and green volume; for dry wood, use 

specific gravity based on ovendry weight and volume at 12% moisture content. Calculated using all data from 

Table 5–3. 

Table 5–11b. Functions relating mechanical properties to specific gravity of 

clear, straight-grained wood (inch–pound)

Specific gravity–strength relationship 

Green wood 

Wood at 12% 

moisture content 

Propertya Softwoods Hardwoods Softwoods Hardwoods 

Static bending 

MOR (lb in–2) 15,890 G1.01 17,210 G1.16 24,760 G1.01 24,850 G1.13

MOE (106 lb in–2) 2.33 G0.76 2.02 G0.72 2.97 G.0.84 2.39 G0.7

WML (in-lbf in–3) 21.33 G1.21 33.2 G1.51 25.9 G1.34 31.8 G1.54

Impact bending (lbf) 79.28 G1.35 94.8 G1.39 77.7 G1.39 95.1 G1.65

Compression parallel (lb in–2) 7,210 G0.94 7,110 G1.11 13,590 G0.97 11,030 G0.89

Compression perpendicular (lb in–2) 1,270 G1.53 2,680 G2.48 2,390 G1.57 3,130 G2.09

Shear parallel (lb in–2) 1,590 G0.73 2,580 G1.24 2,410 G.0.85 3,170 G1.13

Tension perpendicular (lb in–2) 550 G0.78 1,520 G1.37 870 G1.11 1,460 G1.3

Side hardness (lbf) 1,400 G1.41 3,720 G2.31 1,930 G1.49 3,440 G2.09

a

Compression parallel to grain is maximum crushing strength; compression perpendicular to grain is fiber 

stress at proportional limit. MOR is modulus of rupture; MOE, modulus of elasticity; and WML, work to 

maximum load. For green wood, use specific gravity based on ovendry weight and green volume; for dry 

wood, use specific gravity based on ovendry weight and volume at 12% moisture content. Calculated 

using all data from Table 5–3. 

Chapter 5 Mechanical Properties of Wood

5–29
to fiber direction, plotted against angle to the fiber direction 

q. The plot is for a range of values of Q/P and n.

The term slope of grain relates the fiber direction to the 

edges of a piece. Slope of grain is usually expressed by the 

ratio between 25 mm (1 in.) of the grain from the edge or 

long axis of the piece and the distance in millimeters (inches) within which this deviation occurs (tan q). The effect of 

grain slope on some properties of wood, as determined 

from tests, is shown in Table 5–12. The values for modulus 

of rupture fall very close to the curve in Figure 5–4 for 

Q/P = 0.1 and n = 1.5. Similarly, the impact bending values 

fall close to the curve for Q/P = 0.05 and n =1.5, and the 

compression values for the curve for Q/P = 0.1, n = 2.5.

The term cross grain indicates the condition measured by 

slope of grain. Two important forms of cross grain are spiral 

and diagonal (Fig. 5–5). Other types are wavy, dipped, interlocked, and curly.

Spiral grain is caused by winding or spiral growth of wood 

fibers about the bole of the tree instead of vertical growth. 

In sawn products, spiral grain can be defined as fibers lying 

in the tangential plane of the growth rings, rather than parallel to the longitudinal axis of the product (see Fig. 5–5 for a 

simple case). Spiral grain in sawn products often goes undetected by ordinary visual inspection. The best test for spiral 

grain is to split a sample section from the piece in the radial 

direction. A visual method of determining the presence 

of spiral grain is to note the alignment of pores, rays, and 

resin ducts on a flatsawn face. Drying checks on a flatsawn 

surface follow the fibers and indicate the slope of the fiber. 

Relative change in electrical capacitance is an effective 

technique for measuring slope of grain.

Diagonal grain is cross grain caused by growth rings that are 

not parallel to one or both surfaces of the sawn piece. Diagonal grain is produced by sawing a log with pronounced 

taper parallel to the axis (pith) of the tree. Diagonal grain 

also occurs in lumber sawn from crooked logs or logs with 

butt swell.

Cross grain can be quite localized as a result of the disturbance of a growth pattern by a branch. This condition, 

termed local slope of grain, may be present even though the 

branch (knot) may have been removed by sawing. The degree of local cross grain may often be difficult to determine. 

Any form of cross grain can have a deleterious effect on mechanical properties or machining characteristics.

Spiral and diagonal grain can combine to produce a more 

complex cross grain. To determine net cross grain, regardless of origin, fiber slopes on the contiguous surface of a 

piece must be measured and combined. The combined slope 

of grain is determined by taking the square root of the sum 

of the squares of the two slopes. For example, assume 

Figure 5–4. Effect of grain angle on mechanical property of clear wood according to Hankinson-type formula. 

Q/P is ratio of mechanical property across the grain (Q) 

to that parallel to the grain (P); n is an empirically determined constant.

Figure 5–5. Relationship of fiber orientation (O–O) 

to axes, as shown by schematic of wood specimens 

containing straight grain and cross grain. Specimens 

A through D have radial and tangential surfaces; E 

through H do not. Specimens A and E contain no cross 

grain; B, D, F, and H have spiral grain; C, D, G, and H 

have diagonal grain.

5–30

General Technical Report FPL–GTR–190
that the spiral grain slope on the flat-grained surface of 

Figure 5–5D is 1 in 12 and the diagonal-grain slope is 1 in 

18. The combined slope is

or a slope of 1 in 10.

A regular reversal of right and left spiraling of grain in a tree 

stem produces the condition known as interlocked grain. 

Interlocked grain occurs in some hardwood species and 

markedly increases resistance to splitting in the radial plane. 

Interlocked grain decreases both the static bending strength 

and stiffness of clear wood specimens. The data from tests 

of domestic hardwoods shown in Table 5–3 do not include 

pieces that exhibited interlocked grain. Some mechanical 

property values in Table 5–5 are based on specimens with 

interlocked grain because that is a characteristic of some 

species. The presence of interlocked grain alters the relationship between bending strength and compressive strength 

of lumber cut from tropical hardwoods.

Annual Ring Orientation

Stresses perpendicular to the fiber (grain) direction may be 

at any angle from 0° (T direction) to 90° (R direction) to 

the growth rings (Fig. 5–6). Perpendicular-to-grain properties depend somewhat upon orientation of annual rings with 

respect to the direction of stress. The compression perpendicular-to-grain values in Table 5–3 were derived from tests 

in which the load was applied parallel to the growth rings (T

direction); shear parallel-to-grain and tension perpendicularto-grain values are averages of equal numbers of specimens 

with 0° and 90° growth ring orientations. In some species, 

there is no difference in 0° and 90° orientation properties. 

Other species exhibit slightly higher shear parallel or tension perpendicular-to-grain properties for the 0° orientation 

than for the 90° orientation; the converse is true for about an 

equal number of species.

The effects of intermediate annual ring orientations have 

been studied in a limited way. Modulus of elasticity, compressive perpendicular-to-grain stress at the proportional 

limit, and tensile strength perpendicular to the grain tend to 

be about the same at 45° and 0°, but for some species these 

values are 40% to 60% lower at the 45° orientation. For 

those species with lower properties at 45° ring orientation, 

properties tend to be about equal at 0° and 90° orientations. 

For species with about equal properties at 0° and 45° orientations, properties tend to be higher at the 90° orientation.

Reaction Wood

Abnormal woody tissue is frequently associated with leaning boles and crooked limbs of both conifers and hardwoods. Such wood is generally believed to be formed as 

a natural response of the tree to return its limbs or bole to 

a more normal position, hence the term reaction wood. In 

softwoods, the abnormal tissue is called compression wood; 

it is common to all softwood species and is found on the 

lower side of the limb or inclined bole. In hardwoods, the 

abnormal tissue is known as tension wood; it is located on 

the upper side of the inclined member, although in some 

instances it is distributed irregularly around the cross section. Reaction wood is more prevalent in some species than 

in others.

Many of the anatomical, chemical, physical, and mechanical 

properties of reaction wood differ distinctly from those of 

normal wood. Perhaps most evident is the increase in density compared with that of normal wood. The specific gravity 

of compression wood is commonly 30% to 40% greater than 

that of normal wood; the specific gravity of tension wood 

commonly ranges between 5% and 10% greater than that of 

normal wood, but it may be as much as 30% greater.

Compression wood is usually somewhat darker than normal 

wood because of the greater proportion of latewood, and it 

frequently has a relatively lifeless appearance, especially in 

woods in which the transition from earlywood to latewood 

is abrupt. Because compression wood is more opaque than 

Table 5–12. Strength of wood members with 

various grain slopes compared with strength 

of a straight-grained membera

Maximum slope

of grain in 

member 

Modulus

of

rupture

(%)

Impact

bending

(%)

Compression

parallel to 

grain 

(%)

Straight-grained 100 100 100 

1 in 25 96 95 100 

1 in 20 93 90 100 

1 in 15 89 81 100 

1 in 10 81 62 99 

1 in 5 55 36 93 a

Impact bending is height of drop causing complete failure 

(22.7-kg (50-lb) hammer); compression parallel to grain is 

maximum crushing strength.

Figure 5–6. Direction of load in relation to direction of 

annual growth rings: 90° or perpendicular (R), 45°, 0° or 

parallel (T).

Chapter 5 Mechanical Properties of Wood

5–31
normal wood, intermediate stages of compression wood can 

be detected by transmitting light through thin cross sections; 

however, borderline forms of compression wood that merge 

with normal wood can commonly be detected only by microscopic examination.

Tension wood is more difficult to detect than is compression 

wood. However, eccentric growth as seen on the transverse 

section suggests its presence. Also, because it is difficult to 

cleanly cut the tough tension wood fibers, the surfaces of 

sawn boards are “woolly,” especially when the boards are 

sawn in the green condition (Fig. 5–7). In some species, tension wood may be evident on a smooth surface as areas of 

contrasting colors. Examples of this are the silvery appearance of tension wood in sugar maple and the darker color 

of tension wood in mahogany.

Reaction wood, particularly compression wood in the green 

condition, may be stronger than normal wood. However, 

compared with normal wood with similar specific gravity, 

reaction wood is definitely weaker. Possible exceptions to 

this are compression parallel-to-grain properties of compression wood and impact bending properties of tension wood.

Because of the abnormal properties of reaction wood, it 

may be desirable to eliminate this wood from raw material. 

In logs, compression wood is characterized by eccentric 

growth about the pith and the large proportion of latewood 

at the point of greatest eccentricity (Fig. 5–8A). Fortunately, 

pronounced compression wood in lumber can generally be 

detected by ordinary visual examination.

Compression and tension wood undergo extensive longitudinal shrinkage when subjected to moisture loss below the 

fiber saturation point. Longitudinal shrinkage in compression wood may be up to 10 times that in normal wood, and 

in tension wood, perhaps up to 5 times that in normal wood. 

When reaction wood and normal wood are present in the 

same board, unequal longitudinal shrinkage causes internal 

stresses that result in warping. In extreme cases, unequal 

longitudinal shrinkage results in axial tension failure over a 

portion of the cross section of the lumber (Fig. 5–8B). Warp 

sometimes occurs in rough lumber but more often in planed, 

ripped, or resawn lumber (Fig. 5–8C).

Juvenile Wood

Juvenile wood is the wood produced near the pith of the 

tree; for softwoods, it is usually defined as the material 5 to 

20 rings from the pith depending on species. Juvenile wood 

has considerably different physical and anatomical properties than that of mature wood (Fig. 5–9). In clear wood, the 

properties that have been found to influence mechanical 

behavior include fibril angle, cell length, and specific gravity, the latter a composite of percentage of latewood, cell 

wall thickness, and lumen diameter. Juvenile wood has a 

high fibril angle (angle between longitudinal axis of wood 

cell and cellulose fibrils), which causes longitudinal shrinkage that may be more than 10 times that of mature wood. 

Compression wood and spiral grain are also more prevalent 

in juvenile wood than in mature wood and contribute to 

longitudinal shrinkage. In structural lumber, the ratio of 

modulus of rupture, ultimate tensile stress, and modulus 

of elasticity for juvenile to mature wood ranges from 0.5 

Figure 5–7. Projecting tension wood fibers on 

sawn surface of mahogany board.

Figure 5–8. Effects of compression wood. A, eccentric 

growth about pith in cross section containing compression wood—dark area in lower third of cross section is 

compression wood; B, axial tension break caused by excessive longitudinal shrinkage of compression wood; 

C, warp caused by excessive longitudinal shrinkage.

5–32

General Technical Report FPL–GTR–190
to 0.9, 0.5 to 0.95, and 0.45 to 0.75, respectively. Changes 

in shear strength resulting from increases in juvenile wood 

content can be adequately predicted by monitoring changes 

in density alone for all annual ring orientations. The same is 

true for perpendicular-to-grain compressive strength when 

the load is applied in the tangential direction. Compressive 

strength perpendicular-to-grain for loads applied in the radial direction, however, is more sensitive to changes in juvenile wood content and may be up to eight times less than 

that suggested by changes in density alone (Kretschmann 

2008). The juvenile wood to mature wood ratio is lower for 

higher grades of lumber than for lower grades, which indicates that juvenile wood has greater influence in reducing 

the mechanical properties of high-grade structural lumber. 

Only a limited amount of research has been done on juvenile wood in hardwood species.

Compression Failures

Excessive compressive stresses along the grain that produce 

minute compression failures can be caused by excessive 

bending of standing trees from wind or snow; felling of 

trees across boulders, logs, or irregularities in the ground; 

or rough handling of logs or lumber. Compression failures 

should not be confused with compression wood. In some 

instances, compression failures are visible on the surface 

of a board as minute lines or zones formed by crumpling or 

buckling of cells (Fig. 5–10A), although the failures usually appear as white lines or may even be invisible to the 

unaided eye. The presence of compression failures may be 

indicated by fiber breakage on end grain (Fig. 5–10B). Because compression failures are often difficult to detect with 

the unaided eye, special efforts, including optimum lighting, 

may be required for detection. The most difficult cases are 

detected only by microscopic examination.

Products containing visible compression failures have low 

strength properties, especially in tensile strength and shock 

resistance. The tensile strength of wood containing compression failures may be as low as one-third the strength of 

matched clear wood. Even slight compression failures, visible only under a microscope, may seriously reduce strength 

and cause brittle fracture. Because of the low strength 

associated with compression failures, many safety codes 

require certain structural members, such as ladder rails and 

scaffold planks, to be entirely free of such failures.

Pitch Pockets

A pitch pocket is a well-defined opening that contains free 

resin. The pocket extends parallel to the annual rings; it 

is almost flat on the pith side and curved on the bark side. 

Pitch pockets are confined to such species as the pines, 

spruces, Douglas-fir, tamarack, and western larch.

The effect of pitch pockets on strength depends upon their 

number, size, and location in the piece. A large number 

of pitch pockets indicates a lack of bond between annual 

growth layers, and a piece with pitch pockets should be inspected for shake or separation along the grain.

Bird Peck

Maple, hickory, white ash, and a number of other species are 

often damaged by small holes made by woodpeckers. These 

bird pecks often occur in horizontal rows, sometimes encircling the tree, and a brown or black discoloration known as 

a mineral streak originates from each hole. Holes for tapping 

Figure 5–9. Properties of juvenile wood.

Figure 5–10. Compression failures. A, compression failure shown by irregular lines across grain; 

B, fiber breakage in end-grain surfaces of spruce 

lumber caused by compression failures below dark 

line.

Chapter 5 Mechanical Properties of Wood

5–33
maple trees are also a source of mineral streaks. The streaks 

are caused by oxidation and other chemical changes in the 

wood. Bird pecks and mineral streaks are not generally important in regard to strength of structural lumber, although 

they do impair the appearance of the wood.

Extractives

Many wood species contain removable extraneous materials 

or extractives that do not degrade the cellulose–lignin structure of the wood. These extractives are especially abundant 

in species such as larch, redwood, western redcedar, and 

black locust.

A small decrease in modulus of rupture and strength in 

compression parallel to grain has been measured for some 

species after the extractives have been removed. The extent 

to which extractives influence strength is apparently a function of the amount of extractives, the moisture content of the 

piece, and the mechanical property under consideration.

Properties of Timber from Dead Trees

Timber from trees killed by insects, blight, wind, or fire may 

be as good for any structural purpose as that from live trees, 

provided further insect attack, staining, decay, or drying 

degrade has not occurred. In a living tree, the heartwood is 

entirely dead and only a comparatively few sapwood cells 

are alive. Therefore, most wood is dead when cut, regardless 

of whether the tree itself is living or not. However, if a tree 

stands on the stump too long after its death, the sapwood is 

likely to decay or to be attacked severely by wood-boring 

insects, and eventually the heartwood will be similarly affected. Such deterioration also occurs in logs that have been 

cut from live trees and improperly cared for afterwards. Because of variations in climatic and other factors that affect 

deterioration, the time that dead timber may stand or lie in 

the forest without serious deterioration varies.

Tests on wood from trees that had stood as long as 15 years 

after being killed by fire demonstrated that this wood was as 

sound and strong as wood from live trees. Also, the heartwood of logs of some more durable species has been found 

to be thoroughly sound after lying in the forest for many 

years.

On the other hand, in nonresistant species, decay may cause 

great loss of strength within a very brief time, both in trees 

standing dead on the stump and in logs cut from live trees 

and allowed to lie on the ground. The important consideration is not whether the trees from which wood products are 

cut are alive or dead, but whether the products themselves 

are free from decay or other degrading factors that would 

render them unsuitable for use.

Effects of Manufacturing and Service 

Environments

Moisture Content

Many mechanical properties are affected by changes in 

moisture content below the fiber saturation point. Most 

properties reported in Tables 5–3 to 5–5 increase with decrease in moisture content. The relationship that describes 

these changes in clear wood property at about 21 °C (70 °F)

is

(5–3)

where P is the property at moisture content M (%), P12 the 

same property at 12% MC, Pg the same property for green 

wood, and Mp moisture content at the intersection of a 

horizontal line representing the strength of green wood and 

an inclined line representing the logarithm of the strength–

moisture content relationship for dry wood. This assumed 

linear relationship results in an Mp value that is slightly less 

than the fiber saturation point. Table 5–13 gives values of 

Mp for a few species; for other species, Mp = 25 may be 

assumed.

Average property values of P12 and Pg are given for many 

species in Tables 5–3 to 5–5. The formula for moisture content adjustment is not recommended for work to maximum 

load, impact bending, and tension perpendicular to grain. 

These properties are known to be erratic in their response 

to moisture content change.

The formula can be used to estimate a property at any moisture content below Mp from the species data given. For example, suppose you want to find the modulus of rupture of 

white ash at 8% moisture content. Using information from 

Tables 5–3a and 5–13,

Care should be exercised when adjusting properties below 

12% moisture. Although most properties will continue to 

Table 5–13. Intersection 

moisture content values for 

selected speciesa

Species 

Mp

(%)

Ash, white 24 

Birch, yellow 27 

Chestnut, American 24 

Douglas-fir 24 

Hemlock, western 28 

Larch, western 28 

Pine, loblolly 21 

Pine, longleaf 21 

Pine, red 24 

Redwood 21 

Spruce, red 27 

Spruce, Sitka 27 

Tamarack 24 a

Intersection moisture content is point 

at which mechanical properties begin 

to change when wood is dried from 

the green condition.

5–34

General Technical Report FPL–GTR–190
increase while wood is dried to very low moisture content levels, for most species some properties may reach 

a maximum value and then decrease with further drying 

(Fig. 5–11) (Kretschmann and Green 1996, 2008). For clear 

Southern Pine and yellow poplar, the moisture content at 

which a maximum property has been observed is given in 

Table 5–14.

This increase in mechanical properties with drying assumes 

small, clear specimens in a drying process in which no 

deterioration of the product (degrade) occurs. For 51-mm- 

(2-in.-) thick lumber containing knots, the increase in property with decreasing moisture content is dependent upon 

lumber quality. Clear, straight-grained lumber may show 

increases in properties with decreasing moisture content 

that approximate those of small, clear specimens. However, 

as the frequency and size of knots increase, the reduction 

in strength resulting from the knots begins to negate the 

increase in property in the clear wood portion of the lumber. 

Very low quality lumber that has many large knots may be 

insensitive to changes in moisture content. Figures 5–12 and 

5–13 illustrate the effect of moisture content on the properties of lumber as a function of initial lumber strength (Green 

and others 1989). Application of these results in adjusting 

allowable properties of lumber is discussed in Chapter 7. 

Additional information on influences of moisture content 

on dimensional stability is included in Chapter 13.

Temperature

Reversible Effects

In general, the mechanical properties of wood decrease 

when heated and increase when cooled. At a constant moisture content and below approximately 150 °C (302 °F), 

mechanical properties are approximately linearly related 

to temperature. The change in properties that occurs when 

wood is quickly heated or cooled and then tested at that 

condition is termed an immediate effect. At temperatures 

Figure 5–11. Effect of moisture content on wood strength 

properties. A, tension parallel to grain; B, bending; C, compression parallel to grain; D, compression perpendicular to 

grain; and E, tension perpendicular to grain.

Table 5–14. Moisture content for maximum property 

value in drying clear Southern Pine and yellow poplar 

from green to 4% moisture content 

Moisture content 

at which peak 

property occurs 

(%)

Property 

Southern

Pine

Yellow

poplar 

Ultimate tensile stress 

parallel to grain 

12.6 8.6 

Ultimate tensile stress 

perpendicular to grain 

10.2 7.1 

MOE tension perpendicular to grain 4.3 — 

MOE compression parallel to grain 4.3 4.0 

Modulus of rigidity, GRT 10.0 — 

Figure 5–12. Effect of moisture content on tensile strength 

of lumber parallel to grain.

Figure 5–13. Effect of moisture content on compressive 

strength of lumber parallel to grain.

Chapter 5 Mechanical Properties of Wood

5–35
below 100 °C (212 °F), the immediate effect is essentially 

reversible; that is, the property will return to the value at the 

original temperature if the temperature change is rapid.

Figure 5–14 illustrates the immediate effect of temperature on modulus of elasticity parallel to grain, modulus of 

rupture, and compression parallel to grain, 20 °C (68 °F), 

based on a composite of results for clear, defect-free wood. 

This figure represents an interpretation of data from several 

investigators. The width of the bands illustrates variability 

between and within reported trends.

Table 5–15 lists changes in clear wood properties at -50 °C 

(-58 °F) and 50 °C (122 °F) relative to those at 20 °C 

(68 °F) for a number of moisture conditions. The large 

changes at -50 °C (-58 °F) for green wood (at fiber saturation point or wetter) reflect the presence of ice in the wood 

cell cavities.

The strength of dry lumber, at about 12% moisture content, 

may change little as temperature increases from -29 °C 

(-20 °F) to 38 °C (100 °F). For green lumber, strength generally decreases with increasing temperature. However, for 

temperatures between about 7 °C (45 °F) and 38 °C 

(100 °F), the changes may not differ significantly from those 

at room temperature. Table 5–16 provides equations that 

Figure 5–14. Immediate effect of temperature at two 

moisture content levels relative to value at 20 °C 

(68 °F) for clear, defect-free wood: (a) modulus of elasticity parallel to grain, (b) modulus of rupture in bending, (c) compressive strength parallel to grain. The 

plot is a composite of results from several studies. 

Variability in reported trends is illustrated by 

width of bands.

Table 5–15. Approximate middle-trend effects of 

temperature on mechanical properties of clear wood at 

various moisture conditions

Relative change in 

mechanical property 

from 20 °C (68 °F) at:

Property 

Moisture

conditiona

(%)

50 °C 

(58 °F)

(%)

+50 °C

(+122 °F)

(%)

MOE parallel to grain 0 +11 6

12 +17 7

>FSP +50 — 

MOE perpendicular to grain 6 — 20

12 — 35

20 — 38

Shear modulus >FSP — 25

Bending strength 4 +18 10

11–15 +35 20

18–20 +60 25

>FSP +110 25

Tensile strength parallel to grain 0–12 — 4

Compressive strength parallel 0 +20 10

 to grain 12–45 +50 25

Shear strength parallel to grain >FSP — 25

Tensile strength perpendicular 4–6 — 10

 to grain 11–16 — 20

18 — 30

Compressive strength 

 perpendicular to grain at 

 proportional limit 

0–6

10

—

—

20

35

a

>FSP indicates moisture content greater than fiber saturation point. 

5–36

General Technical Report FPL–GTR–190
have been used to adjust some lumber properties for the reversible effects of temperature.

Irreversible Effects

In addition to the reversible effect of temperature on wood, 

there is an irreversible effect at elevated temperature. This 

permanent effect is one of degradation of wood substance, 

which results in loss of weight and strength. The loss depends on factors that include moisture content, heating 

medium, temperature, exposure period, and to some extent, 

species and size of piece involved.

The permanent decrease of modulus of rupture caused by 

heating in steam and water is shown as a function of temperature and heating time in Figure 5–15, based on tests of 

clear pieces of Douglas-fir and Sitka spruce. In the same 

studies, heating in water affected work to maximum load 

more than modulus of rupture (Fig. 5–16). The effect of 

heating dry wood (0% moisture content) on modulus of rupture and modulus of elasticity is shown in Figures 5–17 and 

5–18, respectively, as derived from tests on four softwoods 

and two hardwoods.

Figure 5–19 illustrates the permanent loss in bending 

strength of Spruce–Pine–Fir, Southern Pine, and Douglasfir standard 38- by 89-mm (nominal 2- by 4-in.) lumber 

heated at 66 °C (150 °F) and about 12% moisture content. 

Figure 5–20 illustrates the permanent loss in bending 

strength of Spruce–Pine–Fir, Southern Pine, Douglas-fir, 

and yellow-poplar standard 38- by 89-mm (nominal 2- by 

4-in.) lumber heated at 82 °C (180 °F) and about 12% moisture content. The curves for Spruce–Pine–Fir heated at 

66 °C (150 °F) and about 12% moisture content are included 

for comparison. The trends in Figure 5–20 can be compared 

with the trends in 5–19. In general, there is a greater reduction in MOR with time at the higher temperature. During 

the same time periods shown in Figures 5–19 and 5–20, 

modulus of elasticity barely changed. Acid hydrolysis of 

hemicellulose, especially of arabinose, appears to be the 

fundamental cause of strength loss resulting from thermal 

Table 5–16. Percentage change in bending properties of lumber with 

change in temperaturea

Property 

Lumber 

gradeb

Moisture

content 

((P–P70)/P70)100 = A + BT + CT2

Temperature 

range

A B C Tmin Tmax

MOE All Green 22.0350 0.4578 0 0 32 

Green 13.1215 0.1793 0 32 150 

12% 7.8553 0.1108 0 15 150

MOR SS Green 34.13 0.937 0.0043 20 46

Green 0 0 0 46 100 

12% 0 0 0 20 100

 No. 2 Green 56.89 1.562 0.0072 20 46

 or less Green 0 0 0 46 100 

Dry 0 0 0 20 100 a

For equation, P is property at temperature T in °F; P70, property at 21 °C (70 °F). b

SS is Select Structural. 

Figure 5–15. Permanent effect of heating in water (solid 

line) and steam (dashed line) on modulus of rupture of 

clear, defect-free wood. All data based on tests of Douglas-fir and Sitka spruce at room temperature.

Figure 5–16. Permanent effect of heating in water on work 

to maximum load and modulus of rupture of clear, defectfree wood. All data based on tests of Douglas-fir and Sitka 

spruce at room temperature.

Chapter 5 Mechanical Properties of Wood

5–37
degradation (Green and others 2005). It should be noted that 

most in-service exposures at 66 °C (150 °F) or 82 °C 

(180 °F) would be expected to result in much lower 

moisture content levels.

The permanent property losses discussed here are based on 

tests conducted after the specimens were cooled to room 

temperature and conditioned to a range of 7% to 12% moisture content. If specimens are tested hot, the percentage of 

strength reduction resulting from permanent effects is based 

on values already reduced by the immediate effects. Repeated exposure to elevated temperature has a cumulative effect 

on wood properties. For example, at a given temperature the 

property loss will be about the same after six 1-month exposures as it would be after a single 6-month exposure.

The shape and size of wood pieces are important in analyzing the influence of temperature. If exposure is for only a 

short time, so that the inner parts of a large piece do not 

reach the temperature of the surrounding medium, the immediate effect on strength of the inner parts will be less than 

that for the outer parts. However, the type of loading must 

be considered. If the member is to be stressed in bending, 

the outer fibers of a piece will be subjected to the greatest 

stress and will ordinarily govern the ultimate strength of the 

piece; hence, under this loading condition, the fact that 

the inner part is at a lower temperature may be of little 

significance.

For extended noncyclic exposures, it can be assumed that 

the entire piece reaches the temperature of the heating medium and will therefore be subject to permanent strength 

losses throughout the volume of the piece, regardless of 

size and mode of stress application. However, in ordinary 

construction wood often will not reach the daily temperature 

extremes of the air around it; thus, long-term effects should 

be based on the accumulated temperature experience of 

critical structural parts.

Time Under Load

Rate of Loading

Mechanical property values, as given in Tables 5–3 to 

5–5, are usually referred to as static strength values. Static 

strength tests are typically conducted at a rate of loading or 

rate of deformation to attain maximum load in about 5 min. 

Higher values of strength are obtained for wood loaded at 

a more rapid rate, and lower values are obtained at slower 

rates. For example, the load required to produce failure in 

a wood member in 1 s is approximately 10% higher than 

that obtained in a standard static strength test. Over several 

orders of magnitude of rate of loading, strength is approximately an exponential function of rate. See Chapter 7 for 

application to treated woods.

Figure 5–21 illustrates how strength decreases with time to 

maximum load. The variability in the trend shown is based 

on results from several studies pertaining to bending, compression, and shear.

Figure 5–17. Permanent effect of oven heating at four 

temperatures on modulus of rupture, based on clear 

pieces of four softwood and two hardwood species. All 

tests conducted at room temperature.

Figure 5–18. Permanent effect of oven heating at four 

temperatures on modulus of elasticity, based on clear 

pieces of four softwood and two hardwood species. All 

tests conducted at room temperature.

Figure 5–19. Residual MOR for solid-sawn lumber at 66 °C 

(150 °F) and 75% relative humidity (Green and others 2003).

5–38

General Technical Report FPL–GTR–190
Creep and Relaxation

When initially loaded, a wood member deforms elastically. 

If the load is maintained, additional time-dependent deformation occurs. This is called creep. Creep occurs at even 

very low stresses, and it will continue over a period of years. 

For sufficiently high stresses, failure eventually occurs. This 

failure phenomenon, called duration of load (or creep rupture), is discussed in the next section.

At typical design levels and use environments, after several 

years the additional deformation caused by creep may approximately equal the initial, instantaneous elastic deformation. For illustration, a creep curve based on creep as a 

function of initial deflection (relative creep) at several stress 

levels is shown in Figure 5–22; creep is greater under higher 

stresses than under lower ones.

Ordinary climatic variations in temperature and humidity 

will cause creep to increase. An increase of about 28 °C 

(50 °F) in temperature can cause a two- to threefold increase 

in creep. Green wood may creep four to six times the initial 

deformation as it dries under load.

Unloading a member results in immediate and complete 

recovery of the original elastic deformation and after time, a 

recovery of approximately one-half the creep at deformation 

as well. Fluctuations in temperature and humidity increase 

the magnitude of the recovered deformation.

Relative creep at low stress levels is similar in bending, tension, or compression parallel to grain, although it may be 

somewhat less in tension than in bending or compression 

under varying moisture conditions. Relative creep across the 

grain is qualitatively similar to, but likely to be greater than, 

creep parallel to the grain. The creep behavior of all species 

studied is approximately the same.

If instead of controlling load or stress, a constant deformation is imposed and maintained on a wood member, the initial stress relaxes at a decreasing rate to about 60% to 70% 

of its original value within a few months. This reduction 

of stress with time is commonly called relaxation. In 

limited bending tests carried out between approximately 

18 °C (64 °F) and 49 °C (120 °F) over 2 to 3 months, the 

curve of stress as a function of time that expresses relaxation 

is approximately the mirror image of the creep curve (deformation as a function of time). These tests were carried out 

at initial stresses up to about 50% of the bending strength of 

the wood. As with creep, relaxation is markedly affected by 

fluctuations in temperature and humidity.

Duration of Load

The duration of load, or the time during which a load acts 

on a wood member either continuously or intermittently, is 

an important factor in determining the load that the member 

can safely carry. The duration of load may be affected by 

changes in temperature and relative humidity. The constant 

stress that a wood member can sustain is approximately an 

Figure 5–20. Residual MOR for solid-sawn lumber at 82 °C 

(180 °F) and 80% relative humidity (RH); SPF at 66 °C (150 

°F) and 75% RH shown for comparison. SPF is Spruce–

Pine–Fir; MSR, machine stress rated; DF, Douglas-fir; and 

So. pine, Southern Pine (Green and others 2005).

Figure 5–21. Relationship of ultimate stress at short-time 

loading to that at 5-min loading, based on composite of 

results from rate-of-load studies on bending, compression, and shear parallel to grain. Variability in reported 

trends is indicated by width of band.

Figure 5–22. Influence of four levels of stress on creep 

(Kingston 1962).

Chapter 5 Mechanical Properties of Wood

5–39
exponential function of time to failure, as illustrated in 

Figure 5–22. This relationship is a composite of results of 

studies on small, clear wood specimens, conducted at constant temperature and relative humidity.

For a member that continuously carries a load for a long 

period, the load required to produce failure is much less than 

that determined from the strength properties in Tables 5–3 

to 5–5. Based on Figure 5–23, a wood member under the 

continuous action of bending stress for 10 years may carry 

only 60% (or perhaps less) of the load required to produce 

failure in the same specimen loaded in a standard bending 

strength test of only a few minutes duration. Conversely, if 

the duration of load is very short, the load-carrying capacity 

may be higher than that determined from strength properties 

given in the tables.

Time under intermittent loading has a cumulative effect. 

In tests where a constant load was periodically placed on a 

beam and then removed, the cumulative time the load was 

actually applied to the beam before failure was essentially 

equal to the time to failure for a similar beam under the 

same load applied continuously.

The time to failure under continuous or intermittent loading 

is looked upon as a creep–rupture process; a member has to 

undergo substantial deformation before failure. Deformation at failure is approximately the same for duration of load 

tests as for standard strength tests.

Changes in climatic conditions increase the rate of creep and 

shorten the duration during which a member can support 

a given load. This effect can be substantial for very small 

wood specimens under large cyclic changes in temperature 

and relative humidity. Fortunately, changes in temperature 

and relative humidity are moderate for wood in the typical 

service environment.

Fatigue

In engineering, the term fatigue is defined as the progressive 

damage that occurs in a material subjected to cyclic loading. 

This loading may be repeated (stresses of the same sign; 

that is, always compression or always tension) or reversed 

(stresses of alternating compression and tension). When 

sufficiently high and repetitious, cyclic loading stresses can 

result in fatigue failure.

Fatigue life is a term used to define the number of cycles 

that are sustained before failure. Fatigue strength, the maximum stress attained in the stress cycle used to determine 

fatigue life, is approximately exponentially related to fatigue life; that is, fatigue strength decreases approximately 

linearly as the logarithm of number of cycles increases. 

Fatigue strength and fatigue life also depend on several 

other factors: frequency of cycling; repetition or reversal of 

loading; range factor (ratio of minimum to maximum stress 

per cycle); and other factors such as temperature, moisture 

content, and specimen size. Negative range factors imply 

repeated reversing loads, whereas positive range factors imply nonreversing loads.

Results from several fatigue studies on wood are given in 

Table 5–17. Most of these results are for repeated loading 

with a range ratio of 0.1, meaning that the minimum stress 

per cycle is 10% of the maximum stress. The maximum 

stress per cycle, expressed as a percentage of estimated 

static strength, is associated with the fatigue life given in 

millions of cycles. The first three lines of data, which list 

the same cyclic frequency (30 Hz), demonstrate the effect of 

range ratio on fatigue strength (maximum fatigue stress that 

can be maintained for a given fatigue life); fatigue bending strength decreases as range ratio decreases. Third-point 

bending results show the effect of small knots or slope of 

grain on fatigue strength at a range ratio of 0.1 and frequency of 8.33 Hz. Fatigue strength is lower for wood containing small knots or a 1-in-12 slope of grain than for clear 

straight-grained wood and even lower for wood containing 

a combination of small knots and a 1-in-12 slope of grain. 

Fatigue strength is the same for a scarf joint in tension as 

for tension parallel to the grain, but a little lower for a finger 

joint in tension. Fatigue strength is slightly lower in shear 

than in tension parallel to the grain. Other comparisons do 

not have much meaning because range ratios or cyclic frequency differ; however, fatigue strength is high in compression parallel to the grain compared with other properties. 

Little is known about other factors that may affect fatigue 

strength in wood.

Creep, temperature rise, and loss of moisture content occur 

in tests of wood for fatigue strength. At stresses that cause 

Figure 5–23. Relationship between stress due to constant 

load and time to failure for small clear wood specimens, 

based on 28 s at 100% stress. The figure is a composite 

of trends from several studies; most studies involved 

bending but some involved compression parallel to grain 

and bending perpendicular to grain. Variability in reported trends is indicated by width of band.

5–40

General Technical Report FPL–GTR–190
failure in about 106 cycles at 40 Hz, a temperature rise of 

15 °C (27 °F) has been reported for parallel-to-grain 

compression fatigue (range ratio slightly greater than zero), 

parallel-to-grain tension fatigue (range ratio = 0), and 

reversed bending fatigue (range ratio = -1). The rate of 

temperature rise is high initially but then diminishes to moderate; a moderate rate of temperature rise remains more or 

less constant during a large percentage of fatigue life. During the latter stages of fatigue life, the rate of temperature 

rise increases until failure occurs. Smaller rises in temperature would be expected for slower cyclic loading or lower 

stresses. Decreases in moisture content are probably related 

to temperature rise.

Aging

In relatively dry and moderate temperature conditions where 

wood is protected from deteriorating influences such as decay, the mechanical properties of wood show little change 

with time. Test results for very old timbers suggest that 

significant losses in clear wood strength occur only after 

several centuries of normal aging conditions. The soundness 

of centuries-old wood in some standing trees (redwood, for 

example) also attests to the durability of wood.

Exposure to Chemicals

The effect of chemical solutions on mechanical properties depends on the specific type of chemical. Nonswelling 

liquids, such as petroleum oils and creosote, have no appreciable effect on properties. Properties are lowered in the 

presence of water, alcohol, or other wood-swelling organic 

liquids even though these liquids do not chemically degrade 

the wood substance. The loss in properties depends largely 

on the amount of swelling, and this loss is regained upon 

removal of the swelling liquid. Anhydrous ammonia markedly reduces the strength and stiffness of wood, but these 

properties are regained to a great extent when the ammonia 

is removed. Heartwood generally is less affected than sapwood because it is more impermeable. Accordingly, wood 

treatments that retard liquid penetration usually enhance 

natural resistance to chemicals. 

Chemical solutions that decompose wood substance (by hydrolysis or oxidation) have a permanent effect on strength. 

The following generalizations summarize the effect of 

chemicals:

•	 Some species are quite resistant to attack by dilute mineral and organic acids.

•	 Oxidizing acids such as nitric acid degrade wood more 

than do nonoxidizing acids.

•	 Alkaline solutions are more destructive than are acidic 

solutions.

•	 Hardwoods are more susceptible to attack by both acids 

and alkalis than are softwoods.

•	 Heartwood is less susceptible to attack by both acids and 

alkalis than is sapwood.

Because both species and application are extremely important, reference to industrial sources with a specific history 

of use is recommended where possible. For example, large 

cypress tanks have survived long continuous use where exposure conditions involved mixed acids at the boiling point. 

Wood is also used extensively in cooling towers because of 

its superior resistance to mild acids and solutions of acidic 

salts.

Chemical Treatment

Wood is often treated with chemicals to enhance its fire performance or decay resistance in service. Each set of treatment chemicals and processes has a unique effect on the 

mechanical properties of the treated wood.

Fire-retardant treatments and treatment methods distinctly 

reduce the mechanical properties of wood. Some fire-retardant-treated products have experienced significant in-service degradation on exposure to elevated temperatures when 

used as plywood roof sheathing or roof-truss lumber. New 

performance requirements within standards set by ASTM 

Table 5–17. Summary of reported results on cyclic 

fatiguea

Property 

Range

ratio 

Cyclic

frequency

(Hz)

Maximum

stress per

cycleb

(%)

Approximate 

fatigue 

life 

(106

 cycles)

Bending, clear, 

straight grain 

Cantilever 0.45 30 45 30 

Cantilever 0 30 40 30 

Cantilever 1.0 30 30 30 

Center-point 1.0 40 30 4 

Rotational 1.0 — 28 30 

Third-point 0.1 8-1/3 60 2 

Bending, third-point 

Small knots 0.1 8-1/3 50 2 

Clear, 1:12 slope 

of grain 

0.1 8-1/3 50 2 

Small knots, 1:12 

slope of grain 

0.1 8-1/3 40 2 

Tension parallel 

to grain 

Clear, straight grain 0.1 15 50 30 

Clear, straight grain 0 40 60 3.5 

Scarf joint 0.1 15 50 30 

Finger joint 0.1 15 40 30 

Compression parallel 

to grain 

Clear, straight grain 0.1 40 75 3.5 

Shear parallel to grain 

Glued-laminated 0.1 15 45 30 

a

Initial moisture content about 12% to 15%. 

b

Percentage of estimated static strength. 

Chapter 5 Mechanical Properties of Wood

5–41
International (formerly the American Society for Testing 

and Materials) and American Wood Protection Association 

(AWPA) preclude commercialization of inadequately performing fire-retardant-treated products.

Although preservative treatments and treatment methods 

generally reduce the mechanical properties of wood, any 

initial loss in strength from treatment must be balanced 

against the progressive loss of strength from decay when 

untreated wood is placed in wet conditions. The effects 

of preservative treatments on mechanical properties are 

directly related to wood quality, size, and various pretreatment, treatment, and post-treatment processing factors. The 

key factors include preservative chemistry or chemical type, 

preservative retention, initial kiln-drying temperature, posttreatment drying temperature, and pretreatment incising (if 

required). North American design guidelines address the 

effects of incising on mechanical properties of refractory 

wood species and the short-term duration-of-load adjustments for all treated lumber. These guidelines are described 

in Chapter 7.

Oil-Type Preservatives

Oil-type preservatives cause no appreciable strength loss 

because they do not chemically react with wood cell wall 

components. However, treatment with oil-type preservatives 

can adversely affect strength if extreme in-retort seasoning 

parameters are used (for example, Boultonizing, steaming, 

or vapor drying conditions) or if excessive temperatures or 

pressures are used during the treating process. To preclude 

strength loss, the user should follow specific treatment processing requirements as described in the treatment 

standards.

Waterborne Preservatives

Waterborne preservative treatments can reduce the mechanical properties of wood. Treatment standards include specific processing requirements intended to prevent or limit 

strength reductions resulting from the chemicals and the 

waterborne preservative treatment process. The effects of 

waterborne preservative treatment on mechanical properties 

are related to species, mechanical properties, preservative 

chemistry or type, preservative retention, post-treatment 

drying temperature, size and grade of material, product type, 

initial kiln-drying temperature, incising, and both temperature and moisture in service.

Species—The magnitude of the effect of various waterborne 

preservatives on mechanical properties does not appear to 

vary greatly between different species.

Mechanical property—Waterborne preservatives affect 

each mechanical property differently. If treated according 

to AWPA standards, the effects are as follows: modulus of 

elasticity (MOE), compressive strength parallel to grain, 

and compressive stress perpendicular to grain are unaffected 

or slightly increased; modulus of rupture (MOR) and tensile strength parallel to grain are reduced from 0% to 20%, 

depending on chemical retention and severity of redrying 

temperature; and energy‐related properties (for example, 

work to maximum load and impact strength) are reduced 

from 10% to 50%.

Preservative chemistry or type—Waterborne preservative 

chemical systems differ in regard to their effect on strength, 

but the magnitude of these differences is slight compared 

with the effects of treatment processing factors. Chemistryrelated differences seem to be related to the reactivity of 

the waterborne preservative and the temperature during the 

fixation/precipitation reaction with wood.

Retention—Waterborne preservative retention levels of 

≤16 kg m–3 (≤1.0 lb ft–3) have no effect on MOE or compressive strength parallel to grain and a slight negative 

effect (-5% to -10%) on tensile or bending strength. However, energy-related properties are often reduced from 15% 

to 30%. At a retention level of 40 kg m–3 (2.5 lb ft–3), MOR 

and energy-related properties are further reduced.

Post-treatment drying temperature—Air drying after treatment causes no significant reduction in the static 

strength of wood treated with waterborne preservative at a 

retention level of 16 kg m–3 (1.0 lb ft–3). However, energyrelated properties are reduced. The post-treatment redrying 

temperature used for material treated with waterborne preservative has been found to be critical when temperatures 

exceed 75 °C (167 °F). Redrying limitations in treatment 

standards have precluded the need for an across-the-board 

design adjustment factor for waterborne-preservative‐treated lumber in engineering design standards. The limitation 

on post-treatment kiln-drying temperature is set at 74 °C 

(165 °F).

Size of material—Generally, larger material, specifically 

thicker, appears to undergo less reduction in strength than 

does smaller material. Recalling that preservative treatments 

usually penetrate the treated material to a depth of only 

6 to 51 mm (0.25 to 2.0 in.), depending on species and other 

factors, the difference in size effect appears to be a function 

of the product’s surface-to-volume ratio, which affects the 

relative ratio of treatment-induced weight gain to original 

wood weight.

Grade of material—The effect of waterborne preservative treatment is a quality-dependent phenomenon. Higher 

grades of wood are more affected than lower grades. When 

viewed over a range of quality levels, higher quality lumber 

is reduced in strength to a proportionately greater extent 

than is lower quality lumber.

Product type—The magnitude of the treatment effect on 

strength for laminated veneer lumber conforms closely to 

effects noted for higher grades of solid-sawn lumber. The 

effects of waterborne preservative treatment on plywood 

seem comparable to that on lumber. Fiber-based composite 

5–42

General Technical Report FPL–GTR–190
products may be reduced in strength to a greater extent than 

is lumber. This additional effect on fiber-based composites 

may be more a function of internal bond damage caused by 

waterborne-treatment-induced swelling rather than actual 

chemical hydrolysis.

Initial kiln-drying temperature—Although initial kiln 

drying of some lumber species at 100 to 116 °C (212 to 

240 °F) for short durations has little effect on structural 

properties, such drying results in more hydrolytic degradation of the cell wall than does drying at lower temperature 

kiln schedules. Subsequent preservative treatment and redrying of material initially dried at high temperatures cause 

additional hydrolytic degradation. When the material is subsequently treated, initial kiln drying at 113 °C (235 °F) has 

been shown to result in greater reductions over the entire 

bending and tensile strength distributions than does initial 

kiln drying at 91 °C (196 °F). Because Southern Pine 

lumber, the most widely treated product, is most often initially kiln dried at dry-bulb temperatures near or above 

113 °C (235 °F), treatment standards have imposed a maximum redrying temperature limit of 74 °C (165 °F) to preclude the cumulative effect of thermal processing.

Incising—Incising, a pretreatment mechanical process in 

which small slits (incisions) are punched in the surface of 

the wood product, is used to improve preservative penetration and distribution in difficult-to-treat species. Incising 

may reduce strength; however, because the increase in 

treatability provides a substantial increase in biological 

performance, this strength loss must be balanced against the 

progressive loss in strength of untreated wood from the incidence of decay. Most incising patterns induce some strength 

loss, and the magnitude of this effect is related to the size 

of material being incised and the incision depth and density 

(that is, number of incisions per unit area). In <50-mm- 

(<2-in.-) thick, dry lumber, incising and preservative treatment induces losses in MOE of 5% to 15% and in static 

strength properties of 20% to 30%. Incising and treating 

timbers or tie stock at an incision density of ≤1,500 incisions m–2 (≤140 incisions ft–2) and to a depth of 19 mm 

(0.75 in.) reduces strength by 5% to 10%.

In-service temperature—Both fire-retardant and preservative treatments accelerate the thermal degradation of bending strength of lumber when exposed to temperatures above 

54 °C (130 °F).

In-service moisture content—Current design values apply 

to material dried to ≤19% maximum (15% average) moisture content or to green material. No differences in strength 

have been found between treated and untreated material 

when tested green or at moisture contents above 12%. 

When very dry treated lumber of high grade was tested at 

10% moisture content, its bending strength was reduced 

compared with that of matched dry untreated lumber.

Duration of load—When subjected to impact loads, wood 

treated with chromated copper arsenate (CCA) does not 

exhibit the same increase in strength as that exhibited by 

untreated wood. However, when loaded over a long period, 

treated and untreated wood behave similarly.

Polymerization

Wood is also sometimes impregnated with monomers, such 

as methyl methacrylate, which are subsequently polymerized. Many of the mechanical properties of the resultant 

wood–plastic composite are greater than those of the original wood, generally as a result of filling the void spaces in 

the wood structure with plastic. The polymerization process 

and both the chemical nature and quantity of monomers influence composite properties.

Nuclear Radiation

Wood is occasionally subjected to nuclear radiation. Examples are wooden structures closely associated with nuclear 

reactors, the polymerization of wood with plastic using 

nuclear radiation, and nondestructive estimation of wood 

density and moisture content. Very large doses of gamma 

rays or neutrons can cause substantial degradation of wood. 

In general, irradiation with gamma rays in doses up to about 

10 kGy has little effect on the strength properties of wood. 

As dosage exceeds 10 kGy, tensile strength parallel to grain 

and toughness decrease. At a dosage of 3 MGy, tensile 

strength is reduced about 90%. Gamma rays also affect 

compressive strength parallel to grain at a dosage above 

10 kGy, but higher dosage has a greater effect on tensile 

strength than on compressive strength; only approximately 

one-third of compressive strength is lost when the total 

dose is 3 MGy. Effects of gamma rays on bending and shear 

strength are intermediate between the effects on tensile and 

compressive strength.

Mold and Stain Fungi

Mold and stain fungi do not seriously affect most mechanical properties of wood because such fungi feed on substances within the cell cavity or attached to the cell wall rather 

than on the structural wall itself. The duration of infection 

and the species of fungi involved are important factors in 

determining the extent of degradation.

Although low levels of biological stain cause little loss in 

strength, heavy staining may reduce specific gravity by 1% 

to 2%, surface hardness by 2% to 10%, bending and crushing strength by 1% to 5%, and toughness or shock resistance 

by 15% to 30%. Although molds and stains usually do not 

have a major effect on strength, conditions that favor these 

organisms also promote the development of wood-destroying (decay) fungi and soft-rot fungi (Chap. 14). Pieces with 

mold and stain should be examined closely for decay if they 

are used for structural purposes.

Decay

Unlike mold and stain fungi, wood-destroying (decay) fungi 

seriously reduce strength by metabolizing the cellulose fraction of wood that gives wood its strength.

Chapter 5 Mechanical Properties of Wood

5–43
Early stages of decay are virtually impossible to detect. For 

example, brown-rot fungi may reduce mechanical properties in excess of 10% before a measurable weight loss is 

observed and before decay is visible. When weight loss 

reaches 5% to 10%, mechanical properties are reduced from 

20% to 80%. Decay has the greatest effect on toughness, 

impact bending, and work to maximum load in bending, the 

least effect on shear and hardness, and an intermediate effect on other properties. Thus, when strength is important, 

adequate measures should be taken to (a) prevent decay 

before it occurs, (b) control incipient decay by remedial 

measures (Chap. 14), or (c) replace any wood member in 

which decay is evident or believed to exist in a critical section. Decay can be prevented from starting or progressing if 

wood is kept dry (below 20% moisture content).

No method is known for estimating the amount of reduction in strength from the appearance of decayed wood. 

Therefore, when strength is an important consideration, the 

safe procedure is to discard every piece that contains even 

a small amount of decay. An exception may be pieces in 

which decay occurs in a knot but does not extend into the 

surrounding wood.

Insect Damage

Insect damage may occur in standing trees, logs, and undried (unseasoned) or dried (seasoned) lumber. Although 

damage is difficult to control in the standing tree, insect 

damage can be eliminated to a great extent by proper control methods. Insect holes are generally classified as pinholes, grub holes, and powderpost holes. Because of their 

irregular burrows, powderpost larvae may destroy most of 

a piece’s interior while only small holes appear on the surface, and the strength of the piece may be reduced virtually 

to zero. No method is known for estimating the reduction 

in strength from the appearance of insect-damaged wood. 

When strength is an important consideration, the safe procedure is to eliminate pieces containing insect holes.

Literature Cited

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of 23 species of eastern hardwoods. Res. Note FPL–RN–

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Bodig, J.; Jayne, B.A. 1982. Mechanics of wood and wood 

composites. New York: Van Nostrand Reinhold Company.

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structural lumber products at high temperatures I: 66°C at 

75% RH and 82°C at 30% RH. Wood and Fiber Science. 

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Durability of structural lumber products after exposure at 

82°C and 80% relative humidity. Res. Pap. FPL–RP–631. 

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Green, D.W.; Shelley, B.E.; Vokey, H.P., eds. 1989. In-grade 

testing of structural lumber. Proceedings 47363. Madison, 

WI: Forest Products Society.

Kingston, R.S.T. 1962. Creep, relaxation, and failure of 

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content on shear parallel, compression and tension perpendicular to grain strength and mode I fracture toughness of 

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Kretschmann, D.E.; Green, D.W. 1996. Modeling moisture 

content–mechanical property relationships for clear Southern Pine. Wood and Fiber Science. 28(3): 320–337.

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Additional References

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small clear specimens of timber. ASTM D143–94. West 

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Materials. 

Bendtsen, B.A. 1976. Rolling shear characteristics of nine 

structural softwoods. Forest Products Journal. 26(11): 

51–56.

Bendtsen, B.A.; Freese, F.; Ethington, R.L. 1970. Methods 

for sampling clear, straight-grained wood from the forest. 

Forest Products Journal. 20(11): 38–47.

Bodig, J.; Goodman, J.R. 1973. Prediction of elastic 

parameters for wood. Wood Science. 5(4): 249–264.

Boller, K.H. 1954. Wood at low temperatures. Modern 

Packaging. 28(1): 153–157.

Chudnoff, M. 1984. Tropical timbers of the world. Agriculture Handbook 607. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 464 p.

Coffey, D.J. 1962. Effects of knots and holes on the fatigue 

strength of quarter-scale timber bridge stringers. Madison, 

WI: University of Wisconsin, Department of Civil Engineering. M.S. thesis.

Gerhards, C.C. 1968. Effects of type of testing equipment 

and specimen size on toughness of wood. Res. Pap. 

FPL–RP–97. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 12 p.

Gerhards, C.C. 1977. Effect of duration and rate of loading 

on strength of wood and wood based materials. Res. Pap. 

5–44

General Technical Report FPL–GTR–190
FPL–RP–283. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 24 p.

Gerhards, C.C. 1979. Effect of high-temperature drying on 

tensile strength of Douglas-fir 2 by 4s. Forest Products 

Journal. 29(3): 39–46.

Gerhards, C.C. 1982. Effect of moisture content and temperature on the mechanical properties of wood: an analysis 

of immediate effects. Wood and Fiber. 14(1): 4–36.

Green, D.W.; Evans, J.W. 1994. Effect of ambient temperatures on the flexural properties of lumber. In: PTEC 

94 timber shaping the future: Proceedings, Pacific timber 

engineering conference; 1994 July 11–15; Gold Coast, Australia. Fortitude Valley MAC, Queensland, Australia: Timber 

Research Development and Advisory Council: 190–197. 

Vol. 2.

Green, D.W.; Evans J.W.; Logan J.D.; Nelson, W.J. 1999. 

Adjusting modulus of elasticity of lumber for changes in 

temperature. Madison, WI: Forest Products Society. Forest 

Products Journal. 49(10): 82–94.

Green, D.W.; Rosales, A. 1996. Property relationships for 

tropical hardwoods. In: Proceedings, international wood 

engineering conference; 1996 October 21–31; New Orleans, 

LA. Madison, WI: Forest Products Society: 3-516–3-521.

Hearmon, R.F.S. 1948. The elasticity of wood and plywood. 

Special Rep. 7. London, England: Department of Scientific 

and Industrial Research, Forest Products Research.

Hearmon, R.F.S. 1961. An introduction to applied anisotropic elasticity. London, England: Oxford University Press.

Kollmann, F.F.P.; Cote, W.A., Jr. 1968. Principles of wood 

science and technology. New York: Springer–Verlag.

Koslik, C.J. 1967. Effect of kiln conditions on the strength 

of Douglas-fir and western hemlock. Rep. D–9. Corvallis, 

OR: Oregon State University, School of Forestry, Forestry 

Research Laboratory.

Kretschmann, D.E.; Bendtsen, B.A. 1992. Ultimate tensile 

stress and modulus of elasticity of fast-grown plantation loblolly pine lumber. Wood and Fiber Science. 24(2): 189–203.

Kretschmann, D.E.; Green, D.W.; Malinauskas, V. 1991. 

Effect of moisture content on stress intensity factors in 

Southern Pine. In: Proceedings, 1991 international timber 

engineering conference; 1991 September 2–5; London. 

London: TRADA: 3.391–3.398. Vol. 3.

LeVan, S.L.; Winandy, J.E. 1990. Effects of fire-retardant 

treatments on wood strength: a review. Wood and Fiber 

Science. 22(1): 113–131.

Little, E.L., Jr. 1979. Checklist of United States trees (native 

and naturalized). Agric. Handb. 541. Washington, DC: U.S. 

Department of Agriculture. 375 p.

MacLean, J.D. 1953. Effect of steaming on the strength of 

wood. American Wood-Preservers’ Association. 49: 88–112.

MacLean, J.D. 1954. Effect of heating in water on the 

strength properties of wood. American Wood-Preservers’ 

Association. 50: 253–281.

Mallory, M.P.; Cramer, S. 1987. Fracture mechanics: a tool 

for predicting wood component strength. Forest Products 

Journal. 37(7/8): 39–47.

Mark, R.E.; Adams, S.F.; Tang, R.C. 1970. Moduli of rigidity of Virginia pine and tulip poplar related to moisture content. Wood Science. 2(4): 203–211.

McDonald, K.A.; Bendtsen, B.A. 1986. Measuring localized slope of grain by electrical capacitance. Forest Products 

Journal. 36(10): 75–78.

McDonald, K.A.; Hennon, P.E.; Stevens, J.H.; Green, D.W. 

1997. Mechanical properties of salvaged yellow-cedar in 

southeastern Alaska—Phase I. Res. Pap. FPL–RP–565. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

Millett, M.A.; Gerhards, C.C. 1972. Accelerated aging: residual weight and flexural properties of wood heated in air 

at 115°C to 175°C. Wood Science. 4(4): 193–201.

Nicholas, D.D. 1973. Wood deterioration and its prevention 

by preservative treatments. In: Degradation and Protection 

of Wood. Syracuse, NY: Syracuse University Press. Vol. I.

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detection in ladder rails. Rep. D 1733. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

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measured from plate and tension tests. Wood and Fiber 

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elastic constants for eastern hardwood species. Wood and 

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CCA-treated lumber. Forest Products Journal. 39(5): 64–68.

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I–III. Berlin: Springer–Verlag.

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structures. ANC–18 Bull. Subcommittee on Air Force–Navy 

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early stages of decay on wood strength. Wood and Fiber Science. 9(4): 252–257.

Chapter 5 Mechanical Properties of Wood

5–45
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strength of wood. Journal of Forestry. 19(7): 740–747.

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Agriculture.

Winandy, J.E. 1994. Effects of long-term elevated temperature on CCA-treated Southern Pine lumber. Forest Products 

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treatment on mechanical properties: a review. In: Proceedings, 91st annual meeting of American Wood Preservers’ 

Association; 1995, May 21–24; New York, NY. Woodstock, 

MD: American Wood Preservers’ Association. 91: 17–33.

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strength of CCA treated lumber. Forest Products Journal. 

45(2): 82–85.

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strength of CCA-treated lumber. Wood and Fiber Science. 

27(2): 168–177.

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Pullman, WA: Engineering Extension Service, Washington 

State University.

5–46

General Technical Report FPL–GTR–190
CHAPTER 6

Commercial Lumber, Round Timbers, and Ties

David E. Kretschmann, Research General Engineer

6–1

When sawn, a log yields round timber, ties, or lumber of 

varying quality. This chapter presents a general discussion 

of grading, standards, and specifications for these commercial products.

In a broad sense, commercial lumber is any lumber that is 

bought or sold in the normal channels of commerce. Commercial lumber may be found in a variety of forms, species, 

and types, and in various commercial establishments, both 

wholesale and retail. Most commercial lumber is graded by 

standardized rules that make purchasing more or less uniform throughout the country.

Round timbers and ties represent some of the most efficient uses of our forest resources. They require a minimum 

of processing between harvesting the tree and marketing 

the structural commodity. Poles and piles are debarked or 

peeled, seasoned, and often treated with preservative prior 

to use as structural members. Construction logs are usually 

shaped to facilitate construction. Ties, used for railroads, 

landscaping, and mining, are slab-cut to provide flat surfaces. Because these products are relatively economical to produce compared with glulam, steel, and concrete products, 

they are commonly used throughout the United States.

To enable users to buy the quality that best suits their purposes, lumber, round timbers, and ties are graded into use 

categories, each having an appropriate range in quality.

Generally, the grade of a piece of wood is based on the number, character, and location of features that may lower its 

strength, durability, or utility value. Among the more common visual features are knots, checks, pitch pockets, shake, 

and stain, some of which are a natural part of the tree. Some 

grades are free or practically free from these features. Other 

grades, which constitute the great bulk of solid wood products, contain fairly numerous knots and other features. With 

proper grading, lumber containing these features is entirely 

satisfactory for many uses.

The grading operation for most solid wood products takes 

place at the sawmill. Establishment of grading procedures is 

largely the responsibility of manufacturers’ associations. Because of the wide variety of wood species, industrial practices, and customer needs, different grading practices coexist. 

The grading practices of most interest are considered in the 

sections that follow, under the major categories of hardwood 

lumber and softwood lumber, round timbers, and ties.

Contents 

Hardwood Lumber 6–2

 Factory Lumber 6–2

 Dimension and Component Parts 6–4

 Finished Market Products 6–4

 Lumber Species 6–6

Softwood Lumber 6–6

 Lumber Grades 6–7

 Lumber Manufacture 6–10

 Softwood Lumber Species 6–11

 Softwood Lumber Grading 6–12

Purchase of Lumber 6–12

 Retail Yard Inventory 6–12

 Important Purchase Considerations 6–17

Round Timbers and Ties 6–18

 Standards and Specifications 6–18

 Material Requirements 6–18

 Availability 6–19

 Form 6–20

 Weight and Volume 6–21

 Durability 6–22

 Preservative Treatment 6–22

 Service Life 6–22

Commonly Used Lumber, Round Timber, 

and Tie Abbreviations 6–23

References 6–25
6–2

General Technical Report FPL–GTR–190

Hardwood Lumber

The principal use of hardwood lumber is for remanufacture 

into furniture, cabinetwork, and pallets or direct use as flooring, paneling, moulding, and millwork. Hardwood lumber is 

graded and marketed in three main categories: Factory lumber, dimension parts, and finished market products. Several 

hardwood species are graded under the American Softwood 

Lumber Standard and sold as structural lumber (Chap. 7). 

Also, specially graded hardwood lumber can be used for 

structural glued-laminated lumber.

Prior to 1898, hardwoods were graded by individual mills 

for local markets. In 1898, manufacturers and users formed 

the National Hardwood Lumber Association to standardize grading for hardwood lumber. Between 1898 and 1932, 

grading was based on the number and size of visual features. 

In 1932, the basis for grading was changed to standard 

clear-cutting sizes.

Both Factory lumber and dimension parts are intended to 

serve the industrial customer. The important difference is 

that for Factory lumber, the grades reflect the proportion of 

a piece that can be cut into useful smaller pieces, whereas 

the grades for dimension parts are based on use of the entire 

piece. Finished market products are graded for their unique 

end-use with little or no remanufacture. Examples of finished products include moulding, stair treads, and hardwood 

flooring.

Factory Lumber

Grades

The rules adopted by the National Hardwood Lumber Association are considered standard in grading hardwood lumber 

intended for cutting into smaller pieces to make furniture or 

other fabricated products. In these rules, the grade of a piece 

of hardwood lumber is determined by the proportion of a 

piece that can be cut into a certain number of smaller pieces 

of material, commonly called cuttings, which are generally 

clear on one side, have the reverse face sound, and are not 

smaller than a specified size.

The best grade in the Factory lumber category is termed 

FAS (Firsts and Seconds). The second grade is F1F (FAS 

one face). The third grade is Selects, which is followed by 

No. 1 Common, No. 2A Common, No. 2B Common, No. 

3A Common, No. 3B Common, and Sound Wormy. Except 

for F1F and Selects, the poorer side of a piece is inspected 

for grade assignment. Standard hardwood lumber grades are 

described in Table 6–1. This table illustrates, for example, 

that FAS includes pieces that will allow at least 83-1/3% 

of their surface measure to be cut into clear face material. 

Except for Sound Wormy, the minimum acceptable length, 

width, surface measure, and percentage of piece that must 

work into a cutting decrease with decreasing grade. Figure 

6–1 is an example of grading for cuttings.

This brief summary of grades for Factory lumber should 

not be regarded as a complete set of grading rules, because 

many details, exceptions, and special rules for certain species are not included. The complete official rules of the 

National Hardwood Lumber Association (NHLA) should be 

followed as the only full description of existing grades (see 

Table 6–2 for addresses of NHLA and other U.S. hardwood 

grading associations). Table 6–3 lists names of commercial 

domestic hardwood species that are graded by NHLA rules.

Figure 6–1. Example of hardwood grading for cuttings using No. 1 Common lumber grade. Current grading rules are 

written only in the inch–pound system of measurement. Standard lengths are in 1-ft increments.
6–3

Chapter 6 Commercial Lumber, Round Timbers, and Ties

Standard Dimensions

Standard lengths of hardwood lumber are in 305-mm (1-ft) 

increments from 1.2 to 4.9 m (4 to 16 ft). Standard thickness 

values for hardwood lumber, rough and surfaced on two 

sides (S2S), are given in Table 6–4. The thickness of S1S 

lumber is subject to contract agreement. Abbreviations commonly used in contracts and other documents for the purchase and sale of lumber are listed at the end of this chapter.

Hardwood lumber is usually manufactured to random 

width. The hardwood lumber grades do not specify standard 

widths; however, the grades do specify minimum width for 

each grade as follows: 

Grade

Minimum width 

(mm (in.))

FAS 152 (6)

F1F 152 (6)

Selects 102 (4)

No. 1, 2A, 2B, 3A, 3B Common 76 (3)

Table 6–1. Standard hardwood lumber gradesa,b

Grade and allowable lengths 

Allowable 

width

(in.) 

Allowable 

surface 

measure of 

pieces 

(ft2

)

Minimum 

amount of 

piece in 

clearface 

cuttings 

(%)

Allowable 

cuttings 

Maximum

no.

Minimum

size 

FAS 

8 to 16 ft 

and

F1F 

8 to 16 ftc

6+ 4 to 7 

6 and 7 

8 to 11 

8 to 11 

12 to 15 

12 to 15 

16+

83-1/3

91-2/3

83-1/3

91-2/3

83-1/2

91-2/3

83-1/3

1

2

2

3

3

4

4

4 in. by 5 ft 

or

3 in. by 7 ft 

Selects 6 to 16 ft 4+ 2 and 3 

4+

91-2/3

—d

1 4 in. by 5 ft 

or

3 in. by 7 ft 

No. 1 Common 4 to 16 ft (only 

5% of minimum width is 

allowed) 

3+ 1 

2

3 and 4 

3 and 4 

5 to 7 

5 to 7 

8 to 10 

11 to 13 

14+

100

75

66-2/3

75

66-2/3

75

66-2/3

66-2/3

66-2/3

0

1

1

2

2

3

3

4

5

4 in. by 2 ft 

or

3 in. by 3 ft 

No. 2 Common 4 to 16 ft 3+ 1 

2 and 3 

2 and 3 

4 and 5 

4 and 5 

6 and 7 

6 and 7 

8 and 9 

10 and 11

12 and 13

14+

66-2/3

50

66-2/3

50

66-2/3

50

66-2/3

50

50

50

50

1

1

2

2

3

3

4

4

5

6

7

3 in. by 2 ft 

No. 3A Common 4 to 16 ft 3+ 1+ 33-1/3f —g 3 in. by 2 ft 

No. 3B Common 4 to 16 ft 

Sound Wormye

3+ 1+ 25h

 —g 1-1/2 in. by 

36 in2

a

Current grading rules are written only in the inch–pound system of measurement. b

Inspection made on poorer side of piece, except in Selects grade. c

FAS is a grade that designates Firsts and Seconds. F1F is a grade that designates FAS one face. d

Same as F1F, with reverse side of board not below No. 1 Common. 

e

Sound Wormy grade shall not be below No. 1 Common except that the natural characteristics of worm holes, 

bird pecks, stain, sound knot not exceeding 3/4 in. in diameter are admitted. Other sound defects that do not 

exceed in extent or damage the defects described are admitted in the cuttings. Unless otherwise specified, 

Sound Wormy shall include the full product of the log in No. 1 Common and Better Sound Wormy. f

Also admits pieces that grade not below No. 2 Common on the good face and reverse side of sound cuttings. g

Unlimited. 

h

Cuttings must be sound; clear face not required. 
6–4

If the width is specified by purchase agreement, S1E or S2E 

lumber is 10 mm (3/8 in.) scant of nominal size in lumber 

less than 203 mm (8 in.) wide and 13 mm (1/2 in.) scant in 

lumber ≥203 mm (≥8 in.) wide.

Dimension and Component Parts

The term “dimension parts” for hardwoods signifies stock 

that is processed in specific thickness, width, and length, 

or multiples thereof and ranges from semi-machined to 

completely machined component products. This stock is 

sometimes referred to as “hardwood dimension stock” or 

“hardwood lumber for dimension parts.” This stock should 

not be confused with “dimension lumber,” a term used in the 

structural lumber market to mean lumber standard 38 mm to 

less than 89 mm thick (nominal 2 in. to less than 4 in. thick).

Dimension component parts are normally kiln dried and 

generally graded under the rules of the Wood Components 

Manufacturers Association (WCMA). These rules encompass three classes of material, each of which is classified 

into various grades:

Hardwood

dimension parts

(flat stock)

Solid kiln-dried 

squares

(rough)

Solid kiln-dried

squares

(surfaced)

Clear two faces Clear Clear

Clear one face Select Select

Paint Sound Paint

Core Second

Sound

Each class may be further defined as semifabricated (rough 

or surfaced) or completely fabricated, including edge-glued 

panels. The rough wood component parts are blank-sawn 

and ripped to size. Surfaced semifabricated parts have been 

through one or more manufacturing stages. Completely fabricated parts have been completely processed for their end 

use.

Finished Market Products

Some hardwood lumber products are graded in relatively 

finished form, with little or no further processing anticipated. Flooring is probably the finished market product with 

the highest volume. Other examples are lath, siding, ties, 

planks, carstock, construction boards, timbers, trim, moulding, stair treads, and risers. Grading rules promulgated for 

flooring anticipate final consumer use and are summarized 

in this section. Details on grades of other finished products 

are found in appropriate association grading rules.

Hardwood flooring generally is graded under the rules of 

the Maple Flooring Manufacturers Association (MFMA) 

or the National Oak Flooring Manufacturers Association 

(NOFMA). Tongued-and-grooved, end-matched hardwood 

flooring is commonly furnished. Square-edge, square-endstrip flooring is also available as well as parquet flooring 

suitable for laying with mastic.

The grading rules of the Maple Flooring Manufacturers 

Association cover flooring that is manufactured from hard 

maple, beech, and birch. Each species is graded into four 

categories: 

•	 First grade—one face practically free of all imperfections; variations in natural color of wood allowed

•	 Second grade—tight, sound knots (except on edges or 

ends) and other slight imperfections allowed; must be 

possible to lay flooring without waste

•	 Third grade—may contain all visual features common 

to hard maple, beech, and birch; will not admit voids 

on edges or ends, or holes over 10-mm (3/8-in.) in 

Table 6–2. Hardwood grading associations in United Statesa

Name and address 

Species covered by grading rules 

(products) 

National Hardwood Lumber Association 

P.O. Box 34518 

Memphis, TN 38184–0518 

www.nhla.com 

All hardwood species (furniture 

cuttings, construction lumber) 

Wood Components Manufacturers Association 

741 Butlers Gate, Suite 100 

Marietta, GA 30068 

www.woodcomponents.org 

All hardwood species (hardwood 

furniture dimension, squares, 

laminated stock, interior trim, stair 

treads and risers) 

Maple Flooring Manufacturers Association 

111 Deer lake Road Suite 100 

Deerfield, IL 60015 

www.maplefloor.org 

Maple, beech, birch (flooring) 

National Oak Flooring Manufacturers Association 

22 N. Front St., Suite 1080 

Memphis, TN 38103 

www.nofma.org 

Oak, ash, pecan, hickory, pecan, 

beech, birch, hard maple (flooring, 

including prefinished) 

a

Grading associations that include hardwood species in structural grades are listed in Table 6–5. 

General Technical Report FPL–GTR–190
6–5

Chapter 6 Commercial Lumber, Round Timbers, and Ties

Table 6–3. Nomenclature of commercial hardwood lumber

Commercial 

name for 

lumber Common tree name Botanical name

Commercial 

name for 

lumber Common tree name Botanical name

Alder, red Red alder Alnus rubra Maple, Oregon Big leaf maple Acer macrophyllum 

Ash, black Black ash Fraxinus nigra Maple, soft Red maple Acer rubrum 

Ash, Oregon Oregon ash Fraxinus latifolia Silver maple Acer saccharinum 

Ash, white Blue ash Fraxinus quadrangulata Oak, red Black oak Quercus velutina 

Green ash Fraxinus pennsylvanica Blackjack oak Quercus marilandica 

White ash Fraxinus americana California black oak Quercus kelloggi 

Aspen (popple) Bigtooth aspen Populus grandidentata Cherrybark oak Quercus falcata var. 

 pagodaefolia 

Quaking aspen Populus tremuloides Laurel oak Quercus laurifolia 

Basswood American basswood Tilia americana Northern pin oak Quercus ellipsoidalis 

White basswood Tilia heterophylla Northern red oak Quercus rubra 

Beech American beech Fagus grandifolia Nuttall oak Quercus nuttallii 

Birch Gray birch Betula populifolia Pin oak Quercus palustris 

Paper birch Betula papyrifera Scarlet oak Quercus coccinea 

River birch Betula nigra Shumard oak Quercus shumardii 

Sweet birch Betula lenta Southern red oak Quercus falcata 

Yellow birch Betula alleghaniensis Turkey oak Quercus laevis 

Boxelder Boxelder Acer negundo Willow oak Quercus phellos 

Buckeye Ohio buckeye Aesculus glabra Oak, white Arizona white oak Quercus arizonica 

Yellow buckeye Aesculus octandra Blue oak Quercus douglasii 

Butternut Butternut Juglans cinerea Bur oak Quercus macrocarpa 

Cherry Black cherry Prunus serotina Valley oak Quercus lobata 

Chestnut American chestnut Castanea dentate Chestnut oak Quercus prinus 

Cottonwood Balsam poplar Populus balsamifera Chinkapin oak Quercus muehlenbergii 

Eastern cottonwood Populus deltoids Emory oak Quercus emoryi 

Black cottonwood Populus trichocarpa Gambel oak Quercus gambelii 

Cucumber Cucumbertree Magnolia acuminata Mexican blue oak Quercus oblongifolia 

Dogwood Flowering dogwood Cornus florida Live oak Quercus virginiana 

Pacific dogwood Cornus nuttallii Oregon white oak Quercus garryana 

Elm, rock Cedar elm Ulmus crassifolia Overcup oak Quercus lyrata 

Rock elm Ulmus thomasii Post oak Quercus stellata 

September elm Ulmus serotina Swamp chestnut oak Quercus michauxii 

Winged elm Ulmus alata Swamp white oak Quercus bicolor 

Elm, soft American elm Ulmus Americana White oak Quercus alba 

Slippery elm Ulmus rubra Oregon myrtle California-laurel Umbellularia californica

Gum Sweetgum Liquidambar styraciflua Osage orange Osage-orange Maclura pomifera 

Hackberry Hackberry Celtis occidentalis Pecan Bitternut hickory Carya cordiformis 

Sugarberry Celtis laevigata Nutmeg hickory Carya myristiciformis 

Hickory Mockernut hickory Carya tomentosa Water hickory Carya aquatica 

Pignut hickory Carya glabra Pecan Carya illinoensis 

Shagbark hickory Carya ovata Persimmon Common 

 persimmon 

Diospyros virginiana 

Shellbark hickory Carya lacinosa Poplar Yellow-poplar Liriodendron tulipifera 

Holly American holly Ilex opaca Sassafras Sassafras Sassafras albidum 

Ironwood Eastern 

 hophornbeam 

Ostrya virginiana Sycamore Sycamore Platanus occidentalis 

Locust Black locust Robinia pseudoacacia Tanoak Tanoak Lithocarpus densiflorus 

Honeylocust Gleditsia triacanthos Tupelo Black tupelo, 

 blackgum 

Nyssa sylvatica 

Madrone Pacific madrone Arbutus menziesii Ogeechee tupelo Nyssa ogeche 

Magnolia Southern magnolia Magnolia grandiflora Water tupelo Nyssa aquatica 

Sweetbay Magnolia virginiana Walnut Black walnut Juglans nigra 

Maple, hard Black maple Acer nigrum Willow Black willow Salix nigra 

Sugar maple Acer saccharum Peachleaf willow Salix amygdaloides 
diameter; must permit proper laying of floor and provide a serviceable floor; few restrictions on imperfections; must be possible to lay flooring properly

•	 Fourth grade—may contain all visual features, but must 

be possible to lay a serviceable floor, with some cutting

Combination grades of “Second and Better” and “Third 

and Better” are sometimes specified. There are also special 

grades based on color and species.

The standard thickness of MFMA hard maple, beech, and 

birch flooring is 20 mm (25/32 in.). Face widths are 38, 51, 

57, and 83 mm (1-1/2, 2, 2-1/4, and 3-1/4 in.). Standard 

lengths are 610 mm (2 ft) and longer in First- and Secondgrade flooring and 381 mm (1-1/4 ft) and longer in Thirdgrade flooring.

The Official Flooring Grading Rules of NOFMA cover oak 

(unfinished and prefinished), beech, birch, hard maple, ash, 

and hickory/pecan. Flooring grades are determined by the 

appearance of the face surface. 

Oak is separated as red oak and white oak and by grain direction: plain sawn (all cuts), quartersawn (50% quartered 

character), rift sawn (75% rift character), and quarter/rift 

sawn (a combination). Oak flooring has four main grade 

separations—Clear, Select, No. 1 Common, and No. 2 

Common. Clear is mostly heartwood and accepts a 10-mm 

(3/8-in.) strip of bright sapwood or an equivalent amount 

not more than 25 mm (1 in.) wide along the edge and a 

minimum number of character marks and discoloration, allowing for all natural heartwood color variations. Select allows all color variations of natural heartwood and sapwood 

along with characters such as small knots, pinworm holes, 

and brown streaks. No. 1 Common contains prominent 

variations in coloration, which include heavy streaks, sticker 

stains, open checks, knots, and small knot holes that fill. 

No. 2 Common contains sound natural variation of the forest product and manufacturing imperfections to provide a 

serviceable floor.

Average lengths for unfinished oak grades are as follows:

Grade Standard packaging Shorter packaging

Clear 1.14 m (3-3/4 ft) 1.07 m (3-1/2 ft)

Select 0.99 m (3-1/4 ft) 0.91 m (3 ft)

No. 1 Common 0.84 m (2-3/4 ft) 0.76 m (2-/1/2 ft)

No. 2 Common 0.69 m (2-1/4 ft) 0.61 m (2 ft)

Standard packaging refers to nominal 2.4-m (8-ft) pallets or 

nested bundles. Shorter packaging refers to nominal 2.1-m 

(7-ft) and shorter pallets or nested bundles. 

Standard and special NOFMA grades for species other than 

oak are as follows: 

Species Grade

Standard Grades

Beech, birch,

and hard maple

First, Second, Third, Second & Better, Third 

& Better

Hickory and 

pecan

First, Second, Third, Second & Better, Third 

& Better

Ash Clear, Select, No. 1 Common,

No. 2 Common

Special Grades

Beech and birch First Grade Red

Hard maple First Grade White

Hickory and 

pecan

First Grade White, First Grade Red, Second 

Grade Red

Standard thickness values for NOFMA tongue and groove 

flooring are 19, 13, 10 (3/4, 1/2, 3/8 in.), with 20 and 

26 mm (25/32 and 33/32 in.) for maple flooring. Standard 

face widths are 38, 51, 57, and 83 mm (1-1/2, 2, 2-1/4, and 

3-1/4 in.). Strips are random length from minimum 0.23 m 

to maximum 2.6 m (9 to 102 in.). 

Lumber Species

Names used by the trade to describe commercial lumber in 

the United States are not always the same as names of trees 

adopted as official by the U.S. Forest Service. Table 6–3 

shows the commercial name, the U.S Forest Service tree 

name, and the botanical name. United States agencies and 

associations that prepare rules for and supervise grading of 

hardwoods are given in Table 6–2.

Softwood Lumber

For many years, softwood lumber has demonstrated the 

versatility of wood by serving as a primary raw material for 

6–6

Table 6–4. Standard 

thickness values for rough 

and surfaced (S2S) 

hardwood lumber

Rough Surfaced 

((mm)(in.)) ((mm)(in.)) 

10 (3/8) 5 (3/16) 

13 (1/2) 8 (5/16) 

16 (5/8) 9 (7/16) 

19 (3/4) 14 (9/16) 

25 (1) 21 (13/16) 

32 (1-1/4) 27 (1-1/16)

38 (1-1/2) 33 (1-5/16)

44 (1-3/4) 38 (1-1/2) 

51 (2) 44 (1-3/4) 

63 (2-1/2) 57 (2-1/4) 

76 (3) 70 (2-3/4) 

89 (3-1/2) 83 (3-1/4) 

102 (4) 95 (3-3/4) 

114 (4-1/2) —a —a

127 (5) —a —a

140 (5-1/2) —a —a

152 (6) —a —a

a

Finished size not specified in rules.

Thickness subject to special 

contract. 

General Technical Report FPL–GTR–190
construction and manufacture. In this role, softwood lumber has been produced in a wide variety of products from 

many different species. The first industry-sponsored grading rules (product descriptions) for softwoods, which were 

established before 1900, were comparatively simple because 

sawmills marketed their lumber locally and grades had only 

local significance. As new timber sources were developed 

and lumber was transported to distant points, each producing region continued to establish its own grading rules; thus, 

lumber from various regions differed in size, grade name, 

and allowable grade characteristics. When different species 

were graded under different rules and competed in the 

same consuming areas, confusion and dissatisfaction were 

inevitable.

To minimize unnecessary differences in the grading rules of 

softwood lumber and to improve and simplify these rules, a 

number of conferences were organized by the U.S. Department of Commerce from 1919 to 1925. These meetings 

were attended by representatives of lumber manufacturers, 

distributors, wholesalers, retailers, engineers, architects, 

and contractors. The result was a relative standardization of 

sizes, definitions, and procedures for deriving allowable design properties, formulated as a voluntary American Lumber 

Standard. This standard has been modified several times, 

including addition of hardwood species to the standard 

beginning in 1970. The current edition is the American Softwood Lumber Standard PS–20. Lumber cannot be graded as 

American Standard lumber unless the grade rules have been 

approved by the American Lumber Standard Committee 

(ALSC), Inc., Board of Review.

Softwood lumber is classified for market use by form of 

manufacture, species, and grade. For many products, the 

American Softwood Lumber Standard and the grading rules 

certified through it serve as a basic reference. For specific 

information on other products, reference must be made to 

grade rules, industry marketing aids, and trade journals.

Lumber Grades

Softwood lumber grades can be classified into three major 

categories of use: (a) yard lumber, (b) structural lumber, and 

(c) Factory and Shop lumber. Yard lumber and structural 

lumber relate principally to lumber expected to function as 

graded and sized after primary processing (sawing and planing). Factory and Shop refer to lumber that will undergo a 

number of further manufacturing steps and reach the consumer in a significantly different form.

Yard Lumber

Grading requirements of yard lumber are specifically related 

to the construction uses intended, and little or no further 

grading occurs once the piece leaves the sawmill. Yard lumber can be placed into two basic classifications, Select and 

Common. Select and Common lumber, as categorized here, 

encompass those lumber products in which appearance or 

utility is of primary importance; structural integrity, while 

sometimes important, is a secondary feature. 

Select Lumber—Select lumber is generally non-stressgraded, but it forms a separate category because of the 

distinct importance of appearance in the grading process. 

Select lumber is intended for natural and paint finishes. This 

category of lumber includes lumber that has been machined 

to a pattern and S4S lumber. Secondary manufacture of 

these items is usually restricted to on-site fitting such as 

cutting to length and mitering. The Select category includes 

trim, siding, flooring, ceiling, paneling, casing, base, stepping, and finish boards.

Most Select lumber grades are generally described by letters 

and combinations of letters (B&BTR, C&BTR, C Select, 

D, D Select) or names (Superior, Prime, Supreme, Choice, 

Quality) depending upon the species and the grading rules 

under which the lumber is graded. (See list of commonly 

used lumber abbreviations at the end of this chapter.) The 

specifications FG (flat grain), VG (vertical grain), and MG 

(mixed grain) are offered as a purchase option for some 

select lumber products.

In cedar and redwood, there is a pronounced difference in 

color between heartwood and sapwood. Heartwood also has 

high natural resistance to decay, so some grades are denoted 

as “heart.” Because Select lumber grades emphasize the 

quality of one face, the reverse side may be lower in quality. Select lumber grades are not uniform across species and 

products, so certified grade rules for the species must be 

used for detailed reference.

Common Lumber—Common lumber is normally a nonstress-graded product. The grades of Common lumber are 

suitable for construction and utility purposes. Common lumber is generally separated into three to five different grades 

depending upon the species and grading rules involved. 

Grades may be described by number (No. 1, No. 2, No. 1 

Common, No. 2 Common) or descriptive term (Select Merchantable, Construction, Standard).

Because there are differences in the inherent properties of 

various species and their corresponding names, the grades 

for different species are not always interchangeable. The 

top-grade boards (No. 1, No. 1 Common, Select Merchantable) are usually graded for serviceability, but appearance 

is also considered. These grades are used for such purposes 

as siding, cornice, shelving, and paneling. Features such as 

knots and knotholes are permitted to be larger and more frequent as the grade level becomes lower. Intermediate-grade 

boards are often used for such purposes as subfloors, roof 

and wall sheathing, and rough concrete work. The lower 

grade boards are selected for adequate usability, not appearance. They are used for roof and wall sheathing, subfloor, 

and rough concrete form work (Fig. 6–2).

Grading provisions for other non-stress-graded products 

vary by species, product, and applicable grading rules. For 

detailed descriptions, consult the appropriate grade rule 

for these products (see Table 6–5 for softwood grading

organizations).

Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–7
Structural Lumber—Almost all softwood lumber standard 

38 to 89 mm thick (nominal 2 to 4 in. thick, actual 1-1/2 to 

3-1/2 in. thick) is produced as dimension lumber. Dimension 

lumber is stress graded and assigned allowable properties 

under the National Grading Rule, a part of the American 

Softwood Lumber Standard. For dimension lumber, a single 

set of grade names and descriptions is used throughout the 

United States, although the allowable properties vary with 

species. Timbers (lumber standard 114 mm (nominal 5 in.) 

or more in least dimension) are also structurally graded under ALSC procedures. Unlike grade descriptions for dimension lumber, grade descriptions for structural timbers are not 

standardized across species. For most species, timber grades 

are classified according to intended use. Beams and stringers are members standard 114 mm (nominal 5 in.) or more 

in thickness with a width more than 38 mm (nominal 2 in.) 

greater than the thickness. Beams and stringers are primarily used to resist bending stresses, and the grade description 

of some timber grades for the middle third of the length of 

the beam is more stringent than that for the outer two-thirds. 

Posts and timbers are members standard 114 by 114 mm 

(nominal 5 by 5 in.) and larger, where the width is not more 

than 38 mm (nominal 2 in.) greater than the thickness. Post 

and timbers are primarily used to resist axial stresses. Structural timbers of Southern Pine are graded without regard to 

anticipated use, as with dimension lumber.

Other stress-graded products include decking and some 

boards. Stress-graded lumber may be graded visually or 

mechanically. Stress grades and the National Grading Rule 

are discussed in Chapter 6.

Structural Laminations—Structural laminating grades 

describe the characteristics used to segregate lumber to be 

used in structural glued-laminated (glulam) timbers. Generally, allowable properties are not assigned separately to 

laminating grades; rather, the rules for laminating grades are 

based on the expected effect of that grade of lamination on 

the combined glulam timber.

There are two kinds of graded material: visually graded and 

E-rated. Visually graded material is graded according to one 

of three sets of grading rules: (1) the first set is based on 

the grading rules certified as meeting the requirements of 

the American Softwood Lumber Standard with additional 

requirements for laminating; (2) the second set involves 

laminating grades typically used for visually graded western 

species and includes three basic categories (L1, L2, L3); and 

(3) the third set includes special requirements for tension 

members and outer tension laminations on bending members. The visual grades have provisions for dense, closegrain, medium-grain, or coarsegrain lumber.

The E-rated grades are categorized by a combination of 

visual grading criteria and lumber stiffness. These grades 

are expressed in terms of the size of maximum edge characteristic permitted (as a fraction of the width) along with 

a specified long-span modulus of elasticity (for example, 

1/6–2.2E). 

Radius-Edged Decking—Radius-edged decking is another 

substantial softwood lumber product. Radius-edged decking 

is intended for flatwise use and has oversized eased edges 

of a particular radius. Most often radius-edged decking is 

produced as 25- or 38-mm- (nominal 5/4- or 2-in.-, actual 1- 

or 1-1/2-in.-) thick by 140-mm- (nominal 4- to 6-in.-, actual 

3-1/2- to 5-1/2-in.-) wide pieces of lumber 2.4 to 4.9 m (8 to 

16 ft) in length. The standard radius for 25-mm-thick radiusedged decking product is 6.4 mm (1/4 in.), and 9.5 mm (3/8 

in.) for 38-mm-thick decking. Decking is usually separated 

into a minimum of two grades, most commonly Premium 

and Standard.

Factory and Shop Lumber

A wide variety of species, grades, and sizes of softwood 

lumber is supplied to industrial accounts for cutting to specific smaller sizes, which become integral parts of other 

products. In the secondary manufacturing process, grade 

descriptions, sizes, and often the entire appearance of the 

wood piece are changed. Thus, for Factory and Shop lumber, the role of the grading process is to reflect as accurately 

as possible the yield to be obtained in the subsequent cutting 

operation. Typical of lumber for secondary manufacture are 

the factory grades, industrial clears, box lumber, moulding 

stock, and ladder stock. The variety of species available for 

these purposes has led to a variety of grade names and grade 

definitions. The following sections briefly outline some of 

the more common classifications. For details, reference must 

6–8

Figure 6–2. Typical examples of softwood boards in the 

lower grades.

General Technical Report FPL–GTR–190
Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–9

Table 6–5. Organizations promulgating softwood grades

Name and address Species covered by grading rules 

Cedar Shingle & Shake Bureau 

515 116th Avenue NE, Suite 275 

Bellevue, WA 98004–5294 

Western redcedar (shingles and 

shakes) 

National Hardwood Lumber Association 

P.O. Box 34518 

Memphis, TN 38184–0518 

Baldcypress, eastern redcedar 

National Lumber Grades Authoritya

406 First Capital Place 

960 Quamside Drive 

New Westminister, BC, Canada V3M6G2 

Northern white-cedar, western 

redcedar, yellow-cedar, alpine fir, 

amabilis fir, balsam fir, Douglas-fir, 

grand fir, eastern hemlock, western 

hemlock, western larch, eastern white 

pine, jack pine, lodgepole pine, 

ponderosa pine, red pine, western 

white pine, black spruce, Sitka spruce, 

red spruce, Engelmann spruce, white 

spruce, tamarack, aspen, black 

cottonwood, balsam poplar, red alder, 

white birch 

Northeastern Lumber Manufacturers Association, Inc.a

272 Tuttle Road, P.O. Box 87A 

Cumberland Center, ME 04021 

Balsam fir, eastern white pine, red 

pine, eastern hemlock, black spruce, 

white spruce, red spruce, pitch pine, 

tamarack, jack pine, northern white 

cedar, aspen, mixed maple, beech, 

birch, hickory, mixed oaks, yellow 

poplar 

Northern Softwood Lumber Bureaua

272 Tuttle Road, P.O. Box 87A 

Cumberland Center, ME 04021 

Eastern white pine, jack pine, red pine, 

pitch pine, eastern spruce (red, white, 

and black), balsam fir, eastern 

hemlock, tamarack, eastern 

cottonwood, aspen yellow poplar 

Redwood Inspection Servicea

405 Enfrente Drive, Suite 200 

Novato, CA 94949 

Redwood

Southern Cypress Manufacturers Association 

400 Penn Center Boulevard Suite 530 

Pittsburgh, PA 15235 

Baldcypress 

Southern Pine Inspection Bureaua

4709 Scenic Highway 

Pensacola, FL 32504 

Longleaf pine, slash pine, shortleaf 

pine, loblolly pine, Virginia pine, 

pond pine, sand pine, baldcypress 

West Coast Lumber Inspection Bureaua

Box 23145 

6980 SW. Varns Road 

Portland, OR 97223 

Douglas-fir, western hemlock, western 

redcedar, incense-cedar, Port-Orfordcedar, yellow-cedar, western true firs, 

mountain hemlock, Sitka spruce, 

western larch 

Western Wood Products Associationa

Yeon Building, 522 SW Fifth Avenue 

Portland, OR 97204–2122 

Ponderosa pine, western (Idaho) white 

pine, Douglas-fir, sugar pine, western 

true firs, western larch, Engelmann 

spruce, incense-cedar, western 

hemlock, lodgepole pine, western 

redcedar, mountain hemlock, red 

alder, aspen, subalpine fir, Sitka 

spruce, Port-Orford cedar a

Publishes grading rules certified by the Board of Review of the American Lumber Standard Committee as 

conforming to the American Softwood Lumber Standard PS–20. 
be made to industry sources, such as certified grading rules. 

Availability and grade designation often vary by region and 

species.

Factory (Shop) Grades—Traditionally, softwood lumber 

used for cuttings has been called Factory or Shop. This 

lumber forms the basic raw material for many secondary 

manufacturing operations. Some grading rules refer to these 

grades as Factory, while others refer to them as Shop. All 

impose a somewhat similar nomenclature in the grade structure. Shop lumber is graded on the basis of characteristics 

that affect its use for general cut-up purposes or on the basis 

of size of cutting, such as for sash and doors. Factory Select 

and Select Shop are typical high grades, followed by No. 1 

Shop, No. 2 Shop, and No. 3 Shop.

Grade characteristics of boards are influenced by the width, 

length, and thickness of the basic piece and are based on 

the amount of high-quality material that can be removed by 

cutting. Typically, Factory Select and Select Shop lumber 

would be required to contain 70% of cuttings of specified 

size, clear on both sides. No. 1 Shop would be required to 

have 50% cuttings and No. 2 Shop, 33-1/3%. Because of 

different characteristics assigned to grades with similar nomenclature, the grades of Factory and Shop lumber must be 

referenced to the appropriate certified grading rules.

Industrial Clears—These grades are used for trim, cabinet 

stock, garage door stock, and other product components 

where excellent appearance, mechanical and physical 

properties, and finishing characteristics are important. The 

principal grades are B&BTR, C, and D Industrial. Grading 

is primarily based on the best face, although the influence of 

edge characteristics is important and varies depending upon 

piece width and thickness. In redwood, the Industrial Clear 

All Heart grade includes an “all heart” requirement for decay resistance in the manufacture of cooling towers, tanks, 

pipe, and similar products.

Moulding, Ladder, Pole, Tank, and Pencil Stock—Within 

producing regions, grading rules delineate the requirements 

for a variety of lumber classes oriented to specific consumer 

products. Custom and the characteristics of the wood supply have led to different grade descriptions and terminology. 

For example, in West Coast species, the ladder industry can 

choose from one “ladder and pole stock” grade plus two 

ladder rail grades and one ladder rail stock grade. In Southern Pine, ladder stock is available as Select and Industrial. 

Moulding stock, tank stock, pole stock, stave stock, stadium 

seat stock, box lumber, and pencil stock are other typical 

classes oriented to the final product. Some product classes 

have only one grade level; a few offer two or three levels. 

Special features of these grades may include a restriction 

on sapwood related to desired decay resistance, specific requirements for slope of grain and growth ring orientation for 

high-stress use such as ladders, and particular cutting requirements as in pencil stock. All references to these grades 

should be made directly to current certified grading rules.

Lumber Manufacture

Size

Lumber length is recorded in actual dimensions, whereas 

width and thickness are traditionally recorded in “nominal” 

dimensions—actual dimensions are somewhat less.

Softwood lumber is manufactured in length multiples of 

305 mm (1 ft) as specified in various grading rules. In 

practice, 610-mm (2-ft) multiples (in even numbers) are 

common for most construction lumber. Width of softwood 

lumber varies, commonly from standard 38 to 387 mm 

(nominal 2 to 16 in.). The thickness of lumber can be generally categorized as follows:

•	 Boards—lumber less than standard 38 mm (nominal 

2 in.) in thickness

•	 Dimension—lumber from standard 38 mm (nominal 

2 in.) to, but not including, 114 mm (nominal 5 in.) in 

thickness

•	 Timbers—lumber standard 114 mm (nominal 5 in.) or 

more in thickness in least dimension

To standardize and clarify nominal to actual sizes, the American Softwood Lumber Standard PS–20 specifies the actual 

thickness and width for lumber that falls under the standard. 

The standard sizes for yard and structural lumber are given 

in Table 6–6. Timbers are usually surfaced while “green” 

(unseasoned); however, dry sizes are also given.

Because dimension lumber and boards of some species may 

be surfaced green or dry at the prerogative of the manufacturer, both green and dry standard sizes are given. The 

sizes are such that a piece of green lumber, surfaced to the 

standard green size, will shrink to approximately the standard dry size as it dries to about 15% moisture content. The 

definition of dry boards and dimension is lumber that has 

been seasoned or dried to a maximum moisture content of 

19%. The definition for dry timbers of the various species 

is found in the certified grading rules. Lumber may also be 

designated as kiln dried (KD), meaning the lumber has been 

seasoned in a chamber to a predetermined moisture content 

by applying heat.

Factory and Shop lumber for remanufacture is offered in 

specified sizes to fit end-product requirements. Factory 

(Shop) grades for general cuttings are offered in thickness 

from standard 19 to 89 mm (nominal 1 to 4 in.). Thicknesses of door cuttings start at 29 mm (nominal 1-3/8 in.). Cuttings are of various lengths and widths. Laminating stock is 

sometimes offered oversize, compared with standard dimension sizes, to permit resurfacing prior to laminating. Industrial Clears can be offered rough or surfaced in a variety of 

sizes, starting from standard 38 mm (nominal 2 in.) 

and thinner and as narrow as standard 64 mm (nominal 

3 in.). Sizes for special product grades such as moulding 

stock and ladder stock are specified in appropriate grading 

rules or handled by purchase agreements.

6–10

General Technical Report FPL–GTR–190
Surfacing

Lumber can be produced either rough or surfaced (dressed). 

Rough lumber has surface imperfections caused by the primary sawing operations. It may be greater than target size 

by variable amounts in both thickness and width, depending 

upon the type of sawmill equipment. Rough lumber serves 

as a raw material for further manufacture and also for some 

decorative purposes. A roughsawn surface is common in 

post and timber products. 

Surfaced lumber has been surfaced by a machine on one 

side (S1S), two sides (S2S), one edge (S1E), two edges 

(S2E), or combinations of sides and edges (S1S1E, S2S1E, 

S1S2, S4S). Lumber is surfaced to attain smoothness of 

surface and uniformity of size.

Imperfections or blemishes defined in the grading rules 

and caused by machining are classified as “manufacturing 

imperfections.” For example, chipped and torn grain are 

surface irregularities in which surface fibers have been torn 

out by the surfacing operation. Chipped grain is a “barely 

perceptible” characteristic, while torn grain is classified by 

depth. Raised grain, skip, machine burn and gouge, chip 

marks, and wavy surfacing are other manufacturing imperfections. Manufacturing imperfections are defined in the 

American Softwood Lumber Standard and further detailed 

in the grading rules. Classifications of manufacturing imperfections (combinations of imperfections allowed) are established in the rules as Standard A, Standard B, and so on. For 

example, Standard A admits very light torn grain, occasional 

very light chip marks, and very slight knife marks. These 

classifications are used as part of the grade rule description 

of some lumber products to specify the allowable surface 

quality.

Patterns

Lumber that has been matched, shiplapped, or otherwise 

patterned, in addition to being surfaced, is often classified 

as “worked lumber.” Figure 6–3 shows typical patterns.

Softwood Lumber Species

The names of lumber species adopted by the trade as standard may vary from the names of trees adopted as official 

by the U.S. Forest Service. Table 6–7 shows the American 

Softwood Lumber Standard commercial names for lumber, 

the U.S. Forest Service tree names, and the botanical names. 

Some softwood species are marketed primarily in combinations. Designations such as Southern Pine and Hem–Fir 

represent typical combinations. Grading rule agencies 

Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–11

Table 6–6. American Standard Lumber sizes for yard and structural lumber for construction

Thickness Face width 

Minimum dressed Minimum dressed 

Nominal Dry Green Nominal Dry Green 

Item (in.) (mm (in.)) (mm (in.)) (in.) (mm (in.)) (mm (in.)) 

Boards 1 19 (3/4) 20 (25/32) 2 38 (1-1/2) 40 (1-9/16) 

1-1/4 25 (1) 26 (1-1/32) 3 64 (2-1/2) 65 (2-9/16) 

1-1/2 32 (1-1/4) 33 (1-9/32) 4 89 (3-1/2) 90 (3-9/16) 

 5 114 (4-1/2) 117 (4-5/8) 

 6 140 (5-1/2) 143 (5-5/8) 

 7 165 (6-1/2) 168 (6-5/8) 

 8 184 (7-1/4) 190 (7-1/2) 

 9 210 (8-1/4) 216 (8-1/2) 

 10 235 (9-1/4) 241 (9-1/2) 

 11 260 (10-1/4) 267 (10-1/2) 

 12 286 (11-1/4) 292 (11-1/2) 

 14 337 (13-1/4) 343 (13-1/2) 

 16 387 (15-1/4) 394 (15-1/2) 

Dimension 2 38 (1-1/2) 40 (1-9/16) 2 38 (1-1/2) 40 (1-9/16) 

2-1/2 51 (2) 52 (2-1/16) 3 64 (2-1/2) 65 (2-9/16) 

3 64 (2-1/2) 65 (2-9/16) 4 89 (3-1/2) 90 (3-9/16) 

3-1/2 76 (3) 78 (3-1/16) 5 114 (4-1/2) 117 (4-5/8) 

4 89 (3-1/2) 90 (3-9/16) 6 140 (5-1/2) 143 (5-5/8) 

4-1/2 102 (4) 103 (4-1/16) 8 184 (7-1/4) 190 (7-1/2) 

 10 235 (9-1/4) 241 (9-1/2) 

 12 286 (11-1/4) 292 (11-1/2) 

 14 337 (13-1/4) 343 (13-1/2) 

 16 387 (15-1/4) 394 (15-1/2) 

Timbers 5 & 6 thick 

7–15 thick 

≥ 16 thick 

13 mm 

off 

19 mm 

off 

25 mm 

off 

(1/2 in.

off) 

(3/4 in.

off) 

(1 in. 

off) 

13 mm

off 

13 mm

off 

13 mm

off 

(1/2 in. 

off) 

(1/2 in. 

off) 

(1/2 in. 

off) 

5 & 6 wide

7–15 wide

≥ 16 wide 

13 mm

off 

19 mm

off 

25 mm

off 

(1/2 in. 

off) 

(3/4 in. 

off) 

(1 in. 

off) 

13 mm 

off 

13 mm 

off 

13 mm 

off 

(1/2 in. 

off 

(1/2 in. 

off) 

(1/2 in. 

off) 
(Table 6–5) should be contacted for questions regarding 

combination names and species not listed in Table 6–7. 

Species groups are discussed further in Chapter 7.

Softwood Lumber Grading

Most lumber is graded under the supervision of inspection 

bureaus and grading agencies. These organizations supervise lumber mill grading and provide reinspection services 

to resolve disputes concerning lumber shipments. Some of 

these agencies also write grading rules that reflect the species and products in the geographic regions they represent. 

These grading rules follow the American Softwood Lumber 

Standard (PS–20). This is important because it provides 

for recognized uniform grading procedures. Names and addresses of rules-writing organizations in the United States 

and the species with which they are concerned are listed 

in Table 6–5. Canadian softwood lumber imported into the 

United States and graded by inspection agencies in Canada 

also follows the PS–20 standard. (Names and addresses of 

accredited Canadian grading agencies may be obtained from 

the American Lumber Standard Committee, P.O. Box 210, 

Germantown, MD 20874; email: alsc@alsc.org; 

www.alsc.org.)

Purchase of Lumber

After primary manufacture, most lumber products are 

marketed through wholesalers to remanufacturing plants 

or retail outlets. Because of the extremely wide variety of 

lumber products, wholesaling is very specialized—some 

organizations deal with only a limited number of species 

or products. Where the primary manufacturer can readily 

identify the customers, direct sales may be made. Primary 

manufacturers often sell directly to large retail-chain contractors, manufacturers of mobile and modular housing, and 

truss fabricators.

Some primary manufacturers and wholesalers set up distribution yards in lumber-consuming areas to distribute both 

hardwood and softwood products more effectively. Retail 

yards draw inventory from distribution yards and, in woodproducing areas, from local lumber producers. The wide 

range of grades and species covered in the grade rules may 

not be readily available in most retail outlets.

Transportation is a vital factor in lumber distribution. Often, the lumber shipped by water is green because weight 

is not a major factor in this type of shipping. On the other 

hand, lumber reaching the East Coast from the Pacific Coast 

by rail is usually kiln-dried because rail shipping rates are 

based on weight. A shorter rail haul places southern and 

northeastern species in a favorable economic position in 

regard to shipping costs in this market.

Changing transportation costs have influenced shifts in 

market distribution of species and products. Trucks have 

become a major factor in lumber transport for regional remanufacture plants, for retail supply from distribution yards, 

and for much construction lumber distribution.

The increased production capacity of foreign hardwood 

and softwood manufacturing and the availability of water 

transport have brought foreign lumber products to the U.S. 

market, particularly in coastal areas.

Retail Yard Inventory

Small retail yards throughout the United States carry softwoods for construction purposes and often carry small 

6–12

Figure 6–3. Typical patterns of worked lumber.

General Technical Report FPL–GTR–190
Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–13

Table 6–7. Nomenclature of principal commercial softwood lumber

Commercial species or 

species group names 

under American Softwood 

Lumber Standard 

Tree name used 

in this handbook Botanical name 

Cedar

Alaska yellow-cedar Chamaecyparis nootkatensis 

Eastern red eastern redcedar Juniperus virginiana 

Incense incense-cedar Libocedrus decurrens 

Northern white northern white-cedar Thuja occidentalis 

Port Orford Port-Orford-cedar Chamaecyparis lawsoniana 

Southern white Atlantic white-cedar Chamaecyparis thyoides 

Western red western redcedar Thuja plicata 

Cypress

Baldcypress baldcypress Taxodium distichum 

Pond cypress pond cypress Taxodium distichum var. nutans 

Fir

Alpine subalpine fir (alpine fir) Abies lasiocarpa 

Balsam balsam fir Abies balsamea 

California red California red fir Abies magnifica 

Douglas Fir Douglas-fir Pseudotsuga menziesii 

Fraser Fraser fir Abies fraseri 

Grand grand fir Abies grandis 

Noble Fir noble fir Abies procera 

Pacific Grand Pacific silver fir Abies amabilis 

White white fir Abies concolor

Hemlock

Carolina Carolina hemlock Tsuga caroliniana 

Eastern eastern hemlock Tsuga canadensis 

Mountain mountain hemlock Tsuga mertensiana 

Western western hemlock Tsuga heterophylla 

Juniper

Western alligator juniper Juniperus deppeana 

Rocky Mountain juniper Juniperus scopulorum 

Utah juniper Juniperus osteosperma 

western juniper Juniperus occidentalis 

Larch

 Western western larch Larix occidentalis 

Pine

Bishop bishop pine Pinus muricata 

Coulter Coulter pine Pinus coulteri 

Digger Digger pine Pinus sabibiana 

Knobcone knobcone pine Pinus attenuata 

Idaho white Western white pine Pinus monticola 

Jack jack pine Pinus banksiana 

Jeffrey Jeffrey pine Pinus jeffreyi 

Limber limber pine Pinus flexilis 

Lodgepole lodgepole pine Pinus contorta 

Longleaf longleaf pine Pinus palustris 

slash pine Pinus elliottii 

Northern white eastern white pine Pinus strobus 

Norway red pine Pinus resinosa 

Pitch pitch pine Pinus rigida 

Ponderosa ponderosa pine Pinus ponderosa 

Southern Pine Major loblolly pine Pinus taeda 

longleaf pine Pinus palustris 

shortleaf pine Pinus echinata 

slash pine Pinus elliottii 

Southern Pine Minor pond pine Pinus serotina 

sand pine Pinus clausa 

spruce pine Pinus glabra 

Virginia pine Pinus virginiana 

Southern Pine Mixed loblolly pine Pinus taeda 

longleaf pine Pinus palustris 
6–14

Table 6–7. Nomenclature of principal commercial softwood lumber—con.

pond pine Pinus serotina 

shortleaf pine Pinus echinata 

slash pine Pinus elliottii 

Virginia pine Pinus virginiana 

Radiata/Monterey Pine Monterey pine Pinus radiata 

Sugar sugar pine Pinus lambertiana 

Whitebark whitebark pine Pinus albicaulis 

Redwood

Redwood redwood Sequoia sempervirens 

Spruce

Blue blue spruce Picea pungens 

Eastern black spruce Picea mariana 

red spruce Picea rubens 

white spruce Picea glauca 

Engelmann Engelmann spruce Picea engelmannii 

Sitka Sitka spruce Picea sitchensis 

Tamarack

Tamarack tamarack Larix larcinia 

Yew

Pacific Pacific yew Taxus brevifolia 

Coast Species Douglas-fir Pseudotsuga menziesii 

western larch Larix occidentalis 

Eastern Softwoods black spruce Picea mariana 

red spruce Picea rubens 

white spruce Picea glauca 

balsam fir Abies balsamea 

eastern white pine Pinus strobus 

jack pine Pinus banksiana 

pitch pine Pinus rigida 

red pine Pinus resinosa 

eastern hemlock Tsuga canadensis 

tamarack Larix occidentalis 

Hem–Fir western hemlock Tsuga heterophylla 

California red fir Abies magnifica 

grand fir Abies grandis 

noble fir Abies procera 

Pacific silver fir Abies amabilis 

white fir Abies concolor

Hem–Fir (North) western hemlock Tsuga heterophylla 

Pacific silver fir Abies amabilis 

Northern Pine jack pine Pinus banksiana 

pitch pine Pinus rigida 

red pine Pinus resinosa 

North Species northern white cedar Thuja occidentalis 

western redcedar Thuja plicanta 

yellow-cedar Chamaecyparis nootkatensis 

eastern hemlock Tsuga canadensis 

western hemlock Tsuga heterophylla 

Douglas-fir Pseudotsuga menziesii 

balsam fir Abies balsamea 

grand fir Abies grandis 

Pacific silver fir Abies amabilis 

subalpine (alpine) fir Abies lasiocarpa 

western larch Larix occidentalis 

tamarack Larix laricina 

eastern white pine Pinus strobus 

jack pine Pinus banksiana 

lodgepole pine Pinus contorta 

ponderosa pine Pinus ponderosa 

red pine Pinus resinosa 

western white pine Pinus monticola 

whitebark pine Pinus albicaulis 

General Technical Report FPL–GTR–190

Table 6–7. Nomenclature of principal commercial softwood lumber

Commercial species or 

species group names 

under American Softwood 

Lumber Standard 

Tree name used 

in this handbook Botanical name 

Cedar

Alaska yellow-cedar Chamaecyparis nootkatensis 

Eastern red eastern redcedar Juniperus virginiana 

Incense incense-cedar Libocedrus decurrens 

Northern white northern white-cedar Thuja occidentalis 

Port Orford Port-Orford-cedar Chamaecyparis lawsoniana 

Southern white Atlantic white-cedar Chamaecyparis thyoides 

Western red western redcedar Thuja plicata 

Cypress

Baldcypress baldcypress Taxodium distichum 

Pond cypress pond cypress Taxodium distichum var. nutans 

Fir

Alpine subalpine fir (alpine fir) Abies lasiocarpa 

Balsam balsam fir Abies balsamea 

California red California red fir Abies magnifica 

Douglas Fir Douglas-fir Pseudotsuga menziesii 

Fraser Fraser fir Abies fraseri 

Grand grand fir Abies grandis 

Noble Fir noble fir Abies procera 

Pacific Grand Pacific silver fir Abies amabilis 

White white fir Abies concolor

Hemlock

Carolina Carolina hemlock Tsuga caroliniana 

Eastern eastern hemlock Tsuga canadensis 

Mountain mountain hemlock Tsuga mertensiana 

Western western hemlock Tsuga heterophylla 

Juniper

Western alligator juniper Juniperus deppeana 

Rocky Mountain juniper Juniperus scopulorum 

Utah juniper Juniperus osteosperma 

western juniper Juniperus occidentalis 

Larch

 Western western larch Larix occidentalis 

Pine

Bishop bishop pine Pinus muricata 

Coulter Coulter pine Pinus coulteri 

Digger Digger pine Pinus sabibiana 

Knobcone knobcone pine Pinus attenuata 

Idaho white Western white pine Pinus monticola 

Jack jack pine Pinus banksiana 

Jeffrey Jeffrey pine Pinus jeffreyi 

Limber limber pine Pinus flexilis 

Lodgepole lodgepole pine Pinus contorta 

Longleaf longleaf pine Pinus palustris 

slash pine Pinus elliottii 

Northern white eastern white pine Pinus strobus 

Norway red pine Pinus resinosa 

Pitch pitch pine Pinus rigida 

Ponderosa ponderosa pine Pinus ponderosa 

Southern Pine Major loblolly pine Pinus taeda 

longleaf pine Pinus palustris 

shortleaf pine Pinus echinata 

slash pine Pinus elliottii 

Southern Pine Minor pond pine Pinus serotina 

sand pine Pinus clausa 

spruce pine Pinus glabra 

Virginia pine Pinus virginiana 

Southern Pine Mixed loblolly pine Pinus taeda 

longleaf pine Pinus palustris 

Table 6–7. Nomenclature of principal commercial softwood lumber—con.

pond pine Pinus serotina 

shortleaf pine Pinus echinata 

slash pine Pinus elliottii 

Virginia pine Pinus virginiana 

Radiata/Monterey Pine Monterey pine Pinus radiata 

Sugar sugar pine Pinus lambertiana 

Whitebark whitebark pine Pinus albicaulis 

Redwood

Redwood redwood Sequoia sempervirens 

Spruce

Blue blue spruce Picea pungens 

Eastern black spruce Picea mariana 

red spruce Picea rubens 

white spruce Picea glauca 

Engelmann Engelmann spruce Picea engelmannii 

Sitka Sitka spruce Picea sitchensis 

Tamarack

Tamarack tamarack Larix larcinia 

Yew

Pacific Pacific yew Taxus brevifolia 

Coast Species Douglas-fir Pseudotsuga menziesii 

western larch Larix occidentalis 

Eastern Softwoods black spruce Picea mariana 

red spruce Picea rubens 

white spruce Picea glauca 

balsam fir Abies balsamea 

eastern white pine Pinus strobus 

jack pine Pinus banksiana 

pitch pine Pinus rigida 

red pine Pinus resinosa 

eastern hemlock Tsuga canadensis 

tamarack Larix occidentalis 

Hem–Fir western hemlock Tsuga heterophylla 

California red fir Abies magnifica 

grand fir Abies grandis 

noble fir Abies procera 

Pacific silver fir Abies amabilis 

white fir Abies concolor

Hem–Fir (North) western hemlock Tsuga heterophylla 

Pacific silver fir Abies amabilis 

Northern Pine jack pine Pinus banksiana 

pitch pine Pinus rigida 

red pine Pinus resinosa 

North Species northern white cedar Thuja occidentalis 

western redcedar Thuja plicanta 

yellow-cedar Chamaecyparis nootkatensis 

eastern hemlock Tsuga canadensis 

western hemlock Tsuga heterophylla 

Douglas-fir Pseudotsuga menziesii 

balsam fir Abies balsamea 

grand fir Abies grandis 

Pacific silver fir Abies amabilis 

subalpine (alpine) fir Abies lasiocarpa 

western larch Larix occidentalis 

tamarack Larix laricina 

eastern white pine Pinus strobus 

jack pine Pinus banksiana 

lodgepole pine Pinus contorta 

ponderosa pine Pinus ponderosa 

red pine Pinus resinosa 

western white pine Pinus monticola 

whitebark pine Pinus albicaulis 
Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–15

Table 6–7. Nomenclature of principal commercial softwood lumber—con.

black spruce Picea mariana 

Engelmann spruce Picea engelmannii 

red spruce Picea rubens 

Sitka spruce Picea sitchensis 

bigtooth aspen Populus grandidentata 

quaking aspen Populus tremuloides 

black cottonwood Populus trichocarpa 

balsam poplar Populus balsamifera 

Southern Pine loblolly pine Pinus taeda 

longleaf pine Pinus palustris 

shortleaf pine Pinus echinata 

slash pine Pinus elliottii 

Spruce–Pine–Fir black spruce Picea mariana 

Engelmann spruce Picea engelmannii 

red spruce Picea rubens 

balsam fir Abies balsamea 

subalpine (alpine) fir Abies lasiocarpa 

jack pine Pinus banksiana 

lodgepole pine Pinus contorta 

Spruce–Pine–Fir (South) black spruce Picea mariana 

Engelmann spruce Picea engelmannii 

red spruce Picea rubens 

Sitka spruce Picea sitchensis 

white spruce Picea glauca 

balsam fir Abies balsamea 

jack pine Pinus banksiana 

lodgepole pine Pinus contorta 

red pine Pinus resinosa 

Western Cedars incense-cedar Libocedrus decurrens 

western redcedar Thuja plicata 

Port-Orford-cedar Chamaecyparis lawsoniana 

yellow-cedar Chamaecyparis nootkatensis 

Western Cedar (North) western redcedar Thuja plicata 

yellow-cedar Chamaecyparis nootkatensis 

Western Woods Douglas-fir Pseudotsuga menziesii 

California red fir Abies magnifica 

grand fir Abies grandis 

noble fir Abies procera 

Pacific silver fir Abies amabilis 

subalpine fir Abies lasiocarpa 

white fir Abies concolor

Hemlock mountain Tsuga mertensiana 

western hemlock Tsuga heterophylla 

western larch Larix occidentalis 

Engelmann spruce Picea engelmannii 

Sitka spruce Picea sitchensis 

lodgepole pine Pinus contorta 

ponderosa pine Pinus ponderosa 

sugar pine Pinus lambertiana 

western white pine Pinus monticola 

White Woods California red fir Abies magnifica 

grand fir Abies grandis 

noble fir Abies procera 

Pacific silver fir Abies amabilis 

subalpine fir Abies lasiocarpa 

white fir Abies concolor

mountain hemlock Tsuga mertensiana 

western hemlock Tsuga heterophylla 

Engelmann spruce Picea engelmannii 

Sitka spruce Picea sitchensis 

lodgepole pine Pinus contorta 

ponderosa pine Pinus ponderosa 

sugar pine Pinus lambertiana 

western white pine Pinus monticola 

Table 6–7. Nomenclature of principal commercial softwood lumber

Commercial species or 

species group names 

under American Softwood 

Lumber Standard 

Tree name used 

in this handbook Botanical name 

Cedar

Alaska yellow-cedar Chamaecyparis nootkatensis 

Eastern red eastern redcedar Juniperus virginiana 

Incense incense-cedar Libocedrus decurrens 

Northern white northern white-cedar Thuja occidentalis 

Port Orford Port-Orford-cedar Chamaecyparis lawsoniana 

Southern white Atlantic white-cedar Chamaecyparis thyoides 

Western red western redcedar Thuja plicata 

Cypress

Baldcypress baldcypress Taxodium distichum 

Pond cypress pond cypress Taxodium distichum var. nutans 

Fir

Alpine subalpine fir (alpine fir) Abies lasiocarpa 

Balsam balsam fir Abies balsamea 

California red California red fir Abies magnifica 

Douglas Fir Douglas-fir Pseudotsuga menziesii 

Fraser Fraser fir Abies fraseri 

Grand grand fir Abies grandis 

Noble Fir noble fir Abies procera 

Pacific Grand Pacific silver fir Abies amabilis 

White white fir Abies concolor

Hemlock

Carolina Carolina hemlock Tsuga caroliniana 

Eastern eastern hemlock Tsuga canadensis 

Mountain mountain hemlock Tsuga mertensiana 

Western western hemlock Tsuga heterophylla 

Juniper

Western alligator juniper Juniperus deppeana 

Rocky Mountain juniper Juniperus scopulorum 

Utah juniper Juniperus osteosperma 

western juniper Juniperus occidentalis 

Larch

 Western western larch Larix occidentalis 

Pine

Bishop bishop pine Pinus muricata 

Coulter Coulter pine Pinus coulteri 

Digger Digger pine Pinus sabibiana 

Knobcone knobcone pine Pinus attenuata 

Idaho white Western white pine Pinus monticola 

Jack jack pine Pinus banksiana 

Jeffrey Jeffrey pine Pinus jeffreyi 

Limber limber pine Pinus flexilis 

Lodgepole lodgepole pine Pinus contorta 

Longleaf longleaf pine Pinus palustris 

slash pine Pinus elliottii 

Northern white eastern white pine Pinus strobus 

Norway red pine Pinus resinosa 

Pitch pitch pine Pinus rigida 

Ponderosa ponderosa pine Pinus ponderosa 

Southern Pine Major loblolly pine Pinus taeda 

longleaf pine Pinus palustris 

shortleaf pine Pinus echinata 

slash pine Pinus elliottii 

Southern Pine Minor pond pine Pinus serotina 

sand pine Pinus clausa 

spruce pine Pinus glabra 

Virginia pine Pinus virginiana 

Southern Pine Mixed loblolly pine Pinus taeda 

longleaf pine Pinus palustris 

Table 6–7. Nomenclature of principal commercial softwood lumber—con.

black spruce Picea mariana 

Engelmann spruce Picea engelmannii 

red spruce Picea rubens 

Sitka spruce Picea sitchensis 

bigtooth aspen Populus grandidentata 

quaking aspen Populus tremuloides 

black cottonwood Populus trichocarpa 

balsam poplar Populus balsamifera 

Southern Pine loblolly pine Pinus taeda 

longleaf pine Pinus palustris 

shortleaf pine Pinus echinata 

slash pine Pinus elliottii 

Spruce–Pine–Fir black spruce Picea mariana 

Engelmann spruce Picea engelmannii 

red spruce Picea rubens 

balsam fir Abies balsamea 

subalpine (alpine) fir Abies lasiocarpa 

jack pine Pinus banksiana 

lodgepole pine Pinus contorta 

Spruce–Pine–Fir (South) black spruce Picea mariana 

Engelmann spruce Picea engelmannii 

red spruce Picea rubens 

Sitka spruce Picea sitchensis 

white spruce Picea glauca 

balsam fir Abies balsamea 

jack pine Pinus banksiana 

lodgepole pine Pinus contorta 

red pine Pinus resinosa 

Western Cedars incense-cedar Libocedrus decurrens 

western redcedar Thuja plicata 

Port-Orford-cedar Chamaecyparis lawsoniana 

yellow-cedar Chamaecyparis nootkatensis 

Western Cedar (North) western redcedar Thuja plicata 

yellow-cedar Chamaecyparis nootkatensis 

Western Woods Douglas-fir Pseudotsuga menziesii 

California red fir Abies magnifica 

grand fir Abies grandis 

noble fir Abies procera 

Pacific silver fir Abies amabilis 

subalpine fir Abies lasiocarpa 

white fir Abies concolor

Hemlock mountain Tsuga mertensiana 

western hemlock Tsuga heterophylla 

western larch Larix occidentalis 

Engelmann spruce Picea engelmannii 

Sitka spruce Picea sitchensis 

lodgepole pine Pinus contorta 

ponderosa pine Pinus ponderosa 

sugar pine Pinus lambertiana 

western white pine Pinus monticola 

White Woods California red fir Abies magnifica 

grand fir Abies grandis 

noble fir Abies procera 

Pacific silver fir Abies amabilis 

subalpine fir Abies lasiocarpa 

white fir Abies concolor

mountain hemlock Tsuga mertensiana 

western hemlock Tsuga heterophylla 

Engelmann spruce Picea engelmannii 

Sitka spruce Picea sitchensis 

lodgepole pine Pinus contorta 

ponderosa pine Pinus ponderosa 

sugar pine Pinus lambertiana 

western white pine Pinus monticola 
stocks of one or two hardwoods in grades suitable for finishing or cabinetwork. Special orders must be made for other 

hardwoods. Trim items such as moulding in either softwood 

or hardwood are available cut to standard size and pattern. 

Millwork plants usually make ready-for-installation cabinets, and retail yards carry or catalog many common styles 

and sizes. Hardwood flooring is available to the buyer only 

in standard patterns. Most retail yards carry stress grades of 

lumber.

The assortment of species in general construction items 

carried by retail yards depends to a great extent upon geographic location, and both transportation costs and tradition 

are important factors. Retail yards within, or close to, a 

major lumber-producing region commonly emphasize local 

timber. For example, a local retail yard on the Pacific Northwest Coast may stock only green Douglas Fir and cedar 

in dimension grades, dry pine and hemlock in boards and 

moulding, and assorted special items such as redwood posts, 

cedar shingles and shakes, and rough cedar siding. The only 

hardwoods may be walnut and “Philippine mahogany” (the 

common market name encompassing many species, including tanguile, red meranti, and white lauan). Retail yards 

located farther from a major softwood supply, such as in 

the Midwest, may draw from several growing areas and 

may stock spruce and Southern Pine, for example. Because 

they are located in a major hardwood production area, these 

yards may stock, or have available to them, a different and 

wider variety of hardwoods.

Geography has less influence where consumer demands are 

more specific. For example, where long construction lumber 

(6 to 8 m (20 to 26 ft)) is required, West Coast species are 

often marketed because the height of the trees in several 

species makes long lengths a practical market item. Ease 

of preservative treatability makes treated Southern Pine 

construction lumber available in a wide geographic area.

Structural Lumber for Construction

Dimension lumber is the principal stress-graded lumber 

available in a retail yard. It is primarily framing lumber for 

joists, rafters, and studs. Strength, stiffness, and uniformity 

of size are essential requirements. Dimension lumber is 

stocked in almost all yards, frequently in only one or two 

of the general purpose construction woods such as pine, 

fir, hemlock, or spruce. Standard 38- by 89-mm (nominal 

2- by 4-in.) and wider dimension lumber is found in Select 

Structural, No. 1, No. 2, and No. 3 grades. Standard 38- by 

89-mm (nominal 2- by 4-in.) dimension lumber may also 

be available as Construction, Standard, Utility, and Stud 

grades. Stud grade is also available in wider widths.

Dimension lumber is often found in standard 38-, 89-, 140-, 

184-, 235-, and 286-mm (nominal 2-, 4-, 6-, 8-, 10-, and 

12-in.) widths and 2.4- to 5.5-m (8- to 18-ft) lengths in multiples of 0.6 m (2 ft). Dimension lumber formed by structural end-jointing procedures may be available. Dimension 

lumber thicker than standard 38 mm (nominal 2 in.) and 

longer than 5.5 m (18 ft) may not be commonly available in 

many retail yards.

Other stress-graded products generally available are posts 

and timbers; some beams and stringers may also be in stock. 

Typical grades in these products are Select Structural, No. 1, 

and No. 2.

Yard Lumber for Construction

Boards are the most common non-stress-graded general 

purpose construction lumber in the retail yard. Boards are 

stocked in one or more species, usually in standard 19-mm 

(nominal 1-in.) thickness. Common widths are standard 38, 

64, 89, 140, 184, 235, and 286 mm (nominal 2, 3, 4, 6, 8, 

10, and 12 in.). Grades generally available in retail yards 

are No. 1 Common, No. 2 Common, and No. 3 Common 

(Construction, Standard, No. 1, No. 2, etc.). Boards are sold 

square edged, dressed (surfaced) and matched (tongued and 

grooved), or with a shiplapped joint. Boards formed by endjointing of shorter sections may constitute an appreciable 

portion of the inventory.

Select Lumber

Completion of a construction project usually depends on 

the availability of lumber items in finished or semi-finished 

form. The following items often may be stocked in only a 

few species, finishes, or sizes depending on the lumber yard.

Finish—Finish boards usually are available in a local yard 

in one or two species, principally in grade C&BTR. Cedar 

and redwood have different grade designations: grades such 

as Clear Heart, A, or B are used in cedar; Clear All Heart, 

Clear, and B grade are typical in redwood. Finish boards are 

usually standard 19 mm (nominal 1 in.) thick, surfaced on 

two sides to 19 mm (nominal 1 in.); 38- to 286-mm (nominal 2- to 12-in.) widths are usually stocked, in even 

increments.

Siding—Siding is specifically intended to cover exterior 

walls. Beveled siding is ordinarily stocked in white pine, 

ponderosa pine, western redcedar, cypress, or redwood. 

Drop siding, also known as rustic or barn siding, is usually 

stocked in the same species as is beveled siding. Siding may 

be stocked as B&BTR or C&BTR except in cedar, where 

Clear, A, and B grades may be available, and redwood, 

where Clear All Heart, Clear, and B grades may be found. 

Vertical grain (VG) is sometimes part of the grade designation. Drop siding is also sometimes stocked in C and D 

grades of Southern Pine, Douglas Fir, and hemlock. Drop 

siding may be surfaced and matched, or shiplapped. Knotty 

grades of cedar (Select Tight Knot (STK)) and redwood 

(Rustic) are commonly available.

Flooring—Flooring is made chiefly from hardwoods, such 

as oak and maple, and the harder softwood species, such 

as Douglas-fir, western larch, and Southern Pine. Often, at 

least one softwood and one hardwood are stocked. Flooring 

is usually 19 mm (nominal 1 in.) thick. Thicker flooring 

is available for heavy-duty floors. Thinner flooring is 

6–16

General Technical Report FPL–GTR–190
available, especially for re-covering old floors. Vertical- and 

flat-grained (also called quartersawn and plainsawn) flooring is manufactured from both softwoods and hardwoods. 

Vertical-grained flooring shrinks and swells less than flatgrained flooring, is more uniform in texture, and wears more 

uniformly, and the edge joints have less tendency to open.

Softwood flooring is usually available in B&BTR, C Select, 

or D Select grades. In maple, the chief grades are Clear, 

No. 1, and No. 2. The grades in quartersawn oak are Clear 

and Select, and in plainsawn, Clear, Select, and No. 1 

Common. Quartersawn hardwood flooring has the same 

advantages as does vertical-grained softwood flooring. In 

addition, the silver or flaked grain of quartersawn flooring is 

frequently preferred to the figure of plainsawn flooring.

Casing and Base—Casing and base are standard items 

in the more important softwoods and are stocked in most 

yards in at least one species. The chief grade, B&BTR, is 

designed to meet the requirements of interior trim for dwellings. Many casing and base patterns are surfaced to 17 by 

57 mm (11/16 by 2-1/4 in.); other sizes include 14 mm 

(9/16 in.) by 76 mm (3 in.), by 83 mm (3-1/4 in.), and by 

89 mm (3-1/2 in.). Hardwoods for the same purposes, such 

as oak and birch, may be carried in stock in the retail yard 

or obtained on special order.

Shingles and Shakes—Commonly available shingles are 

sawn from western redcedar and northern white-cedar. For 

western redcedar, the shingle grades are No. 1, No. 2, and 

No. 3; for northern white-cedar, Extra, Clear, 2nd Clear, 

Clearwall, and Utility.

Shingles that contain only heartwood are more resistant to 

decay than are shingles that contain sapwood. Edge-grained 

shingles are less likely to warp and split than flat-grained 

shingles, thick-butted shingles less likely than thin-butted 

shingles, and narrow shingles less likely than wide shingles. 

The standard thickness values of thin-butted shingles are 

described as 4/2, 5/2-1/4, and 5/2 (four shingles to 51 mm 

(2 in.) of butt thickness, five shingles to 57 mm (2-1/4 in.) 

of butt thickness, and five shingles to 51 mm (2 in.) of butt 

thickness). Lengths may be 406, 457, or 610 mm (16, 18, or 

24 in.). Random widths and specified (“dimension” shingle) 

widths are available in western redcedar, redwood, and 

cypress.

Shingles are usually packed four bundles to a square. A 

square of shingles will cover roughly 9 m2 (100 ft2) of roof 

area when the shingles are applied at standard weather 

exposures.

Shakes are hand split or hand split and resawn from western 

redcedar. Shakes are of a single grade and must be 100% 

clear. In the case of hand split and resawn material, shakes 

are graded from the split face. Hand-split shakes are graded 

from the best face. Shakes must be 100% heartwood. The 

standard thickness of shakes ranges from 9.5 to 32 mm 

(3/8 to 1-1/4 in.). Lengths are 457 and 610 mm (18 and 

Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–17

24 in.), with a special “Starter–Finish Course” length of 

381 mm (15 in.).

Pallet and Container Stock—Wood is often manufactured 

into lengths and sizes for wooden pallets and containers. 

As with other uses of wood, pallet and container stock 

must meet minimum wood quality requirements for checks, 

splits, shakes, wane, cross grain, decay, knots, and warp 

that are specific to their intended application. A detailed 

description of the recognized minimum quality requirements for wood used in the principal types of wood pallets 

is documented in Uniform Standard for Wood Pallets, and 

that for packaging is detailed in the Uniform Standard for 

Wood Containers produced by the National Wooden Pallet 

and Container Association (NWPCA 2007, 2009). See these 

documents for a more complete description of terms commonly understood among manufacturers, repairers, distributors, and users of wood pallets and containers. The specifications are specific to the expected number of uses, single 

or multiple, the item being manufactured is expected to see. 

Important Purchase Considerations

Some points to consider when ordering lumber or timbers 

are the following:

1. Quantity—Lineal measure, board measure, surface measure, number of pieces of definite size and length. Consider that the board measure depends on the thickness 

and width nomenclature used and that the interpretation 

of these must be clearly delineated. In other words, such 

features as nominal or actual dimensions and pattern 

size must be considered.

2. Size—Thickness in millimeters or inches—nominal 

or actual if surfaced on faces; width in millimeters or 

inches—nominal or actual if surfaced on edges; length 

in meters or feet—may be nominal average length, 

limiting length, or a single uniform length. Often a 

trade designation, “random” length, is used to denote a 

nonspecified assortment of lengths. Such an assortment 

should contain critical lengths as well as a range. The 

limits allowed in making the assortment random can be 

established at the time of purchase.

3. Grade—As indicated in grading rules of lumber manufacturing associations. In softwoods that are in compliance with the American Softwood Lumber Standard, 

each piece of lumber may be grade stamped with its 

official grade designation, species identification, a name 

or number identifying the producing mill, the dryness at 

the time of surfacing, and a symbol identifying the inspection agency supervising the grading inspection. The 

grade designation stamped on a piece indicates the quality at the time the piece was graded. Subsequent exposure to unfavorable storage conditions, improper drying, 

or careless handling may cause the material to fall below 

its original grade.

Working or recutting a graded product to a pattern may 

change or invalidate the original grade. The purchase 
specification should be clear in regard to regrading or 

acceptance of worked lumber. In softwood lumber, 

grades for dry lumber generally are determined after 

kiln drying and surfacing. However, this practice is not 

general for hardwood Factory lumber, where the grade is 

generally based on quality and size prior to kiln drying. 

To be certain the product grade is correct, refer to the 

grading rule by number and paragraph.

4. Species or species group of wood—Such as Douglas 

Fir, Southern Pine, Hem–Fir. Some species have been 

grouped for marketing convenience; others are sold 

under a variety of names. Be sure the species or species 

group is correctly and clearly described on the purchase 

specification.

5. Product—Such as flooring, siding, timbers, boards. 

Nomenclature varies by species, region, and grading 

association. To be certain the nomenclature is correct 

for the product, refer to the grading rule by number and 

paragraph.

6. Condition of seasoning—Such as air dry, kiln dry. Softwood lumber less than 114 mm (nominal 5 in.) in thickness dried to 19% moisture content or less is defined 

as dry by the American Softwood Lumber Standard. 

Kiln-dried lumber is lumber that has been seasoned in a 

chamber to a predetermined moisture content by applying heat. For lumber of nominal 5-in. or greater in thickness, some species are defined as dry having a maximum 

moisture content of greater than 19%. Green lumber is 

lumber less than 114 mm (nominal 5 in.) in thickness 

that has a moisture content in excess of 19%. For lumber 

of nominal 5-in. or greater thickness, green shall be defined in accordance with the provision of the applicable 

grading rules. If the moisture requirement is critical, the 

level of moisture content and the method by which it 

will be achieved must be specified.

7. Surfacing and working—Rough (unplaned), surfaced 

(dressed, planed), or patterned stock. Specify condition. 

If surfaced, indicate code (S4S, S1S1E). If patterned, list 

pattern number with reference to appropriate grade rules.

8. Grading rules—Official grading agency name and name 

of official rules under which product is graded, product 

identification, paragraph and page number of rules, and 

date of rules or official rule edition may be specified by 

the buyer.

9. Manufacturer—Name of manufacturer or trade name of 

specific product or both. Most lumber products are sold 

without reference to a specific manufacturer. If proprietary names or quality features of a manufacturer are 

required, this must be stipulated clearly on the purchase 

agreement.

10. Structural lumber and timbers should be stamped by an 

agency accredited by the Board of Review of the American Lumber Standard Committee.

11. Reinspection—Procedures for resolution of purchase 

disputes. The American Softwood Lumber Standard 

provides for procedures to be followed in resolution of 

manufacturer–wholesaler–consumer conflicts over quality or quantity of ALS lumber grades. The dispute may 

be resolved by reinspecting the shipment. Time limits, 

liability, costs, and complaint procedures are outlined 

in the grade rules of both softwood and hardwood agencies under which the disputed shipment was graded 

and purchased.

Round Timbers and Ties

Standards and Specifications

Material standards and specifications listed in Table 6–8 

were created through the joint efforts of producers and users to ensure compatibility between product quality and end 

use. These guidelines include recommendations for production, treatment, and engineering design. They are updated 

periodically to conform to changes in material and design 

technology.

Material Requirements

Round timber and tie material requirements vary with intended use. The majority of uses involve exposure to harsh 

Table 6–8. Standards and specifications for round timbers and tiesa

Engineering design stresses 

Product

Material 

requirements

Preservative 

treatment Procedures Design values 

Utility poles ANSI O5.1 AWPA Commodity 

Specification D 

— ANSI O5.1 

Construction poles ANSI O5.1 AWPA Commodity 

Specification D 

ASTM D 3200 ASAE EP 388 

Piles ASTM D 25 AWPA Commodity 

Specification E 

ASTM D 2899 NDS

Construction logs (See material supplier) — ASTM D 3957 (See material supplier) 

Ties AREA AWPA Commodity 

Specification C, AREA 

— AREA

a

ANSI, American National Standards Institute; ASTM, ASTM International; ASAE, American Society of Agricultural Engineers; 

AREA, American Railway Engineers Association; NDS, National Design Specification (for Wood Construction); AWPA, 

American Wood Protection Association. 

6–18

General Technical Report FPL–GTR–190
environments. Thus, in addition to availability, form, and 

weight, durability is also an important consideration for 

the use of round timbers and ties. Availability reflects the 

economic feasibility of procuring members of the required 

size and grade. Form or physical appearance refers to visual 

characteristics, such as straightness and occurrence of knots 

and spiral grain. Weight affects shipping and handling costs 

and is a function of volume, moisture content, and wood 

density. Durability is directly related to expected service life 

and is a function of treatability and natural decay resistance. 

Finally, regardless of the application, any structural member 

must be strong enough to resist imposed loads with a reasonable factor of safety. Material specifications available for 

most applications of round timbers and ties contain guidelines for evaluating these factors.

Availability

Material evaluation begins with an assessment of availability. For some applications, local species of timber may 

be readily available in an acceptable form and quality. 

However, this is not normally the case. Pole producers and 

tie mills are scattered throughout heavily forested regions. 

Their products are shipped to users throughout North 

America.

Poles

Most structural applications of poles require timbers that 

are relatively straight and free of large knots. Poles used to 

support electric utility distribution and transmission lines 

(Fig. 6–4) range in length from 6 to 38 m (20 to 125 ft) and 

from 0.13 to 0.76 m (5 to 30 in.) in diameter, 1.8 m (6 ft) 

from the butt. Poles used to support local area distribution 

lines are normally <15 m (<50 ft) long and are predominately Southern Pine. 

Hardwood species can be used for poles when the trees are 

of suitable size and form; their use is limited, however, by 

their weight, by their excessive checking, and because of the 

lack of experience in preservative treatment of hardwoods. 

Thus, most poles are softwoods.

The Southern Pine lumber group (principally loblolly, 

longleaf, shortleaf, and slash) accounts for roughly 80% of 

poles treated in the United States. Three traits of these pines 

account for their extensive use: thick and easily treated 

sapwood, favorable strength properties and form, and availability in popular pole sizes. In longer lengths, Southern 

Pine poles are in limited supply, so Douglas-fir, and to some 

extent western redcedar, ponderosa pine, and western larch, 

are used to meet requirements for 15-m (50-ft) and longer 

transmission poles.

Douglas-fir is used throughout the United States for transmission poles and is used in the Pacific Coast region for 

distribution and building poles. Because the heartwood 

of Douglas-fir is resistant to preservative penetration and 

has limited decay and termite resistance, serviceable poles 

need a well-treated shell of sapwood that is free of checking. To minimize checking after treatment, poles should be 

adequately seasoned or conditioned before treatment. With 

these precautions, the poles should compare favorably with 

treated Southern Pine poles in serviceability.

A small percentage of the poles treated in the United States 

are of western redcedar, produced mostly in British Columbia. The number of poles of this species used without 

treatment is not known but is considered to be small. Used 

primarily for utility lines in northern and western United 

States, well-treated redcedar poles have a service life that 

compares favorably with poles made from other species 

and could be used effectively in pole-type buildings.

Lodgepole pine is also used in small quantities for treated 

poles. This species is used both for utility lines and for 

pole-type buildings. It has a good service record when well 

treated. Special attention is necessary, however, to obtain 

poles with sufficient sapwood thickness to ensure adequate 

penetration of preservative, because the heartwood is not 

usually penetrated and is not decay resistant. The poles must 

also be well seasoned prior to treatment to avoid checking 

and exposure of unpenetrated heartwood to attack by decay 

fungi.

Western larch poles produced in Montana and Idaho came 

into use after World War II because of their favorable size, 

shape, and strength properties. Western larch requires preservative treatment full length for use in most areas and, as 

in the case of lodgepole pine poles, must be selected for adequate sapwood thickness and must be well seasoned prior 

to treatment. Other species occasionally used for poles are 

listed in the American National Standards Institute (ANSI) 

O5.1 standard. These minor species make up a very small 

portion of pole production and are used locally. Gluedlaminated, or glulam, poles are also available for use where 

Figure 6–4. 

An example 

of round 

timber poles 

used for 

electrical 

utility 

distribution.

Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–19
special sizes or shapes are required. The ANSI Standard 

O5.2 provides guidelines for specifying these poles.

Piles

Material available for timber piles is more restricted than 

that for poles. Most timber piles used in the eastern half of 

the United States are Southern Pine, while those used in 

western United States are coast Douglas-fir. Oak, red pine, 

and cedar piles are also referenced in timber pile literature 

but are not as widely used as Southern Pine and Douglas-fir.

Construction Logs

Round timbers have been used in a variety of structures, including bridges, log cabins, and pole buildings. Log stringer 

bridges (Fig. 6–5) are generally designed for a limited life 

on logging roads intended to provide access to remote areas. 

In Alaska where logs may exceed 1 m (3 ft) in diameter, 

bridge spans may exceed 9 m (30 ft). Building poles, on the 

other hand, are preservative-treated logs in the 0.15- to 

0.25-m- (6- to 10-in.-) diameter range. These poles rarely 

exceed 9 m (30 ft) in length. Although poles sold for this application are predominately Southern Pine, there is potential 

for competition from local species in this category. Finally, 

log cabin logs normally range from 0.2 to 0.25 m (8 to 

10 in.) in diameter, and the availability of logs in this size 

range is not often a problem. However, because logs are 

not normally preservative-treated for this application, those 

species that offer moderate to high natural decay resistance, 

such as western redcedar, are preferred. Pole buildings, 

which incorporate round timbers as vertical columns and 

cantilever supports, require preservative-treated wood. 

Preservative-treated poles for this use may not be readily 

available.

Ties

The most important availability consideration for railroad 

cross ties is quantity. Ties are produced from most native 

species of timber that yield log lengths >2.4 m (8 ft) with 

diameters >0.18 m (7 in.). The American Railway Engineering Association (AREA) lists 26 U.S. species that may be 

used for ties. Thus, the tie market provides a use for many 

low-grade hardwood and softwood logs.

Form

Natural growth properties of trees play an important role in 

their use as structural round timbers. Three important form 

considerations are cross-sectional dimensions, straightness, 

and the presence of surface characteristics such as knots.

Poles and Piles

Standards for poles and piles have been written with the 

assumption that trees have a round cross section with a circumference that decreases linearly with height. Thus, the 

shape of a pole or pile is often assumed to be that of the 

frustum of a cone. Actual measurements of tree shape indicate that taper is rarely linear and often varies with location 

along the height of the tree. Average taper values from the 

ANSI O5.1 standard are shown in Table 6–9 for the more 

popular pole species. Guidelines to account for the effect 

of taper on the location of the critical section above the 

groundline are given in ANSI O5.1. The standard also tabulates pole dimensions for up to 15 size classes of 11 major 

pole species.

Taper also affects construction detailing of pole buildings. 

Where siding or other exterior covering is applied, poles are 

generally set with the taper to the interior side of the structures to provide a vertical exterior surface (Fig. 6–6). 

Another common practice is to modify the round poles by 

slabbing to provide a continuous flat face. The slabbed face 

permits more secure attachment of sheathing and framing 

members and facilitates the alignment and setting of intermediate wall and corner poles. The slabbing consists of a 

minimum cut to provide a single continuous flat face from 

the groundline to the top of intermediate wall poles and two 

continuous flat faces at right angles to one another from the 

groundline to the top of corner poles. However, preservative 

penetration is generally limited to the sapwood of most species; therefore slabbing, particularly in the groundline area 

of poles with thin sapwood, may result in somewhat less 

protection than that of an unslabbed pole. All cutting and 

sawing should be confined to that portion of the pole above 

the groundline and should be performed before treatment.

Figure 6–5. Logs are used to construct logging bridges in 

remote forest areas.

Table 6–9. Circumference taper

Species 

Change in 

circumference

per meter 

(cm)

Change in 

circumference

per foota

(in.)

Western redcedar 3.7 0.38

Ponderosa pine 2.4 0.29

Jack, lodgepole, and red pine 2.5 0.30

Southern Pine 2.1 0.25

Douglas-fir, larch 1.7 0.21

Western hemlock 1.7 0.20 a

Taken from ANSI O5.1. 

6–20

General Technical Report FPL–GTR–190
The ASTM International (formerly American Society for 

Testing and Materials) standard ASTM D 25 provides tables 

of pile sizes for either friction piles or end-bearing piles. 

Friction piles rely on skin friction rather than tip area for 

support, whereas end-bearing piles resist compressive force 

at the tip. For this reason, a friction pile is specified by butt 

circumference and may have a smaller tip than an endbearing pile. Conversely, end-bearing piles are specified by 

tip area and butt circumference is minimized.

Straightness of poles or piles is determined by two form 

properties: sweep and crook. Sweep is a measure of bow or 

gradual deviation from a straight line joining the ends of the 

pole or pile. Crook is an abrupt change in direction of the 

centroidal axis. Limits on these two properties are specified 

in both ANSI O5.1 and ASTM D 25.

Construction Logs

Logs used in construction are generally specified to meet the 

same criteria for straightness and knots as poles and piles 

(ASTM D 25). For log stringer bridges, the log selection 

criteria may vary with the experience of the person doing 

the selection, but straightness, spiral grain, wind shake, and 

knots are limiting criteria. Although no consensus standard 

is available for specifying and designing log stringers, the

Design Guide for Native Log Stringer Bridges was prepared 

by the U.S. Forest Service.

Logs used for log cabins come in a wide variety of crosssectional shapes (Fig. 6–7). Commercial cabin logs are usually milled so that their shape is uniform along their length. 

The ASTM D 3957 standard, a guide for establishing stress 

grades for building logs, recommends stress grading on the 

basis of the largest rectangular section that can be inscribed 

totally within the log section. The standard also provides 

commentary on the effects of knots and slope of grain.

Ties

Railroad ties are commonly shaped to a fairly uniform section along their length. The American Railway Engineering 

Association (AREA) publishes specifications for the sizes, 

which include seven size classes ranging from 0.13 by 

0.13 m (5 by 5 in.) to 0.18 by 0.25 m (7 by 10 in.). These 

tie classes may be ordered in any of three standard lengths: 

2.4 m (8 ft), 2.6 m (8.5 ft), or 2.7 m (9 ft).

Weight and Volume

The weight of any wood product is a function of its volume, density, moisture content, and any retained treatment 

substance. An accurate estimate of volume of a round pole 

would require numerous measurements of the circumference 

and shape along the length, because poles commonly exhibit 

neither a uniform linear taper nor a perfectly round shape. 

The American Wood Protection Association (AWPA) Factor 

3 section therefore recommends volume estimates be based 

on the assumption that the pole is shaped as the frustum of 

a cone (that is, a cone with the top cut perpendicular to the 

axis), with adjustments dependent on species. The volume 

in this case is determined as the average cross-sectional area 

A times the length. Estimates of average cross-sectional area 

may be obtained either by measuring the circumference at 

mid-length (A = Cm2/4π) or taking the average of the butt 

and tip diameters (A = π(D + d)2/16) to estimate the area of 

a circle. The AWPA recommends that these estimates then 

be adjusted by the following correction factors for the given 

species and application:

Oak piles 0.82

Southern Pine piles 0.93

Southern Pine and red pine poles 0.95

Tables for round timber volume are given in AWPA Factor 3 tables. The volume of a round timber differs little 

whether it is green or dry. Drying of round timbers causes 

checks to open, but there is little reduction of the gross diameter of the pole.

Wood density also differs with species, age, and growing 

conditions. It will even vary along the height of a single 

tree. Average values, tabulated by species, are normally expressed as specific gravity (SG), which is density expressed 

as a ratio of the density of water (see Chap. 5). For commercial species grown in the United States, SG varies from 

Figure 6–6. Poles provide economical foundation and wall 

systems for agricultural and storage buildings.

Figure 6–7. Construction logs can be formed in a variety 

of shapes for log homes. Vertical surfaces may be varied 

for aesthetic purposes, while the horizontal surfaces 

generally reflect structural and thermal considerations.

Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–21
0.32 to 0.65. If you know the green volume of a round 

timber and its SG, its dry weight is a product of its SG, 

its volume, and the unit weight of water (1,000 kg m–3

(62.4 lb ft–3)). Wood moisture content can also be highly 

variable. A pole cut in the spring when sap is flowing may 

have a moisture content exceeding 100% (the weight of the 

water it contains may exceed the weight of the dry wood 

substance). If you know the moisture content (MC) of the 

timber, multiply the dry weight by (1 + MC/100) to get the 

wet weight.

Finally, in estimating the weight of a treated wood product 

such as a pole, pile, or tie, you must take into account the 

weight of the preservative. Recommended preservative retentions are listed in Table 15–1 in Chapter 15. By knowing 

the volume, the preservative weight can be approximated 

by multiplying volume by the recommended preservative 

retention. This estimation will err on the side of overestimating preservative weight because the actual retention 

specifications are based on an outer assay zone and not the 

entire volume. 

Durability

For most applications of round timbers and ties, durability 

is primarily a question of decay resistance. Some species are 

noted for their natural decay resistance; however, even these 

may require preservative treatment, depending upon the 

environmental conditions under which the material is used 

and the required service life. For some applications, natural 

decay resistance is sufficient. This is the case for temporary 

piles, marine piles in fresh water entirely below the permanent water level, and construction logs used in building construction. Any wood members used in ground contact should 

be pressure treated, and the first two or three logs above 

a concrete foundation should be brush treated with a 

preservative–sealer.

Preservative Treatment

The American Wood Protection Association (AWPA) standards covers the inspection and treatment requirements for 

various wood products including poles, piles, and ties. Federal Specification TT–W–571 (U.S. Federal Supply Service 

(USFSS)) is no longer current, and government specifiers 

now use AWPA standards.) AWPA Standard T1 contains 

general pressure treatment specifications, Commodity 

Specification A covers treatment of lumber timbers, Commodity Specification C covers treatment of ties, Commodity 

Specification D covers pressure and thermal treatment of 

poles, and Commodity Specification E covers round timber 

piles. The AREA specifications for cross ties and switch ties 

also cover preservative treatment. Retention and types of 

various preservatives recommended for various applications 

are given in Table 15–1. 

Inspection and treatment of poles in service has been effective in prolonging the useful life of untreated poles and 

those with inadequate preservative penetration or retention. 

The Forest Research Laboratory at Oregon State University 

has published guidelines for developing an in-service pole 

maintenance program.

Service Life

Service conditions for round timbers and ties vary from mild 

for construction logs to severe for cross ties. Construction 

logs used in log homes may last indefinitely if kept dry and 

properly protected from insects. Most railroad ties, on the 

other hand, are continually in ground contact and are subject 

to mechanical damage.

Poles

The life of poles can vary within wide limits, depending 

upon properties of the pole, preservative treatments, service 

conditions, and maintenance practices. In distribution or 

transmission line supports, however, service life is often 

limited by obsolescence of the line rather than the physical 

life of the pole.

It is common to report the average life of untreated or treated poles based on observations over a period of years. These 

average life values are useful as a rough guide to the service 

life to be expected from a group of poles, but it should be 

kept in mind that, within a given group, 60% of the poles 

will have failed before reaching an age equal to the average 

life.

Early or premature failure of treated poles can generally be 

attributed to one or more of three factors: (a) poor penetration and distribution of preservative, (b) an inadequate retention of preservative, or (c) use of a substandard preservative. Properly treated poles can last 50 years or longer.

Western redcedar is one species with a naturally decayresistant heartwood. If used without treatment, however, 

the average life is somewhat less than 20 years. 

Piles

The expected life of a pile is also determined by treatment and use. Wood that remains completely submerged 

in water does not decay, although bacteria may cause some 

degradation; therefore, decay resistance is not necessary in 

all piles, but it is necessary in any part of the pile that may 

extend above the permanent water level. When piles that 

support the foundations of bridges or buildings are to be cut 

off above the permanent water level, they should be pressure treated to conform to recognized specifications such as 

AWPA Commodity Specification E. The untreated surfaces 

exposed at the cutoffs should also be given protection by 

thoroughly brushing the cut surface with copper naphthenate 

containing at least 1% elemental copper. A coat of pitch, asphalt, or similar material may then be applied over the creosote and a protective sheet material, such as metal, roofing 

felt, or saturated fabric, should be fitted over the pile cut-off 

in accordance with AWPA Standard M4. Correct application 

and maintenance of these materials are critical in maintaining the integrity of piles.

6–22

General Technical Report FPL–GTR–190
Piles driven into earth that is not constantly wet are subject 

to about the same service conditions as apply to poles but 

are generally required to last longer. Preservative retention 

requirements for piles are therefore sometimes greater than 

for poles (Table 15–1). Piles used in salt water are subject to 

destruction by marine borers even though they do not decay 

below the waterline. The most effective practical protection 

against marine borers has been a treatment first with a waterborne preservative, followed by seasoning with a creosote 

treatment. Other preservative treatments of marine piles are 

covered in AWPA Commodity Specification E and shown in 

Table 15–2.

Ties

The life of ties in service depends on their ability to resist 

decay and mechanical destruction. Under sufficiently light 

traffic, heartwood ties of naturally durable wood, even if of 

low strength, may give 10 or 15 years of average service 

without preservative treatment; under heavy traffic without 

adequate mechanical protection, the same ties might fail in 

2 or 3 years. Advances in preservatives and treatment processes, coupled with increasing loads, are shifting the primary cause of tie failure from decay to mechanical damage. 

Well-treated ties, properly designed to carry intended loads, 

should last from 25 to 40 years on average. Records on life 

of treated and untreated ties are occasionally published in 

the annual proceedings of the AREA and AWPA.

Commonly Used Lumber, Round 

Timber, and Tie Abbreviations

The following standard lumber abbreviations are commonly 

used in contracts and other documents for purchase and sale 

of lumber.

AAR Association of American Railroads

AD air dried

ADF after deducting freight

AF alpine fir

ALS American Lumber Standard

AST antistain treated; at ship tackle (western 

softwoods)

AV or avg Average

AW&L all widths and lengths

B1S see EB1S, CB1S, and E&CB1S

B2S see EB2S, CB2S, and E&CB2S

B&B, B&BTR B and Better

B&S beams and stringers

BD Board

BD FT board feet

BDL Bundle

BEV bevel or beveled

BH boxed heart

B/L, BL bill of lading

BM board measure

BSND bright sapwood, no defect

BTR Better

CB center beaded

CB1S center bead on one side

CB2S center bead on two sides

CC cubical content

cft or cu. ft. cubic foot or feet

CF cost and freight

CIF cost, insurance, and freight

CIFE cost, insurance, freight, and exchange

CG2E center groove on two edges

C/L carload

CLG ceiling

CLR clear

CM center matched

Com Common

CONST construction

CS caulking seam

CSG casing

CV center V

CV1S center V on one side

CV2S center V on two sides

DB Clg double-beaded ceiling (E&CB1S)

DB Part double-beaded partition (E&CB2S)

DET double end-trimmed

DF Douglas-fir

DF–L Douglas-fir plus larch

DIM dimension

DKG decking

D/S, DS, D/Sdg drop siding

D1S, D2S see S1S and S2S

D&M dressed and matched

D&CM dressed and center matched

D&SM dressed and standard matched

D2S&CM dressed two sides and center matched

D2S&SM dressed two sides and standard matched

E edge

EB1S edge bead one side

EB2S, SB2S edge bead on two sides

EE eased edges

EG edge (vertical or rift) grain

EM end matched

EV1S, SV1S edge V one side

EV2S, SV2S edge V two sides

E&CB1S edge and center bead one side

E&CB2S, DB2S, 

BC&2S

edge and center bead two sides

E&CV1S, 

DV1S, V&CV1S

edge and center V one side

E&CV2S, 

DV2S, V&CV2S

edge and center V two sides

ES Engelmann spruce

Fb, Ft, Fc, Fv, Fcx allowable stress (MPa (lb/in2)) in bending; 

tension, compression and shear parallel to 

grain; and in compression perpendicular to 

grain, respectively

FA facial area

Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–23
Fac factory

FAS free alongside (vessel)

FAS Firsts and Seconds

FAS1F Firsts and Seconds one face

FBM, Ft. BM feet board measure

FG flat or slash grain

FJ finger joint; end-jointed lumber using fingerjoint configuration

FLG, Flg flooring

FOB free on board (named point)

FOHC free of heart center

FOK free of knots

FRT, Frt freight

FT, ft foot, feet

FT. SM feet surface measure

G girth

GM grade marked

G/R grooved roofing

HB, H.B. hollow back

HEM hemlock

H-F mixed hemlock and fir (Hem–Fir)

Hrt heart

H&M hit and miss

H or M hit or miss

IC incense cedar

IN, in. inch, inches

Ind industrial

IWP Idaho white pine

J&P joists and planks

JTD jointed

KD kiln dried

KDAT kiln-dried after treatment

L western larch

LBR, Lbr lumber

LCL less than carload

LGR longer

LGTH length

Lft, Lf lineal foot, feet

LIN, Lin lineal

LL longleaf

LNG, Lng lining

LP lodgepole pine

M thousand

MBM, MBF, 

M.BM

thousand (feet) board measure

MC, M.C. moisture content

MERCH, Merch merchantable

MFMA Maple Flooring Manufacturers Association 

MG medium grain or mixed grain

MH mountain hemlock

MLDG, Mldg moulding

Mft thousand feet

M-S mixed species

MSR machine stress rated

N nosed

NBM net board measure

NOFMA National Oak Flooring Manufacturers 

Association

No. number

N1E or N2E nosed one or two edges

Ord Order

PAD partially air-dried

PAR, Par paragraph

PART, Part partition

PAT, Pat pattern

Pcs. pieces

PE plain end

PET precision end-trimmed

PP ponderosa pine

P&T posts and timbers

P1S, P2S see S1S and S2S

RDM random

REG, Reg regular

Rfg. roofing

RGH, Rgh rough

R/L, RL random lengths

R/W, RW random widths

RES resawn

SB1S single bead one side

SDG, Sdg siding

S-DRY surfaced dry; lumber ≤19% moisture content 

per ALS for softwood

SE square edge

SEL, Sel Select or Select grade

SE&S square edge and sound

SG slash or flat grain

S-GRN surfaced green; lumber unseasoned, >19% 

moisture content per ALS for softwood

SGSSND sapwood, gum spots and streaks, no defect

SIT. SPR Sitka spruce

S/L, SL, S/Lap shiplap

SM surface measure

Specs specifications

SP sugar pine

SQ square

SQRS squares

SRB stress-rated board

STD, Std standard

Std. lgths. standard lengths

STD. M standard matched

SS Sitka spruce

SSE sound square edge

SSND sap stain, no defect (stained)

STK Select tight knot

STK stock

STPG stepping

STR, STRUCT structural

SYP Southern Pine

S&E side and edge (surfaced on)

S1E surfaced one edge

S2E surfaced two edges

6–24

General Technical Report FPL–GTR–190
S1S surfaced one side

S2S surfaced two sides

S4S surfaced four sides

S1S&CM surfaced one side and center matched

S2S&CM surfaced two sides and center matched

S4S&CS surfaced four sides and caulking seam

S1S1E surfaced one side, one edge

S1S2E surfaced one side, two edges

S2S1E surfaced two sides, one edge

S2S&SL surfaced two sides and shiplapped

S2S&SM surfaced two sides and standard matched

TBR timber

T&G tongued and grooved

TSO treating service only (nonconforming to 

standard)

UTIL utility

VG vertical (edge) grain

V1S see EV1S, CV1S, and E&CV1S

V2S see EV2S, CV2S, and E&CV2S

WC western cedar

WCH West Coast hemlock

WCW West Coast woods

WDR, wdr wider

WF white fir

WHAD worm holes (defect)

WHND worm holes (no defect)

WT weight

WTH width

WRC western redcedar

WW white woods (Engelmann spruce, any true firs, 

any hemlocks, any pines)

References

ANSI. [Current edition]. ANSI O5.1. Specifications and 

dimensions for wood poles. New York, NY: American National Standards Institute.

ASTM. [Current edition]. ASTM D 3957. Standard methods 

for establishing stress grades for structural members used in 

log buildings.West Conshohocken, PA: ASTM International.

AWPA. [Current edition]. Book of standards (includes standards on pressure and thermal treatment of poles, piles, and 

ties). American Wood-Preserver’s Bureau official quality 

control standards. Bethesda, MD: American Wood Protection Association.

DOC. [Current edition]. American softwood lumber standard. Voluntary Product Standard PS–20–94. Washington, 

DC: U.S. Department of Commerce.

Muchmore, F.W. 1977. Design guide for native log stringer 

bridges. Juneau, AK: U.S. Department of Agriculture, Forest 

Service, Region 10.

Chapter 6 Commercial Lumber, Round Timbers, and Ties

6–25

NWPCA 2007. Uniform Standard for Wood Pallets. Alexandria, VA: National Wooden Pallet and Container Association. 49 p.

NWPCA 2009. Uniform Standard for Wood Containers. 

Alexandria, VA: National Wooden Pallet and Container 

Association. 44 p.

Smith, L.W.; Wood, L.W. 1964. History of yard lumber size 

standards. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 59 p.

CHAPTER 7

Stress Grades and Design Properties for Lumber, 

Round Timber, and Ties 

David E. Kretschmann, Research General Engineer

7–1

Round timbers, ties, and lumber sawn from a log, regardless 

of species and size, are quite variable in mechanical properties. Pieces may differ in strength by several hundred percent. For simplicity and economy in use, pieces of wood of 

similar mechanical properties are placed in categories called 

stress grades, which are characterized by (a) one or more 

sorting criteria, (b) a set of properties for engineering design, and (c) a unique grade name. The most familiar system 

is that for lumber. Sorting criteria have also been established 

for round timbers and ties. This chapter briefly discusses the 

stress grades and design properties for lumber, round timber, 

and ties.

Lumber

The U.S. Department of Commerce American Softwood 

Lumber Standard PS 20 describes sorting criteria for two 

stress-grading methods and the philosophy of how properties for engineering design are derived. The derived properties are then used in one of two design formats: (a) the load 

and resistance factor design (LRFD), which is based on a 

reference strength at the lower 5th percentile 5-min stress 

(AF&PA [current edition]), or (b) the allowable stress design (ASD), which is based on a design stress at the lower 

5th percentile 10-year stress. The properties depend on the 

particular sorting criteria and on additional factors that are 

independent of the sorting criteria. Design properties are 

lower than the average properties of clear, straight-grained 

wood tabulated in Chapter 5.

From one to six design properties are associated with a 

stress grade: bending modulus of elasticity for an edgewise 

loading orientation and stress in tension and compression 

parallel to the grain, stress in compression perpendicular to 

the grain, stress in shear parallel to the grain, and extreme 

fiber stress in bending. As is true of the properties of any 

structural material, the allowable engineering design properties must be either inferred or measured nondestructively. 

In wood, the properties are inferred through visual grading 

criteria, nondestructive measurement such as flatwise bending stiffness or density, or a combination of these properties. 

These nondestructive tests provide both a sorting criterion 

and a means of calculating appropriate mechanical 

properties.

The philosophies contained in this chapter are used by a 

number of organizations to develop visual and machine 

stress grades. References are made to exact procedures 

Contents 

Lumber 7–1

 Responsibilities and Standards for Stress 

 Grading 7–2

 Visually Graded Structural Lumber 7–3

 Machine-Graded Structural Lumber 7–7

 Adjustment of Properties for Design Use 7–10

Round Timbers and Ties 7–13

 Strength Properties 7–13

 Poles 7–13

 Piles 7–14

 Construction Logs 7–14

 Ties 7–14

Literature Cited 7–14

Additional References 7–15

 Lumber 7–15

 General 7–15

 Poles 7–15

 Piles 7–16

 Construction Logs 7–16

 Ties 7–16
7–2

General Technical Report FPL–GTR–190

National Grading Rule, which specifies grading characteristics for different grade specifications.

Organizations that write and publish grading rule books 

containing stress-grade descriptions are called rules-writing 

agencies. Grading rules that specify American Softwood 

Lumber Standard PS 20 must be certified by the ALSC 

Board of Review for conformance with this standard. Organizations that write grading rules, as well as independent 

agencies, can be accredited by the ALSC Board of Review 

to provide grading and grade-marking supervision and 

reinspection services to individual lumber manufacturers. 

Accredited rules-writing and independent agencies are listed 

in Table 7–1. The continued accreditation of these organizations is under the scrutiny of the ALSC Board of Review.

Most commercial softwood species lumber manufactured in 

the United States is stress graded under American Lumber 

Standard practice and is called American Lumber Standard 

(ALS) program lumber. Distinctive grade marks for each 

species or species grouping are provided by accredited 

agencies. The principles of stress grading are also applied to 

several hardwood species under provisions of the American 

Softwood Lumber Standard. Lumber found in the marketplace may be stress graded under grading rules developed in 

accordance with methods approved by the ALSC or by some 

other stress-grading rule, or it may not be stress graded. 

Only those stress grades that meet the requirements of the 

voluntary American Softwood Lumber Standard system are 

discussed in this chapter.

Table 7–1. Sawn lumber grading agenciesa

Rules-writing agencies

Northeastern Lumber Manufacturers Association (NeLMA)

Northern Softwood Lumber Bureau (NSLB) 

Redwood Inspection Service (RIS) 

Southern Pine Inspection Bureau (SPIB) 

West Coast Lumber Inspection Bureau (WCLIB) 

Western Wood Products Association (WWPA) 

National Lumber Grades Authority (NLGA) 

Independent agencies 

American Institute of Timber Construction 

Continental Inspection Agency, LLC 

Pacific Lumber Inspection Bureau, Inc. 

Renewable Resource Associates, Inc. 

Stafford Inspection and Consulting, LLC 

Renewable Resource Associates, Inc. 

Timber Products Inspection 

Alberta Forest Products Association 

Canadian Lumbermen’s Association 

Canadian Mill Services Association 

Canadian Softwood Inspection Agency, Inc. 

Central Forest Products Association 

Council of Forest Industries 

MacDonald Inspection 

Maritime Lumber Bureau 

Newfoundland and Labrador Lumber Producers Association

Quebec Forest Industry Council 

a

For updated information, contact American Lumber Standard 

Committee, P.O. Box 210, Germantown, MD 20875; 

alsc@alsc.org; www.alsc.org. 

and the resulting design stresses, but these are not presented 

in detail.

Responsibilities and Standards for Stress 

Grading

An orderly, voluntary, but circuitous system of responsibilities has evolved in the United States for the development, 

manufacture, and merchandising of most stress-graded 

lumber. The system is shown schematically in Figure 7–1. 

Stress-grading principles are developed from research findings and engineering concepts, often within committees and 

subcommittees of ASTM International (formerly the American Society for Testing and Materials).

American Lumber Standard Committee

Voluntary product standards are developed under procedures 

published by the U.S. Department of Commerce. The Department of Commerce National Institute of Standards and 

Technology (NIST), working with rules-writing agencies, 

lumber inspection agencies, lumber producers, distributors 

and wholesalers, retailers, end users, members of Federal 

agencies, and others, works through the American Lumber 

Standard Committee (ALSC) to maintain a voluntary consensus softwood standard, the American Softwood Lumber 

Standard (PS 20). The PS 20 Standard prescribes the ways 

in which stress-grading principles can be used to formulate 

grading rules designated as conforming to the American 

Lumber Standard. Under the auspices of the ALSC is the 

Figure 7–1. Voluntary system of responsibilities for 

stress grading under the American Softwood Lumber 

Standard.
7–3

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties

National Grading Rule

Stress grading under the auspices of the ALSC is applied 

to many sizes and patterns of lumber that meet the American Softwood Lumber Standard provision. However, most 

stress-graded lumber is dimension lumber (standard 38 mm 

to 89 mm (nominal 2 to 4 in., actual 1.5 to 3.5 in.) thick) 

and is governed by uniform specifications under the National Grading Rule. The National Grading Rule provides 

guidelines for writing grading rules for lumber in this thickness range and specifies grading characteristics for different 

grade specifications. American Softwood Lumber Standard 

dimension lumber in this thickness range is required to 

conform to the National Grading Rule, except for special 

products such as scaffold planks. Grade rules for other sizes, 

such as structural timbers (standard 114-mm and larger 

(nominal 5-in. and larger) thick) may vary between ruleswriting agencies or species.

The National Grading Rule establishes the lumber classifications and grade names for visually stress-graded dimension 

lumber (Table 7–2). The ALSC Machine Grading Policy 

provides for the grading of dimension lumber by a combination of machine and visual methods. Visual requirements for 

this type of lumber are developed by the respective ruleswriting agencies for particular species grades.

Standards

Table 7–2 also shows associated minimum bending strength 

ratios to provide a comparative index of quality. The 

strength ratio is the hypothetical ratio of the strength of a 

piece of lumber with visible strength-reducing growth characteristics to its strength if those characteristics were absent. 

Formulas for calculating strength ratios are given in ASTM 

standard D 245. The corresponding visual description of the 

dimension lumber grades can be found in the grading rule 

books of the rules-writing agencies listed in Table 7–1. Design properties will vary by size, species, and grade and are 

published in the appropriate rule books and in the National 

Design Specification for Wood Construction (AF&PA).

Grouping of Species

Most species are grouped together and the lumber from 

them treated as equivalent. Species are usually grouped 

when they have about the same mechanical properties, 

when the wood of two or more species is very similar in 

appearance, or for marketing convenience. For visual stress 

grades, ASTM D 2555 contains procedures for calculating 

clear wood properties for groups of species to be used with 

ASTM D 245. ASTM D 1990 contains procedures for calculating design properties for groups of species tested as fullsized members. The properties assigned to a group by such 

procedures will often be different from those of any species 

that make up the group. The group will have a unique identity, with nomenclature approved by the Board of Review 

of the ALSC. The identities, properties, and characteristics 

of individual species of the group are found in the grading 

rules for any particular species or species grouping. In the 

case of machine stress grading, the inspection agency that 

supervises the grading certifies by testing that the design 

properties in that grade are appropriate for the species or 

species grouping and the grading process.

Foreign Species

Currently, the importation of structural lumber is governed 

by two ALSC guidelines that describe the application of the 

American Lumber Standard and ASTM D 1990 procedures 

to foreign species. The approval process is outlined in 

Table 7–3.

Visually Graded Structural Lumber

Visual Sorting Criteria

Visual grading is the original method for stress grading. It is 

based on the premise that mechanical properties of lumber 

differ from mechanical properties of clear wood because 

many growth characteristics affect properties and these 

characteristics can be seen and judged by eye. Growth characteristics are used to sort lumber into stress grades. The 

typical visual sorting criteria discussed here are knots, slope 

of grain, checks and splits, shake, density, decay, annual ring 

count and percentage latewood, pitch pockets, and wane.

Knots

Knots cause localized cross grain with steep slopes. A very 

damaging aspect of knots in sawn lumber is that the continuity of the grain around the knot is interrupted by the 

sawing process.

Table 7–2. Visual grades described in National 

Grading Rule

Lumber classificationa Grade name 

Bending 

strength 

ratio

(%) 

Light framingb Construction 34 

Standard 19 

Utility 9 

Structural light framingb Select Structural 67 

1 55 

2 45 

3 26 

Studc Stud 26 

Structural joists and planksd Select Structural 65 

1 55 

2 45 

3 26 a

Contact rules-writing agencies for additional information. b

Standard 38 to 89 mm (nominal 2 to 4 in.) thick and wide. Widths 

narrower than 89 mm (4 in. nominal) may have different strength 

ratio than shown. 

c

Standard 38 to 89 mm (nominal 2 to 4 in.) thick, ≥38 mm (≥4 in. 

nominal) wide. 

d

Standard 38 to 89 mm (nominal 2 to 4 in.) thick, ≥114 mm (≥5 in.

nominal) wide. 
7–4

Checks and Splits

Checks are separations of the wood that normally occur 

across or through the annual rings, usually as a result of seasoning. Splits are a separation of the wood through the piece 

to the opposite surface or to an adjoining surface caused 

by tearing apart of the wood cells. As opposed to shakes, 

checks and splits are rated by only the area of actual opening. An end-split is considered equal to an end-check that 

extends through the full thickness of the piece. The effects 

of checks and splits on strength and the principles of their 

limitation are the same as those for shake.

Shake

Shake is a separation or a weakness of fiber bond, between 

or through the annual rings, that is presumed to extend 

lengthwise without limit. Because shake reduces resistance 

to shear in members subjected to bending, grading rules 

therefore restrict shake most closely in those parts of a bending member where shear stresses are highest. In members 

with limited cross grain, which are subjected only to tension or compression, shake does not affect strength greatly. 

Shake may be limited in a grade because of appearance and 

because it permits entrance of moisture, which results in 

decay.

Density

Strength is related to the mass per unit volume (density) of 

clear wood. Properties assigned to lumber are sometimes 

modified by using the rate of growth and percentage of 

latewood as measures of density. Typically, selection for 

density requires that the rings per unit length on the cross 

section and the percentage of latewood be within a specified range. Some very low-strength pieces may be excluded 

Table 7–3. Approval process for acceptance of design 

values for foreign species

1 Rules-writing agency seeks approval to include species in 

grading rule book. 

2 Agency develops sampling and testing plan, following 

American Lumber Standard Committee (ALSC) foreign 

importation guidelines, which must then be approved by 

ALSC Board of Review. 

3 Lumber is sampled and tested in accordance with approved 

sampling and testing plan. 

4 Agency analyzes data by ALSC Board of Review, ASTM 

D 1990 procedures, and other appropriate criteria (if 

needed). 

5 Agency submits proposed design values to ALSC Board of 

Review.

6 Submission is reviewed by ALSC Board of Review and 

USDA Forest Service, Forest Products Laboratory. 

7 Submission is available for comment by other agencies and 

interested parties. 

8 ALSC Board of Review approves (or disapproves) design 

values, with modification (if needed) based on all available 

information.

9 Agency publishes new design values for species. 

In general, knots have a greater effect on strength in tension 

than compression; in bending, the effect depends on whether 

a knot is in the tension or compression side of a beam (knots 

along the centerline have little or no effect). Intergrown 

(or live) knots resist (or transmit) some kinds of stress, but 

encased knots (unless very tight) or knotholes resist (or 

transmit) little or no stress. On the other hand, distortion of 

grain is greater around an intergrown knot than around an 

encased (or dead) knot of equivalent size. As a result, overall strength effects are roughly equalized, and often no distinction is made in stress grading between intergrown knots, 

dead knots, and knotholes.

The zone of distorted grain (cross grain) around a knot has 

less “parallel to piece” stiffness than does straight-grained 

wood; thus, localized areas of low stiffness are often associated with knots. However, such zones generally constitute 

only a minor part of the total volume of a piece of lumber. 

Because overall stiffness of a piece reflects the character of 

all parts, stiffness is not greatly influenced by knots.

The presence of a knot has a greater effect on most strength 

properties than on stiffness. The effect on strength depends 

approximately on the proportion of the cross section of the 

piece of lumber occupied by the knot, knot location, and 

distribution of stress in the piece. Limits on knot sizes are 

therefore made in relation to the width of the face and location on the face in which the knot appears. Compression 

members are stressed about equally throughout, and no 

limitation related to location of knots is imposed. In tension, 

knots along the edge of a member cause an eccentricity that 

induces bending stresses, and they should therefore be more 

restricted than knots away from the edge. In simply supported structural members subjected to bending, stresses are 

greater in the middle of the length and at the top and bottom 

edges than at midheight. These facts are recognized in some 

grades by differing limitations on the sizes of knots in different locations.

Knots in glued-laminated structural members are not continuous as in sawn structural lumber, and different methods 

are used for evaluating their effect on strength (Chap. 12).

Slope of Grain 

Slope of grain (cross grain) reduces the mechanical properties of lumber because the fibers are not parallel to the 

edges. Severely cross-grained pieces are also undesirable 

because they tend to warp with changes in moisture 

content. Stresses caused by shrinkage during drying are 

greater in structural lumber than in small, clear straightgrained specimens and are increased in zones of sloping or 

distorted grain. To provide a margin of safety, the reduction 

in design properties resulting from cross grain in visually 

graded structural lumber is considerably greater than that 

observed in small, clear specimens that contain similar 

cross grain.

General Technical Report FPL–GTR–190
7–5

from a grade by excluding those that are exceptionally low 

in density.

Decay

Decay in most forms should be prohibited or severely restricted in stress grades because the extent of decay is difficult to determine and its effect on strength is often greater 

than visual observation would indicate. Decay of the pocket 

type (for example, Fomes pini) can be permitted to some 

extent in stress grades, as can decay that occurs in knots but 

does not extend into the surrounding wood.

Heartwood and Sapwood

Heartwood does not need to be taken into account in stress 

grading because heartwood and sapwood have been assumed to have equal mechanical properties. However, 

heartwood is sometimes specified in a visual grade because 

the heartwood of some species is more resistant to decay 

than is the sapwood; heartwood may be required if untreated 

wood will be exposed to a decay hazard. On the other hand, 

sapwood takes preservative treatment more readily than 

heartwood and it is preferable for lumber that will be treated 

with preservatives.

Pitch Pockets

Pitch pockets ordinarily have so little effect on structural 

lumber that they can be disregarded in stress grading if they 

are small and limited in number. The presence of a large 

number of pitch pockets, however, may indicate shake or 

weakness of bond between annual rings.

Wane

Wane refers to bark or lack of wood on the edge or corner of 

a piece of lumber, regardless of cause (except manufactured 

eased edges). Requirements of appearance, fabrication, or 

ample bearing or nailing surfaces generally impose stricter 

limitations on wane than does strength. Wane is therefore 

limited in structural lumber on that basis.

Procedures for Deriving Design Properties

The mechanical properties of visually graded lumber may 

be established by (a) tests of a representative sample of fullsize members (ASTM D 1990 in-grade testing procedure) 

or (b) appropriate modification of test results conducted on 

clear specimens (ASTM D 245 procedure for small clear 

wood). Design properties for the major commercial softwood dimension lumber species given in current design 

specification and codes in the United States have been derived from full-size member test results. However, design 

properties for some species of softwood and most species 

of hardwood dimension lumber (standard 38- to 89-mm 

(nominal 2- to 4-in.) thick) and all species of structural timbers (standard 114-mm and larger (nominal 5-in. and larger) 

thick) are still derived using results of tests on small clear 

samples.

Procedure for Clear Wood

The derivation of mechanical properties of visually graded 

lumber was historically based on clear wood properties 

with appropriate modifications for the lumber characteristics allowed by visual sorting criteria. Sorting criteria that 

influence mechanical properties are handled with “strength 

ratios” for the strength properties and with “quality factors” 

for the modulus of elasticity.

Piece to piece variation occurs in both the clear wood properties and the occurrence of growth characteristics. The 

influence of this variability on lumber properties is handled 

differently for strength properties than for modulus of 

elasticity.

Strength Properties—Each strength property of a piece 

of lumber is derived from the product of the clear wood 

strength for the species and the limiting strength ratio. The 

strength ratio is the hypothetical ratio of the strength of a 

piece of lumber with visible strength-reducing growth characteristics to its strength if those characteristics were absent. 

The true strength ratio of a piece of lumber is never known 

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties

Figure 7–2. Effect of edge knot: A, edge knot in lumber; B, assumed loss of cross section (cross-hatched 

area).

Figure 7–3. Relation between bending strength ratio 

and size of edge knot expressed as fraction of face 

width. k is knot size; h, width of face containing the 

knot.
The range of strength ratios in a grade and the natural variation in clear wood strength give rise to variation in strength 

between pieces in the grade. To account for this variation 

and to ensure safety in design, it is intended that the actual 

strength of at least 95% of the pieces in a grade exceed the 

design properties (before reduction for duration of load and 

safety) assigned to that grade. In visual grading, according 

to ASTM D 245, this is handled by using a near-minimum 

clear wood strength as a base value and multiplying it by the 

minimum strength ratio permitted in the grade to obtain the 

grade strength property. The near-minimum value is called 

the 5% exclusion limit. ASTM D 2555 provides clear wood 

strength data and gives a method for estimating the 5% exclusion limit.

For example, suppose a 5% exclusion limit for the clear 

wood bending strength of a species in the green condition is 

48 MPa (7,000 lb in–2). Suppose also that among the characteristics allowed in a grade of lumber, one characteristic 

(a knot, for example) provides the lowest strength ratio in 

bending—assumed in this example as 40%. Using the numbers, the bending strength for the grade is estimated by multiplying the strength ratio (0.40) by 48 MPa (7,000 lb in–2), 

equaling 19 MPa (2,800 lb in–2) (Fig. 7–4). The bending 

strength in the green condition of 95% of the pieces in this 

species in a grade that has a strength ratio of 40% is expected to be ≥19 MPa (≥2,800 lb in–2). Similar procedures are 

followed for other strength properties, using the appropriate 

clear wood property value and strength ratio. Additional 

multiplying factors are then applied to produce properties 

for design, as summarized later in this chapter.

Modulus of Elasticity—Modulus of elasticity E is a measure of the ability of a beam to resist deflection or of a column to resist buckling. The assigned E is an estimate of the 

average modulus, adjusted for shear deflection, of the lumber grade when tested in static bending. The average modulus of elasticity for clear wood of the species, as recorded in 

ASTM D 2555, is used as a base. The clear wood average is 

Figure 7–4. Example of relation between strength and 

strength ratio.

Figure 7–5. Histogram of modulus of elasticity observed in a single visual grade, from pieces selected 

over a broad geographical range.

7–6

and must be estimated. Therefore, the strength ratio assigned 

to a growth characteristic serves as a predictor of lumber 

strength. Strength ratio is expressed as a percentage, ranging 

from 0 to 100.

Estimated strength ratios for cross grain and density have 

been obtained empirically; strength ratios for other growth 

characteristics have been derived theoretically. For example, 

to account for the weakening effect of knots, the assumption 

is made that the knot is effectively a hole through the piece, 

reducing the cross section, as shown in Figure 7–2. For a 

beam containing an edge knot, the bending strength ratio 

can be idealized as the ratio of the bending moment that can 

be resisted by a beam with a reduced cross section to that of 

a beam with a full cross section:

where SR is strength ratio, k knot size, and h width of face 

containing the knot. This is the basic expression for the effect of a knot at the edge of the vertical face of a beam that 

is deflected vertically. Figure 7–3 shows how strength ratio 

changes with knot size according to the formula.

Strength ratios for all knots, shakes, checks, and splits are 

derived using similar concepts. Strength ratio formulas are 

given in ASTM D 245. The same reference contains guidelines for measuring various growth characteristics.

An individual piece of lumber will often have several characteristics that can affect any particular strength property. 

Only the characteristic that gives the lowest strength ratio is 

used to derive the estimated strength of the piece. In theory, 

a visual stress grade contains lumber ranging from pieces 

with the minimum strength ratio permitted in the grade up 

to pieces with the strength ratio just below the next higher 

grade. In practice, there are often pieces in a grade with 

strength ratios of a higher grade. This is a result of grade 

reduction for appearance factors such as wane that do not 

affect strength.

General Technical Report FPL–GTR–190
multiplied by empirically derived “quality factors” to represent the reduction in modulus of elasticity that occurs by 

lumber grade for pieces tested in an edgewise orientation. 

This procedure is outlined in ASTM D 245.

For example, assume a clear wood average modulus of elasticity of 12.4 GPa (1.8 × 106 lb in–2) for the example shown 

earlier. The limiting bending strength ratio was 40%. ASTM 

D 245 assigns a quality multiplying factor of 0.80 for lumber with this bending strength ratio. The modulus of elasticity for that grade would be the product of the clear wood 

modulus and the quality factor; that is, 12.4 × 0.8 = 9.9 GPa 

(1.8 × 0.8 = 1.44 × 106 lb in–2).

Actual modulus of elasticity of individual pieces of a grade 

varies from the average assumed for design (Fig. 7–5). 

Small individual lots of lumber can be expected to deviate 

from the distribution shown by this histogram. The additional multiplying factors used to derive final design values 

of modulus of elasticity are discussed later in this chapter.

In-Grade Procedure

To establish the mechanical properties of specified grades 

of lumber from tests of full-size specimens, a representative sample of the lumber population is obtained following 

procedures in ASTM D 2915 and D 1990. The specimens 

are tested using appropriate procedures given in ASTM D 

198 or D 4761. Because the range of quality with any one 

specific grade may be large, it is necessary to assess the 

grade quality index (GQI) of the sampled material in relation to the assumed GQI. In the North American In-Grade 

Program, GQI was the strength ratio calculated according to 

formulas in ASTM D 245. The sample GQI and the assumed 

GQI are compared to see if adjustment to the test data is 

necessary. An average value for the edgewise modulus of 

elasticity or a near-minimum estimate of strength properties 

is obtained using ASTM D 1990 procedures. The grade GQI 

is also used as a scaling parameter that allows for modeling 

of strength and modulus of elasticity with respect to grade. 

These properties are further modified for design use by consideration of service moisture content, duration of load, and 

safety.

Machine-Graded Structural Lumber

Machine-graded lumber is lumber evaluated by a machine 

using a nondestructive test followed by visual grading to 

evaluate certain characteristics that the machine cannot or 

may not properly evaluate. Machine-stress-rated (MSR) 

lumber and machine-evaluated-lumber (MEL) are two types 

of machine-graded lumber used in North America. MSR is 

lumber that has modulus of elasticity E evaluated by mechanical stress equipment, with each piece being marked 

to indicate the modulus of elasticity E. MEL is lumber that 

has a parameter, often density, nondestructively evaluated 

by mechanical grading equipment approved by the ALSC 

Board of Review to predict certain mechanical properties. 

The MEL machine evaluates each piece and sorts each 

piece into various strength classification grade categories. 

Machine-graded lumber allows for better sorting of material 

for specific applications in engineered structures. The basic 

components of a machine-grading system are as follows:

Table 7–4. Common grades for machine-graded lumbera

Fb E Ft Fc║

Grade name (MPa (lb in–2)) (GPa (106 lb in–2)) (MPa (lb in–2)) (MPa (lb in–2)) 

MSR 

1350f–1.3E 9.3 (1,350) 9.0 (1.3) 5.2 (750) 11.0 (1,600) 

1450f–1.3E 10.0 (1,450) 9.0 (1.3) 5.5 (800) 11.2 (1,625) 

1650f–1.5E 11.4 (1,650) 10.3 (1.5) 7.0 (1,020) 11.7 (1,700) 

1800f–1.6E 12.4 (1,800) 11.0 (1.6) 8.1 (1,175) 12.1 (1,750) 

1950f–1.7E 13.4 (1,950) 11.7 (1.7) 9.5 (1,375) 12.4 (1,800) 

2100f–1.8E 14.5 (2,100) 12.4 (1.8) 10.9 (1,575) 12.9 (1,875) 

2250f–1.9E 15.5 (2,250) 13.1 (1.9) 12.1 (1,750) 13.3 (1,925) 

2400f–2.0E 16.5 (2,400) 13.8 (2.0) 13.3 (1,925) 13.6 (1,975) 

2550f–2.1E 17.6 (2,550) 14.5 (2.1) 14.1 (2,050) 14.0 (2,025) 

2700f–2.2E 18.6 (2,700) 15.2 (2.2) 14.8 (2,150) 14.4 (2,100) 

2850f–2.3E 19.7 (2,850) 15.9 (2.3) 15.9 (2,300) 14.8 (2,150) 

MEL

M–10 9.7 (1,400) 8.3 (1.2) 5.5 (800) 11.0 (1,600) 

M–11 10.7 (1,550) 10.3 (1.5) 5.9 (850) 11.5 (1,675) 

M–14 12.4 (1,800) 11.7 (1.7) 6.9 (1,000) 12.1 (1,750) 

M–19 13.8 (2,000) 11.0 (1.6) 9.0 (1,300) 12.6 (1,825) 

M–21 15.9 (2,300) 13.1 (1.9) 9.7 (1,400) 13.4 (1,950) 

M–23 16.5 (2,400) 12.4 (1.8) 13.1 (1,900) 13.6 (1,975) 

M–24 18.6 (2,700) 13.1 (1.9) 12.4 (1,800) 14.5 (2,100) 

a

Forest Products Society (1997). Other grades are available and permitted. 

Fb is allowable 10-year load duration bending stress parallel to grain. 

E is modulus of elasticity. 

Ft is allowable 10-year load duration tensile stress parallel to grain. 

Fc║ is allowable 10-year load duration compressive stress parallel to grain. 

7–7

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties
In the United States and Canada, MSR and MEL lumber 

are also subjected to a visual assessment because the size 

of edge knots in combination with E is a better predictor of 

strength than is E alone. Maximum edge knots are limited 

to a specified proportion of the cross section, depending 

on grade level. Other visual restrictions, which are primarily appearance rather than strength criteria, are placed on 

checks, shake, skips (portions of board “skipped” by the 

planer), splits, wane, and warp.

Procedures for Deriving Design Properties

Mechanical properties of machine-graded structural lumber 

may be established using ASTM D 6570.

Allowable Stress for Bending

A stress grade derived for machine-graded lumber relates 

design strength to a nondestructive parameter such as E or 

Figure 7–6. Schematic E sort: (a) using a regression 

line as the predictor showing four categories: 1—accepted correctly; 2—accepted incorrectly; 3—rejected 

correctly; and 4—rejected correctly; (b) using a lower 

confidence line as the predictor and showing the 

relatively low proportion of material in the accepted 

incorrectly category (lower right).

7–8

•	 Sorting and prediction of strength through machine-measured nondestructive determination of properties coupled 

with visual assessment of growth characteristics

•	 Assignment of design properties based on strength 

prediction

•	 Quality control to ensure that assigned properties are 

being obtained

The quality control procedures ensure 

(a) proper operation of the machine used to make the non 

destructive measurements,

(b) appropriateness of the predictive parameter–bending 

strength relationship, and

(c) appropriateness of properties assigned for tension and 

compression.

The MSR and MEL systems differ in grade names, quality 

control, and coefficient of variation (COV) for E values. 

Grade names for MSR lumber are a combination of the 

design bending stress and average modulus of elasticity, 

whereas grade names for MEL lumber start with an M designation. For quality control, MSR requires pieces to be 

tested daily for at least one strength property and bending 

modulus of elasticity in an edgewise orientation, whereas 

MEL requires daily tension quality control and edgewise 

bending strength and stiffness testing. Finally, MSR grades 

are assigned a COV = 11% on E, whereas MEL grades are 

assigned a COV ≤ 15% on E. Grade names for a wide range 

of machine-graded lumber commonly available across North 

America are given in Table 7–4. Not all grades are available 

in all sizes or species.

Machine Sorting Criteria

The most common method of sorting machine-graded lumber is modulus of elasticity E. When used as a sorting criterion for mechanical properties of lumber, E can be measured 

in a variety of ways. Usually, the apparent E, or deflection 

related to stiffness, is actually measured. Because lumber is 

heterogeneous, the apparent E depends on span, orientation 

(edgewise or flatwise in bending), load speed of test (static 

or dynamic), and method of loading (tension, bending, concentrated, or uniform). Any of the apparent E values can be 

used, as long as the grading machine is properly calibrated, 

to assign the graded piece to a “not to exceed” grade 

category. Most grading machines in the United States are 

designed to detect the lowest flatwise bending E that occurs 

in any approximately 1.2-m (4-ft) span and the average flatwise E for the entire length of the piece.

Another method of sorting machine-graded lumber is using 

density measurements to estimate knot sizes and frequency. 

X-ray sources in conjunction with a series of detectors are 

used to determine density information. Density information 

is then used to assign the graded piece to a “not to exceed” 

grade category.

General Technical Report FPL–GTR–190
density. For this example, it will be considered to be E. Because E is an imperfect predictor of strength, lumber sorted 

solely by average E falls into one of four categories, one of 

which is sorted correctly and three incorrectly (Fig. 7–6).

Consider, for example, the simplest case (sometimes referred to as “go” or “no go”) where lumber is sorted into 

two groups: one with sufficient strength and stiffness for a 

specific application, the other without. In Figure 7–6a, a regression line relating E and strength is used as the prediction 

model. The “accept–reject” groups identified by the regression sort can be classified into four categories:

•	 Category 1—Material that has been accepted correctly, 

that is, pieces have sufficient strength and stiffness as 

defined

•	 Category 2—Material that has been accepted incorrectly, 

that is, pieces do not have sufficient strength

•	 Category 3—Material that has been rejected correctly 

because it does not have sufficient strength

•	 Category 4—Material that has been rejected correctly 

because it does not have sufficient stiffness

Thus, the sort shown in Figure 7–6a has worked correctly 

for categories 1, 3, and 4 but incorrectly for category 2. 

Pieces in category 2 present a problem. These pieces are 

accepted as having sufficient strength but in reality they do 

not, and they are mixed with the accepted pieces of category 1. The number of problem pieces that fall in category 2 

depends on the variability in the prediction model.

To minimize the material that falls into category 2, adjustments are made to the property assignment claims made 

about the sorted material. An appropriate model is one that 

minimizes the material in category 2 or at least reduces it 

to a lower risk level. Additional grading criteria (edge-knot 

limitations, for example) are also added to improve the efficiency of the sorting system relative to the resource and the 

claimed properties.

Commonly, a lower confidence line is used as the prediction model (Fig. 7–6b). The number of pieces that fall into 

category 2 is now low compared with the regression line 

model. Furthermore, the probability of a piece (and thus the 

number of pieces) falling into category 2 is controlled by the 

confidence line selected.

In actual MSR systems, the lumber is sorted (graded) into 

E classes. In the United States and Canada, the number of 

grades has increased as specific market needs have developed for MSR lumber. Today, individual grading agencies 

list as many as 13 E classifications and more than 20 different grades. The grades are designated by the recommended 

extreme fiber stress in bending Fb and edgewise modulus of 

elasticity E. For example, “2100F–1.8E” designates an MSR 

grade with a design stress Fb = 14 MPa (2,100 lb in–2) and 

E = 12.4 GPa (1.8 × 106 lb in–2).

In theory, any F–E combination can be marketed that can 

be supported by test data. In practice, a mill will usually 

produce only a few of the possible existing F–E classifications depending on the potential of the timber being harvested, mill production capabilities, and product or market 

demand. When a mill has determined the grades it would 

like to produce (based on their lumber resource and marketing issues), grade boundary machine settings are used to 

separate the lumber into F–E classifications. A qualification 

sample of lumber is tested by a grading agency for strength 

and stiffness, to verify that the proper machine settings are 

being used. After initial qualification, additional quality control tests are performed during production.

Figure 7–7 illustrates how Fb–E classifications have been 

developed historically for species groups. Data for a particular species group are collected, the relationship of E 

and modulus of rupture (MOR) is evaluated, and a lower 

confidence line is established for the species, as illustrated 

in Figure 7–6b. Using the lower confidence line of this relationship, a MOR value corresponding to the “minimum 

E” assigned to the grade is determined. The “minimum E” 

assigned to the grade represents the 5th percentile of the E 

distribution. The 5th percentile value is expected to be exceeded by 95% of the pieces in a grade or class. In 

this example, for a grade with an assigned E of 13.8 GPa 

(2.0 × 106 lb in–2), the “minimum E” is 11.3 GPa 

(1.64 × 106 lb in–2). The corresponding MOR value from 

the lower confidence line prediction model, approximately a 

5th percentile MOR value, is 34.8 MPa (5.04 × 103 lb in–2). 

This value is then adjusted by a factor (2.1) for assumed 

10-year duration of load and safety to obtain Fb. This factor 

Figure 7–7. Typical assignment of Fb–E values for 

MSR lumber in United States (solid lines are minimum 

E for the Fb–E classification and bending strengths 

predicted by minimum E values).

7–9

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties
design stress (Fb or Ft

) for the assigned Fb–E classification. 

In bending, the pieces are loaded on a random edge with the 

maximum-edge defect within the maximum moment area 

(middle one-third span in third-point loading) or as near to 

that point as possible. In tension, the pieces are tested with a 

2.4-m (8-ft) gauge length.

If the number of pieces in the sample failing the proof-test 

load indicates a high probability that the population from 

which the pieces came does not meet the minimum grade 

criteria, a second sampling and proof test are conducted immediately. If the second sample confirms the results of the 

first sample, the MSR grading system is declared “out of 

control” and the operation is shut down to isolate and correct the problem. The lumber that was incorrectly labeled is 

then correctly labeled.

Cumulative machine calibration records are useful for detecting trends or gradual change in machine operation that 

might coincide with use and wear of machine parts. The 

proof-test results are also accumulated. Standard statistical 

quality control procedures (such as control charts) are used 

to monitor the production process so that it can be modified 

as needed in response to change in the timber resource, and 

to make the output fit the assumed model.

Too many failures in one, or even consecutive, samples do 

not necessarily indicate that the system is out of control. If 

the prediction line is based on 95% confidence, it can be 

expected by chance alone that 1 sample in 20 will not meet 

the proof-load requirements. One or more out-of-control 

samples may also represent a temporary aberration in material properties (E–strength relationship). In any event, this 

situation would call for inspection of the cumulative quality 

control records for trends to determine if machine adjustment might be needed. A “clean” record (a period when the 

system does not go out of control) rectifies the evaluation of 

a system thought to be out of control.

Adjustment of Properties for Design Use

The mechanical properties associated with lumber quality 

are adjusted to give design unit stresses and a modulus of 

elasticity suitable for engineering uses. First, a lower confidence level is determined for the material, and this value 

is then adjusted for shrinkage, size, duration of load, and in 

ASD, an additional factor of safety. These adjustment factors are discussed in the following text (specific adjustments 

are given in ASTM D 245 and D 1990).

Shrinkage

As described in Chapter 4, lumber shrinks and swells with 

changes in moisture content. The amount of dimensional 

change depends on a number of factors, such as species and 

ring angle. The American Softwood Lumber Standard PS 20 

lists specific shrinkage factors from green to 15% moisture 

content that were used historically to set green lumber dimensions for most species (2.35% for thickness and 2.80% 

for width). The standard does provide a means of adjusting 

7–10

applied to an estimated 5th percentile MOR value of 

34.8 MPa (5.04 × 103 lb in–2) yields an Fb of 16.5 MPa 

(2.40 × 103 lb in–2) for the 2.0E grade; in other words, a 

2400f –2.0E MSR grade.

Design Stresses for Other Properties

Properties in tension and compression are commonly developed from relationships with bending rather than estimated 

directly by the nondestructive parameter E. In Canada and 

the United States, the relationships between the 5th percentile 10-year bending stress and those in tension and compression are based upon limited lumber testing for the three 

properties but supported by years of successful experience 

in construction with visual stress grades of lumber. For tension, it is assumed that the ratio of design bending stress Fb

to design tensile stress Ft is between 0.5 and 0.8, depending 

on the grade, whereas the relationship between Fb and fiber 

stress in design compressive parallel-to-grain stress Fc is 

assumed to be

Fc = [0.338(2.1Fb) + 2060.7]/1.9

Strength in shear parallel to the grain and in compression 

perpendicular to the grain is poorly related to modulus of 

elasticity. Therefore, in machine stress grading these properties are assumed to be grade-independent and are assigned 

the same values as those for visual lumber grades, except 

when predicted from specific gravity on a mill-by-mill basis. 

It is permissible to assign higher allowable stress for shear 

parallel to grain and compression perpendicular to grain to 

specific grades based on additional specific gravity research.

Quality Control

Quality control procedures are necessary to ensure that 

stresses assigned by a machine-grading system reflect the 

actual properties of the lumber graded. These procedures 

must check for correct machine operation. Verification of 

the relationships between bending and other properties may 

also be required by the rules-writing agency, particularly for 

fiber stress in tension Ft.

Daily or even more frequent calibration of machine operation may be necessary. Depending upon machine principle, 

calibration may involve operating the machine on a calibration bar of known stiffness, comparing grading machine 

E values to those obtained on the same pieces of lumber 

by calibrated laboratory test equipment, determining if 

machine-predicted density matches a calibration sample 

density, or in some instances, using two or more procedures. 

Machine operation should be certified for all sizes of lumber 

being produced. Machine settings may need to be adjusted 

to produce the same grade material from different widths.

Quality control procedures of the MSR prediction model 

(E–bending strength relationship) have been adopted in 

Canada and the United States. Daily or more frequently, 

lumber production is representatively sampled and proofloaded, usually in bending, with supplementary testing in 

tension. The pieces are proof-loaded to at least twice the 

General Technical Report FPL–GTR–190
lumber dimensions to other moisture content by recognizing an allowance of a tolerance below or above minimum 

standard dry sizes on a basis of 1% shrinkage or expansion for each 4% change in moisture content. (See sections 

6.2.3.1 and 6.2.5.1 of PS 20 for additional information.) The 

standard also provides specific shrinkage factors for species 

such as redwood and the cedars, which shrink less than most 

species. Using the PS 20 recommendations and an assumed 

green moisture content Mg, we derive equations that can be 

used with most species to calculate the shrinkage of lumber 

as a function of percentage moisture content M. The equation is applicable to lumber of all annual ring orientations. 

For dimension lumber, the dimensions at different moisture 

contents can be estimated with the following equation:

 

d2 = d1

1− (a − bM2 ) /100

1 − (a − bM1 ) /100

where d1 is dimension (mm, in.) at moisture content M1, d2

dimension (mm, in.) at moisture content M2, M1 moisture 

content (%) at d1, M2 moisture content (%) at d2, and a and

b are variables from Table 7–5.

Size Factor

In general, a size effect causes small members to have greater unit strength than that of large members. Two procedures 

can be used for calculating size-adjustment factors—small 

clear and In-grade.

Small Clear Procedure

ASTM D 245 provides only a formula for adjusting bending 

strength. The bending strength for lumber is adjusted to a 

new depth Fn other than 2 in. (51 mm) using the formula

o

9

1

n

o n F

d

d F 













 =

where do is original depth (51 mm, 2 in.), dn new depth, and 

Fo original bending strength.

This formula is based on an assumed center load and a spanto-depth ratio of 14. A depth effect formula for two equal 

concentrated loads applied symmetrical to the midspan 

points is given in Chapter 9.

In–Grade Test Procedures

ASTM D 1990 provides a formula for adjusting bending, 

tension, and compression parallel to grain. No size 

adjustments are made to modulus of elasticity or for thickness effects in bending, tension, and compression. The size 

adjustments to dimension lumber are based on volume using 

the formula

w l

L

L

W

W P P 





























 =

2

1

2

1

1 2

where P1 is property value (MPa, lb in–2) at volume 1, P2 

property value (MPa, lb in–2) at volume 2, W1 width (mm, 

in.) at P1, W2 width (mm, in.) at P2, L1 length (mm, in.) at 

P1, and L2 length (mm, in.) at P2. Exponents are defined in 

Table 7–6.

Moisture Adjustments

For lumber ≤102 mm (≤4 in.) thick that has been dried, 

strength properties have been shown to be related quadratically to moisture content. Two relationships for modulus of 

rupture at any moisture content are shown in Figure 7–8. 

Both models start with the modulus of elasticity of green 

lumber. The curves with solid dots represent a precise quadratic model fit to experimental results. In typical practice, 

adjustments are made to correspond to average moisture 

contents of 15% and 12% with expected maximum moisture 

contents of 19% and 15%, respectively, using simplified expressions represented by the open dot curves. Below about 

8% moisture content, some properties may decrease with 

decreasing moisture content values, and care should be exercised in these situations. Equations applicable to adjusting 

properties to other moisture levels between green and 

10% moisture content are as follows:

For MOR, ultimate tensile stress (UTS), and ultimate 

compressive stress (UCS), the following ASTM D 1990 

equations apply:

For MOR ≤ 16.7 MPa (2,415 lb in–2)

UTS ≤ 21.7 MPa (3,150 lb in–2)

UCS ≤ 9.7 MPa (1,400 lb in–2)

P1 = P2

Thus, there is no adjustment for stresses below these levels.

Table 7–5. Coefficients for equations to determine 

dimensional changes with moisture content change 

in dimension lumber

Width Thickness

Species a b a b Mg

a

Redwood,

western redcedar,

and northern 

white cedar 

3.454 0.157 2.816 0.128 22 

Other species 6.031 0.215 5.062 0.181 28 

a

Mg is assumed green moisture content.

Table 7–6. Exponents for 

adjustment of dimension lumber 

mechanical properties with 

change in sizea

Exponent MOR UTS UCS 

w 0.29 0.29 0.13 

l 0.14 0.14 0 a

MOR, modulus of rupture; UTS, ultimate 

tensile stress; and UCS, ultimate 

compressive parallel-to-grain stress. 

7–11

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties

2
stress design, design stresses are based on an assumed 

10-year loading period (called normal loading). If duration 

of loading, either continuously or cumulatively, is expected 

to exceed 10 years, design stresses are reduced 10%. If the 

expected duration of loading is for shorter periods, published design stresses can be increased using Figure 7–9. Ultimate limit-state design stresses are based on a 5-min loading period. If the duration of loading is expected to exceed 

5 min, limit-state design stresses are reduced by applying 

the time effects factor. Intermittent loading causes cumulative effects on strength and should be treated as continuous 

load of equivalent duration. The effects of cyclic loads of 

short duration must also be considered in design (see discussion of fatigue in Chap. 5). These duration of load modifications are not applicable to modulus of elasticity.

In many design circumstances, several loads bear on the 

structure, some acting simultaneously and each with a 

Table 7–7. Coefficients for moisture adjustment 

of dimension lumber mechanical properties with 

change in moisture contenta

Property (MPa (lb in–2))

Coefficients MOR UTS UCS 

B1 16.6 (2,415) 21.7 (3,150) 9.6 (1,400) 

B2 0.276 (40) 0.552 (80) 0.234 (34) 

a

MOR is modulus of rupture; UTS, ultimate tensile stress; and 

UCS, ultimate compressive parallel-to-grain stress. 

Figure 7–9. Relation of strength to duration of load.

Table 7–8. Example of duration of load adjustments for ASD

Time

(year) 

Total load 

(kPa (lb ft–2)) 

Load 

adjustmenta

Equivalent 10-year 

design load 

(kPa (lb ft–2)) 

 1 4.8 (100) + 0.96 (20) = 5.7 (120) 0.93 5.36 (112) 

50 0.96 (20) 1.04 1.0 (21) 

a

Figure 7–9. 

7–12

For MOR > 16.6 MPa (2,415 lb in–2) 

UTS > 21.7 MPa (3,150 lb in–2)

UCS > 9.7 MPa (1,400 lb in–2)

( ) 1 2

2 1

1 1

2 1 M M

B M

P B P P − 















−

− = +

where M1 is moisture content 1 (%), M2 is moisture content 

2 (%), and B1, B2 are constants from Table 7–7.

For E, the following equation applies:

















−

− = 1.857 (0.0237 )1.857 (0.0237 )

1

2

1 2 M

M E E

where E1 is property (MPa, lb in–2) at moisture content 1 

and E2 is property (MPa, lb in–2) at moisture content 2.

For lumber thicker than 102 mm (4 in.), often no adjustment 

for moisture content is made because properties are assigned 

on the basis of wood in the green condition. This lumber is 

usually put in place without drying, and it is assumed that 

drying degrade offsets the increase in strength normally associated with loss in moisture.

Duration of Load

Design may be based on either design stresses and a duration of load factor or on ultimate limit state design stresses 

and a time effects factor. Both the duration of load and time 

effects factor describe the same phenomenon. In allowable 

Figure 7–8. Modulus of rupture as a function of 

moisture content for dimension lumber. Open dots 

represent the ASTM D 1990 model, and solid dots 

represent the more precise quadratic surface model 

on which the ASTM D 1990 model was based.

General Technical Report FPL–GTR–190
different duration. When loads of different time duration are 

applied, the load duration factor corresponding to the shortest time duration is used. Each increment of time during 

which the total load is constant should be treated separately, 

and the most severe condition governs the design. Either the 

design stress or the total design load (but not both) can be 

adjusted using Figure 7–9.

For example, suppose a structure is expected to support a 

load of 4.8 kPa (100 lb ft–2) on and off for a cumulative duration of 1 year. Also, it is expected to support its own dead 

load of 0.96 kPa (20 lb ft–2) for the anticipated 50-year life 

of the structure. The adjustments to be made to arrive at 

an equivalent 10-year design load for ASD are listed in 

Table 7–8.

The more severe design load is 5.36 kPa (112 lb ft–2), and 

this load and the design stress for lumber would be used to 

select members of suitable size. In this case, it was convenient to adjust the loads on the structure, although the same 

result can be obtained by adjusting the design stress.

Treatment Effects

Treatments have been shown to affect the final strength of 

wood (see Chap. 5 for detailed discussion). There is a 5% 

reduction in E and a 15% reduction in strength properties of 

incised and treated dimension lumber for both dry- and wetuse conditions in the United States. In Canada, a 10% reduction in E and a 30% reduction in all strength properties from 

incising are applied to dry-use conditions, whereas 5% and 

15% reductions are used for wet-use conditions. The wetuse factors are applied in addition to the traditional wet-use 

service factor. Reductions in energy-related properties are 

about 1.5 to 2 times those reported for static strength 

properties. There is no difference in long-term duration 

of load behavior between treated and untreated material 

(Fig. 7–10). Current design standards prohibit increases in 

design stresses beyond the 1.6 factor for short-term duration 

of load when considering impact-type loading for material 

treated with waterborne preservative.

Temperature Effects

As wood is cooled below normal temperatures, its properties 

increase. When heated, its properties decrease. The magnitude of the change depends upon moisture content. Up to 

65 °C (150 °F), the effect of temperature is assumed by 

design codes to be reversible. For structural members that 

will be exposed to temperatures up to 65 °C (150 °F), design values are multiplied by the factors given in Table 7–9 

(AF&PA). Prolonged exposure to heat can lead to a permanent loss in strength (see Chap. 5).

Round Timbers and Ties

Strength Properties

Allowable strength properties of round timbers have been 

developed and published in several standards. In most cases, 

published values are based on strength of clear test samples. 

Allowable stresses are derived by adjusting clear test values 

for effects of growth characteristics, conditioning, shape, 

and load conditions as discussed in applicable standards. 

In addition, published values for some species of poles and 

piles reflect results of full-sized tests.

Poles

Most poles are used as structural members in support structures for distribution and transmission lines. For this application, poles may be designed as single-member or guyed 

cantilevers or as structural members of a more complex 

structure. Specifications for wood poles used in single pole 

structures have been published by the American National 

Standards Institute (ANSI) in Standard O5.1. Guidelines for 

the design of pole structures are given in the ANSI National 

Electric Safety Code (NESC) (ANSI C2).

Figure 7–10. Load duration factor for material untreated and treated with waterborne preservative.

Table 7–9. Property adjustment factors for in-service temperature exposures 

In-service 

moisture 

content 

Factor 

Design values 

T ≤ 37 °C 

(T ≤ 100 °F) 

37 °C < T ≤ 52 °C 

(100 °F < T ≤ 125 °F) 

52 °C < T ≤ 65 °C 

(125 °F < T ≤ 150 °F) 

Ft, E Wet or dry 1.0 0.9 0.9 

Fb, Fv, Fc, Fcୄ Dry 1.0 0.8 0.7 

Wet 1.0 0.7 0.5 

7–13

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties

Fc

┴
pile foundations under similar conditions or from the results 

of static-load tests.

Working stresses for piles are governed by building code requirements and by recommendations of ASTM D 2899. This 

standard gives recommendations for adjusting small clear 

strength values listed in ASTM D 2555 for use in the design 

of full-sized piles. In addition to adjustments for properties 

inherent to the full-sized pile, the ASTM D 2899 standard 

provides recommendations for adjusting allowable stresses 

for the effects of pretreatment conditioning.

Design stresses for timber piles are tabulated in the NDS for 

wood construction. The NDS values include adjustments for 

the effects of moisture content, load duration, and preservative treatment. Recommendations are also given to adjust 

for lateral support conditions and factors of safety.

Construction Logs

Design values for round timbers used as structural members in pole or log buildings may be determined following 

standards published by ASTM International. The ASTM 

standard D 3200 refers pole designers to the same standard 

used to derive design stresses for timber piles (D 2899). 

Derivation of design stresses for construction logs used in 

log homes is covered in ASTM D 3957, which provides a 

method of establishing stress grades for structural members 

of any of the more common log configurations. Manufacturers can use this standard to develop grading specifications 

and derive engineering design stresses for their construction 

logs.

Ties

Railroad cross and switch ties have historically been overdesigned from the standpoint of rail loads. Tie service life 

was limited largely by deterioration rather than mechanical 

damage. However, because of advances in decay-inhibiting 

treatment and increased axle loads, adequate structural design is becoming more important in increasing railroad tie 

service life.

Rail loads induce stresses in bending and shear as well as 

in compression perpendicular to the grain in railroad ties. 

The American Railway Engineering and Maintenance-ofWay Association (AREMA) manual gives recommended 

limits on ballast bearing pressure and allowable stresses for 

cross ties. This information may be used by the designer to 

determine adequate tie size and spacing to avoid premature 

failure due to mechanical damage.

Specific gravity and compressive strength parallel to the 

grain are also important properties to consider in evaluating 

cross tie material. These properties indicate the resistance of 

the wood to both pull out and lateral thrust of spikes.

Literature Cited

AF&PA. [Current edition]. Washington, DC: American 

Forest & Paper Association.

7–14

The ANSI O5.1 standard gives values for fiber stress in 

bending for species commonly used as transmission or 

distribution poles. These values represent the near-ultimate 

fiber stress for poles used as cantilever beams. For most species, these values are based partly on full-sized pole tests 

and include adjustments for moisture content and pretreatment conditioning. The values in ANSI O5.1 are compatible 

with the ultimate strength design philosophy of the NESC, 

but they are not compatible with the working stress design 

philosophy of the National Design Specification (NDS).

Reliability-based design techniques have been developed for 

the design of distribution–transmission line systems. This 

approach requires a strong database on the performance of 

pole structures. Supporting information for these design procedures is available in a series of reports published by the 

Electric Power Research Institute (EPRI).

Piles

Bearing loads on piles are sustained by earth friction along 

their surface (skin friction) or by bearing of the tip on a 

solid stratum. Wood piles, because of their tapered form, are 

particularly efficient in supporting loads by skin friction. 

Bearing values that depend upon friction are related to the 

stability of the soil and generally do not approach the ultimate strength of the pile. Where wood piles sustain foundation loads by bearing of the tip on a solid stratum, loads may 

be limited by the compressive strength of the wood parallel 

to the grain. If a large proportion of the length of a pile extends above ground, its bearing value may be limited by its 

strength as a long column. Side loads may also be applied to 

piles extending above ground. In such instances, however, 

bracing is often used to reduce the unsupported column 

length or to resist the side loads.

The most critical loads on piles often occur during driving. 

Under hard driving conditions, piles that are too dry (<18% 

moisture content at a 51-mm (2-in.) depth) have literally 

exploded under the force of the driving hammers. Steel 

banding is recommended to increase resistance to splitting, 

and driving the piles into predrilled holes reduces driving 

stresses.

The reduction in strength of a wood column resulting from 

crooks, eccentric loading, or any other condition that will 

result in combined bending and compression is not as great 

as would be predicted with the NDS interaction equations. 

This does not imply that crooks and eccentricity should be 

without restriction, but it should relieve anxiety as to the 

influence of crooks, such as those found in piles. Design 

procedures for eccentrically loaded columns are given in 

Chapter 9.

There are several ways to determine bearing capacity of 

piles. Engineering formulas can estimate bearing values 

from the penetration under blows of known energy from the 

driving hammer. Some engineers prefer to estimate bearing 

capacity from experience or observation of the behavior of 

General Technical Report FPL–GTR–190
Design values for wood construction—a supplement to 

the national design specification for wood construction.

National design specification for wood construction.

Forest Products Society. 1997. Machine-graded lumber. 

Madison, WI: Forest Products Society. Wood Design Focus. 

8(2): 1–24.

Additional References

Lumber

ALSC. 1995. Germantown, MD: American Lumber Standards Committee.

ASTM. [Current edition]. Section 04—Construction; Volume 10—Wood. West Conshohocken, PA: American Society for Testing and Materials.

ASTM D 198–05a. Standard methods of static tests of 

timbers in structural sizes.

ASTM D 245–06. Standard methods for establishing 

structural grades for visually-graded lumber.

ASTM D 1990–07. Standard methods for establishing 

allowable properties for visually-graded dimension 

lumber from in-grade tests of full-size specimens.

ASTM D 2555–06. Standard methods for establishing 

clear wood strength values.

ASTM D 2915–03. Standard practice for evaluating 

properties for stress grades of structural lumber.

ASTM D 4761–05. Standard test methods for mechanical properties of lumber and wood-base structural 

materials.

ASTM D 6570–04. Standard practice for assigning allowable properties for mechanically graded lumber.

DOC. [Current edition]. American softwood lumber standard. Voluntary Product Standard PS–20. Washington, DC: 

U.S. Department of Commerce.

Galligan, W.L.; Green, D.W.; Gromala, D.S.; Haskell, J.H. 

1980. Evaluation of lumber properties in the United States 

and their application to structural research. Forest Products 

Journal. 30(10): 45–51.

Gerhards, C.C. 1977. Effect of duration and rate of loading 

on strength of wood and wood-based materials. Res. Pap. 

FPL–RP–283. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 27 p.

Green, D.W. 1989. Moisture content and the shrinkage of 

lumber. Res. Pap. FPL–RP–489. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 11 p.

Green, D.W.; Evans, J.W. 1987. Mechanical properties of 

visually graded dimension lumber. PB–88–159–371. 

Springfield, VA: National Technical Information Service. 

Vol. 1–Vol. 7.

Green, D.W.; Kretschmann, D.E. 1992. Properties and 

grading of Southern Pine timbers. Forest Products Journal. 

47(9): 78–85.

Green, D.W.; Shelley, B.E. 2006. Guidelines for assigning 

allowable properties to visually graded foreign species. Germantown, MD: Board of Review, American Lumber Standards Committee. 10 p.

Green, D.W.; Shelley, B.E. 2006. Guidelines for assigning 

allowable properties to mechanically graded foreign species. 

Germantown, MD: American Lumber Standard Committee. 

12 p.

Green, D.W.; Shelley, B.E.; Vokey, H.P. 1989. In-grade testing of structural lumber. In: Proceedings of workshop sponsored by In-grade Testing Committee and Forest Products 

Society. Proceedings 47363. Madison, WI: Forest Products 

Society.

Kretschmann, D.E.; Green D.W. 1996. Modeling moisture 

content–mechanical property relationships for clear Southern Pine. Wood Fiber Science. 28(3): 320–337.

Kretschmann, D.E.; Evans, J.W. 2010. History of lumber 

submissions under ASTM D 1990 since the North American 

in-grade testing program. Gen. Tech. Rep. FPL–GTR–189. 

Madison, WI: U.S. Department of Agriculture, Forest Service. Forest Products Laboratory. 52 p.

Winandy, J.E. 1995. The influence of time–to–failure on the 

strength of CCA-treated lumber. Forest Products Journal. 

45(2): 82–85.

General

AWPA. [Current edition]. Book of standards (includes standards on pressure and thermal treatment of poles, piles, and 

ties). American Wood-Preserver’s Bureau official quality 

control standards. Bethesda, MD: American Wood Protection Association.

Engineering Data Management and Colorado State University. 1989–1998. International conference—Wood poles and 

piles. Conference proceedings. Fort Collins, CO: Engineering Data Management and Colorado State University.

USFSS. [Current edition]. Poles and piles, wood. Federal Specification MM–P–371c—ties, railroad (cross and 

switch); Federal Specification MM–T–371d—wood preservation: treating practice; Federal Specification TT–W–571. 

Washington, DC: U.S. Federal Supply Service.

Poles

ANSI. [Current edition]. New York, NY: American National 

Standards Institute.

ANSI O5.1. Specifications and dimensions for wood 

poles.

ANSI C2. National electrical safety code.

7–15

Chapter 7 Stress Grades and Design Properties for Lumber, Round Timber, and Ties
AREA. 1982. Timber structures. In: Manual for railway engineering. Washington, DC: American Railway Engineering 

Association. Chapter 7.

Armstrong, R.M. 1979. Structural properties of timber piles. 

In: Behavior of deep foundations. ASTM STP670. Philadelphia, PA: American Society for Testing and Materials: 

118–152.

ASTM. [Current edition]. West Conshohocken, PA: American Society for Testing and Materials.

ASTM D 25–99. Standard specification for round 

timber piles.

ASTM D 2899–03. Establishing design stresses for 

round timber piles.

AWPI. 1969. Pile foundations know-how. Washington, DC: 

American Wood Preservers Institute.

Thompson, W.S. 1969. Factors affecting the variation in 

compressive strength of Southern Pine piling. Washington, 

DC: American Wood-Preservers’ Association.

Construction Logs

ASTM. [Current edition]. Standard methods for establishing 

stress grades for structural members used in log buildings. 

ASTM D 3957. West Conshohocken, PA: American Society 

for Testing and Materials.

Muchmore, F.W. 1977. Design guide for native log stringer 

bridges. Juneau, AK: U.S. Department of Agriculture, Forest 

Service, Region 10. 12 p.

Rowell, R.M.; Black, J.M.; Gjovik, L.R.; Feist, W.C. 1977. 

Protecting log cabins from decay. Gen. Tech. Rep. FPL–11. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 16 p.

Ties

AREMA. 2008. Ties. In: Manual for railway engineering. 

Washington, DC: American Railway Engineering and Maintenance-of-Way Association. Volume 1, Chapter 30.

7–16

ANSI O5.2. Structural glued laminated timber for utility structures.

ASTM. [Current edition]. West Conshohocken, PA: American Society for Testing and Materials.

ANSI/ASTM D 1036–99. Standard methods of static 

tests of wood poles.

ASTM D 3200–74. Standard specification and methods 

for establishing recommended design stresses for round 

timber construction poles.

Carson, J.M.; Dougherty, M., eds. 1997. Post-frame building handbook: materials, design considerations, construction 

procedures. Ithaca, NY: Northeast Regional Agricultural 

Engineering Service. 78 p.

EPRI. 1981. Probability-based design of wood transmission structures. (a) Vol. 1: Strength and stiffness of wood 

utility poles. (b) Vol. 2: Analysis and probability-based 

design of wood utility structures. (c) Vol. 3: Users’ manual. 

POLEDA–80. Pole design and analysis. Prepared by: Research Institute of Colorado for Electric Power Research 

Institute. Palo Alto, CA: Electric Power Research Institute. 

EL1–2040. Vols. 1–3, Proj. 1352–1. September.

EPRI. 1985. Wood pole properties. Vol. 1: Background and 

Southern Pine data. Prepared by: Research Institute of Colorado for Electric Power Research Institute. Palo Alto, CA: 

Electric Power Research Institute. EL–4109. Proj. 1352–2. 

July.

EPRI. 1986. Wood pole properties. (a) Vol. 2: Douglas-fir 

data. (b) Vol. 3: Western redcedar. Prepared by: Research 

Institute of Colorado for Electric Power Research Institute. 

Palo Alto, CA: Electric Power Research Institute. EL–4109. 

Proj. 1352–2. January.

Morrell, J.J. 1996. Wood pole maintenance manual. Corvallis, OR: Oregon State University, College of Forestry, Forest 

Research Laboratory. 47 p.

NRAES. 1997. Post-frame building construction. Ithaca, 

NY: Northeast Regional Agricultural Engineering Service.

Thompson, W.S. 1969. Effect of steaming and kiln drying 

on properties of Southern Pine poles. Part I—mechanical 

properties. Forest Products Journal. 19(1): 21–28.

Wood, L.W.; Erickson, E.C.O.; Dohr, A.W. 1960. Strength 

and related properties of wood poles. Philadelphia, PA: 

American Society for Testing and Materials. 83 p.

Wood, L.W.; Markwardt, L.J. 1965. Derivation of fiber 

stresses from strength values of wood poles. Res. Pap. FPL 

39. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 12 p.

Piles

AF&PA. [Current edition]. National design specification for 

wood construction. Washington, DC: American Forest & 

Paper Association.

General Technical Report FPL–GTR–190
CHAPTER 8

Fastenings

Douglas R. Rammer, Research General Engineer

8–1

Contents 

Nails 8–1

 Withdrawal Resistance 8–2 

 Lateral Resistance 8–6

Spikes 8–9

Staples 8–9 

Drift Bolts 8–10

Wood Screws 8–10

 Withdrawal Resistance 8–10

 Lateral Resistance 8–11

Lag Screws 8–12

 Withdrawal Resistance 8–12

 Lateral Resistance 8–13

Bolts 8–14

 Bearing Stress of Wood under Bolts 8–14

 Loads at an Angle to the Grain 8–15

 Steel Side Plates 8–15

 Bolt Quality 8–15

 Effect of Member Thickness 8–16

 Two Member, Multiple Member Joints 8–16

 Spacing, Edge, and End Distance 8–16

 Effect of Bolt Holes 8–16

 Pre-1991 Allowable Loads 8–17

 Post-1991 Yield Model 8–17

Connector Joints 8–19

 Parallel-to-Grain Loading 8–19

 Perpendicular-to-Grain Loading 8–20

 Design Loads 8–20

 Modifications 8–21

 Net Section 8–23

 End Distance and Spacing 8–23

 Placement of Multiple Connectors 8–23

 Cross Bolts 8–24

 Multiple-Fastener Joints 8–24

Metal Plate Connectors 8–25

Joist Hangers 8–25

Fastener Head Embedment 8–26

Literature Cited 8–27

Additional References 8–27

The strength and stability of any structure depend heavily on the fastenings that hold its parts together. One prime 

advantage of wood as a structural material is the ease with 

which wood structural parts can be joined together with a 

wide variety of fastenings—nails, spikes, screws, bolts, lag 

screws, drift pins, staples, and metal connectors of various 

types. For utmost rigidity, strength, and service, each type of 

fastening requires joint designs adapted to the strength properties of wood along and across the grain and to dimensional 

changes that may occur with changes in moisture content.

Maximum lateral resistance and safe design load values for 

small-diameter (nails, spikes, and wood screws) and largediameter dowel-type fasteners (bolts, lag screws, and drift 

pins) were based on an empirical method prior to 1991. Research conducted during the 1980s resulted in lateral resistance values that are currently based on a yield model theory. This theoretical method was adapted for the 1991 edition 

of the National Design Specification for Wood Construction 

(NDS). Because literature and design procedures exist that 

are related to both the empirical and theoretical methods, we 

refer to the empirical method as pre-1991 and the theoretical 

method as post-1991 throughout this chapter. Withdrawal 

resistance methods have not changed, so the pre- and post1991 refer only to lateral resistance.

The information in this chapter represents primarily Forest 

Products Laboratory research results. A more comprehensive discussion of fastenings is given in the American Society of Civil Engineers Manuals and Reports on Engineering 

Practice No. 84, Mechanical Connections in Wood Structures. The research results of this chapter are often modified 

for structural safety, based on judgment or experience, and 

thus information presented in design documents may differ 

from information presented in this chapter. Additionally, 

research by others serves as a basis for some current design 

criteria. Allowable stress design and limit states design criteria are presented in the National Design Specification for 

Wood Construction published by the American Forest and 

Paper Association.

Nails

Nails are the most common mechanical fastenings used in 

wood construction. There are many types, sizes, and forms 

of nails (Fig. 8–1). Most load equations presented in this 

section apply for bright, smooth, common steel wire nails 

driven into wood when there is no visible splitting. For nails 

other than common wire nails, the loads can be adjusted by 

factors given later in the chapter.
8–2

General Technical Report FPL–GTR–190

Nails in use resist withdrawal loads, lateral loads, or a combination of the two. Both withdrawal and lateral resistance 

are affected by the wood, the nail, and the condition of use. 

In general, however, any variation in these factors has a 

more pronounced effect on withdrawal resistance than on 

lateral resistance. The serviceability of joints with nails laterally loaded does not depend greatly on withdrawal resistance unless large joint distortion is tolerable.

The diameters of various penny or gauge sizes of bright 

common nails are given in Table 8–1. The penny size designation should be used cautiously. International nail producers sometimes do not adhere to the dimensions of Table 8–1. 

Thus penny sizes, although still widely used, are obsolete. 

Specifying nail sizes by length and diameter dimensions is 

recommended. Bright box nails are generally of the same 

length but slightly smaller diameter (Table 8–2), whereas 

cement-coated nails such as coolers, sinkers, and coated box 

nails are slightly shorter (3.2 mm (1/8 in.)) and of smaller 

diameter than common nails of the same penny size. Helically and annularly threaded nails generally have smaller 

diameters than common nails for the same penny size 

(Table 8–3).

Withdrawal Resistance

The resistance of a nail shank to direct withdrawal from a 

piece of wood depends on the density of the wood, the diameter of the nail, and the depth of penetration. The surface 

condition of the nail at the time of driving also influences 

the initial withdrawal resistance.

For bright common wire nails driven into the side grain of 

seasoned wood or unseasoned wood that remains wet, the 

results of many tests have shown that the maximum withdrawal load is given by the empirical equation

(metric) (8–1a)

(inch–pound) (8–1b)

where p is maximum load (N, lb), L depth (mm, in.) of penetration of the nail in the member holding the nail point, 

G specific gravity of the wood based on ovendry weight and 

volume at 12% moisture content (see Chap. 5, Tables 5–2 to 

5–5), and D diameter of the nail (mm, in.). (The NDS uses 

ovendry weight and volume as a basis.)

The loads expressed by Equation (8–1) represent average 

data. Certain wood species give test values that are 

somewhat greater or less than the equation values. A 

Figure 8–1. Various types of nails: (left to right) bright 

smooth wire nail, cement coated, zinc-coated, annularly threaded, helically threaded, helically threaded 

and barbed, and barbed.

Table 8–1. Sizes of bright common wire 

nails

Size Gauge

Length 

(mm (in.)) 

Diameter 

(mm (in.)) 

6d 11-1/2 50.8 (2) 2.87 (0.113)

8d 10-1/4 63.5 (2-1/2) 3.33 (0.131)

10d 9 76.2 (3) 3.76 (0.148)

12d 9 82.6 (3-1/4) 3.76 (0.148)

16d 8 88.9 (3-1/2) 4.11 (0.162)

20d 6 101.6 (4) 4.88 (0.192)

30d 5 114.3 (4-1/2) 5.26 (0.207)

40d 4 127.0 (5) 5.72 (0.225)

50d 3 139.7 (5-1/2) 6.20 (0.244)

60d 2 152.4 (6) 6.65 (0.262)

Table 8–2. Sizes of smooth box nails

Size Gauge

Length 

(mm (in.)) 

Diameter 

(mm (in.)) 

3d 14-1/2 31.8 (1-1/4) 1.93 (0.076)

4d 14 38.1 (1-1/2) 2.03 (0.080)

5d 14 44.5 (1-3/4) 2.03 (0.080)

6d 12-1/2 50.8 (2) 2.49 (0.099)

7d 12-1/2 57.2 (2-1/4) 2.49 (0.099)

8d 11-1/2 63.5 (2-1/2) 2.87 (0.113)

10d 10-1/2 76.2 (3) 3.25 (0.128)

16d 10 88.9 (3-1/2) 3.43 (0.135)

20d 9 101.6 (4) 3.76 (0.148)

Table 8–3. Sizes of helically 

and annularly threaded nails

Size 

Length 

(mm (in.)) 

Diameter

(mm (in.))

6d 50.8 (2) 3.05 (0.120)

8d 63.5 (2-1/2) 3.05 (0.120)

10d 76.2 (3) 3.43 (0.135)

12d 82.6 (3-1/4) 3.43 (0.135)

16d 88.9 (3-1/2) 3.76 (0.148)

20d 101.6 (4) 4.50 (0.177)

30d 114.3 (4-1/2) 4.50 (0.177)

40d 127.0 (5) 4.50 (0.177)

50d 139.7 (5-1/2) 4.50 (0.177)

60d 152.4 (6) 4.50 (0.177)

70d 177.8 (7) 5.26 (0.207)

80d 203.2 (8) 5.26 (0.207)

90d 228.6 (9) 5.26 (0.207)
8–3

Chapter 8 Fastenings

typical load–displacement curve for nail withdrawal 

(Fig. 8–2) shows that maximum load occurs at relatively 

small values of displacement.

Although the equation for nail-withdrawal resistance indicates that the dense, heavy woods offer greater resistance 

to nail withdrawal than do the lower density ones, lighter 

species should not be disqualified for uses requiring high 

resistance to withdrawal. As a rule, the less dense species 

do not split as readily as the denser ones, thus offering an 

opportunity for increasing the diameter, length, and number 

of the nails to compensate for the wood’s lower resistance to 

nail withdrawal.

The withdrawal resistance of nail shanks is greatly affected 

by such factors as type of nail point, type of shank, time the 

nail remains in the wood, surface coatings, and moisture 

content changes in the wood.

Effect of Seasoning

With practically all species, nails driven into green wood 

and pulled before any seasoning takes place offer about the 

same withdrawal resistance as nails driven into seasoned 

wood and pulled soon after driving. However, if common 

smooth-shank nails are driven into green wood that is allowed to season, or into seasoned wood that is subjected 

to cycles of wetting and drying before the nails are pulled, 

they lose a major part of their initial withdrawal resistance. 

The withdrawal resistance for nails driven into wood that 

is subjected to changes in moisture content may be as low 

as 25% of the values for nails tested soon after driving. On 

the other hand, if the wood fibers deteriorate or the nail corrodes under some conditions of moisture variation and time, 

withdrawal resistance is erratic; resistance may be regained 

or even increased over the immediate withdrawal resistance. 

However, such sustained performance should not be relied 

on in the design of a nailed joint.

In seasoned wood that is not subjected to appreciable moisture content changes, the withdrawal resistance of nails may 

also diminish due to relaxation of the wood fibers with time. 

Under all these conditions of use, the withdrawal resistance 

of nails differs among species and shows variation within 

individual species.

Effect of Nail Form

The surface condition of nails is frequently modified during 

the manufacturing process to improve withdrawal resistance. Such modification is usually done by surface coating, surface roughening, or mechanical deformation of the 

shank. Other factors that affect the surface condition of the 

nail are the oil film remaining on the shank after manufacture or corrosion resulting from storage under adverse conditions; but these factors are so variable that their influence 

on withdrawal resistance cannot be adequately evaluated.

Surface Modifications—A common surface treatment 

for nails is the so-called cement coating. Cement coatings, 

contrary to what the name implies, do not include cement 

as an ingredient; they generally are a composition of resin 

applied to the nail to increase the resistance to withdrawal 

by increasing the friction between the nail and the wood. 

If properly applied, they increase the resistance of nails to 

withdrawal immediately after the nails are driven into the 

softer woods. However, in the denser woods (such as hard 

maple, birch, or oak), cement-coated nails have practically 

no advantage over plain nails, because most of the coating is removed in driving. Some of the coating may also be 

removed in the side member before the nail penetrates the 

main member.

Good-quality cement coatings are uniform, not sticky to the 

touch, and cannot be rubbed off easily. Different techniques 

of applying the cement coating and variations in its ingredients may cause large differences in the relative resistance 

to withdrawal of different lots of cement-coated nails. Some 

nails may show only a slight initial advantage over plain 

nails. In the softer woods, the increase in withdrawal resistance of cement-coated nails is not permanent but drops off 

significantly after a month or so. Cement-coated nails are 

used primarily in construction of boxes, crates, and other 

containers usually built for rough handling and relatively 

short service.

Nails that have galvanized coatings, such as zinc, are intended primarily for uses where corrosion and staining resistance are important factors in permanence and appearance. 

If the zinc coating is evenly applied, withdrawal resistance 

may be increased, but extreme irregularities of the coating 

may actually reduce it. The advantage that uniformly coated 

galvanized nails may have over nongalvanized nails in resistance to initial withdrawal is usually reduced by repeated 

cycles of wetting and drying.

Nails have also been made with plastic coatings. The usefulness and characteristics of these coatings are influenced by 

Figure 8–2. Typical load–displacement curve for direct withdrawal of a nail.
8–4

General Technical Report FPL–GTR–190

the quality and type of coating, the effectiveness of the bond 

between the coating and base fastener, and the effectiveness 

of the bond between the coating and wood fibers. Some 

plastic coatings appear to resist corrosion or improve resistance to withdrawal, while others offer little improvement.

Fasteners with properly applied nylon coating tend to retain 

their initial resistance to withdrawal compared with other 

coatings, which exhibit a marked decrease in withdrawal 

resistance within the first month after driving.

A chemically etched nail has somewhat greater withdrawal 

resistance than some coated nails, as the minutely pitted 

surface is an integral part of the nail shank. Under impact 

loading, however, the withdrawal resistance of etched nails 

is little different from that of plain or cement-coated nails 

under various moisture conditions.

Sand-blasted nails perform in much the same manner as 

chemically etched nails.

Shape Modifications—Nail shanks may be varied from 

a smooth, circular form to give an increase in surface 

area without an increase in nail weight. Special nails with 

barbed, helically or annularly threaded, and other irregular 

shanks (Fig. 8–1) are commercially available.

The form and magnitude of the deformations along the 

shank influence the performance of the nails in various 

wood species. In wood remaining at a uniform moisture 

content, the withdrawal resistance of these nails is generally somewhat greater than that of common wire nails of 

the same diameter. From tests in which nails were driven in 

the side grain of seasoned wood, bright annularly threaded 

nails, with shank-to-thread-crest diameter difference greater 

than 0.2 mm (0.008 in.) and thread spacing between 1.27 

mm (0.05 in.) and 1.96 mm (0.077 in.), the immediate maximum withdrawal load is given by the empirical equation

(metric) (8–2a)

(inch–pound) (8–2b)

where p is maximum load (N, lb), L depth (mm, in.) of penetration of the nail in the member holding the nail point, G

specific gravity of the wood based on ovendry weight and 

volume and oven dry moisture content (see Chap. 5, Tables 

5–2 to 5–5), and D shank diameter of the nail (mm, in.). 

The expression is valid only for the threaded portion of the 

nail. Comparison of Equations (8–1) and (8–2) indicates 

that the bright annularly threaded nail can have withdrawal 

resistances that are double the values of common nails. For 

galvanized annularly threaded nails, the immediate withdrawal strength is slightly lower. However, under conditions 

involving changes in moisture content of the wood, some 

special nail forms provide considerably greater withdrawal 

resistance than the common wire nail—about four times 

greater for annularly and helically threaded nails of the same 

diameter. This is especially true of nails driven into green 

wood that subsequently dries. In general, annularly threaded 

nails sustain larger withdrawal loads, and helically threaded 

nails sustain greater impact withdrawal work values than do 

the other nail forms (Fig. 8–3).

Nails with deformed shanks are sometimes hardened by heat 

treatments for use where driving conditions are difficult or 

to obtain improved performance, such as in pallet assembly. 

Hardened nails are brittle and care should be exercised to 

avoid injuries from fragments of nails broken during

driving.

Nail Point—A smooth, round shank nail with a long, sharp 

point will usually have a greater withdrawal resistance, 

particularly in the softer woods, than the common wire nail 

(which usually has a diamond point). However, sharp points 

accentuate splitting in certain species, which may reduce 

withdrawal resistance. A blunt or flat point without taper 

reduces splitting, but its destruction of the wood fibers when 

driven reduces withdrawal resistance to less than that of the 

common wire nail. A nail tapered at the end and terminating 

in a blunt point will cause less splitting. In heavier woods, 

such a tapered, blunt-pointed nail will provide about the 

same withdrawal resistance, but in less dense woods, its resistance to withdrawal is less than that of the common nail.

Nail Head—Nail head classifications include flat, oval, 

countersunk, deep-countersunk, and brad. Nails with all 

types of heads, except the deep-countersunk, brad, and some 

of the thin flathead nails, are sufficiently strong to withstand 

the force required to pull them from most woods in direct 

withdrawal. One exception to this statement is for annularly 

threaded nails. Due to the increased withdrawal capacity 

for these type nails, nail head can be pulled into or through 

wood members. The deep-countersunk and brad nails are 

usually driven below the wood surface and are not intended 

to carry large withdrawal loads. In general, the thickness 

Figure 8–3. Typical load–displacement curves deformed and smooth shank for direct withdrawal of a 

nail.
8–5

Chapter 8 Fastenings

and diameter of the heads of the common wire nails increase 

as the size of the nail increases.

The development of some pneumatically operated portable 

nailers has introduced nails with specially configured heads, 

such as T-nails and nails with a segment of the head cut off.

Corrosion and Staining

In the presence of moisture, metals used for nails may corrode when in contact with wood treated with certain preservative or fire-retardant treatments (Chaps. 15 and 18). Use 

of certain metals or metal alloys will reduce the amount of 

corrosion. Nails of copper, silicon bronze, and 300 series 

stainless steels have performed well in wood treated with 

ammoniacal copper arsenate and chromated copper arsenate. 

Similarly, 300 series stainless steel nails have performed 

well in wood treated with copper azole and alkaline copper 

quaternary. The choice of metals for use with fire-retardanttreated woods depends upon the particular fire-retardant 

chemical.

With the greater use of metal connecters, such as joist hangers, in outdoor environments, an additional corrosion concern is possible. Both the joist hanger and fastener should 

be of the same metal type; if not, the corrosion rate of either 

the fastener or hanger may increase due to galvanic (mixed 

metal) corrosion between the hanger and fastener.

Organic coated fasteners, such as polymer coatings, resist 

corrosion on the principle of isolation. Any damage that occurs to the coating during insertion can give the corrosive 

environment a path to the substrate, and pitting or crevice 

corrosion will occur at these sites.

Staining caused by the reaction of certain wood extractives 

(Chap. 3) and steel in the presence of moisture is a problem 

if appearance is important, such as with naturally finished 

siding. Use of stainless steel, aluminum, or hot-dipped galvanized nails can alleviate staining.

In general, the withdrawal resistance of copper, other alloy, 

and polymer-coated nails is comparable with that of common steel wire nails when pulled soon after driving.

Driving

The resistance of nails to withdrawal is generally greatest 

when they are driven perpendicular to the grain of the wood. 

When a bright nail is driven parallel to the wood fibers (that 

is, into the end of the piece) withdrawal resistance in wood 

ranges between 50% to 75% of the resistance obtained when 

the nail is driven perpendicular to the grain. The ratio between the immediate end- and side-grain withdrawal loads 

is nearly constant for all specific gravities. In contrast to the 

immediate withdrawal case, nails pulled after a time interval 

or after moisture content changes experience a decreased 

load in both side and end grain. For most species the decrease in the side grain withdrawal load is greater than in the 

end grain; therefore the resulting end- to side-grain ratio is 

larger.

Toe nailing, a common method of joining wood framework, 

involves slant driving a nail or group of nails through the 

end or edge of an attached member and into a main member. 

Toe nailing requires greater skill in assembly than does ordinary end nailing but provides joints of greater strength and 

stability. Tests show that the maximum strength of toenailed 

joints under lateral and uplift loads is obtained by (a) using 

the largest nail that will not cause excessive splitting, 

(b) allowing an end distance (distance from the end of the 

attached member to the point of initial nail entry) of approximately one-third the length of the nail, (c) driving the nail 

at a slope of 30° with the attached member, and (d) burying 

the full shank of the nail but avoiding excessive mutilation 

of the wood from hammer blows.

The results of withdrawal tests with multiple nail joints in 

which the piece attached is pulled directly away from the 

main member show that slant driving is usually superior to 

straight driving when nails are driven into dry wood and 

pulled immediately, and decidedly superior when nails are 

driven into green or partially dry wood that is allowed to 

season for a month or more. However, the loss in depth of 

penetration due to slant driving may, in some types of joints, 

offset the advantages of slant nailing. Cross slant driving of 

groups of nails through the side grain is usually somewhat 

more effective than parallel-slant driving through the end 

grain.

Nails driven into lead holes with a diameter slightly smaller 

(approximately 90%) than the nail shank have somewhat 

greater withdrawal resistance than nails driven without lead 

holes. Lead holes also prevent or reduce splitting of the 

wood, particularly for dense species.

Clinching

The withdrawal resistance of smooth-shank, clinched nails 

is considerably greater than that of unclinched nails. The 

point of a clinched nail is bent over where the nail protrudes 

through the side member. The ratio between the loads for 

clinched and unclinched nails varies enormously, depending 

upon the moisture content of the wood when the nail is driven and withdrawn, the species of wood, the size of nail, and 

the direction of clinch with respect to the grain of the wood.

In dry or green wood, a clinched nail provides 45% to 170% 

more withdrawal resistance than an unclinched nail when 

withdrawn soon after driving. In green wood that seasons 

after a nail is driven, a clinched nail gives 250% to 460% 

greater withdrawal resistance than an unclinched nail. However, this improved strength of a clinched-nail joint does not 

justify the use of green lumber, because the joints may loosen as the lumber seasons. Furthermore, laboratory tests were 

made with single nails, and the effects of drying, such as 

warping, twisting, and splitting, may reduce the efficiency 

of a joint that has more than one nail. Clinching of nails is 

generally confined to such construction as boxes and crates 

and other container applications.
Nails clinched across the grain have approximately 20% 

more resistance to withdrawal than nails clinched along the 

grain.

Fastening of Plywood

The nailing characteristics of plywood are not greatly different from those of solid wood except for plywood’s greater 

resistance to splitting when nails are driven near an edge. 

The nail withdrawal resistance of plywood is 15% to 30% 

less than that of solid wood of the same thickness. The reason is that fiber distortion is less uniform in plywood than in 

solid wood. For plywood less than 12.5 mm (1/2 in.) thick, 

the greater splitting resistance tends to offset the lower 

withdrawal resistance compared with solid wood. The withdrawal resistance per unit length of penetration decreases as 

the number of plies per unit length increases. The direction 

of the grain of the face ply has little influence on the withdrawal resistance from the face near the end or edge of a 

piece of plywood. The direction of the grain of the face ply 

may influence the pull-through resistance of staples or nails 

with severely modified heads, such as T-heads. Fastener 

design information for plywood is available from APA–

The Engineered Wood Association.

Allowable Loads

The preceding discussion dealt with maximum withdrawal 

loads obtained in short-time test conditions. For design, 

these loads must be reduced to account for variability, 

duration-of-load effects, and safety. A value of one-sixth 

the average maximum load has usually been accepted as the 

allowable load for long-time loading conditions. For normal 

duration of load, this value may be increased by 10%. Normal duration of load is defined as a load of 10-year duration.

Lateral Resistance

Pre-1991

Test loads at joint slips of 0.38 mm (0.015 in.) (approximate 

proportional limit load) for bright common wire nails in 

lateral resistance driven into the side grain (perpendicular 

to the wood fibers) of seasoned wood are expressed by the 

empirical equation

 p = KD3/ 2 (8–2)

where p is lateral load per nail, K a coefficient, and D

diameter of the nail. Values of coefficient K are listed in 

Table 8–4 for ranges of specific gravity of hardwoods and 

softwoods. The loads given by the equation apply only 

where the side member and the member holding the nail 

point are of approximately the same density. The thickness 

of the side member should be about one-half the depth of 

penetration of the nail in the member holding the point.

The ultimate lateral nail loads for softwoods may approach 

3.5 times the loads expressed by the equation, and for 

hardwoods they may be 7 times as great. The joint slip at 

maximum load, however, is more than 20 times 0.38 mm 

(0.015 in.). This is demonstrated by the typical load–slip 

curve shown in Figure 8–4. To maintain a sufficient ratio 

between ultimate load and the load at 0.38 mm (0.015 in.), 

the nail should penetrate into the member holding the point 

by not less than 10 times the nail diameter for dense woods 

(specific gravity greater than 0.61) and 14 times the diameter for low-density woods (specific gravity less than 0.42). 

For species having densities between these two ranges, the 

penetration may be found by straight line interpolation.

Post-1991

The yield model theory selects the worst case of yield 

modes based on different possibilities of wood bearing and 

nail bending. It does not account for nail head effects, friction between the main and side member, or axial forces 

transmitted along the length of the fastener. A description 

of the various combinations is given in Figure 8–5. 

Mode I is a wood bearing failure in either the main or side 

member; mode II is a rotation of the fastener in the joint 

without bending; modes III and IV are a combination of 

wood bearing failure and one or more plastic hinge yield 

formations in the fastener. Modes Im and II have not been 

8–6

General Technical Report FPL–GTR–190

Table 8–4. Coefficients for computing test loads for 

fasteners in seasoned wooda

 (pre-1991)

Specific 

gravity

rangeb

Lateral load coefficient K (metric (inch–pound)) 

Nailsc Screws 

Lag 

screws 

Hardwoods 

0.33–0.47 50.04 (1,440) 23.17 (3,360) 26.34 (3,820) 

0.48–0.56 69.50 (2,000) 31.99 (4,640) 29.51 (4,280) 

0.57–0.74 94.52 (2,720) 44.13 (6,400) 34.13 (4,950) 

Softwoods 

0.29–0.42 50.04 (1,440) 23.17 (3,360) 23.30 (3,380) 

0.43–0.47 62.55 (1,800) 29.79 (4,320) 26.34 (3,820) 

0.48–0.52 76.45 (2,200) 36.40 (5,280) 29.51 (4,280) 

a

Wood with a moisture content of 15%. 

b

Specific gravity based on ovendry weight and volume at 

12% moisture content. 

c

Coefficients based on load at joint slip of 0.38 mm (0.015 in.) 

Figure 8–4. Typical relation between lateral load and 

slip in the joint and 5% offset definition.
observed in nail and spike connections. The yield model 

theory is applicable to all types of dowel fasteners (nails, 

screws, bolts, lag screws), and thus the wood bearing capacity is described by a material property called the dowel bearing strength.

The yield mode equations (Table 8–5) are entered with the 

dowel bearing strength and dimensions of the wood members and the bending yield strength and diameter of the 

fastener.

The dowel bearing strength of the wood is experimentally 

determined by compressing a dowel into a wood member. 

The strength basis is the load representing a 5% diameter 

offset on the load–deformation curve (Fig. 8–4). Dowel 

bearing strength Fe (MPa, lb in–2) is empirically related to 

specific gravity G by

 

Fe = 114.5G1.84 (metric) (8–3a)

 

Fe = 16, 600G1.84 (inch–pound) (8–3b)

where specific gravity is based on ovendry weight and 

volume.

Bending yield strengths for the common nails are 

determined by ASTM F 1575 tests with typical values 

ranging between 551 MPa (80,000 lb in–2) and 689 MPa 

(100,000 lb in–2) for common nails. Smaller diameter nails 

have higher bending yield strength due to surface hardening 

during fabrication.

Spacing

End distance, edge distance, and spacing of nails should be 

such as to prevent unusual splitting. As a general rule, nails 

should be driven no closer to the edge of the side member 

than one-half its thickness and no closer to the end than the 

thickness of the piece. Smaller nails can be driven closer 

to the edges or ends than larger ones because they are less 

likely to split the wood.

8–7

Chapter 8 Fastenings

Figure 8–5. Various combinations of wood-bearing and fastener-bending yields for (a) two-member connections and (b) three-member connections.

Table 8–5. The 5% offset lateral yield strength (Z)

for nails and screws for a two-member joint

Mode Z value for nails Z value for screws 

Is DtsFes DtsFes

IIIm

1 2 e

1 em

+ R

k DpF

—

IIIs

e

2 s em

2 R

k Dt F

 Re

k Dt F

2 

3 s em

IV

3(1 )

2 2

e

em yb

R

F F

D  3(1 )

1.75 2

e

em yb

R

F F

D 

Definitions

D nail, spike, or screw diameter, mm (in.) (for 

 annularly threaded nails, D is thread-root diameter; 

 for screws, D is either the shank diameter or the root

 diameter if the threaded portion of the screw is in the

 shear plane) 

Fem dowel bearing stress of main member 

 (member holding point), MPa (lb in–2)

Fes dowel bearing stress of side member, MPa (lb in–2)

Fyb bending yield stress of nail, spike, or screw, MPa 

 (lb in–2)

p penetration of nail or spike in main member, mm (in.)

ts thickness of side member, mm (in.) 

Z offset lateral yield strength 

Re = Fem/Fes 

 2 em

2 yb e

1 e 3

2 (1 2 ) 1 2(1 )

F p

F R D

k R      

 2 em s

2 yb e

e

e 2 3

2(1 ) 2 (2 ) 1

F t

F R D

R

R k      

 2 em s

2 e

e

e 3 2

2(1 ) (2 ) 1 yb

F t

F R D

R

R k      
Grain Direction Effects

The lateral load for side-grain nailing applies whether the 

load is in a direction parallel to the grain of the pieces 

joined or at right angles to it. When nails are driven into the 

end grain (parallel with the wood fibers), limited data on 

softwood species indicate that their maximum resistance to 

lateral displacement is about two-thirds that for nails driven 

into the side grain. Although the average proportional limit 

loads appear to be about the same for end- and side-grain 

nailing, the individual results are more erratic for end-grain 

nailing, and the minimum loads approach only 75% of corresponding values for side-grain nailing.

Moisture Content Effects

Nails driven into the side grain of unseasoned wood give 

maximum lateral resistance loads approximately equal to 

those obtained in seasoned wood, but the lateral resistance 

loads at 0.38 mm (0.015 in.) joint slip are somewhat less. 

To prevent excessive deformation, lateral loads obtained for 

seasoned wood should be reduced by 25% for unseasoned 

wood that will remain wet or be loaded before seasoning 

takes place.

When nails are driven into green wood, their lateral proportional limit loads after the wood has seasoned are also less 

than when they are driven into seasoned wood and loaded. 

The erratic behavior of a nailed joint that has undergone one 

or more moisture content changes makes it difficult to establish a lateral load for a nailed joint under these conditions. 

Structural joints should be inspected at intervals, and if it is 

apparent that the joint has loosened during drying, the joint 

should be reinforced with additional nails.

Deformed-Shank Nails

Deformed-shank nails carry somewhat higher maximum 

lateral loads than do the same pennyweight common wire 

nails, but both perform similarly at small distortions in 

the joint. It should be noted that the same pennyweight 

deformed-shank nail has a different diameter than that of the 

common wire nail. These nails often have higher bending 

yield strength than common wire nails, resulting in higher 

lateral strength in modes III and IV.

Lateral Load–Slip Models

A considerable amount of work has been done to describe, 

by mathematical models, the lateral load–slip curve of nails. 

These models have become important because of their need 

as input parameters for advanced methods of structural 

analysis.

One theoretical model, which considers the nail to be a 

beam supported on an elastic foundation (the wood), 

describes the initial slope of the curve:

 (8–4)

where P is the lateral load and d is the joint slip. The factors L1, L2, J1, J2, K1, and K2 (Table 8–6) are combinations 

of hyperbolic and trigonometric functions of the quantities 

l1a and l2b in which a and b are the depth of penetration of 

the nail in members 1 and 2, respectively. For smooth round 

nails,

 (8–5)

where k0 is elastic bearing constant, D nail diameter, and 

E modulus of elasticity of the nail. For seasoned wood, the 

elastic bearing constant k0 (N mm–3, lb in–3) has been shown 

to be related to average species specific gravity G if no lead 

hole is used by

 

k0 = 582G (metric) (8–6a)

 

k0 = 2,144, 000G (inch–pound) (8–6b)

If a prebored lead hole equal to 90% of the nail diameter is 

used,

 

k0 = 869G (metric) (8–7a)

 

k0 = 3, 200, 000G (inch–pound) (8–7b)

Other empirically derived models attempt to describe the 

entire load–slip curve. Two such expressions are

 

P = A log10 (1+ Bd) (8–8a)

8–8

General Technical Report FPL–GTR–190

Table 8–6. Expressions for factors 

in Equation (8–4)

Factor Expressiona

b b

b b+ b b

k

K

a a

a a+ a a

k

K

b b

b+ b

k

J

a a

a+ a

k

J

b b

b b b b

k

L

a a

a a a a

k

L

2 2 2 2

2 2 2 2

2

3

2 2

1 2 1 2

1 1 1 1

1

3

1 1

2 2 2 2

2 2 2 2

2

2

2 2

1 2 1 2

1 2 1 2

1

2

1 1

2 2 2 2

2 2 2 2

2

2 2

1 2 1 2

1 1 1 1

1

1 1

sinh sin

sinh cosh sin cos

sinh sin

sinh cosh sin cos

sinh sin

sinh sin

sinh sin

sinh sin

sinh sin

sinh cosh sin cos

sinh sin

sinh cosh sin cos

 

    

 

    

 

  

 

  

 

    

 

    

















a k1 = k01d and k2 = k02d, where k1 and k2 are 

the foundation moduli of members 1 and 2, 

respectively.
where the parameters A and B are empirically fitted, and 

a second model, which includes a load reducing behavior, 

(8–8b)

where the parameters K0, r1, r2, and P0 are empirically determined; d u is deformation at ultimate load, and d F is deformation at failure.

The previous two expressions represent fastener loading 

deformation response for monotonic loading. Recently, the 

load–deformation behavior of nails subjected to cyclic load 

has become of interest. The behavior of wood structures to 

dynamic or repeated loading condition from high wind or 

earthquakes is strongly linked to nail fastener models that 

consider the reversal of loading as an extension of the previous monotonic fastener load deformation model to a specific cyclic loading protocol as shown in Figure 8–6, where 

load–displacement paths OA and CD follow the monotonic 

envelope curve as expressed by Equation (8–8b). All other 

paths are assumed to exhibit a linear relationship between 

force and deformation. Unloading off the envelope curve 

follows a path such as AB with stiffness r3K0. Here, both the 

connector and wood are unloading elastically. Under continued unloading, the response moves onto path BC, which 

has reduced stiffness r4K0. Along this path, the connector 

loses partial contact with the surrounding wood because of 

permanent deformation that was produced by previous loading, along path OA in this case. The slack response along 

this path characterizes the pinched hysteresis displayed by 

dowel connections under cyclic loading. Loading in the opposite direction for the first time forces the response onto 

the envelope curve CD. Unloading off this curve is assumed 

elastic along path DE, followed by a pinched response along 

path EF, which passes through the zero-displacement intercept FI

, with slope r4K0. Continued reloading follows path 

FG with degrading stiffness Kp. Hysteretic fastener models 

are not single analytical expressions and are typically used 

in computer models.

Spikes

Common wire spikes are manufactured in the same manner 

as common wire nails. They have either a chisel point or a 

diamond point and are made in lengths of 76 to 305 mm (3 

to 12 in.). For corresponding lengths in the range of 76 to 

152 (3 to 6 in.), they have larger diameters (Table 8–7) than 

common wire nails, and beyond the 60d size they are usually designated by diameter.

The withdrawal and lateral resistance equations and limitations given for common wire nails are also applicable to 

spikes, except that in calculating the withdrawal load for 

spikes, the depth of penetration is taken as the length of the 

spike in the member receiving the point, minus two-thirds 

the length of the point.

Staples

Different types of staples have been developed with various modifications in points, shank treatment and coatings, 

gauge, crown width, and length. These fasteners are available in clips or magazines for use in pneumatically operated 

portable staplers. Most factors that affect the withdrawal and 

lateral loads of nails similarly affect the loads on staples. 

The withdrawal resistance, for example, varies almost directly with the circumference and depth of penetration when 

the type of point and shank are similar to nails. Thus, Equation (8–1) has been used to predict the withdrawal load for 

one leg of a staple, but no verification tests have been done.

The load in lateral resistance varies approximately as the 

3/2 power of the diameter when other factors, such as quality of metal, type of shank, and depth of penetration, are 

similar to nails. The diameter of each leg of a two-legged 

8–9

Chapter 8 Fastenings

Figure 8–6. Load deformation curve for nails for a 

specific cyclic loading protocol.

Table 8–7. Sizes of common wire 

spikes

Length Diameter 

Size (mm (in.)) (mm (in.)) 

10d 76.2 (3) 4.88 (0.192)

12d 82.6 (3-1/4) 4.88 (0.192)

16d 88.9 (3-1/2) 5.26 (0.207)

20d 101.6 (4) 5.72 (0.225)

30d 114.3 (4-1/2) 6.20 (0.244)

40d 127.0 (5) 6.68 (0.263)

50d 139.7 (5-1/2) 7.19 (0.283)

60d 152.4 (6) 7.19 (0.283)

5/16 in. 177.8 (7) 7.92 (0.312)

3/8 in. 215.9 (8-1/2) 9.53 (0.375)
staple must therefore be about two-thirds the diameter of a 

nail to provide a comparable load. Equation (8–2) has been 

used to predict the lateral resistance of staples. However, 

yield model theory equations have not yet been experimentally verified for staples.

In addition to the immediate performance capability of 

staples and nails as determined by test, factors such as corrosion, sustained performance under service conditions, and 

durability in various uses should be considered in evaluating 

the relative usefulness of a stapled connection.

Drift Bolts

A drift bolt (or drift pin) is a long pin of iron or steel, with 

or without head or point. It is driven into a bored hole 

through one timber and into an adjacent one, to prevent the 

separation of the timbers connected and to transmit lateral 

load. The hole in the second member is drilled sufficiently 

deep to prevent the pin from hitting the bottom.

The ultimate withdrawal load of a round drift bolt or pin 

from the side grain of seasoned wood is given by

(metric) (8–9a)

 (inch–pound) (8–9b)

where p is the ultimate withdrawal load (N, lb), G specific 

gravity based on the ovendry weight and volume at 12% 

moisture content of the wood, D diameter of the drift bolt 

(mm, in.), and L length of penetration of the bolt (mm, in.). 

(The NDS uses ovendry weight and volume as a basis.)

This equation provides an average relationship for all species, and the withdrawal load for some species may be 

above or below the equation values. It also presumes that 

the bolts are driven into prebored holes having a diameter 

3.2 mm (1/8 in.) less than the bolt diameter.

Data are not available on lateral resistance of drift bolts. 

The yield model should provide lateral strength prediction, 

but the model has not been experimentally verified for drift 

bolts. Designers have used bolt data and design methods 

based on experience. This suggests that the load for a drift 

bolt driven into the side grain of wood should not exceed, 

and ordinarily should be taken as less than, that for a bolt 

of the same diameter. Bolt design values are based on the 

thickness of the main member in a joint. Thus the depth of 

penetration of the drift bolt must be greater than or equal to 

the main-member thickness on which the bolt design value 

is based. However, the drift bolt should not fully penetrate 

its joint.

Wood Screws

The common types of wood screws have flat, oval, or round 

heads. The flathead screw is most commonly used if a flush 

surface is desired. Ovalhead and roundhead screws are used 

for appearance, and roundhead screws are used when countersinking is objectionable. The principal parts of a screw 

are the head, shank, thread, and core (Fig. 8–7). The root 

diameter for most sizes of screws averages about two-thirds 

the shank diameter. Wood screws are usually made of steel, 

brass, other metals, or alloys, and may have specific finishes 

such as nickel, blued, chromium, or cadmium. They are 

classified according to material, type, finish, shape of head, 

and diameter or gauge of the shank.

Current trends in fastenings for wood also include tapping 

or self-drilling screws. Tapping screws have threads the full 

length of the shank and may have some advantage for certain specific uses. Self-drilling screws have a drill-shaped tip 

to cut through both wood and steel material, eliminating the 

need for pre-drilling.

Withdrawal Resistance

Experimental Loads

The resistance of wood screw shanks to withdrawal from 

the side grain of seasoned wood varies directly with the 

square of the specific gravity of the wood. Within limits, the 

withdrawal load varies directly with the depth of penetration 

of the threaded portion and the diameter of the screw, provided the screw does not fail in tension. The screw will fail 

in tension when its strength is exceeded by the withdrawal 

strength from the wood. The limiting length to cause a tension failure decreases as the density of the wood increases 

since the withdrawal strength of the wood increases with 

density. The longer lengths of standard screws are therefore 

superfluous in dense hardwoods.

The withdrawal resistance of type A tapping screws, commonly called sheet metal screws, is in general about 10% 

greater than that for wood screws of comparable diameter 

and length of threaded portion. The ratio between the withdrawal resistance of tapping screws and wood screws varies 

from 1.16 in denser woods, such as oak, to 1.05 in lighter 

woods, such as redwood.

Ultimate test values for withdrawal loads of wood screws 

inserted into the side grain of seasoned wood may be expressed as

 (metric) (8–10a)

8–10

General Technical Report FPL–GTR–190

Figure 8–7. Common types of wood screws: A, 

flathead; B, roundhead; and C, ovalhead.
(inch–pound) (8–10b)

where p is maximum withdrawal load (N, lb), G specific 

gravity based on ovendry weight and volume at 12% moisture content, D shank diameter of the screw (mm, in.), and 

L length of penetration of the threaded part of the screw 

(mm, in.). (The NDS uses ovendry weight and volume as a 

basis.) These values are based on reaching ultimate load in 

5- to 10-min.

This equation is applicable when screw lead holes have a 

diameter of about 70% of the root diameter of the threads in 

softwoods, and about 90% in hardwoods.

The equation values are applicable to the screw sizes listed 

in Table 8–8. (Shank diameters are related to screw gauges.)

For lengths and gauges outside these limits, the actual values are likely to be less than the equation values.

The withdrawal loads of screws inserted in the end grain of 

wood are somewhat erratic, but when splitting is avoided, 

they should average 75% of the load sustained by screws 

inserted in the side grain.

Lubricating the surface of a screw with soap or similar lubricant is recommended to facilitate insertion, especially in 

dense woods, and it will have little effect on ultimate withdrawal resistance.

Fastening of Particleboard

Tapping screws are commonly used in particleboard where 

withdrawal strength is important. Care must be taken when 

tightening screws in particleboard to avoid stripping the 

threads. The maximum amount of torque that can be applied to a screw before the threads in the particleboard are 

stripped is given by

(metric) (8–11a)

 (inch–pound) (8–11b)

where T is torque (N–m, in–lb) and X is density of the particleboard (kg m–3, lb ft–3). Equation (8–11) is for 8-gauge 

screws with a depth of penetration of 15.9 mm (5/8 in.). The 

maximum torque is fairly constant for lead holes of 0% to 

90% of the root diameter of the screw.

Ultimate withdrawal loads P (N, lb) of screws from particleboard can be predicted by

 (8–12)

where D is shank diameter of the screw (mm, in.), L depth 

of embedment of the threaded portion of the screw (mm, 

in.), and G specific gravity of the board based on ovendry 

weight and volume at current moisture content. For metric 

measurements, K = 41.1 for withdrawal from the face of the 

board and K = 31.8 for withdrawal from the edge; for inch–

pound measurements, K = 2,655 for withdrawal from the 

face and K = 2,055 for withdrawal from the edge. Equation 

(8–12) applies when the setting torque is between 60% to 

90% of T (Eq. ( 8–11)).

Withdrawal resistance of screws from particleboard is not 

significantly different for lead holes of 50% to 90% of the 

root diameter. A higher setting torque will produce a somewhat higher withdrawal load, but there is only a slight difference (3%) in values between 60% to 90% setting torques 

(Eq. (8–11)). A modest tightening of screws in many cases 

provides an effective compromise between optimizing withdrawal resistance and stripping threads.

Equation (8–12) can also predict the withdrawal of screws 

from fiberboard with K = 57.3 (metric) or 3,700 (inch–

pound) for the face and K = 44.3 (metric) or 2,860 (inch–

pound) for the edge of the board.

Lateral Resistance

Pre-1991

The proportional limit loads obtained in tests of lateral resistance for wood screws in the side grain of seasoned wood 

are given by the empirical equation

 p = KD2

 (8–13)

where p is lateral load, D diameter of the screw shank, and 

K a coefficient depending on the inherent characteristics of 

the wood species. Values of screw shank diameters for various screw gauges are listed in Table 8–9.

Values of K are based on ranges of specific gravity of hardwoods and softwoods and are given in Table 8–4. They 

apply to wood at about 15% moisture content. Loads computed by substituting these constants in the equation are 

expected to have a slip of 0.18 to 0.25 mm (0.007 to 

0.010 in.), depending somewhat on the species and density 

of the wood.

Equation (8–13) applies when the depth of penetration of 

the screw into the block receiving the point is not less 

than seven times the shank diameter and when the side 

8–11

Chapter 8 Fastenings

Table 8–8. Screw sizes

appropriate for 

Equation (8–10) 

Screw length 

(mm (in.)) 

Gauge

limits 

12.7 (1/2) 1 to 6 

19.0 (3/4) 2 to 11 

25.4 (1) 3 to 12 

38.1 (1-1/2) 5 to 14 

50.8 (2) 7 to 16 

63.5 (2-1/2) 9 to 18 

76.2 (3) 12 to 20 
member and the main member are approximately of the 

same density. The thickness of the side member should be 

about one-half the depth of penetration of the screw in the 

member holding the point. The end distance should be no 

less than the side member thickness, and the edge distances 

no less than one-half the side member thickness.

This depth of penetration (seven times shank diameter) 

gives an ultimate load of about four times the load obtained 

by the equation. For a depth of penetration of less than 

seven times the shank diameter, the ultimate load is reduced 

about in proportion to the reduction in penetration, and the 

load at the proportional limit is reduced somewhat less rapidly. When the depth of penetration of the screw in the holding block is four times the shank diameter, the maximum 

load will be less than three times the load expressed by the 

equation, and the proportional limit load will be approximately equal to that given by the equation. When the screw 

holds metal to wood, the load can be increased by about 

25%.

For these lateral loads, the part of the lead hole receiving the 

shank should be the same diameter as the shank or slightly 

smaller; that part receiving the threaded portion should be 

the same diameter as the root of the thread in dense species 

or slightly smaller than the root in low-density species.

Screws should always be turned in. They should never be 

started or driven with a hammer because this practice tears 

the wood fibers and injures the screw threads, seriously reducing the load carrying capacity of the screw.

Post-1991

Screw lateral strength is determined by the yield model 

theory (Table 8–5). Modes I, III, and IV failures may occur 

(Fig. 8–5). The dowel bearing strength values are based on 

the same specific gravity equation used to establish values 

for nails (Eq. (8–3)). Further discussion of screw lateral 

strength is found in ASCE Manual No. 84, Mechanical 

Connections in Wood Structures.

Lag Screws

Lag screws are commonly used because of their convenience, particularly where it would be difficult to fasten a 

bolt or where a nut on the surface would be objectionable. 

Commonly available lag screws range from about 5.1 to 

25.4 mm (0.2 to 1 in.) in diameter and from 25.4 to 406 mm 

(1 to 16 in.) in length. The length of the threaded part varies 

with the length of the screw and ranges from 19.0 mm 

(3/4 in.) with the 25.4- and 31.8-mm (1- and 1-1/4-in.) 

screws to half the length for all lengths greater than 254 mm 

(10 in.). Lag screws have a hexagonal-shaped head and are 

tightened by a wrench (as opposed to wood screws, which 

have a slotted head and are tightened by a screw driver). 

The following equations for withdrawal and lateral loads 

are based on lag screws having a base metal average tensile 

yield strength of about 310.3 MPa (45,000 lb in–2) and

 an average ultimate tensile strength of 530.9 MPa 

(77,000 lb in–2).

Withdrawal Resistance

The results of withdrawal tests have shown that the maximum direct withdrawal load of lag screws from the side 

grain of seasoned wood may be computed as

 p = 125.4G3 /2

D3 /4L (metric) (8–14a)

 

p = 8,100G3/ 2

D3/ 4L (inch–pound) (8–14b)

where p is maximum withdrawal load (N, lb), D shank diameter (mm, in.), G specific gravity of the wood based on 

ovendry weight and volume at 12% moisture content, and 

L length (mm, in.) of penetration of the threaded part. (The 

NDS use ovendry weight and volume as a basis.) Equation 

(8–14) was developed independently of Equation (8–10) but 

gives approximately the same results.

Lag screws, like wood screws, require prebored holes of the 

proper size (Fig. 8–8). The lead hole for the shank should 

be the same diameter as the shank. The diameter of the 

lead hole for the threaded part varies with the density of the 

wood: For low-density softwoods, such as the cedars and 

white pines, 40% to 70% of the shank diameter; for Douglas-fir and Southern Pine, 60% to 75%; and for dense hardwoods, such as oaks, 65% to 85%. The smaller percentage 

in each range applies to lag screws of the smaller diameters 

and the larger percentage to lag screws of larger diameters. 

Soap or similar lubricants should be used on the screw to 

facilitate turning, and lead holes slightly larger than those 

recommended for maximum efficiency should be used with 

long screws.

In determining the withdrawal resistance, the allowable 

tensile strength of the lag screw at the net (root) section 

should not be exceeded. Penetration of the threaded part 

to a distance about seven times the shank diameter in the 

8–12

General Technical Report FPL–GTR–190

Table 8–9. Screw shank 

diameters for various screw 

gauges

Screw number

or gauge 

Diameter 

(mm (in.)) 

4 2.84 (0.112) 

5 3.18 (0.125) 

6 3.51 (0.138) 

7 3.84 (0.151) 

8 4.17 (0.164) 

9 4.50 (0.177) 

10 4.83 (0.190) 

11 5.16 (0.203) 

12 5.49 (0.216) 

14 6.15 (0.242) 

16 6.81 (0.268) 

18 7.47 (0.294) 

20 8.13 (0.320) 

24 9.45 (0.372) 
denser species (specific gravity greater than 0.61) and 10 to 

12 times the shank diameter in the less dense species (specific gravity less than 0.42) will develop approximately the 

ultimate tensile strength of the lag screw. Penetrations at 

intermediate densities may be found by straight-line interpolation.

The resistance to withdrawal of a lag screw from the endgrain surface of a piece of wood is about three-fourths as 

great as its resistance to withdrawal from the side-grain surface of the same piece.

Lateral Resistance

Pre-1991

The experimentally determined lateral loads for lag screws 

inserted in the side grain and loaded parallel to the grain of a 

piece of seasoned wood can be computed as

 

p = KD2

 (8–15)

where p is proportional limit lateral load (N, lb) parallel to 

the grain, K a coefficient depending on the species specific 

gravity, and D shank diameter of the lag screw (mm, in.). 

Values of K for a number of specific gravity ranges can be 

found in Table 8–4. These coefficients are based on average 

results for several ranges of specific gravity for hardwoods 

and softwoods. The loads given by this equation apply when 

the thickness of the side member is 3.5 times the shank 

diameter of the lag screw, and the depth of penetration in 

the main member is seven times the diameter in the harder 

woods and 11 times the diameter in the softer woods. For 

other thicknesses, the computed loads should be multiplied 

by the factors listed in Table 8–10.

The thickness of a solid wood side member should be about 

one-half the depth of penetration in the main member.

When the lag screw is inserted in the side grain of wood and 

the load is applied perpendicular to the grain, the load given 

by the lateral resistance equation should be multiplied by the 

factors listed in Table 8–11.

For other angles of loading, the loads may be computed 

from the parallel and perpendicular values by using the 

Hankinson formula for determining the bearing strength of 

wood at various angles to the grain,

 

N = PQ

Psin 2 q + Q cos2 q (8–16)

where P is load or stress parallel to the grain, Q load or 

stress perpendicular to the grain, and N load or stress at an 

inclination q with the direction of the grain.

Values for lateral resistance as computed by the preceding methods are based on complete penetration of the unthreaded shank into the side member but not into the main 

member. When the shank penetrates the main member, the 

permitted increases in loads are given in Table 8–12.

8–13

Chapter 8 Fastenings

Figure 8–8. A, Clean-cut, deep penetration of thread 

made by lag screw turned into a lead hole of proper 

size, and B, rough, shallow penetration of thread made 

by lag screw turned into oversized lead hole.

Table 8–10. Multiplication 

factors for loads computed 

from Equation (8–15) 

Ratio of thickness of 

side member to shank

diameter of lag screw Factor 

2 0.62 

2.5 0.77 

3 0.93 

3.5 1.00 

4 1.07 

4.5 1.13 

5 1.18 

5.5 1.21 

6 1.22 

6.5 1.22 

Table 8–11. Multiplication 

factors for loads applied 

perpendicular to grain 

computed from 

Equation (8–15) with lag 

screw in side grain of 

wood 

Shank diameter 

of lag screw 

(mm (in.)) Factor 

4.8 (3/16) 1.00 

6.4 (1/4) 0.97 

7.9 (5/16) 0.85 

9.5 (3/8) 0.76 

11.1 (7/16) 0.70 

12.7 (1/2) 0.65 

15.9 (5/8) 0.60 

19.0 (3/4) 0.55 

22.2 (7/8) 0.52 

25.4 (1) 0.50 
When lag screws are used with metal plates, the lateral loads 

parallel to the grain may be increased 25%, provided the 

plate thickness is sufficient so that the bearing capacity of 

the steel is not exceeded. No increase should be made when 

the applied load is perpendicular to the grain.

Lag screws should not be used in end grain, because splitting may develop under lateral load. If lag screws are so 

used, however, the loads should be taken as two-thirds those 

for lateral resistance when lag screws are inserted into side 

grain and the loads act perpendicular to the grain.

The spacings, end and edge distances, and net section for 

lag screw joints should be the same as those for joints with 

bolts (discussed later) of a diameter equal to the shank diameter of the lag screw.

Lag screws should always be inserted by turning with a 

wrench, not by driving with a hammer. Soap, beeswax, or 

other lubricants applied to the screw, particularly with the 

denser wood species, will facilitate insertion and prevent 

damage to the threads but will not affect performance of the 

lag screw.

Post-1991

Lag screw lateral strength is determined by the yield model 

theory table similar to the procedure for bolts. Modes I, 

III, and IV yield may occur (Fig. 8–5). The dowel bearing 

strength values are based on the same parallel- and perpendicular-to-grain specific gravity equations used to establish 

values for bolts.

For other angles of loading, the dowel bearing strength values for use in the yield model are determined by the Hankinson equation, where P and Q are the values of dowel bearing parallel and perpendicular to grain, respectively.

Bolts

Bearing Stress of Wood under Bolts

The bearing stress under a bolt is computed by dividing the 

load on a bolt by the product LD, where L is the length of a 

bolt in the main member and D is the bolt diameter. Basic 

parallel-to-grain and perpendicular-to-grain bearing stresses 

have been obtained from tests of three-member wood joints 

where each side member is half the thickness of the main 

member. The side members were loaded parallel to grain for 

both parallel- and perpendicular-to-grain tests. Prior to 1991, 

bearing stress was based on test results at the proportional 

limit; since 1991, bearing stress is based on test results at a 

yield limit state, which is defined as the 5% diameter offset 

on the load–deformation curve (similar to Fig. 8–4).

The bearing stress at proportional limit load is largest when 

the bolt does not bend, that is, for joints with small L/D values. The curves of Figures 8–9 and 8–10 show the reduction 

in proportional limit bolt-bearing stress as L/D increases. 

The bearing stress at maximum load does not decrease as 

L/D increases, but remains fairly constant, which means 

that the ratio of maximum load to proportional limit load 

increases as L/D increases. To maintain a fairly constant 

ratio between maximum load and design load for bolts, the 

relations between bearing stress and L/D ratio have been 

adjusted as indicated in Figures 8–9 and 8–10.

The proportional limit bolt-bearing stress parallel to grain 

for small L/D ratios is approximately 50% of the small clear 

crushing strength for softwoods and approximately 60% for 

hardwoods. For bearing stress perpendicular to the grain, the 

ratio between bearing stress at proportional limit load and 

the small clear proportional limit stress in compression perpendicular to grain depends upon bolt diameter (Fig. 8–11) 

for small L/D ratios.

Species compressive strength also affects the L/D ratio relationship, as indicated in Figure 8–10. Relatively higher bolt 

proportional-limit stress perpendicular to grain is obtained 

with wood low in strength (proportional limit stress of 

3,930 kPa (570 lb in–2) than with material of high strength 

8–14

General Technical Report FPL–GTR–190

Table 8–12. Permitted increases in 

loads when lag screw unthreaded 

shank penetrates foundation 

member

Ratio of penetration of

shank into foundation 

member to shank 

diameter 

Increase 

in load 

(%)

1 8 

2 17 

3 26 

4 33 

5 36 

6 38 

7 39 

Figure 8–9. Variation in bolt-bearing stress at the 

proportional limit parallel to grain with L/D ratio. 

Curve A, relation obtained from experimental evaluation; curve B, modified relation used for establishing 

design loads.
(proportional limit stress of 7,860 kPa (1,140 lb in–2)). This 

effect also occurs for bolt-bearing stress parallel to grain, 

but not to the same extent as for perpendicular-to-grain 

loading.

The proportional limit bolt load for a three-member joint 

with side members half the thickness of the main member 

may be estimated by the following procedures.

For parallel-to-grain loading, (a) multiply the species small 

clear compressive parallel strength (Tables 5–3, 5–4, or 5–5) 

by 0.50 for softwoods or 0.60 for hardwoods, (b) multiply 

this product by the appropriate factor from Figure 8–9 for 

the L/D ratio of the bolt, and (c) multiply this product by

LD.

For perpendicular-to-grain loading, (a) multiply the species 

compression perpendicular-to-grain proportional limit stress 

(Tables 5–3, 5–4, or 5–5) by the appropriate factor from Figure 8–11, (b) multiply this product by the appropriate factor 

from Figure 8–10, and (c) multiply this product by LD.

Loads at an Angle to the Grain

For other angles of loading, the dowel bearing strength values for use in the yield model are determined by the Hankinson equation, where P and Q are the values of dowel bearing parallel and perpendicular to grain, respectively.

Steel Side Plates

When steel side plates are used, the bolt-bearing stress 

parallel to grain at joint proportional limit is approximately 

25% greater than that for wood side plates. The joint deformation at proportional limit is much smaller with steel 

side plates. If loads at equivalent joint deformation are compared, the load for joints with steel side plates is approximately 75% greater than that for wood side plates. Pre-1991 

design criteria included increases in connection strength 

with steel side plates; post-1991 design criteria include steel 

side plate behavior in the yield model equations.

For perpendicular-to-grain loading, the same loads are obtained for wood and steel side plates.

Bolt Quality

Both the properties of the wood and the quality of the bolt 

are factors in determining the strength of a bolted joint. The 

percentages given in Figures 8–9 and 8–10 for calculating bearing stress apply to steel machine bolts with a yield 

stress of 310 MPa (45,000 lb in–2). Figure 8–12 indicates the 

8–15

Chapter 8 Fastenings

Figure 8–10. Variation in bolt-bearing stress at the 

proportional limit perpendicular to grain with L/D ratio. Relations obtained from experimental evaluation 

for materials with average compression perpendicular stress of 7,860 kPa (1,140 lb in–2) (curve A–1) and 

3,930 kPa (570 lb in–2) (curve A–2). Curves B–1 and 

B–2, modified relations used for establishing design 

loads.

Figure 8–11. Bearing stress perpendicular to the 

grain as affected by bolt diameter.

Figure 8–12. Variation in the proportional limit boltbearing stress parallel to grain with L/D ratio. Curve 

A, bolts with yield stress of 861.84 MPa (125,000 lb 

in–2); curve B, bolts with yield stress of 310.26 MPa 

(45,000 lb in–2).
increase in bearing stress parallel to grain for bolts with a 

yield stress of 862 MPa (125,000 lb in–2).

Effect of Member Thickness

The proportional limit load is affected by the ratio of the 

side member thickness to the main member thickness 

(Fig. 8–13).

Pre-1991 design values for bolts are based on joints with 

the side member half the thickness of the main member. 

The usual practice in design of bolted joints is to take no 

increase in design load when the side members are greater 

than half the thickness of the main member. When the side 

members are less than half the thickness of the main member, a design load for a main member that is twice the thickness of the side member is used. Post-1991 design values 

include member thickness directly in the yield model 

equations.

Two-Member, Multiple-Member Joints

In pre-1991 design, the proportional limit load was taken as 

half the load for a three-member joint with a main member 

the same thickness as the thinnest member for two-member 

joints.

For four or more members in a joint, the proportional limit 

load was taken as the sum of the loads for the individual 

shear planes by treating each shear plane as an equivalent 

two-member joint.

Post-1991 design for joints with four or more members also 

results in values per shear plane. Connection strength for 

any number of members is conservatively found by multiplying the value for the weakest shear plane by the number 

of shear planes.

Spacing, Edge, and End Distance

The center-to-center distance along the grain should be 

at least four times the bolt diameter for parallel-to-grain 

loading. The minimum center-to-center spacing of bolts in 

the across-the-grain direction for loads acting through metal 

side plates and parallel to the grain need only be sufficient 

to permit the tightening of the nuts. For wood side plates, 

the spacing is controlled by the rules applying to loads acting parallel to grain if the design load approaches the boltbearing capacity of the side plates. When the design load 

is less than the bolt-bearing capacity of the side plates, the 

spacing may be reduced below that required to develop their 

maximum capacity.

When a joint is in tension, the bolt nearest the end of a timber should be at a distance from the end of at least seven 

times the bolt diameter for softwoods and five times for 

hardwoods. When the joint is in compression, the end margin may be four times the bolt diameter for both softwoods 

and hardwoods. Any decrease in these spacings and margins 

will decrease the load in about the same ratio.

For bolts bearing parallel to the grain, the distance from the 

edge of a timber to the center of a bolt should be at least 

1.5 times the bolt diameter. This margin, however, will usually be controlled by (a) the common practice of having an 

edge margin equal to one-half the distance between bolt 

rows and (b) the area requirements at the critical section. 

(The critical section is that section of the member taken at 

right angles to the direction of load, which gives the maximum stress in the member based on the net area remaining 

after reductions are made for bolt holes at that section.) For 

parallel-to-grain loading in softwoods, the net area remaining at the critical section should be at least 80% of the total 

area in bearing under all the bolts in the particular joint under consideration; in hardwoods it should be 100%.

For bolts bearing perpendicular to the grain, the margin between the edge toward which the bolt pressure is acting and 

the center of the bolt or bolts nearest this edge should be at 

least four times the bolt diameter. The margin at the opposite 

edge is relatively unimportant.

The aforementioned prescriptive spacing recommendations 

are based on experimental information and have been found 

to be sufficient for a majority of designed connections. 

There is still a need to validate the design spacing using a 

mechanics-based method that considers wood strength. The 

prescriptive spacing requirement may not agree with the 

strength-based method, and the large spacing requirement 

should be used. One method for the design of fastener 

joints loaded in tension parallel to grain is highlighted in 

appendix E of the National Design Specification for Wood 

Construction.

Effect of Bolt Holes

The bearing strength of wood under bolts is affected considerably by the size and type of bolt holes into which the bolts 

are inserted. A bolt hole that is too large causes nonuniform 

bearing of the bolt; if the bolt hole is too small, the wood 

will split when the bolt is driven. Normally, bolts should 

fit so that they can be inserted by tapping lightly with a 

8–16

General Technical Report FPL–GTR–190

Figure 8–13. Proportional limit load related to side 

member thickness for three-member joints. Center 

member thickness was 50.8 mm (2 in.).
wood mallet. In general, the smoother the hole, the higher 

the bearing values will be (Fig. 8–14). Deformations accompanying the load are also less with a smoother bolt-hole 

surface (Fig. 8–15).

Rough holes are caused by using dull bits and improper 

rates of feed and drill speed. A twist drill operated at a peripheral speed of approximately 38 m min–1 (1,500 in min–1) 

produces uniformly smooth holes at moderate feed rates. 

The rate of feed depends upon the diameter of the drill and 

the speed of rotation but should enable the drill to cut, rather 

than tear, the wood. The drill should produce shavings, not 

chips.

Proportional limit loads for joints with bolt holes the same 

diameter as the bolt will be slightly higher than for joints 

with a 1.6-mm (1/16-in.) oversized hole. However, if drying 

takes place after assembly of the joint, the proportional limit 

load for snug-fitting bolts will be considerably less due to 

the effects of shrinkage.

Pre-1991 Allowable Loads

The following procedures are used to calculate allowable 

bolt loads for joints with wood side members, each half the 

thickness of the main member.

Parallel to Grain—The starting point for parallel-to-grain 

bolt values is the maximum green crushing strength for the 

species or group of species. Procedures outlined in ASTM 

D 2555 are used to establish a 5% exclusion value. The 

exclusion value is divided by a factor of 1.9 to adjust to 

a 10-year normal duration of load and provide a factor of 

safety. This value is multiplied by 1.20 to adjust to a seasoned strength. The resulting value is called the basic boltbearing stress parallel to grain.

The basic bolt-bearing stress is then adjusted for the effects 

of L/D ratio. Table 8–13 gives the percentage of basic stress 

for three classes of species. The particular class for the species is determined from the basic bolt-bearing stress as indicated in Table 8–14. The adjusted bearing stress is further 

multiplied by a factor of 0.80 to adjust to wood side plates. 

The allowable bolt load in pounds is then determined by 

multiplying by the projected bolt area, LD.

Perpendicular to Grain—The starting point for perpendicular-to-grain bolt values is the average green proportional 

limit stress in compression perpendicular to grain. Procedures in ASTM D 2555 are used to establish compression 

perpendicular values for groups of species. The average 

proportional limit stress is divided by 1.5 for ring position 

(growth rings neither parallel nor perpendicular to load during test) and a factor of safety. This value is then multiplied 

by 1.20 to adjust to a seasoned strength and by 1.10 to 

adjust to a normal duration of load. The resulting value is 

called the basic bolt-bearing stress perpendicular to grain.

The basic bolt-bearing stress is then adjusted for the effects 

of bolt diameter (Table 8–15) and L/D ratio (Table 8–13). 

The allowable bolt load is then determined by multiplying 

the adjusted basic bolt-bearing stress by the projected bolt 

area, LD.

Post-1991 Yield Model

The empirical design approach used prior to 1991 was based 

on a tabular value for a single bolt in a wood-to-wood, 

three-member connection where the side members are each 

8–17

Chapter 8 Fastenings

Figure 8–14. Effect of rate of feed and drill speed on 

the surface condition of bolt holes drilled in Sitka 

spruce. A, hole was bored with a twist drill rotating 

at a peripheral speed of 7.62 m min–1 (300 in min–1); 

feed rate was 1.52 m/min (60 in min–1). B, hole was 

bored with the same drill at a peripheral speed of 

31.75 m min–1 (1,250 in min–1); feed rate was 50.8 mm 

min–1 (2 in min–1).

Figure 8–15. Typical load–deformation curves showing the effect of surface condition of bolt holes, resulting from a slow feed rate and a fast feed rate, on 

the deformation in a joint when subjected to loading 

under bolts. The surface conditions of the bolt holes 

were similar to those illustrated in Figure 8–14.
a minimum of one-half the thickness of the main member. 

The single-bolt value must then be modified for any variation from these reference conditions. The theoretical approach, after 1991, is more general and is not limited to 

these reference conditions.

The theoretical approach is based on work done in Europe 

(Johansen 1949) and is referred to as the European Yield 

Model (EYM). The EYM describes a number of possible 

yield modes that can occur in a dowel-type connection 

(Fig. 8–5). The yield strength of these different modes is 

determined from a static analysis that assumes the wood 

and the bolt are both perfectly plastic. The yield mode that 

results in the lowest yield load for a given geometry is the 

theoretical connection yield load.

Equations corresponding to the yield modes for a threemember joint are given in Table 8–16. (Equations for 

two-member allowable values are given in the NDS.) The 

nominal single-bolt value is dependent on the joint geometry (thickness of main and side members), bolt diameter 

and bending yield strength, dowel bearing strength, and direction of load to the grain. The equations are equally valid 

8–18

General Technical Report FPL–GTR–190

Table 8–13. Percentage of basic bolt-bearing stress used for calculating 

allowable bolt loads

Ratio of bolt 

length to 

diameter 

(L/D)

L/D adjustment factor by classa

Parallel 

to grain 

Perpendicular 

to grain 

1 2 3 1 2 3 4 

1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 

2 100.0 100.0 100.0 100.0 100.0 100.0 100.0 

3 100.0 100.0 99.0 100.0 100.0 100.0 100.0 

4 99.5 97.4 92.5 100.0 100.0 100.0 100.0 

5 95.4 88.3 80.0 100.0 100.0 100.0 100.0 

6 85.6 75.8 67.2 100.0 100.0 100.0 96.3 

7 73.4 65.0 57.6 100.0 100.0 97.3 86.9 

8 64.2 56.9 50.4 100.0 96.1 88.1 75.0 

9 57.1 50.6 44.8 94.6 86.3 76.7 64.6 

10 51.4 45.5 40.3 85.0 76.2 67.2 55.4 

11 46.7 41.4 36.6 76.1 67.6 59.3 48.4 

12 42.8 37.9 33.6 68.6 61.0 52.0 42.5 

13 39.5 35.0 31.0 62.2 55.3 45.9 37.5 a

Class determined from basic bolt-bearing stress according to Table 8–14. 

Table 8–14. L/D adjustment class associated with basic bolt-bearing stress

Basic bolt-bearing stress for species group (MPa (lb in–2)) 

Loading direction Softwoods Hardwoods 

L/D adjustment 

(Table 8–13) 

Parallel <7.93 (<1,150) <7.33 (<1,063) 1 

7.93–10.37 (1,150–1,504) 7.33–9.58 (1,063–1,389) 2 

>10.37 (>1,504) >9.58 (>1,389) 3 

Perpendicular <1.31 (<190) <1.44 (<209) 1 

1.31–2.00 (190–290) 1.44–2.20 (209–319) 2 

2.00–2.59 (291–375) 2.21–2.84 (320–412) 3 

>2.59 (>375) >2.84 (>412) 4 

Table 8–15. Factors for adjusting 

basic bolt-bearing stress 

perpendicular to grain for bolt 

diameter when calculating 

allowable bolt loads 

Bolt diameter 

(mm (in.)) 

Adjustment

factor 

6.35 (1/4) 2.50 

9.53 (3/8) 1.95 

12.70 (1/2) 1.68 

15.88 (5/8) 1.52 

19.05 (3/4) 1.41 

22.23 (7/8) 1.33 

25.40 (1) 1.27 

31.75 (1-1/4) 1.19 

38.10 (1-1/2) 1.14 

44.45 (1-3/4) 1.10 

50.80 (2) 1.07 

63.50 (2-1/2) 1.03 

>76.20 (>3 or over) 1.00 
for wood or steel side members, which is taken into account 

by thickness and dowel bearing strength parameters. The 

equations are also valid for various load-to-grain directions, 

which are taken into account by the Kq and Fe parameter.

The dowel bearing strength is a material property not generally familiar to structural designers. The dowel bearing 

strength of the wood members is determined from tests that 

relate species specific gravity and dowel diameter to bearing 

strength. Empirical equations for these relationships are as 

follows:

Parallel to grain

 (metric) (8–17a)

 (inch–pound) (8–17b)

Perpendicular to grain

 (metric) (8–18a)

 (inch–pound) (8–18b)

where Fe is dowel bearing strength (MPa, lb in–2), G specific 

gravity based on ovendry weight and volume, and D bolt 

diameter (mm, in.).

For other angles of loading, the dowel bearing strength 

values for use in the yield model are determined by the 

Hankinson equation, where P and Q are the values of dowel 

bearing parallel and perpendicular to grain, respectively.

Connector Joints

Several types of connectors have been devised that increase 

joint bearing and shear areas by utilizing rings or plates 

around bolts holding joint members together. The primary 

load-carrying portions of these joints are the connectors; the 

bolts usually serve to prevent transverse separation of the 

members but do contribute some load-carrying capacity.

The strength of the connector joint depends on the type and 

size of the connector, the species of wood, the thickness and 

width of the member, the distance of the connector from the 

end of the member, the spacing of the connectors, the direction of application of the load with respect to the direction 

of the grain of the wood, and other factors. Loads for wood 

joints with steel connectors—split ring (Fig. 8–16) and shear 

plate (Fig. 8–17)—are discussed in this section. These connectors require closely fitting machined grooves in the wood 

members.

Parallel-to-Grain Loading

Tests have demonstrated that the density of the wood is a 

controlling factor in the strength of connector joints. For 

split-ring connectors, both maximum load and proportional 

limit load parallel to grain vary linearly with specific gravity (Figs. 8–18 and 8–19). For shear plates, the maximum 

load and proportional limit load vary linearly with specific 

gravity for the less dense species (Figs. 8–20 and 8–21). In 

the higher density species, the shear strength of the bolts 

becomes the controlling factor. These relations were 

8–19

Chapter 8 Fastenings

Table 8–16. The 5% offset yield lateral strength (Z)

for three-member bolted joints

Mode

Z value for threemember bolted joint 

Mode Im K

DtmFem

Mode Is K

DtsFes 2

Mode IIIs  R )K

k Dt F

e

4 s em

(2

2



Mode IV 3(1 )

2 2

e

em yb 2

R

F F

K

D

θ 

Definitions

D nominal bolt diameter, mm (in.) 

Fem dowel bearing strength of main (center) 

 member, MPa (lb in–2)

Fes dowel bearing strength of side members, 

 MPa (lb in–2)

Fyb bending yield strength of bolt, MPa (lb in–2)

K 1 + /360 

tm thickness of main (center) member, mm (in.) 

ts thickness of side member, mm (in.) 

Z nominal single bolt design value 

 angle of load to grain (degrees) 

Re = Fem/Fes

2 em s

2 yb e

e

e 4 3

2(1 ) 2 (2 ) 1

F t

F R D

R

R k      

Figure 8–16. Joint with split-ring connector showing 

connector, precut groove, bolt, washer, and nut.
obtained for seasoned members, approximately 12% moisture content.

Perpendicular-to-Grain Loading

Loads for perpendicular-to-grain loading have been established using three-member joints with the side members 

loaded parallel to grain. Specific gravity is a good indicator of perpendicular-to-grain strength of timber connector 

joints. For split-ring connectors, the proportional limit loads 

perpendicular to grain are 58% of the parallel-to-grain proportional limit loads. The joint deformation at proportional 

limit is 30% to 50% more than for parallel-to-grain loading.

For shear-plate connectors, the proportional limit and maximum loads vary linearly with specific gravity (Figs. 8–22 

and 8–23). The wood strength controls the joint strength for 

all species.

Design Loads

Design loads for parallel-to-grain loading have been established by dividing ultimate test loads by an average factor 

of 4. This gives values that do not exceed five-eighths of the 

proportional limit loads. The reduction accounts for variability in material, a reduction to long-time loading, and a 

factor of safety. Design loads for normal duration of load are 

10% higher.

For perpendicular-to-grain loading, ultimate load is given 

less consideration and greater dependence placed on load at 

proportional limit. For split rings, the proportional limit load 

is reduced by approximately half. For shear plates, the design loads are approximately five-eighths of the proportional 

limit test loads. These reductions again account for material 

variability, a reduction to long-time loading, and a factor of 

safety.

Design loads are presented in Figures 8–18 to 8–23. In practice, four wood species groups have been established, based 

8–20

General Technical Report FPL–GTR–190

Figure 8–19. Relation between load bearing parallel 

to grain and specific gravity (ovendry weight, volume at test) for two 101.6-mm (4-in.) split rings and a 

single 19.1-mm- (3/4-in.-) diameter bolt in air-dry material. Center member thickness was 127.0 mm (5 in.) 

and side member thickness was 63.5 mm (2-1/2 in.).

Figure 8–17. 

Joints with 

shear-plate 

connectors 

with (A) wood 

side plates and 

(B) steel side 

plates.

Figure 8–18. Relation between load bearing parallel 

to grain and specific gravity (ovendry weight, volume 

at test) for two 63.5-mm (2-1/2-in.) split rings with a 

single 12.7-mm (1/2-in.) bolt in air-dry material. Center member was thickness 101.6 mm (4 in.) and side 

member thickness was 50.8 mm (2 in.).

Figure 8–20. Relation between load bearing parallel to 

grain and specific gravity (ovendry weight, volume at 

test) for two 66.7-mm (2-5/8-in.) shear plates in air-dry 

material with steel side plates. Center member thickness was 76.2 mm (3 in.).
primarily on specific gravity, and design loads assigned 

for each group. Species groupings for connectors are presented in Table 8–17. The corresponding design loads (for 

long-continued load) are given in Table 8–18. The National 

Design Specification for Wood Construction gives design 

values for normal-duration load for these and additional 

species.

Modifications

Some factors that affect the loads of connectors were taken 

into account in deriving the tabular values. Other varied and 

extreme conditions require modification of the values.

Steel Side Plates

Steel side plates are often used with shear-plate connectors. 

The loads parallel to grain have been found to be approximately 10% higher than those with wood side plates. The 

perpendicular-to-grain loads are unchanged.

Exposure and Moisture Condition of Wood

The loads listed in Table 8–18 apply to seasoned members 

used where they will remain dry. If the wood will be more 

or less continuously damp or wet in use, two-thirds of the 

tabulated values should be used. The amount by which 

the loads should be reduced to adapt them to other conditions of use depends upon the extent to which the exposure 

favors decay, the required life of the structure or part, the 

frequency and thoroughness of inspection, the original cost 

and the cost of replacements, the proportion of sapwood and 

durability of the heartwood of the species (if untreated), and 

the character and efficacy of any treatment. These factors 

should be evaluated for each individual design. Industry recommendations for the use of connectors when the condition 

of the lumber is other than continuously wet or continuously 

dry are given in the National Design Specification for Wood 

Construction.

8–21

Chapter 8 Fastenings

Figure 8–21. Relation between load bearing parallel 

to grain and specific gravity (ovendry weight, volume 

at test) for two 101.6-mm (4-in.) shear plates in airdry material with steel side plates. Center member 

thickness was 88.9 mm (3-1/2 in.).

Figure 8–22. Relation between load bearing perpendicular to grain and specific gravity (ovendry weight, 

volume at test) for two 66.7-mm (2-5/8-in.) shear 

plates in air-dry material with steel side plates. Center member thickness was 76.2 mm (3 in.).

Figure 8–23. Relation between load bearing perpendicular to grain and specific gravity (ovendry weight, 

volume at test) for two 101.6-mm (4-in.) shear plates 

in air-dry material with steel side plates. Center 

member thickness was 88.9 mm (3-1/2 in.).

Table 8–17. Species groupings for connector loadsa

Connector Species or species group 

Group 1 Aspen Basswood Cottonwood 

 Western 

redcedar 

Balsam fir White fir 

 Eastern hemlock Eastern white pine Ponderosa pine 

 Sugar pine Western white pine Engelmann 

spruce 

Group 2 Chestnut Yellow-poplar Baldcypress 

 Yellow-cedar Port-Orford-cedar Western 

hemlock 

 Red pine Redwood Red spruce 

 Sitka spruce White spruce 

Group 3 Elm, American Elm, slippery Maple, soft 

 Sweetgum Sycamore Tupelo 

 Douglas-fir Larch, western Southern Pine 

Group 4 Ash, white Beech Birch 

 Elm, rock Hickory Maple, hard 

 Oak 

a

Group 1 woods provide the weakest connector joints; group 4 woods, the 

strongest. 
Ordinarily, before fabrication of connector joints, members 

should be seasoned to a moisture content corresponding as 

nearly as practical to that which they will attain in service. 

This is particularly desirable for lumber for roof trusses and 

other structural units used in dry locations and in which 

shrinkage is an important factor. Urgent construction needs 

sometimes result in the erection of structures and structural 

units employing green or inadequately seasoned lumber 

with connectors. Because such lumber subsequently dries 

out in most buildings, causing shrinkage and opening the 

joints, adequate maintenance measures must be adopted. 

The maintenance for connector joints in green lumber 

should include inspection of the structural units and tightening of all bolts as needed during the time the units are coming to moisture equilibrium, which is normally during the 

first year.

Grade and Quality of Lumber

The lumber for which the loads for connectors are applicable should conform to the general requirements in regard to 

quality of structural lumber given in the grading rule books 

of lumber manufacturers’ associations for various commercial species.

The loads for connectors were obtained from tests of joints 

whose members were clear and free from checks, shakes, 

8–22

General Technical Report FPL–GTR–190

and splits. Cross grain at the joint should not be steeper than 

1 in 10, and knots in the connector area should be accounted 

for as explained under Net Section.

Loads at Angle with Grain

The loads for the split-ring and shear-plate connectors for 

angles of 0° to 90° between direction of load and grain may 

be obtained by the Hankinson equation (Eq. (8–16)).

Thickness of Member

The relationship between loads for different thicknesses of 

lumber is based on test results for connector joints. The least 

thickness of member given in Table 8–18 for the various 

sizes of connectors is the minimum to obtain optimum load. 

The loads listed for each type and size of connector are the 

maximum loads to be used for all thicker lumber. The loads 

for wood members of thicknesses less than those listed can 

be obtained by the percentage reductions indicated in Figure 8–24. Thicknesses below those indicated by the curves 

should not be used.

When one member contains a connector in only one face, 

loads for thicknesses less than those listed in Table 8–18 can 

be obtained by the percentage reductions indicated in Figure 

8–24 using an assumed thickness equal to twice the actual 

member thickness.

Table 8–18. Design loads for one connector in a jointa

Load (N (lb)) 

Minimum thickness of wood 

member (mm (in.)) Group 1 woods Group 2 woods Group 3 woods Group 4 woods 

Connector 

With one 

connector 

only 

With two 

connectors in 

opposite faces,

one boltb

Minimum

width all 

members

(mm (in.)) 

At 0° 

angle

to

grain 

At 90°

angle

to

grain 

At 0° 

angle

to

grain 

At 90°

angle

to

grain 

At 0° 

angle

to

grain 

At 90° 

angle

to

grain 

At 0°

angle

to

grain 

At 90°

angle

to

grain 

Split ring 

63.5-mm (2-1/2-in.) 

diameter, 19.0 mm 

(3/4 in.) wide, with 

12.7-mm (1/2-in.) 

bolt 

25 (1) 51 (2) 89 (3-1/2) 7,940

(1,785) 

4,693 

(1,055) 

9,274

(2,085) 

5,471

(1,230)

11,032 

(2,480) 

6,561 

(1,475) 

12,789

(2,875)

7,673

(1,725)

101.6-mm (4-in.) 

diameter, 25.4 mm 

(1 in.) wide, with 

19.0-mm (3/4-in.) 

bolt 

38 (1-1/2) 76 (3) 140 (5-1/2) 15,324

(3,445) 

8,874 

(1,995) 

17,726

(3,985) 

10,275

(2,310)

21,262 

(4,780) 

12,344 

(2,775) 

24,821

(5,580)

14,390

(3,235)

Shear plate 

66.7-mm (2-5/8-in.) 

diameter, 10.7 mm 

(0.42 in.) wide, 

with 19.0-mm 

(3/4-in.) bolt 

38 (1-1/2) 67 (2-5/8) 89 (3-1/2) 8,407

(1,890) 

4,871 

(1,095) 

9,742

(2,190) 

5,649

(1,270)

11,699 

(2,630) 

6,784 

(1,525) 

11,854

(2,665)

7,918

(1,780)

101.6-mm (4-in.) 

diameter, 16.2 mm 

(0.64 in.) wide, with 

19.0-mm or 22.2-mm 

(3/4- or 7/8-in.) bolt 

44 (1-3/4) 92 (3-518) 140 (5-1/2) 12,677

(2,850) 

7,362 

(1,655) 

14,701

(3,305) 

8,518

(1,915)

17,637 

(3,965) 

10,231 

(2,300) 

20,573

(4,625)

11,943

(2,685)

a

The loads apply to seasoned timbers in dry, inside locations for a long-continued load. It is also assumed that the joints are properly designed with 

respect to such features as centering of connectors, adequate end distance, and suitable spacing. Group 1 woods provide the weakest connector joints, 

group 4 woods the strongest. Species groupings are given in Table 8–17. 

b

A three-member assembly with two connectors takes double the loads indicated. 
8–23

Chapter 8 Fastenings

Width of Member

The width of member listed for each type and size of connector is the minimum that should be used. When the connectors are bearing parallel to the grain, no increase in load 

occurs with an increase in width. When they are bearing 

perpendicular to the grain, the load increases about 10% for 

each 25-mm (1-in.) increase in width of member over the 

minimum widths required for each type and size of connector, up to twice the diameter of the connectors. When 

the connector is placed off center and the load is applied 

continuously in one direction only, the proper load can be 

determined by considering the width of member as equal to 

twice the edge distance (the distance between the center of 

the connector and the edge of the member toward which the 

load is acting). The distance between the center of the connector and the opposite edge should not, however, be less 

than half the permissible minimum width of the member.

Net Section

The net section is the area remaining at the critical section 

after subtracting the projected area of the connectors and 

bolt from the full cross-sectional area of the member. For 

sawn timbers, the stress in the net area (whether in tension 

or compression) should not exceed the stress for clear wood 

in compression parallel to the grain. In using this stress, it is 

assumed that knots do not occur within a length of half the 

diameter of the connector from the net section. If knots are 

present in the longitudinal projection of the net section within a length from the critical section of one-half the diameter 

of the connector, the area of the knots should be subtracted 

from the area of the critical section.

In laminated timbers, knots may occur in the inner laminations at the connector location without being apparent from 

the outside of the member. It is impractical to ensure that 

there are no knots at or near the connector. In laminated 

construction, therefore, the stress at the net section is limited 

to the compressive stress for the member, accounting for the 

effect of knots.

End Distance and Spacing

The load values in Table 8–18 apply when the distance of 

the connector from the end of the member (end distance e) 

and the spacing s between connectors in multiple joints are 

not factors affecting the strength of the joint (Fig. 8–25A). 

When the end distance or spacing for connectors bearing 

parallel to the grain is less than that required to develop the 

full load, the proper reduced load may be obtained by multiplying the loads in Table 8–18 by the appropriate strength 

ratio given in Table 8–19. For example, the load for a 102-

mm (4-in.) split-ring connector bearing parallel to the grain, 

when placed 178 mm or more (7 in. or more) from the end 

of a Douglas-fir tension member that is 38 mm (1-1/2 in.) 

thick is 21.3 kN (4,780 lb). When the end distance is only 

133 mm (5-1/4 in.), the strength ratio obtained by direct interpolation between 178 and 89 mm (7 and 3-1/2 in.) in Table 8–19 is 0.81, and the load equals 0.81 times 21.3 (4,780) 

or 17.2 kN (3,870 lb).

Placement of Multiple Connectors

Preliminary investigations of the placement of connectors 

in a multiple-connector joint, together with the observed 

behavior of single-connector joints tested with variables that 

simulate those in a multiple-connector joint, are the basis for 

some suggested design practices.

Figure 8–24. Effect of thickness of wood member on the optimum load capacity 

of a timber connector.
8–24

General Technical Report FPL–GTR–190

When two or more connectors in the same face of a member are in a line at right angles to the grain of the member 

and are bearing parallel to the grain (Fig. 8–25C), the clear 

distance c between the connectors should not be less than 

12.7 mm (1/2 in.). When two or more connectors are acting 

perpendicular to the grain and are spaced on a line at right 

angles to the length of the member (Fig. 8–25B), the rules 

for the width of member and edge distances used with one 

connector are applicable to the edge distances for multiple 

connectors. The clear distance c between the connectors 

should be equal to the clear distance from the edge of the 

member toward which the load is acting to the connector 

nearest this edge.

In a joint with two or more connectors spaced on a line parallel to the grain and with the load acting perpendicular to 

the grain (Fig. 8–25D), the available data indicate that the 

load for multiple connectors is not equal to the sum of the 

loads for individual connectors. Somewhat more favorable 

results can be obtained if the connectors are staggered so 

that they do not act along the same line with respect to the 

grain of the transverse member. Industry recommendations 

for various angle-to-grain loadings and spacings are given in 

the National Design Specification for Wood Construction.

Cross Bolts

Cross bolts or stitch bolts placed at or near the end of members joined with connectors or at points between connectors 

will provide additional safety. They may also be used to 

reinforce members that have, through change in moisture 

content in service, developed splits to an undesirable degree.

Multiple-Fastener Joints

When fasteners are used in rows parallel to the direction of 

loading, total joint load is unequally distributed among fasteners in the row. Simplified methods of analysis have been 

developed to predict the load distribution among the fasteners in a row below the proportional limit. These analyses 

indicate that the elastic load distribution is a function of 

(a) the extensional stiffness EA of the joint members, where 

E is modulus of elasticity and A is gross cross-sectional 

area, (b) the fastener spacing, (c) the number of fasteners, 

and (d) the single-fastener load–deformation characteristics.

Theoretically, the two end fasteners carry a majority of the 

load. For example, in a row of six bolts, the two end bolts 

will carry more than 50% of the total joint load. Adding 

bolts to a row tends to reduce the load on the less heavily 

loaded interior bolts. The most even distribution of bolt 

loads occurs in a joint where the extensional stiffness of the 

main member is equal to that of both splice plates. Increasing the fastener spacing tends to put more of the joint load 

on the end fasteners. Load distribution tends to be worse for 

stiffer fasteners.

The actual load distribution in field-fabricated joints is difficult to predict. Small misalignment of fasteners, variations 

in spacing between side and main members, and variations 

in single-fastener load–deformation characteristics can 

cause the load distribution to be different than predicted by 

the theoretical analyses.

For design purposes, modification factors for application to 

a row of bolts, lag screws, or timber connectors have been 

developed based on the theoretical analyses. Tables are 

given in the National Design Specification for Wood Construction.

A design equation was developed to replace the double entry 

required in the National Design Specification for Wood Construction tables. This equation was obtained by algebraic 

simplification of the Lantos analysis that these tables are 

based on:

 

 (8–19)

where Cg is modification factor, n number of fasteners in 

a row, REA the lesser of (EsAs)/(EmAm) or (EmAm)/(EsAs),

Em modulus of elasticity of main member, Es modulus of 

elasticity of side members, Am gross cross-sectional area of 

main member, As sum of gross cross-sectional areas of side 

Figure 8–25. Types of multiple-connector joints: 

A, joint strength depends on end distance e and 

connector spacing s; B, joint strength depends on 

e, clear c, and edge a distances; C, joint strength 

depends on end e and clear c distances; D, joint 

strength depends on end e, clear c, and edge a

distances.
8–25

Chapter 8 Fastenings

members, m = 1 2 u − u − , u = 1 + g(s/2)(1/EmAm + 

1/EsAs), s center-to-center spacing between adjacent fasteners in a row, and g load/slip modulus for a single fastener 

connection. For 102-mm (4-in.) split-ring or shear-plate 

connectors,

g = 87,560 kN m–1 (500,000 lb in–1)

For 64-mm (2-1/2-in.) split ring or 67-mm (2-5/8-in.) split 

ring or shear plate connectors,

g = 70,050 kN m –1 (400,000 lb in–1)

For bolts or lag screws in wood-to-wood connections,

g = 246.25 D1.5 (metric) 

 = 180,000 D1.5 (inch–pound)

For bolts or lag screws in wood-to-metal connections,

g = 369.37 D1.5 (metric) 

= 270,000 D1.5 (inch–pound)

where D is diameter of bolt or lag screw.

Metal Plate Connectors

Metal plate connectors, commonly called truss plates, have 

become a popular means of joining, especially in trussed 

rafters and joists. These connectors transmit loads by means 

of teeth, plugs, or nails, which vary from manufacturer 

to manufacturer. Examples of such plates are shown in 

Figure 8–26. Plates are usually made of light-gauge galvanized steel and have an area and shape necessary to transmit 

the forces on the joint. Installation of plates usually requires 

a hydraulic press or other heavy equipment, although some 

plates can be installed by hand.

Basic strength values for plate connectors are determined 

from load–slip curves from tension tests of two butted wood 

members joined with two plates. Some typical curves are 

shown in Figure 8–27. Design values are expressed as load 

per tooth, nail, plug, or unit area of plate. The smallest value 

as determined by two different means is the design load 

for normal duration of load: (1) the average load of at least 

five specimens at 0.38-mm (0.015-in.) slip from plate to 

wood member or 0.76-mm (0.030-in.) slip from member to 

member is divided by 1.6; (2) the average ultimate load of at 

least five specimens is divided by 3.0.

The strength of a metal plate joint may also be controlled by 

the tensile or shear strength of the plate.

Joist Hangers

Joist hangers have become a popular means of joining 

wood-based joists to header beams or columns. Hangers are 

usually made of light-gauge steel or welded from plate steel 

with shape and configuration necessary to transmit forces 

through the joint. Loads are transmitted from the joist to 

the hanger primarily through direct bearing of the joist, but 

for the uplift forces, load transfer is due to lateral loading 

of fasteners. How loads are transferred from the hanger to 

Table 8–19. Strength ratio for connectors for various longitudinal spacings and end 

distancesa

Connector 

diameter 

(mm (in.)) 

Spacing 

strength ratio

End distanceb

 (mm (in.)) 

Spacingc

(mm (in.)) 

Tension 

member 

Compression 

member 

End distance

strength ratio

Split-ring 

63.5 (2-1/2) 171.4+ (6-3/4+) 100 139.7+ (5-1/2+) 101.6+ (4+) 100 

63.5 (2-1/2) 85.7 (3-3/8) 50 69.8 (2-3/4) 63.5 (2-1/2) 62 

101.6 (4) 228.6+ (9+) 100 177.8+ (7+) 139.7+ (5-1/2+) 100 

101.6 (4) 123.8 (4-7/8) 50 88.9 (3-1/2) 82.6 (3-1/4) 62 

Shear-plate 

66.7 (2-5/8) 171.4+ (6-3/4+) 100 139.7+ (5-1/2+) 101.6+ (4+) 100 

66.7 (2-5/8) 85.7 (3-3/8) 50 69.8 (2-3/4) 63.5 (2-1/2) 62 

101.6 (4) 228.6+ (9+) 100 177.8+ (7+) 139.7+ (5-1/2+) 100 

101.6 (4) 114.3 (4-1/2) 50 88.9 (3-1/2) 82.6 (3-1/4) 62 

a

Strength ratio for spacings and end distances intermediate to those listed may be obtained by interpolation and multiplied 

by the loads in Table 8–18 to obtain design load. The strength ratio applies only to those connector units affected by the 

respective spacings or end distances. The spacings and end distances should not be less than the minimum shown. b

End distance is distance from center of connector to end of member (Fig. 8–25A). c

Spacing is distance from center to center of connectors (Fig. 8–25A). 

Figure 8–26. Some typical metal plate connectors.
8–26

General Technical Report FPL–GTR–190

the header differs depending on whether the joist hanger is 

a face mount or top mount. Face-mount hangers transmit 

loads through lateral loading of dowel-type fasteners; topmount hangers transmit loads by bearing on the top of the 

header and lateral loading of the dowel type fasteners. Design of the joist hanger varies from manufacturer to manufacturer. Examples of such plates are shown in Figure 8–28.

Design loads are limited to the lowest values determined by 

experiment or by calculations. By experiment, design loads 

for joist hangers are determined from tests in which a joist 

is loaded at midspan and supported by two joist hangers 

attached to headers, following ASTM D 7147 procedures. 

The smallest value as determined by two different means 

is the test design load for normal duration of load: (1) the 

average load at 3.2-mm (0.125-in.) deformation between the 

joist and header of at least six specimens and (2) the average 

ultimate load of at least six specimens divided by 3.0 or the 

least ultimate load for lower than six replicates divided by 

3.0. Design loads for calculations are also highlighted in 

ASTM D 7147.

Fastener Head Embedment

The bearing strength of wood under fastener heads is important in such applications as the anchorage of building 

framework to foundation structures. When pressure tends 

to pull the framing member away from the foundation, the 

fastening loads could cause tensile failure of the fastenings, 

withdrawal of the fastenings from the framing member, or 

embedment of the fastener heads in the member. The fastener head could even be pulled completely through.

The maximum load for fastener head embedment is related 

to the fastener head perimeter, whereas loads at low embedments (1.27 mm (0.05 in.)) are related to the fastener 

head bearing area. These relations for several species at 

10% moisture content are shown in Figures 8–29 and 8–30.

When annularly threaded nails are used for withdrawal applications, fastener head embedment or pull-through can be 

a limiting condition and should be considered in design.

Figure 8–27. Typical load–slip curves for two types of 

metal plate connectors loaded in tension.

Figure 8–28. Typical joist hanger connectors. 

Figure 8–29. Relation between maximum embedment load and fastener perimeter for several species of wood.

Figure 8–30. Relation between load at 1.27-mm (0.05-

in.) embedment and fastener bearing area for several species.
8–27

Chapter 8 Fastenings

Literature Cited

Johansen, K.W. 1949. Theory of timber connections. Zurich, 

Switzerland: Publication of International Association for 

Bridge and Structural Engineering. 9: 249–262.

Additional References

AF&PA. 2005. National design specification for wood construction. Washington, DC: American Forest & Paper Association.

Anderson, L.O. 1959. Nailing better wood boxes and crates. 

Agric. Handb. 160. Washington, DC: U.S. Department of 

Agriculture.

Anderson, L.O. 1970. Wood-frame house construction. 

Agric. Handb. 73 (rev.). Washington, DC: U.S. Department 

of Agriculture.

ASCE. 1996. Mechanical connections in wood structures. 

Washington, DC: American Society of Civil Engineers.

ASTM. [Current edition]. West Conshohocken, PA: 

American Society for Testing and Materials.

ASTM D 1761–88. Standard test methods for mechanical 

fasteners in wood.

ASTM D 2555–96. Standard methods for establishing clear 

wood strength values.

ASTM D 5652–95. Standard test methods for bolted connections in wood and wood-base products.

ASTM D 7147–05 Standard specifications for testing and 

establishing allowable loads of joist hangers.

ASTM F 547. Standard terminology of nails for use with 

wood and wood-base materials.

ASTM F 1575–03 Standard test method for determining 

bending yield moment of nails.

ASTM F 1667. Specification for driven fasteners: nails, 

spiker, staples.

Cramer, C.O. 1968. Load distribution in multiple-bolt tension joints. Proc. Pap. 5939. Journal of Structural Division, 

American Society of Civil Engineers. 94(ST5): 1101–1117.

Doyle, D.V.; Scholten, J.A. 1963. Performance of bolted 

joints in Douglas-fir. Res. Pap. FPL 2. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Eckelman, C.A. 1975. Screw holding performance in 

hardwoods and particleboard. Forest Products Journal. 

25(6): 30–35.

Fairchild, I.J. 1926. Holding power of wood screws. Technol. Pap. 319. Washington, DC: U.S. National Bureau of 

Standards.

Folz, B.; Filiatrault, A. 2001. Cyclic analysis of wood shear 

walls. ASCE Journal of Structural Engineering. 127(4): 

433–441.

FPL. 1962. General observations on the nailing of wood. 

FPL Tech. Note 243. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

FPL. 1964. Nailing dense hardwoods. Research Note FPL–

037. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory.

FPL. 1965. Nail withdrawal resistance of American woods. 

Res. Note FPL–RN–033. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

Goodell, H.R.; Philipps, R.S. 1944. Bolt-bearing strength 

of wood and modified wood: effect of different methods of 

drilling bolt holes in wood and plywood. FPL Report 1523. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

Jordan, C.A. 1963. Response of timber joints with metal 

fasteners to lateral impact loads. FPL Rep. 2263. Madison, 

WI: U.S. Department Agriculture, Forest Service, Forest 

Products Laboratory.

Kuenzi, E.W. 1951. Theoretical design of a nailed or bolted 

joint under lateral load. FPL Rep. 1951. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Kurtenacker, R.S. 1965. Performance of container fasteners 

subjected to static and dynamic withdrawal. Res. Pap. FPL 

29. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory.

Lantos, G. 1969. Load distribution in a row of fasteners subjected to lateral load. Madison, WI: Wood Science. 

1(3): 129–136.

Markwardt, L.J. 1952. How surface condition of nails affects their holding power in wood. FPL Rep. D1927. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory.

Markwardt, L.J.; Gahagan, J.M. 1930. Effect of nail points 

on resistance to withdrawal. FPL Rep. 1226. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

Markwardt, L.J.; Gahagan, J.M. 1952. Slant driving of nails. 

Does it pay? Packing and Shipping. 56(10): 7–9,23,25.

McLain, T.E. 1975. Curvilinear load-slip relations in 

laterally-loaded nailed joints. Fort Collins, CO: Department 

of Forestry and Wood Science, Colorado State University. 

Ph.D. dissertation.

Newlin, J.A.; Gahagan, J.M. 1938. Lag screw joints: their 

behavior and design. Tech. Bull. 597. Washington, DC: 

U.S. Department of Agriculture.
8–28

General Technical Report FPL–GTR–190

NPA. 1968. Screw holding of particleboard. Tech. Bull. 3. 

Washington, DC: National Particleboard Association.

Perkins, N.S.; Landsem, P.; Trayer, G.W. 1933. Modern connectors for timber construction. Washington, DC: U.S. Department of Commerce, National Committee on Wood Utilization, and U.S. Department of Agriculture, Forest Service.

Rammer, D.R.; Mendez, A.M. 2008. Withdrawal strength of 

post-frame ring shank nails. Frame Building News. April. 

p. 59–67.

Rammer, D.R.; Zelinka, S.L. 2004. Review of end grain 

nail withdrawal research. Gen. Tech. Rep. FPL–GTR–151. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

Rammer, D.R.; Winistorfer, S.G.; Bender, D.A. 2001. Withdrawal strength of threaded nails. Journal of Structural Engineering. 127(4): 442–449. April.

Scholten, J.A. 1944. Timber-connector joints, their strength 

and design. Tech. Bull. 865. Washington, DC: U.S. Department of Agriculture.

Scholten, J.A. 1946. Strength of bolted timber joints. FPL 

Report R1202. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

Scholten, J.A. 1950. Nail-holding properties of southern 

hardwoods. Southern Lumberman. 181(2273): 208–210.

Scholten, J.A.; Molander, E.G. 1950. Strength of 

nailed joints in frame walls. Agricultural Engineering. 

31(11): 551–555.

Skulteti, M.J.; Bender, D.A.; Winistorfer, S.G.; Pollock, 

D.G. 1997. Withdrawal strength of ring-shank nails embedded in southern pine lumber. Transactions of the ASAE, 

American Society of Agricultural Engineers. 40(2): 451–

456. February.

Soltis, L.A.; Wilkinson, T.L. 1987. Bolted connection design. Gen. Tech. Rep. FPL–GTR–54. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Stern, E.G. 1940. A study of lumber and plywood joints 

with metal split-ring connectors. Bull. 53. State College, 

PA: Pennsylvania Engineering Experiment Station.

Stern, E.G. 1950. Nails in end-grain lumber. Timber News 

and Machine Woodworker. 58(2138): 490–492.

Trayer, G.W. 1932. Bearing strength of wood under bolts. 

Tech. Bull. 332. Washington, DC: U.S. Department of 

Agriculture.

Truss Plate Institute. [N.d.] Design specification for metal 

plate connected wood trusses. TPI–78. Madison, WI: Truss 

Plate Institute.

Wilkinson, T.L. 1971. Bearing strength of wood under embedment loading of fasteners. Res. Pap. FPL 163. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

Wilkinson, T.L. 1971. Theoretical lateral resistance of nailed 

joints. Proceedings of American Society of Civil Engineering. Journal of Structural Division. ST5(97): (Pap. 8121): 

1381–1398.

Wilkinson, T.L. 1978. Strength of bolted wood joints with 

various ratios of member thicknesses. Res. Pap. FPL 314. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

Wilkinson, T.L. 1980. Assessment of modification factors 

for a row of bolts or timber connectors. Res. Pap. FPL 376. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

Wilkinson, T.L. 1991. Bolted connection allowable loads 

based on the European yield model. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Wilkinson, T.L. 1991. Dowel bearing strength. Res. Pap. 

FPL–RP–505. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

Wilkinson, T.L.; Laatsch, T.R. 1970. Lateral and withdrawal 

resistance of tapping screws in three densities of wood. Forest Products Journal. 20(7): 34–41.

Zahn, J.J. 1991. Design equation for multiple-fastener 

wood connections. New York, NY: Journal of Structural 

Engineering, American Society of Civil Engineers. 

Vol. 117(11): 3477–3485. November.

Zelinka, S.L.; Rammer, D.R. Corrosion rates of fasteners in 

treated wood exposed to 100% relative humidity. Journal of 

Materials in Civil Engineering, American Society of Civil 

Engineers. 21(12): 758–763. December.
CHAPTER 9

Structural Analysis Equations

Douglas R. Rammer, Research General Engineer

9–1

Contents 

Deformation Equations 9–1

 Axial Load 9–1

 Bending 9–1

 Combined Bending and Axial Load 9–3

 Torsion 9–4

Stress Equations 9–4

 Axial Load 9–4

 Bending 9–4

 Combined Bending and Axial Load 9–6

 Torsion 9–7

Stability Equations 9–7

 Axial Compression 9–7

 Bending 9–8

 Interaction of Buckling Modes 9–9

Literature Cited 9–10

Additional References 9–10

 

Equations for deformation and stress, which are the basis 

for tension members and beam and column design, are discussed in this chapter. The first two sections cover tapered 

members, straight members, and special considerations such 

as notches, slits, and size effect. A third section presents 

stability criteria for members subject to buckling and for 

members subject to special conditions. 

Note that this chapter focuses primarily on presenting fundamental mechanics-based equations. For design procedures, 

the reader is encouraged to contact appropriate industry 

trade associations or product manufacturers. Current design 

information can be readily obtained form their web sites, 

technical handbooks, and bulletins.

Deformation Equations

Equations for deformation of wood members are presented 

as functions of applied loads, moduli of elasticity and rigidity, and member dimensions. They may be solved to determine minimum required cross-sectional dimensions to meet 

deformation limitations imposed in design. Average moduli 

of elasticity and rigidity are given in Chapter 5. Consideration must be given to variability in material properties and 

uncertainties in applied loads to control reliability of the 

design.

Axial Load

The deformation of an axially loaded member is not usually 

an important design consideration. More important considerations will be presented in later sections dealing with 

combined loads or stability. Axial load produces a change of 

length given by

(9–1)

where d is change of length, L length, A cross-sectional 

area, E modulus of elasticity (EL when grain runs parallel to 

member axis), and P axial force parallel to grain.

Bending

Straight Beam Deflection

The deflection of straight beams that are elastically stressed 

and have a constant cross section throughout their length is 

given by

(9–2)

where d is deflection, W total beam load acting perpendicular to beam neutral axis, L beam span, kb and ks constants 
9–2

General Technical Report FPL–GTR–190

dependent upon beam loading, support conditions, and 

location of point whose deflection is to be calculated, I beam 

moment of inertia, A′ modified beam area, E beam modulus 

of elasticity (for beams having grain direction parallel to 

their axis, E = EL), and G beam shear modulus (for beams 

with flat-grained vertical faces, G = GLT, and for beams with 

edge-grained vertical faces, G = GLR). Elastic property 

values are given in Tables 5–1 and 5–2 (Chap. 5). The first 

term on the right side of Equation (9–2) gives the bending 

deflection and the second term the shear deflection. Values 

of kb and ks for several cases of loading and support are 

given in Table 9–1.

The moment of inertia I of the beams is given by

 

 for beam of rectangular cross section

(9–3)

 for beam of circular cross section

where b is beam width, h beam depth, and d beam diameter. 

The modified area A′ is given by

 for beam of rectangular cross section

(9–4)

 for beam of circular cross section

If the beam has initial deformations such as bow (lateral 

bend) or twist, these deformations will be increased by the 

bending loads. It may be necessary to provide lateral or torsional restraints to hold such members in line. (See Interaction of Buckling Modes section.)

Tapered Beam Deflection

Figures 9–1 and 9–2 are useful in the design of tapered 

beams. The ordinates are based on design criteria such as 

span, loading, difference in beam height (hc - h0) as required 

by roof slope or architectural effect, and maximum allowable deflection, together with material properties. From this, 

the value of the abscissa can be determined and the smallest 

beam depth h0 can be calculated for comparison with that 

given by the design criteria. Conversely, the deflection of a 

beam can be calculated if the value of the abscissa is known. 

Tapered beams deflect as a result of shear deflection in addition to bending deflections (Figs. 9–1 and 9–2), and this 

shear deflection Ds can be closely approximated by

 for uniformly distributed load

(9–5)

 for midspan-concentrated load

The final beam design should consider the total deflection 

as the sum of the shear and bending deflection, and it may 

be necessary to iterate to arrive at final beam dimensions. 

Equations (9–5) are applicable to either single-tapered or 

double-tapered beams. As with straight beams, lateral or 

torsional restraint may be necessary.

Effect of Notches and Holes

The deflection of beams is increased if reductions in crosssection dimensions occur, such as by holes or notches. The 

deflection of such beams can be determined by considering 

them of variable cross section along their length and appropriately solving the general differential equations of the 

elastic curves, EI(d2y/dx 2) = M, to obtain deflection expressions or by the application of Castigliano’s theorem. (These 

procedures are given in most texts on strength of materials.)

Effect of Time: Creep Deflections

In addition to the elastic deflections previously discussed, 

wood beams usually sag in time; that is, the deflection increases beyond what it was immediately after the load was 

first applied. (See the discussion of creep in Time under 

Load in Chap. 5.)

Green timbers, in particular, will sag if allowed to dry under load, although partially dried material will also sag to 

some extent. In thoroughly dried beams, small changes in 

deflection occur with changes in moisture content but with 

little permanent increase in deflection. If deflection under 

longtime load with initially green timber is to be limited, 

it has been customary to design for an initial deflection of 

about half the value permitted for longtime deflection. If 

deflection under longtime load with initially dry timber is to 

be limited, it has been customary to design for an initial deflection of about two-thirds the value permitted for longtime 

deflection.

Water Ponding

Ponding of water on roofs already deflected by other loads 

can cause large increases in deflection. The total short-term 

Table 9–1. Values of kb and ks for several beam loadings 

Loading Beam ends Deflection at kb ks

Uniformly distributed Both simply supported Midspan 5/384 1/8 

Both clamped Midspan 1/384 1/8 

Concentrated at midspan Both simply supported Midspan 1/48 1/4 

Both clamped Midspan 1/192 1/4 

Concentrated at outer Both simply supported Midspan 11/768 1/8 

quarter span points Both simply supported Load point 1/96 1/8 

Uniformly distributed Cantilever, one free, one clamped Free end 1/8 1/2 

Concentrated at free end Cantilever, one free, one clamped Free end 1/3 1 
9–3

Chapter 9 Structural Analysis Equations

deflection D due to design load plus ponded water can be 

closely estimated by

(9–6)

where D0 is deflection due to design load alone, S beam 

spacing, and Scr critical beam spacing (Eq. (9–31)).

Combined Bending and Axial Load

Concentric Load

Addition of a concentric axial load to a beam under loads 

acting perpendicular to the beam neutral axis causes increase in bending deflection for added axial compression 

and decrease in bending deflection for added axial tension. 

The deflection under combined loading at midspan for pinended members can be estimated closely by

(9–7)

where the plus sign is chosen if the axial load is tension 

and the minus sign if the axial load is compression, D is 

midspan deflection under combined loading, D0 beam 

midspan deflection without axial load, P axial load, and 

Pcr a constant equal to the buckling load of the beam under 

axial compressive load only (see Axial Compression in Stability Equations section.) based on flexural rigidity about the 

neutral axis perpendicular to the direction of bending loads. 

This constant appears regardless of whether P is tension or 

compression. If P is compression, it must be less than Pcr

to avoid collapse. When the axial load is tension, it is conservative to ignore the P/Pcr term. (If the beam is not supported against lateral deflection, its buckling load should be 

checked using Eq. (9–35).)

Eccentric Load

If an axial load is eccentrically applied to a pin-ended member, it will induce bending deflections and change in length 

given by Equation (9–1). Equation (9–7) can be applied to 

find the bending deflection by writing the equation in the 

form

(9–8)

where δb is the induced bending deflection at midspan

and ε0 the eccentricity of P from the centroid of the 

cross section.

Figure 9–1. Graph for determining tapered beam 

size based on deflection under uniformly distributed load.

Figure 9–2. Graph for determining tapered beam 

size on deflection under concentrated midspan 

load.
9–4

Torsion

The angle of twist of wood members about the longitudinal 

axis can be computed by

(9–9)

where q is angle of twist in radians, T applied torque, L

member length, G shear modulus (use , or approximate G by EL/16 if measured G is not available), and K a 

cross-section shape factor. For a circular cross section, K is 

the polar moment of inertia:

(9–10)

where D is diameter. For a rectangular cross section,

(9–11)

where h is larger cross-section dimension, b is smaller crosssection dimension, and j is given in Figure 9–3.

Stress Equations

The equations presented here are limited by the assumption 

that stress and strain are directly proportional (Hooke’s law) 

and by the fact that local stresses in the vicinity of points of 

support or points of load application are correct only to the 

extent of being statically equivalent to the true stress distribution (St. Venant’s principle). Local stress concentrations 

must be separately accounted for if they are to be limited in 

design.

Axial Load

Tensile Stress

Concentric axial load (along the line joining the centroids of 

the cross sections) produces a uniform stress:

(9–12)

where ft is tensile stress, P axial load, and A cross-sectional 

area.

Short-Block Compressive Stress

Equation (9–12) can also be used in compression if the 

member is short enough to fail by simple crushing without 

deflecting laterally. Such fiber crushing produces a local 

“wrinkle” caused by microstructural instability. The member 

as a whole remains structurally stable and able to bear load.

Bending

The strength of beams is determined by flexural stresses 

caused by bending moment, shear stresses caused by shear 

load, and compression across the grain at the end bearings 

and load points.

Straight Beam Stresses

The stress due to bending moment for a simply supported 

pin-ended beam is a maximum at the top and bottom edges. 

The concave edge is compressed, and the convex edge is 

under tension. The maximum stress is given by

(9–13)

where fb is bending stress, M bending moment, and Z beam 

section modulus (for a rectangular cross section, Z = bh2/6; 

for a circular cross section, Z = pD3/32).

This equation is also used beyond the limits of Hooke’s 

law with M as the ultimate moment at failure. The resulting 

pseudo-stress is called the “modulus of rupture,” values of 

which are tabulated in Chapter 5. The modulus of rupture 

has been found to decrease with increasing size of member. 

(See Size Effect section.)

The shear stress due to bending is a maximum at the centroidal axis of the beam, where the bending stress happens to be 

zero. (This statement is not true if the beam is tapered—see 

following section.) In wood beams this shear stress may 

produce a failure crack near mid-depth running along the 

axis of the member. Unless the beam is sufficiently short 

and deep, it will fail in bending before shear failure can 

develop; but wood beams are relatively weak in shear, and 

shear strength can sometimes govern a design. The maximum shear stress is

A

V f = k s (9–14)

where fs is shear stress, V vertical shear force on cross section, A cross-sectional area, and k = 3/2 for a rectangular 

cross section or k = 4/3 for a circular cross section.

Tapered Beam Stresses

For beams of constant width that taper in depth at a slope 

less than 25°, the bending stress can be obtained from Equation (9–13) with an error of less than 5%. The shear stress, 

however, differs markedly from that found in uniform 

beams. It can be determined from the basic theory presented 

Figure 9–3. Coefficient φ for determining torsional 

rigidity of rectangular member (Eq. (9 –11)).

General Technical Report FPL–GTR–190
9–5

Chapter 9 Structural Analysis Equations

by Maki and Kuenzi (1965). The shear stress at the tapered 

edge can reach a maximum value as great as that at the neutral axis at a reaction.

Consider the example shown in Figure 9–4, in which concentrated loads farther to the right have produced a support 

reaction V at the left end. In this case the maximum stresses 

occur at the cross section that is double the depth of the 

beam at the reaction. For other loadings, the location of the 

cross section with maximum shear stress at the tapered edge 

will be different.

For the beam depicted in Figure 9–4, the bending stress is 

also a maximum at the same cross section where the shear 

stress is maximum at the tapered edge. This stress situation 

also causes a stress in the direction perpendicular to the 

neutral axis that is maximum at the tapered edge. The effect 

of combined stresses at a point can be approximately accounted for by an interaction equation based on the Henky–

von Mises theory of energy due to the change of shape. This 

theory applied by Norris (1950) to wood results in

1 2

2

2

2

2

2

+ + =

y

y

xy

xy

x

x

F

f

F

f

F

f (9–15)

where fx is bending stress, fy stress perpendicular to the neutral axis, and fxy shear stress. Values of Fx, Fy, and Fxy are 

corresponding stresses chosen at design values or maximum 

values in accordance with allowable or maximum values 

being determined for the tapered beam. Maximum stresses 

in the beam depicted in Figure 9–4 are given by

(9–16)

Substitution of these equations into the interaction Equation 

(9–15) will result in an expression for the moment capacity 

M of the beam. If the taper is on the beam tension edge, the 

values of fx and fy are tensile stresses.

Example: Determine the moment capacity (newtonmeters) of a tapered beam of width b = 100 mm, depth 

h0 = 200 mm, and taper tan q = 1/10. Substituting these 

dimensions into Equation (9–16) (with stresses in pascals) 

results in

Substituting these into Equation (9–15) and solving for M

results in

where appropriate allowable or maximum values of the F

stresses are chosen.

Size Effect

The modulus of rupture (maximum bending stress) of wood 

beams depends on beam size and method of loading, and the 

strength of clear, straight-grained beams decreases as size 

increases. These effects were found to be describable by statistical strength theory involving “weakest link” hypotheses 

and can be summarized as follows: For two beams under 

two equal concentrated loads applied symmetrical to the 

midspan points, the ratio of the modulus of rupture of beam 

1 to the modulus of rupture of beam 2 is given by

(9–17)

where subscripts 1 and 2 refer to beam 1 and beam 2, R is 

modulus of rupture, h beam depth, L beam span, a distance 

between loads placed a/2 each side of midspan, and m a 

constant. For clear, straight-grained Douglas-fir beams, 

m = 18. If Equation (9–17) is used for beam 2 size (Chap. 5) 

Figure 9–4. Shear stress distribution for a tapered beam.

xy

xy
loaded at midspan, then h2 = 5.08 mm (2 in.), L2 = 71.112 

mm (28 in.), and a2 = 0 and Equation (9–17) becomes

(metric) (9–18a)

(inch–pound) (9–18b)

Example: Determine modulus of rupture for a beam 10 in. 

deep, spanning 18 ft, and loaded at one-third span points 

compared with a beam 2 in. deep, spanning 28 in., and 

loaded at midspan that had a modulus of rupture of 10,000 

lb in–2. Assume m = 18. Substituting the dimensions into 

Equation (9–18) produces

Application of the statistical strength theory to beams under 

uniformly distributed load resulted in the following relationship between modulus of rupture of beams under uniformly 

distributed load and modulus of rupture of beams under concentrated loads:

(9–19)

where subscripts u and c refer to beams under uniformly 

distributed and concentrated loads, respectively, and other 

terms are as previously defined.

Shear strength for non-split, non-checked, solid-sawn, and 

glulam beams also decreases as beam size increases. A 

relationship between beam shear t and ASTM shear block 

strength tASTM, including a stress concentration factor for 

the re-entrant corner of the shear block, Cf, and the shear 

area A, is

1/ 5

1.9 f ôASTM ô

A

C = (metric) (9–20a)

1/ 5

1.3 f ôASTM ô

A

C = (inch–pound) (9–20b)

where t is beam shear (MPa, lb in–2), Cf stress concentration 

factor, tASTM ASTM shear block strength (MPa, lb in–2), and 

A shear area (cm2, in2).

This relationship was determined by empirical fit to test 

data. The shear block re-entrant corner concentration factor 

is approximately 2; the shear area is defined as beam width 

multiplied by the length of beam subjected to shear force.

Effect of Notches, Slits, and Holes

In beams having notches, slits, or holes with sharp interior 

corners, large stress concentrations exist at the corners. The 

local stresses include shear parallel to grain and tension perpendicular to grain. As a result, even moderately low loads 

can cause a crack to initiate at the sharp corner and propagate along the grain. An estimate of the crack-initiation load 

can be obtained by the fracture mechanics analysis of Murphy (1979) for a beam with a slit, but it is generally more 

economical to avoid sharp notches entirely in wood beams, 

especially large wood beams, since there is a size effect: 

sharp notches cause greater reductions in strength for larger 

beams. A conservative criterion for crack initiation for a 

beam with a slit is

(9–21)

where h is beam depth, b beam width, M bending moment, 

and V vertical shear force, and coefficients A and B are 

presented in Figure 9–5 as functions of a/h, where a is slit 

depth. The value of A depends on whether the slit is on the 

tension edge or the compression edge. Therefore, use either 

At or Ac as appropriate. The values of A and B are dependent 

upon species; however, the values given in Figure 9–5 are 

conservative for most softwood species.

Effects of Time: Creep Rupture, Fatigue, and Aging

See Chapter 5 for a discussion of fatigue and aging. Creep 

rupture is accounted for by duration-of-load adjustment in 

the setting of allowable stresses, as discussed in Chapters 5 

and 7.

Water Ponding

Ponding of water on roofs can cause increases in bending 

stresses that can be computed by the same amplification factor (Eq. (9–6)) used with deflection. (See Water Ponding in 

the Deformation Equations section.)

Combined Bending and Axial Load

Concentric Load

Equation (9–7) gives the effect on deflection of adding an 

end load to a simply supported pin-ended beam already bent 

by transverse loads. The bending stress in the member is 

modified by the same factor as the deflection:

(9–22)

Figure 9–5. Coefficients A and B for crack-initiation 

criterion (Eq. (9–21)).

9–6

General Technical Report FPL–GTR–190

t

t

t

t
where the plus sign is chosen if the axial load is tension and 

the minus sign is chosen if the axial load is compression, fb

is net bending stress from combined bending and axial load, 

fb0 bending stress without axial load, P axial load, and Pcr

the buckling load of the beam under axial compressive load 

only (see Axial Compression in the Stability Equations section), based on flexural rigidity about the neutral axis perpendicular to the direction of the bending loads. This Pcr is 

not necessarily the minimum buckling load of the member. 

If P is compressive, the possibility of buckling under combined loading must be checked. (See Interaction of Buckling 

Modes.)

The total stress under combined bending and axial load is 

obtained by superposition of the stresses given by Equations 

(9–12) and (9–22).

Example: Suppose transverse loads produce a bending 

stress fb0 tensile on the convex edge and compressive on the 

concave edge of the beam. Then the addition of a tensile 

axial force P at the centroids of the end sections will produce a maximum tensile stress on the convex edge of

and a maximum compressive stress on the concave edge of

where a negative result would indicate that the stress was in 

fact tensile.

Eccentric Load

If the axial load is eccentrically applied, then the bending 

stress fb0 should be augmented by ±Pe0/Z, where e0 is 

eccentricity of the axial load.

Example: In the preceding example, let the axial load be 

eccentric toward the concave edge of the beam. Then the 

maximum stresses become

Torsion

For a circular cross section, the shear stress induced by 

torsion is

(9–23)

where T is applied torque and D diameter. For a rectangular 

cross section,

(9–24)

where T is applied torque, h larger cross-section dimension, 

and b smaller cross-section dimension, and b is presented in 

Figure 9–6.

Stability Equations

Axial Compression

For slender members under axial compression, stability is 

the principal design criterion. The following equations are 

for concentrically loaded members. For eccentrically loaded 

columns, see Interaction of Buckling Modes section.

Long Columns

A column long enough to buckle before the compressive 

stress P/A exceeds the proportional limit stress is called a 

“long column.” The critical stress at buckling is calculated 

by Euler’s formula:

(9–25)

where EL is elastic modulus parallel to the axis of the member, L unbraced length, and r least radius of gyration (for 

a rectangular cross section with b as its least dimension, 

r = b/ 12 , and for a circular cross section, r = d/4). Equation (9–25) is based on a pinned-end condition but may be 

used conservatively for square ends as well.

Short Columns

Columns that buckle at a compressive stress P/A beyond the 

proportional limit stress are called “short columns.” Usually 

the short column range is explored empirically, and appropriate design equations are proposed. Material of this nature 

is presented in USDA Technical Bulletin 167 (Newlin and 

Gahagan 1930). The final equation is a fourth-power parabolic function that can be written as

(9–26)

Figure 9–6. Coefficient β for computing maximum 

shear stress in torsion of rectangular member 

(Eq. (9–24)).

9–7

Chapter 9 Structural Analysis Equations

12
where Fc is compressive strength and remaining terms are 

defined as in Equation (9–25). Figure 9–7 is a graphical representation of Equations (9–25) and (9–26).

Short columns can be analyzed by fitting a nonlinear function to compressive stress–strain data and using it in place 

of Hooke’s law. One such nonlinear function proposed by 

Ylinen (1956) is































 = - - -

c c

c å (1 )log 1

F

f c

F

f c

E

F e L

(9–27)

where e is compressive strain, f compressive stress, c a 

constant between 0 and 1, and EL and Fc are as previously 

defined. Using the slope of Equation (9–27) in place of EL

in Euler’s formula (Eq. (9–25)) leads to Ylinen’s buckling 

equation

(9–28)

where Fc is compressive strength and fe buckling stress 

given by Euler’s formula (Eq. (9–25)). Equation (9–28) 

can be made to agree closely with Figure 9–7 by choosing 

c = 0.97.

Comparing the fourth-power parabolic function Equation 

(9–26) to experimental data indicates the function is nonconservative for intermediate L/r range columns. Using 

Ylinen’s buckling equation with c = 0.8 results in a better 

approximation of the solid-sawn and glued-laminated data, 

whereas c = 0.9 for strand lumber seems appropriate.

Built-Up and Spaced Columns

Built-up columns of nearly square cross section with the 

lumber nailed or bolted together will not support loads as 

great as if the lumber were glued together. The reason is that 

shear distortions can occur in the mechanical joints.

If built-up columns are adequately connected and the axial 

load is near the geometric center of the cross section, Equation (9–28) is reduced with a factor that depends on the type 

of mechanical connection. The built-up column capacity is

(9–29)

where Fc, fe, and c are as defined for Equation (9–28). Kf

is the built-up stability factor, which accounts for the efficiency of the connection; for bolts, Kf

 = 0.75, and for nails, 

Kf = 0.6, provided bolt and nail spacing requirements meet 

design specification approval.

If the built-up column is of several spaced pieces, the spacer 

blocks should be placed close enough together, lengthwise 

in the column, so that the unsupported portion of the spaced 

member will not buckle at the same or lower stress than that 

of the complete member. “Spaced columns” are designed 

with previously presented column equations, considering 

each compression member as an unsupported simple column; the sum of column loads for all the members is taken 

as the column load for the spaced column.

Columns with Flanges

Columns with thin, outstanding flanges can fail by elastic 

instability of the outstanding flange, causing wrinkling of 

the flange and twisting of the column at stresses less than 

those for general column instability as given by Equations 

(9–25) and (9–26). For outstanding flanges of cross sections 

such as I, H, +, and L, the flange instability stress can be 

estimated by

(9–30)

where E is column modulus of elasticity, t thickness of the 

outstanding flange, and b width of the outstanding flange. If 

the joints between the column members are glued and reinforced with glued fillets, the instability stress increases to as 

much as 1.6 times that given by Equation (9–30).

Bending

Beams are subject to two kinds of instability: lateral–

torsional buckling and progressive deflection under water 

ponding, both of which are determined by member stiffness.

Water Ponding

Roof beams that are insufficiently stiff or spaced too far 

apart for their given stiffness can fail by progressive deflection under the weight of water from steady rain or another 

continuous source. The critical beam spacing Scr is given by

(9–31)

where E is beam modulus of elasticity, I beam moment of 

inertia, r density of water (1,000 kg m–3, 0.0361 lb in–3), L

beam length, and m = 1 for simple support or m = 16/3 for 

9–8

General Technical Report FPL–GTR–190

e

Figure 9–7. Graph for determining critical buckling stress 

of wood columns.
fixed-end condition. To limit the effect of ponding, the beam 

spacing must be less than Scr.

Lateral–Torsional Buckling

Because beams are compressed on the concave edge when 

bent under load, they can buckle by a combination of lateral 

deflection and twist. Because most wood beams are rectangular in cross section, the equations presented here are for 

rectangular members only. Beams of I, H, or other built-up 

cross section exhibit a more complex resistance to twisting and are more stable than the following equations would 

predict.

Long Beams—Long slender beams that are restrained 

against axial rotation at their points of support but are otherwise free to twist and to deflect laterally will buckle when 

the maximum bending stress fb equals or exceeds the following critical value:

(9–32)

where a is the slenderness factor given by

(9–33)

where EIy is lateral flexural rigidity equal to , h

is beam depth, b beam width, GK torsional rigidity defined 

in Equation (9–9), and Le effective length determined by 

type of loading and support as given in Table 9–2. Equation 

(9–32) is valid for bending stresses below the proportional 

limit.

Short Beams—Short beams can buckle at stresses beyond 

the proportional limit. In view of the similarity of Equation 

(9–32) to Euler’s formula (Eq. (9–25)) for column buckling, 

it is recommended that short-beam buckling be analyzed by 

using the column buckling criterion in Figure 9–7 applied 

with a in place of L/r on the abscissa and 

f bcr /Fb in place 

of fcr /Fc on the ordinate. Here Fb is beam modulus of 

rupture.

Effect of Deck Support—The most common form of 

support against lateral deflection is a deck continuously 

attached to the top edge of the beam. If this deck is rigid 

against shear in the plane of the deck and is attached to the 

compression edge of the beam, the beam cannot buckle. In 

regions where the deck is attached to the tension edge of 

the beam, as where a beam is continuous over a support, the 

deck cannot be counted on to prevent buckling and restraint 

against axial rotation should be provided at the support 

point.

If the deck is not very rigid against in-plane shear, as for 

example standard 38-mm (nominal 2-in.) wood decking, 

Equation (9–32) and Figure 9–7 can still be used to check 

stability except that now the effective length is modified by 

dividing by q, as given in Figure 9–8. The abscissa of this 

figure is a deck shear stiffness parameter t given by

(9–34)

where EIy is lateral flexural rigidity as in Equation (9–33), 

S beam spacing, GD in-plane shear rigidity of deck (ratio 

of shear force per unit length of edge to shear strain), and L

actual beam length. This figure applies only to simply supported beams. Cantilevers with the deck on top have their 

tension edge supported and do not derive much support 

from the deck.

Interaction of Buckling Modes

When two or more loads are acting and each of them has a 

critical value associated with a mode of buckling, the combination can produce buckling even though each load is less 

than its own critical value.

The general case of a beam of unbraced length le includes a 

primary (edgewise) moment M1, a lateral (flatwise) moment 

M2, and axial load P. The axial load creates a secondary 

moment on both edgewise and flatwise moments due to the 

deflection under combined loading given by Equation (9–7). 

In addition, the edgewise moment has an effect like the secondary moment effect on the flatwise moment.

The following equation contains two moment modification 

factors, one on the edgewise bending stress and one on the 

flatwise bending stress that includes the interaction of biaxial bending. The equation also contains a squared term for 

axial load to better predict experimental data:

(9–35)

Table 9–2. Effective length for checking lateral–

torsional stability of beamsa

Support Load

Effective

length Le

Simple support Equal end moments L

Concentrated force at center 

h L

L

1 2

0.742



Uniformly distributed force 

h L

L

1 2

0.887



Cantilever Concentrated force at end 

h L

L

1 2

0.783



Uniformly distributed force 

h L

L

1 2

0.489



a

These values are conservative for beams with a width-to-depth 

ratio of less than 0.4. The load is assumed to act at the top edge of 

the beam. 

9–9

Chapter 9 Structural Analysis Equations
where f is actual stress in compression, edgewise bending,

or flatwise bending (subscripts c, b1, or b2, respectively), 

F buckling strength in compression or bending (a single 

prime denotes the strength is reduced for slenderness), e/d

ratio of eccentricity of the axial compression to member 

depth ratio for edgewise or flatwise bending (subscripts 1 

or 2, respectively), and qc moment magnification factors for 

edgewise and flatwise bending, given by

(9–36)

(9–37)

(9–38)

(9–39)

(9–40)

where le is effective length of member and S and Scr are 

previously defined ponding beam spacing.

Literature Cited

Maki, A.C.; Kuenzi, E.W. 1965. Deflection and stresses 

of tapered wood beams. Res. Pap. FPL–RP–34. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 56 p.

Murphy, J.F. 1979. Using fracture mechanics to predict failure of notched wood beams. In: Proceedings of first international conference on wood fracture; 1978 August 14–16; 

Banff, AB. Vancouver, BC: Forintek Canada Corporation. 

159: 161–173.

Newlin, J.A.; Gahagan, J.M. 1930. Tests of large timber 

columns and presentation of the Forest Products 

Laboratory column formula. Tech. Bull. 167. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 44 p.

Norris, C.B. 1950. Strength of orthotropic materials subjected to combined stresses. Rep. 1816. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 40 p.

Ylinen, A. 1956. A method of determining the buckling 

stress and the required cross-sectional area for centrally 

loaded straight columns in elastic and inelastic range. Zurich, Switzerland: Publication of the IABSA (International 

Association for Bridge and Structural Engineering). Vol. 16.

Additional References

ASTM. [Current edition]. Standard methods for testing clear 

specimens of timber. ASTM D 143–94. West Conshohocken, PA: American Society for Testing and Materials.

Bohannan, B. 1966. Effect of size on bending strength of 

wood members. Res. Pap. FPL–RP–56. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 30 p.

Gerhardt, T.D.; Liu, J.Y. 1983. Orthotropic beams under 

normal and shear loading. American Society of Civil 

Engineers (ASCE). Journal of Engineering Mechanics. 

109(2): 394–410.

Kuenzi, E.W.; Bohannan, B. 1964. Increases in deflection 

and stress caused by ponding of water on roofs. Forest 

Products Journal. 14(9): 421–424.

Liu, J.Y. 1980. Shear strength of wood beams: a Weibull 

analysis. American Society of Civil Engineers (ASCE). 

Journal of Structural Division. 106(ST10): 2035–2052.

Liu, J.Y. 1981. Shear strength of tapered wood beams. 

American Society of Civil Engineers (ASCE). Journal of 

Structural Division. 107(ST5): 719–731.

Liu, J.Y. 1982. A Weibull analysis of wood member bending 

strength. Transactions, American Society of Mechanical Engineers. Journal of Mechanical Design. 104: 572–579.

Liu, J.Y. 1984. Evaluation of the tensor polynomial strength 

theory for wood. Journal of Composite Materials. 18(3): 

216–226. (May).

Liu, J.Y.; Cheng, S. 1979. Analysis of orthotropic beams. 

Res. Pap. FPL–RP–343. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

37 p.

Malhorta, S.K.; Sukumar, A.P. 1989. A simplified procedure 

for built-up wood compression members. Annual conference. St. John’s, Newfoundland: Canadian Society for Civil 

Engineering: 1–18 (June).

Newlin, J.A.; Trayer, G.W. 1924. Deflection of beams 

with special reference to shear deformations. Rep. 180. 

Figure 9–8. Increase in buckling stress resulting 

from attached deck; simply supported beams. To 

apply this graph, divide the effective length by θ.

9–10

General Technical Report FPL–GTR–190
Washington, DC: U.S. National Advisory Committee on 

Aeronautics. 18 p.

Rammer, D.R.; Soltis, L.A. 1994. Experimental shear 

strength of glued-laminated beams. Res. Rep. FPL–RP–527. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 40 p.

Rammer, D.R.; Soltis, L.A.; Lebow, P.K. 1996. Experimental shear strength of unchecked solid sawn Douglas-fir. 

Res. Pap. FPL–RP–553. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

35 p.

Soltis, L.A.; Rammer, D.R. 1997. Bending to shear 

ratio approach to beam design. Forest Products Journal. 

47(1): 104–108.

Trayer, G.W. 1930. The torsion of members having sections 

common in aircraft construction. Rep. 334. Washington, 

DC: U.S. National Advisory Committee on Aeronautics. 

49 p.

Trayer, G.W.; March, H.W. 1931. Elastic instability of 

members having sections common in aircraft construction. 

Rep. 382. Washington, DC: U.S. National Advisory Committee on Aeronautics. 38 p.

Zahn, J.J. 1973. Lateral stability of wood beam-and-deck 

systems. American Society of Civil Engineers (ASCE). 

Journal of Structural Division. 99(ST7): 1391–1408.

Zahn, J.J. 1986. Design of wood members under combined 

loads. American Society of Civil Engineers (ASCE). Journal 

of Structural Engineering. 112(ST9): 2109–2126.

Zahn, J.J. 1988. Combined-load stability criterion for 

wood beam-columns. American Society of Civil Engineers 

(ASCE). Journal of Structural Engineering. 114(ST11): 

2612–2628.

9–11

Chapter 9 Structural Analysis Equations

CHAPTER 10

Adhesives with Wood Materials 

Bond Formation and Performance

Charles R. Frihart, Research Chemist

Christopher G. Hunt, Research Chemist

10–1

Adhesive bonding of wood plays an increasing role in the 

forest products industry and is a key factor for efficiently 

utilizing our timber resource. The main use of adhesives 

is in the manufacture of building materials, including plywood, oriented strandboard, particleboard, fiberboard, structural composite lumber, doors, windows and frames, and 

factory-laminated wood products. Adhesives are also used 

in the assembly of furniture and cabinets, manufacture of 

engineered wood products, and construction of residential 

and commercial structures.

Adhesives transfer and distribute loads between components, thereby increasing the strength and stiffness 

of wood products. Effective transfer of stress from one 

member to another depends on the strength of the links in 

an imaginary chain across the adhesive-bonded joint 

(Fig. 10–1). Thus, the performance of a bonded joint depends on how well the complex factors that contribute to the 

properties of the individual links (wood, adhesive, and interphase regions of wood and adhesive) are controlled during 

product assembly, which ultimately determines the strength 

of the chain.

Adhesion involves both mechanical and chemical factors 

that control the adhesive’s ability to hold together two wood 

surfaces. Because wood is porous, one mechanism of adhesion is mechanical interlocking. Effective mechanical interlocking takes place when an adhesive penetrates beyond 

the surface debris and damaged fibers into sound wood two 

to six cells deep. Further penetration into the cell wall microstructure increases the mechanical interlocking and the 

surface area for adhesive contact with the wood. With many 

adhesives, the most durable, water-resistant bonds develop 

when the adhesive flows deeply into cell cavities and infiltrates inside the cell walls. The standard for excellent bonds 

is that the wood breaks away from the adhesive joint and 

that the bond strength is equal to the strength of the solid 

wood.

Attractive forces between molecules of adhesive and wood 

contribute greatly to adhesion. Although covalent bonds—

chemical bonds between the adhesive and wood—seem 

plausible with some adhesives, no evidence exists that they 

contribute to the strength of adhesive bonds. However, intermolecular attractive forces, such as Van der Waal’s forces, 

Contents 

Surface Properties of Wood for Bonding 10–2

 Lumber Surfaces 10–2

 Veneer Surfaces 10–3

 Chemical Interference to Bonding 10–4

 Bonding of Wood Composite Products and 

 Nonwood Materials 10–5

Physical Properties of Wood for Bonding 10–5

 Density and Porosity 10–5

 Moisture Content and Dimensional Changes 10–6

Adhesives 10–8

 Composition 10–8

 Strength and Durability 10–9

 Selection 10–10

 Health and Safety 10–14

Bonding Process 10–14

 Moisture Content Control 10–14

 Surface Preparation 10–15

 Spreading of Adhesive 10–15

 Assembly and Pressing 10–16

 Post-Cure Conditioning 10–17

Bonded Joints 10–18

 Edge-Grain Joints 10–18

 End-Grain Joints 10–18

 End-to-Edge-Grain Joints 10–18

 Construction Joints 10–19

Testing and Performance 10–19

 Analytical, Chemical, and Mechanical Testing 

 of Polymers 10–19

 Mechanical Testing of Bonded Assemblies 10–20

 Short- and Long-Term Performance 10–21

 Product Quality Assurance 10–22

Standards 10–22

References 10–23
10–2

dipole–dipole forces, and hydrogen bonding, occur so frequently that they must be very important for bond strength, 

especially given the high contact area of the adhesive with 

the wood. With some wood surfaces, such as teak, wood 

extractives can interfere with the direct adhesive contact, 

leading to a chemically weak boundary effect and poor bond 

strength.

For maximum adhesive bond strength, the liquid adhesive 

must “wet” the wood surface, flowing over and penetrating into the wood. Molecules of adhesive must come into 

direct contact with molecules of wood to provide the best 

mechanical interlock and intermolecular attraction between adhesive and wood. Wood surfaces may appear to be 

smooth and flat, but microscopic examination shows peaks, 

valleys, and crevices littered with loose fibers and other 

debris. Such surface conditions cause air pockets and blockages that prevent complete wetting by the adhesive and 

introduce stress concentrations when the adhesive has cured. 

In addition, different characteristics of wood (such as grain 

angle, natural defects, and extractives) lead to widely different surface energies, roughness, and chemistry. (Surface 

wetting is discussed in more detail in the section on Chemical Interference to Bonding.) In addition to wetting, or completely covering these different surfaces, adhesives must be 

fluid enough to flow into the microscopic holes, or capillary 

structure, of wood. Pressure enhances wetting by forcing 

liquid adhesive to flow over the surfaces, displace air blockages, and penetrate to the sound wood.

The adhesive bond forms once the adhesive solidifies, but 

full strength may take from hours to days to develop. The 

applied adhesive changes from liquid to solid by one or 

more of three mechanisms: (a) loss of solvent from adhesive 

through evaporation and diffusion into the wood, (b) cooling of a molten adhesive, or (c) chemical polymerization 

into cross-linked structures that resist softening on heating. 

Because water is a common carrier for most wood adhesives, loss of water and chemical polymerization often occur 

simultaneously.

Surface Properties of Wood for 

Bonding

Because adhesives bond by surface attachment, the physical 

and chemical conditions of the wood’s surface are extremely 

important to satisfactory bond performance. The wood 

surface should be smooth, flat, and free of machine marks 

and other surface irregularities, including planer skips and 

crushed, torn, or chipped grain. The surface should be free 

of burnishes, exudates, oils, dirt, and other debris that form 

a weak boundary between the adhesive and the wood.

Both mechanical and chemical properties of a wood surface influence the quality of adhesive bonds. Wood whose 

surface is highly fractured or crushed cannot form a strong 

bond even if the adhesive forms a strong bond with the 

surface. The weak wood underneath the surface is the weak 

link in the chain and the location of failure in the bonded assembly. In other cases, poor bond strength is due to chemical properties of the surface. Sometimes natural extractives, 

overdrying, or chemicals added to modify the wood alter 

the surface chemistry enough to harm adhesive bond performance. Physical deterioration and chemical contamination 

interfere with essential wetting, flow, and penetration of 

adhesive, and contamination sometimes interferes with the 

cure of the adhesive and resulting cohesive strength of the 

bond.

Lumber Surfaces

Surfacing or resurfacing the wood within 24 h before bonding removes extractives and provides a more wettable 

surface. Surfacing also removes any unevenness that may 

have occurred from changes in moisture content. Parallel 

and flat surfaces allow the adhesive to flow freely and form 

a uniformly thin layer that is essential to optimal adhesive 

performance.

Experience and testing have proven that a smooth, knife-cut 

surface is best for bonding. Surfaces made using saws are 

usually rougher than those made using planers and jointers. 

However, surfaces sawn with special blades on properly set 

straight-line ripsaws are satisfactory for both structural and 

nonstructural joints. Furniture manufacturers commonly use 

precision sawing of wood joints rather than two-step sawing and jointing to reduce costs for labor, equipment, and 

material. Unless the saws and feed works are well maintained, however, joints made with sawed surfaces will be 

weaker and less uniform in strength than those made with 

sharp planer or jointer knives. Dull cutting edges of planer 

or jointer knives crush and burnish the cells on the wood 

surface. Not only are these cells weaker, they also inhibit 

adhesive wetting and penetration. Damage to the surface can 

be revealed by wiping a very wet rag over a portion of 

the surface, waiting for a minute or more, removing any 

remaining water with a dry paper towel, and comparing the 

roughness of the wet and dry surfaces. If the wetted area 

General Technical Report FPL–GTR–190

Figure 10–1. Imaginary links of adhesive bond between two pieces of wood using the schematic from 

Marra (1980). 
10–3

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

is much rougher than the dry area, then the machining has 

damaged the surface. A weak joint results if the adhesive 

does not completely penetrate crushed cells to restore their 

original strength.

Abrasive planing with grit sizes from 24 to 60 causes surface and subsurface crushing of wood cells. The adhesive 

industry typically recommends 60–80-grit sanding as acceptable for wood bonding as this equates to 24 to 30 knife 

marks per inch when planing. Generally, anything above 

200 grit fuzzes the wood surface and is not recommended. 

Figure 10–2 shows bondlines of undamaged, knifeplaned Douglas-fir lumber (A) compared with bondlines 

between surfaces damaged by abrasive planing (B). Such 

damaged surfaces are inherently weak and result in poor 

bond strength. If abrasive planing is to be used before bonding, belts must be kept clean and sharp, and sanding dust 

must be removed completely from the surface. However, 

abrasive planing is not recommended for structural joints 

that will be subjected to high swelling and shrinkage stresses from water soaking and drying.

Veneer Surfaces

The desired properties of wood veneer are essentially 

similar to those of lumber, but manufacturing processes, 

including cutting, drying, and laminating into plywood, can 

drastically change physical and chemical surface properties 

of veneer. Special knowledge and attention to these properties are required to ensure good wetting and penetration of 

the adhesive.

Rotary-cut veneer is produced by rotating a log by its ends 

against a knife, which results in continuous sheets of flatgrain veneer. As the knife peels veneer from the log, the 

knife forces the veneer away from the log at a sharp angle, 

fracturing (checking) the veneer on the knife side. The 

checked side is commonly called the loose side, and the opposite side without checking is called the tight side. When 

rotary-cut veneer is used for faces in plywood, the loose side 

should be bonded and the tight side finished. Otherwise, 

open checks in the faces produce imperfections in the finish. 

Adhesive overpenetration into lathe checks usually is not a 

problem if the adhesive spread rate is set correctly.

Sliced veneer is produced in long strips by moving a 

squared log, called a flitch, against a knife. As in rotary cutting, the knife forces the veneer away from the flitch at a 

sharp angle, causing fine checking of the veneer on the knife 

side. This checked surface will show imperfections in a 

finished surface, so the loose side should be bonded and the 

tight side finished. For book-matched face veneers, where 

grain patterns of adjacent veneers are near mirror images, 

half the veneers will be loosely cut and must be finished 

so the veneer must be cut as tightly as possible. Generally, 

hardwood face veneers are sliced to reveal the most attractive grain patterns.

Sawn veneer is produced in long narrow strips from flitches 

that have been selected and sawn for attractive grain patterns. The two sides of sawn veneer are free from knife 

checks, so either surface may be bonded with satisfactory 

results.

Veneer is dried promptly after cutting, using continuous, 

high-temperature dryers that are heated with either steam 

or hot gases from wood-residue- or gas-fired burners. 

Drying temperatures range from 170 to 230 °C (330 to 

446 °F) for short periods. Drying to very low moisture 

levels at very high temperatures or at moderate temperatures for prolonged periods inactivates the veneer surfaces, 

causing poor wetting of veneer and hence poor bonding. 

Residues deposited on veneer surfaces from incomplete 

Figure 10–2. Cross sections of bonded joints involving undamaged and damaged Douglas-fir surfaces. 

The dark area at the center of micrograph is the 

adhesive bondline. Image A involves two undamaged surfaces from planing with sharp knife (120×) 

and shows open wood cells with their distinct walls. 

Image B involves two damaged surfaces abrasively 

planed with 36-grit sandpaper and shows crushed 

cells with their indistinct walls in and adjacent to the 

bondline.
10–4

combustion of gases and fuel oils can cause serious adhesion problems in plywood production.

Veneer selected for its attractive appearance, or for use in 

sanded grades of plywood, should be uniform in thickness, 

smooth, and flat; be free from deep checks, knots, holes, 

and decay; and have face grain suitable for the intended face 

grade. For lower grade plywood, defect standards are not 

as strict. For example, loosely cut veneer with many deep 

checks and large defects is suitable for structural plywood, 

but this veneer requires more adhesive than does tightly cut 

veneer.

Chemical Interference to Bonding

Chemical interference that reduces the bondability of wood 

is more complicated and more difficult to detect than the 

mechanical weakening of wood surfaces. This interference 

can be from natural causes (migration of extractives to the 

surface), inadvertent wood alteration (overdrying of the 

wood surface), or intentional alteration (wood modification). A simple water test can reveal much about the state of 

a wood surface and any difficulties for wetting and bonding 

with an adhesive. This test allows estimation of the degree 

of surface inactivation of veneer towards wetting and penetration by placing a drop of water on the wood surface and 

observing how fast the drop spreads over the wood. A drop 

of water is placed in an area on the earlywood of a flat-grain 

surface that does not have checks or splits. A surface with 

good wettability and penetrability will absorb the drop within 20 s. If the drop spreads out but some water remains on 

the surface after 40 s, then the surface has good wettability 

and poor penetration, and may be difficult to bond. If after 

40 s the water drop retains much of its original shape with 

little spreading, then bonding problems from surface inactivation (poor wettability and penetrability) is a certainty. 

Figure 10–3 shows how the inactivated surface of veneer 

can be removed by sanding of the surface to allow the droplet to flow into a wider droplet on the surface instead of 

staying as a bead.

Extractives on wood surfaces contribute to surface inactivation through both physical and chemical means. Most wood 

adhesives are waterborne; therefore, they do not properly 

wet and penetrate extractive-covered surfaces. Particularly 

troublesome extractives are pitch, especially in the southern 

pines and Douglas-fir, and oil, such as in teak. When subjected to high temperatures during processing, extractives 

migrate to the surface where they concentrate and physically 

block adhesive contact with wood. Furthermore, pitchy and 

oily extractives are hydrophobic (that is, they repel water). 

The acidity of extractives of some Southeast Asian hardwoods and oak species can interfere with the chemical cure 

of some adhesives. In contrast, alkaline extractives can retard normal polymerization of an acid-cured adhesive, such 

as urea-formaldehyde, which would compromise the integrity of the adhesive film and bond.

Overdrying and overheating interfere with adhesion by 

causing extractives to diffuse to the surface, by reorienting 

surface molecules and exposing the less polar portion, by 

oxidizing or pyrolyzing the wood, or by irreversibly closing 

the larger micropores of cell walls. Airborne chemical contaminants can also inactivate a wood surface.

To reduce decay, wood is treated with a variety of preservatives, including creosote, pentachlorophenol, chromated 

copper arsenate (CCA), copper azole, ammoniacal copper 

quat, and boron compounds. These treatments generally decrease the ability of the adhesive to wet the wood; the effect 

is greater with some treatments than others. Poor wetting 

reduces contact area and thus bond strength between adhesive and wood. In addition, some treatments are known to 

alter the curing of adhesives. By understanding the properties of these modified woods, adhesive companies have been 

able to alter the adhesives and bonding process to provide 

sufficiently durable products.

The most common fire-retarding chemicals used for wood 

are inorganic salts based on phosphorous, nitrogen, and boron. These acid salts release acid at elevated temperatures 

to decrease flammable volatiles and increase char in wood, 

thereby effectively reducing flame spread. The elevated 

temperature and moisture conditions of hot-press curing can 

release some of these acids, inhibiting the cure of alkaline 

phenolic adhesives. Alkaline resins can still make durable 

bonds after some of these treatments by priming the wood 

with certain alkaline aqueous solutions or by selecting resins 

of appropriate molecular-size distribution.

Chemical modification of wood by acetylation drastically 

reduces moisture-related dimensional changes and the 

rate of biodeterioration. Acetic anhydride reacts with the 

Figure 10–3. A simple water drop test shows differences in wettability of yellow birch veneer surface. 

Three drops were applied to surface simultaneously 

and then photographed after 30 s. Left drop retained 

a large contact angle on aged and unsanded surface; 

center drop had a smaller contact angle and improved 

wettability after the surface was renewed by two passes with 320-grit sandpaper; right drop showed a small 

contact angle and good wettability after four passes 

with the sandpaper.

General Technical Report FPL–GTR–190
10–5

hydroxyl groups of wood. The conversion of hydroxyl 

groups to acetyl groups results in a lower affinity for water. 

Room-temperature-curing resorcinolic and acid-catalyzed 

phenolic hot-press adhesives develop durable bonds to 

acetylated wood. Most other wood adhesives develop poorer 

bonds with acetylated wood than with untreated wood.

Bonding of Wood Composite Products and 

Nonwood Materials

The surfaces of wood composites such as plywood, oriented 

strandboard, particleboard, fiberboard, and hardboard generally have poor wettability relative to that of freshly cut, 

polar wood surfaces. Surfaces of these materials may appear glazed, indicating that they have been inactivated by 

pressing at high temperatures. During hot pressing, resinous 

extractives and added waxes migrate to the surface, adhesive on the outer surfaces of particles and fibers cures, and 

caul release agents remain on the surfaces—all of which 

reduce wetting by waterborne wood adhesives. Surfaces of 

composite products typically are more difficult to bond than 

surfaces of solid wood products. Lightly sanding with 320-

grit sandpaper often improves adhesion to composite panel 

products having poor wettability (Fig. 10–3). Too much 

sanding can create an uneven surface and perhaps produce 

too much loose-fiber debris that can interfere with adhesion. 

Furthermore, the internal strength of composites often limits 

the strength of adhesive bonds.

Products incorporating wood composites bonded to metal or 

plastic are becoming more common because of property and 

cost advantages, but they present special challenges. Metal 

foils and plastic films laminated to wood composites do not 

require high cohesive strength for indoor applications, but 

the adhesives still must be compatible with both the wood 

and nonwood surfaces. If a structural bond is required between wood and metal or plastic, then only epoxy, polyurethane, and isocyanate-based adhesives may be sufficiently 

compatible. Even then, good adhesion often requires cleaning of the nonwood surfaces to remove contaminants or applying coupling agents, primers, or other special treatments 

to chemically activate the surfaces.

The difficulty with bonding metals to wood is usually metal 

surface inactivation. The surface energy of clean metals 

is higher than that of wood, but with exposure to air, metals quickly adsorb contaminants and form metal oxides to 

produce a low-energy, weak boundary layer at the surface. 

A series of cleaning procedures is required to regenerate the 

high-energy surface and create microscale roughness necessary for structural bonding. Steps in surface preparation may 

include abrasion by sandblasting, cleaning with liquid or 

vapor organic solvents, alkaline washing, chemical etching, 

and/or priming with adhesive solutions or coupling agents.

Plastic surfaces are difficult to bond because they are generally low energy, nonpolar, and hydrophobic. Plastics are 

organic polymers that may be either thermoplastic (soften 

on heating) or thermosetting (cross-linked and resist softening on heating). Thermoplastics generally are not as strong 

and stiff as wood, but the properties of thermoset materials 

approximate and even exceed the mechanical properties of 

wood. When plastics containing fibrous reinforcing materials such as fiberglass are bonded to woods, strength and 

stiffness of the composite materials can be greater than that 

of wood. Reinforced plastics that are effectively bonded to 

wood offer strong and cost-effective structural composites. 

Traditional waterborne wood adhesives do not bond well 

to plastics because they are polar and hydrophilic. Epoxies, 

polyurethanes, and isocyanate-based adhesives are capable 

of bonding many plastics to wood. Adhesion to plastic surfaces occurs primarily by physical intermolecular attraction 

forces and, in some cases, hydrogen bonding. Abrading and 

chemical etching of plastic surfaces increase adhesion by 

providing some mechanical interlocking. Coupling agents 

have molecules that are capable of reacting with both the 

adhesive and the surface, making them particularly useful 

for bridging dissimilar materials. Plasma treatment of plastic 

surfaces can clean and activate surfaces for enhanced adhesion. Grafting of monomers onto cleaned plastic surfaces by 

means of plasma polymerization creates a polar surface that 

is more compatible with adhesives.

Physical Properties of Wood for 

Bonding

Density and Porosity

Surface properties are not the only factors to control bonding in wood. Bond quality is also affected by the bulk physical properties of wood, particularly density, porosity, moisture content, strength, and swelling–shrinking properties.

Solid wood cell walls have a density of 1,500 kg m–3

(94 lb ft–3), regardless of the wood species. However, 

density varies greatly with void volume and thickness of 

cell walls between wood species and within a species, and 

between earlywood and latewood growth (as discussed in 

Chap. 3). High-density wood has thick walls and small lumina, whereas low-density wood has thin walls and large 

lumina. Thus, higher density wood contains more material 

per unit of volume and can carry more load.

Adhesively bonded wood assemblies typically increase in 

strength with wood density up to a range of 700 to 800 kg 

m–3 (44 to 50 lb ft–3) (moisture content 12%). Below this 

level, adhesion is usually easy and the strength of the wood 

limits the assembly strength. Above this level, high-strength 

joints with high wood failure are hard to produce consistently. Wood failure refers to the percentage of the total failure 

area that is wood, rather than adhesive. High wood failure is 

preferred because the load design values can be based upon 

the known wood strength and not reduced because of the 

quality of the bondline.

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance
High-density woods are difficult to bond for several reasons. 

Because of their thicker cell walls and smaller diameter lumens, adhesives do not easily penetrate into the wood, limiting mechanical interlock to less than two cells deep. Much 

greater pressure is required to compress stronger, stiffer, 

high-density wood to bring contact between wood surfaces 

and adhesive. Higher concentration of extractives that may 

interfere with the cure of adhesives is common in high-density species, particularly domestic oaks and imported tropical hardwoods. High-density woods are strong and allow 

high loads to be placed upon the bondline. Finally, highdensity woods tend to swell and shrink more with changes 

in moisture content than do low-density woods.

Density is perhaps a crude indicator, but as previously noted, it is useful for estimating the bondability of a great variety of wood species. Table 10–1 categorizes commonly used 

domestic and imported species according to their relative 

ease of bonding. The bondability categories for domestic 

woods are based on the average strength of side-grain joints 

of lumber as determined in laboratory tests and industrial 

experience. The laboratory tests included animal, casein, 

starch, urea-formaldehyde, and resorcinol-formaldehyde 

adhesives. The categories for imported woods are based on 

information found in the literature on bond strength, species 

properties, extractives content, and industrial experience. 

In most cases, fewer data are available for imported woods 

than domestic woods. Beware that a species that bonds 

poorly with one adhesive may develop much better bonds 

with another adhesive. A similar type of adhesive with 

somewhat different working, penetration, curing, and even 

strength properties can often dramatically improve bondability of a given species. Adhesive suppliers quite often adjust 

adhesive formulations to solve specific adhesion problems.

Wood density and anatomy control wood porosity, which 

usually affects penetration and bond performance. To attain the highest joint strength, the adhesive must penetrate 

and interlock several cells deep into sound, undamaged cell 

structure. In wood, porosity varies according to the grain 

direction. End-grain surfaces are many times more porous 

than radial or tangential surfaces. Adhesives penetrate so 

easily into the open lumens along the grain that overpenetration often occurs when gluing end-grain. This overpenetration is a primary reason why it is so difficult to form strong, 

load-bearing bonds in butt joints. Across the grain, paths for 

adhesive flow are fewer and smaller, so overpenetration generally is not a problem with a properly formulated adhesive.

The porosity and resulting adhesive flow into wood varies 

greatly, both between hardwoods and softwoods and within 

each of these groups. In Figure 10–4, cross-section micrographs demonstrate the large differences in lumen volume 

between three diffuse-porous hardwood species. Softwoods 

have longitudinal tracheid lumens connected by bordered 

pits. Pits are the small openings between fibers that permit 

lateral transfer of fluids in living trees. Adhesives might use 

the network of pits to penetrate deeply, even in tangential 

and radial directions. In hardwoods, the thin-walled, relatively large longitudinal vessels have porous end walls, so 

adhesive can penetrate deeply along the end grain. Where 

two vessels are in lateral contact, multiple inter-vessel pitting can occur, which allows for lateral flow between vessels. The remaining thick-walled fibers have relatively few 

pits for lateral transfer of adhesive. Some species, such as 

red oaks, have large numbers of radially oriented rays that 

can allow excessive flow and overpenetration. Adhesives 

provided for customers who use large volume are specifically formulated for hardwoods or softwoods, and for specific 

species within the groups, and have adjustable properties for 

specific manufacturing situations.

Moisture Content and Dimensional Changes

Water occurs naturally in living trees and affects wood 

properties and adhesive bond strength dramatically. 

Depending on extractives levels and wood chemistry, wood 

can typically take up 25% to 30% of its dry weight in water. 

The point at which wood cannot adsorb any more water is 

called the fiber saturation point. As wood dries below the 

fiber saturation point, it begins to shrink and become stiffer. 

Above the fiber saturation point, excess water simply fills 

lumens and makes wood heavier. Wood in service will 

10–6

Figure 10–4. Cross sections of three different species 

showing openness of cellular structure. Basswood is 

in the “bond easily” category in Table 10–1, soft maple 

“bond well,” and hard maple “bond satisfactorily.” The 

more easily bonded wood has greater lumen volume 

for adhesive penetration and less cell wall volume. The 

lower density of the basswood compared with the hard 

maple makes the wood weak, and therefore less force 

can be applied to the bondline.

General Technical Report FPL–GTR–190
10–7

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

Table 10–1. Categories of selected wood species according to 

ease of bonding 

U.S. hardwoods U.S. softwoods Imported woods 

Bond easilya

Alder Fir Balsa Hura 

Aspen White Cativo Purpleheart 

Basswood Grand Courbaril Roble 

Cottonwood Noble Determab

Chestnut, American Pacific 

Magnolia Pine 

Willow, black Eastern white 

 Western white 

Redcedar, western 

Redwood 

Spruce, Sitka 

Bond wellc

Butternut Douglas-fir Afromosia Meranti (lauan)

Elm Larch, westernd

 Andiroba Light red 

 American Pine Angelique White 

 Rock Sugar Avodire Yellow 

Hackberry Ponderosa Banak Obeche 

Maple, soft Redcedar, eastern Iroko Okoume 

Sweetgum Jarrah Opepe 

Sycamore Limba Peroba rosa 

Tupelo Mahogany Sapele 

Walnut, black African Spanish-cedar 

Yellow-poplar American Sucupira 

 Wallaba 

Bond satisfactorilye

Ash, white Yellow-cedar Angelin Meranti (lauan), 

 dark red 

Beech, American Port-Orford-cedar Azobe Pau marfim 

Birch Pines, southern Benge Parana-pine 

 Sweet Bubinga Pine 

 Yellow Karri Caribbean 

Cherry Radiata 

Hickory Ramin 

 Pecan 

 True 

Madrone 

Maple, hard 

Oak 

 Redb 

 Whiteb

 

Bond with difficultyf

Osage-orange Balata Keruing 

Persimmon Balau Lapacho 

 Greenheart Lignumvitae 

 Kaneelhart Rosewood 

 Kapur Teak 

a

Bond very easily with adhesives of a wide range of properties and under a wide 

range of bonding conditions. 

b

Difficult to bond with some phenol-formaldehyde adhesives. 

c

Bond well with a fairly wide range of adhesives under a moderately wide range of 

bonding conditions. 

d

Wood from butt logs with high extractive content is difficult to bond. 

e

Bond satisfactorily with good-quality adhesives under well-controlled bonding 

conditions. 

f

Satisfactory results require careful selection of adhesives and very close control of 

bonding conditions; may require special surface treatment. 
shrink and swell as it loses and gains moisture from the air; 

under typical indoor conditions, wood contains 5% to 12% 

moisture. The shrinking and swelling (dimensional changes) 

are different for the three principal directions in wood. Longitudinal dimensional change (along the grain, or up and 

down in the standing tree) is the least and amounts to less 

than 1% between fiber saturation and ovendry. Tangential 

dimensional change is the greatest, typically 6% to 12%, 

while radial dimensional change is typically about half of 

the tangential movement. Wood with low density tends to 

have the smallest dimensional change. Chapter 4 provides a 

detailed discussion of wood– moisture relations.

Wood dimensional changes that accompany changes in 

moisture content have broad-ranging and important consequences on the performance of bonded joints. As wood 

in bonded assemblies swells and shrinks, stresses develop 

that can damage the adhesive bond or wood. Damage may 

occur when moisture content changes in adjacent pieces of 

wood that have different swelling or shrinkage coefficients. 

This can arise with different species, different heartwood, 

sapwood, or juvenile wood content, or grain type, such as 

radial grain bonded to tangential or end grain bonded to 

cross grain. Even more stressful is when only one part of an 

assembly changes moisture content. Dimensional changes 

associated with water are a common cause of adhesive failure. Moisture-driven stresses can be minimized by bonding 

pieces of wood with compatible grain directions and low 

shrinkage coefficients and by bonding at the moisture content expected during service.

The moisture in wood combined with water in adhesive will 

greatly influence the wetting, flow, penetration, and cure of 

waterborne wood adhesives. In general, optimum adhesive 

properties occur when the wood is between 6% and 14% 

moisture content. Special formulations are often used outside this range. Aqueous adhesives tend to dry out when 

applied to wood below 6% moisture content. Wood absorbs 

water from the adhesive so quickly that adhesive flow and 

penetration into the wood are drastically inhibited, even 

under high pressure. Wood may become so dry below 3% 

moisture content that it temporarily resists wetting.

Wood with too much moisture is also difficult to bond with 

normal waterborne adhesives. Water and low-molecularweight portions of the adhesive migrate less effectively into 

wet wood cell walls than into drier cell walls. This leaves 

the adhesive more runny and prone to squeeze-out when 

pressure is applied. The extra adhesive mobility can also 

lead to overpenetration and starvation of the bond. In many 

adhesives, low-molecular-weight components infiltrating 

the cell walls are necessary for long-term durability. Control 

of moisture content is particularly critical when adhesive 

is cured in a hot press because the excess moisture turns to 

high-pressure steam inside the product. This pressurized 

steam can blast channels through the wood product or cause 

large internal voids, called blows, in panel products. Even 

if blows do not occur, excess moisture within thermosetting 

adhesives can prevent complete cross linking, thereby weakening the adhesive. Appropriate moisture content levels of 

wood for bonding by hot-press methods are well known, 

as are target moisture content levels for wood products 

throughout the United States. However, controlling moisture 

content during bonding of wood materials is not always easy 

(as discussed in the Moisture Content Control section).

Adhesives

Composition

During the 20th century, wood adhesives shifted from 

natural to synthetic organic polymers. A polymer is a large 

molecule constructed of many small repeated units. Natural polysaccharide and protein polymers in blood, hide, 

casein, soybean, starch, dextrin, and other biomass have 

been used as adhesives for centuries. These polymers are 

still in use today, although they have been largely replaced 

by petrochemical and natural-gas-based systems. The first 

wood adhesives based on synthetic polymers were produced 

commercially during the 1930s. Synthetic polymers can be 

made stronger, more rigid, and more durable than wood, and 

they generally have much greater water resistance than do 

traditional adhesives from natural polymers. However, recent advances in biomass-based adhesives have made them 

more competitive with fossil-fuel-based adhesives than are 

traditional ones.

Whether a synthetic adhesive is thermoplastic or thermosetting has a major influence on its performance in service. 

Thermoplastics are long-chain polymers that soften and 

flow on heating and then harden again upon cooling. They 

generally have less resistance to heat, moisture, and longterm static loading than do thermosetting polymers. Common thermoplastic adhesives for wood include poly(vinyl 

acetate) emulsions, elastomerics, contacts, and hot-melts. 

Thermosetting polymers make excellent structural adhesives because they undergo irreversible chemical change 

when cured, and on reheating, they do not soften and flow 

again. They form cross-linked polymers that can have high 

strength, have resistance to moisture and other chemicals, 

and are rigid enough to support high, long-term static loads 

without deforming. Phenol-formaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, urea-formaldehyde, isocyanate, and epoxy adhesives are examples of thermosetting 

polymers.

When delivered, adhesives usually contain a mixture of 

several chemically active and inert materials, each added for 

specific properties such as working characteristics, strength 

properties, shelf life, or durability. Solvents dissolve or 

disperse adhesive polymers, act as carriers of polymer and 

additives, aid in wetting, and control flow and penetration of 

10–8

General Technical Report FPL–GTR–190
the adhesive. Water is the carrier for most wood adhesives, 

primarily because water readily absorbs into wood, is inexpensive, and does not have adverse effects on the environment. Organic solvents are still used with elastomeric and 

contact adhesives, although waterborne adhesive systems 

are becoming more important in these markets as well. Reinforcing fibers, mostly inert organics, can enhance mechanical properties of the adhesive film, especially toughness, 

impact resistance, and shrinkage. Fillers of both organic and 

inorganic origins contribute to rheological control of the 

fluid system, particularly in reducing the spreading and penetrating of the adhesive into wood. Extenders are like fillers, 

in that they control flow and working characteristics, but are 

different in that they do not reduce bond strength.

Certain chemicals are added to plasticize adhesive polymers, enhance tackiness, improve heat resistance, or lower 

costs. Plasticizers, for example dibutyl phthalate, are used to 

soften the brittle vinyl acetate homopolymer in poly(vinyl 

acetate) emulsion adhesives. This is necessary to facilitate 

adhesive spreading and formation of a flexible adhesive film 

from the emulsion at and below room temperature. Phenolic polymers are used as tackifiers and adhesion promoters 

in neoprene and nitrile rubber contact adhesives. Reactive 

polymeric fortifiers, such as melamine-formaldehyde, can 

be substituted into urea-formaldehyde adhesives to improve 

resistance to moisture and heat. Substituting phenol-formaldehyde for resorcinol-formaldehyde reduces adhesive costs 

without sacrificing adhesive strength and durability.

Catalysts are chemicals used to accelerate the rate of chemical reaction of polymeric components. Acids, bases, salts, 

peroxides, and sulfur compounds are a few examples of 

catalysts. Catalysts do not become a part of the reacted compound; they simply increase the rate of reaction. Usually, 

hardeners are added to base polymers as reactive components, and they do become a part of the reacted compound. 

Examples are an amine hardener added to epoxy and formaldehyde added to resorcinol—both produce cross-linking 

reactions to solidify the adhesive. For curing urea-formaldehyde and melamine-formaldehyde adhesives, hardeners are 

actually catalysts in that they cure the adhesive but do not 

become part of the polymer. Other chemicals, such 

as antioxidants, acid scavengers, preservatives, wetting 

agents, defoamers, or colorants, may be added to control or 

eliminate some of the less desirable characteristics of certain 

adhesive formulations.

Strength and Durability

Table 10–2 loosely classifies adhesives according to how 

much load they can bear and how long they can sustain the 

load without deforming when exposed to water, heat, or 

other environmental conditions. In building construction, 

adhesives that contribute strength and stiffness to the structure during its life are considered structural. These adhesives 

generally are stronger and stiffer than the wood that they 

bond. Structural bonds are critical because bond failure 

could result in serious damage to the structure or its occupants. Examples of structural applications include gluedlaminated beams, prefabricated I-joists, and stressed-skin 

panels. Structural adhesives that maintain their strength and 

rigidity under the most severe cyclic water saturation and 

drying are considered fully exterior adhesives. Adhesives 

that degrade faster than wood under severe conditions, particularly water exposure, are considered interior adhesives. 

Between exterior and interior adhesives are the intermediate 

adhesives, which maintain strength and rigidity in shortterm water soaking but deteriorate faster than wood during 

long-term exposure to water and heat. Unfortunately, adhesives that are the strongest, most rigid, and most resistant 

to deterioration in service are typically the least tolerant of 

wide variations in wood surface condition, wood moisture 

content, and assembly conditions, including pressures, temperatures, and curing conditions.

Semistructural adhesives impart strength and stiffness to an 

adhesive-bonded assembly, and in some instances, they may 

be as strong and rigid as wood. However, semistructural adhesives generally do not withstand long-term static loading 

without deformation. They are capable of short-term exposure to water although some do not withstand long-term 

saturation, hence their limited exterior classification. Another semistructural adhesive application is the nailed–glued 

assembly where failure of the bond would not cause serious 

loss of structural integrity because the load would be carried 

by mechanical fasteners.

Nonstructural adhesives typically support the dead weight 

of the material being bonded and can equal the strength and 

rigidity of wood in the dry condition. On exposure to water 

or high humidity, most nonstructural adhesives continue to 

support the weight of the material sufficiently, though a few 

lose the ability to transfer load. A major market for nonstructural adhesives is furniture assembly.

Elastomeric construction adhesives are categorized as nonstructural but are normally used for field assembly of panelized floor and wall systems in the light-frame construction 

industry. These adhesive joints are much stiffer than mechanically fastened joints, resulting in stiffer panels. In addition to the adhesive, mechanical fasteners are used to carry 

the load in case of adhesive failure.

Some adhesives listed in Table 10–2 could be easily 

included in more than one category because they can be 

formulated for a broad range of applications. Isocyanate and 

polyurethane adhesives are examples. Polymeric methylene 

diphenyl diisocyanate, with a low molecular weight, develops highly durable bonds in structural strandboard, even 

though strandboard products deteriorate from swelling and 

shrinkage stresses. One-part polyurethane adhesives have 

highly durable adhesive films, but as molecular weight 

10–9

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance
increases, adhesion to porous wood generally decreases and 

bonds become increasingly susceptible to deterioration from 

swelling and shrinkage stresses. Soybean-based adhesives 

have limited wet strength on their own, but cross-linking 

agents can be added to increase water resistance.

Selection

Many factors need to be considered when selecting the best 

adhesive for a particular application. The adhesive must 

be applied, wet the surface, penetrate into the wood, cure, 

and maintain strength for sufficient time under different 

loads and environmental conditions. Table 10–3 describes 

the typical form, properties, preparation, and uses of many 

adhesive families, though considerable variation may occur 

within each family. A manufacturer and adhesive supplier 

should completely review the product, its intended service 

environment, and all production processes and equipment 

before choosing an appropriate adhesive. Whatever the approach to adhesive selection might be, the following points 

are important.

Strength—The amount of load the adhesive will be required 

to carry must be considered.

Durability—The kind of environment the bond will be exposed to (liquid water, humidity, heat, cold, chemicals, light, 

loading level) and the length of exposure will determine 

durability.

Wetting—As discussed in the introduction, the chemistry of 

the surface and adhesive must be compatible. A waterborne 

adhesive on an oily surface is unlikely to spread out unless 

the adhesive contains surfactants, organic solvents, or other 

materials to help it spread and make molecular contact with 

the surface.

Timing—Several timing factors must be considered. Pot 

life relates to the duration of time before the adhesive is applied to the wood. Open time is the time between applying 

the adhesive and joining the pieces. Closed time refers to 

the time between joining the pieces and applying pressure. 

Clamp time is determined by the duration of set time until 

the finished piece can be unclamped. Increasing temperature usually shortens set and cure time. Emulsion polymer 

isocyanates set very rapidly, which is an advantage in wood 

I-beam assemply. After hot or cold pressing, adhesives typically need hours or weeks to completely cure.

Consistency— The consistency, or viscosity, of the adhesive 

must be compatible with the application equipment, whether 

it be brush, spatula, extruder, curtain coater, spray, or powder metering device. In addition, the adhesive must be fluid 

enough to enter the void spaces in the wood but not so fluid 

that most of the adhesive is squeezed out of the bondline, 

causing a starved joint.

Mixing—If water, a hardener, catalyst, filler, or extender 

must be mixed with a resin, appropriate equipment must be 

available.

10–10

Table 10–2. Wood adhesives categorized according to their expected 

structural performance at various levels of environmental exposurea,b

Structural integrity Service environment Adhesive type 

Structural Fully exterior Phenol-formaldehyde 

(withstands long-term water 

soaking and drying) 

Resorcinol-formaldehyde 

Phenol-resorcinol-formaldehyde 

Emulsion polymer isocyanate 

Melamine-formaldehyde 

Isocyanate 

Limited exterior Melamine-urea-formaldehyde 

(withstands short-term water 

soaking)

Epoxy 

Polyurethane 

Interior Urea-formaldehyde 

(withstands short-term high 

humidity) 

Casein 

Semistructural Limited exterior Cross-linked poly(vinyl acetate) 

Cross-linked soybean 

Nonstructural Interior Poly(vinyl acetate) 

Animal 

Elastomeric construction 

Elastomeric contact 

Hot-melt 

Starch a

Assignment of an adhesive type to only one structural/service environment category does not 

exclude certain adhesive formulations from falling into the next higher or lower category. b

Priming wood surfaces with hydroxymethylated resorcinol coupling agent improves resistance 

to delamination of epoxy, isocyanate, emulsion polymer isocyanate, melamine and urea, 

phenolic, and resorcinolic adhesives in exterior service environment, particularly bonds to treated 

lumber. 

General Technical Report FPL–GTR–190
10–11

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

Table 10–3. Working and strength properties of adhesives, with typical uses

Type Form and color Preparation and application Strength properties Typical uses 

Natural origin

Animal, protein Solid and liquid; brown 

to white bondline 

Solid form added to water, 

soaked, and melted; adhesive 

kept warm during application; 

liquid form applied directly; 

both pressed at room temperature; 

bonding process must be adjusted 

for small changes in temperature 

High dry strength; low 

resistance to water and damp 

atmosphere

Assembly of furniture 

and stringed 

instruments; repairs of

antique furniture 

Blood, protein Solid and partially dried 

whole blood; dark red to 

black bondline 

Mixed with cold water, lime, 

caustic soda, and other chemicals; 

applied at room temperature; 

pressed either at room 

temperature or 120 °C (250 °F)

and higher 

High dry strength; moderate 

resistance to water and damp 

atmosphere and to 

microorganisms

Interior-type softwood 

plywood, sometimes 

in combination with 

soybean adhesive; 

mostly replaced by 

phenolic adhesive 

Casein, protein Powder with added 

chemicals; white to tan 

bondline

Dissolved in water under basic 

conditions; applied and pressed 

at room temperature 

High dry strength; moderate 

resistance to water, damp 

atmospheres, and high 

temperatures; not suitable for 

exterior uses 

Mainly in interior 

doors, especially fire 

doors; used in 

laminated timbers 

Cross-linked

soybean, 

protein 

Powder or dispersion 

with added chemicals; 

white to tan, similar color 

in bondline 

Either dissolve the solid in water 

with other added chemicals or 

use a pre-dispersed soy, mix with 

cross linker prior to application; 

cured in hot press at 120 to 150 °C

(250 to 300 °F) 

Good dry strength and decent 

resistance to water and damp 

atmospheres; good resistance to 

elevated temperatures 

Decorative plywood 

for interior use, 

laminated flooring, 

particleboard, and 

oriented strandboard 

Lignins and 

tannins 

Powder or liquid; may be 

blended with phenolic 

adhesive; dark brown 

bondline

Blended with extender and filler 

by user; adhesive cured in hotpress 130 to 150 °C (266 to 

300 °F) similar to phenolic 

adhesive

Good dry strength; moderate to 

good wet strength; durability 

improved by blending with 

phenolic adhesive 

Partial replacement 

for phenolic adhesive 

in composite and 

plywood panel 

products

Soybean, 

protein 

Powder with added 

chemicals; white to tan, 

similar color in bondline 

Mixed with cold water, lime, 

caustic soda, and other 

chemicals; applied and pressed at 

room temperatures, but more 

frequently hot pressed when 

blended with blood adhesive 

Moderate to low dry strength; 

moderate to low resistance to 

water and damp atmospheres; 

moderate resistance to 

intermediate temperatures 

Decorative plywood 

for interior use; 

combinations with 

phenolics gives good 

moisture durability 

Synthetic origin 

Cross-linked

poly(vinyl 

acetate) 

emulsion

Liquid, similar to 

poly(vinyl acetate) 

emulsions but includes 

copolymers capable of 

cross linking with a 

separate catalyst; white 

to tan with colorless 

bondline

Liquid emulsion mixed with 

catalyst; cure at room 

temperature or at elevated 

temperature in hot press and 

radio-frequency press 

High dry strength; improved 

resistance to warm 

temperatures and moisture, 

particularly long-term 

performance in moist 

environment

Interior and exterior 

doors; molding and 

architectural 

woodwork; cellulosic 

overlays 

Elastomeric 

contact

Viscous liquid, typically 

neoprene or styrenebutadine elastomers in 

organic solvent or water 

emulsion; tan to yellow 

Liquid applied directly to both 

surfaces, partially dried after 

spreading and before pressing; 

roller-pressing at room 

temperature produces instant 

bonding

Strength develops immediately 

upon pressing, increases slowly 

over a period of weeks; dry 

strengths much lower than 

those of conventional wood 

adhesives; low resistance to 

water and damp atmospheres; 

adhesive film readily yields 

under static load 

On-the-job bonding of

decorative tops to 

kitchen counters; 

factory lamination of 

wood, paper, metal, 

and plastic sheet 

materials 
10–12

Table 10–3. Working and strength properties of adhesives, with typical usescon.

Type Form and color Preparation and application Strength properties Typical uses 

Elastomeric 

mastic 

(construction

adhesive) 

Putty-like consistency, 

synthetic or natural 

elastomers in organic 

solvent or latex 

emulsions; tan, yellow, 

gray 

Mastic extruded in bead to 

framing members by caulking 

gun or like pressure equipment; 

nailing required to hold materials 

in place during setting and 

service 

Strength develops slowly over 

several weeks; dry strength 

lower than conventional wood 

adhesives; resistant to water 

and moist atmospheres; tolerant 

of outdoor assembly conditions; 

gap-filling; nailing required to 

ensure structural integrity 

Lumber to plywood or 

strandboard in floor 

and wall systems; 

laminating gypsum 

board and rigid foam 

insulating; assembly 

of panel system in 

manufactured homes 

Emulsion

polymer 

isocyanate 

Liquid emulsion and 

separate isocyanate 

hardener; white with 

hardener; colorless 

bondline

Emulsion and hardener mixed by 

user; reactive on mixing with 

controllable pot-life and curing 

time; cured at room and elevated 

temperatures; radio-frequency 

curable; high pressure required 

High dry and wet strength; very 

resistant to water and damp 

atmosphere; very resistant to 

prolonged and repeated wetting 

and drying; adheres to metals 

and plastics 

Laminated beams for 

interior and exterior 

use; lamination of 

plywood to steel 

metals and plastics; 

doors and 

architectural materials

Epoxy Liquid resin and 

hardener supplied as two 

parts; completely reactive, 

mainly solvent-free; clear 

to amber; colorless 

bondline

Resin and hardener mixed by 

user; reactive with limited 

pot-life; cured at room or 

elevated temperatures; only low 

pressure required for bond 

development

High dry and wet strength to 

wood, metal, glass, and plastic; 

formulations for wood resist 

water and damp atmospheres; 

delaminate with repeated 

wetting and drying; gap-filling 

Laminating veneer 

and lumber in coldmolded wood boat 

hulls; assembly of 

wood components in 

aircraft; lamination of 

architectural railings 

and posts; repair of 

laminated wood 

beams and 

architectural building 

components;

laminating sports 

equipment; general 

purpose home and 

shop

Hot melt Solid blocks, pellets, 

ribbons, rods, or films; 

solvent-free; white to 

tan; near colorless 

bondline

Solid form melted for spreading; 

bond formed on solidification; 

requires special application 

equipment for controlling melt 

and flow 

Develops strength quickly on 

cooling; lower strength than 

conventional wood adhesives; 

moderate resistance to moisture; 

gap-filling with minimal 

penetration 

Edge-banding of 

panels; plastic 

lamination; patching; 

film and paper 

overlays; furniture 

assembly; general 

purpose home and 

shop

Isocyanate Liquid containing 

monomers and oligomers 

of methylene diphenyl 

diisocyanate; light brown 

liquid and clear bondline 

Adhesive applied directly by 

spray; reactive with water; 

requires high temperature and 

high pressure for best bond 

development in flakeboards 

High dry and wet strength; very 

resistant to water and damp 

atmosphere; adheres to metals 

and plastics 

Flakeboards;

particleboard, strandwood products 

Melamine- and 

melamine-ureaformaldehyde 

Powder with blended 

catalyst; may be blended 

up to 40% with urea; 

white to tan; colorless 

bondline

Dissolved in water; cured in hot 

press with platens at 120 to 

150 °C (250 to 300 °F) and 

lower internal temperatures; 

particularly suited for fast curing 

in high-frequency presses 

High dry and wet strength; very 

resistant to water and damp 

atmospheres

Melamine–ureaformaldehyde primary 

adhesive for durable 

bonds in hardwood 

plywood; end-jointing 

and edge-gluing of 

lumber; and scarf 

joining softwood 

plywood, ultra-low 

emitting formaldehyde adhesive for 

particleboard and 

fiberboard 

General Technical Report FPL–GTR–190
Pressure—Pressure is applied to joints to ensure close contact between the parts. Typically, most wood adhesives do 

not fill gaps well and so require high pressure. Pressure also 

helps the adhesive to wet and penetrate the wood surface by 

forcing it into the void spaces of wood. However, too high a 

pressure, such that the adhesive largely squeezes out, should 

be avoided.

Temperature—The adhesive should work under different 

temperature conditions. The temperature of the surrounding environment can affect adhesive pot life, duration of 

open time, and curing. Phenol-formaldehyde, melamineformaldehyde, urea-formaldehyde, and isocyanate adhesives 

must be cured at high temperatures and require expensive, 

heated presses. Some of these are cured within minutes in 

expensive, high-frequency, heated presses. Emulsion polymer isocyanates, poly(vinyl acetate), epoxy, polyurethanes, 

and resocinol-containing adhesives cure well at room temperatures.

Moisture content—Many adhesives need low wood moisture content to penetrate the wood. However, isocyanates 

and polyurethanes are less sensitive and may even perform 

better at higher moisture contents.

Color and finishing properties—In furniture and interior 

millwork where appearance is critical, adhesive color, ability to absorb stains and finishes, and freedom from bleeding and staining are critical factors. Adhesives used in the 

furniture industry are usually formulated to produce a tan or 

colorless joint.

10–13

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

Table 10–3. Working and strength properties of adhesives, with typical usescon.

Type Form and color Preparation and application Strength properties Typical uses 

Phenolformaldehyde Liquid, powder, and dry 

film; dark red bondline 

Liquid blended with extenders 

and fillers by user; film inserted 

directly between laminates; liquid 

or powder applied directly to 

flakes in composites; all 

formulations cured in hot press at 

120 to 150 °C (250 to 300 °F) up 

to 200 °C (392 °F) in flakeboards 

High dry and wet strength; very 

resistant to water and damp 

atmospheres; more resistant than 

wood to high temperatures and 

chemical aging 

Primary adhesive for 

exterior softwood 

plywood, flakeboard, 

hardboard, and low 

emission particleboard

Poly(vinyl 

acetate) 

emulsion

Liquid ready to use; often 

polymerized with other 

polymers; white to tan to 

yellow; colorless bondline 

Liquid applied directly; pressed 

at room temperatures and in highfrequency press 

High dry strength; low 

resistance to moisture and 

elevated temperatures; joints 

yield under continued stress 

Furniture; flush doors; 

plastic laminates; 

panelized floor and 

wall systems in 

manufactured 

housing; general 

purpose in home and 

shop

Polyurethane Low viscosity liquid to 

high viscosity mastic; 

supplied as one-part or 

two-part systems; 

completely reactive; 

color varies from clear 

to brown; colorless 

bondline

Adhesive applied directly to one 

surface, preferably to watermisted surface; reactive with 

moisture on surface and in air; 

cures at room temperature; high 

pressure required, but mastic 

required only pressure from 

nailing 

High dry and wet strength; 

resistant to water and damp 

atmosphere; limited resistance 

to prolonged and repeated 

wetting and drying; gap-filling 

General purpose home 

and shop; construction 

adhesive for panelized 

floor and wall 

systems; laminating 

plywood to metal and 

plastic sheet 

materials; specialty 

laminates; installation 

of gypsum board 

Resorcinoland phenolresorcinolformaldehyde 

Liquid resin and 

powdered hardener 

supplied as two parts; 

phenol may be 

copolymerized with 

resorcinol; dark red 

bondline

Liquid mixed with powdered or 

liquid hardener; resorcinol 

adhesives cure at room 

temperatures; phenol-resorcinols 

cure at temperatures from 21 to 

66 °C (70 to 150 °F) 

High dry and wet strength; very 

resistant to moisture and damp 

atmospheres; more resistant 

than wood to high temperature 

and chemical aging 

Primary adhesives for 

laminated timbers and 

assembly joints that 

must withstand severe 

service conditions 

Ureaformaldehyde Powder and liquid forms; 

may be blended with 

melamine or other more 

durable resins; white to 

tan resin with colorless 

bondline

Powder mixed with water, 

hardener, filler, and extender by 

user; some formulations cure at 

room temperatures, others require 

hot pressing at about 120 °C 

(250 °F) for plywood and 210 °C 

(410 °F) for fiberboard and 

particleboard; curable with 

high-frequency heating 

High dry and wet strength; 

moderately durable under damp 

atmospheres; moderate to low 

resistance to temperatures in 

excess of 50 °C (122 °F) 

Hardwood plywood; 

furniture; medium 

density fiberboard; 

particleboard; 

underlayment; flush 

doors; furniture cores
Ease and simplicity—One-part adhesives, such as 

poly(vinyl acetate), one-part polyurethane, hot-melt, and 

phenol-formaldehyde, are the simplest to use because there 

is no chance for error in weighing and mixing components. 

Waterborne adhesives are easy to clean up. Two- or multiple-part adhesives require careful measuring and mixing of 

components and often require special solvents for cleanup 

after bonding. High water resistance often means more difficult cleanup when cured.

Cost—Given that adhesives are more expensive than the 

wood, the cost of adhesive and cost of related application 

equipment and labor must all be considered.

Safety and environment—Many adhesives cure by chemical 

reactions and therefore are hazardous in the uncured state. 

Even waterborne adhesives can have organic chemical components that evaporate, causing health concerns for workers 

and consumers. Frequently, adhesives are toxic to the skin 

or give off toxic fumes. Formaldehyde hardener for resorcinol, phenol, melamine, and urea adhesives is a severe irritant. Amine hardeners in some epoxy adhesives are strong 

skin sensitizers. Chemical sensitivity can be caused by 

repeated exposure to uncured adhesives. State and Federal 

regulations continue to require adhesives suppliers to reduce 

air emissions. In recent years, the cost of organic solvents 

and the cost of recovering volatiles to prevent air pollution have increased. Substitute waterborne systems can be 

less expensive because of the low cost of the water solvent; 

however, raising of the wood grain, slower drying, and final 

product performance must be considered.

Health and Safety

Uncured adhesives can be harmful and require safety precautions, while cured adhesives are usually safe for human 

contact. A notable exception is urea-formaldehyde adhesive, 

which can release low concentrations of formaldehyde gas 

from bonded wood products, especially under hot, moist 

conditions. Formaldehyde can react with proteins of the 

body to cause irritation and inflammation of membranes 

of eyes, nose, and throat, and may be a carcinogen. Driven 

by regulations mandating lower formaldehyde emissions, 

considerable research has led to new adhesive formulations 

with significantly reduced levels of formaldehyde emissions 

in both manufacturing operations and bonded wood products. New standards in the United States and other countries 

have reduced the acceptable upper limit for formaldehyde 

emissions. Phenol(resorcinol)-formaldehyde adhesives, 

which are used to manufacture plywood, strandboard, and 

laminated beams, also contain formaldehyde. However, the 

highly durable phenol-formaldehyde, resorcinol-formaldehyde, and phenol-resorcinol-formaldehyde polymers do not 

chemically break down in service; thus, no detectable formaldehyde is released. Although not quite as durable in bond 

strength as the phenolics, melamine-formaldehyde polymers 

do not break down to yield formaldehyde. New ultra-low 

emitting formaldehyde (ULEF) adhesives are formulated 

to reduce formaldehyde emissions. Poly(vinyl acetate), isocyanate, and soy adhesives address the formaldehyde issue 

by being no added formaldehyde (NAF). Unless detailed 

knowledge of the safety of the adhesive is available, it 

should be assumed that uncured adhesives can be harmful at 

high concentrations or with chronic exposure.

Diisocyanates are sensitizers that are capable of causing occupational asthma. They also are highly reactive chemicals 

that polymerize rapidly on contact with strong alkali, mineral acids, and water. Because polymeric methylene diphenyl 

diisocyanate (pMDI) adhesives develop strong and durable 

bonds to wood, they have gained acceptance in composite 

wood products. Any isocyanate is potentially hazardous if 

mishandled, but the low vapor pressure of pMDI adhesives 

coupled with adequate ventilation to remove airborne pMDI 

on dust particles permits manufacturing plants to operate 

safely. Emulsion polymerized isocyanates (EPI) and polyurethanes also contain the reactive isocyanate group, and 

so chronic contact with these uncured adhesives should be 

avoided. Properly cured isocyanate adhesives are not considered hazardous in bonded wood products.

Thermoplastic adhesives are generally of low toxicity, but 

any added solvents may be toxic. Construction and contact 

adhesives contain organic solvents with low flash points. 

When used in small, unventilated spaces, the solvent can accumulate in the air and cause an explosion if ignited. Some 

adhesive producers offer less flammable formulations based 

on chlorinated solvents. Solvents in these adhesives are generally toxic, but harmful effects can be avoided by providing 

adequate ventilation and following the manufacturer’s safety 

instructions.

Health and safety regulations require that toxic and hazardous chemicals have a visible label to warn of their dangers. 

Material safety data sheets (MSDS) or instructions are provided with adhesive products to advise of proper handling 

procedures, protective gear and clothing, and procedures for 

dealing with spills and fire and to offer guidance for first-aid 

and professional treatment of injuries. The statements made 

in this book concerning the safety of adhesives and effects 

on the health of the user are general and not meant to be 

all-inclusive. The user should consult the MSDS and follow 

the manufacturer’s instructions and precautions before using 

any adhesive.

Bonding Process

Moisture Content Control

After wood and adhesive selection, the next most important 

factor contributing to trouble-free service of adhesive bonds 

is control of wood moisture content before and during the 

bonding process. Moisture content strongly affects the final 

strength and durability of joints, development of surface 

checks in wood, and dimensional stability of the bonded 

assembly. Large changes in moisture content after bonding 

10–14

General Technical Report FPL–GTR–190
will cause shrinking or swelling that can seriously weaken 

both wood and joint and can cause warping, twisting, and 

surface irregularities. Wood should not be bonded at high 

moisture content, particularly high-density wood that has a 

large coefficient of shrinkage, unless the in-service moisture 

content is also expected to be high.

The moisture content of wood products should be targeted 

to the equilibrium moisture content (EMC) that the product 

will experience in service. The regional average EMC values of wood in building interiors are shown in Figure 10–5. 

The average moisture content for most of the United States 

is 8%. Average moisture content increases to 11% along the 

Atlantic and Gulf coastal regions; in the arid southwest, the 

EMC is relatively low at 6%. The moisture content of wood 

outdoors averages near 12% and ranges from 7% to 14% 

in most of the United States. During winter in the northern 

states, heating of indoor air that is normally dry lowers 

wood EMC to 4% to 5% but can raise the moisture levels 

within the walls. Furniture manufactured in the southeast 

at 11% EMC, then sold or moved to northern states where 

EMC drops to 4%, may experience some splitting, delamination of joints, or other noticeable appearance defects. 

Manufacturers of bonded wood products must be aware of 

these regional and seasonal variations to condition the wood 

and bond it at moisture content levels consistent with regional service conditions.

Wood should be dry enough so that even if moisture is 

added during bonding, the moisture content of the product 

is at about the level expected for the assembly in service. 

In lumber laminates, the proportion of glue to wood is so 

low that a waterborne adhesive adds only 1% to 2% to the 

total moisture content of the laminate. In particleboard or 

fiberboard, however, the water in the adhesive can be 3% 

to 7% of the wood weight. During hot pressing, some water 

evaporates when the board is removed from the press; to 

minimize deformation and prevent steam blisters or blows, 

the total moisture content of the assembly should not exceed 

10%. Lumber moisture content of 6% to 7%, assuming 1% 

to 2% will be added by aqueous adhesives, is satisfactory 

for cold pressing of furniture and interior millwork. Lumber 

laminated for exterior use should contain 10% to 12% moisture before bonding. Moisture content of 3% to 5% in veneer at the time of hot pressing is satisfactory for hardwood 

plywood intended for furniture and interior millwork and for 

softwood plywood intended for construction and industrial 

uses.

Lumber that has been kiln dried to the approximate average moisture content intended for bonding may nonetheless 

vary in moisture content level between boards and within 

individual boards. Large differences in moisture content 

between adjacent boards result in considerable stress on 

the common joint as the boards equalize toward a common 

moisture content. Best results are achieved when differences 

in moisture content are not greater than about 5% for lower 

density species and 2% for high-density species.

Surface Preparation

The section Surface Properties of Wood for Bonding covers 

the detailed relationships between surface condition and adhesive bond performance. Wood surfaces are best prepared 

for maximum adhesive wetting, flow, and penetration by 

removing all materials that might interfere with bond formation to sound wood. Ideally, wood should be knife-planed 

within 24 h of adhesive spreading. However, other surfacing 

methods have been used successfully for certain types of 

bonded joints, including sawing for furniture and millwork, 

knife-cutting for veneer, and abrasive-planing for panels. All 

methods must produce smooth, flat, parallel surfaces, free 

from machining irregularities, such as burnishes, skips, and 

crushed, torn, and chipped grain. Properly planed flat surfaces help ensure uniform adhesive spread rate.

Spreading of Adhesive

Regardless of method used for lamination, the purpose in 

spreading the adhesive is to distribute uniformly an adequate amount of adhesive over the bonding area, so that 

under pressure, the adhesive will flow into a uniformly thin 

layer. The amount of adhesive needed will depend on wood 

species, surface quality of wood, moisture content, type of 

adhesive, temperature and humidity of the air, assembly 

time, and application of adhesive to one or both surfaces. 

Adhesives can be spread by hand with brush, roller, or beadextruder, but in manufacturing, adhesives are applied mechanically, such as by roll-spreader, extruder, curtain-coater, 

or spray. Instead of applying a uniform film, extruders apply 

continuous, uniformly spaced beads of discreet diameter 

and flow rate (Fig. 10–6). Figure 10–7 shows the use of a 

pressurized extruder in the field to apply a single bead of 

elastomeric construction adhesive to joists for a plywood 

floor system.

For composite manufacturing involving flakes, strands, particles, or fibers, the adhesive is applied as a slow stream or 

as droplets using a spray nozzle or spinning disc, and then 

10–15

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

Figure 10–5. Average equilibrium moisture content 

(EMC) for wood in building interiors in U.S. regions.
distributed with a drum blender, kneader, or tube blender. 

These binder adhesives hold the product together by a series of joints similar to spot welds rather than a continuous 

film. Microscopic analysis of droplet size and distribution 

illustrates adhesive distribution and its influence on board 

properties.

Assembly and Pressing

Adhesive viscosity is important during application, open 

time, closed time, and pressing. Sometimes keeping 

the viscosity correct throughout this process requires 

balancing a variety of factors. The relationship between adhesive viscosity and bonding pressure is illustrated in Figure 

10–8. Viscosity strongly affects wetting, flow, penetration, 

and, particularly, transfer of adhesive to opposing wood 

surfaces when pressure is applied to the assembly. Adhesive 

viscosity depends upon type of adhesive, type and quantity 

of solvent, age of adhesive mixture, and temperature. After 

application, adhesive viscosity will change depending on 

the amount of adhesive spread; species, moisture content, 

and temperature of wood; temperature and humidity of surrounding air; and evaporation and absorption of solvent. 

When the adhesive-covered surfaces remain open before 

assembly (open assembly), the adhesive thickens by losing solvent to the air by evaporation and to the wood by 

absorption. Bringing the adhesive-covered surfaces together 

(closed assembly) stops evaporation but not absorption. 

Cold-setting waterborne wood adhesives lose water by absorption and evaporation, so that viscosity steadily increases 

until the adhesives eventually set. Thermosetting waterborne 

adhesives also dry out, but they continue to flow to some 

extent in the presence of heat, eventually hardening by 

chemical reaction.

Pressure during bond assembly serves several useful 

purposes by

•	 forcing trapped air from the joint,

•	 bringing adhesive into molecular contact with the wood 

surfaces,

•	 forcing adhesive to penetrate into the wood structure for 

more effective surface adhesion and mechanical interlocking,

•	 squeezing the adhesive into a thin film, and

•	 holding the assembly in position while the adhesive 

cures.

If pressure is too high, however, the adhesive can be forced 

so deeply into (or in some cases out of) the wood that there 

is insufficient adhesive to fill the bondline. These conditions 

of overpenetration and excess squeeze-out result in a starved 

joint and produce inferior bond strength (Fig. 10–8). Overpenetration is especially common in low-density woods, 

whereas excess squeeze-out is common in high-density 

woods. The strongest joints are made with moderately high 

clamping pressure for the wood density, using adhesive with 

viscosity high enough to avoid overpenetration and excess 

squeeze-out at that pressure.

Low pressures near 0.7 MPa (100 lb in–2) are suitable for 

low-density wood because the surfaces easily conform to 

each other, thus ensuring intimate contact between adhesive 

and wood. High pressures up to 1.7 MPa (247 lb in–2) are 

required for the highest density woods, which are difficult 

to compress. Small areas of flat, well-planed surfaces can be 

bonded satisfactorily at lower pressures.

Because adhesives become thicker after they are applied 

to the wood and some start to cure immediately, assembly 

times can be very important. Some adhesives require time 

10–16

Figure 10–6. An extruder applies continuous and uniformly sized and spaced beads of adhesive to veneer 

for laminating into laminated veneer lumber (LVL).

Figure 10–7. A pressurized extruder applies a single 

bead of elastomeric construction adhesive to floor 

joists for assembly of a plywood floor system.

General Technical Report FPL–GTR–190
before pressing to allow solvents to evaporate or adsorb into 

the wood, so that the adhesive is thick enough to avoid 

overpenetration when pressure is applied. On the other hand, 

adhesives that dry or cure too much before pressing do not 

transfer, or wet the opposite surface, resulting in thick, weak 

bondlines.

Bonded material should be kept under pressure until the 

adhesive is strong enough to resist any forces that may 

cause parts to shift or open gaps in the bondline. When coldpressing lumber under normal conditions, this stage can be 

reached in as little as 15 min or as long as 24 h, depending 

on adhesive temperature and curing characteristics and the 

absorptive characteristics of the wood. During hot pressing, 

the time under pressure varies with temperature of platens, 

thickness of the assembly and species of wood, and adhesive formulation. Typical hot-pressing times are 2 to 15 min, 

and up to 30 min for very thick laminates. High-frequency 

heating can reduce the time under pressure to less than 

3 min. High-power radio frequency energy can travel 

through wood but is strongly absorbed by the water in 

adhesives, causing selective heating of the adhesive. Highfrequency curing is commonly used for bonding lumber; 

forming end- and edge-grain joints; patching, scarfing, and 

fingerjointing plywood; and manufacturing various panel 

products. With high frequency, press times can be shorter 

than 30 s, as with parquet production using 4- by 10- by 

2-mm plies. Careful control of power and press time is essential to prevent arcing, or to control the more common 

problem of steam pressure that could blow apart the product.

With the stiff structural adhesives (phenol-, resorcinol-, 

melamine-formaldehyde), the strongest bonds generally 

have bondlines between 0.08 and 0.15 mm (1/32 and 

1/16 in.) thick. Thinner bondlines do not effectively transfer 

stresses, particularly stresses from moisture-induced dimensional changes. As these bondlines become thicker, they become weaker and fracture more easily. These adhesives also 

contain solvents, which cause the adhesive to shrink upon 

curing and even leave voids. Thick bondlines result from 

inadequate pressure or incorrect adhesive consistency. When 

rough, warped, or poorly mated surfaces are joined, pressure 

will be uneven along the bondline. As a result, the adhesive 

flow from the areas of very high pressure to those of little 

to no pressure will result in very thick bondlines. Both the 

starved and thick areas of the bondline lead to weak bonds.

For composites, the adhesive must have enough strength to 

withstand the steam pressure inside the panel as the applied 

press pressure is released. If the adhesive is not sufficiently 

strong, the internal steam pressure will cause a large delamination (blow) within the product. As the size of the composite increases, there is less relative area for steam escape and 

the chance of delamination increases. Dry wood, high solids 

adhesives, less adhesive with better distribution, and faster 

curing adhesives can decrease the problem of delamination.

Post-Cure Conditioning

In the process of bonding edge-grain joints, the wood in the 

joint absorbs moisture from the adhesive, then swells. If the 

bonded assembly is surfaced before this excess moisture is 

evaporated or absorbed uniformly, more wood is removed 

along the swollen joint than elsewhere. Later, when the 

added moisture evaporates, the wood in the joint shrinks 

beneath the surface. These sunken bondlines become very 

conspicuous under a high-gloss finish. This is particularly 

important when using adhesives containing large amounts 

of water. Moisture can be redistributed by conditioning the 

bonded assembly for 24 h at 70 °C (158 °F), for 4 days at 

50 °C (122 °F), or at least 7 days at room temperature before surfacing. In each case, the relative humidity must be 

adjusted to prevent drying the wood below the target moisture content.

Conditioning to the moisture content of service is especially 

important for plywood, veneers, and other composites made 

of thin layers. During room-temperature bonding, water 

often needs to be removed, which can be done by controlling humidity on a time schedule. If room-temperaturebonded products are dried too much, warping, checking, 

and debonding increase markedly. Softwood plywood is 

often very dry after hot pressing, which can be corrected by 

spraying the hot panels and stacking them tightly to allow 

the moisture to diffuse uniformly. This process also restores 

some of the panel thickness lost by compression during 

hot pressing and apparently minimizes warping in service. 

Many composite panels need time after pressing for the adhesive to cure completely and for the moisture to equilibrate 

throughout the panel.

10–17

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

Figure 10–8. Relationship between adhesive consistency and bonding pressure for affect on bond formation using thermosetting adhesive.
Bonded Joints

Edge-Grain Joints

Edge-grain joints (Fig. 10–9A) can be almost as strong as 

the wood in shear parallel to the grain, tension across 

the grain, and cleavage. The tongue-and-groove joint 

(Fig. 10–9B) and other shaped edge-grain joints have a 

theoretical strength advantage because of greater surface 

area than the straight, edge-grain joints, but they do not 

produce higher strength. The theoretical advantage is lost, 

wholly or partly, because the shaped sides of the two mating 

surfaces cannot be machined precisely enough to produce 

the perfect fit that will distribute pressure uniformly over 

the entire joint area. Because of poor contact, the effective 

bonding area and strength can actually be less in a shaped 

joint than on a flat surface. Tongue-and-groove and other 

shaped joints have the advantage that the parts can be quickly aligned in clamps or presses. A shallow-cut tongue-andgroove is just as useful in this respect as a deeper cut, and 

less wood is wasted.

End-Grain Joints

It is practically impossible to make end-grain butt joints 

(Fig. 10–10A) strong enough to meet the requirements of 

ordinary service with conventional bonding techniques. 

Even with special techniques, butt joints reach only about 

25% of the tensile strength of the wood parallel-to-grain. To 

approximate the tensile strength of clear solid wood, a scarf 

joint or fingerjoint (Fig. 10–10B–E) should have a surface 

area at least 10 times greater than the cross-sectional area of 

the piece, because wood is approximately 10 times stronger 

in tension than in shear. Joints cut with a slope of 1 in 12 or 

flatter (12 times the cross-sectional area) produce the highest strength. In plywood scarf and finger joints, a slope of 

1 in 8 (8 times the cross-sectional area) is typical for structural products. For nonstructural, low-strength joints, these 

requirements are unnecessary.

When fingerjoints are cut with a high slope, such as 1 in 12, 

the tip thickness must be no greater than 0.8 mm (1/32 in.). 

A thickness of 0.4 to 0.8 mm (1/64 to 1/32 in.) is about the 

practical minimum for machined tips. Sharper tips are possible using dies that are forced into the end grain of the board.

Fingerjoints can be cut with the profile showing either on 

the wide face (vertical joint) (Fig. 10–10C) or on the edge 

(horizontal joint) (Fig. 10–10E). Vertical joints have greater 

area for designing shapes of fingers but require a longer cutting head with more knives. Vertical joints also cure faster 

than horizontal joints in high-frequency heating. A nonstructural fingerjoint, with fingers much shorter than in the two 

structural fingerjoints, is shown in Figure 10–10E.

A well-manufactured scarf, finger, or lap joint in end grain 

can have up to 90% of the tensile strength of clear wood and 

exhibit behavior much like that of clear wood. However, the 

cycles-to-failure for a well-manufactured end joint are often 

lower than for clear wood.

End-to-Edge-Grain Joints

It is difficult to design a plain end-to-edge-grain joint 

(Fig. 10–11A) capable of carrying appreciable loading. As a 

result, it is necessary to design these joints with interlocking 

surfaces so that edge grain of the interlocking piece bonds 

to the edge grain of the adjoining piece. Increasing the joint 

surface area also helps by providing more bondline to transfer load. Some examples of strong connections are dowels, 

mortise and tenons, and rabbets (Fig. 10–11). Because wood 

swells so much more across the grain than along the grain, 

moisture changes in these joints produce large internal 

stresses. All end-to-edge-grain joints should be protected 

from changes in moisture content in service.

10–18

Figure 10–9. Edge-grain joints: A, plain; B, tongueand-groove.

Figure 10–10. End-grain joints: A, butt; B, plain 

scarf; C, vertical structural fingerjoint; D, horizontal 

structural fingerjoint; E, nonstructural fingerjoint.

General Technical Report FPL–GTR–190
10–19

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

Construction Joints

Elastomeric construction adhesives are commonly used 

in the light-frame construction industry for field assembly 

of panelized floor and wall systems. Structural panels are 

bonded to floor joists and wall studs with mastic adhesives 

that have the unique capability of bridging gaps up to 

6.5 mm (1/4 in.) between rough and poorly fitting surfaces 

(Fig. 10–12). Without any premixing, the adhesive is extruded in a bead along framing members with a hand-held caulking gun or a pressurized dispenser similar to that shown in 

Figure 10–7. Nails or screws provide the only pressure for 

bonding, and they hold materials in position while the adhesive sets. Elastomerics are also uniquely tolerant of the temperature and moisture content variations at field construction 

sites. Although they do not deliver the strength and durability of conventional structural adhesives, elastomerics are 

strong and flexible enough to give long-term performance 

under most conditions of installation and service.

Construction adhesives enable a nailed or screwed floor 

system to act to some degree as a composite assembly with 

increased stiffness. Greater stiffness permits joists to be 

longer and spaced more widely, with one layer of plywood 

subflooring replacing two. Floors are less bouncy with fewer 

squeaks and nail pops or screw pulls. However, structural 

design of the composite assembly is based only on the increased stiffness of nailed or screwed panel and framing materials. The strength contributed by the adhesive cannot be 

factored into the engineering design but provides increased 

value to the homeowner.

Testing and Performance

Testing is necessary to ensure that adhesively bonded materials hold together within a given service environment for 

the life of the structure. Many methods are available to test 

bonding performance, particularly for bonded assemblies. 

Generally, these testing methods attempt to predict how 

bonded joints are likely to perform in a specific loading 

mode (shear, tensile, creep) in an assembly at specific 

temperature and moisture conditions for a specific time.

Most performance tests are short term. They are based on 

chemical, mechanical, and rheological laboratory tests of 

adhesive polymers, adhesives, and bonds. Intermediate-term 

tests of products that are conducted in pilot operations and 

field experiments are integrated with short-term laboratory 

tests in an effort to extrapolate these data into long-term performance. Long-term tests of bonded assemblies under actual environmental exposures are conducted, but this information may not be available for 10 to 30 years. Therefore, 

short-term tests are extensively used to predict long-term 

performance. As we learn the relationships between chemical structure and mechanical performance, and as companies 

are under continued pressure to launch new products, the 

reliance on short-term testing is increasing.

Analytical, Chemical, and Mechanical Testing 

of Polymers

Although many methods of characterizing adhesives are 

available, this section only briefly mentions some of the 

most important and common methods.

Figure 10–11. End-to-edge-grain joints: A, plain; B, 

miter; C, dowel; D, mortise and tenon; E, dado tongue 

and rabbet; F, slip or lock corner; G, dovetail; H, 

blocked; I, tongue-and-groove.

Figure 10–12. Gap-filling construction adhesive in 

field-assembled plywood floor system.
Nuclear magnetic resonance (NMR) spectroscopy and other 

spectroscopic techniques help characterize the molecular 

structures of adhesive polymers. Molecular size distribution 

is commonly measured by gel permeation chromatography (GPC), also known as size exclusion chromatography 

(SEC). Differential scanning calorimetry (DSC) and gel 

times provide information on rates of chemical curing reactions. The rheological properties of curing and cured adhesives are characterized by dynamic mechanical analysis 

(DMA) and torsional-braid analysis (TBA). Sophisticated 

fracture mechanics techniques are used to measure toughness of adhesive bonds as they fail in a cleavage mode. 

High-magnification microscopes, including scanning electron microscope, transmission electron microscope, and 

atomic force microscope, enable scientists to see wood and 

adhesive surfaces in minute detail before, during, and after 

fracture. Fluorescent and confocal microscopes provide excellent information on adhesive distribution, adhesive penetration, and bond fracture surfaces because of their ability 

to distinguish between wood and adhesive.

Although much can be learned from measurements of chemical, mechanical, and rheological properties of polymers and 

adhesives before their application to wood, no correlation 

between laboratory test and product performance is perfect. 

There is no substitute for testing performance in bonded 

assemblies prepared with specific adhesives and materials 

and tested under specific loading modes, environmental 

conditions, and duration of loading. When adhesives are 

formulated through a blend of scientific analysis and art of 

formulation, they are tested for strength and durability in 

the laboratory and field, usually by industry-accepted standard test methods and product specifications.

Mechanical Testing of Bonded Assemblies

In an attempt to promote communication and understanding, there are many standardized test methods for evaluating 

and comparing the performance of different materials. Most 

test methods, specifications, and practices for adhesives and 

bonded assemblies are consensus standards. ASTM International publishes a book of standard methods each year 

(ASTM [Current edition]). Several trade associations have 

their own specifications and performance standards that apply to their specific wood products. The Federal government 

also has specifications that are used by the General Services 

Administration to purchase products. In all test modes, 

specific materials, conditions of materials and testing, and 

testing procedures are completely specified to ensure repeatability and enable valid comparisons of data.

Two basic failure modes, shear and cleavage, are commonly 

used to test adhesive bonds to determine strength levels 

during impact, flexure, fatigue, and creep under long-term 

stress (Fig. 10–13). The following describes the basic stress 

modes in adhesive-bonded joints:

•	 Shear, resulting from forces applied parallel to the 

bondline, either in compression or tension

•	 Cleavage, resulting from forces applied perpendicular to the bondline. These forces may be applied by a 

wedge or other crack-opening device, by pulling on a 

double cantilever beam, or by pulling two faces apart, 

such as in a section of particleboard. Tensile loads often 

result in cleavage failures.

As the names imply, impact, fatigue, and creep are tests that 

pertain to the rate at which loads are applied. Standard testing is done so that load continues to increase until failure, 

typically occurring between 1 and 5 min. Impact loads are 

sudden; for example, hitting a specimen with a swinging 

arm. Fatigue is the loss in strength from repeated loading 

and unloading to reflect bond deterioration from mechanical stresses. Sometimes, environmental stresses such as 

moisture and temperature are added as well. Creep loads are 

static loads applied for long times, from a few days to years, 

usually under extreme environmental conditions.

The common measures used to estimate potential performance of bonded wood joints are strength, wood failure, and 

delamination. The highest performance level after exposure 

to severe environmental conditions is bond strength greater 

than wood strength, wood failure in more than 85% of the 

bonded area, and less than 5% or 8% delamination of the 

joint, for softwoods and hardwoods, respectively. These performance values reflect how wood, adhesive, and environmental exposure interact in response to loading.

Exceeding the strength of wood is an essential performance criterion, often more important than measured shear 

strength. Percentage wood failure is the amount of wood 

that fails as a percentage of the area of the bonded joint. In 

general, strong and durable bonds give high wood failure 

and fracture deep into the grain of the wood. If wood failure 

is shallow with only wood fibers remaining attached to the 

adhesive film, bond strength and probably durability are 

lacking. Thus, a consistently high level of wood failure, 

above 75% in some standards and above 85% in others, 

means that the shear strength associated with these bonds is 

a good estimate of the load-carrying capability of the joint. 

Figure 10–13. Failure modes of adhesive bonds.

10–20

General Technical Report FPL–GTR–190
High levels of wood failure in a wet and hot environment 

suggest that the adhesive bond is as strong as the wood. If 

cycles of alternate wetting and drying were included with 

cycles of wet and hot conditions, then high wood failure 

would indicate even more durable bonds. High wood failure 

in shear tests of water-saturated bonds is also a strong indicator of bond durability. Wood failure is considered a valid 

measure of bond strength only to solid wood, not to reconstituted products made of bonded wood particles.

High shear strength and high wood failure are not sufficient 

indicators of bond durability. Delamination is an indicator 

of how well the bonded joint withstands severe swelling and 

shrinking stresses in the presence of high moisture and heat. 

Delamination is the separation between laminates because 

of adhesive failure, either in the adhesive or at the interface 

between adhesive and wood. If adhesives are able to resist 

delaminating forces, any wood failure will occur adjacent to 

the bondline, but not within the adhesive. Delamination of 

adhesives in structural laminated wood products exposed to 

the cyclic delamination test in ASTM D 2559 cannot exceed 

5% in softwoods and 8% in hardwoods.

Bonds in structural assemblies are expected to exceed the 

strength of the wood, so in traditional design of joints, adhesive strength has been ignored. Traditionally, adhesives that 

are not as strong as the wood simply have not been used in 

structural applications because methods for determining allowable mechanical properties of adhesives for engineering 

design had not been developed. One such method now exists—ASTM D 5574.

Short- and Long-Term Performance

In the short term, mechanical properties of wood, adhesives, 

and bonded products vary with specific environmental exposure. In most cases, all properties decrease as temperature 

and moisture levels increase. Strength and stiffness may 

return to their original levels if the yield points of the materials are not exceeded while under load. Wood properties 

degrade faster under heat and moisture than do rigid thermosetting adhesives like resorcinol-, phenol-, and melamineformaldehyde, but this is not true for urea-formaldehyde. 

Therefore, evaluating short-term performance of products 

made with these adhesives is simply a matter of testing 

bonds at room temperature in dry and wet conditions. With 

increased moisture and/or heat, thermoplastic adhesives 

such as poly(vinyl acetate), elastomerics, hot-melts, pressure-sensitive adhesives, soy and casein tend to lose stiffness and strength more rapidly than does wood. These adhesives must be tested dry, dry after water soaking, and after 

prolonged exposure to high humidity environments. In addition, some specifications require testing bonded structural 

and nonstructural products at elevated temperatures similar 

to what might be encountered in roofs or enclosed shipping 

containers. A short-term dead-load test at elevated 

temperatures may also be required. Adhesive specifications for structural products such as laminated beams and 

plywood require high minimum strength and wood failure 

values after several different water exposure tests. Adhesive 

bonds in laminated beams must show very little delamination after exposure to severe cyclic moisture content and 

temperature changes.

Long-term deterioration of wood, adhesives, and bonded 

products is determined by the levels of temperature, moisture, and stress, and, in some instances, by concentrations 

of chemicals and presence of microorganisms. Long-term 

performance is the ability of a product to resist loss of a 

measured mechanical property over the time of exposure. 

A durable bonded product is one that shows no greater loss 

of properties during its life in service than does solid wood 

of the same species and quality.

Many adhesives in bonded products have decades of 

documented performance in many environments. Thus, it 

is possible to predict with a high degree of certainty the 

long-term performance of similar products. Well-designed 

and well-made joints with any of the commonly used woodworking adhesives will retain their strength indefinitely if 

the moisture content of the wood does not exceed approximately 15% and if the temperature remains within the range 

of human comfort. However, some adhesives deteriorate 

when exposed either intermittently or continuously to temperatures greater than 38 °C (100 °F) for long periods. Low 

temperatures seem to have no significant effect on strength 

of bonded joints.

Products made with phenol-formaldehyde, resorcinol-formaldehyde, and phenol-resorcinol-formaldehyde adhesives 

have proven to be more durable than wood when exposed to 

warm and humid environments, water, alternate wetting and 

drying, and even temperatures high enough to char wood. 

These adhesives are adequate for use in products that are 

exposed to the weather indefinitely (Fig. 10–14).

Well-made products with melamine-, melamine-urea-, and 

urea-formaldehyde resin adhesives have proven to be less 

durable than wood. Melamine-formaldehyde is only slightly 

less durable than phenol-formaldehyde or resorcinolformaldehyde and is considered acceptable for structural 

products. Although considered less durable, melamineurea-formaldehyde is also accepted in structural products 

at a melamine:urea ratio of 60:40. Urea-formaldehyde 

resin is susceptible to deterioration by heat and moisture 

(Fig. 10–14).

Products bonded with poly(vinyl acetate) and protein-based 

adhesives will not withstand prolonged exposure to water 

or repeated high–low moisture content cycling in bonds of 

high-density woods. However, if properly formulated, these 

adhesives are durable in a normal interior environment. The 

use of poly(vinyl acetate) adhesives is prohibited for some 

structural applications.

Some isocyanate, epoxy, polyurethane, emulsion polymer 

isocyanates, and cross-linked poly(vinyl acetate) adhesives 

10–21

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance
are durable enough to use on lower density species even 

under exterior conditions, but for most of these adhesives, 

exterior exposure must be limited. Some elastomer-based 

adhesives may be durable enough for limited exposure to 

moisture with lower density species in nonstructural applications or in structural applications when used in conjunction with approved nailing schedules. Polyurethane adhesives that chemically cure and remain flexible are among the 

most durable construction adhesives.

New adhesives do not have a history of long-term performance in service environments, so accelerated laboratory 

exposures that include cycles of heat, moisture, and stress 

are used to estimate long-term performance. However, laboratory exposures cannot duplicate the actual conditions of a 

service environment. Estimates of long-term performance 

can be obtained by exposing specimens outdoors for up to 

30 years. Outdoor exposures may be intensified by facing 

specimens south at an angle perpendicular to the noonday 

sun and by establishing exposure sites in regions with the 

most extreme service environments, for example, southern 

coastal and arid southwestern regions. Only four long-term 

laboratory aging methods have been standardized, and none 

specifies minimum performance levels because the bonded 

product is the item that must meet code standards. Therefore, performance of any new adhesive or bonded product 

must be compared with performance of established adhesives or products tested in the same laboratory exposure.

Product Quality Assurance

After the short- and long-term performance of a product has 

been established, maintenance of the manufacturing process 

to ensure that the product will be made and perform at that 

level is the major concern of a quality-assurance program, 

which consists of three parts:

1. Establishing limits on bonding process factors that will 

ensure acceptable joints and product

2. Monitoring production processes and bond quality in 

joints and product

3. Detecting unacceptable joints and product, determining 

the cause, and correcting the problem

The structural panel, laminated-beam, particleboard, millwork, and other industrial trade associations have established quality-assurance programs that effectively monitor 

the joint and product performance at the time of manufacture for compliance with voluntary product standards. 

Product performance is usually evaluated immediately after 

manufacture by subjecting specimens from the product to a 

series of swell–shrink cycles. The treatments are more rigorous for products intended for exterior exposure. For example, exterior softwood plywood is subjected to two boil–dry 

cycles, while interior plywood is subjected to a single soak–

dry cycle at room temperature. After exposure, specimens 

are evaluated for delamination, percentage wood failure, or 

both. Test results are compared with the minimum requirement in the trade association’s standards. Lengthy experience and correlations between exterior performance and 

accelerated laboratory tests have shown that products with 

at least the minimum values will probably perform satisfactorily in service. If the product meets the requirement, it is 

certified by the association as meeting the standard for satisfactory performance.

Standards

AITC. [Current edition]. Inspection manual for structural 

glued laminated timber, AITC 200. Englewood, CO: American Institute of Timber Construction.

ANSI/AITC. [Current edition]. American national standard 

for wood products–structural glued laminated timber, ANSI/

AITC A190.1. Englewood, CO: American Institute of Timber Construction.

APA. [Current edition]. Performance standards and policies for structural-use panels, APA PRP–108. Tacoma, WA: 

APA–The Engineered Wood Association.

APA. [Current edition]. U.S. product standard PS1-07 for 

construction and industrial plywood with typical APA 

trademarks. Tacoma, WA: APA–The Engineered Wood 

Association.

ASTM. [Current edition]. Annual book of ASTM standards, 

Vol. 15.06 Adhesives. West Conshohocken, PA: American 

Society for Testing and Materials. Many of the standards 

are related to wood bonding for specific types of 

applications.

CPA. [Current edition.] Particleboard, ANSI A208.1. Leesburg, VA: Composite Panel Association.

CPA. [Current edition]. Medium density fiberboard. ANSI 

A208.2. Leesburg, VA: Composite Panel Association.

Figure 10–14. Relative rates of deterioration of 

bond strength of small specimens exposed directly to weather.

10–22

General Technical Report FPL–GTR–190
Hardwood Plywood & Veneer Association. [Current edition]. American national standard for hardwood and decorative plywood, ANSI/HPVA HP–1. Reston, VA.

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Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance
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10–24

General Technical Report FPL–GTR–190
CHAPTER 11

Wood-Based Composite Materials

Panel Products, Glued-Laminated Timber, Structural 

Composite Lumber, and Wood–Nonwood Composite 

Materials

Nicole M. Stark, Research Chemical Engineer

Zhiyong Cai, Supervisory Research Materials Engineer

Charles Carll, Research Forest Products Technologist

11–1

Contents 

Scope 11–2

Conventional Wood-Based Composite Panels 11–2

 Elements 11–2

 Adhesives 11–3

 Additives 11–5

 Plywood 11–5

 Oriented Strandboard 11–7

 Particleboard 11–10

 Fiberboard 11–12

 Speciality Composite Materials 11–15

 Performance and Standards 11–15

Glulam Timber 11–17

 Advantages 11–17

 Types of Glulam Combinations 11–17

 Standards and Specifications 11–18

 Manufacture 11–19

Structural Composite Lumber 11–20

 Laminated Veneer Lumber 11–21

 Parallel Strand Lumber 11–21

 Laminated Strand Lumber and Oriented Stand 

 Lumber 11–21

 Advantages and Uses 11–21

 Standards and Specifications 11–22

Wood–Nonwood Composite Materials 11–22

 Inorganic-Bonded Composite Materials 11–23

 Wood–Thermoplastic Composite Materials 11–25

Literature Cited 11–26

The term composite is being used in this chapter to describe 

any wood material adhesively bonded together. Wood-based 

composites encompass a range of products, from fiberboard 

to laminated beams. Wood-based composites are used for a 

number of nonstructural and structural applications in product lines ranging from panels for interior covering purposes 

to panels for exterior uses and in furniture and support structures in buildings (Fig. 11–1). Maloney (1986) proposed 

a classification system to logically categorize the array of 

wood-based composites. The classification in Table 11-1 

reflects the latest product developments.

The basic element for wood-based composites is the fiber, 

with larger particles composed of many fibers. Elements 

used in the production of wood-based composites can be 

made in a variety of sizes and shapes. Typical elements include fibers, particles, flakes, veneers, laminates, or lumber. 

Figure 11–2 shows the variation and relative size of wood 

elements. Element size and geometry largely dictate the 

product manufactured and product performance. Performance standards are in place for many conventional woodbased composite products (Table 11–2).

A variety of wood sources are appropriate for use in woodbased composites. Wood with localized defects (such as 

knots) can often be used effectively in wood-based composites. Reducing wood with defects to wood elements mitigates the influence of these characteristics in the manufactured products. Recovered wood from construction waste or 

industrial manufacturing processes, and wood derived from 

small-diameter timber, forest residues, or exotic and invasive species, may also be effectively used in wood-based 

composites. Because natural wood properties vary among 

species, between trees of the same species, and between 

pieces from the same tree, solid wood cannot match composite products in the uniformity and range of properties 

that can be controlled. 
11–2

Scope

This chapter gives an overview of the general types and 

composition of wood-based composite products and the materials and processes used to manufacture them. It describes 

conventional wood-based composite panels and structural 

composite materials intended for general construction, 

interior use, or both. This chapter also describes wood–

nonwood composites. Mechanical properties of these types 

of composites are presented and discussed in Chapter 12. 

Because wood-based composites come in a variety of forms, 

we briefly describe several of the most common commercial 

products. 

This chapter is organized into three sections. The first section covers conventional wood-based composite panels. 

Materials, adhesives, and additives common to conventional 

wood-based composites are summarized. Specific products 

addressed include panel products such as plywood, oriented 

strandboard, particleboard, and fiberboard. Specialty 

composites are also discussed. The second section covers 

structural composite lumber, including glued-laminated 

timber, laminated veneer lumber, parallel strand lumber, 

laminated strand lumber, and oriented strand lumber. Wood–

nonwood composites are discussed in the third section, 

including inorganic-bonded composites and wood–thermoplastic composites. Books have been written about each of 

these categories, and the constraints of this chapter necessitate that the discussion be general and brief. References are 

provided for more detailed information.

Conventional Wood-Based 

Composite Panels

Conventional wood-based composites are manufactured 

products made primarily from wood with only a few percent 

resin and other additives. A useful way to classify conventional wood-based composites based on specific gravity, 

density, raw material, and processing methods is shown in 

Figure 11–3, which presents an overview of the most common types of commercial panel products discussed in this 

chapter and a quick reference to how these composite materials compare with solid wood from the standpoint of density and general processing considerations. The raw material 

classifications of fibers, particles, and veneers are shown on 

the left y-axis. Specific gravity and density are shown on 

the top and bottom horizontal axes (x-axes), respectively. 

The right y-axis, wet and dry processes, describes in general 

terms the processing method used to produce a particular 

product. Selection of wood elements, adhesives, and processing techniques all contribute to product performance. 

Figure 11–4 shows examples of some commercial woodbased composites. 

Elements

The primary component of wood-based composites is the 

wood element, often 94% or more by mass. Common elements for conventional wood-based composites include 

veneers, strands, particles, and fibers. The physical characteristics of common elements can be seen in Figure 11–5. 

Properties of composite materials can be changed by changing the size and geometry of the elements and by combining, reorganizing, or stratifying elements. 

Table 11–1. Classification of woodbased compositesa

Veneer-based material 

Plywood 

Laminated veneer lumber (LVL) 

Parallel-strand lumber (PSL) 

Laminates

Glue-laminated timbers 

Overlayed materials 

Laminated wood–nonwood compositesb

Multiwood composites (COM-PLYc

)

Composite material 

Fiberboard (low-, medium-, or high-density) 

Cellulosic fiberboard 

Hardboard

Particleboard

Waferboard

Flakeboard

Oriented strandboard (OSB) 

Laminated strand lumber (LSL) 

Oriented strand lumber (OSL) 

Wood–nonwood composites 

Wood fiber–polymer composites 

Inorganic-bonded composites 

a

Adapted from Maloney (1986). 

b

Panels or shaped materials combined with nonwood 

materials such as metal, plastic, and fiberglass. 

c

Registered trademark of APA–The Engineered 

Wood Association. 

Figure 11–1. Wood-based composites used in the new 

Centennial Research Facility at the Forest Products 

Laboratory. Glulam timbers support composite I-joists 

and plywood sheathing. (Photo by Steve Schmieding, 

Forest Products Laboratory.)

General Technical Report FPL–GTR–190
11–3

Chapter 11 Wood-Based Composite Materials

Adhesives

Bonding in most conventional wood-based composites 

is provided by thermosetting (heat-curing) adhesive

resins. Chapter 9 provides a more thorough discussion of 

thermoset adhesive resins. Commonly used resin–binder 

systems include phenol-formaldehyde, urea-formaldehyde, 

melamine-formaldehyde, and isocyanate.

Phenol-Formaldehyde

Phenol-formaldehyde (PF) resins are typically used in the 

manufacture of construction plywood and oriented strandboard where exposure to weather during construction is a 

concern. Other moisture exposure situations, such as temporary weather exposure, occasional plumbing leaks, or wet 

foot traffic, may also necessitate the use of PF resins. PF 

resins are commonly referred to as phenolic resins. Phenolic 

resins are relatively slow-curing compared with other thermosetting resins. In hot-pressed wood-based composites, 

use of phenolic resin necessitates longer press times and 

higher press temperatures. Hot-stacking of pressed material 

shortly after emergence from the press is a fairly common 

industrial practice, used to attain adequate resin cure without greatly extending press time. Significant heat exposure 

associated with pressing of phenolic-bonded composites 

commonly results in a noticeable reduction in their hygroscopicity. Cured phenolic resins remain chemically stable 

at elevated temperatures. Their bonds also are sometimes 

referred to as being “boil-proof” because of their ability to 

maintain composite dimensional and mechanical properties 

under wet conditions. The inherently darker color of PF resin compared with other resins may make them aesthetically 

unsuitable for product applications such as interior paneling 

and furniture.

Urea-Formaldehyde

Urea-formaldehyde (UF) resins are typically used in the 

manufacture of products used in interior applications, primarily particleboard and medium-density fiberboard (MDF), 

because moisture exposure leads to a breakdown of the 

bond-forming reactions. Excessive heat exposure will also 

result in chemical breakdown of cured UF resins, therefore 

UF-bonded panels are typically cooled after emergence 

Table 11–2. Commercial product or performance standards for wood-based composites 

Product category Applicable standard Name of standard Source 

Plywood PS 1–07 Voluntary product standard PS 1–07 

construction and industrial plywood 

NIST 2007 

PS 2–04 Voluntary product standard PS 2–04 

performance standard for wood-based 

structural-use panels 

NIST 2004 

HP–1–2004 Voluntary product standard 

HP–1–2004 hardwood and decorative 

plywood 

HPVA 2004 

Oriented strandboard 

(OSB) 

PS 2–04 Voluntary product standard PS 2–04 

performance standard for wood-based 

structural-use panels 

NIST 2004 

Particleboard ANSI A 208.1–2009 Particleboard standard CPA 2009a 

Fiberboard ANSI A 208.2–2009 MDF standard CPA 2009b 

ANSI A 135.4–2004 Basic hardboard CPA 2004a 

ANSI A 135.5–2004 Pre-finished hardboard paneling CPA 2004b 

ANSI A 135.6–2006 Hardboard siding CPA 2006 

ASTM C 208–08a Cellulosic fiberboard ASTM 2008c 

Glued-laminated 

timber (glulam) 

ANSI/AITC 190.1 American National Standard for Wood 

Products—structural glued-laminated 

timber 

AITC 2007a 

Structural composite 

lumber (including 

laminated veneer 

lumber (LVL), 

laminated strand 

lumber (LSL), and 

parallel strand 

lumber (PSL)) 

ASTM D 5456–07 Standard specification for evaluation 

of structural composite lumber products 

ASTM 2008b
11–4

from the press. Advantages of UF resins include lower curing temperatures than PF resins and ease of use under a 

variety of curing conditions. UF resins are the lowest cost 

thermosetting adhesive resins. They offer light color, which 

often is a requirement in the manufacture of decorative 

products. However, the release of formaldehyde from products bonded with UF is a growing health concern.

Melamine-Formaldehyde

Melamine-formaldehyde (MF) resins are used primarily 

for decorative laminates, paper treating, and paper 

coating. They are typically more expensive than PF resins. 

MF resins may, despite their high cost, be used in bonding 

conventional wood-based composites. MF resins are often 

used in combination with UF. MF–UF resins are used when 

an inconspicuous (light color) adhesive is needed and when 

greater water resistance than can be attained with UF resin 

is required.

Isocyanates

The isocyanate wood adhesive is a polymeric methylene diisocyanate (pMDI). It is used as an alternative to PF resin, 

primarily in composite products fabricated from strands. 

pMDI resins are typically more costly than PF resins but 

have more rapid cure rates and will tolerate higher moisture 

contents in the wood source. pMDI resin is sometimes used 

in core layers of strand-based composites, with slowercuring PF resin used in surface layers. Facilities that use 

pMDI are required to take special precautionary protective 

measures because the uncured resin can result in chemical 

sensitization of persons exposed to it. Cured pMDI resin 

poses no recognized health concerns.

Bio-Based Adhesives

Bio-based adhesives, primarily protein glues, were widely 

used prior to the early 1970s in construction plywood. In the 

mid-1970s, they were supplanted by PF adhesives, on the 

basis of the superior bond durability provided by phenolics. 

The move toward “green” products has led to a renewed 

interest in bio-based adhesives. Several soy-protein-based 

resin systems, with bond durabilities similar to those provided by PF resins, have recently been developed and commercialized. Durable adhesive systems may also be derived 

from tannins or from lignin. Tannins are natural phenol 

compounds that are present in the bark of a number of tree 

species. The tannins can be extracted from bark, modified, 

and reacted with formaldehyde to produce an intermediate polymer that is a satisfactory thermosetting adhesive. 

Lignin-based resins have also been developed from spent 

pulping liquor, which is generated when wood is pulped for 

paper or chemical feedstocks. In the manufacture of wetprocess fiberboard, lignin, which is an inherent component 

of lignocellulosic material, is frequently used as binder 

(Suchsland and Woodson 1986), although “natural” lignin 

bonding is sometimes augmented with small amounts of

PF resin.

Figure 11–2. Basic wood elements, from largest to smallest (Kretschmann and others 2007).

General Technical Report FPL–GTR–190
11–5

Resin Choice

Often a particular resin will dominate for a particular product, but each has its advantages. Factors taken into account 

include materials to be bonded together, moisture content 

at time of bonding, mechanical property and durability requirements of the composite products, anticipated end-use 

of the product, and resin system costs.

PF, UF, and pMDI resins systems are expected to remain 

the dominant adhesives used for bonded wood-based 

composites. However, cost and reliable availability of 

petrochemicals may affect the relative predominance of PF, 

UF, and pMDI adhesives versus bio-based adhesives. More 

stringent regulation concerning emissions from formaldehyde-containing products (driven by concern over indoor air 

quality) may affect the continued commercial predominance 

of UF resin in interior products. For example, the California 

Air Resources Board (CARB) has established formaldehyde 

emission standards that cover hardwood plywood, particleboard, and MDF. As a result, bio-based adhesive and resin 

systems may gain market share compared with petroleumbased synthetic resins.

Additives

A number of additives are used in the production of conventional composite products. The most common additive 

is wax, which is used to provide products with some resistance to liquid water absorption. In flake-, particle-, and 

fiberboard products, wax emulsions provide limited-term 

water resistance and dimensional stability when the board is 

wetted. Even small amounts (0.5% to 1%) act to retard the 

rate of liquid water pickup for limited time periods. These 

improved short-term water penetration properties are important for ensuring the success of subsequent secondary gluing 

operations and for providing protection upon accidental wetting of the product during and after construction. The addition of wax has practically no effect on water vapor sorption 

or dimensional changes associated with changes in humidity. Other additives used for specialty products include 

preservatives, moldicides, and fire retardants. Composites 

containing additives are more thoroughly discussed in the 

section on Specialty Composite Materials.

Plywood

Plywood is a panel product built up wholly or primarily of 

sheets of veneer called plies. It is constructed with an odd 

number of layers with the grain direction of adjacent layers 

oriented perpendicular to one another. A layer can consist 

of a single ply or of two or more plies laminated with their 

grain direction parallel. A panel can contain an odd or even 

number of plies but always an odd number of layers. The 

outside plies are called faces, or face and back plies. Inner 

plies are plies other than the face or back plies. The outer 

layers and all odd-numbered layers have their grain direction oriented parallel to the long dimension of the panel. The 

grain in even-numbered layers is perpendicular to the length 

of the panel. Inner plies whose grain direction runs parallel 

to that of the faces are termed “centers” whereas inner plies 

Chapter 11 Wood-Based Composite Materials

Figure 11–3. Classification of wood composite panels by particle size, density, and process 

(Suchsland and Woodson 1986). Note that insulation board is now known as cellulosic fiberboard.
whose grain direction runs perpendicular to that of the faces 

are termed “crossbands.” To distinguish the number of plies 

(individual sheets of veneer in a panel) from the number 

of layers (number of times the grain orientation changes), 

panels are sometimes described as three-ply, three-layer or 

four-ply, three-layer, etc. The center layer may be veneer, 

lumber, particleboard, or fiberboard; however, all-veneer 

construction is most common in construction and industrial 

plywood.

Plywood panels are used in various applications, including 

construction sheathing, furniture, and cabinet panels. 

Plywood is also used as a component in other engineered 

wood products and systems in applications such as prefabricated I-joists, box beams, stressed-skin panels, and panelized roofs.

Characteristics

The properties of plywood depend on the quality of the 

veneer plies, the order of layers, the adhesive used, and the 

degree to which bonding conditions are controlled during 

production. The durability of the adhesive-to-wood bond 

depends largely on the adhesive used but also on control of 

bonding conditions and on veneer quality. The grade of the 

panel depends upon the quality of the veneers used, particularly of the face and back.

Plywood panels have significant bending strength both 

along the panel and across the panel, and the differences in 

strength and stiffness along the panel length versus across 

the panel are much smaller than those differences in solid 

wood. Plywood also has excellent dimensional stability 

along its length and across its width. Minimal edge-swelling 

makes plywood a good choice for adhesive-bonded tongueand-groove joints, even where some wetting is expected. 

Unlike most panels fabricated from particles, it undergoes 

minimal irreversible thickness swelling if wetted. The alternating grain direction of its layers makes plywood resistant 

to splitting, allowing fasteners to be placed very near the 

edges of a panel. In uses where internal knotholes and voids 

may pose a problem, such as in small pieces, plywood can 

be ordered with a solid core and face veneers.

Figure 11–4. Examples of various composite products. From top left, clockwise: LVL, PSL, LSL, plywood, OSB, particleboard, and fiberboard.

11–6

General Technical Report FPL–GTR–190

Figure 11–5. Common wood elements used in woodbased composites from top left, clockwise: shavings, 

sawdust, fiber, large particles, wafers, and strands.
Classes of Plywood

Two classes of plywood are commonly available, covered 

by separate standards: (a) construction and industrial plywood and (b) hardwood and decorative plywood.

Most construction and industrial plywood used in the United 

States is produced domestically, and U.S. manufacturers 

export some material. The bulk of construction and industrial plywood is used where performance is more important 

than appearance. However, some grades of construction and 

industrial plywood are made with faces selected primarily 

for appearance and are used either with clear natural finishes 

or lightly pigmented finishes. Construction and industrial 

plywood has traditionally been made from softwoods such 

as Douglas-fir and southern yellow pine. However, true firs, 

western hemlock, and western pines are also used (Bowyer 

and others 2007). A large number of hardwoods qualify for 

use under the standard. PF resin is the primary adhesive type 

used in construction and industrial plywood. Construction 

and industrial plywood is categorized by exposure capability 

and grade using Voluntary Product Standard PS 1–07 (NIST 

2007).

Hardwood and decorative plywood is made of many different species, both in the United States and overseas. Well 

over half of all panels used in the United States are imported. Hardwood plywood is normally used in applications 

including decorative wall panels and furniture and cabinet 

panels where appearance is more important than strength. 

Most of the production is intended for interior or protected 

uses, although a very small proportion is made with adhesives suitable for exterior service, such as in marine applications. A substantial portion of all hardwood plywood 

is available completely finished. Hardwood and decorative 

plywood is categorized by species and characteristics of 

face veneer, bond durability, and composition of center 

layers (veneer, lumber, particleboard, MDF, or hardboard) 

(HP–1–2004, HPVA 2004).

Exposure Capability

Construction and industrial plywood is classified as either 

Exposure 1 or Exterior in Voluntary Product Standard PS 

1–07 (NIST 2007). Exposure 1 plywood is intended for 

applications not permanently exposed to weather, whereas 

Exterior plywood is suitable for repeated wetting and drying, or long-term exposure to weather. Bond quality of plywood of either bond classification (Exposure 1 or Exterior) 

is evaluated by the same test procedure, but a higher level 

of performance in the test procedure is required for Exterior 

plywood. The test procedure involves water saturation, boiling, and high-temperature exposure (in excess of boiling 

temperature). The majority of construction and industrial 

plywood sold in North America is of Exposure 1 classification. Exposure 1 panels may undergo rain-wetting during 

building construction but will be protected from wetting 

after the building is enclosed.

Two exposure classes of hardwood and decorative plywood 

are recognized by ANSI/HPVA HP–1–2004, Exterior and 

Interior. The standard actually lists two different Exterior 

classes, Technical and Type I, but the bond performance 

requirements for these classes, as determined by test procedures outlined in the standard, are the same.

Plywood Grades

Plywood grades may indicate the intended use, a type of 

surface treatment, or the grades of the face and back veneers, and in some cases, a combination of these. Agencies 

that provide quality certification services for plywood mills 

have coined their own trademarked grade names for specified end uses through proprietary product standards. Grade 

stamps are used to identify plywood products (Figs. 11–6 

and 11–7). An example of plywood CARB third-party identification is also shown in Figure 11–7.

Veneer quality is a factor in construction and industrial plywood based on visually observable characteristics. Knots, 

decay, splits, insect holes, surface roughness, number of 

surface repairs, and other defects are considered. Veneer 

species and characteristics are also a major factor in categorization of hardwood and decorative plywood.

Specialty Plywood Panels

Plywood is easily pressure-treated with waterborne preservatives and fire retardants. Because plywood is not prone 

to irreversible thickness swelling, its bond integrity is unaffected by pressure treatment with waterborne chemicals. 

Treatment is typically performed by commercial entities 

specializing in treatment rather than by the plywood manufacturer. Treatments for plywood have been standardized 

(AWPA 2007a,b). This allows specification by reference to 

a commercial standard. Special grades of plywood are produced for specific uses such as boat construction, concrete 

form work, or special exterior applications such as highway 

signage.

Oriented Strandboard

Oriented strandboard (OSB) is an engineered structural-use 

panel manufactured from thin wood strands bonded together 

with water-resistant resin, typically PF or pMDI. It is used 

extensively for roof, wall, and floor sheathing in residential 

and commercial construction. The wood strands typically 

have an aspect ratio (strand length divided by width) of at 

least 3. OSB panels are usually made up of three layers of 

strands, the outer faces having longer strands aligned in the 

long-direction of the panel and a core layer that is counteraligned or laid randomly using the smaller strands or fines. 

The orientation of different layers of aligned strands gives 

OSB its unique characteristics, including greater bending 

strength and stiffness in the oriented or aligned direction. 

Control of strand size, orientation, and layered construction 

allows OSB to be engineered to suit different uses.

OSB technology and the raw material used originally 

evolved from waferboard technology, for which aspen was 

11–7

Chapter 11 Wood-Based Composite Materials
the predominant wood species used. As the industry learned 

to control strand size, placement, and orientation, the performance and utility of OSB products improved to the 

point that their performance was similar to that of structural 

plywood. As a result, product acceptance and the industry 

expanded as OSB began to replace softwood plywood in 

construction applications.

Raw Materials

In North America, aspen is the predominant wood used 

for OSB. Species other than aspen, such as Southern Pine, 

spruce, birch, yellow-poplar, sweetgum, sassafrass, and 

beech, are also suitable raw materials for OSB production. 

High-density species such as beech and birch are often 

mixed with low-density species such as aspen to maintain 

panel properties (Bowyer and others 2007).

Manufacturing Process

To manufacture OSB, debarked logs are sliced into long, 

thin wood elements called strands. The strands are dried, 

blended with resin and wax, and formed into thick, loosely 

consolidated mats that are pressed under heat and pressure 

into large panels. Figure 11–8 shows an OSB manufacturing process. A more detailed description of each individual 

manufacturing step follows.

During stranding, logs are debarked and then sent to a 

soaking pond or directly to the stranding process. Long log 

disk or ring stranders are commonly used to produce wood 

strands typically measuring 114 to 152 mm (4.5 to 6 in.) 

long, 12.7 mm (0.5 in.) wide, and 0.6 to 0.7 mm (0.023 to 

0.027 in.) thick. Green strands are stored in wet bins and 

dried in a traditional triple-pass dryer, a single-pass dryer, a 

Figure 11–6. Typical grade stamps for plywood and OSB. (Courtesy of 

TECO, Sun Prairie, Wisconsin, and APA–The Engineered Wood Association, Tacoma, Washington. Used by permission.)

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General Technical Report FPL–GTR–190
Figure 11–7. Typical grade stamps for hardwood plywood. (Courtesy of Hardwood Plywood & Veneer Association, Reston, Virginia. Used by permission.)

11–9

Chapter 11 Wood-Based Composite Materials
combination triple-pass/single-pass dryer, or a three-section 

conveyor dryer. A recent development is a continuous chain 

dryer, in which the strands are laid on a chain mat that is 

mated with an upper chain mat and the strands are held in 

place as they move through the dryer. New drying techniques allow the use of longer strands, reducing surface 

inactivation of strands, and lowering dryer outfeed temperatures. Dried strands are screened and sent to dry bins.

Dried strands are blended with adhesive and wax in a highly 

controlled operation, with separate rotating blenders used 

for face and core strands. Typically, different resin formulations are used for face and core layers. Face resins may be 

liquid or powdered phenolics, whereas core resins may be 

phenolics or isocyanates. Several different resin application 

systems are used; spinning disk resin applicators are the 

most common.

The strands with adhesive applied are sent to mat formers. 

Mat formers take on a number of configurations, ranging 

from electrostatic equipment to mechanical devices containing spinning disks to align strands along the panel’s length 

and star-type cross-orienters to position strands across the 

panel’s width. All formers use the long and narrow characteristic of the strand to place it between spinning disks 

or troughs before it is ejected onto a moving screen or 

conveyor belt below the forming heads. Oriented layers of 

strands within the mat are dropped sequentially onto a moving conveyor. The conveyor carries the loose, layered mat 

into the press.

Once the mat is formed, it is hot-pressed. Hot-pressing 

consolidates the mat by heating it at 177 to 204 °C (350 to 

400 °F), which cures the resin in 3–5 minutes. As many as 

sixteen 3.7- by 7.3-m (12- by 24-ft) panels may be formed 

simultaneously in a multiple-opening press. A more recent 

development is the continuous press which presses the mat 

between rollers as it is conveyed.

OSB Grade Marks and Product Certification

OSB that has been grade marked is manufactured to comply 

with voluntary industry product performance standards. 

Inspection or certification programs also generally require 

that the quality control system of a production plant meet 

specified criteria. OSB panels conforming to product performance standards are marked with grade stamps (Fig. 11–6).

Particleboard

Particleboard is produced by mechanically reducing the 

wood raw material into small particles, applying adhesive to 

the particles, and consolidating a loose mat of the particles 

with heat and pressure into a panel product. The particleboard industry initially used cut flakes as a raw material. 

However, economic concerns prompted development of 

the ability to use sawdust, planer shavings, and to a lesser 

extent, mill residues and other waste materials. To manufacture particleboard with good strength, smooth surfaces, and 

equal swelling, manufacturers ideally use a homogeneous 

raw material.

Figure 11–8. Schematic of OSB manufacturing process. (Courtesy of TECO, Sun Prairie, Wisconsin. 

Used by permission.)

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General Technical Report FPL–GTR–190
Particleboard is typically made in layers. But unlike OSB, 

the faces of particleboard usually consist of fine wood particles and the core is made of coarser material. The result is 

a smoother surface for laminating, overlaying, painting, or 

veneering. Particleboard is readily made from virtually any 

wood material and from a variety of agricultural residues. 

Low-density insulating or sound-absorbing particleboard 

can be made from kenaf core or jute stick. Low-, medium-, 

and high-density panels can be produced with cereal straw, 

which has been used in North America. Rice husks are commercially manufactured into medium- and high-density 

products in the Middle East.

All other things being equal, reducing lignocellulosic materials to particles requires less energy than reducing the same 

material into fibers. However, particleboard is generally not 

as strong as fiberboard because the fibrous nature of lignocellulosics (that is, their high aspect ratio) is not exploited 

as well. Particleboard is widely used in furniture, where it 

is typically overlaid with other materials for decorative 

purposes. It is the predominant material used in ready-toassemble furniture. Particleboard can also be used in flooring systems, in manufactured houses, and as underlayment. 

Thin panels can also be used as a paneling substrate. Since 

most applications are interior, particleboard is usually 

bonded with a UF resin, although PF and MF resins are 

sometimes used for applications requiring more moisture 

resistance.

Manufacturing Process

Manufacturing particleboard is a dry process. The steps 

involved in particleboard manufacturing include particle 

preparation, particle classification and drying, adhesive application, mat formation, pressing, and finishing.

Standard particleboard plants use combinations of hogs, 

chippers, hammermills, ring flakers, ring mills, and attrition 

mills to obtain particles. Particles are classified and separated to minimize negative effect on the finished product. 

Very small particles (fines) increase particle surface area 

and thus increase resin requirements. Oversized particles 

can adversely affect the quality of the final product because 

of internal flaws in the particles. While some particles are 

classified through the use of air streams, screen classification methods are the most common. In screen classification, 

the particles are fed over a vibrating flat screen or a series of 

screens. The screens may be wire cloth, plates with holes or 

slots, or plates set on edge. Particles are typically conveyed 

by mechanical means. Sometimes damp conditions are 

maintained to reduce break-up of particles during 

conveying.

Desirable particles have a high degree of slenderness (long, 

thin particles), no oversize particles, no splinters, and no 

dust. Depending on the manufacturing process and board 

configurations, specifications for the ideal particle size are 

different. For a common three-layer board, core particles are 

longer and surface particles shorter, thinner, and smaller. For 

a five-layer board, the particles for the intermediate layer 

between surface and core are long and thin, which builds a 

good carrier for the fine surface and gives the boards high 

bending strength and stiffness. Particleboard used for quality 

furniture uses much smaller core particles. The tighter core 

gives a better quality edge which allows particleboard to 

compete more favorably with MDF.

The raw materials (or furnish) do not usually arrive at the 

plant at a low enough moisture content for immediate use. 

Furnish that arrives at the plant can range from 10% to 

200% dry basis moisture content. For use with liquid resins, 

for example, the furnish must be reduced to about 2% to 7% 

moisture content. The moisture content of particles is critical during hot-pressing operations and depends on whether 

resin is to be added dry or as a solution or emulsion. The 

moisture content of materials leaving the dryers is usually 

in the range of 4% to 8%. The main methods used to dry 

particles are rotary, disk, and suspension drying. A triplepass rotary dryer consists of a large horizontal, heated, rotating drum. Operating temperatures depend on the moisture 

content of the incoming furnish. The drum is set at a slight 

angle, and material is fed into the high end and discharged 

at the low end. A series of flights forces the furnish to flow 

from one end to the other three times before being discharged. The rotary movement of the drum moves the material from input to output.

Frequently used resins for particleboard include UF and, to 

a much lesser extent, PF, MF, and isocyanates. The type and 

amount of resin used for particleboard depends on the type 

of product desired. Based on the weight of dry resin solids 

and ovendry weight of the particles, the overall resin content 

can range between 4% and 10%, but usually ranges between 

6% and 9% for UF resins. The resin content of the outer layers is usually higher (about 8% to 15%) than that of the core 

(about 4% to 8%). UF resin is usually introduced in water 

solutions containing about 50% to 65% solids. Besides resin, wax is added to improve short-term moisture resistance. 

The amount of wax ranges from 0.3% to 1% based on the 

ovendry weight of the particles.

After the particles have been prepared, they are laid into an 

even and consistent mat to be pressed into a panel. This is 

accomplished in batch mode or by continuous formation. 

The batch system traditionally employs a caul or tray on 

which a deckle frame is placed. The mat is formed by the 

back-and-forth movement of a tray or hopper feeder. The 

production of three-layer boards requires three or more 

forming stations. 

In more common continuous mat-forming systems the particles are distributed in one or several layers on traveling 

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Chapter 11 Wood-Based Composite Materials
cauls or on a moving belt. Mat thickness is controlled volumetrically. Batch-formed or continuous-formed mats are 

often pre-pressed to reduce mat height and help consolidate 

the mat for pressing.

After pre-pressing, the mats are hot-pressed into panels. 

Presses can be divided into platen and continuous types. 

Further development in the industry has made possible the 

construction of presses for producing increasingly larger 

panel sizes in both single- and multi-opening presses. Both 

of these types of presses can be as wide as 3.7 m (12 ft). 

Multi-opening presses can be as long as 10 m (33 ft), and 

continuous presses, up to 30 m (100 ft) long.

After pressing, panels are trimmed to obtain the desired 

length and width and to square the edges. Trim losses usually amount to 0.5% to 8%, depending on the size of the 

panel, the process employed, and the control exercised. 

Trimmers usually consist of saws with tungsten carbide 

tips. After trimming, the panels are sanded or planed prior 

to packaging and shipping. Particleboards may also be veneered or overlaid with other materials to provide a decorative surface, or they may be finished with lacquer or paint. 

Treatments with fire-resistant chemicals are also available.

Particleboard Grade Marks and Product Certification

A grade mark on particleboard ensures that the product 

has been periodically tested for compliance with voluntary 

industry product performance standards. Inspection or certification programs also generally require that the quality control system of a production plant meets strict criteria. Particleboard panels conforming to these product performance 

standards are marked with grade stamps (Fig. 11–9).

Fiberboard

The term fiberboard includes hardboard, medium-density 

fiberboard (MDF), and cellulosic fiberboard. Several things 

differentiate fiberboard from particleboard, most notably the 

physical configuration of the wood element. Because wood 

is fibrous by nature, fiberboard exploits the inherent strength 

of wood to a greater extent than does particleboard.

To make fibers for composites, bonds between the wood 

fibers must be broken. Attrition milling, or refining, is the 

easiest way to accomplish this. During refining, material 

is fed between two disks with radial grooves. As the material is forced through the preset gap between the disks, it is 

sheared, cut, and abraded into fibers and fiber bundles. Refiners are available with single- or double-rotating disks, as 

well as steam-pressurized and unpressurized configurations. 

Refining can be augmented by steaming or chemical treatments. Steaming the lignocellulosic weakens the lignin 

bonds between the cellulosic fibers. As a result, fibers are 

more readily separated and are usually less damaged than 

fibers processed by dry processing methods. Chemical treatments, usually alkali, are also used to weaken the lignin 

bonds. Although treatments help increase fiber quality and 

reduce energy requirements, they may also reduce yield and 

modify the fiber chemistry. For MDF, steam-pressurized 

refining is typical.

Fiberboard is normally classified by density and can be 

made by either dry or wet processes (Fig. 11–3). Dry 

processes are applicable to boards with high density 

(hardboard) and medium density (MDF). Wet processes are 

applicable to both high-density hardboard and low-density 

Figure 11–9. Examples of grade stamps for particleboard. (Courtesy of TECO, Sun Prairie, Wisconsin, and Composite Panel Association, Leesburg, Virginia. Used by permission.)

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General Technical Report FPL–GTR–190
cellulosic fiberboard. The following subsections briefly describe the manufacturing of high- and medium-density dryprocess fiberboard, wet-process hardboard, and wet-process 

low-density cellulosic fiberboard. Suchsland and Woodson 

(1986) and Maloney (1993) provide more detailed 

information.

Dry-Process Fiberboard

Dry-process fiberboard is made in a similar fashion to particleboard. Resin (UF or MF–UF) and other additives are 

applied to the fibers by spraying in short-retention blenders 

or introduced as wet fibers are fed from the refiner into a 

blow-line dryer. Alternatively, some fiberboard plants add 

the resin in the refiner. The adhesive-coated fibers are then 

air-laid into a mat for subsequent pressing, much the same 

as mat formation for particleboard.

Pressing procedures for dry-process fiberboard differ somewhat from particleboard procedures. After the fiber mat is 

formed, it is typically pre-pressed in a band press. The densified mat is then trimmed by disk cutters and transferred to 

caul plates for the hardboard pressing operation; for MDF, 

the trimmed mat is transferred directly to the press. Many 

dry-formed boards are pressed in multi-opening presses. 

Continuous pressing using large, high-pressure band presses 

is also gaining in popularity. Panel density is constantly 

monitored by moisture sensors using infrared light as an 

indicator of panel quality.

MDF is frequently used in place of solid wood, plywood, 

and particleboard in many furniture applications. It is also 

used for interior door skins, mouldings, and interior trim 

components. ANSI A208.2 classifies MDF by physical and 

mechanical properties, and identifies dimensional tolerances 

and formaldehyde emission limits (CPA 2009b). An example of an MDF formaldehyde emissions certification tag 

is shown in Figure 11–10.

Wet-Process Hardboard

Wet-process hardboards differ from dry-process fiberboards 

in several significant ways. First, water is used as the fiber 

distribution medium for mat formation. The technology is 

really an extension of paper manufacturing technology. Secondly, some wet-process boards are made without additional 

binders. If the lignocellulosic contains sufficient lignin and 

if lignin is retained during the refining operation, lignin can 

serve as the binder. Under heat and pressure, lignin will flow 

and act as a thermosetting adhesive, enhancing the naturally 

occurring hydrogen bonds.

Refining is an important step for developing strength in wetprocess hardboards. The refining operation must also yield 

a fiber of high “freeness” (that is, it must be easy to remove 

water from the fibrous mat). The mat is typically formed 

on a Fourdrinier wire, like papermaking, or on cylinder 

formers. The wet process employs a continuously traveling 

mesh screen, onto which the soupy pulp flows rapidly and 

smoothly. Water is drawn off through the screen and then 

through a series of press rolls.

Wet-process hardboards are pressed in multi-opening presses heated by steam. The press cycle consists of three phases 

and lasts 6 to 15 min. The first phase is conducted at high 

pressure, and it removes most of the water while bringing 

the board to the desired thickness. The primary purpose of 

the second phase is to remove water vapor. The third phase 

is relatively short and results in the final cure. A maximum 

pressure of about 5 MPa (725 lb in–2) is used in the first 

and third phases. Heat is essential during pressing to induce 

fiber-to-fiber bond. A high temperature of up to 210 °C 

(410 °F) is used to increase production by causing faster 

vaporization of the water. Insufficient moisture removal 

during pressing adversely affects strength and may result in 

“springback” or blistering.

Wet-formed composite technology has lost market share 

compared with dry-formed technology over the past few 

decades because of processing speed and perceived 

environmental issues related to process water. However, 

wet-formed technology does offer unique opportunities for 

forming geometric shapes that yield enhanced structural 

performance and decrease weight, elimination of fiber drying prior to forming, and reduced need for adhesive resins. 

It also greatly increases the ability to use recovered paper 

and some other woody fibers. Recent advances in process 

wastewater recycling and remediation also bode well for 

wet-formed technologies. Wet-formed composite technology may become more important because of reduced energy 

demands, increased composite structural performance and 

decreased weight, and the virtual elimination of (or drastic 

reduction in) process water concerns.

Several treatments are used to increase dimensional stability 

and mechanical performance of hardboard. Heat treatment, 

tempering, and humidification may be done singularly or 

in conjunction with one another. Heat treatment—exposure 

of pressed fiberboard to dry heat—improves dimensional 

stability and mechanical properties, reduces water adsorption, and improves interfiber bonding. Tempering is the heat 

treatment of pressed boards, preceded by the addition of oil. 

Tempering improves board surface hardness, resistance to 

abrasion, scratching, scarring, and water. The most common 

oils used include linseed oil, tung oil, and tall oil. Humidification is the addition of moisture to bring the board 

moisture content to levels roughly equivalent to those anticipated in its end-use environment. Air of high humidity 

is forced through the stacks where it provides water vapor 

to the boards. Another method involves spraying water on 

the back side of the board. Typical hardboard products are 

prefinished paneling, house siding, floor underlayment, and 

concrete form board. A typical grade stamp for hardboard 

siding is shown in Figure 11–11.

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Chapter 11 Wood-Based Composite Materials
Cellulosic Fiberboard

Cellulosic fiberboards are low-density, wet-laid panel products. In the manufacture of cellulosic fiberboard, the need 

for refining and screening is a function of the raw material 

available, the equipment used, and the desired end-product. 

Cellulosic fiberboards typically do not use a binder, and they 

rely on hydrogen bonds to hold the board components together. Sizing agents are usually added to the furnish (about 

1%) to provide the finished board with a modest degree of 

water resistance and dimensional stability.

As in the manufacture of wet-process hardboard, cellulosic 

fiberboard manufacture is a modification of papermaking. 

A thick fibrous sheet is made from a low-consistency pulp 

suspension in a process known as wet felting. Felting can 

be accomplished through use of a deckle box, Fourdrinier 

screen, or cylinder screen. A deckle box is a bottomless 

frame that is placed over a screen. A measured amount of 

pulp suspension is put in the box and a vacuum is applied to 

remove most of the water. The use of a Fourdrinier screen 

for felting is similar to that for papermaking, except that line 

speeds are reduced to 8 to 18 m min–1 (25 to 60 ft min–1).

Cellulosic fiberboard formed in a deckle box is coldpressed to remove the free water after the mat is formed. 

Compression rollers on the Fourdrinier machines squeeze 

out the free water. The wet mats are then dried to the final 

moisture content. Dryers may be a continuous tunnel or a 

multideck arrangement. The board is generally dried in 

stages at temperatures ranging from 120 to 190 °C 

(248 to 374 °F). Typically, about 2 to 4 h is required 

to reduce moisture content to about 1% to 3%.

After drying, some boards are treated for specific applications. Boards may be given tongue-and-groove or shiplap 

edges or grooved to produce a plank effect. Other boards are 

laminated by means of asphalt to produce roof insulation.

Cellulosic fiberboard products include sound-deadening 

board, insulation boards, structural and nonstructural sheathings, backer board, and roof decking in various thicknesses. 

An example of a grade mark stamp for these cellulosic fiberboard products conforming to ASTM C 208 (ASTM 2008c) 

is shown in Figure 11–12.

Finishing Techniques

Several techniques are used to finish fiberboard: trimming, 

sanding, surface treatment, punching, and embossing. Trimming consists of reducing products into standard sizes and 

shapes. Generally, double-saw trimmers are used to saw the 

panels. Trimmers consist of overhead-mounted saws or multiple saw drives. If thickness tolerance is critical, hardboard 

Figure 11–10. Example of MDF formaldehyde emissions certification tag. 

(Courtesy of Composite Panel Association, Leesburg, Virginia. Used by 

permission.)

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General Technical Report FPL–GTR–190
is sanded prior to finishing. S1S (smooth on one side) panels 

require this process. Sanding reduces thickness variation and 

improves surface paintability. Single-head, wide-belt sanders are used with 24- to 36-grit abrasive. Surface treatments 

improve the appearance and performance of boards. Panels 

are cleaned by spraying with water and then dried at about 

240 °C (464 °F) for 30 s. Panel surfaces are then modified 

with paper overlay, paint, or stain or are printed directly on 

the panel. Punching changes panels into perforated sheets 

used as peg board. Embossing consists of pressing the unconsolidated mat of fibers with a textured form. This process 

results in a slightly contoured panel surface that can enhance 

the resemblance of the panel to that of sawn or weathered 

wood, brick, and other materials.

Specialty Composite Materials

Special-purpose composite materials are produced to obtain 

enhanced performance properties such as water resistance, 

mechanical strength, acidity control, and fire, decay, and 

insect resistance. Overlays and veneers can also be added 

to enhance both structural properties and appearance 

(Fig. 11–13).

Water-Repellant Composites

Sizing agents are used to increase the water repellency of 

wood-based composites. Sizing agents cover the surface of 

fibers, reduce surface energy, and increase fiber hydrophobicity. Sizing agents can be applied in two ways. In the first 

method, water is used as a medium to ensure thorough mixing of sizing and fiber. The sizing is precipitated from the 

water and is fixed to the fiber surface. In the second method, 

the sizing is applied directly to the fibers.

Common sizing agents include rosin, wax, and asphalt. 

Rosin is obtained from living pine trees, from pine stumps, 

and as a by-product of kraft pulping of pines. Rosin is added 

in amounts of less than 3% solids based on dry fiber weight. 

Waxes are high-molecular-weight hydrocarbons derived 

from crude oil. Wax is used in solid form in dry-process 

fiberboard production. For wet processes, wax is added in 

solid form or as an emulsion. Wax tends to lower strength 

properties to a greater extent than rosin does. Asphalt is also 

used to increase water resistance, especially in low-density 

wet-process cellulosic fiberboard. Asphalt is a black–brown 

solid or semi-solid material that liquefies when heated. Asphalt is added to the process water as an emulsion and precipitated onto fiber by the addition of alum.

Flame-Retardant Composites

Two general application methods are available for improving the fire performance of composites with fire-retardant 

chemicals. One method consists of pressure impregnating 

the wood with waterborne or organic solvent-borne fire 

retardant chemicals (AWPA 2007a). The second method 

consists of applying fire-retardant chemical coatings to the 

wood surface. The pressure impregnation method is usually 

more effective and longer lasting; however, this technique is 

standardized only for plywood. It is not generally used with 

structural flake, particle, or fiber composites, because it can 

cause swelling that permanently damages the wood–adhesive bonds in the composite and results in the degradation of 

some physical and mechanical properties of the composite. 

For wood in existing constructions, surface application of 

fire-retardant paints or other finishes offers a possible method to reduce flame spread. 

Preservative-Treated Composites

Composites can be protected from attack by decay fungi 

and harmful insects by applying selected chemicals as wood 

preservatives. The degree of protection obtained depends 

on the kind of preservative used and the ability to achieve 

proper penetration and retention of the chemicals. Wood 

preservatives can be applied using pressurized or non-pressurized processes (AWPA 2007b). As in the application of 

fire-retardant chemicals, the pressurized application of wood 

preservatives is generally performed after manufacture and 

is standardized for plywood. Post-manufacture pressurized 

treatments are not standardized for all types of flake, particle, or fiber composite due to the potential for swelling. 

Preservatives can be added during the composite manufacturing process, but the preservative must be resistant to 

vaporization during hot pressing. Proprietary flakeboard and 

fiberboard products with incorporated nonvolatile preservatives have been commercialized. Common preservative 

treatments include ammoniacal copper quat (ACQ), copper 

azol (CA), and boron compounds.

Performance and Standards

Standards for conventional wood-based composite products 

are typically established under a series of internationally 

Figure 11–11. Typical grade stamp for hardboard siding. (Courtesy of Composite Panel Association, Leesburg, Virginia. Used by permission.)

Figure 11–12. Typical grade 

stamp for cellulosic fiberboard. (Courtesy American 

Fiberboard Association, 

Palatine, Illinois. Used by 

permission.)

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Chapter 11 Wood-Based Composite Materials
accredited consensus review processes involving users, producers, and general interests. Then, commercial wood composites are manufactured to conform to these commercial 

product or performance standards (Table 11–2). These product or performance standards are cited in the International 

Building Code (IBC) and other similar documents. The IBC 

is the model building code produced by the International 

Code Council (ICC), a non-profit organization established in 

1994 dedicated to developing a single set of comprehensive 

and coordinated construction codes. Since the adoption of 

the first edition of the IBC in 2000, most building codes in 

the U.S. are based on the IBC. Previously, there were three 

model code organizations that produced model codes with 

largely regional acceptance. The ICC also produces the International Residential Code (IRC) and other related model 

code documents. 

The ICC–Evaluation Service (ICC–ES) issues advisory 

reports on their evaluation of building products, components, methods, and materials for their compliance with the 

IBC. The ICC–ES was created in 2003 with the merger of 

the four existing evaluation services. ICC–ES also issues 

ICC–ES Acceptance Criteria documents for specific types 

of products that specify the performance criteria and the 

data that needs to be submitted for the evaluations. These 

evaluation reports and the acceptance criteria documents are 

available on the web site of the ICC–ES (www.icc-es.org). 

Types of Product Standards

Product standards may be classified as manufacturing 

standards or performance standards. The best example of a 

manufacturing method standard is Voluntary Product Standard PS 1–07 for construction and industrial plywood (NIST 

2007). Specifications in the standard include which wood 

species and grades of veneer may be used, what repairs are 

permissible, and how repairs must be made. Limited performance-based evaluations may also be specified in manufacturing standards for issues relating to adhesive durability 

and strength.

Standard PS 2–04 (NIST 2004) is a performance-based 

standard because it applies to all structural-use wood-based 

panels, including plywood and OSB. This standard includes 

performance requirements and test methods suitable for a 

given application (qualification) and also require that properties of a qualified product remain in control during production over time (certification). With respect to plywood, 

the PS 2–04 standard is not a replacement for PS 1–07, 

which contains necessary veneer-grade and adhesive-bond 

requirements as well as lay-up provisions, but it is broad 

enough to also include many plywood grades not covered 

under PS 1–07. PS 2–04 specifies that certification be performed by an independent third party.

The American National Standards for particleboard and 

MDF (ANSI A208.1 and A208.2 respectively) are sponsored 

by the Composite Panel Association (CPA) in Leesburg, 

Virginia (CPA 2009a, b). These performance standards 

require the composite products to show certain minimally 

acceptable physical and mechanical properties. The test requirements give some indication of product quality, but the 

tests were not specifically developed to correlate with performance of whole panels in specific end uses.

Role of Standards in Construction

Conventional wood-based composite panels and lumber elements manufactured in conformance with product standards 

(Table 11–2) are approved under the International Building Code. These wood-based composites can be used for 

construction applications such as sheathing for roofs, subflooring, and walls. Similarly, many types of wood-based 

composite lumber can be used for joists, purlins, stringers, 

beams, and columns.

Design properties and basic installation guidelines of 

structural-use panels have become standardized by the ICC. 

The ICC requires independent third-party certification of 

these panels, and several such third-party certification agencies exist, such as APA–The Engineered Wood Association 

(www.apawood.org) and TECO (www.tecotested.com). 

These agencies and others offer a variety of technical information on the proper selection, design, and installation of 

structural-use panels. 

Plywood panels conforming to PS 1–07 are marked with 

grade stamps (Fig. 11–6a,b). Flake-based composites, such 

as OSB, are usually marketed as conforming to a product 

standard for sheathing or single-layer sub-flooring or underlayment and are also marked with grade stamps (Fig. 

11–6c,d). 

Structural-use panels are also span-rated. Span ratings refer to on-center spacing of support members (expressed in 

inches), with the long panel dimension (in plywood this is 

the same direction as the face grain) placed across the supports, assuming that there are at least two spans (a minimum 

of three supports). Span-rating of construction plywood 

and OSB simplifies materials specification in light-frame 

Figure 11–13. Medium-density fiberboard with veneer 

overlay. Edges can be shaped and finished as required 

by end product.

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General Technical Report FPL–GTR–190
construction by allowing specification without resorting to 

specific structural engineering design calculations. Panels in 

PS 2–04 are designated by application (wall, roof, sub-floor, 

or single floor) and by span rating. Specification by application and span is more convenient for builders than specification by species or species group, veneer grade, and panel 

thickness. A panel may be suitable for use as either roof 

sheathing or sub-flooring, with different span ratings for the 

two applications. Such panels will have a dual span-rating, 

the first (and larger) number indicating allowable span when 

used as roof sheathing, the second number indicating the 

allowable span when used as sub-flooring.

Glulam Timber

Structural glued-laminated timber (glulam) is one of the 

oldest glued engineered wood products. Glulam is an engineered, stress-rated product that consists of two or more 

layers of lumber that are glued together with the grain of all 

layers, which are referred to as laminations, parallel to the 

length. Glulam is defined as a material that is made from 

suitably selected and prepared pieces of wood either in a 

straight or curved form, with the grain of all pieces essentially parallel to the longitudinal axis of the member. The 

maximum lamination thickness permitted is 50 mm (2 in.), 

and the laminations are typically made of standard 25- or 

50-mm- (nominal 1- or 2-in.-) thick lumber. North American 

standards require that glulam be manufactured in an approved manufacturing plant. Because the lumber is joined 

end to end, edge to edge, and face to face, the size of glulam 

is limited only by the capabilities of the manufacturing plant 

and the transportation system.

Douglas Fir–Larch, Southern Pine, Hem–Fir, and Spruce–

Pine–Fir (SPF) are commonly used for glulam in the United 

States. Nearly any species can be used for glulam timber, 

provided the mechanical and physical properties are suitable 

and gluing properties acceptable. Industry standards cover 

many softwoods and hardwoods, and procedures are in 

place for including other species.

Advantages

Compared with sawn timbers as well as other structural 

materials, glulam has several distinct advantages. These include size capability, architectural effects, seasoning, variation of cross sections, grades, and effect on the environment.

Size Capabilities

Glulam offers the possibility of manufacturing structural 

timbers that are much larger than the trees from which the 

component lumber was sawn. In the past, the United States 

had access to large trees that could produce relatively large 

sawn timbers. However, the present trend is to harvest 

smaller diameter trees on much shorter rotations, and nearly 

all new sawmills are built to accommodate relatively small 

logs. By combining the lumber in glulam, the production of 

large structural elements is possible. Straight members up 

to 30 m (100 ft) long are not uncommon, and some span up 

to 43 m (140 ft). Sections deeper than 2 m (7 ft) have been 

used. Thus, glulam offers the potential to produce large timbers from small trees.

Architectural Effects

By curving lumber during the manufacturing process, a 

variety of architectural effects can be obtained with glulam 

that are impossible or very difficult with other materials 

(Fig. 11–14). The degree of curvature is controlled by the 

thickness of the laminations. Thus, glulam with moderate 

curvature is generally manufactured with standard 19-mm- 

(nominal 1-in.-) thick lumber. Low curvatures are possible 

with standard 38-mm (nominal 2-in.) lumber, whereas 

13 mm (1/2 in.) or thinner material may be required for very 

sharp curves. As noted later in this chapter, the radius of 

curvature is limited to between 100 and 125 times the 

lamination thickness.

Seasoning Advantages

The lumber used in the manufacture of glulam must be seasoned or dried prior to use, so the effects of checking and 

other drying defects are minimized. This allows design on 

the basis of seasoned wood, which permits greater design 

values than can be assigned to unseasoned timber.

Varying Cross Sections

Structural elements can be designed with varying cross 

sections along their length as determined by strength and 

stiffness requirements. Similarly, arches often have varying 

cross sections as determined by design requirements.

Varying Grades

One major advantage of glulam is that a large quantity 

of lower grade lumber can be used within the less highly 

stressed laminations of the beams. Grades are often varied 

within the beams so that the highest grades are used in the 

highly stressed laminations near the top and bottom edges, 

with the lower grades used in the inner section (toward the 

center) of the beams. Species can also be varied to match 

the structural requirements of the laminations.

Types of Glulam Combinations

Bending Members

The configuring of various grades of lumber to form a 

glulam cross section is commonly referred to as a glulam 

combination. Glulam combinations subjected to flexural 

loads, called bending combinations, were developed to 

provide the most efficient and economical section for resisting bending stress caused by loads applied perpendicular 

to the wide faces of the laminations. This type of glulam is 

commonly referred to as a horizontally laminated member. 

Lower grades of laminating lumber are commonly used for 

the center portion of the combination, or core, where bending stress is low, while a higher grade of material is placed 

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Chapter 11 Wood-Based Composite Materials
on the outside faces where bending stress is relatively high. 

To optimize the bending stiffness of this type of glulam 

member, equal amounts of high-quality laminations on the 

outside faces should be included to produce a “balanced” 

combination. To optimize bending strength, the combination 

can be “unbalanced” with more high-quality laminations 

placed on the tension side of the member compared with the 

quality used on the compression side. For high-quality lumber placed on the tension side of the glulam combination, 

stringent requirements are placed on knot size, slope 

of grain, and lumber stiffness.

For compression-side laminations, knot size and slope-ofgrain requirements are less stringent and only lumber stiffness is given high priority. In the case where the glulam 

member is used over continuous supports, the combination would need to be designed as a balanced member for 

strength and stiffness because of the exposure of both the 

top and bottom of the beam to tensile stresses. The knot and 

slope-of-grain requirements for this type of combination 

are generally applied equally to both the top and bottom 

laminations.

Axial Members

Glulam axial combinations were developed to provide the 

most efficient and economical section for resisting axial 

forces and flexural loads applied parallel to the wide faces 

of the laminations. Members having loads applied parallel 

to the wide faces of the laminations are commonly referred 

to as vertically laminated members. Unlike the practice for 

bending combinations, the same grade of lamination is used 

throughout the axial combination. Axial combinations 

may also be loaded perpendicular to the wide face of the 

laminations, but the nonselective placement of material 

often results in a less efficient and less economical member 

than does the bending combination. As with bending combinations, knot and slope-of-grain requirements apply based 

on whether the axial member will be used as a tension or 

compression member.

Curved Members

Efficient use of lumber in cross sections of curved glulam 

combinations is similar to that in cross sections of straight, 

horizontally laminated combinations. Tension and compression stresses are analyzed as tangential stresses in the curved 

portion of the member. A unique behavior in these curved 

members is the formation of radial stresses perpendicular to 

the wide faces of the laminations. As the radius of curvature 

of the glulam member decreases, the radial stresses formed 

in the curved portion of the beam increase. Because of the 

relatively low strength of lumber in tension perpendicularto-the-grain compared with tension parallel-to-the-grain, 

these radial stresses become a critical factor in designing 

curved glulam combinations. Curved members are commonly manufactured with standard 19- and 38-mm- (nominal 

1- and 2-in.-) thick lumber. Naturally, the curvature that is 

obtainable with the standard 19-mm- (nominal 1-in.-) 

thick lumber will be sharper than that for the standard 

38-mm- (nominal 2-in.-) thick lumber.

Tapered Straight Members

Glulam beams are often tapered to meet architectural requirements, provide pitched roofs, facilitate drainage, and 

lower wall height requirements at the end supports. The 

taper is achieved by sawing the member across one or more 

laminations at the desired slope. It is recommended that 

the taper cut be made only on the compression side of the 

glulam member, because violating the continuity of the 

tension-side laminations would decrease the overall strength 

of the member.

Standards and Specifications

Manufacture

The ANSI/AITC A190.1 standard of the American National 

Standards Institute (AITC 2007a) contains requirements for 

the production, testing, and certification of structural glulam 

timber in the United States. A standard for glulam poles, 

ANSI O5.2 (ANSI 2006), addresses special requirements for 

utility uses.

Derivation of Design Values

ASTM D 3737 (ASTM 2008a) covers procedures to establish design values for structural glulam timber. Properties 

considered include bending, tension, compression parallel to 

grain, modulus of elasticity, horizontal shear, radial tension, 

and compression perpendicular to grain.

Figure 11–14. Erected in 1934 at the Forest Products 

Laboratory in Madison, Wisconsin, this building is one 

of the first constructed with glued-laminated timbers 

arched, designed, and built using engineering principles.

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General Technical Report FPL–GTR–190
Design Values and Procedures

Manufacturers of glulam timber have standardized the target 

design values in bending for beams. For softwoods, these 

design values are given in AITC 117, “Standard Specifications for Structural Glued-Laminated Timber of Softwood 

Species” (AITC 2004a). This specification contains design 

values and recommended modification of stresses for the 

design of glulam timber members in the United States. A 

comparable specification for hardwoods is AITC 119, “Standard Specifications for Structural Glued-Laminated Timber 

of Hardwood Species” (AITC 1996). For additional design 

information, see the Timber Construction Manual (AITC 

2004b).

Manufacture

The manufacture of glulam timber must follow recognized 

national standards to justify the specified engineering design 

values. When glulam is properly manufactured, both the 

quality of the wood and the adhesive bonds should demonstrate a balance in structural performance.

The ANSI/AITC standard A190.1 (AITC 2007a) has a twophase approach to all phases of manufacturing. First is the 

qualification phase, in which all equipment and personnel 

critical to the production of a quality product are thoroughly 

examined by a third-party agency and the strength of samples of glued joints is determined. In the second phase, after 

successful qualification, daily quality assurance procedures 

and criteria are established, which are targeted to keep each 

of the critical phases of the process under control. An employee is assigned responsibility for supervising the daily 

testing and inspection. The third-party agency makes unannounced visits to the plants to monitor the manufacturing 

process and the finished product and to examine the daily 

records of the quality assurance testing.

The manufacturing process can be divided into four major 

parts: (a) drying and grading the lumber, (b) end jointing the 

lumber, (c) face bonding, and (d) finishing and fabrication. 

In instances where the glulam will be used in high-moisturecontent conditions, the member must also be pressuretreated with preservative.

A final critical step in ensuring the quality of glulam is protection of the glulam timber during transit and storage.

Lumber Drying and Grading

To minimize dimensional changes following manufacture 

and to take advantage of the increased structural properties 

assigned to lumber compared with large sawn timbers, the 

lumber must be properly dried prior to glulam manufacture. 

This generally means kiln drying. Matching the moisture 

content of the glulam timber at the time of manufacture to 

that which it will attain in application minimizes shrinkage 

and swelling, the main causes of checking. Most manufacturers use a continuous in-line moisture meter to check the 

moisture content of each piece of lumber as it enters the 

manufacturing process. Pieces that have a high moisture 

level are removed and redried.

Grading standards published by the regional lumber grading 

associations describe the characteristics that are permitted 

in various grades of lumber. Manufacturing standards for 

glulam timber describe the combination of lumber grades 

necessary for specific design values (AITC 117) (AITC 

2004). The rules for visually graded lumber are based entirely upon the characteristics that are readily apparent. The 

lumber grade description consists of limiting characteristics 

for knot sizes, slope of grain, wane, and several other characteristics.

Manufacturers generally purchase graded lumber and verify 

the grades through visual inspection of each piece and, if Erated, testing of a sample. To qualify the material for some 

of the higher design stresses for glulam timber, manufacturers must also conduct additional grading for material to be 

used in the tension zone of certain beams. Another option 

is to purchase lumber manufactured under a quality assurance system that meets the required tensile strength. Another 

option practiced by at least one manufacturer has been to 

use laminated veneer lumber (LVL) to provide the required 

tensile strength.

End Jointing

To manufacture glulam timber in lengths beyond those commonly available for lumber, laminations must be made by 

end jointing lumber to the proper length. The most common 

end joint, a fingerjoint, is about 28 mm (1.1 in.) long. Other 

configurations are also acceptable, provided they meet specific strength and durability requirements. The advantages 

of fingerjoints are that they require only a short length of 

lumber to manufacture (thus reducing waste) and continuous 

production equipment is readily available. Well-made joints 

are critical to ensure adequate performance of glulam timber. Careful control at each stage of the process—determining lumber quality, cutting the joint, applying the adhesive, 

mating, applying end pressure, and curing—is necessary to 

produce consistent high strength joints.

Face Bonding

The assembly of laminations into full-depth members is another critical stage in manufacture. To obtain clear, parallel, 

and gluable surfaces, laminations must be planed to strict 

tolerances. The best procedure is to plane the two wide faces 

of the laminations just prior to the gluing process. This ensures that the final assembly will be rectangular and that the 

pressure will be applied evenly. Adhesives that have been 

pre-qualified are then spread, usually with a glue extruder. 

Phenol resorcinol is the most commonly used adhesive for 

face gluing, but other adhesives that have been adequately 

evaluated and proven to meet performance and durability 

requirements may also be used.

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Chapter 11 Wood-Based Composite Materials
The laminations are then assembled into the required layup; 

after the adhesive is given the proper open assembly time, 

pressure is applied. The most common method for applying 

pressure is with clamping beds; the pressure is applied with 

either a mechanical or hydraulic system. This results in a 

batch-type process, and the adhesive is allowed to cure at 

room temperature from 6 to 24 h. Some newer automated 

clamping systems include continuous hydraulic presses and 

radio-frequency curing to shorten the face gluing process 

from hours to minutes. Upon completion of the face bonding process, the adhesive is expected to have attained 90% 

or more of its bond strength. During the next few days, curing continues, but at a much slower rate.

The face bonding process is monitored by controls in the 

lumber planing, adhesive mixing, and adhesive spreading 

and clamping processes. Performance is evaluated by conducting shear tests on samples cut off as end trim from the 

finished glulam timber. Thus, the adhesive bonds are expected to develop nearly the full strength of the wood soon 

after manufacture.

Finishing and Fabrication

After the glulam timber is removed from the clamping 

system, the wide faces are planed to remove the adhesive 

that has squeezed out between adjacent laminations and to 

smooth out any slight irregularities between the edges of 

adjacent laminations. As a result, the finished glulam timber 

is slightly narrower than nominal dimension lumber. The 

remaining two faces of the member can be lightly planed or 

sanded.

The appearance requirements of the beam dictate the additional finishing necessary at this point. Historically, three 

classifications of finishing have been included in the industry standard, AITC 110: Industrial, Architectural, and 

Premium (AITC 2001). Industrial appearance is generally 

applicable when appearance is not a primary concern, such 

as industrial plants and warehouses. Architectural appearance is suitable for most applications where appearance is 

an important requirement. Premium appearance is the highest classification. The primary difference among these classifications is the amount of knot holes and occasional planer 

skips that are permitted. A recently introduced classification, 

called Framing, consists of hit-and-miss planing and permits 

a significant amount of adhesive to remain on the surface. 

This finishing is intended for uses that require one member 

to have the same width as the lumber used in manufacture 

for framing into walls. These members are often covered in 

the finished structure.

The next step in the manufacturing process is fabrication, 

where the final cuts are made, holes are drilled, connectors are added, and a finish or sealer is applied, if specified. 

For various members, different degrees of prefabrication 

are done at this point. Trusses may be partially or fully 

assembled. Moment splices can be fully fabricated, then 

disconnected for transportation and erection. End sealers, 

surface sealers, primer coats, and wrapping with waterproof 

paper or plastic all help to stabilize the moisture content of 

the glulam timber between the time it is manufactured and 

installed. The extent of protection necessary depends upon 

the end use and must be specified.

Preservative Treatment

In instances where the moisture content of the finished 

glulam timber will approach or exceed 20% (in most 

exterior and some interior uses), the glulam timber should 

be preservative-treated following AITC 109 (AITC 2007b). 

Three main types of preservatives are available: creosote, 

oilborne, and waterborne. Creosote and oilborne preservatives are applied to the finished glulam timbers. Some light 

oil solvent treatments can be applied to the lumber prior to 

gluing, but the suitability must be verified with the manufacturer. Waterborne preservatives are best applied to the 

lumber prior to the laminating and manufacturing process 

because they can lead to excessive checking if applied to 

large finished glulam timbers.

Structural Composite Lumber

Structural composite lumber (SCL) was developed in response to the increasing demand for high-quality lumber at 

a time when it was becoming difficult to obtain this type of 

lumber from the forest resource. Structural composite lumber products are characterized by smaller pieces of wood 

glued together into sizes common for solid-sawn lumber.

One type of SCL product is manufactured by laminating 

veneer with all plies parallel to the length. This product is 

called laminated veneer lumber (LVL) and consists of specially graded veneer. Another type of SCL product consists 

of strands of wood or strips of veneer glued together under 

high pressures and temperatures. Depending upon the component material, this product is called laminated strand lumber (LSL), parallel strand lumber (PSL), or oriented strand 

lumber (OSL). These types of SCL products can be manufactured from raw materials, such as aspen or other underutilized species, that are not commonly used for structural 

applications. Different widths of lumber can be ripped from 

SCL for various uses. Compared with similar size solidsawn lumber, SCL often provides a stronger, more reliable 

structural member that can often span greater distances and 

has less dimensional change.

Structural composite lumber is a growing segment of the engineered wood products industry. It is used as a replacement 

for lumber in various applications and in the manufacture of 

other engineered wood products, such as prefabricated wood 

I-joists, which take advantage of engineering design values 

that can be greater than those commonly assigned to sawn 

lumber.

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General Technical Report FPL–GTR–190
Laminated Veneer Lumber

Work in the 1940s on LVL targeted the production of highstrength parts for aircraft structures using Sitka spruce veneer. Research on LVL in the 1970s was aimed at defining 

the effects of processing variables for veneer up to 12.7 mm 

(1/2 in.) thick. Since the 1990s, production of LVL uses 

veneers 3.2 to 2.5 mm (1/8 to 1/10 in.) thick, which are hot 

pressed with phenol-formaldehyde adhesive into lengths 

from 2.4 to 18.3 m (8 to 60 ft) or more. Today LVL is commonly used as the flanges in composite I-joists.

Veneer for the manufacture of LVL must be carefully selected for the product to achieve the desired engineering 

properties. Veneers are often sorted using ultrasonic testing 

to ensure that the finished product will have the desired engineering properties.

End joints between individual veneers may be staggered 

along the product to minimize their effect on strength. These 

end joints may be butt joints, or the veneer ends may overlap for some distance to provide load transfer. Some producers provide structural end joints in the veneers using either 

scarf or fingerjoints. LVL may also be made in 2.4-m (8-ft) 

lengths, having no end joints in the veneer; longer pieces 

are then formed by end-jointing these pieces to create the 

desired length.

Sheets of LVL are commonly produced in 0.6- to 1.2-m 

(2- to 4-ft) widths in a thickness of 38 mm (1.5 in.). Continuous presses can be used to form a potentially endless sheet, 

which is cut to the desired length. Various widths of lumber 

can be manufactured at the plant or the retail facility.

Parallel Strand Lumber

Parallel strand lumber (PSL) is defined as a composite of 

wood strand elements with wood fibers oriented primarily 

along the length of the member. The least dimension of the 

strands must not exceed 6.4 mm (0.25 in.), and the average 

length of the strands must be a minimum of 150 times the 

least dimension. PSL is a proprietary product, sold as Parallam®. It is often used for large beams and columns, typically 

as a replacement of solid-sawn lumber or glulam.

Parallel strand lumber is manufactured using veneer about 

3 mm (1/8 in.) thick, which is then clipped into strands 

about 19 mm (3/4 in.) wide. These strands are commonly 

at least 0.6 m (24 in.) long. The manufacturing process was 

designed to use the material from roundup of the log in the 

veneer cutting operation as well as other less than full-width 

veneer (Fig. 11–15). Thus, the process can utilize waste material from a plywood or LVL operation. Species commonly 

used for PSL include Douglas-fir, southern pines, western 

hemlock, and yellow-poplar, but there are no restrictions 

on using other species.

The strands are coated with a waterproof structural adhesive, commonly phenol-resorcinol formaldehyde, and 

oriented in a press using special equipment to ensure proper 

orientation and distribution. The pressing operation results 

in densification of the material, and the adhesive is cured 

using microwave technology. Billets larger than those 

of LVL are commonly produced; a typical size is 0.28 by 

0.48 m (11 by 19 in.). This product can then be sawn into 

smaller pieces, if desired. As with LVL, a continuous press 

is used so that the length of the product is limited by handling restrictions.

Laminated Strand Lumber and Oriented 

Strand Lumber

Laminated strand lumber (LSL) and oriented strand lumber 

(OSL) products are an extension of the technology used to 

produce oriented strandboard (OSB) structural panels. The 

products have more similarities than differences. The main 

difference is that the aspect ratio of strands used in LSL 

is higher than for OSL (AF&PA 2006). One type of LSL 

uses strands that are about 0.3 m (12 in.) long, which is 

somewhat longer than the strands commonly used for OSB. 

Waterproof adhesives are used in the manufacture of LSL. 

One type of product uses an isocyanate type of adhesive 

that is sprayed on the strands and cured by steam injection. 

This product needs a greater degree of alignment of the 

strands than does OSB and higher pressures, which result in 

increased densification. Both LSL and OSL are proprietary 

products; LSL is sold as TimberStrand®. Applications such 

as studs and millwork are common.

Advantages and Uses

In contrast with sawn lumber, the strength-reducing characteristics of SCL are dispersed within the veneer or strands 

and have much less effect on strength properties. Thus, 

relatively high design values can be assigned to strength 

properties for both LVL and PSL. Whereas both LSL and 

OSL have somewhat lower design values, they have the 

advantage of being produced from a raw material that need 

not be in a log size large enough for peeling into veneer. All 

SCL products are made with structural adhesives and are 

dependent upon a minimum level of strength in these bonds.

All SCL products are made from veneers or strands that are 

dried to a moisture content that is slightly less than that for 

most service conditions. Thus, little change in moisture content will occur in many protected service conditions. When 

used indoors, this results in a product that is less likely to 

warp or shrink in service. However, the porous nature of 

both LVL and PSL means that these products can quickly 

absorb water unless they are provided with some protection.

All types of SCL products can be substituted for sawn lumber products in many applications. Laminated veneer lumber 

is used extensively for scaffold planks and in the flanges of 

prefabricated I-joists, which take advantage of the relatively 

high design properties. Both LVL and PSL beams are used 

as headers and major load-carrying elements in construction. The LSL and OSL products are used for band joists in 

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Chapter 11 Wood-Based Composite Materials
floor construction and as substitutes for studs and rafters in 

wall and roof construction. Various types of SCL are also 

used in a number of nonstructural applications, such as the 

manufacture of windows and doors.

Standards and Specifications

The ASTM D 5456 (ASTM 2008b) standard provides methods to develop design properties for SCL products as well as 

requirements for quality assurance during production. Each 

manufacturer of SCL products is responsible for developing 

the required information on properties and ensuring that the 

minimum levels of quality are maintained during production. An independent inspection agency is required to monitor the quality assurance program.

Unlike lumber, no standard grades or design stresses have 

been established for SCL. Each manufacturer may have 

unique design properties and procedures. Thus, the designer 

should consult information provided by the manufacturer.

Wood–Nonwood Composite 

Materials

Wood may be combined with inorganic materials and 

with plastics to produce composite products with unique 

properties. Wood–nonwood composites typically contain 

wood elements suspended in a matrix material (for example 

in fiber-reinforced gypsum board, or in thermoplastic material), in which the proportion of wood elements may account 

for less than 60% of product mass.

The primary impetus for developing such products has come 

from one or more of the following research and development goals:

•	 Develop “green” or “environmentally benign” products 

with enhanced sustainability.

•	 Reduce material costs by combining a lower cost material (acting as a filler or extender) with an expensive 

material.

•	 Develop products that can utilize recycled materials and 

be recyclable in themselves.

•	 Produce composite products that exhibit specific properties that are superior to those of the component materials alone (for example, increased strength-to-weight 

ratio, improved abrasion resistance, enhance resistance 

to fire, decay, and insects).

Composites made from wood and other non-wood materials create enormous opportunities to match product performance to end-use requirements. The following discussion 

includes the most common type of wood–nonwood composites: inorganic-bonded and wood–thermoplastic composites.

Inorganic-Bonded Composite Materials

Inorganic-bonded wood composites have a long and varied 

history that started with commercial production in Austria 

in 1914. They are now used in many countries in the world, 

mostly in panel form. Applications include panel products, 

siding, roofing tiles, and precast building members.

Figure 11–15. Generalized process for manufacturing PSL. (Courtesy of iLevel by Weyerhaeuser, 

Federal Way, Washington. Used by permission.)

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General Technical Report FPL–GTR–190
Inorganic-bonded wood composites are molded products or 

boards that contain between 10% and 70% by weight wood 

particles or fibers and conversely 90% to 30% inorganic 

binder. Acceptable properties of an inorganic-bonded wood 

composite can be obtained only when the wood particles are 

fully encased, and the binder is a continuous matrix material. This differs considerably from conventional wood-based 

composites, where flakes or particles are “spot welded” by 

a binder applied as a finely distributed spray or powder. 

Because of this difference and because hardened inorganic 

binders have a higher density than most thermosetting 

resins, the required amount of inorganic binder per unit 

volume of composite material is much higher than that of 

resin-bonded wood composites. The properties of inorganicbonded wood composites are significantly influenced by the 

amount and type of the inorganic binder and the wood element as well as the density of the composites.

Inorganic-bonded composites include gypsum-bonded, 

cement-bonded, and ceramic-bonded composites. Magnesia 

and Portland cement are the most common cement binders. 

Gypsum and magnesia cement are sensitive to moisture, 

and their use is generally restricted to interior applications. 

Composites bonded with Portland cement are more durable 

than those bonded with gypsum or magnesia cement and are 

used in both interior and exterior applications. Inorganicbonded composites are made by blending wood elements 

with inorganic materials in the presence of water and allowing the inorganic material to cure or “set up” to make a rigid 

composite. Some inorganic-bonded composites are very resistant to deterioration by decay fungi, insects, and vermin. 

Most have appreciable fire resistance.

An advantage of inorganic-bonded composites is that their 

manufacture is adaptable to either end of the cost and technology spectrum. This is facilitated by the fact that no heat 

is required to cure the inorganic material. This versatility 

makes inorganic-bonded composites ideally suited to a variety of lignocellulosic materials. With a very small capital 

investment, satisfactory inorganic-bonded lignocellulosic 

composite building materials can be produced on a small 

scale using mostly unskilled labor. If the market for such 

composites increases, technology can be introduced to increase manufacturing throughput. The labor force can be 

trained concurrently with the gradual introduction of more 

sophisticated technology.

Gypsum-Bonded Composite Materials

Paper-faced gypsum boards have been widely used since the 

1950s for the interior lining of walls and ceilings. They are 

commonly called drywall because they often replaced wet 

plaster systems. These panels are critical for good fire ratings in walls and ceilings. Paper-faced gypsum boards also 

find use as exterior wall sheathing. Gypsum sheathing panels are primarily used in commercial construction, usually 

over steel studding, and are distinguished from gypsum drywall by their water repellent additives in the paper facings 

and gypsum core. The facings of drywall and of gypsum 

sheathing panels are adhered to the gypsum core, providing 

the panels with impact resistance, and bending strength and 

stiffness. The paper facings of gypsum panels are derived 

from recycled paper fiber.

An alternative to adhered facings is to incorporate lignocellulosic fiber (typically recycled paper fiber) in the gypsum 

core to make what are termed fiber-reinforced gypsum panels. In the production process, a paste of gypsum and water 

is mixed with the recycled paper fiber and extruded into a 

panel without facings. Shortly after formation, the panel is 

dried in an oven. Bonding occurs between the gypsum and 

the fiber as hydrate crystals form.

Fiber-reinforced gypsum panels are typically stronger and 

more resistant to abrasion and indentation than paper-faced 

drywall panels and also have a moderate fastener-holding 

capability. They are marketed for use as interior finish panels (drywall). Additives can provide a moderate degree of 

water resistance, for use as sheathing panels, floor underlayment, roof underlayment, or tile-backer board.

Cement-Bonded Composite Materials

The properties of cement-bonded composites are influenced 

by wood element characteristics (species, size, geometry, 

chemical composition), cement type, wood–water–cement 

ratio, environmental temperature, and cure time (Jorge and 

others 2004). They are heavier than conventional woodbased composites but lighter than concrete. Therefore they 

can replace concrete in construction, specifically in applications that are not subjected to loads. Wood–cement composites provide an option for using wood residues, or even 

agricultural residues. However, species selection can be important because many species contain sugars and extractives 

that retard the cure of cement (Bowyer and others 2007).

Magnesia-Cement-Bonded Composite Materials

Fewer boards bonded with magnesia cement have been 

produced than Portland-cement-bonded panels, mainly because of price. However, magnesia cement does offer some 

manufacturing advantages over Portland cement. First, the 

various sugars in lignocellulosics do not have as much effect on the curing and bonding. Second, magnesia cement is 

more tolerant of high water content during production. This 

opens up possibilities to use lignocellulosics not amenable 

to Portland cement composites, without leaching or other 

modification, and to use alternative manufacturing processes 

and products. Although composites bonded with magnesia 

cement are considered water sensitive, they are much less so 

than gypsum-bonded composites.

One successful application of magnesia cement is a lowdensity panel made for interior ceiling and wall applications. 

In the production of this panel product, wood wool (excelsior) is laid out in a low-density mat. The mat is then sprayed 

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Chapter 11 Wood-Based Composite Materials
with an aqueous solution of magnesia cement, pressed, and 

cut into panels (Fig. 11–16).

Other processes have been suggested for manufacturing 

magnesia-cement-bonded composites. For example, a slurry 

of magnesia cement, water, and lignocellulosic fiber may be 

sprayed onto existing structures as fireproofing. Extrusion 

into a pipe-type profile or other profiles is also possible.

Portland-Cement-Bonded Composite Materials

The most widely used cement-bonded composites are those 

bonded with Portland cement. Portland cement, when 

combined with water, reacts in a process called hydration 

to solidify into a solid stone-like mass and bind aggregate 

materials. Successfully marketed Portland-cement-bonded 

composites consist of both low-density products made with 

excelsior and high-density products made with particles and 

fibers.

Low-density products may be used as interior ceiling and 

wall panels in commercial buildings. In addition to the 

advantages described for low-density magnesia-bonded 

composites, low-density composites bonded with Portland cement offer sound control and can be decorative. In 

some parts of the world, these panels function as complete 

wall and roof decking systems. The exterior of the panels 

is coated with stucco, and the interior is plastered. Highdensity panels can be used as flooring, roof sheathing, fire 

doors, load-bearing walls, and cement forms. Fairly complex shapes, such as decorative roofing tiles or non-pressure 

pipes, can be molded or extruded.

The largest volume of cement-bonded wood-based composite materials manufactured in North America is fiber-cement 

siding. Fiber-cement siding incorporates delignified wood 

fiber into a Portland cement matrix.

Problems and Solutions of Cement-Bonded Composite 

Materials

The use of cement for wood-based composites involves limitations and tradeoffs. Embrittlement of the lignocellulosic 

component is known to occur and is caused by the alkaline 

environment provided by the cement matrix. In addition, 

hemicellulose, starch, sugar, tannins, and lignin, each to a 

varying degree, affect the cure rate and ultimate strength of 

these composites. To make strong and durable composites, 

measures must be taken to ensure long-term stability of the 

lignocellulosic in the cement matrix. To overcome these 

problems, various schemes have been developed. The most 

common is leaching, whereby the lignocellulosic is soaked 

in water for 1 or 2 days to extract some of the detrimental 

components. However, in some parts of the world, the water 

containing the leachate is difficult to dispose of. Low water–

cement ratios are helpful, as is the use of curing accelerators 

like calcium carbonate. Conversely, low-alkali cements have 

been developed, but they are not readily available throughout the world. Two other strategies involve the use of natural pozzolans and carbon dioxide treatment.

Pozzolans—Pozzolans are defined as siliceous or siliceous 

and aluminous materials that can react chemically with 

calcium hydroxide (slaked lime) at normal temperatures in 

the presence of water to form cement compounds. Some 

common pozzolanic materials include volcanic ash, fly ash, 

rice husk ash, and condensed silica fume. All these materials 

can react with lime at normal temperatures to make a natural 

water-resistant cement.

In general, when pozzolans are blended with Portland cement, they increase the strength of the cement but slow the 

cure time. More importantly, pozzolans decrease the alkalinity of the product.

Carbon Dioxide Treatment—In the manufacture of a 

cement-bonded lignocellulosic composite, the cement hydration process normally requires from 8 to 24 h to develop 

sufficient board strength and cohesiveness to permit the 

release of consolidation pressure. By exposing the cement to 

carbon dioxide, the initial hardening stage can be reduced to 

less than 5 min. This phenomenon results from the chemical 

reaction of carbon dioxide with calcium hydroxide to form 

calcium carbonate and water.

Reduction of initial cure time of the cement-bonded lignocellulosic composite is not the only advantage of using carbon dioxide injection. Certain species of wood have various 

amounts of sugars and tannins that interfere with the hydration or setting of Portland cement. Research has shown that 

the use of carbon dioxide injection reduces the likelihood 

that these compounds will inhibit the hydration process, 

Figure 11–16. Commercial cement-bonded composite 

panel. (Courtesy of Ty-Mawr Lime Ltd., UK. Used by 

permission.)

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General Technical Report FPL–GTR–190
thus allowing the use of a wider range of species. In addition, research has demonstrated that composites treated with 

carbon dioxide can be twice as stiff and strong as untreated 

composites (Geimer and others 1992). Finally, carbondioxide-treated composites do not experience efflorescence 

(migration of calcium hydroxide to surface of material), 

so the appearance of the surface of the final product is not 

changed over time.

Ceramic-Bonded Composite Materials

In the last few years a new class of inorganic binders, nonsintered ceramic inorganic binders, has been developed. 

These non-sintered ceramic binders are formed by an acid–

base aqueous reaction between a divalent or trivalent oxide 

and an acid phosphate or phosphoric acid. The reaction slurry hardens rapidly, but the rate of setting can be controlled. 

With suitable selection of oxides and acid-phosphates, a 

range of binders may be produced. Recent research suggests that phosphates may be used as adhesives, cements, or 

surface augmentation materials to manufacture wood-based 

composites (Jeong and Wagh 2003, Wagh and Jeong 2003).

As adhesives, the reaction slurry resulting from the acid–

base reaction may be used as an adhesive similar to the current polymer resins. Thus, phosphate adhesives can be used 

to coat individual fibers and form a composite by binding 

the fibers to each other. The adhesives will behave much 

like current polymer resins and may be used with existing 

equipment. The binder content is typically 15% to 20 % by 

weight.

As a cement, phosphate binders can be used to produce bulk 

composites. When conventional cement is used in fiberbased products, typical cement loading is approximately 

30% or higher; phosphate cements may be used in a similar 

manner. The slurry formed by the acid–base reaction may 

be mixed with fiber or any other extender to produce solid 

composites (Jeong and Wagh 2003).

Phosphate binders may also be used for coating wood-based 

composite panels to enhance surface properties. The phosphate slurry is very smooth; thin (<1 mm) coatings can be 

applied, suitable for providing fire or water resistance.

Wood–Thermoplastic Composite Materials

In North America and Europe, wood elements have been 

combined with thermoplastics for several decades. However, it is only in the past decade that wood–thermoplastic 

composites have become a widely recognized commercial 

product in construction, automotive, furniture, and other 

consumer applications (Oksman Niska and Sain 2008). 

Commercialization in North America has been primarily due 

to penetration into the construction industry, first as decking 

and window profiles, followed by railing, siding, and roofing. Interior molding applications are also receiving attention. The automotive industry in Europe has been a leader 

in using wood–thermoplastic composites for interior panel 

parts and is leading the way in developing furniture applications. Manufacturers in Asia are targeting the furniture 

industry, in addition to interior construction applications. 

Continued research and development will expand the available markets and each application will penetrate the global 

marketplace.

Materials

Broadly defined, a thermoplastic softens when heated and 

hardens when cooled. Thermoplastics selected for use with 

wood generally melt or soften at or below the thermal degradation temperature of the wood element, normally 200 to 

220 °C (392 to 428 °F). These thermoplastics include polypropylene, polystyrene, vinyls, and low- and high-density 

polyethylenes.

The term wood–thermoplastic composites is broad, and the 

class of materials can include fibers derived from wood or 

other natural sources. Geographical location often dictates 

the raw material choice. In North America, wood is the most 

common raw material, in Europe natural fibers such as jute, 

hemp, and kenaf are preferred, while rice hull flour and 

bamboo fiber are typical in Asia. The wood is incorporated 

as either fiber bundles with low aspect ratio (wood flour) or 

as single fibers with higher aspect ratio (wood fiber). Wood 

flour is processed commercially, often from post-industrial 

materials such as planer shavings, chips, and sawdust. Several grades are available depending upon wood species and 

particle size. Wood fibers, although more difficult to process 

than wood flour, can lead to superior composite properties and act more as a reinforcement than as a filler. A wide 

variety of wood fibers are available from both virgin and 

recycled resources.

Other materials can be added to affect processing and product performance of wood–thermoplastic composites. These 

additives can improve bonding between the thermoplastic 

and wood component (for example, coupling agents), product performance (impact modifiers, ultraviolet (UV) light 

stabilizers, flame retardants), and processability (lubricants).

Wood–thermoplastic composites are of two main types. In 

the first type, the wood element serves as a reinforcing agent 

or filler in a continuous thermoplastic matrix. In the second 

type, the thermoplastic serves as a binder to the wood elements much like conventional wood-based composites. The 

presence or absence of a continuous thermoplastic matrix 

may also determine the processability of the composite material. In general, if the matrix is continuous, conventional 

thermoplastic processing equipment may be used to process 

composites; however, if the matrix is not continuous, other 

processes may be required. For the purpose of discussion, 

we present two scenarios—composites with high and low 

thermoplastic content.

11–25

Chapter 11 Wood-Based Composite Materials
Composite Materials with High Thermoplastic Content

The vast majority of commercially available wood–thermoplastic composites have high thermoplastic content. In 

composites with high thermoplastic content, the thermoplastic component is a continuous matrix and the wood element 

serves as a reinforcement or filler. The wood content is typically less than 60% by weight. In the great majority of reinforced thermoplastic composites available commercially, 

inorganic materials (for example, glass, clays, and minerals) 

are used as reinforcements or fillers. Wood-based materials 

offer some advantages over inorganic materials: they are 

lighter, much less abrasive, and renewable. Wood elements 

reinforce the thermoplastic by stiffening and strengthening 

and can improve thermal stability of the product compared 

with that of unfilled material.

The manufacture of thermoplastic composites is usually a 

two-step process. The raw materials are first mixed together 

in one step, and the composite blend is then formed into a 

product in the second step. The combination of these steps is 

called in-line processing, and the result is a single processing step that converts raw materials to end products. In-line 

processing can be very difficult because of control demands 

and processing trade-offs. As a result, it is often easier and 

more economical to separate the processing into a compounding step and a forming step (Clemons 2002).

Compounding is the feeding and dispersing of the wood element in a molten thermoplastic to produce a homogeneous 

material. Various additives are added and moisture is removed during compounding. Compounding may be accomplished using either batch mixers (for example, internal and 

thermokinetic mixers) or continuous mixers (for example, 

extruders and kneaders).

The compounded material can be immediately pressed or 

shaped into an end product while still in its molten state 

or pelletized into small, regular pellets for future reheating 

and forming. The most common types of product-forming 

methods for wood–thermoplastic composites involve forcing molten material through a die (sheet or profile extrusion) 

or into a cold mold (injection molding), or pressing in calenders (calendering) or between mold halves (thermoforming and compression molding). Most wood–thermoplastic 

composites in North America are formed using profile extrusion. Products such as decking, siding, railings, and window 

profiles readily lend themselves to extrusion through a twodimensional die (Fig. 11–17). Injection-molded applications 

such as consumer household goods and furniture parts are 

gaining importance (Fig. 11–18). Thermoforming or compression molding is the forming method of choice for the 

automotive industry.

Several factors must be considered when processing 

wood with thermoplastics. Moisture can disrupt many 

thermoplastic processes, resulting in poor surface quality, 

voids, and unacceptable parts. Either materials must be 

pre-dried or vented equipment must be used to remove 

moisture. The low thermal degradation temperature of wood 

must also be considered. As a general rule, melt temperatures should be kept below 200 °C (392 °F), except for short 

periods. Higher temperatures can result in the release of 

volatiles, discoloration, odor, and embrittlement of the wood 

component. Although processing wood flour in thermoplastics is relatively easy, the low bulk density and difficulty of 

dispersing fibrous materials in thermoplastics is more difficult. More intensive mixing and the use of special feeding 

equipment may be necessary to handle longer fibers.

Composite Materials with Low Thermoplastic Content

In composites with low thermoplastic content, the thermoplastic component is not continuous, acting more as a binder 

for the fiber much the same way as binders in conventional 

wood-based composites. Thermoplastic content is typically 

Figure 11–17. Extruded wood–thermoplastic composites being evaluated for a siding application (Clemons 

and Stark 2007).

Figure 11–18. Injection-molded wood–thermoplastic 

composites in a variety of shapes and forms.

11–26

General Technical Report FPL–GTR–190
less than 30% by weight. In their simplest form, lignocellulosic particles or fibers can be dry-blended with thermoplastic granules, flakes, or fibers and pressed into panel products. An alternative is to use the thermoplastic in the form of 

a textile fiber. The thermoplastic textile fiber enables a variety of lignocellulosics to be incorporated into a low-density, 

non-woven, textile-like mat. The mat may be a product in 

itself, or it may be consolidated into a high-density product.

Because the thermoplastic component remains molten when 

hot, different pressing strategies must be used than when 

thermosetting binders are used. Two options have been 

developed to accommodate these types of composites. In 

the first, the material is placed in the hot press at ambient 

temperature. The press then closes and consolidates the material, and heat is used to melt the thermoplastic component, 

which flows around the lignocellulosic component. The 

press is then cooled, “freezing” the thermoplastic so that the 

composite can be removed from the press. Alternatively, the 

material can be first heated in an oven or hot press. The hot 

material is then transferred to a cool press where it is quickly consolidated and cooled to make a rigid panel. Some 

commercial nonstructural wood–thermoplastic composite 

panels are made in this way.

Literature Cited

AF&PA. 2006. Structural composite lumber & glued laminated timber awareness guide. Washington, DC: American 

Forest & Paper Association.

AITC. 1996. Standard specifications for structural glued 

laminated timber of hardwood species. AITC 119. Centennial, CO: American Institute of Timber Construction.

AITC. 2001. Standard appearance grades for structural 

glued laminated timber. AITC 110. Centennial, CO: American Institute of Timber Construction.

AITC. 2004a. Standard specifications for structural glued 

laminated timber of softwood species. AITC 117. Centennial, CO: American Institute of Timber Construction.

AITC. 2004b. Timber Construction Manual. American Institute of Timber Construction. 5th ed. New York: John Wiley 

& Sons.

AITC. 2007a. Standard for wood products: structural gluelaminated timber. ANSI/AITC 190.1. Centennial, CO: 

American Institute of Timber Construction.

AITC. 2007b. Standard for preservative treatment of 

structural glued laminated timber. AITC 109. Centennial, 

CO: American Institute of Timber Construction.

ANSI. 2006. Structural glued laminated timber for utility 

structures. ANSI 05.2. New York: American National Standards Institute.

ASTM. 2008a. ASTM D 3737–08. Standard practice for 

establishing allowable properties for structural glued laminated timber (glulam). Annual Book of ASTM Standards. 

Philadelphia, PA: American Society for Testing and 

Materials.

ASTM. 2008b. ASTM D 5456–03. Standard specification 

for structural composite lumber. Annual Book of ASTM 

Standards. Philadelphia, PA: American Society for Testing 

and Materials.

ASTM. 2008c. ASTM C 208–08a. Standard specification 

for cellulosic fiber insulating board. Annual Book of ASTM 

Standards. Philadelphia, PA: American Society for Testing 

and Materials.

AWPA. 2007a. Commodity specification H: fire retardants. 

AWPA Standard U–1: USS: User standard for treated wood. 

Birmingham, AL: American Wood Protection Association.

AWPA. 2007b. Commodity specification F: wood composites. AWPA Standard U–1: USS: User standard for treated 

wood. Birmingham, AL: American Wood Protection Association.

Bowyer, J.L.; Shmulsky, R.; Haygreen, J.G. 2007. Forest 

Products and Wood Science. 5th ed. Ames, IA: Blackwell 

Publishing Professional. 558 p.

Clemons, C.M. 2002. Wood-plastic composites in the United States: the interfacing of two industries. Forest Products 

Journal. 52(6): 10–18.

Clemons, C.; Stark, N. 2007. Use of saltcedar and Utah 

juniper as fillers in wood-plastic composites. Res. Pap. FPL 

RP–641. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 17 p.

CPA. 2004a. Basic hardboard. ANSI A 135.4–2004.

 Leesburg, VA: Composite Panel Association.

CPA. 2004b. Prefinished hardboard paneling. ANSI 

A 135.5–2004. Leesburg, VA: Composite Panel Association.

CPA. 2006. Hardboard siding. ANSI A 135.6–2006. 

Leesburg, VA: Composite Panel Association.

CPA. 2009a. Particleboard. ANSI A 208.1–2009. Leesburg, 

VA: Composite Panel Association.

CPA. 2009b. Medium density fiberboard (MDF). ANSI 

A 208.2–2009. Leesburg, VA: Composite Panel Association.

Geimer, R.L.; Souza, M.R.; Moslemi, A.A.; Simatupang, 

M.H. 1992. Carbon dioxide application for rapid production 

of cement particleboard. In: Moslemi, A., ed. Proceedings, 

inorganic bonded wood and fiber composite materials conference. Spokane, WA. September 27–30, 1992.

HPVA. 2004. American national standard for hardwood and 

decorative plywood. ANSI/HPVA HP–1–2004. Reston, VA: 

Hardwood Plywood & Veneer Association.

IRC. 2009. International residential code for one- and twofamily dwellings. Washington, DC: International Code 

Council.

11–27

Chapter 11 Wood-Based Composite Materials
Jeong, S.; Wagh, A. 2003. Cementing the gap between ceramics, cements and polymers. Materials Technology. 18(3): 

162–168.

Jorge, F.C.; Pereira, C.; Ferreira, J.M.F. 2004. Wood-cement 

composites: a review. Holz als Roh-und Werkstoff. 62: 

370–377.

Kretschmann, D.E.; Winandy, J.; Clausen, C.; Wiemann, 

M.; Bergman, R.; Rowell, R.; Zerbe, J.; Beecher, J.; White, 

R.; McKeever, D.; Howard, J. 2007. Wood. In: Kirk-Othmer 

Encyclopedia of Chemical Technology. New York: John 

Wiley & Sons, Inc. 60 p.

Maloney, T.M. 1986. Terminology and products definitions—a suggested approach to uniformity worldwide. In: 

Proceedings, 18th international union of forest research organization world congress; September. Ljubljana: Yugoslavia. IUFRO World Congress Organizing Committee.

Maloney, T.M. 1993. Modern particleboard and dry-process 

fiberboard manufacturing. San Francisco, CA: Miller Freeman Publishing, Inc. 681 p.

NIST. 2004. Voluntary product standard PS 2–04. Performance standard for wood-based structural-use panels. National Institute of Standards and Technology. Gaithersburg, 

MD: U.S. Department of Commerce.

NIST. 2007. Voluntary product standard PS 1–07. Structural 

plywood. National Institute of Standards and Technology. 

Gaithersburg, MD: U.S. Department of Commerce.

Oksman Niska, K.; Sain, M., eds. 2008. Wood-polymer 

composites. Cambridge, UK: Woodhead Publishing, Ltd. 

384 p.

Suchsland, O.; Woodson, G.E. 1986. Fiberboard manufacturing practices in the United States. Agric. Handb. 640. 

Washington, DC: U.S. Department of Agriculture. 263 p.

Wagh, A.S.; Jeong, S.Y. 2003. Chemically bonded phosphate ceramics: I, a dissolution model of formation. Journal 

of the American Ceramic Society. 86(11): 1838–44.

11–28

General Technical Report FPL–GTR–190
CHAPTER 12

Mechanical Properties of Wood-Based 

Composite Materials

Zhiyong Cai, Supervisory Research Materials Engineer

Robert J. Ross, Supervisory Research General Engineer

12–1

Contents 

Elastic Properties 12–2

 Modulus of Elasticity 12–2

 Shear Modulus 12–3

Strength Properties 12–3

Panel Products 12–3

 Plywood 12–3

 Oriented Strandboard (OSB) 12–4

 Particleboard 12–4

 Hardboard 12–4

 Medium-Density Fiberboard 12–5

Timber Elements/Structural Composite Lumber 12–5

 Glued-Laminated Timber 12–5

 Structural Composite Lumber 12–6

Wood–Nonwood Composites 12–7

 Wood–Plastic Composite 12–7

 Inorganic-Bonded Composites 12–9

Testing Standards 12–9

Literature Cited 12–10

The term composite is used to describe any wood material 

bonded together with adhesives. The current product mix 

ranges from fiberboard to laminated beams and components. 

In this chapter, wood-based composite materials are classified into the following categories: panel products (plywood, 

oriented strandboard (OSB), particleboard, fiberboard, 

medium-density fiberboard (MDF), hardboard); structural 

timber products (glued-laminated timber (glulam), laminated veneer lumber (LVL), laminated strand lumber, parallel strand lumber); and wood–nonwood composites (wood 

fiber–thermoplastics, inorganic-bonded composites).

Wood-based composites are used for a number of structural 

and nonstructural applications. Product lines include panels 

for both interior and exterior uses, furniture components, 

and support structures in buildings. Knowledge of the mechanical properties of these products is of critical importance to their proper use.

Wood-based composites are made from a wide range of 

materials—from fibers obtained from underutilized smalldiameter or plantation trees to structural lumber. Regardless 

of the raw material used in their manufacture, wood-based 

composites provide uniform and predictable in-service 

performance, largely as a consequence of standards used 

to monitor and control their manufacture. The mechanical 

properties of wood composites depend upon a variety of 

factors, including wood species, forest management regimes 

(naturally regenerated, intensively managed), the type of adhesive used to bind the wood elements together, geometry of 

the wood elements (fibers, flakes, strands, particles, veneer, 

lumber), and density of the final product (Cai 2006).

A wide range of engineering properties are used to characterize the performance of wood-based composites. Mechanical properties are typically the most frequently used to evaluate wood-based composites for structural and nonstructural 

applications. Elastic and strength properties are the primary 

criteria to select materials or to establish design or product 

specifications. Elastic properties include modulus of elasticity (MOE) in bending, tension, and compression. Strength 

properties usually reported include modulus of rupture 

(MOR, bending strength), compression strength parallel to 

surface, tension strength parallel to surface, tension strength 

perpendicular to surface (internal bond strength), shear 

strength, fastener holding capacity, and hardness. Model 
12–2

building codes in the United States stipulate that plywood 

used for structural applications such as subflooring and 

sheathing must meet the requirements of certain U.S. Department of Commerce standards. Voluntary Product Standard PS 1–07 for construction and industrial plywood (NIST 

2007) and Performance Standard PS 2–04 for wood-based 

structural-use panels (NIST 2004) spell out the ground rules 

for manufacturing plywood and establishing plywood or 

OSB properties, respectively. These standards have evolved 

over time from earlier documents (O’Halloran 1979, 1980; 

APA 1981) and represent a consensus opinion of the makers, sellers, and users of plywood products as well as other 

concerned parties.

Many of the questions that arise with wood-based composites have to do with their mechanical properties, especially 

how properties of one type of material compare with those 

of clear wood and other wood products. Although an extensive review that compares all properties of wood-based 

materials and products is beyond the scope of this chapter, 

Table 12–1 provides some insight to how static bending 

properties of these materials vary and how their properties 

compare with those of solid, clear wood. Although the mechanical properties of most wood composites might not be 

as high as those of solid wood, they provide very consistent 

and uniform performance.

The mechanical property data presented in this chapter were 

obtained from a variety of reports of research conducted 

to develop basic property information for a wide range of 

wood-based composite materials. The wood-based composites industry is very dynamic, with changes occurring 

frequently in the manufacture of these materials and corresponding changes in design information. Consequently, 

this chapter primarily focuses on presenting fundamental 

mechanical property information for wood-based composite 

materials. For design procedures and values, the reader is 

encouraged to contact the appropriate industry trade association or product manufacturers. Current design information can be readily obtained from their websites, technical 

handbooks, and bulletins.

The organization of this chapter follows closely that of 

Chapter 5. Basic mechanical property information is presented following a brief background discussion of these 

products. A discussion of performance and testing standards 

covering their manufacture and use is also presented.

Elastic Properties

Modulus of Elasticity 

Elasticity implies that deformations produced by low stress 

below the proportional limit are completely recoverable 

after loads are removed. When loaded to stress levels above 

the proportional limit, plastic deformation or failure occurs. 

Typically, the stress–strain curve for wood-based composites is linear below the proportional limit. The slope of the 

linear curve is the MOE. In compression or tensile tests, this 

slope is sometime referred to as Young’s modulus to differentiate it from bending MOE. Bending MOE is a measure 

of the resistance to bending deflection, which is relative to 

the stiffness. Young’s modulus is a measure of resistance 

to elongation or shortening of a member under tension or 

compression. The procedure to determine MOE is fully described in ASTM D 1037 for fiber- and particle-based panel 

products, ASTM D 3043 for structural wood-based panels, 

ASTM D 5456 for structural composite lumber products, 

General Technical Report FPL–GTR–190

Table 12–1. Static bending properties of different wood and wood-based composites

Material 

Specific 

gravity

Static bending properties 

Modulus of elasticity Modulus of rupture 

GPa (×106 lb in–2) MPa (lb in–2)

Clear wood 

White oak 0.68 12.27 (1.78) 104.80 (15,200) 

Red maple 0.54 11.31 (1.64) 92.39 (13,400) 

Douglas-fir (Coastal) 0.48 13.44 (1.95) 85.49 (12,400) 

Western white pine 0.38 10.07 (1.46) 66.88 (9,700) 

Longleaf pine 0.59 13.65 (1.98) 99.97 (14,500) 

Panel products 

Hardboard 0.9–1.0 3.10–5.52 (0.45–0.80) 31.02–56.54 (4,500–8,200) 

Medium-density fiberboard 0.7–0.9 3.59 (0.52) 35.85 (5,200) 

Particleboard 0.6–0.8 2.76–4.14 (0.40–0.60) 15.17–24.13 (2,200–3,500) 

Oriented strandboard 0.5–0.8 4.41–6.28 (0.64–0.91) 21.80–34.70 (3,161–5,027) 

Plywood 0.4–0.6 6.96–8.55 (1.01–1.24) 33.72–42.61 (4,890–6,180) 

Structural timber products 

Glued-laminated timber 0.4–0.6 9.00–14.50 (1.30–2.10) 28.61–62.62 (4,150–9,080) 

Laminated veneer lumber 0.4–0.7 8.96–19.24 (1.30–2.79) 33.78–86.18 (4,900–12,500)

Wood–nonwood composites 

Wood plastic 1.53–4.23 (0.22–0.61) 25.41–52.32 (3,684–7,585) 
12–3

Chapter 12 Mechanical Properties of Wood-Based Composite Materials

ASTM D 7031 for wood–plastic composites, and ASTM D 

7341 for glulam products.

Shear Modulus

Shear modulus, also called modulus of rigidity, indicates 

the resistance to deflection of a member caused by shear 

stresses. Shear stress is different from tension or compression stress in that it tends to make one side of a member slip 

past the other side of a member adjacent to it. There are two 

main types of shear in different planes of wood-based panels: interlaminar shear and edgewise shear or shear throughthe-thickness. Interlaminar shear is also commonly called 

planar shear (or rolling shear, or horizontal shear) in plywood panels to describe stress that acts between the veneers 

that are glued with grain direction in adjacent pieces perpendicular to one another. For example, when the plywood 

panel is loaded in the middle with its two ends simply supported, the layers or veneers tend to slip horizontally past 

each other as the panel bends. The glue-bonding between 

the laminates of veneers resists the slipping and often dictates the panel stiffness. Edgewise shear is also commonly 

called racking shear. The moduli of rigidity vary within and 

between species, resin application, moisture content, and 

specific gravity. The procedure to determine different shear 

moduli for fiber- and particle-based panels is described in 

ASTM D 1037 and for structural panels in ASTM D 3044.

Strength Properties

Strength refers to the maximum stress that can be developed 

in a member due to applied loads prior to failure. Mechanical properties most commonly measured and represented as 

“strength properties” for design include modulus of rupture 

in bending, tension strength parallel-to-surface, tension 

strength perpendicular-to-surface, compression strength 

parallel-to-surface, shear strength, fastener holding strength, 

and hardness. Strength tests are typically made on specimens at moisture equilibrium under prescribed conditions 

or after soaking. The procedures to determine strengths for 

wood-based composites are described in ASTM D 1037, 

ASTM D 3044, ASTM D 5456, ASTM D 3737, and 

ASTM D 7031.

Modulus of rupture reflects the maximum load-carrying 

capacity of a member in bending and is proportional to maximum moment borne by the specimen. Modulus of rupture 

is an accepted measure of strength, although it is not a true 

stress because the formula by which it is computed is valid 

only to the elastic limit (McNatt 1973).

Tension strength parallel-to-surface is the maximum stress 

sustained by a specimen from a test with tension forces applied parallel to the surface. Tests are made with the long dimension of the specimen cut both parallel and perpendicular 

to the long dimension of the board to determine the strength 

in each of the primary panel directions.

Tension strength perpendicular-to-surface (internal bond 

strength) is the maximum stress sustained by a specimen 

from a test with tension forces applied perpendicular to the 

surface. Tests are made on specimens in the dry condition 

to determine the resistance of the specimen to delamination 

or splitting in the direction perpendicular to the plane of the 

board.

Compression strength parallel-to-surface is the maximum 

stress sustained by a specimen from a test with compression forces applied parallel to the surface. Tests are made 

with the long dimension of the specimen cut both parallel 

and perpendicular to the long dimension of the board to determine the material’s resistance to crushing in each of the 

primary panel directions.

Interlaminar shear (planar shear) indicates the ability to 

resist internal slipping of one layer upon another within the 

panel. It is used to describe the glue line or bonding performance inside or between the test materials.

Hardness is measured as resistance to indentation using a 

modified Janka hardness test, measured by the load required 

to embed an 11.3-mm (0.444-in.) diameter ball to one-half 

its diameter.

Fastener holding strength is the maximum resistance to 

separate or withdraw a fastener in a plane normal to the 

testing face. It usually contains three tests: nail withdrawal, 

nail-head pull-through, and direct screw withdrawal.

Panel Products

Plywood 

Plywood is separated into two general classes: (a) construction and industrial plywood and (b) hardwood and decorative plywood. Construction and industrial plywood are 

covered by Product Standard PS 1–07 (NIST 2007), and 

hardwood and decorative plywood are covered by American National Standard ANSI/HPVA–1–2004 (HPVA 2004). 

Each standard recognizes different exposure durability classifications, which are primarily based on moisture resistance 

of the adhesive and the grade of veneer used. In addition, 

model building codes require that plywood manufacturers 

be inspected and their products certified for conformance 

to PS 1–07, PS 2–04, APA PRP–108, or TECO PRP–133 

(TECO 1991) by qualified independent third-party agencies 

on a periodic unannounced basis. With PS 1–07, as long as 

a plywood panel is manufactured using the veneer grades, 

adhesive, and construction established in the standard’s prescriptive requirements, the panel is by definition acceptable.

All hardwood plywood represented as conforming to American National Standard ANSI/HPVA–1–2004 (HPVA 2004) 

is identified by one of two methods: by marking each panel 

with the Hardwood Plywood & Veneer Association (HPVA) 

plywood grade stamp or by including a written statement 

with this information with the order or shipment.
12–4

If design calculations are desired, a design guide is provided 

by the APA–The Engineered Wood Association in Plywood 

Design Specification (PDS) and APA Technical Note N375B 

(APA 1995a,b). The design guide contains tables of grade 

stamp references, section properties, and allowable stresses 

for plywood used in construction of buildings and similar 

structures. Table 12–2 shows selected properties of various 

species of plywood.

Oriented Strandboard (OSB)

Oriented strandboard is an engineered, structural-use panel 

manufactured from thin wood strands bonded together with 

water-resistant adhesive under heat and pressure. It is used 

extensively for roof, wall, and floor sheathing in residential 

and commercial construction. Design capacities of performance-rated products, which include OSB and waferboard, 

can be determined by using procedures outlined in Technical Note N375B (APA 1995a). In this reference, allowable 

design strength and stiffness properties, as well as nominal 

thickness and section properties, are specified based on the 

span rating of the panel. Additional adjustment factors 

based on panel grade and construction are also provided. 

Table 12–3 shows selected properties of OSB obtained from 

the literature.

Under PS 2–04, a manufacturer is required to enter into an 

agreement with an accredited testing agency to demonstrate 

that its panels conform to the requirements of the chosen 

standard. The manufacturer must also maintain an in-plant 

quality control program in which panel properties are 

regularly checked, backed by a quality assurance program 

administered by an independent third-party. The third-party 

agency must visit the mill on a regular unannounced basis. 

The agency must confirm that the in-plant quality control 

program is being maintained and that panels meet the minimum requirements of the standard.

Particleboard

Particleboard is typically made in three layers. The faces 

of the board consist of fine wood particles, and the core is 

made of the coarser material (Chap. 11). Particleboard is 

used for furniture cores and case goods, where it is typically 

overlaid with other materials for decorative purposes. Particleboard can be used in flooring systems, in manufactured 

houses, for stair treads, and as underlayment. Requirements 

for grades of particleboard and particleboard flooring products are specified by the American National Standard for 

Particleboard A208.1-1999 (CPA 1999). Table 12–4 represents some of selected properties of different particleboard 

manufacturers.

Hardboard

Basic hardboard physical properties for selected products 

are presented in ANSI A135.4–2004 (CPA 2004a). The uses 

for hardboard can generally be grouped as construction, furniture and furnishings, cabinet and store work, appliances, 

and automotive and rolling stock. Typical hardboard products are prefinished paneling (ANSI A135.5–2004 

(CPA 2004b)), house siding (ANSI A135.6–2006 (CPA 

2006)), floor underlayment, and concrete form board. 

Table 12–5 shows selected physical and mechanical properties of hardboard from different manufacturers. Hardboard 

siding products come in a great variety of finishes and textures (smooth or embossed) and in different sizes. For application purposes, the Composite Panel Association (CPA) 

classifies siding into three basic types:

Lap siding—boards applied horizontally, with each board 

overlapping the board below it

Square edge panels—siding intended for vertical application in full sheets

Shiplap edge panel siding—siding intended for vertical application, with the long edges incorporating shiplap joints

The type of panel dictates the application method. The CPA 

administers a quality conformance program for hardboard 

for both panel and lap siding. Participation in this program 

is voluntary and is open to all (not restricted to CPA members). Under this program, hardboard siding products are 

General Technical Report FPL–GTR–190

Table 12–2. Selected properties of plywood sheathing productsa

Species 

Specific 

gravity

Static bending 

Rail shear 

strength 

Glue line shear 

MOE MOR strength 

Fiber stress at 

proportional

limit 

GPa

(×106

lb in–2) MPa (lb in–2) GPa (lb in–2) MPa (lb in–2) MPa (lb in–2)

Baldcypress 0.50 7.58 (1.10) 39.23 (5,690) 29.4 (4,260) 5.6 (805) 2.7 (389) 

Douglas-fir 0.53 7.45 (1.08) 41.37 (6,000) 39.3 (5,700) 3.8 (556) 1.4 (207) 

Lauan 0.44 7.43 (1.08) 33.72 (4,890) 28.1 (4,070) 4.3 (628) 1.3 (192) 

Western 

redcedar 

0.41 8.55 (1.24) 37.37 (5,420) 33.3 (4,830) 4.6 (674) 1.7 (240) 

Redwood 0.41 6.96 (1.01) 42.61 (6,180) 37.4 (5,420) 5.3 (769) 1.5 (220) 

Southern Pine 0.57 7.70 (1.12) 37.09 (5,380) 26.2 (3,800) 5.5 (800) 1.6 (233) 

a

From Biblis (2000). 
12–5

tested by an independent laboratory in accordance with 

product standard ANSI A135.6.

Medium-Density Fiberboard

Minimum property requirements for MDF are specified by 

the American National Standard for MDF, ANSI A208.2-

2002 (CPA 2002), and some of selected properties are given 

in Table 12–6 from different manufacturers. Medium-density fiberboard is frequently used in furniture applications. 

It is also used for interior door skins, moldings, flooring 

substrate, and interior trim components (Cai and others 

2006, Youngquist and others 1993).

Timber Elements/Structural 

Composite Lumber

Glued-Laminated Timber

Structural glued-laminated timber (glulam) is an engineered, 

stress-rated product that consists of two or more layers of 

lumber that are glued together with the grain of all layers, 

Chapter 12 Mechanical Properties of Wood-Based Composite Materials

Table 12–3. Selected properties of oriented strandboard (OSB) products

Reference Species 

Mill 

no.

Specific 

gravity

Bending MOE Bending MOR 

Parallel Perpendicular Parallel Perpendicular 

Internal

bond

GPa

(×106

lb in–2) GPa

(×106

lb in–2) MPa (lb in–2) MPa (lb in–2) MPa (lb in–2)

Biblis 

(1989)

Southern

Pine

1 0.80 4.41 (0.640) 2.89 (0.419) 23.8 (3,445) 24.2 (3,515) 0.57 (83) 

2 0.70 4.78 (0.694) 2.61 (0.378) 26.0 (3,775) 22.1 (3,205) 0.28 (41) 

3 0.68 5.75 (0.834) 3.17 (0.460) 32.0 (4,645) 23.8 (3,445) 0.32 (47) 

Pu and 

others

(1992)

Southern

Pine

4 0.51 4.41 (0.640) 2.40 (0.348) 21.8 (3,161) 25.4 (3,685) 0.23 (34) 

5 0.60 5.67 (0.822) 2.61 (0.378) 27.8 (4,039) 27.1 (3,925) 0.28 (41) 

6 0.58 4.41 (0.640) 2.97 (0.431) 23.9 (3,473) 28.7 (4,165) 0.26 (38) 

Aspen 7 0.65 6.28 (0.911) 2.03 (0.294) 32.2 (4,672) 30.4 (4,405) 0.43 (62) 

8 0.66 5.69 (0.825) 1.92 (0.278) 31.6 (4,584) 32.0 (4,645) 0.41 (60) 

9 0.74 6.31 (0.915) 2.79 (0.404) 34.7 (5,027) 33.7 (4,885) 0.34 (50) 

Wang and 

others

(2003a) 

Southern

Pine

10 0.63 5.01 (0.726) 2.26 (0.327) 30.2 (4,379) 16.8 (2,436) 0.36 (52) 

11 0.66 5.30 (0.769) 2.32 (0.336) 28.1 (4,075) 14.4 (2,088) 0.43 (62) 

12 0.67 5.12 (0.742) 2.56 (0.371) 30.7 (4,452) 21.1 (3,060) 0.32 (46) 

13 0.66 4.91 (0.712) 2.24 (0.325) 28.3 (4,104) 19.8 (2,871) 0.38 (55) 

Hardwood

mixture

14 0.68 5.15 (0.747) 1.77 (0.257) 26.9 (3,901) 11.8 (1,711) 0.28 (40) 

15 0.67 5.87 (0.851) 1.40 (0.204) 33.9 (4,916) 7.8 (1,131) 0.23 (33) 

16 0.70 6.73 (0.976) 2.25 (0.326) 36.9 (5,351) 15.8 (2,291) 0.45 (66) 

Aspen 17 0.63 6.50 (0.943) 3.10 (0.450) 38.0 (5,510) 21.5 (3,118) 0.28 (41) 

18 0.62 7.90 (1.146) 3.10 (0.450) 38.8 (5,626) 23.2 (3,364) 0.46 (66) 

19 0.61 6.10 (0.885) 2.50 (0.363) 30.7 (4,452) 19.7 (2,857) 0.34 (49) 

20 0.61 6.50 (0.943) 1.80 (0.261) 35.5 (5,148) 13.7 (1,987) 0.25 (36) 

21 0.66 6.75 (0.979) 2.45 (0.356) 37.3 (5,409) 19.3 (2,799) 0.38 (55) 

22 0.63 5.80 (0.840) 2.40 (0.348) 26.9 (3,901) 17.9 (2,596) 0.40 (58) 

Table 12–4. Selected properties of industrial particleboard productsa

Mill 

Moisture

content 

(%)

Specific 

gravity

Static bending properties Tensile properties 

Modulus of 

elasticity 

Modulus of 

rupture

Modulus of 

elasticity 

Ultimate tensile 

stress Internal bond 

GPa

(×106

lb in–2) MPa (lb in–2) GPa 

(×106

lb in–2) MPa (lb in–2) MPa (lb in–2)

A 8.7 0.71 3.0 (0.44) 16.8 (2,430) 2.2 (0.32) 7.72 (1,120) 0.79 (115) 

B 9.1 0.72 3.5 (0.51) 20.6 (2,990) 2.6 (0.38) 9.38 (1,360) 1.07 (155) 

C 9.8 0.76 3.5 (0.51) 18.9 (2,740) 2.3 (0.34) 8.27 (1,200) 1.00 (145) 

H 8.0 0.77 4.0 (0.58) 22.8 (3,310) 3.0 (0.44) 10.89 (1,580) 1.17 (170) 

J 8.5 0.72 3.0 (0.43) 17.2 (2,500) 1.9 (0.28) 7.45 (1,080) 0.45 (65) 

K 9.1 0.68 2.8 (0.40) 15.2 (2,206) 1.6 (0.23) 5.58 (810) 0.31 (45) 

L 9.3 0.62 3.2 (0.46) 17.0 (2,470) 1.8 (0.26) 6.69 (970) 0.48 (70) 

M 9.7 0.65 3.6 (0.52) 18.9 (2,740) 2.2 (0.32) 8.07 (1,170) 0.69 (100) 

N 8.3 0.60 3.1 (0.45) 17.0 (2,470) 3.7 (0.54) 8.00 (1,160) 0.31 (45) 

a

From McNatt (1973). 
which are referred to as laminations, parallel to the length. 

Table 12–7 provides some selected properties of glulam 

products from different research studies.

Douglas–Fir–Larch, Southern Pine, yellow-cedar, Hem–Fir, 

and Spruce–Pine–Fir are commonly used for glulam in the 

United States. Nearly any species can be used for glulam 

timber, provided its mechanical and physical properties are 

suitable and it can be properly glued. Industry standards 

cover many softwoods and hardwoods, and procedures are 

in place for using other species.

Manufacturers of glulam timber have standardized the target 

design values in bending for beams. For softwoods, these 

design values are given in “Standard for Wood Products: 

Structural Glued-Laminated Timber” (AITC 2007). This 

specification contains design values and recommended modification of stresses for the design of glulam timber members 

in the United States. The National Design Specification for 

Wood Construction (NDS) summarizes the design information in ANSI/AITC 190.1 and defines the practice to be 

followed in structural design of glulam timbers (AF&PA 

2005). APA–The Engineered Wood Association has also developed design values for glulam under National Evaluation 

Report 486, which is recognized by all the building codes.

Structural Composite Lumber

Structural composite lumber (SCL) products are characterized by smaller pieces of wood glued together into sizes 

common for solid-sawn lumber. One type of SCL product is 

manufactured by laminating veneer with all plies parallel to 

the length. This product is called laminated veneer lumber 

(LVL) and consists of specially graded veneer. Another type 

of SCL product consists of strands of wood or strips of veneer glued together under high pressures and temperatures. 

Depending upon the component material, this product is 

called laminated strand lumber (LSL), parallel strand lumber (PSL), or oriented strand lumber (OSL).

In contrast with sawn lumber, the strength-reducing characteristics of SCL are dispersed within the veneer or strands 

and have much less of an effect on strength properties. Thus, 

12–6

General Technical Report FPL–GTR–190

Table 12–5. Selected properties of hardboard productsa

Mill 

Type of 

hardboard 

Moisture

content 

(%)

Specific

gravity

Modulus of 

elasticity 

Modulus of 

rupture

Ultimate tensile 

stress Internal bond 

GPa

(×106

lb in–2) MPa (lb in–2) MPa (lb in–2) MPa (lb in–2)

A 1/8-in. 

standard 

4.6 0.9 3.83 (556) 31.44 (4,560) 23.24 (3,370) 1.24 (180) 

B 6.5 1.02 4.36 (633) 33.92 (4,920) 23.17 (3,360) 2.76 (400) 

C 5.2 0.94 4.20 (609) 45.85 (6,650) 37.58 (5,450) 2.17 (315) 

D 5.6 0.9 3.32 (482) 38.75 (5,620) 28.61 (4,150) 1.55 (225) 

E 6.5 0.95 3.55 (515) 47.50 (6,890) 32.96 (4,780) 3.52 (510) 

F 7.7 0.91 3.23 (468) 37.85 (5,490) 25.72 (3,730) 1.93 (280) 

B 1/4-in. 

standard 

6.4 1.02 4.45 (645) 33.85 (4,910) 22.61 (3,280) 1.86 (270) 

E 6.0 0.90 3.88 (563) 38.96 (5,650) 23.65 (3,430) 1.65 (240) 

A 1/4-in. 

tempered 

4.9 0.99 5.30 (768) 53.02 (7,690) 31.58 (4,580) 1.79 (260) 

F 1/4-in. 

tempered 

6.9 0.98 5.14 (745) 55.57 (8,060) 30.61 (4,440) 1.86 (270) 

a

From McNatt and Myers (1993). 

Table 12–6. Selected properties of medium-density fiberboard productsa

Mill

no.

Density

(g cm–3)

Modulus of 

rupture

Modulus of 

elasticity Internal bond 

Screwholding 

edge Capacity face 

MPa (lb in–2) GPa 

(×106

lb in–2) MPa (lb in–2) kg (lb) kg (lb) 

1 0.73 33.6 (4,873) 3.21 (466) 0.86 (125) 117 (257) 148 (326) 

2 0.90 34.0 (4,932) 3.97 (576) 0.94 (136) 147 (325) 185 (407) 

3 0.79 23.2 (3,366) 2.98 (432) 1.94 (282) 150 (330) 202 (445) 

4 0.82 39.3 (5,703) 4.38 (635) 0.83 (121) 114 (252) 148 (326) 

5 0.95 24.6 (3,565) 3.56 (517) 0.92 (133) 184 (405) 231 (509) 

6 0.80 36.4 (5,278) 3.99 (578) 0.71 (103) 143 (315) 183 (404) 

7 0.77 37.4 (5,421) 3.94 (572) 1.23 (179) 163 (360) 210 (464) 

8 0.71 35.2 (5,107) 3.34 (485) 1.09 (158) 147 (324) 189 (416) 

a

From Suchsland and others (1979). 
relatively high design values can be assigned to strength 

properties for both LVL and PSL. Whereas both LSL and 

OSL have somewhat lower design values, they have the advantage of being produced from a raw material that need not 

be in a log size large enough for peeling into veneer.

All types of SCL products can be substituted for sawn lumber products in many applications. Laminated veneer lumber is used extensively for scaffold planks and in the flanges 

of prefabricated I-joists. Both LVL and PSL beams are used 

as headers and major load-carrying elements in construction. The LSL and OSL products are used for band joists in 

floor construction and as substitutes for studs and rafters in 

wall and roof construction. Various types of SCL are also 

used in a number of nonstructural applications, such as the 

manufacture of windows and doors. Table 12–8 provides 

some selected properties of LVL products from different 

research studies.

Wood–Nonwood Composites

Wood–Plastic Composite

The use of wood–plastic composite lumber in North America has experienced tremendous growth in the past decade, 

largely because of residential construction applications. 

Common applications in North America include decking, 

railings, window profiles, roof tiles, and siding. These lumber products are generally manufactured using profile extrusion. Some generalizations can be made regarding the performance of wood–plastic composites, but there are exceptions. Flexural and tensile properties of wood–plastic composite lumber generally fall between those of solid wood 

lumber and unfilled plastics. Most commercial wood–plastic 

composites are considerably less stiff than solid wood but 

are stiffer than unfilled plastic (Clemons 2002). Compared 

with solid wood lumber, wood–plastic composites have better decay resistance and dimensional stability when exposed 

to moisture. Compared with unfilled plastics, wood–plastic 

composites are stiffer and have better dimensional stability 

when exposed to changes in temperature.

Table 12–9 shows mechanical properties of unfilled polypropylene and several wood–polypropylene composites. 

One of the primary reasons to add wood filler to unfilled 

plastics is to improve stiffness. Strength of the unfilled plastic can also increase but only if the wood component acts as 

reinforcement with good bonding between the two components. Table 12–9 illustrates how wood–plastic composite 

properties can vary with changing variables. For example, 

adding wood fiber instead of wood flour to polypropylene 

12–7

Chapter 12 Mechanical Properties of Wood-Based Composite Materials

Table 12–7. Selected properties of glulam products 

Reference Species 

Moisture

content

(%)

Number of

laminations 

Static bending properties 

Modulus of 

elasticity 

Modulus of 

rupture

GPa

(×106

lb in–2) MPa (lb in–2)

Manbeck 

and others 

(1993)

Red maple 12 8 12.3 (1.78) 62.6 (9,080) 

12 12 12.2 (1.77) 55.0 (7,980) 

12 16 12.3 (1.78) 54.2 (7,860) 

Moody 

and others 

(1993)

Yellow

poplar 

8.2 8 13.0 (1.89) 55.6 (8,060) 

7.5 12 13.4 (1.94) 52.1 (7,560) 

8 17 12.3 (1.79) 45.3 (6,570) 

Shedlauskus

and others 

(1996)

Red oak 12.8 8 13.0 (1.88) 60.5 (8,770) 

11.1 18 12.8 (1.86) 46.0 (6,670) 

Janowiak

and others 

(1995)

Red maple 12.6 12 12.2 (1.77) 55.0 (7,980) 

8.9 5 12.8 (1.86) 

8.9 5 12.9 (1.87) 45.7 (6,630) 

Hernandez

and others 

(2005)

Ponderosa

pine

8.8 8 9.44 (1.37) 31.4 (4,560) 

8.8 13 9.07 (1.32) 29.6 (4,290) 

Hernandez

and Moody 

(1992)

Southern

Pine

— 10 14.1 (2.04) 61.7 (8,950) 

— 17 13.5 (1.96) 49.8 (7,230) 

Marx and 

Moody 

(1981 a,b) 

Southern

Pine

10 4, 8, 10 11.2 (1.63) 46.5 (6,740) 

10 4, 8, 11 10.8 (1.56) 33.9 (4,920) 

Douglas-fir–

larch 

11 4, 8, 12 13.9 (2.02) 47.2 (6,840) 

11 4, 8, 13 13.6 (1.97) 40.7 (5,910) 

Moody 

(1974)

Southern

Pine

11.8 17 9.3 (1.35) 28.6 (4,150) 

11.9 17 10.3 (1.49) 31.4 (4,560) 
12–8

General Technical Report FPL–GTR–190

Table 12–8. Selected properties of laminated veneer lumber for structural composite lumber products Reference Species 

Static bending properties Tensile properties 

Modulus of elasticity Modulus of rupture Modulus of 

elasticity

Ultimate tensile 

stress Edge Flat Edge Flat

GPa 

(×106

lb in–2) GPa 

(×106

lb in–2) MPa (lb in–2) MPa (lb in–2) GPa 

(×106

lb in–2) MPa (lb in–2)

Bohlen (1974) 

Douglas-fir — — — — — — — — 15.2 (2.20) 28.99 (4,205) 

Youngquist and others (1984) 

Douglas-fir — — — — — — — — 14.0–15.0 (2.03–2.17) 28.1–39.0 (4,080– 5,650) 

 11.1–12.4 (1.61–1.80) 18.3–38.1 (2,660–

5,520) 

Jung (1982) 

Douglas-fir 15.5–19.2 (2.25–2.79) 15.4–19.3 (2.23–2.80) 58.0–71.7 (8,420–

10,400) 

54.2–62.5 (7,860–

9,060) 

15.6–20.3 (2.27–2.94) 37.9–46.2 (5,500–

6,700) 

Kunesh (1978) 

Douglas-fir 15.9 (2.31) 16.1 (2.34) — — 78.8 (11,430) 14.1 (2.04) 44.4 (6,435) 

Koch(1973) 

Southern Pine 13.2 (1.91) 0.0 0.00 64.2 (9,310) — — — — — — 

Moody (1972) 

Douglas-fir — — — — — — — — 14.3 (2.07) 37.6 (5,450) 

Southern Pine — — — — — — — — 13.5 (1.96) 34.6 (5,025) 

Moody and Peters (1972) 

Southern Pine 14.1 (2.04) 14.7 (2.13) 80.8 (11,720) 86.0 (12,480) — — — — 

Wang and others (2003b) 

Red maple 10.8 (1.56) 11.3 (1.64) 83.3 (12,081) — — — — — — 

Hindman and others (2006) 

Southern Pine 15.8 (2.29) 17.4 (2.54) — — — — — — — — 

Kretschmann and others (1993) 

Douglas-fir 9.0–12.8 (1.30–1.86) 9.0–13.7 (1.30–1.98) 37.9–67.9 (5,500–

9,850) 

33.8–63.9 (4,900–

9,270) 

8.5–12.8 (1.24–1.86) 20.8–49.1 (3,020–

7,100) 

Southern Pine 9.8–13.7 (1.34–1.98) 8.8–13.0 (1.27–1.89) 51.9–70.3 (7,530–

10,190) 

47.8–66.5 (6,940–

9,650) 

9.6–13.6 (1.39–1.97) 36.6–51.2 (5,310–

7,430) 
improved the strength and stiffness. Generally, adding a 

coupling agent to the mix also improved mechanical properties. Adding wood to polypropylene was not without 

tradeoffs. Impact resistance of such composites decreased 

compared with that of unfilled polypropylene. 

In addition to these commercial deck products, wood–plastic composites are being developed for structural applications such as foundation elements, deck substructures, industrial decking, and shoreline structures (Bender and others 

2006). Table 12–10 shows the range of average mechanical 

properties of extruded wood–plastic composites by polymer 

type. In general, polyvinylcholoride and polyethylene formulations produce higher mechanical properties than those 

produced from polyethylene alone. Formulations that use 

coupling agents with either polypropylene or high-density 

polyethylene result in improved strength, stiffness, and reduced moisture absorption properties.

Properties of wood–plastic composites can vary greatly depending upon such variables as type, form, weight fractions 

of constituents, type of additives, and processing methods 

(Stark and Rowlands 2003, Wolcott and others 2006). Because formulations from each commercial manufacture are 

proprietary, design data should be obtained directly from the 

manufacturer.

Inorganic-Bonded Composites

Inorganic-bonded wood composites are molded products or 

boards that contain between 10% and 70% by weight wood 

particles or fibers and conversely 90% to 30% inorganic 

binder. Acceptable properties of an inorganic-bonded wood 

composite can be obtained only when the wood particles are 

fully encased with the binder to make a coherent material. 

This differs considerably from the technique used to manufacture thermosetting-resin-bonded boards, where flakes or 

particles are “spot welded” by a binder applied as a finely 

distributed spray or powder. Because of this difference and 

because hardened inorganic binders have a higher density 

than that of most thermosetting resins, the required amount 

of inorganic binder per unit volume of composite material 

is much higher than that of resin-bonded wood composites. 

The properties of inorganic-bonded wood composites are 

significantly influenced by the amount and nature of the inorganic binder and the woody material as well as the density 

of the composites.

Inorganic binders fall into three main categories: gypsum, 

magnesia cement, and Portland cement. Gypsum and magnesia cement are sensitive to moisture, and their use is 

generally restricted to interior applications. Composites 

bonded with Portland cement are more durable than those 

bonded with gypsum or magnesia cement and are used in 

both interior and exterior applications. Inorganic-bonded 

composites are made by blending proportionate amounts of 

lignocellulosic fiber with inorganic materials in the presence 

of water and allowing the inorganic material to cure or “set 

up” to make a rigid composite. All inorganic-bonded composites are very resistant to deterioration, particularly by 

insects, vermin, and fire. Typical properties of low-density 

cement-wood composite fabricated using an excelsior-type 

particle are shown in Table 12–11.

Testing Standards

The physical and mechanical properties of wood-based 

composite materials are usually determined by standard 

ASTM test methods. The following are the commonly used 

methods described in ASTM (2009):

ASTM C 208–08. Standard specification for cellulosic 

fiber insulating board.

12–9

Chapter 12 Mechanical Properties of Wood-Based Composite Materials

Table 12–9. Selected properties of wood–plastic productsa

Composite

Specific 

gravity

Tensile properties Flexural properties Izod impact 

energy 

(J m–1 Strength Modulus Strength Modulus )

MPa (lb in–2) GPa 

(×106

lb in–2) MPa (lb in–2) GPa 

(×106

lb in–2) Notched Unnotched 

Polypropylene 

(PP) 

0.90 28.5 (4,134) 1.53 (0.22) 38.30 (5,555) 1.19 (0.17) 20.9 656 

PP + 40% wood 

flour

1.05 25.4 (3,684) 3.87 (0.56) 44.20 (6,411) 3.03 (0.44) 22.2 73 

PP + 40% wood 

flour + 3% 

coupling agent 

1.05 32.3 (4,685) 4.10 (0.59) 53.10 (7,702) 3.08 (0.45) 21.2 78 

PP + 40% wood 

fiber 

1.03 28.2 (4,090) 4.20 (0.61) 47.90 (6,947) 3.25 (0.47) 23.2 91 

PP + 40% wood 

fiber + 3% 

coupling agent 

1.03 52.3 (7,585) 4.23 (0.61) 72.40 (10,501) 3.22 (0.47) 21.6 162 

a

From Stark and Rowlands (2003). 
12–10

General Technical Report FPL–GTR–190

ASTM D 1037–06a. Standard test methods for evaluating the properties of wood-based fiber and particle 

panel materials.

ASTM D 2718–00 (2006). Standard test method for 

structural panels in planar shear (rolling shear).

ASTM D 2719–89 (2007). Standard test methods for 

structural panels in shear through-the-thickness.

ASTM D 3043–00 (2006). Standard test methods of 

testing structural panels in flexure.

ASTM D 3044–94 (2006). Standard test method for 

shear modulus of wood-based structural plywood.

ASTM D 3500–90 (2003). Standard test methods for 

structural panels in tension.

ASTM D 3501–05a. Standard test methods of testing 

plywood in compression.

ASTM D 3737–08. Standard practice for establishing 

allowable properties for structural glued laminated timber (glulam).

ASTM D 5456–09. Specification for evaluation of 

structural composite lumber products.

ASTM D 7031–04. Standard guide for evaluating mechanical and physical properties of wood-plastic composite products.

ASTM D 7032–08. Standard specification for establishing performance ratings for wood-plastic composite 

deck boards and guardrail systems.

ASTM D 7341–09. Standard practice for establishing 

characteristic values for flexural properties of structural 

glued laminated timber by full-scale testing.

ASTM E 1333–96 (2002). Test method for determining 

formaldehyde concentration in air and emission rate 

from wood products using a large chamber.

Literature Cited

AF&PA. 2005. Guideline—wood I-joist ASD manual for 

engineering wood construction. ANSI/AF&PA NDS–2005. 

National Design Specification® (NDS®) for Wood Construction. Washington, DC: American Forest & Paper Association.

AITC. 2007. Standard for wood products: structural gluedlaminated timber. ANSI/AITC 190.1–2007. Centennial, CO: 

American Institute of Timber Construction.

APA. 1981. Performance standards and policies for APA 

structural use panels. Tacoma, WA: American Plywood Association. 1 p.

APA. 1991. Performance standards and policies for structural use panels. APA PRP–108. Tacoma, WA: APA – The 

Engineered Wood Association.

APA. 1995a. Design capacities of APA performance-rated 

structural-use panels. Tech. Note N375 B. Tacoma, WA: 

APA – The Engineered Wood Association. 12 p.

APA. 1995b. Plywood design specification. Tacoma, WA: 

APA – The Engineered Wood Association. 10 p.

ASTM. 2007. Annual book of ASTM standards. Philadelphia, PA: American Society for Testing and Materials.

Table 12–11. General properties of lowdensity cement–wood composite fabricated 

using an excelsior-type particlea,b

Property 

Value range (MPa (lb in–2)) 

Low High 

Bending strength 1.7 (250) 5.5 (800) 

Modulus of 

elasticity 

621 (90,000) 1,241 (180,000)

Tensile strength 0.69 (100) 4.1 (600) 

Compression 

strength 

0.69 (100) 5.5 (800) 

Shearc 0.69 (100) 1.4 (200) 

E/G ratiod 40.0 100.0 a

Data present compilation of raw data from a variety of 

sources for range of board properties. Variables include 

cement–wood mix, particle configuration, density, and 

forming and curing method. 

b

Specific gravity range from 0.5 to 1.0. 

c

Shear strength data are limited to small samples having a 

specific gravity of 0.5 to 0.65. 

b

E/G is ratio of bending modulus of elasticity to shear 

modulus. For wood, this ratio is about 16. 

Table 12–10. Selected properties of extruded wood–plastic products

Composite

Tensile strength 

(MPa (lb in–2))

Compression 

strength 

(MPa (lb in–2))

Bending strength 

(GPa (×106 lb in–2))

Bending modulus 

(MPa (lb in–2))

Shear strength 

(MPa (lb in–2))

Dowel bearing 

strength 

(MPa (lb in–2))

Polypropylene 

(PP)a, b

20.0 (2,900) 55.2 (8,000) 3.49–5.97 

(0.506–0.866)

22.2–60.8 

(3,220–8,820)

22.0 (3,190) 84.8 (12,300)

High-density 

polyethylene 

(HDPE)c

5.5–15.2 

(800–2,200)

11.7–26.9 

(1,700–3,900)

1.79–5.17 

(0.260–0.750)

10.3–25.5 

(1,500–3,700)

7.79–10.3 

(1,130–1,500)

35.7 (5,180)

Polyvinylchloride 

(PVC)c

25.1 (3,640) 61.2 (8,880) 4.81–7.58 

(0.697–1.100)

35.9–54.5 

(5,200–7,900)

20.2 (2,930) 72.4–128.2 

(10,500–18,600)

a

From Slaughter (2004). 

b

From Kobbe (2005). 

c

From Wolcott (2001).
12–11

Chapter 12 Mechanical Properties of Wood-Based Composite Materials

Bender, D.A.; Wolcott, M.P.; Dolan, J.D. 2006. Structural 

design and applications with wood–plastic composites. 

Wood Design Focus. 16(3): 13–16. 

Biblis, E.J. 1989. Properties of commercial OSB from three 

Southern Pine mills. Forest Products Journal. 39(1): 55–57.

Biblis, E.J. 2000. Effect of weathering on surface quality 

and structural properties of six species of untreated commercial plywood siding after 6 years of exposure in Alabama. 

Forest Products Journal. 50(5): 47–50.

Bohlen, J.C. 1974. Tensile strength of Douglas-fir laminated-veneer lumber. Forest Products Journal. 24(1): 54–58.

Cai, Z. 2006. Selected properties of MDF and flakeboard 

overlaid with fiberglass mats. Forest Products Journal. 

56(11/12): 142–146.

Cai, Z.; Muehl, J.H.; Winandy, J.E. 2006. Effects of panel 

density and mat moisture content on processing medium 

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Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 3 p.
CHAPTER 13

Drying and Control of Moisture Content 

and Dimensional Changes

Richard Bergman, Research Forest Products Technologist

13–1

Contents 

Determination of Moisture Content 13–1

 Oven-Drying Method 13–2

 Electrical Method 13–2

Recommended Moisture Content 13–3

 Timbers 13–3

 Lumber 13–3

 Glued Wood Products 13–3

Drying of Wood 13–5

 Air Drying 13–6

 Accelerated Air Drying and Pre-Drying 13–6

 Kiln Drying 13–6

 Drying Mechanism 13–6

 Drying Stresses 13–7

 Dry Kilns 13–8

 Kiln Schedules 13–8

 Drying Defects 13–9

 Moisture Content of Dried Lumber 13–11

Moisture Control during Transit and Storage 13–13

 Plywood and Structural Items 13–14

 Finish and Factory Lumber 13–14

Dimensional Changes in Wood 13–15

 Estimation Using Dimensional Change 

 Coefficient 13–15

 Calculation Based on Green Dimensions 3–17

Design Factors Affecting Dimensional Change 13–17

 Framing Lumber in House Construction 13–17

 Heavy Timber Construction 13–18

 Interior Finish 13–18

 Flooring 13–18

Wood Care and Installation during 

Construction 13–18

 Lumber and Trusses 13–18

 Exterior Trim and Millwork 13–19

 Finished Flooring 13–19

 Interior Trim 13–19

 Plastering 13–19

Literature Cited 13–19

In the living tree, wood contains large quantities of water. 

As green wood dries, most of the water is removed. The 

moisture remaining in the wood tends to come to equilibrium with the relative humidity of the surrounding air. Correct 

drying, handling, and storage of wood will minimize moisture content changes that might occur after drying when the 

wood is in service. If moisture content is controlled within 

reasonable limits by such methods, major problems from 

dimensional changes can usually be avoided.

The discussion in this chapter is concerned with moisture 

content determination, recommended moisture content values, drying methods, methods of calculating dimensional 

changes, design factors affecting such changes in structures, 

and moisture content control during transit, storage, and 

construction. Data on green moisture content, fiber saturation point, shrinkage, and equilibrium moisture content 

are given with information on other physical properties in 

Chapter 4.

Wood in service is always undergoing slight changes in 

moisture content. These changes that result from daily 

humidity changes are often small and usually of no consequence. Changes that occur because of seasonal variation, 

although gradual, tend to be of more concern. Protective 

coatings can retard dimensional changes in wood but do not 

prevent them. In general, no significant dimensional changes 

will occur if wood is fabricated or installed at a moisture 

content corresponding to the average atmospheric conditions to which it will be exposed. When incompletely dried 

material is used in construction, some minor dimensional 

changes can be tolerated if the proper design is used.

Determination of Moisture Content

The amount of moisture in wood is ordinarily expressed as a 

percentage of wood mass when oven-dried. Four methods of 

determining moisture content are covered in ASTM D 4442 

(ASTM 2007). Two of these—the oven-drying and the electrical methods—are described in this chapter.

The oven-drying method has been the most universally 

accepted method for determining moisture content, but it 

is slow and necessitates cutting the wood. In addition, the 

oven-drying method may give values slightly greater than 

true moisture content with woods containing volatile extrac-
13–2

tives. The electrical method is rapid, does not require cutting 

the wood, and can be used on wood installed in a structure. 

However, considerable care must be taken to use and interpret the results correctly. Use of the electrical method is 

generally limited to moisture content values less than 30%.

Oven-Drying Method

In the oven-drying method, specimens are taken from representative boards or pieces of a quantity of lumber. With 

lumber, obtain the specimens at least 500 mm (20 in.) from 

the end of the pieces. They should be free from knots and 

other irregularities, such as bark and pitch pockets. Specimens from lumber should be full cross sections and 25 mm 

(1 in.) long. Specimens from larger items may be representative sectors of such sections or subdivided increment borer 

or auger chip samples. Convenient amounts of chips and 

particles can be selected at random from larger batches, with 

care taken to ensure that the sample is representative of the 

batch. Select veneer samples from four or five locations in a 

sheet to ensure that the sample average will accurately indicate the average of the sheet.

To prevent drying or uptake of moisture, weigh each specimen immediately. If the specimen cannot be weighed immediately, place it in a plastic bag or tightly wrapped in 

metal foil to protect it from moisture change until it can be 

weighed. After weighing, place the specimen in an oven 

heated to 101 to 105 °C (214 to 221 °F), and keep it there 

until no appreciable weight change occurs in 4-h weighing 

intervals. A lumber section 25 mm (1 in.) along the grain 

will reach a constant weight in 12 to 48 h. Smaller specimens will take less time. The constant or oven-dry mass 

and the (original) mass of the specimen when cut are used 

to determine the percentage of moisture content (MC) using 

the formula

(13–1)

Electrical Method

The electrical method of determining the moisture content 

of wood uses the relationships between moisture content 

and measurable electrical properties of wood, such as conductivity (or its inverse, resistivity), dielectric constant, or 

power-loss factor. These properties vary in a definite and 

predictable way with changing moisture content, but correlations are not perfect. Therefore, moisture determinations 

using electrical methods are always subject to some uncertainty.

Electric moisture meters are available commercially and 

are based on each of these properties and identified by the 

property measured. Conductance-type (or resistance) meters measure moisture content in terms of the direct current 

conductance of the specimen. Dielectric-type meters are of 

two types. Those based principally on dielectric constant are 

called capacitance or capacitive admittance meters; those 

based on loss factor are called power-loss meters.

The principal advantages of the electrical method compared 

with the oven-drying method are speed and convenience. 

Only a few seconds are required for the determination, and 

the piece of wood being tested is not cut or damaged, except 

for driving electrode needle points into the wood when using conductance-type meters. Thus, the electrical method is 

adaptable to rapid sorting of lumber on the basis of moisture 

content, measuring the moisture content of wood installed in 

a building, or establishing the moisture content of a quantity 

of lumber or other wood items, when used in accordance 

with ASTM D 4442.

For conductance meters, needle electrodes (pins) of various 

lengths are driven into the wood. The two general types of 

electrodes are insulated and uninsulated. Uninsulated electrodes will sense the lowest resistance (highest conductance) 

along their length, thus highest moisture content level. 

Moisture gradients between the surface and the interior can 

lead to confusion; therefore, insulating the electrode except 

the tip is useful to show moisture gradients. If the wood is 

wetter near the center than the surface, which is typical for 

drying wood, the reading will correspond to the depth of the 

tip of the insulated electrodes. If a meter reading increases 

as the electrodes are being driven in, then the moisture gradient is typical. In this case, drive the pins about one-fifth to 

one-fourth the thickness of the wood to reflect the average 

moisture content of the entire piece. Dried or partially dried 

wood sometimes regains moisture in the surface fibers from 

rewetting therefore the surface moisture content is greater 

than that of the interior. An example of this is when dried 

wood is rained on. In this case, the meter with the uninsulated pins will read the higher moisture content surface, possibly causing a significant deviation from the average moisture content. To guard against this problem, electrodes with 

insulated shanks have been developed. They measure moisture content of only the wood at the tips of the electrodes.

Dielectric-type meters are fitted with surface contact electrodes designed for the type of specimen material being tested. The electric field from these electrodes penetrates well 

into the specimen, but with a strength that decreases rapidly 

with depth of penetration. For this reason, the surface layers 

of the specimen influence the readings of dielectric (pinless) meters predominantly, and the meter reading may not 

adequately represent the material near the core if there is a 

large moisture content gradient.

To obtain accurate moisture content values, use each instrument in accordance with its manufacturer’s instructions. 

The electrodes should be appropriate for the material 

being tested and properly oriented according to the meter 

manufacturer’s instructions. Take the readings after inserting 

General Technical Report FPL–GTR–190
13–3

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

the electrode. Apply a species correction supplied with the 

instrument when appropriate. Make temperature corrections 

if the temperature of the wood differs considerably from 

the temperature of calibration used by the manufacturer. 

Approximate corrections for conductance-type (resistance) 

meters are made by adding or subtracting about 0.5% for 

each 5.6 °C (10 °F) the wood temperature differs from the 

calibration temperature. Add the correction factors to the 

readings for temperatures less than the calibration temperature and subtract from the readings for temperatures greater 

than the calibration temperature. Temperature corrections 

for older dielectric meters are rather complex and are best 

made from published charts (James 1988). Newer dielectric meters perform this temperature calibration internally, 

although newer dielectric meters require a specific gravity 

adjustment.

Although some meters have scales that go up to 120%, the 

range of moisture content that can be measured reliably 

is 4% to about 30% for commercial dielectric meters and 

about 6% to 30% for resistance meters. The precision of the 

individual meter readings decreases near the limits of these 

ranges. Readings greater than 30% must be considered only 

qualitative. When the meter is properly used on a quantity 

of lumber dried to a constant moisture content below fiber 

saturation, the average moisture content from the corrected 

meter readings should be within 1% of the true average.

Recommended Moisture Content

Install wood at the moisture content levels that the wood 

will experience in service. This minimizes the seasonal 

variation in moisture content, thus dimensional changes, 

after installation, avoiding problems such as floor buckling 

or cracks in furniture. The in-service moisture content of 

exterior wood (siding, wood trim) primarily depends on 

the outdoor relative humidity and exposure to rain or sun. 

The in-service moisture content of interior wood primarily depends on indoor relative humidity, which in turn is a 

complex function of moisture sources, ventilation rate, dehumidification (for example, air conditioning), and outdoor 

humidity conditions.

Recommended values for interior wood presented in this 

chapter are based on measurements in well-ventilated buildings without unusual moisture sources and without air conditioning. In air-conditioned buildings, moisture conditions 

depend largely on the proper sizing of the air-conditioning 

equipment. Installing wood in basements or over a crawl 

space may experience moisture contents greater than the 

range given. Wood in insulated walls or roofs and attics may 

experience moisture contents greater or less than the range. 

Nevertheless, the recommended values for installation provide a useful guideline.

Timbers

Ideally, dry solid timbers to the average moisture content 

the material will reach in service. Although this optimum is 

possible with lumber less than 76 mm (3 in.) thick, it is seldom practical to obtain fully dried timbers, thick joists, and 

planks. When thick solid members are used, some shrinkage 

of the assembly should be expected. In the case of builtup assemblies, such as roof trusses, it may be necessary to 

tighten bolts or other fastenings occasionally to maintain 

full bearing of the connectors as the members shrink.

Lumber

Match the recommended moisture content of wood as closely as is practical to the equilibrium moisture content (EMC) 

conditions in service. Table 13–1 shows the EMC conditions 

in outdoor exposure in various U.S. cities for each month. 

The EMC data are based on the average relative humidity 

and temperature data (30 or more years) available from the 

National Climatic Data Center of the National Oceanic and 

Atmospheric Administration. The relative humidity data are 

the averages of the morning and afternoon values. In most 

cases, these values are representative of the EMC attained 

by the wood. However, in some locations, early morning 

relative humidity may occasionally reach 100%. Under 

these conditions, condensation may occur on the wood 

surface, therefore surface fibers will exceed the EMC. The 

moisture content requirements are more exacting for finished lumber and wood products used inside heated and airconditioned buildings than those for lumber used outdoors 

or in unheated buildings. For various areas of the United 

States, the recommended moisture content values 

for wood used inside heated buildings are shown in Figure 13–1. Values and tolerances for both interior and exterior uses of wood in various forms are given in Table 13–2. 

If the average moisture content is within 1% of that recommended and all pieces fall within the individual limits, the 

entire lot is probably satisfactory (Simpson 1998).

General commercial practice is to kiln dry wood for some 

products, such as flooring and furniture, to a slightly lower 

moisture content than service conditions demand. This 

anticipates a moderate increase in moisture content during 

processing, transportation, and construction. This practice is 

intended to ensure uniform distribution of moisture among 

the individual pieces. Common grades of softwood lumber 

and softwood dimension lumber are not normally dried to 

the moisture content values indicated in Table 13–2. Dry 

lumber, as defined in the American Softwood Lumber Standard, has a maximum moisture content of 19%. Some industry grading rules provide for an even lower maximum. For 

example, to be grade marked KD 15, the maximum moisture 

content permitted is generally 15%.

Glued Wood Products

When veneers are bonded with cold-setting adhesives 

to make plywood, they take up comparatively large 
13–4

General Technical Report FPL–GTR–190

Table 13–1. Equilibrium moisture content for outside conditions in several U.S. locations prior to 1997

Equilibrium moisture contenta

 (%) 

State City Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

AK Juneau 16.5 16.0 15.1 13.9 13.6 13.9 15.1 16.5 18.1 18.0 17.7 18.1

AL Mobile 13.8 13.1 13.3 13.3 13.4 13.3 14.2 14.4 13.9 13.0 13.7 14.0

AZ Flagstaff 11.8 11.4 10.8 9.3 8.8 7.5 9.7 11.1 10.3 10.1 10.8 11.8

AZ Phoenix 9.4 8.4 7.9 6.1 5.1 4.6 6.2 6.9 6.9 7.0 8.2 9.5

AR Little Rock 13.8 13.2 12.8 13.1 13.7 13.1 13.3 13.5 13.9 13.1 13.5 13.9

CA Fresno 16.4 14.1 12.6 10.6 9.1 8.2 7.8 8.4 9.2 10.3 13.4 16.6

CA Los Angeles 12.2 13.0 13.8 13.8 14.4 14.8 15.0 15.1 14.5 13.8 12.4 12.1

CO Denver 10.7 10.5 10.2 9.6 10.2 9.6 9.4 9.6 9.5 9.5 11.0 11.0

DC Washington 11.8 11.5 11.3 11.1 11.6 11.7 11.7 12.3 12.6 12.5 12.2 12.2

FL Miami 13.5 13.1 12.8 12.3 12.7 14.0 13.7 14.1 14.5 13.5 13.9 13.4

GA Atlanta 13.3 12.3 12.0 11.8 12.5 13.0 13.8 14.2 13.9 13.0 12.9 13.2

HI Honolulu 13.3 12.8 11.9 11.3 10.8 10.6 10.6 10.7 10.8 11.3 12.1 12.9

ID Boise 15.2 13.5 11.1 10.0 9.7 9.0 7.3 7.3 8.4 10.0 13.3 15.2

IL Chicago 14.2 13.7 13.4 12.5 12.2 12.4 12.8 13.3 13.3 12.9 14.0 14.9

IN Indianapolis 15.1 14.6 13.8 12.8 13.0 12.8 13.9 14.5 14.2 13.7 14.8 15.7

IA Des Moines 14.0 13.9 13.3 12.6 12.4 12.6 13.1 13.4 13.7 12.7 13.9 14.9

KS Wichita 13.8 13.4 12.4 12.4 13.2 12.5 11.5 11.8 12.6 12.4 13.2 13.9

KY Louisville 13.7 13.3 12.6 12.0 12.8 13.0 13.3 13.7 14.1 13.3 13.5 13.9

LA New Orleans 14.9 14.3 14.0 14.2 14.1 14.6 15.2 15.3 14.8 14.0 14.2 15.0

ME Portland 13.1 12.7 12.7 12.1 12.6 13.0 13.0 13.4 13.9 13.8 14.0 13.5

MA Boston 11.8 11.6 11.9 11.7 12.2 12.1 11.9 12.5 13.1 12.8 12.6 12.2

MI Detroit 14.7 14.1 13.5 12.6 12.3 12.3 12.6 13.3 13.7 13.5 14.4 15.1

MN Minneapolis–St. Paul 13.7 13.6 13.3 12.0 11.9 12.3 12.5 13.2 13.8 13.3 14.3 14.6

MS Jackson 15.1 14.4 13.7 13.8 14.1 13.9 14.6 14.6 14.6 14.1 14.3 14.9

MO St. Louis 14.5 14.1 13.2 12.4 12.8 12.6 12.9 13.3 13.7 13.1 14.0 14.9

MT Missoula 16.7 15.1 12.8 11.4 11.6 11.7 10.1 9.8 11.3 12.9 16.2 17.6

NE Omaha 14.0 13.8 13.0 12.1 12.6 12.9 13.3 13.8 14.0 13.0 13.9 14.8

NV Las Vegas 8.5 7.7 7.0 5.5 5.0 4.0 4.5 5.2 5.3 5.9 7.2 8.4

NV Reno 12.3 10.7 9.7 8.8 8.8 8.2 7.7 7.9 8.4 9.4 10.9 12.3

NM Albuquerque 10.4 9.3 8.0 6.9 6.8 6.4 8.0 8.9 8.7 8.6 9.6 10.7

NY New York 12.2 11.9 11.5 11.0 11.5 11.8 11.8 12.4 12.6 12.3 12.5 12.3

NC Raleigh 12.8 12.1 12.2 11.7 13.1 13.4 13.8 14.5 14.5 13.7 12.9 12.8

ND Fargo 14.2 14.6 15.2 12.9 11.9 12.9 13.2 13.2 13.7 13.5 15.2 15.2

OH Cleveland 14.6 14.2 13.7 12.6 12.7 12.7 12.8 13.7 13.8 13.3 13.8 14.6

OK Oklahoma City 13.2 12.9 12.2 12.1 13.4 13.1 11.7 11.8 12.9 12.3 12.8 13.2

OR Pendleton 15.8 14.0 11.6 10.6 9.9 9.1 7.4 7.7 8.8 11.0 14.6 16.5

OR Portland 16.5 15.3 14.2 13.5 13.1 12.4 11.7 11.9 12.6 15.0 16.8 17.4

PA Philadelphia 12.6 11.9 11.7 11.2 11.8 11.9 12.1 12.4 13.0 13.0 12.7 12.7

SC Charleston 13.3 12.6 12.5 12.4 12.8 13.5 14.1 14.6 14.5 13.7 13.2 13.2

SD Sioux Falls 14.2 14.6 14.2 12.9 12.6 12.8 12.6 13.3 13.6 13.0 14.6 15.3

TN Memphis 13.8 13.1 12.4 12.2 12.7 12.8 13.0 13.1 13.2 12.5 12.9 13.6

TX Dallas–Ft. Worth 13.6 13.1 12.9 13.2 13.9 13.0 11.6 11.7 12.9 12.8 13.1 13.5

TX El Paso 9.6 8.2 7.0 5.8 6.1 6.3 8.3 9.1 9.3 8.8 9.0 9.8

UT Salt Lake City 14.6 13.2 11.1 10.0 9.4 8.2 7.1 7.4 8.5 10.3 12.8 14.9

VA Richmond 13.2 12.5 12.0 11.3 12.1 12.4 13.0 13.7 13.8 13.5 12.8 13.0

WA Seattle–Tacoma 15.6 14.6 15.4 13.7 13.0 12.7 12.2 12.5 13.5 15.3 16.3 16.5

WI Madison 14.5 14.3 14.1 12.8 12.5 12.8 13.4 14.4 14.9 14.1 15.2 15.7

WV Charleston 13.7 13.0 12.1 11.4 12.5 13.3 14.1 14.3 14.0 13.6 13.0 13.5

WY Cheyenne 10.2 10.4 10.7 10.4 10.8 10.5 9.9 9.9 9.7 9.7 10.6 10.6

a

EMC values were determined from the average of 30 or more years of relative humidity and temperature data available from the 

National Climatic Data Center of the National Oceanic and Atmospheric Administration. 
quantities of moisture. To keep the final moisture content 

low and to minimize the need for re-drying the plywood, 

the initial moisture content of the veneer should be as low 

as practical. However, dry veneer is brittle and difficult to 

handle without damage, so the minimum practical moisture 

content is about 4%. Freshly glued plywood intended for 

interior service should be dried to the moisture content values given in Table 13–2.

Hot-pressed plywood and other board products, such as 

particleboard and hardboard, usually do not have the same 

moisture content as lumber. The high temperatures used in 

hot presses cause these products to assume a lower moisture 

content for a given relative humidity. Because this lower 

equilibrium moisture content varies widely, depending on 

the specific type of hot-pressed product, it is recommended 

that such products be conditioned at 30% to 40% relative 

humidity for interior use and 65% for exterior use.

Lumber used in the manufacture of large laminated members should be dried to a moisture content slightly less than 

the moisture content expected in service. This is done so 

that the moisture adsorbed from the adhesive will not cause 

the moisture content of the product to exceed the service 

value. The range of moisture content between laminations 

assembled into a single member should not exceed 5 percentage points.

Although laminated members are often massive and respond 

rather slowly to changes in environmental conditions, it is 

desirable to follow the recommendations in Table 13–2 for 

moisture content at time of installation.

Drying of Wood

Drying is required for wood to be used in most products. 

Dried lumber has many advantages over green lumber for 

producers and consumers. Removal of excess water reduces 

weight, thus shipping and handling costs. Proper drying 

reduces shrinking and swelling of wood while in use to 

manageable amounts under all but extreme conditions of 

relative humidity or rewetting such as flooding. As wood 

dries, most of its strength properties increase, as well as its 

electrical and thermal insulating properties. Properly dried 

lumber can be cut to precise dimensions and machined more 

easily and efficiently; wood parts can be more securely fitted 

and fastened together with nails, screws, bolts, and adhesives; warping, splitting, checking, and other harmful effects 

of uncontrolled drying are largely eliminated; and paint, 

varnish, and other finishes are more effectively applied and 

maintained. Wood must be relatively dry before gluing or 

treating with decay-preventing and fire-retardant chemicals.

The key to successful and efficient drying is control of the 

drying process. Timely application of optimum or at least 

adequate temperature, relative humidity, and air circulation 

conditions is critical. Uncontrolled drying leads to drying 

defects that can adversely affect the serviceability and economics of the product. The usual strategy is to dry as fast as 

the particular species, thickness, and end-product requirements allow without damaging the wood. Slower drying can 

be uneconomical and can introduce the risk of stain.

13–5

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

Figure 13–1. Recommended average moisture content for interior use of wood products in various 

areas of the United States.

Table 13–2. Recommended moisture content values for various wood products at time of 

installation 

Recommended moisture content (%) for areas in the United States 

Most areas of the 

United States 

Dry 

southwestern areaa

Damp, warm 

coastal areaa

Use of wood Averageb

Individual

pieces Averageb

Individual

pieces Averageb

Individual

pieces 

Interior: woodwork, flooring, 

furniture, wood trim 

8 6–10 6 4–9 11 8–13 

Exterior: siding, wood trim, 

sheathing, laminated timbers 

12 9–14 9 7–12 12 9–14 

a

Major areas are indicated in Figure 13–1. 

b

To obtain a realistic average, test at least 10% of each item. If the quantity of a given item is small, make several tests. 

For example, in an ordinary dwelling containing 60 floor joists, at least six tests should be made on joists selected at 

random. 
Softwood lumber intended for framing in construction is 

usually targeted for drying to an average moisture content 

of 15%, not to exceed 19%. Softwood lumber for many appearance grade uses is dried to a lower moisture content of 

10% to 12% and to 7% to 9% for furniture, cabinets, and 

millwork. Hardwood lumber for framing in construction, although not in common use, should also be dried to an average moisture content of 15%, not to exceed 19%. Hardwood 

lumber for furniture, cabinets, and millwork is usually dried 

to 6% to 8% moisture content.

Lumber drying is usually accomplished by some combination of air drying, accelerated air drying or pre-drying, and 

kiln drying. Wood species, initial moisture content, lumber 

thickness, economics, and end use are often the main factors 

in determining the details of the drying process.

Air Drying

The main purpose of air drying lumber is to evaporate as 

much of the water as possible before end use or prior to 

kiln-drying. Air drying lumber down to 20% to 25% moisture content prior to kiln-drying is common. Sometimes, 

depending on a mill’s scheduling, air drying may be cut 

short at a higher moisture content before the wood is sent 

to the dry kiln. Air drying saves energy costs and reduces 

required dry kiln capacity. Limitations of air drying are generally associated with uncontrolled drying. The drying rate 

is very slow during the cold winter months. At other times, 

hot, dry winds may increase degrade and volume losses as 

a result of severe surface checking and end splitting. End 

coating may alleviate end checking and splitting. Warm, 

humid periods with little air movement may encourage 

the growth of fungal stains, as well as aggravate chemical 

stains. Another limitation of air drying is the high cost of 

carrying a large inventory of high value lumber for extended 

periods. Air drying time to 20% to 25% moisture content 

varies widely, depending on species, thickness, location, and 

the time of year the lumber is stacked. Some examples of 

extremes for 25-mm- (1-in.-) thick lumber are 15 to 30 days 

for some of the low-density species, such as pine, spruce, 

red alder, and soft maple, stacked in favorable locations and 

favorable times of the year; 200 to 300 days for slow-drying 

species, such as sinker hemlock and pine, oak, and birch, in 

northern locations and stacked at unfavorable times of the 

year. Details of important air-drying considerations, such as 

lumber stacking and air drying yard layout, are covered in 

Air Drying of Lumber: A Guide to Industry Practices (Rietz 

and Page 1971).

Accelerated Air Drying and Pre-Drying

The limitations of air drying have led to increased use of 

technology that reduces drying time and introduces some 

control into drying (green) wood. Accelerated air drying 

involves the use of fans to force air through lumber piles 

in a shed. This protects the lumber from the elements and 

improves air circulation compared with air drying, thus 

improving quality. Heat is sometimes added to reduce the 

relative humidity and slightly increase the shed temperature 

to aid drying. Pre-dryers take this acceleration and control 

a step further by providing control of both temperature and 

relative humidity and providing forced air circulation in a 

completely enclosed compartment. Typical conditions in 

a pre-dryer are 27 to 38 °C (80 to 100 °F) and 65% to 

85% relative humidity.

Kiln Drying

In kiln drying, higher temperatures and faster air circulation 

are used to significantly increase the drying rate. Specific 

kiln schedules have been developed to control temperature 

and relative humidity in accordance with the moisture content and stress situation within the wood, thus minimizing 

shrinkage-caused defects (Boone and others 1988).

Drying Mechanism

Water in wood normally moves from high to low zones of 

moisture content, which means that the surface of the wood 

must be drier than the interior if moisture is to be removed. 

Drying can be broken down into two phases: movement of 

water from the interior to the wood surface and evaporation 

of water from the surface. The surface fibers of most species 

reach moisture equilibrium with the surrounding air soon after drying begins. This is the beginning of the development 

of a typical moisture gradient (Fig. 13–2), that is, the difference in moisture content between the inner and outer portions of a board. If air circulation is too slow, a longer time 

is required for the wood surface to reach moisture equilibrium. This is one reason why air circulation is so important 

in kiln drying. If air circulation is too slow, the drying rate 

is also slower than necessary and mold could develop on the 

surface of lumber. If drying is too fast, electrical energy in 

running the fans is wasted, and in certain species, surface 

checking and other drying defects can develop if relative 

humidity and air velocity are not coordinated.

Water moves through the interior of wood as a liquid or 

vapor through various air passageways in the cellular structure of the wood, as well as through the wood cell walls. 

Moisture moves in these passageways in all directions, both 

across and with the grain. In general, lighter species dry 

faster than heavier species because the structure of lighter 

wood contains more openings per unit volume, and moisture 

moves through air faster than through wood cell walls. Water moves by two main mechanisms: capillary action (liquid) and diffusion of bound water (vapor). Capillary action 

causes the free water to flow through cell cavities and the 

small passageways that connect adjacent cell cavities. Diffusion of bound water moves moisture from areas of high 

concentration to areas of low concentration. Diffusion in the 

longitudinal direction is about 10 to 15 times faster than radial or tangential diffusion, and radial diffusion is somewhat 

faster than tangential diffusion. This explains why flatsawn 

lumber generally dries faster than quartersawn lumber. Al13–6

General Technical Report FPL–GTR–190
though longitudinal diffusion is much faster than diffusion 

across the grain, it generally is not of practical importance in 

lumber that is many times longer than it is thick. In addition, 

a direct result of longitudinal diffusion may be end-checking 

or splitting without proper care.

Because chemical extractives in heartwood plug up passageways, moisture generally moves more freely in sapwood 

than in heartwood; thus, sapwood generally dries faster than 

heartwood. However, the heartwood of many species is lower in moisture content than is the sapwood. Thus heartwood 

can reach final moisture content as fast as the sapwood.

The rate at which moisture moves in wood depends on the 

relative humidity of the surrounding air, the steepness of the 

moisture gradient, and the temperature of the wood. Lower 

relative humidity increases capillary flow. Low relative 

humidity also stimulates diffusion by lowering the moisture content at the surface, thereby steepening the moisture 

gradient and increasing the diffusion rate. The greater the 

temperature of the wood, the faster moisture will move from 

the wetter interior to the drier surface, thus the steeper the 

moisture gradient. If relative humidity is too low in the early 

stages of drying, excessive shrinkage may occur, resulting 

in surface and end checking. If the temperature is too high, 

collapse, honeycomb, or strength reduction can occur.

Drying Stresses

Drying stresses are the main cause of nonstain-related drying defects. Understanding these stresses provides a means 

for minimizing and recognizing the damage they can cause. 

The cause of drying stresses is the differential shrinkage 

between the outer part of a board (the shell) and the interior 

part (the core) that can result in drying defects. Early in 

drying, the fibers in the shell dry first and begin to shrink. 

However, the core has not yet begun to dry and shrink; 

consequently, the core prevents the shell from shrinking 

fully. Thus, the shell goes into tension and the core into 

compression (Fig. 13–3). If the shell dries too rapidly, it is 

stressed beyond the elastic limit and dries in a permanently 

stretched (set) condition without attaining full shrinkage. 

Sometimes surface cracks, or checks, occur from this initial 

stage of drying and can be a serious defect for many uses. 

As drying progresses, the core begins to dry and attempts to 

shrink. However, the shell is set in a permanently expanded 

condition and prevents normal shrinkage of the core. This 

causes the stresses to reverse; the core goes into tension and 

the shell into compression. The change in the shell and core 

stresses and in the moisture content level during drying is 

shown in Figure 13–4. These internal tension stresses may 

be severe enough to cause internal cracks (honeycomb).

Differential shrinkage caused by differences in radial, tangential, and longitudinal shrinkage is a major cause of warp. 

The distortions shown in Figure 4–3 in Chapter 4 are due 

to differential shrinkage. When juvenile or reaction wood is 

present on one edge or face of a board and normal wood is 

13–7

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

Figure 13–2. Typical moisture gradient in lumber 

during drying at time increasing from t1 to t3.

Figure 13–3. End view of board showing development of drying stresses (a) early and (b) later in 

drying.
present on the opposite side, the difference in their longitudinal shrinkage can also cause warp.

Dry Kilns

Most dry kilns are thermally insulated compartments designed for a batch process in which the kiln is completely 

loaded with lumber in one operation and the lumber remains 

stationary during the entire drying cycle. Temperature and 

relative humidity are kept as uniform as possible throughout 

the kiln and can be controlled over a wide range. As the 

wood dries, kiln temperature and relative humidity change 

based on a schedule that takes into account the moisture 

content or the drying rate, or both, of the lumber. All dry 

kilns use some type of forced-air circulation, with air moving through the lumber perpendicular to the length of the 

lumber and parallel to the spacers (stickers) that separate 

each layer of lumber in a stack. This forced-air circulation 

allows for uniform air flow in the dry kiln.

Three general types of kilns are in common use. One is the 

track-loaded type (Fig. 13–5), where lumber is stacked on 

kiln trucks that are rolled in and out of the kiln on tracks. 

Most softwood lumber in the United States is dried in this 

kiln type. Another major type is the package-loaded kiln 

(Fig. 13–6), where individual stacks of lumber are forklifted into place in the kiln. Package-loaded kilns are commonly used for drying hardwood lumber. Indirect-steam 

heat is common for these two types although softwood 

lumber kilns are sometimes directly heated using combustion gases from burning fuel. A third common type of kiln, 

usually package loaded, is the dehumidification kiln. Instead 

of venting humid air to remove water, as the other two types 

of kilns do, water is removed by condensation on cold dehumidifier coils (Fig. 13–7).

Kiln Schedules

A kiln schedule is a carefully developed compromise 

between the need to dry lumber as fast as possible for 

economic efficiency and the need to avoid severe drying 

conditions that will lead to drying defects. A kiln schedule 

is a series of temperatures and relative humidities that are 

applied at various stages of drying. In most schedules, the 

temperature is gradually increased and the relative humidity 

decreased, thus lowering the EMC. The schedule for Southern Pine structural lumber is an exception to this general 

rule. This is lumber usually dried at a constant temperature 

and relative humidity. Temperatures are chosen to balance 

the highest drying rate with the avoidance of objectionable 

drying defects. The stresses that develop during drying are 

the limiting factor in determining the kiln schedule. The 

schedule must be developed so that the drying stresses do 

not exceed the strength of the wood at any given temperature and moisture content. Otherwise, the wood will crack 

either on the surface or internally or be crushed by forces 

that collapse the wood cells. Wood generally becomes 

stronger as the moisture content decreases, and to a lesser 

13–8

General Technical Report FPL–GTR–190

Figure 13–4. Moisture–stress relationship during six 

stages of kiln drying 50-mm- (2-in.-) thick red oak.

Figure 13–5. Lineshaft, double-track, compartment 

kiln with alternately opposing fans. Vents are over 

fan shaft between fans. Vent on high pressure side of 

fans becomes fresh air inlet when direction of circulation is reversed.
extent, it becomes weaker as temperature increases. The 

net result is that as wood dries it becomes stronger because 

of the decreasing moisture content and can tolerate higher 

drying temperatures and lower relative humidities without 

cracking. This is a fortunate circumstance because as wood 

dries, its drying rate decreases at any given temperature, and 

the ability to increase drying temperature helps maintain a 

reasonably fast drying rate. Thus, rapid drying is achieved in 

kilns by the use of temperatures as high as possible and relative humidities as low as possible.

Drying schedules vary by species, thickness, grade, moisture content, and end use of lumber. The two general types 

of kiln schedules are moisture content schedules and timebased schedules. Most hardwood lumber is dried by moisture content schedules. This means that the temperature and 

relative humidity conditions are changed according to the 

percentage moisture content of the lumber during drying. 

A typical hardwood schedule might begin at 49 °C (120 °F) 

and 80% relative humidity when the lumber is green. By the 

time the lumber has reached 15% moisture content, the temperature is as high as 82 °C (180 °F). A typical hardwood 

drying schedule is shown in Table 13–3. Some method of 

monitoring moisture content during drying is required for 

schedules based on moisture content. One common method 

is the use of kiln samples that are periodically weighed, 

usually manually but potentially remotely with load cells. 

Alternatively, imbedded electrodes in sample boards sense 

the change in electrical conductivity with moisture content. 

This system is limited to moisture content values less than 

30% (Simpson 1991, Denig and others 2000).

Softwood kiln schedules generally differ from hardwood 

schedules in that changes in kiln temperature and relative 

humidity are made at predetermined times rather than moisture content levels. Examples of time-based schedules, both 

conventional temperature (<100 °C (<212 °F)) and high 

temperature (>110 °C (>230 °F )), are given in Table 13–3. 

Some hardwoods used as structural lumber also use a timebased schedule as shown in Table 13–3 (Simpson and Wang 

2001, Ross and Erickson 2005).

Drying Defects

Most drying defects or problems that develop in wood products during drying can be classified as fracture or distortion, 

warp, or discoloration. Defects in any one of these categories are caused by an interaction of wood properties with 

processing factors. Wood shrinkage is mainly responsible 

for wood ruptures and distortion of shape. Cell structure and 

chemical extractives in wood contribute to defects associated with uneven moisture content, undesirable color, and 

undesirable surface texture. Drying temperature is the most 

important processing factor because it can be responsible for 

defects in each category.

13–9

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

Figure 13–6. Package-loaded kiln with fans connected directly to motors.
Fracture or Distortion

Surface checks occur early in drying when the shell of a 

board is stressed in tension enough to fracture the wood. 

These checks occur most often on the face of flatsawn 

boards and are illustrated in Figure 13–8. End checks 

(Fig. 13–9) are similar to surface checks but appear on the 

ends of boards and logs. End checks occur because the 

rapid longitudinal movement of moisture causes the end 

to dry very quickly and develop high stresses, therefore 

fracturing. End coatings, on either the log or freshly sawn 

(green) lumber, are an effective preventative measure. Collapse is a distortion, flattening, or crushing of wood cells. 

In severe cases (Fig. 13–10), collapse usually shows up as 

grooves or corrugations, a washboarding effect. Less severe 

collapse shows up as excessive thickness shrinkage and may 

not be a serious problem. Honeycomb (Fig. 13–11) is an 

internal crack that occurs in the later stages of kiln drying 

when the core of a board is in tension. This internal defect 

is caused when the core is still at a relatively high moisture 

content and drying temperatures are too high for too long 

during this critical drying period. It may go unnoticed until 

the lumber is machined. Nondestructive testing methods, 

using speed of sound, have been found to be effective in detecting the presence of these cracks in dried lumber. Knots 

may loosen during drying because of the unequal shrinkage 

between the knot and the surrounding wood (Fig. 13–12).

Warp

Warp in lumber is any deviation of the face or edge of a 

board from flatness or any edge that is not at right angles to 

the adjacent face or edge. Warp can be traced to two causes: 

(a) differences between radial, tangential, and longitudinal 

shrinkage in the piece as it dries or (b) growth stresses. 

Warp is aggravated by irregular or distorted grain and the 

presence of abnormal types of wood, such as juvenile and 

reaction wood. The six major types of warp are bow, crook, 

twist, oval, diamond, and cup (Fig. 13–13).

Discoloration

Discoloration impairs the use of dried wood products, particularly when the end use requires a clear, natural finish. 

Unwanted discoloration can develop in the tree, during 

storage of logs and green lumber, or during drying. The two 

general types of discoloration are chemical and fungal.

Chemical discoloration is the result of oxidative and enzymatic reactions with chemical compounds in wood. 

Discolorations range from pinkish, bluish, and yellowish 

hues through gray and reddish brown to dark brown shades. 

Brown stain in pines and darkening in many hardwoods is 

a common problem when drying temperatures are too high 

(Fig. 13–14). A deep grayish-brown chemical discoloration 

can occur in many hardwood species if initial drying is too 

slow or too high of an initial kiln temperature (Fig. 13–15) 

(Wiemann and others 2009).

Fungal stains, often referred to as blue or sap stain, are 

caused by fungi that grow in the sapwood (Fig. 13–16). 

Blue-stain fungi do not cause decay of the sapwood, and 

fungi generally do not grow in heartwood. Blue stain can 

develop if initial drying is too slow.

Another common type of stain develops under stickers 

(Fig. 13–17). This stain results from contact of the sticker 

with the board. Sticker stains (sometimes called shadow) 

are imprints of the sticker that are darker or lighter than 

the wood between the stickers and can be caused by either 

chemical or fungal action, or both.

13–10

General Technical Report FPL–GTR–190

Figure 13–7. A typical dehumidification kiln (top) and 

dehumidification drying system (bottom).
Moisture Content of Dried Lumber

Although widely used, the trade terms “shipping dry,” “air 

dry,” and “kiln dry” may not have identical meanings as 

to moisture content in the different producing regions. Despite the wide variations in the use of these terms, they are 

sometimes used to describe dried lumber. The following 

statements, which are not exact definitions, outline these 

categories.

Shipping Dry

Shipping dry means lumber that has been partially dried to 

prevent stain or mold during brief periods of transit; ideally 

the outer 3.2 mm (1/8 in.) is dried to 25% or less moisture 

content (McMillen 1978).

Air Dry

Air dry means lumber dried by exposure to the air outdoors 

or in a shed or by forced circulation of air that has not been 

13–11

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

Table 13–3. Typical dry kiln schedules for lumber

Moisture-content-based schedule for 25-mm (1-in.) (4/4) black 

walnut, dried to 7% moisture content

Moisture

content 

(%)

Temperature (°C (°F)) 

Relative

humidity

(%)

Equilibrium 

moisture content 

Dry-bulb Wet-bulb (%)

Above 50 49.0 (120) 45.0 (113) 80 14.4 

50 to 40 49.0 (120) 43.5 (110) 72 12.1 

40 to 35 49.0 (120) 40.5 (105) 60 9.6 

35 to 30 49.0 (120) 35.0 (95) 40 6.5 

30 to 25 54.5 (130) 32.0 (90) 22 4.0 

25 to 20 60.0 (140) 32.0 (90) 15 2.9 

20 to 15 65.5 (150) 37.5 (100) 18 3.2 

15 to 7 82.2 (180) 54.4 (130) 27 3.7 

Equalize 82.2 (180) 58.3 (137) 30 3.8 

Condition 82.2 (180) 76.7 (170) 79 11.1 

Time-based schedule for 25- to 50-mm (1- to 2-in.) (4/4 to 8/4) 

Douglas-fir, upper grades, dried to 12% moisture content

Time 

(h)

Temperature (°C (°F)) Relative

humidity

(%)

Equilibrium 

moisture content 

Dry-bulb Wet-bulb (%)

0 to 12 76.5 (170) 73.5 (164) 86 14.1 

12 to 24 76.5 (170) 71.0 (160) 78 11.4 

24 to 48 79.5 (175) 71.0 (160) 69 9.1 

48 to 72 82.2 (180) 71.0 (160) 62 7.7 

72 to 96 

or until dry 

82.2 (180) 60.0 (140) 36 4.5 

High-temperature schedule for 50- by 100-mm to 50- by 

250-mm (2- by 4-in. to 2- by 10-in.) Southern Pine, dried 

to 15% moisture content

Time 

(h)

Temperature (°C (°F)) 

Relative

humidity

(%)

Equilibrium 

moisture content 

(%) Dry-bulb Wet-bulb 

0 until dry 116 (240) 82.2 (180) 29 2.5 

Time-based schedule for 50- by 150-mm (2- by 6-in.) sugar 

maple, dried to 15% moisture content in 5 days

Time 

(h)

Temperature (°C (°F)) Relative

humidity

(%)

Equilibrium 

moisture content 

Dry-bulb Wet-bulb (%)

0 to 24 71.0 (160) 67.2 (153) 84 14.1 

24 to 48 71.0 (160) 65.6 (150) 78 12.1 

48 to 60 71.0 (160) 62.8 (145) 69 10.1 

60 to 72 71.0 (160) 57.2 (135) 52 7.4 

72 to 84 76.7 (170) 54.4 (130) 35 4.9 

84 to 115 82.2 (180) 54.4 (130) 27 3.7 
heated above 49 °C (120 °F). Commercial air-dry stock generally has an average moisture content low enough for rapid 

kiln drying or rough construction use. Moisture content is 

generally in the range of 20% to 25% for dense hardwoods 

and 15% to 20% for softwoods and low-density hardwoods. 

Extended exposure can bring standard 19- and 38-mm 

(nominal 1- and 2-in.) lumber within one or two percentage 

points of the average exterior equilibrium moisture content 

of the region. For much of the United States, the minimum 

moisture content of thoroughly air-dried lumber is 12% to 

15%.

Kiln Dry

Kiln dry means lumber that has been dried in a kiln or by 

some special drying method to an average moisture content 

13–12

General Technical Report FPL–GTR–190

Figure 13–8. Surface checking on white oak 

5/4 lumber.

Figure 13–9. End checking in red pine logs.

Figure 13–10. Severe collapse in western redcedar.

Figure 13–11. Red cedar timber end shows honeycomb (top). Surface of the timber shows no honeycomb (bottom).

Figure 13–12. Large knot in treated Southern Pine.
specified or understood to be suitable for a certain use. The 

average moisture content should have upper and lower tolerance limits, and all values should fall within these limits. If 

the moisture contents fall outside these limits, use the dry 

kiln to equalize the lumber until the moisture is inside these 

limits. Kiln-dried softwood dimension lumber generally 

has an average moisture content of 19% or less; the average 

moisture content for many other softwood uses is 10% 

to 20%. Hardwood and softwood lumber for furniture, 

cabinetry, and millwork usually has a final moisture content of 6% to 8% and can be specified to be free of drying 

stresses. Drying stresses built up during the drying cycle are 

relieved by conditioning inside the dry kiln. The importance 

of suitable moisture content values is recognized, and provisions covering them are now incorporated in some softwood 

standards as grading rules. Moisture content values in the 

general grading rules may or may not be suitable for a 

specific use; if not, a special moisture content specification 

should be made (USDC 2005).

Moisture Control during Transit 

and Storage

Lumber and other wood items may change in moisture content and dimension while awaiting shipment, during fabrication, in transit, and in storage.

When standard 19-mm (nominal 1-in.) dry softwood lumber 

is shipped in tightly closed boxcars, shipping containers, or 

trucks or in packages with complete and intact wrappers, 

average moisture content changes for a package can generally be held to 0.2% or less per month. In holds or between 

decks of ships, dry material usually adsorbs about 1.5% 

moisture during normal shipping periods. If green material 

is included in the cargo, the moisture regain of the dry lumber may be doubled. On the top deck, if unprotected from 

the elements, the moisture regain can be as much as 7%.

When standard 19-mm (nominal 1-in.) softwood lumber, 

kiln dried to 8% or less, is piled solid under a good pile roof 

in a yard in warm, humid weather, average moisture content 

of a pile can increase at the rate of about 2% per month during the first 45 days. A moisture uptake rate of about 1% per 

month can then be sustained throughout a humid season. 

Comparable initial and sustaining moisture uptake rates are 

about 1% per month in open (roofed) sheds and 0.3% per 

month in closed sheds. Stock piled for a year in an open 

13–13

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

Figure 13–13. Various types of warp that can develop 

in boards during drying.

Figure 13–14. Brown sapwood stain in Southern Pine 

lumber.

Figure 13–15. Soft maple sapwood boards (surface, 

end, edge) showing patches of oxidative stain.
shed in a western location increased 2.7% on the inside of 

solid piles and 3.5% on the outside of the piles. Protect all 

manufactured stock from precipitation and spray, because 

liquid water on a solid pile tends to be absorbed by the 

wood instead of evaporating. The extent to which additional 

control of the storage environment is required depends upon 

the final use of the wood and the corresponding moisture 

content recommendations. It is important to determine the 

moisture content of all stock when received. If moisture 

content is not as specified or required, stickered storage in 

an appropriate condition could ultimately bring the stock 

within the desired moisture content range. If a large degree 

of moisture change is required, the stock must be redried 

(Rietz 1978).

Plywood and Structural Items

It is good practice to open-pile green or partially dried lumber and timbers using stickers and protect from sunshine and 

precipitation by a tight roof. Framing lumber and plywood 

with 20% or less moisture content can be solid-piled (no 

stickers) in a shed that has good protection against sunshine 

and direct or wind-driven precipitation. However, a better 

practice for stock with greater than 12% moisture content is 

the use of stickered piling to bring moisture content more in 

line with the moisture content in use. Dry lumber piled solid 

in the open for relatively short periods with a minimum pile 

cover of waterproofed paper whenever possible. Because 

keeping rain out completely is difficult, storing solid-piled 

lumber in the open for long periods is not recommended. If 

framing lumber must be stored in the open for a long time, 

pile on stickers with good base support and cover the piles. 

Re-pile using stickers for solid-piled material that has become wet again is good practice.

Finish and Factory Lumber

Keep kiln-dried items such as exterior finish, siding, and exterior millwork in a closed unheated shed. Place material on 

supports raised above the floor, at least 150 mm (6 in.) high 

if the floor is paved or 300 mm (12 in.) if not paved. Interior 

trim, flooring, cabinet work, and lumber for processing into 

furniture should be stored in a room or closed shed where 

relative humidity is controlled. In addition, store kiln-dried 

and machined hardwood dimension or softwood cut stock 

under controlled humidity conditions.

Dried and machined hardwood dimension or softwood lumber intended for remanufacture should also be stored under 

controlled humidity conditions. Under uncontrolled conditions, the ends of such stock may attain a higher moisture 

content than the rest of the stock. Then, when the stock is 

straight-line ripped or jointed before edge gluing, subsequent shrinkage will cause splitting or open glue joints at 

the ends of panels. The simplest way to reduce relative humidity in storage areas of all sizes is to heat the closed space 

to a temperature slightly higher than that of the outside air. 

Dehumidifiers can be used in small, well-enclosed spaces.

If the heating method is used, and there is no source of 

moisture except that contained in the air, the equilibrium 

moisture content can be maintained by increasing the 

temperature of the storage area greater than the outside 

temperature by the amounts shown in Table 13–4. When a 

dehumidifier is used, monitor or control if needed the average temperature in the storage space. Select the proper relative humidity in Table 4–2 in Chapter 4 to give the desired 

average moisture content. Wood in a factory awaiting or following manufacture can become too dry if the area is heated 

to 21 °C (70 °F) or greater when the outdoor temperature is 

low. This often occurs in the northern United States during 

the winter. Under such circumstances, exposed ends and 

surfaces of boards or cut pieces will tend to dry to the low 

equilibrium moisture content condition, causing shrinkage 

and warp. In addition, an equilibrium moisture content of 

4% or more below the moisture content of the core of freshly crosscut boards can cause end checking. Simple remedies 

are to cover piles of partially manufactured items with plastic film and lower the shop temperature during non-work 

hours. Increased control can be obtained in critical shop and 

13–14

General Technical Report FPL–GTR–190

Figure 13–16. Sap stain in Ponderosa Pine. Color ranges from bluish gray to black.
13–15

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

storage areas by humidification. In warm weather, cooling 

can increase relative humidity and dehumidification may be 

necessary (FPL 1972).

Dimensional Changes in Wood

Dry wood undergoes small changes in dimension with normal changes in relative humidity. More humid air will cause 

slight swelling, and drier air will cause slight shrinkage. 

These changes are considerably smaller than those involved 

with shrinkage from the green condition. Equation (13–2) 

can be used to approximate dimensional changes caused by 

shrinking and swelling by using the total shrinkage coefficient from green to oven-dry. However, the equation assumes that the shrinkage–moisture content relationship is 

linear. Figure 4–4 (Chap. 4) shows that this is not the case, 

so some error is introduced. The error is in the direction of 

underestimating dimensional change, by about 5% of the 

true change. Many changes of moisture content in use are 

over the small moisture content range of 6% to 14%, where 

the shrinkage–moisture content relationship is linear 

(Chap. 4, Fig.4–4). Therefore, a set of shrinkage coefficients based on the linear portion of the shrinkage–moisture 

content curve has been developed (Table 13–5). Estimating 

approximate changes in dimension using this simple equation utilizes these dimensional change coefficients, from 

Table 13–5, when moisture content remains within the range 

of normal use. (Dimensional changes are further discussed 

in Chaps. 4 and 7.)

Estimation Using Dimensional Change 

Coefficient

The change in dimension within the moisture content limits 

of 6% to 14% can be estimated satisfactorily by using a 

dimensional change coefficient based on the dimension at 

10% moisture content:

[ ( )] DD = DI CT MF - MI (13–2)

where DD is change in dimension, DI dimension in units of 

length at start of change, CT dimensional change coefficient 

tangential direction (for radial direction, use CR), MF moisture content (%) at end of change, and MI moisture content 

(%) at start of change.

Values for CT and CR, derived from total shrinkage values, 

are given in Table 13–5. When MF < MI, the quantity 

(MF - MI) will be negative, indicating a decrease in dimension; when greater, it will be positive, indicating an increase 

in dimension.

As an example, assuming the width of a flat-grained white 

fir board is 232 mm (9.15 in.) at 8% moisture content, its 

change in width at 11% moisture content is estimated as

DD = 232[0.00245(11 - 8)]

= 232(0.00735)

= 1.705 mm

DD = 9.15[0.00245(11 - 8)]

= 9.15[0.00735]

= 0.06725 or 0.067 in.

Figure 13–17. Sticker stain in sapwood of sugar maple 

after planing.

Table 13–4. Increase in storage area temperature above outside 

temperature to maintain the desired wood moisture content

Outside

relative 

humidity 

(%)

Temperature differential (°C (°F)) for desired wood moisture content 

6% 7% 8% 9% 10% 11% 12% 

90 18.3 (33) 16.1 (29) 12.8 (23) 10.0 (18) 8.3 (15) 6.1 (11) 5.0 (9) 

80 16.7 (30) 13.9 (25) 10.5 (19) 7.8 (14) 6.1 (11) 4.4 (8) 3.3 (6) 

70 13.9 (25) 11.1 (20) 8.3 (15) 5.6 (10) 3.9 (7) 2.2 (4) 1.7 (3) 

60 11.1 (20) 8.3 (15) 5.0 (9) 3.3 (6) 1.7 (3) — — 

50 8.3 (15) 5.6 (10) 2.8 (5) 0.6 (1) — — — 
13–16

General Technical Report FPL–GTR–190

Table 13–5. Dimensional change coefficients (CR, radial; CT, tangential) for shrinking or 

swelling within moisture content limits of 6% to 14%

Dimensional

change

coefficienta

Dimensional

change

coefficienta

Species CR CT Species CR CT

Hardwoods

Alder, red 0.00151 0.00256 Honeylocust 0.00144 0.00230 

Apple 0.00205 0.00376 Locust, black 0.00158 0.00252 

Ash, black 0.00172 0.00274 Madrone, Pacific 0.00194 0.00451 

Ash, Oregon 0.00141 0.00285 Magnolia, cucumbertree 0.00180 0.00312 

Ash, pumpkin 0.00126 0.00219 Magnolia, southern 0.00187 0.00230 

Ash, white 0.00169 0.00274 Magnolia, sweetbay 0.00162 0.00293 

Ash, green 0.00169 0.00274 Maple, bigleaf 0.00126 0.00248 

Aspen, quaking 0.00119 0.00234 Maple, red 0.00137 0.00289 

Basswood, American 0.00230 0.00330 Maple, silver 0.00102 0.00252 

Beech, American 0.00190 0.00431 Maple, black 0.00165 0.00353 

Birch, paper 0.00219 0.00304 Maple, sugar 0.00165 0.00353 

Birch, river 0.00162 0.00327 Oak, black 0.00123 0.00230 

Birch, yellow 0.00256 0.00338 Red Oak, commercial 0.00158 0.00369 

Birch, sweet 0.00256 0.00338 Red oak, California 0.00123 0.00230 

Buckeye, yellow 0.00123 0.00285 Red oak: water, laurel, willow 0.00151 0.00350 

Butternut 0.00116 0.00223 White Oak, commercial 0.00180 0.00365 

Catalpa, northern 0.00085 0.00169 White oak, live 0.00230 0.00338 

Cherry, black 0.00126 0.00248 White oak, Oregon white 0.00144 0.00327 

Chestnut, American 0.00116 0.00234 White oak, overcup 0.00183 0.00462 

Cottonwood, black 0.00123 0.00304 Persimmon, common 0.00278 0.00403 

Cottonwood, eastern 0.00133 0.00327 Sassafras 0.00137 0.00216 

Elm, American 0.00144 0.00338 Sweetgum 0.00183 0.00365 

Elm, rock 0.00165 0.00285 Sycamore, American 0.00172 0.00296 

Elm, slippery 0.00169 0.00315 Tanoak 0.00169 0.00423 

Elm, winged 0.00183 0.00419 Tupelo, black 0.00176 0.00308 

Elm, cedar 0.00183 0.00419 Tupelo, water 0.00144 0.00267 

Hackberry 0.00165 0.00315 Walnut, black 0.00190 0.00274 

Hickory, pecan 0.00169 0.00315 Willow, black 0.00112 0.00308 

Hickory, true 0.00259 0.00411 Willow, Pacific 0.00099 0.00319 

Holly, American 0.00165 0.00353 Yellow-poplar 0.00158 0.00289 

Softwoods

Baldcypress 0.00130 0.00216 Pine, eastern white 0.00071 0.00212 

Cedar, yellow- 0.00095 0.00208 Pine, jack 0.00126 0.00230 

Cedar, Atlantic white- 0.00099 0.00187 Pine, loblolly 0.00165 0.00259 

Cedar, Eastern Red 0.00106 0.00162 Pine, pond 0.00165 0.00259 

Cedar, incense 0.00112 0.00180 Pine, lodgepole 0.00148 0.00234 

Cedar, northern white-b

 0.00101 0.00229 Pine, Jeffrey 0.00148 0.00234 

Cedar, Port-Orford- 0.00158 0.00241 Pine, longleaf 0.00176 0.00263 

Cedar, western redb

 0.00111 0.00234 Pine, ponderosa 0.00133 0.00216 

Douglas-fir, Coast-type 0.00165 0.00267 Pine, red 0.00130 0.00252 

Douglas-fir, Interior north 0.00130 0.00241 Pine, shortleaf 0.00158 0.00271 

Douglas-fir, Interior west 0.00165 0.00263 Pine, slash 0.00187 0.00267 

Fir, balsam 0.00099 0.00241 Pine, sugar 0.00099 0.00194 

Fir, California red 0.00155 0.00278 Pine, Virginia 0.00144 0.00252 

Fir, noble 0.00148 0.00293 Pine, western white 0.00141 0.00259 

Fir, Pacific silver 0.00151 0.00327 Redwood, old-growthb 0.00120 0.00205 

Fir, subalpine 0.00088 0.00259 Redwood, second-growthb

 0.00101 0.00229 

Fir, grand 0.00112 0.00245 Spruce, black 0.00141 0.00237 

Fir, white 0.00112 0.00245 Spruce, Engelmann 0.00130 0.00248 
13–17

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

Then, dimension at end of change 

DI + DD = 232 + 1.7 (= 9.15 + 0.067)

= 233.7 mm (= 9.217 in.)

The thickness of the same board at 11% moisture content 

can be estimated by using the coefficient CR = 0.00112.

Because commercial lumber is often not perfectly flatsawn 

or quartersawn, this procedure will probably overestimate 

width shrinkage and underestimate thickness shrinkage. 

Note also that if both a size change and percentage moisture 

content are known, Equation (13–2) can be used to calculate 

the original moisture content.

Calculation Based on Green Dimensions

Approximate dimensional changes associated with moisture content changes greater than 6% to 14%, or when one 

moisture content value is outside of those limits, can be 

calculated by

(13–3)

where ST is tangential shrinkage (%) from green to ovendry 

(Chap. 4 Tables 4–3 and 4–4) (use radial shrinkage SR when 

appropriate).

Neither MI nor MF should exceed 30%, the assumed moisture content value when shrinkage starts for most species.

Design Factors Affecting 

Dimensional Change

Framing Lumber in House Construction

Ideally, house framing lumber should be dried to the moisture content it faces in use to minimize dimensional changes 

as a result of frame shrinkage. This ideal condition is difficult to achieve, but some drying and shrinkage of the frame 

may take place without being visible or causing serious 

defects after the house is completed. If, at the time the wall 

and ceiling finish is applied, the moisture content of the 

framing lumber is not more than about 5% above that which 

it will reach in service, there will be little or no evidence of 

defects caused by shrinkage of the frame. For heated houses 

in cold climates, joists over heated basements, studs, and 

ceiling joists may reach a moisture content as low as 6% to 

7% (Table 13–2). In mild climates, the minimum moisture 

content will be greater.

The most common signs of excessive shrinkage are cracks 

in plastered walls, truss rise, open joints, and nail pops in 

dry-wall construction; distortion of door openings; uneven 

floors; and loosening of joints and fastenings. The extent of 

vertical shrinkage after the house is completed is proportional to the depth of wood used as supports in a horizontal 

position, such as girders, floor joists, and plates. After all, 

Table 13–5. Dimensional change coefficients (CR, radial; CT, tangential) for shrinking or 

swelling within moisture content limits of 6% to 14%—con.

Dimensional

change

coefficienta

Dimensional

change

coefficienta

Species CR CT CR CT

Hemlock, eastern 0.00102 0.00237 Spruce, red 0.00130 0.00274 

Hemlock, western 0.00144 0.00274 Spruce, white 0.00130 0.00274 

Larch, western 0.00155 0.00323 Spruce, Sitka 0.00148 0.00263 

 Tamarack 0.00126 0.00259 

Imported Woods

Andiroba, crabwood 0.00137 0.00274 Light red “Philippine mahogany” 0.00126 0.00241 

Angelique 0.00180 0.00312 Limba 0.00151 0.00187 

Apitong, keruingb

(all Dipterocarpus spp.) 

0.00243 0.00527 Mahoganyb

Meranti 

0.00172

0.00126

0.00238

0.00289 

Avodire 0.00126 0.00226 Obeche 0.00106 0.00183 

Balsa 0.00102 0.00267 Okoume 0.00194 0.00212 

Banak 0.00158 0.00312 Parana, pine 0.00137 0.00278 

Cativo 0.00078 0.00183 Paumarfim 0.00158 0.00312 

Cuangare 0.00183 0.00342 Primavera 0.00106 0.00180 

Greenheartb 0.00390 0.00430 Ramin 0.00133 0.00308 

Irokob 0.00153 0.00205 Santa Maria 0.00187 0.00278 

Khaya 0.00141 0.00201 Spanish-cedar 0.00141 0.00219 

Kokroduab 0.00148 0.00297 Teakb 0.00101 0.00186 

Lauans: dark red 

“Philippine mahogany” 

0.00133 0.00267 

a

Per 1% change in moisture content, based on dimension at 10% moisture content and a straight-line relationship between 

moisture content at which shrinkage starts and total shrinkage. (Shrinkage assumed to start at 30% for all species except 

those indicated by footnote b.) 

b

Shrinkage assumed to start at 22% moisture content. 
13–18

General Technical Report FPL–GTR–190

shrinkage occurs primarily in the width and thickness of 

members, not the length.

Thoroughly consider the type of framing best suited to the 

whole building structure. Methods should be chosen that 

will minimize or balance the use of wood across the grain 

in vertical supports. These involve variations in floor, wall, 

and ceiling framing. The factors involved and details of 

construction are covered extensively in Wood-Frame House 

Construction (Sherwood and Stroh 1991).

Heavy Timber Construction

In heavy timber construction, a certain amount of shrinkage 

is to be expected. A column that bears directly on a wood 

girder can result in a structure settling as a result of the perpendicular-to-grain shrinkage of the girder. If not provided 

for in the design, shrinkage may cause weakening of the 

joints or uneven floors or both. One means of eliminating 

part of the shrinkage in mill buildings and similar structures 

is to use metal post caps; the metal in the post cap separates 

the upper column from the lower column. The same thing is 

accomplished by bolting wood corbels (tassels or braggers) 

to the side of the lower column to support the girders.

When joist hangers are installed, the top of the joist should 

be above the top of the girder; otherwise, when the joist 

shrinks in the stirrup, the floor over the girder will be higher 

than that bearing upon the joist. Heavy planking used for 

flooring should be near 12% moisture content to minimize openings between boards as they approach moisture 

equilibrium. When standard 38- or 64-mm (nominal 2- or 

3-in.) joists are nailed together to provide a laminated floor 

of greater depth for heavy design loads, the joist material 

should be somewhat less than 12% moisture content if the 

building is to be heated.

Interior Finish

Normal seasonal changes in the moisture content of interior 

finish are not enough to cause serious dimensional change if 

the woodwork was properly installed. Large members, such 

as ornamental beams, cornices, newel posts, stair stringers, 

and handrails, should be built up from comparatively small 

pieces. Wide door and window trim and base should be hollow-backed. Backband trim, if mitered at the corners, should 

be glued and splined before erection; otherwise butt joints 

should be used for the wide faces. Design and install large, 

solid pieces, such as wood paneling, so that the panels are 

free to move across the grain. Narrow widths are preferable.

Flooring

Flooring is usually dried to the moisture content expected 

in service so that shrinking and swelling are minimized and 

buckling or large gaps between boards do not occur. For 

basement, large hall, or gymnasium floors, however, leave 

enough space around the edges to allow for some expansion.

Wood Care and Installation during 

Construction

Lumber and Trusses

Although it is good housekeeping practice, lumber is often 

not protected from the weather at construction sites. Lumber 

is commonly placed on the ground in open areas near the 

building site as bulked and strapped packages. Place supports under such packages that elevate the packages at least 

150 mm (6 in.) off the ground to prevent wetting from mud 

and ground water. In addition, cover the packages with plastic tarpaulins for protection from rain.

Pile lumber that is green or nearly green on stickers under a 

roof for additional drying before building into the structure. 

The same procedure is required for lumber treated with a 

waterborne preservative but not fully re-dried. Prefabricated 

building parts, such as roof trusses, sometimes lie unprotected on the ground at the building site. In warm, rainy weather, moisture regain can result in fungal staining. Wetting of 

the lumber also results in swelling, and subsequent shrinkage of the framing may contribute to structural distortions. 

Extended storage of lumber at moisture contents greater 

than 20% without drying can allow decay to develop.

If framing lumber has a greater moisture content when 

installed than that recommended in Table 13–2, shrinkage 

can be expected. Framing lumber, even thoroughly air-dried 

stock, will generally have a moisture content greater than 

that recommended when it is delivered to the building site. 

If carelessly handled in storage at the site, the lumber can 

take up more moisture. Builders can schedule their work 

so an appreciable amount of drying can take place during 

the early stages of construction. This minimizes the effects 

of additional drying and shrinkage after completion. When 

the house has been framed, sheathed, and roofed, the framing is so exposed that in time it can dry to a lower moisture 

content than could be found in yard-dried lumber. The 

application of the wall and ceiling finish is delayed while 

wiring and plumbing are installed. If this delay is about 

30 days in warm, dry weather, the framing lumber should 

lose enough moisture so that any additional drying in place 

will be minimal. In cool, damp weather, or if wet lumber is 

used, the period of exposure should be extended. Checking 

moisture content of door and window headers and floor and 

ceiling joists at this time with an electric moisture meter is 

good practice. When these members approach an average of 

12% moisture content, interior finish and trim can normally 

be installed. Closing the house and using the heating system 

will hasten the rate of drying.

Before the wall finish is applied, the frame should be examined and defects that may have developed during drying, 

such as warped or distorted studs, shrinkage of lintels (header) over openings, or loosened joints, should be corrected.
13–19

Chapter 13 Drying and Control of Moisture Content and Dimensional Changes

Exterior Trim and Millwork

Exterior trim, such as cornice and rake mouldings, fascia 

boards, and soffit material, is typically installed before the 

shingles are laid. Protect trim, siding, and window and door 

frames on the site by storing in the house or garage until 

time of installation. Although items such as window frames 

and sashes are usually treated with some type of waterrepellent preservative to resist absorption of water, store in a 

protected area if they cannot be installed soon after delivery. 

Wood siding is often received in packaged form and can ordinarily remain in the package until installation.

Finished Flooring

Cracks develop in flooring if the material takes up moisture 

either before or after installation, then shrinks when the 

building is heated. Such cracks can be greatly reduced by 

observing the following practices:

•	 Specify flooring manufactured according to association 

rules and sold by dealers that protect the material properly during storage and delivery.

•	 Measure random pieces of flooring using a non-penetrating meter to ensure moisture content is correct upon 

arrival and prior to installation.

•	 Have flooring delivered after masonry and plastering 

are completed and fully dry, unless a dry storage space 

is available.

•	 Install the heating plant before flooring is delivered.

•	 Break open flooring bundles and expose all sides of 

flooring to the atmosphere inside the structure.

•	 Close up the house at night and increase the temperature about 8 °C (15 °F) greater than the outdoor temperature for about 3 days before laying the floor. 

•	 If the house is not occupied immediately after the floor 

is laid, keep the house closed at night or during damp 

weather and supply some heat if necessary.

Better and smoother sanding and finishing can be done 

when the house is warm and the wood has been kept dry 

(FPL 1961).

Interior Trim

In a building under construction, average relative humidity 

will be greater than that in an occupied house because of 

the moisture that evaporates from wet concrete, brickwork, 

plaster, and even the structural wood members. The average 

temperature will be lower because workers prefer a lower 

temperature than is common in an occupied house. Under 

such conditions, the interior trim tends to have greater moisture content during construction than it will have during 

occupancy.

Before the interior trim is delivered, the outside doors and 

windows should be kept closed at night. In this way, interior 

conditions are held as close as possible to the higher 

temperature and lower humidity that ordinarily occurs during the day. Such protection may be sufficient during dry 

warm weather, but during damp or cool weather, it is highly 

desirable to heat the house, particularly at night. Whenever 

possible, the heating plant should be placed in the house 

before the interior trim is installed, to be available for supplying the necessary heat. Portable heaters can also be used. 

Keep the inside temperature during the night about 8 °C 

(15 °F) greater than the outside temperature but not below 

about 21 °C (70 °F) during the summer or 17 °C (62 °F) 

when the outside temperature is below freezing.

After buildings have thoroughly dried, less heat is needed, 

but unoccupied houses, new or old, should have some heat 

during the winter. A temperature of about 8 °C (15 °F) 

greater than the outside temperature and above freezing at 

all times will keep the woodwork, finish, and other parts of 

the house from being affected by dampness or frost.

Plastering

During a plastering operation in a moderate-sized, six-room 

house, approximately 450 kg (1,000 lb) of water is used, 

all of which must dissipate before the house is ready for 

the interior finish. Adequate ventilation removes the evaporated moisture and keeps it from being adsorbed by the 

framework. In houses plastered in cold weather, the excess 

moisture can also cause paint to blister on exterior finish and 

siding. During warm, dry weather, with the windows wide 

open, the moisture will be gone within a week after the final 

coat of plaster is applied. During damp, cold weather, the 

heating system or portable heaters are used to prevent freezing of plaster and to hasten its drying. Provide adequate ventilation constantly because a large volume of air is required 

to carry away the amount of water involved. Even in the 

coldest weather, the windows on the side of the house away 

from the prevailing winds should be opened 50 to 75 mm 

(2 to 3 in.), preferably from the top.

Literature Cited

ASTM. 2007. Standard test methods for direct moisture 

content measurement of wood and wood-base materials. 

Standard D 4442. West Conshohocken, PA: ASTM 

International. 

Boone, R.S.; Kozlik, C.J.; Bois, P.J.; Wengert, E.M. 1988. 

Dry kiln schedules for commercial woods: temperate and 

tropical. Gen. Tech. Rep. FPL–GTR–57. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 158 p.

Denig, J.; Wengert E.M.; Simpson, W.T. 2000. Drying hardwood lumber. Gen. Tech. Rep. FPL–GTR–118. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 138 p.

FPL. 1961. Wood floors for dwellings. Agric. Handb. 204. 

Washington, DC: U.S. Department of Agriculture, Forest 

Service. 44 p.
13–20

General Technical Report FPL–GTR–190

FPL. 1972. Methods of controlling humidity in woodworking plants. Res. Note FPL–RN–0218. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 6 p.

James, W.L. 1988. Electric moisture meters for wood. Gen. 

Tech. Rep. FPL–GTR–6. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

17 p.

McMillen, J.M. 1978. Overseas protection of logs and lumber. Madison, WI: U.S. Department of Agriculture,Forest 

Service, Forest Products Laboratory. 4 p.

Rietz, R.C. 1978. Storage of lumber. Agric. Handb. 531. 

Washington, DC: U.S. Department of Agriculture. 63 p.

Rietz, R.C.; Page, R.H. 1971. Air drying of lumber: A guide 

to industry practices. Agric. Handb. 402. Washington, DC: 

U.S. Department of Agriculture. 120 p.

Ross, R.J.; Erickson, J.R. 2005. Undervalued Hardwoods 

for Engineered Materials and Components. Forest Products 

Society. Madison, WI. 144 p.

Sherwood, G.E.; Stroh, R.C. 1991. Wood-frame house construction. Agric. Handb. 73. Washington, DC: U.S. 

Department of Agriculture. 268 p.

Simpson, W.T., ed. 1991. Dry kiln operator’s manual. Agric. 

Handb. 188. Washington, DC: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 274 p.

Simpson, W.T. 1998. Equilibrium moisture content of wood 

in outdoor locations in the United States and worldwide. 

Res. Note FPL–RN–268. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

11 p.

Simpson, W.T.; Wang, X. 2001. Time-based kiln drying 

schedule for sugar maple for structural uses. Res. Note 

FPL–RN–0279. Madison, WI: USDA Forest Service, Forest 

Products Laboratory. 4 p. 

USDC. 2005. American softwood lumber standard. NIST 

Voluntary Product Standard PS 20–05. Washington, DC: 

U.S. Department of Commerce. 50 p.

Wiemann, M.C.; Bergman, R.D.; Knaebe, M.; Bowe, S.A. 

2009. Exploring methods for prevention of oxidative stain 

in soft maple. Res. Pap. FPL–RP–654. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 6 p.
CHAPTER 14

Biodeterioration of Wood

Carol A. Clausen, Supervisory Research Microbiologist

14–1

Under proper conditions, wood will give centuries of service. However, under conditions that permit the development of wood-degrading organisms, protection must be 

provided during processing, merchandising, and use.

The organisms that can degrade wood are principally fungi, 

insects, bacteria, and marine borers.

Molds, most sapwood stains, and decay are caused by 

fungi, which are microscopic, thread-like microorganisms 

that must have organic material to live. For some of them, 

wood offers the required food supply. The growth of fungi 

depends on suitably mild temperatures, moisture, and air 

(oxygen). Chemical stains, although they are not caused by 

organisms, are mentioned in this chapter because they resemble stains caused by fungi.

Insects also may damage wood and in many situations must 

be considered in protective measures. Termites are the major 

insect enemy of wood, but on a national scale, they are a 

less serious threat than fungi.

Bacteria in wood ordinarily are of little consequence, but 

some may make the wood excessively absorptive. In addition, some may cause strength losses over long periods of 

exposure, particularly in forest soils.

Marine borers can attack susceptible wood rapidly in salt 

water harbors, where they are the principal cause of damage 

to piles and other wood marine structures.

Wood degradation by organisms has been studied extensively, and many preventive measures are well known and 

widely practiced. By taking ordinary precautions with the 

finished product, the user can contribute substantially to ensuring a long service life.

Fungus Damage and Control

Fungus damage to wood may be traced to three general 

causes: (a) lack of suitable protective measures when storing 

logs or bolts; (b) improper seasoning, storing, or handling 

of the raw material produced from the log; and (c) failure to 

take ordinary simple precautions in using the final product. 

The incidence and development of molds, decay, and stains 

caused by fungi depend heavily on temperature and moisture conditions (Fig. 14–1).

Molds and Fungal Stains

Molds and fungal stains are confined to a great extent to 

sapwood and are of various colors. The principal fungal 

Contents 

Fungus Damage and Control 14–1

 Molds and Fungal Stains 14–1 

 Chemical Stains 14–2

 Decay 14–3

 Prevention of Mold, Stain, and Decay 14–6 

Bacteria 14–9

Insect Damage and Control 14–9

 Beetles 14–9

 Termites 14–11

 Carpenter Ants 14–13

 Carpenter Bees 14–13

Marine Borer Damage and Control 14–13

 Shipworms 14–13

 Pholads 14–14

 Limnoria and Sphaeroma 14–14

 Protection from Marine Borers 14–14

References 14–15
14–2

pored hardwoods (for example, oaks), the wood beneath 

the surface growth is commonly stained too deeply to be 

surfaced off. The staining tends to occur in spots of various 

concentration and size, depending on the kind and pattern of 

the superficial growth.

Under favorable moisture and temperature conditions, staining and molding fungi may become established and develop 

rapidly in the sapwood of logs shortly after they are cut. 

In addition, lumber and such products as veneer, furniture 

stock, and millwork may become infected at any stage of 

manufacture or use if they become sufficiently moist. 

Freshly cut or unseasoned stock that is piled during warm, 

humid weather may be noticeably discolored within 5 or 

6 days. Recommended moisture control measures are given 

in Chapter 13.

Ordinarily, stain and mold fungi affect the strength of the 

wood only slightly; their greatest effect is usually confined 

to strength properties that determine shock resistance or 

toughness (Chap. 5). They increase the absorbency of wood, 

and this can cause over-absorption of glue, paint, or wood 

preservative during subsequent processing. Increased porosity also makes wood more wettable, which can lead to subsequent colonization by typical wood-decay fungi.

Stain- and mold-infected stock is practically unimpaired 

for many uses in which appearance is not a limiting factor, 

and a small amount of stain may be permitted by standard 

grading rules. Stock with stain and mold may not be entirely 

satisfactory for siding, trim, and other exterior millwork because of its greater water absorbency. Also, incipient decay 

may be present, though inconspicuous, in the discolored 

areas. Both of these factors increase the possibility of decay 

in wood that is rain-wetted unless the wood has been treated 

with a suitable preservative.

Chemical Stains

Nonmicrobial or chemical stains are difficult to control and 

represent substantial loss in wood quality. These stains, 

which should not be confused with fungal brown stain, 

include a variety of discolorations in wood that are often promoted by slow drying of lumber and warm to hot 

temperatures. Such conditions allow naturally occurring 

chemicals in wood to react with air (enzymatic oxidation) 

to form a new chemical that is typically dark in color. Common chemical stains include (a) interior sapwood graying, 

prevalent in oak, hackberry, ash, and maple, (b) brown stain 

in softwoods, and (c) pinking and browning in the interior 

of light-colored woods such as maple. Another common 

discoloration, iron stain, is caused by the interaction of iron 

with tannins in wood. Iron stain is more prevalent in hardwoods (for example, oak and many tropical hardwoods) and 

in some softwoods such as Douglas-fir. Control is achieved 

by eliminating the source of iron.

stains are usually referred to as sap stain or blue stain. The 

distinction between molding and staining is made primarily 

on the basis of the depth of discoloration. With some molds 

and the lesser fungal stains, there is no clear-cut differentiation. Typical sap stain or blue stain penetrates into the sapwood and cannot be removed by surfacing. Also, the discoloration as seen on a cross section of the wood often appears 

as pie-shaped wedges oriented radially, corresponding to the 

direction of the wood rays (Fig. 14–2). The discoloration 

may completely cover the sapwood or may occur as specks, 

spots, streaks, or patches of various intensities of color. The 

so-called blue stains, which vary from bluish to bluish black 

and gray to brown, are the most common, although various 

shades of yellow, orange, purple, and red are sometimes 

encountered. The exact color of the stain depends on the 

infecting organisms and the species and moisture condition 

of the wood. The fungal brown stain mentioned here should 

not be confused with chemical brown stain.

Mold discolorations usually become noticeable as fuzzy or 

powdery surface growths, with colors ranging from light 

shades to black. Among the brighter colors, green and yellowish hues are common. On softwoods, though the fungus 

may penetrate deeply, the discoloring surface growth often 

can easily be brushed or surfaced off. However, on largeFigure 14–1. Climate index for decay hazard. Higher numbers indicate greater decay hazard.

Figure 14–2. 

Typical radial penetration of log by 

stain. The pattern 

is a result of more 

rapid penetration 

by the fungus 

radially (through 

the ray) than 

tangentially.

General Technical Report FPL–GTR–190
14–3

Chapter 14 Biodeterioration of Wood

Decay

Decay-producing fungi may, under conditions that favor 

their growth, attack either heartwood or sapwood in most 

wood species (Fig. 14–3). The result is a condition 

designated as decay, rot, dote, or doze. Fresh surface 

growths of decay fungus may appear as fan-shaped patches 

(Fig. 14–4), strands, or root-like structures that are usually 

white or brown in color. Sometimes fruiting bodies are produced that take the form of mushrooms, brackets, or crusts. 

The fungus, in the form of microscopic, threadlike strands 

called hyphae, permeates the wood and uses parts of it as 

food. Some fungi live largely on cellulose, whereas others 

use lignin and cellulose.

Certain decay fungi colonize the heartwood (causing heart 

rot) and rarely the sapwood of living trees, whereas others 

confine their activities to logs or manufactured products, 

such as sawn lumber, structural timbers, poles, and ties. 

Most fungi that attack trees cease their activities after the 

trees have been cut, as do the fungi causing brown pocket 

(peck) in baldcypress or white pocket in Douglas-fir and 

other conifers. Relatively few fungi continue their destruction after the trees have been cut and worked into products and then only if conditions remain favorable for their 

growth. Although heartwood is more susceptible to decay 

than is sapwood in living trees, for many species, the sapwood of wood products is more susceptible to decay than is 

the heartwood.

Most decay can progress rapidly at temperatures that favor 

growth of plant life in general. For the most part, decay is 

relatively slow at temperatures below 10 °C (50 °F) and 

above 35 °C (95 °F). Decay essentially ceases when the 

temperature drops as low as 2 °C (35 °F) or rises as high

as 38 °C (100 °F).

Serious decay occurs only when the moisture content of 

the wood is above the fiber saturation point (average 30%). 

Only when previously dried wood is contacted by water in 

the form of rain or condensation or is in contact with wet 

ground will the fiber saturation point be reached. By itself, 

the water vapor in humid air will not wet wood sufficiently 

to support significant decay, but it will permit development 

of some mold fungi. Fully air-dried wood usually will have 

a moisture content not exceeding 20% and should provide 

a reasonable margin of safety against fungal damage. Thus, 

wood will not decay if it is kept air dry, and decay already 

present from prior infection will not progress.

Wood can be too wet for decay as well as too dry. If the 

wood is water-soaked, the supply of air to the interior of a 

piece may not be adequate to support development of typical decay fungi. For this reason, foundation piles buried 

beneath the water table and logs stored in a pond or under a 

suitable system of water sprays are not subject to decay by 

typical wood-decay fungi.

Figure 14–3. The decay cycle (top to bottom). 

Thousands of spores produced in a fungal fruiting body are distributed by wind or insects. On 

contacting moist, susceptible wood, spores germinate and fungal hypha create new infections in 

the wood cells. In time, serious decay develops 

that may be accompanied by formation of new 

fruiting bodies. 

Figure 14–4. Mycelial fans on a wood door.
14–4

outward dimensions, does not shrink or collapse, and often 

feels spongy (Fig. 14–5). Brown-rot fungi commonly colonize softwoods, and white-rot fungi commonly occur on 

hardwoods, but both brown- and white-rot fungi occasionally colonize both types of wood.

Brown, crumbly rot, in the dry condition, is sometimes 

called dry rot, but the term is incorrect because wood must 

be damp to decay, although it may become dry later. A few 

fungi, however, have water-conducting strands; such fungi 

are capable of carrying water (usually from the soil) into 

buildings or lumber piles, where they moisten and rot wood 

that would otherwise be dry. They are sometimes referred to 

technically as dry-rot fungi or water-conducting fungi. The 

latter term better describes the true situation because these 

fungi, like the others, must have water.

A third and generally less important kind of decay is known 

as soft rot. Soft rot is caused by fungi related to the molds 

rather than those responsible for brown and white rot. Soft 

rot typically is relatively shallow, primarily affecting the 

outer surface of wood; the affected wood is greatly degraded 

and often soft when wet, but immediately beneath the zone 

of rot, the wood may be firm (Fig. 14–5). Because soft rot 

usually is rather shallow, it is most damaging to relatively 

thin pieces of wood such as slats in cooling towers. It is favored by wet situations but is also prevalent on surfaces that 

have been alternately wet and dry over a substantial period. 

Heavily fissured surfaces, familiar to many as weathered 

wood, generally have been quite degraded by soft-rot fungi.

Decay Resistance of Wood

The heartwood of common native species of wood has various degrees of natural decay resistance. Untreated sapwood 

of essentially all species has low resistance to decay and 

usually has a short service life under conditions favoring 

decay. The natural decay resistance of heartwood is greatly 

affected by differences in preservative qualities of the wood 

extractives, the attacking fungus, and the conditions of exposure. Considerable differences in service life can be obtained from pieces of wood cut from the same species, even 

the same tree, and used under apparently similar conditions. 

There are further complications because, in a few species, 

such as the spruces and the true firs (not Douglas-fir), heartwood and sapwood are so similar in color that they cannot 

be easily distinguished.

Precise ratings of decay resistance of heartwood of different species are not possible because of differences within 

species and the variety of service conditions to which wood 

is exposed. However, broad groupings of many native species, based on service records, laboratory tests, and general 

expertise, are helpful in choosing heartwood for use under 

conditions favorable to decay. Groupings by natural resistance of some domestic and imported wood species to decay 

fungi are shown in Table 14–1, which ranks the heartwood 

of a grouping of species according to decay resistance. The 

The early or incipient stages of decay are often accompanied 

by a discoloration of the wood, which can be difficult to 

recognize but is more evident on freshly exposed surfaces 

of unseasoned wood than on dry wood. Abnormal mottling 

of the wood color, with either unnatural brown or bleached 

areas, is often evidence of decay infection. Many fungi that 

cause heart rot in the standing tree produce incipient decay 

that differs only slightly from the normal color of the wood 

or gives a somewhat water-soaked appearance to the wood.

Typical or late stages of decay are easily recognized, because the wood has undergone definite changes in color and 

properties, the character of the changes depending on the 

organism and the substances it removes.

Two kinds of major decay fungi are recognized: brown rot 

and white rot. With brown-rot fungi, only the cellulose is 

extensively removed, the wood takes on a browner color, 

and it can crack across the grain, shrink, collapse, and be 

crushed into powder (Fig. 14–5). With white-rot fungi, both 

lignin and cellulose usually are removed, the wood may lose 

color and appear “whiter” than normal, it does not crack 

across the grain, and until severely degraded, it retains its 

Figure 14–5. Representative samples of four common types of fungal growth on wood: (a) mold 

discoloration; (b) brown rotted pine (note the dark 

color and cubical checking in the wood); (c) white rot 

in maple (note the bleached appearance); (d) softrotted preservative-treated pine utility pole (note the 

shallow depth of decay).

General Technical Report FPL–GTR–190
14–5

Chapter 14 Biodeterioration of Wood

extent of variations in decay resistance of individual trees 

or wood samples of a particular species is much greater for 

most of the more resistant species than for the slightly or 

nonresistant species.

Natural resistance of wood to fungi is important only where 

conditions conducive to decay exist or may develop. Where 

decay hazard exists, heartwood of a species in the resistant 

category generally gives satisfactory service for wood used 

above-ground, while those in the very resistant category 

generally give satisfactory performance in contact with the 

ground. Heartwood of species in the other two categories 

will usually require some form of preservative treatment. 

For mild decay conditions, a simple preservative treatment—such as a short soak in preservative after all cutting 

and boring operations are complete—may be adequate for 

wood low in decay resistance. For more severe decay hazards, pressure treatment is often required. Even the very decay-resistant species may require preservative treatment for 

important structural uses or other uses where failure would 

Table 14–1. Grouping of some domestic and imported woods according to average 

heartwood decay resistancea

Very 

resistant Resistant 

Moderately

resistant 

Slightly

or nonresistant 

Domestic

Black locust Baldcypress, old growth Baldcypress, young growth Alder, red 

Mulberry, red Catalpa Cherry, black Ashes 

Osage-orange Cedar Douglas-fir Aspens 

Yew, Pacific Atlantic white Honey locust Beech 

 Eastern redcedar Larch, western Birches 

 Incense Pine, eastern white, old growth Buckeye 

 Northern white Pine, longleaf, old growth Butternut 

 Port-Orford Pine, slash, old growth Cottonwood 

 Western redcedar Redwood, young growth Elms 

 Yellow Tamarack Basswood 

Chestnut Firs, true 

Cypress, Arizona Hackberry 

Junipers Hemlocks 

Mesquite Hickories 

Oaks, whiteb Magnolia 

Redwood, old growth Maples 

Sassafras Pines (other than those listed)b

Walnut, black Spruces 

Sweetgum

Sycamore 

Tanoak 

Willows 

Yellow-poplar 

Imported

Angelique Aftotmosia (Kokrodua) Andiroba Balsa 

Azobe Apamate (Roble) Avodire Banak 

Balata Balaub Benge Cativo 

Goncalo alves Courbaril Bubinga Ceiba 

Greenheart Determa Ehie Hura 

Ipe (lapacho) Iroko Ekop Jelutong 

Jarrah Kapur Keruingb Limba 

Lignumvitae Karri Mahogany, African Meranti, light redb

Purpleheart Kempas Meranti, dark redb Meranti, yellowb

Teak, old growth Mahogany, American Mersawab Meranti, whiteb

Manni Sapele Obeche 

Spanish-cedar Teak, young growth Okoume 

Sucupira Tornillo Parana pine 

Wallaba Ramin 

Sande 

Sepitir 

Seraya, white 

a

Decay resistance may be less for members placed in contact with the ground and/or used in warm, humid climates. 

Substantial variability in decay resistance is encountered with most species, and limited durability data were available for 

some species listed. Use caution when using naturally durable woods in structurally critical or ground-contact applications. b

More than one species included, some of which may vary in resistance from that indicated.
drying practices will usually dry the wood fast enough to 

protect it, particularly if the protection afforded by drying 

is supplemented by dip or spray treatment of the stock with 

an EPA-approved fungicidal solution. Successful control by 

this method depends not only upon immediate and adequate 

treatment but also upon proper handling of the lumber after 

treatment. However, kiln drying is the most reliable method 

of rapidly reducing moisture content.

Air-drying yards should be kept as sanitary and as open 

as possible to air circulation (Fig. 14–7). Recommended 

practices include locating yards and sheds on well-drained 

ground; removing debris (which serves as a source of infection) and weeds (which reduce air circulation); and employing piling methods that permit rapid drying of the lumber 

and protect against wetting. Storage sheds should be constructed and maintained to prevent significant wetting of the 

stock. Ample roof overhang on open sheds is desirable. In 

areas where termites or water-conducting fungi may be troublesome, stock to be held for long periods should be set on 

foundations high enough so that the wood can be inspected 

from beneath.

The user’s best assurance of receiving lumber free from 

decay other than light stain is to buy stock marked by a 

lumber association in a grade that eliminates or limits such 

14–6

Figure 14–6. Spraying logs with water protects them 

against fungal stain and decay.

Figure 14–7. A sanitary, well-drained air-drying yard.

endanger life or require expensive repairs. When selecting naturally decay-resistant wood species for applications 

where conditions are conducive to decay, it is important 

to utilize heartwood. Marketable sizes of some species are 

primarily second growth and contain a high percentage of 

sapwood. Consequently, substantial quantities of heartwood 

lumber of these species are not available. If wood is subjected to severe decay conditions, pressure-treated wood, rather 

than resistant heartwood, is generally recommended. Preservative treatments and methods are discussed in Chapter 15.

Effect of Decay on Strength of Wood

Decay initially affects toughness, or the ability of wood to 

withstand impacts. This is generally followed by reductions 

in strength values related to static bending. Eventually, all 

strength properties are seriously reduced.

Strength losses during early stages of decay can be considerable, depending to a great extent upon the fungi involved 

and, to a lesser extent, upon the type of wood undergoing 

decay. In laboratory tests, losses in toughness ranged from 

6% to >50% by the time 1% weight loss had occurred in the 

wood as a result of fungal attack. By the time weight losses 

resulting from decay have reached 10%, most strength losses may be expected to exceed 50%. At such weight losses 

(10% or less), decay is detectable only microscopically. It 

may be assumed that wood with visually discernible decay 

has been greatly reduced in all strength values.

Prevention of Mold, Stain, and Decay

Logs, Poles, Piles, and Ties

The wood species, geographic region, and time of year 

determine what precautions must be taken to avoid serious 

damage from fungi in logs, poles, piles, ties, and similar 

thick products during seasoning or storage. In dry climates, 

rapid surface seasoning of poles and piles will retard development of mold, stain, and decay. The bark is peeled from 

the pole and the peeled product is decked on high skids or 

piled on high, well-drained ground in the open to air-dry. In 

humid regions, such as the Gulf States, these products often 

do not air-dry fast enough to avoid losses from fungi. Preseasoning treatments with approved preservative solutions 

can be helpful in these circumstances.

For logs, rapid conversion into lumber or storage in water 

or under a water spray (Fig. 14–6) is the surest way to avoid 

fungal damage. Preservative sprays promptly applied to the 

wood will protect most timber species during storage for 

2 to 3 months, except in severe decay hazard climates, such 

as in Mississippi (Fig. 14–1). For longer storage, an end 

coating is needed to prevent seasoning checks, through 

which infection can enter the log.

Lumber

Growth of decay fungi can be prevented in lumber and 

other wood products by rapidly drying them to a moisture 

content of 20% or less and keeping them dry. Standard airGeneral Technical Report FPL–GTR–190
quality-reducing features. Surface treatment for protection 

at the drying yard is only temporarily effective. Except for 

temporary structures, lumber to be used under conditions 

conducive to decay should be all heartwood of a naturally 

durable wood species or should be adequately treated with 

a wood preservative (Chap. 15).

Buildings

The lasting qualities of properly constructed wood buildings 

are apparent in all parts of the country. Serious decay problems are almost always a sign of faulty design or construction, lack of reasonable care in the handling the wood, or 

improper maintenance of the structure.

Construction principles that ensure long service and avoid 

decay in buildings include (a) building with dry lumber, free 

of incipient decay and not exceeding the amounts of mold 

and blue stain permitted by standard grading rules; (b) using construction details and building designs that will keep 

exterior wood and wood-based building components dry and 

that will promote their drying if they become wet; (c) using 

wood treated with a preservative or heartwood of a decayresistant species for parts exposed to aboveground decay 

hazards; and (d) using pressure-treated wood for the high 

hazard situation associated with ground contact.

A building site that is dry or for which drainage is provided 

will reduce the possibility of decay. Grading around the 

building is an important consideration, as is adequate planning for management of roof runoff (Chap. 17). Stumps, 

wood debris, stakes, or wood concrete forms are frequently 

subject to decay if left under or near a building and may become a source for decay infestation for the building.

Wet or infected wood should not be enclosed until it is thoroughly dried. Wet wood includes green (unseasoned) lumber, lumber that has been inadequately dried, or dried lumber that has been rewetted as a result of careless storage and 

handling. Wood can become infected because of improper 

handling at the sawmill or retail yard or after delivery to the 

job site.

Untreated wood parts of substructures should not be permitted to contact the soil. Minimums of 200 mm (8 in.) clearance between soil and framing and 150 mm (6 in.) between 

soil and siding are recommended. Where frequent hard rains 

occur, a foundation height above grade of 300 to 460 mm 

(12 to 18 in.) is advocated. An exception may be made for 

certain temporary constructions. If contact with soil is unavoidable, the wood should be pressure treated (Chap. 15).

Sill plates and other wood resting on a concrete slab foundation generally should be pressure treated and protected by 

installing a moisture-resistant membrane, such as polyethylene, beneath the slab. Girder and joist openings in masonry 

walls should be big enough to ensure an air space around 

the ends of these wood members. If the members are below 

the outside soil level, moisture proofing the outer face of the 

wall is essential.

In buildings without basements but with crawl spaces, wetting of the floor framing and sheathing by condensation 

may result in serious decay damage. The primary source of 

condensation is soil moisture. Isolating the crawl space from 

soil moisture can be achieved by laying a barrier such as 

polyethylene on the soil. To facilitate inspection of the crawl 

space, a minimum 460-mm (18-in.) clearance should be left 

under wood joists.

Wood and wood-based building components should also be 

protected from rain during construction. Continuous protection from rainwater or condensation in walls and roofs will 

prevent the development of decay. Thus, design, work quality, and maintenance of wall and roofing systems are critical, 

particularly at roof edges and points where roofs interface 

with walls. A fairly wide roof overhang (0.6 m (2 ft)) 

with gutters and downspouts that are kept free of debris 

is desirable.

The use of sound, dry lumber is equally important for the 

interior of buildings. Primary sources for interior moisture 

are humidity and plumbing leaks. Interior humidity control 

is discussed in Chapter 17. Plumbing leaks can result in serious decay problems within buildings, particularly if they are 

undetected for long periods.

Where service conditions in a building are such that the 

wood cannot be kept dry, the use of preservative-treated 

wood (Chap. 15) or heartwood of a durable species is advised. Examples include porches, exterior steps, and decking 

platforms and such places as textile mills, pulp and paper 

mills, and cold storage plants.

In making repairs necessitated by decay, every effort should 

be made to correct the moisture condition that led to the 

damage. If the condition cannot be corrected, all infected 

parts should be replaced with preservative-treated wood or 

with all-heartwood lumber of a naturally decay-resistant 

wood species. If the sources of moisture that caused the 

decay are entirely eliminated, it is necessary only to replace 

the weakened wood with dry lumber.

Other Structures and Products

In general, the principles underlying the prevention of mold, 

stain, or decay damage to veneer, plywood containers, 

boats, and other wood products and structures are similar 

to those described for buildings—dry the wood rapidly and 

keep it dry, or treat it with approved protective and preservative solutions. Interior grades of plywood should not be 

used where the plywood will be exposed to moisture; the 

adhesives, as well as the wood, may be damaged by fungi 

and bacteria and degraded by moisture. With exterior-type 

panels, joint construction should be carefully designed to 

prevent the entrance and entrapment of rainwater.

In treated bridge or wharf timbers, checking may occur and 

may expose untreated wood to fungal attack. Annual inplace treatment of these checks will provide protection from 

Chapter 14 Biodeterioration of Wood

14–7
14–8

Table 14–2. Types of damage caused by wood-attacking insects

Type of 

damage Description Causal agent 

Damage 

Begins Ends 

Pin holes 0.25 to 6.4 mm (1/100 to 1/4 in.) in diameter, 

usually circular 

Tunnels open:

Holes 0.5 to 3 mm (1/50 to 1/8 in.) in diameter, 

usually centered in dark streak or ring in 

surrounding wood 

Ambrosia beetles In living trees and 

unseasoned logs and 

lumber

During seasoning 

Holes variable sizes; surrounding wood rarely 

dark stained; tunnels lined with wood-colored 

substance

Timber worms In living trees and 

unseasoned logs and 

lumber

Before seasoning 

Tunnels usually packed with fine sawdust: 

Exit holes 0.8 to 1.6 mm (1/32 to 1/16 in.) in 

diameter; in sapwood of large-pored hardwoods; 

loose floury sawdust in tunnels 

Lyctid powder-post 

beetles

During or after 

seasoning

Reinfestation continues 

until sapwood destroyed 

Exit holes 1.6 to 3 mm (1/16 to 1/8 in.) in 

diameter; primarily in sapwood, rarely in 

heartwood; tunnels loosely packed with fine 

sawdust and elongate pellets 

Anobiid powder-post 

beetles

Usually after wood in 

use (in buildings) 

Reinfestation continues; 

progress of damage very 

slow

Exit holes 2.5 to 7 mm (3/32 to 9/32 in.) in 

diameter; primarily sapwood of hardwoods, 

minor in softwoods; sawdust in tunnels fine to 

coarse and tightly packed 

Bostrichid powderpost beetles Before seasoning or if 

wood is rewetted 

During seasoning or 

redrying

Exit holes 1.6 to 2 mm (1/16 to 1/12 in.) in 

diameter; in slightly damp or decayed wood; 

very fine sawdust or pellets tightly packed in 

tunnels

Wood-boring weevils In slightly damp wood 

in use 

Reinfestation continues 

while wood is damp 

Grub holes 3 to 13 mm (1/8 to 1/2 in.) in diameter, circular or 

oval

Exit holes 3 to 13 mm (1/8 to 1/2 in.) in diameter; 

circular; mostly in sapwood; tunnels with coarse to 

fibrous sawdust or it may be absent 

Roundheaded borers 

(beetles) 

In living trees and 

unseasoned logs and 

lumber

When adults emerge 

from seasoned wood or 

when wood is dried 

Exit holes 3 to 13 mm (1/8 to 1/2 in.) in diameter; 

mostly oval; in sapwood and heartwood; sawdust 

tightly packed in tunnels 

Flatheaded borers 

(beetles) 

In living trees and 

unseasoned logs and 

lumber

When adults emerge 

from seasoned wood or 

when wood is dried 

Exit holes ~6 mm (~1/4 in.) in diameter; circular; 

in sapwood of softwoods, primarily pine; tunnels 

packed with very fine sawdust 

Old house borers 

(a roundheaded 

borer)

During or after 

seasoning

Reinfestation continues 

in seasoned wood in use 

Exit holes perfectly circular, 4 to 6 mm (1/6 to 1/4 

in.) in diameter; primarily in softwoods; tunnels 

tightly packed with coarse sawdust, often in decay 

softened wood 

Woodwasps In dying trees or fresh 

logs

When adults emerge 

from seasoned wood, 

usually in use, or when 

kiln-dried 

Nest entry hole and tunnel perfectly circular ~13 

mm (~1/2 in.) in diameter; in soft softwoods in 

structures 

Carpenter bees In structural timbers, 

siding

Nesting reoccurs 

annually in spring at 

same and nearby 

locations

Network of 

galleries 

Systems of interconnected tunnels and chambers Social insects with 

colonies

Walls look polished; spaces completely clean of 

debris

Carpenter ants Usually in damp partly 

decayed, or softtextured wood in use 

Colony persists unless 

prolonged drying of 

wood occurs 

Walls usually speckled with mud spots; some 

chambers may be filled with “clay” 

Subterranean termites In wood structures Colony persists 

Chambers contain pellets; areas may be walled off 

by dark membrane 

Dry-wood termites 

(occasionally damp 

wood termites) 

In wood structures Colony persists 

Pitch pocket Openings between growth rings containing pitch Various insects In living trees In tree 

Black check Small packets in outer layer of wood Grubs of various 

insects 

In living trees In tree 

Pith fleck Narrow, brownish streaks Fly maggots or adult 

weevils

In living trees In tree 

Gum spot Small patches or streaks of gum-like substances Grubs of various 

insects 

In living trees In tree 

Ring

distortion 

Double growth rings or incomplete annual layers 

of growth 

Larvae of defoliating 

insects or flatheaded 

cambium borers 

In living trees In tree 

Stained area more than 25.4 mm (1 in.) long 

introduced by insects in trees or recently felled 

logs

Staining fungi With insect wounds With seasoning 

General Technical Report FPL–GTR–190
vary in length from approximately 1.5 to 6.5 mm (1/16 to 

1/4 in.) They bore through the outer bark to the soft inner 

part, where they make tunnels in which they lay their 

eggs. In making tunnels, bark beetles push out fine 

brownish-white sawdust-like particles. If many beetles are 

present, their extensive tunneling will loosen the bark and 

permit it to fall off in large patches, making the structure 

unsightly.

To avoid bark beetle damage, logs may be debarked rapidly, 

sprayed with an approved insecticidal solution, stored in 

water or under a water spray, or cut during the dormant season (October or November, for instance). If cut during this 

period, logs should immediately be piled off the ground and 

arranged for good air movement to promote rapid drying of 

the inner bark. This should occur before the beetles begin to 

fly in the spring. Drying the bark will almost always prevent 

damage by insects that prefer freshly cut wood.

Ambrosia beetles, roundheaded and flatheaded borers, and 

some powder-post beetles that get into freshly cut timber 

Chapter 14 Biodeterioration of Wood

14–9

Figure 14–8. Types of insect damage most likely 

to occur in a building. Upper left—Termite attack; 

feeding galleries (often parallel to the grain) contain excrement and soil. Upper right—Powder-post 

beetle attack; exit holes usually filled with wood 

flour and not associated with discolored wood. 

Lower left—Carpenter ant attack; nesting galleries 

usually cut across grain and are free of residue. 

Lower right—Beetle attack; feeding galleries 

(made in the wood while green) free of residue 

and surrounding wood darkly stained.

decay. Similarly, pile tops may be protected by treatment 

with a wood preservative followed by application of a suitable capping compound or shield.

Wood boats present certain problems that are not encountered in other uses of wood. The parts especially subject to 

decay are the stem, knighthead, transom, and frameheads, 

which can be reached by rainwater from above or condensation from below. Frayed surfaces are more likely to decay 

than are exposed surfaces, and in salt water service, hull 

members just below the weather deck are more vulnerable than those below the waterline. Recommendations for 

avoiding decay include (a) using only heartwood of durable 

species, free of infection, and preferably below 20% moisture content; (b) providing and maintaining ventilation in 

the hull and all compartments; (c) keeping water out as 

much as is practicable, especially fresh water; and (d) where 

it is necessary to use sapwood or nondurable heartwood, 

impregnating the wood with an approved preservative and 

treating the fully cut, shaped, and bored wood before installation by soaking it for a short time in preservative solution. 

Where such mild soaking treatment is used, the wood most 

subject to decay should also be flooded with an approved 

preservative at intervals of 2 or 3 years. During subsequent 

treatment, the wood should be dry so that joints are relatively loose.

Bacteria

Most wood that has been wet for a considerable length of 

time probably will contain bacteria. The sour smell of logs 

that have been held under water for several months, or of 

lumber cut from them, manifests bacterial action. Usually, 

bacteria have little effect on wood properties, except over 

long periods, but some may make the wood excessively absorptive. This can result in excessive absorption of moisture, 

adhesive, paint, or preservative during treatment or use. This 

effect has been a problem in the sapwood of millwork cut 

from pine logs that have been stored in ponds. There also is 

evidence that bacteria developing in pine veneer bolts held 

under water or sprayed with water may cause noticeable 

changes in the physical character of the veneer, including 

some strength loss. Additionally, a mixture of different bacteria and fungi was found capable of accelerating decay of 

treated cooling tower slats and mine timbers.

Insect Damage and Control

The more common types of damage caused by woodattacking insects are shown in Table 14–2 and Figure 14–8. 

Methods of controlling and preventing insect attack of wood 

are described in the following paragraphs.

Beetles

Bark beetles may damage the surface of the components of 

logs and other rustic structures from which the bark has not 

been removed. These beetles are reddish brown to black and 
tion under conditions that ensure the center of the wood 

will be held at 56 °C (133 °F) for 30 min will effectively 

kill insects in infested lumber. Those conditions vary with 

moisture content, size, and dimension of wood—see Chapter 20 for further information on heat sterilization. A 3-min 

soaking in a petroleum oil solution containing an insecticide 

is also effective for checking infestation or preventing attack on lumber up to standard 19 mm (nominal 1 in.) thick. 

Small dimension stock also can be protected by brushing or 

spraying with approved chemicals. For infested furniture or 

finished woodwork in a building, the same insecticides may 

be used, but they should be dissolved in refined petroleum 

oil, such as mineral spirits. Because Lyctus beetles lay their 

eggs in the open pores of wood, infestation can be prevented 

by covering the entire surface of each piece of wood with a 

suitable finish.

Powder-post beetles in the family Anobiidae, depending 

on the species, infest hardwoods and softwoods. Their life 

cycle takes 2 to 3 years, and they require wood moisture 

content around 15% or greater for viable infestation. Therefore, in most modern buildings, the wood moisture content 

is generally too low for anobiids. When ventilation is inadequate or in more humid regions of the United States, wood 

components of a building can reach the favorable moisture 

conditions for anobiids. This is especially a problem in airconditioned buildings where water condenses on cooled 

exterior surfaces. Susceptibility to anobiid infestation can be 

alleviated by lowering the moisture content of wood through 

improved ventilation and the judicious use of insulation and 

vapor barriers. Insecticides registered for use against these 

beetles are generally restricted for exterior applications to 

avoid potential safety hazards indoors. Wood being reused 

or recycled from older structures often has lyctid or anobiid 

larvae in it. Such wood should be fumigated or kiln dried 

before use in another structure.

Beetles in the family Bostrichidae and weevils in the family 

Curculionidae are associated with wood moisture contents 

favorable for wood-infesting fungi because they may benefit 

nutritionally from the fungi. Thus, protection against these 

insects consists of the same procedures as for protection 

against wood-decay fungi.

A roundheaded beetle, commonly known as the old house 

borer, causes damage to seasoned, coniferous building materials. The larvae reduce the sapwood to a powdery or granular consistency and make a ticking sound while at work. 

When mature, the beetles make an oval hole approximately 

6.5 mm (1/4 in.) in diameter in the surface of the wood and 

emerge. Anobiid powder-post beetles, which make holes 

1.6 to 3.2 mm (1/16 to 1/8 in.) in diameter, also cause damage to pine joists. Infested wood should be drenched with a 

solution of one of the currently recommended insecticides in 

a highly penetrating solvent. Beetles nesting in wood behind 

plastered or paneled walls can be eliminated through fumigation of the building by a licensed operator.

14–10

can cause considerable damage to wood in rustic structures 

and some manufactured products. Certain beetles may complete development and emerge several years after the wood 

is dry, often raising a question as to the origin of the 

infestation.

Proper cutting practices, rapid debarking, storing under 

water, and spraying with an approved chemical solution, as 

recommended for bark beetles, will control these insects. 

Damage by ambrosia beetles can be prevented in freshly 

sawn lumber by dipping the product in a chemical solution. 

The addition of one of the sap-stain preventives approved 

for controlling molds, stains, and decay will keep the lumber bright. Powder-post beetles attack both hardwoods and 

softwoods and both freshly cut and seasoned lumber and 

timber. Powder-post damage is indicated by holes made in 

the surface of the wood by the winged adults as they emerge 

and by the fine powder that may fall from the wood. The 

powder-post beetles that cause most of the damage to dry 

hardwood lumber belong to the genus Lyctus. They attack 

the sapwood of ash, hickory, oak, and other large-pored 

hardwoods as it begins to season. Eggs are laid in pores of 

the wood, and the larvae burrow through the wood, making tunnels from 1.5 to 2 mm (1/16 to 1/12 in.) in diameter, 

which they leave packed with a fine powder. Species of anobiid beetles colonize coniferous materials.

Susceptible hardwood lumber used for manufacturing 

purposes should be protected from powder-post beetle attack as soon as it is sawn and when it arrives at the plant. 

An approved insecticide applied in water emulsion to the 

green lumber will provide protection. Such treatment may 

be effective even after the lumber is kiln dried, until it is 

surfaced. Heat sterilization is another way to kill insects in 

green lumber. To effectively kill insects in lumber or timbers, heat sterilization requires that the center of the wood 

be held at 56 °C (133 °F) for 30 min. The time required to 

reach that temperature is highly variable and depends on 

thickness of boards, dimension of timbers, and moisture 

content of the wood (Chap. 20). For example, heating time 

increases with increasing board thickness or increasing 

cross-sectional dimension.

Good plant sanitation is extremely important in alleviating 

the problem of infestation. Proper sanitation measures can 

often eliminate the necessity for other preventative steps. 

Damage to manufactured items frequently is traceable to 

infestation that occurred before the products were placed on 

the market, particularly if a finish is not applied to the surface of the items until they are sold. Once wood is infested, 

the larvae will continue to develop, even though the surface 

is subsequently painted, oiled, waxed, or varnished.

When selecting hardwood lumber for building or manufacturing purposes, any evidence of powder-post infestation 

should not be overlooked, because the beetles may continue 

to be active long after the wood is put to use. Heat sterilizaGeneral Technical Report FPL–GTR–190
Termites

Termites superficially resemble ants in size, general appearance, and habit of living in colonies. About 56 species are 

known in the United States. From the standpoint of their 

methods of attack on wood, termites can be grouped into 

two main classes: (a) ground-inhabiting or subterranean 

termites and (b) wood-inhabiting or nonsubterranean 

termites.

Subterranean Termites

Subterranean termites are responsible for most of the termite 

damage to wood structures in the United States. This damage can be prevented. Subterranean termites are more prevalent in the southern than in the northern states, where low 

temperatures do not favor their development (Fig. 14–9). 

The hazard of infestation is greatest (a) beneath buildings 

without basements that were erected on a concrete slab 

foundation or were built over a crawl space that is poorly 

drained and lacks a moisture barrier (see Chap. 17) and (b) 

in any substructure wood component close to the ground or 

an earth fill (for example, an earth-filled porch).

Subterranean termites develop their colonies and maintain 

their headquarters in the ground. They build their tunnels 

through earth and around obstructions to reach the wood 

they need for food. They also must have a constant source 

of moisture, whether from the wood on which they are feeding or the soil where they nest. The worker members of the 

colony cause destruction of wood. At certain seasons of the 

year, usually spring, male and female winged forms swarm 

from the colony, fly a short time, lose their wings, mate, and 

if successful in locating a suitable home, start new colonies. 

The appearance of “flying ants” or their shed wings is an 

indication that a termite colony may be near and causing 

serious damage. Not all “flying ants” are termites; therefore, 

suspicious insects should be identified before investing in 

eradication (Fig. 14–10).

Subterranean termites normally do not establish themselves 

in buildings by being carried there in lumber; they primarily 

enter from ground nests after the building has been constructed. An introduced species, the Formosan termite, is 

adept at initiating aboveground infestations and nests 

in structures where wood remains wet for prolonged 

periods, such as from roof leaks. Telltale signs of subterranean termite presence are the earthen tubes or runways built 

by these insects over the surfaces of the foundation or other 

exposed areas to reach the wood above. Another sign is the 

swarming of winged adults early in the spring or fall. In the 

wood itself, the termites make galleries that generally follow the grain, leaving a shell of sound wood to conceal their 

activities. Because the galleries seldom show on the wood 

surfaces, probing with a pick or knife is advisable if the 

presence of termites is suspected.

The best protection for wood in areas where subterranean 

termites are prevalent is to prevent the termites from gaining 

hidden access to a building. The foundations should be of 

concrete, pressure-treated wood, or other material through 

which the termites cannot penetrate. With brick, stone, or 

concrete block, cement mortar should be used because termites can work through some other kinds of mortar. Also, 

Chapter 14 Biodeterioration of Wood

14–11

Figure 14–9. A, the northern limit of recorded damage 

done by subterranean termites in the United States; B, 

the northern limit of damage done by dry-wood termites.

Figure 14–10. A, winged termite; B, winged 

ant (both greatly enlarged). The wasp waist 

of the ant and the long wings of the termite 

are distinguishing characteristics.
Key West, and to the islands of Hawaii. They also are a localized problem in Arizona and New Mexico.

The nonsubterranean termites, especially the dry-wood type, 

do not multiply as rapidly as the subterranean termites and 

have a somewhat different colony life and habits. The total 

amount of destruction they cause in the United States is 

much less than that caused by the subterranean termites. The 

ability of dry-wood termites to live in dry wood without outside moisture or contact with the ground, however, makes 

them a definite menace in the regions where they occur. 

Their destruction is not rapid, but they can thoroughly riddle 

timbers with their tunneling if allowed to work undisturbed 

for many years. Nonsubterranean termites are often moved 

from structure to structure in infested items such as

furniture.

In constructing a building in localities where the dry-wood 

type of nonsubterranean termite is prevalent, it is good 

practice to inspect the lumber carefully to see that it was not 

infested before arrival at the building site. If the building is 

constructed during the swarming season, the lumber should 

be watched during the course of construction, because infestation by colonizing pairs can easily take place. Because 

paint is a good protection against the entrance of dry-wood 

termites, exposed wood (except that which is preservative treated) should be kept covered with a paint film. Fine 

screen should be placed over any openings to the interior 

unpainted parts of the building. As in the case of groundnesting termites, dead trees, old stumps, posts, or wood 

debris of any kind that could serve as sources of infestation 

should be removed from the premises.

If a building is infested with dry-wood termites, badly damaged wood should be replaced. If the wood is only slightly 

damaged or is difficult to replace, further termite activity 

can be arrested by injecting a small amount of an approved 

pesticidal dust or liquid formulation into each nest. Current 

recommendations for such formulations can be found from 

state pest control associations. Buildings heavily infested 

with nonsubterranean termites can be successfully fumigated. This method is quicker than the use of poisonous liquids 

and dusts and does not require finding all of the colonies. 

However, it does not prevent the termites from returning 

because no poisonous residue is left in the tunnels. Fumigation is very dangerous and should be conducted only by licensed professional fumigators. Infested pieces of furniture, 

picture frames, and other small pieces can be individually 

fumigated, heated, or placed in a freezer for a short time. 

In localities where dry-wood termites do serious damage 

to posts and poles, the best protection for these and similar 

forms of outdoor timbers is full-length pressure treatment 

with a preservative.

Naturally Termite-Resistant Woods

Only a limited number of woods grown in the United States 

offer any marked degree of natural resistance to termite 

14–12

it is a good precaution to cap the foundation with 100 mm 

(4 in.) of reinforced concrete. Posts supporting floor girders 

should, if they bear directly on the ground, be of concrete. 

If there is a basement, it should be floored with concrete. 

Untreated posts in such a basement should rest on concrete 

piers extending a few inches above the basement floor. 

However, pressure-treated posts can rest directly on the 

basement floor. With the crawl-space type of foundation, 

wood floor joists should be kept at least 460 mm (18 in.) and 

girders 300 mm (12 in.) from the earth with a polyethylene 

vapor barrier covering exposed soil and extending partially 

up the foundation wall. Moisture condensation on the floor 

joists and subflooring, which may cause conditions favorable to decay and contribute to infestation by termites, can 

be avoided by covering the soil below with a moisture barrier and assuming proper drainage of rainwater away from all 

sides of a structure by managing rain and roof runoff with 

gutters, downspouts, and proper grading around the foundation. All concrete forms, stakes, stumps, and wastewood 

should be removed from the building site because they are 

possible sources of infestation. Generally, the precautions 

effective against subterranean termites are also helpful 

against decay.

The principal method of protecting buildings in high termite areas is to thoroughly treat the soil adjacent to the 

foundation walls and piers beneath the building with a soil 

insecticide. When concrete slab floors are laid directly on 

the ground, all soil under the slab should be treated with an 

approved insecticide before the concrete is poured. Furthermore, insulation containing cellulose that is used as a filler 

in expansion joints should be impregnated with an approved 

chemical toxic to termites. Sealing the top 13 mm (1/2 in.) 

of the expansion joint with roofing-grade coal-tar pitch also 

provides effective protection from ground-nesting termites. 

Several soil treatments and insecticidal bait control methods are currently available. Information on current control 

methods is available from national pest control operator associations. These organizations should be consulted to take 

advantage of the latest technology in termite control.

To control termites already in a building, contact between 

the termite colony in the soil and the woodwork must be 

broken. This can be done by blocking the runways from soil 

to wood, treating the soil, repairing leaks that keep wood 

within the structure wet (for example, plumbing leaks), or 

some combination of these techniques. Possible reinfestation can be guarded against by frequent inspections for 

signs of termites.

Nonsubterranean Termites

In the United States, nonsubterranean termites have been 

found only in a narrow strip of territory extending from central California around the southern edge of the continental 

United States to Virginia (Fig. 14–9) and in the West Indies 

and Hawaii. Their principal damage is confined to an area 

in southern California, to parts of southern Florida, notably 

General Technical Report FPL–GTR–190
attack. The close-grained heartwood of California redwood 

has some resistance, especially when used above ground. 

Very resinous heartwood of Southern Pine is practically immune to attack, but it is not available in large quantities and 

is seldom used.

Carpenter Ants

Carpenter ants are black or brown. They usually occur in 

stumps, trees, or logs but sometimes damage poles, structural timbers, or buildings. One form is easily recognized by 

its giant size relative to other ants. Carpenter ants use wood 

for shelter rather than for food, usually preferring wood that 

is naturally soft or has been made soft by decay. They may 

enter a building directly by crawling or may be carried there 

in firewood. If left undisturbed, they can, in a few years, 

enlarge their tunnels to the point where replacement or extensive repairs are necessary. The parts of dwellings they 

frequent most often are porch columns, porch roofs, window 

sills, and sometimes the wood plates in foundation walls. 

They often nest in hollow-core doors. The logs of rustic cabins are also attacked.

Precautions that prevent attack by decay and termites are 

usually effective against carpenter ants. Decaying or infested wood, such as logs, stumps, or retaining walls, should 

be removed from the premises, and crevices present in the 

foundation or woodwork of the building should be sealed. 

Particularly, leaks in porch roofs should be repaired because 

the decay that may result makes the wood more desirable to 

the ants.

When carpenter ants are found in a structure, any badly 

damaged timbers should be replaced. Because the carpenter 

ant needs high humidity in its immature stages, alterations in 

the construction may also be required to eliminate moisture 

from rain or condensation. In wood not sufficiently damaged 

to require replacement, the ants can be killed by injection of 

approved insecticide into the nest galleries. Carpenter ant 

nests are relatively easy to find because they keep their internal nest sites very clean and free of debris. As particles of 

wood are removed to create galleries or as pieces of insects 

that have been fed upon accumulate, the debris is removed 

from the nest and then accumulates below the nest opening.

Carpenter Bees

Carpenter bees resemble large bumblebees, but the top of 

their abdomen is hairless, causing their abdomens to shine, 

unlike bumblebees. The females make large (13-mm- (1/2-

in.-) diameter) tunnels into unfinished soft wood for nests. 

They partition the hole into cells; each cell is provided with 

pollen and nectar for a single egg. Because carpenter bees 

reuse nesting sites for many years, a nesting tunnel into a 

structural timber may be extended several feet and have 

multiple branches. In thin wood, such as siding, the holes 

may extend the full thickness of the wood. They nest in 

wood that has been finished with a stain or thin paint film, 

or light preservative salt treatments, as well as in bare wood. 

A favorite nesting site is in unfinished exterior wood not 

directly exposed to sunlight (for example, the undersides of 

porch roofs, and grape arbors).

Control is aimed at discouraging the use of nesting sites 

in and near buildings. The tunnel may be injected with an 

insecticide labeled for bee control and plugged with caulk. 

Treating the surface around the entry hole will discourage 

reuse of the tunnel during the spring nesting period. A good 

paint film or pressure preservative treatment protects exterior wood surfaces from nesting damage. Bare interior wood 

surfaces, such as in garages, can be protected by screens and 

tight-fitting doors.

Marine Borer Damage and Control

Damage by marine-boring organisms to wood structures in 

salt or brackish waters is practically a worldwide problem. 

Evidence of attack is sometimes found in rivers even above 

the region of brackishness. The rapidity of attack depends 

upon local conditions and the kinds of borers present. Along 

the Pacific, Gulf, and South Atlantic Coasts of the United 

States, attack is rapid, and untreated pilings may be completely destroyed in a year or less. Along the coast of the 

New England States, the rate of attack is slower because 

of cold water temperatures but is still sufficiently rapid to 

require protection of wood where long life is desired. The 

principal marine borers from the standpoint of wood damage in the United States are described in this section. Control measures discussed in this section are those in use at 

the time this handbook was revised. Regulations should be 

reviewed at the time control treatments are being considered 

so that approved practices will be followed.

Shipworms

Shipworms are the most destructive of the marine borers. 

They are mollusks of various species that superficially are 

worm-like in form. The group includes several species of 

Teredo and several species of Bankia, which are especially 

damaging. These mollusks are readily distinguishable on 

close observation but are all very similar in several respects. 

In the early stages of their life, they are minute, free-swimming organisms. Upon finding suitable lodgment on wood, 

they quickly develop into a new form and bury themselves 

in the wood. A pair of boring shells on the head grows rapidly in size as the boring progresses, while the tail part or 

siphon remains at the original entrance. Thus, the animal 

grows in length and diameter within the wood but remains 

a prisoner in its burrow, which it lines with a shell-like 

deposit. It lives on the wood borings and the organic matter extracted from the sea water that is continuously being 

pumped through its system. The entrance holes never grow 

large, and the interior of wood may be completely honeycombed and ruined while the surface shows only slight 

perforations. When present in great numbers, shipworms 

grow only a few centimeters before the wood is so completely occupied that growth is stopped. However, when not 

Chapter 14 Biodeterioration of Wood

14–13
levels, piles heavily attacked by Limnoria characteristically 

wear within this zone to an hourglass shape (Fig. 14–11). In 

heavily infested harbors, untreated piling can be destroyed 

by Limnoria within a year.

Sphaeroma are somewhat larger, sometimes reaching a 

length of 13 mm (1/2 in.) and a width of 6 mm (1/4 in.). In 

general appearance and size, they resemble the common 

sow bug or pill bug that inhabits damp places. Sphaeroma 

are widely distributed but are not as plentiful as Limnoria 

and cause much less damage, although damage caused by 

Sphaeroma action resembles that of Limnoria. Nevertheless, 

piles in some structures have been ruined by them. It has 

been reported that Sphaeroma attack salt-treated wood in 

Florida. Occasionally, they have been found in fresh water.

The average life of well-creosoted structures in areas susceptible to marine borer attack is many times the average 

life obtained from untreated structures. However, even 

thorough creosote treatment will not always stop Martesia, 

Sphaeroma, and especially Limnoria; shallow or erratic 

creosote penetration affords only slight protection. The spots 

with poor protection are attacked first, and from there, the 

borers spread inward and destroy the untreated interior of 

the pile.

When wood is to be used in salt water, avoidance of cutting 

or injuring the surface after treatment is even more important than when wood is to be used on land. No cutting or 

injury of any kind for any purpose should be permitted in 

the underwater part of the pile. Where piles are cut to grade 

above the waterline, the exposed surfaces should be protected from decay. This may be accomplished by in-place 

application of a wood preservative followed by a suitable 

capping compound.

Protection from Marine Borers

No wood is immune to marine-borer attack, and no commercially important wood of the United States has sufficient 

marine-borer resistance to justify its use untreated in any 

important structure in areas where borers are active. The 

heartwood of several foreign species, such as greenheart, 

jarrah, azobe, and manbarklak, has shown resistance to 

marine-borer attack. Service records on these woods, however, do not always show uniform results and are affected by 

local conditions. Borer damage to wooden marine structures 

can be prevented, but knowing the type of borer present in 

the geographic location is important for selection of proper 

preservative and treatment retention to protect the structure 

from surface erosion.

Protection of Permanent Structures

The best practical protection for piles in sea water with shipworms and moderate Limnoria hazard is heavy treatment 

with coal-tar creosote or creosote coal-tar solution. Where 

severe Limnoria hazard exists, dual treatment (copperarsenate-containing waterborne preservatives followed by 

coal-tar creosote) is recommended. The treatment must be 

14–14

crowded, they can grow to lengths of 0.3 to 1.2 m (1 to 4 ft) 

depending on the species.

Pholads

Another group of wood-boring mollusks is the pholads, 

which clearly resemble clams and therefore are not included 

with the shipworms. They are entirely encased in their 

double shells. The Martesia are the best-known species, 

but another well-known group is the Xylophaga. Like the 

shipworms, the Martesia enter the wood when they are very 

small, leaving a small entrance hole, and grow larger as they 

burrow into the wood. They generally do not exceed 64 mm 

(2-1/2 in.) long and 25 mm (1 in.) in diameter but are capable of doing considerable damage. Their activities in the 

United States appear to be confined to the Gulf Coast, San 

Diego, and Hawaii.

Limnoria and Sphaeroma

Another distinct group of marine borers are crustaceans, 

which are related to lobsters and shrimp. The principal borers in this group are species of Limnoria and Sphaeroma. 

Their attack differs from that of the shipworms and the 

Martesia in that the bore hole is quite shallow; the result is 

that the wood gradually is thinned from the surface inward 

through erosion by the combined action of the borers and 

water erosion. Also, the Limnoria and Sphaeroma do not 

become imprisoned in the wood but may move freely from 

place to place.

Limnoria are small, 3 to 4 mm (1/8 to 1/6 in.) long, and 

bore small burrows in the surface of wood. Although they 

can change their location, they usually continue to bore in 

one place. When great numbers of Limnoria are present, 

their burrows are separated by very thin walls of wood that 

are easily eroded by the motion of the water or damaged by 

objects floating upon it. This erosion causes the Limnoria to 

burrow continually deeper; otherwise, the burrows would 

probably not become greater than 51 mm (2 in.) long or 13 

mm (1/2 in.) deep. Because erosion is greatest between tide 

Figure 14–11. Limnoria damage to 

piling.

General Technical Report FPL–GTR–190
thorough, the penetration as deep as possible, and the retention high to give satisfactory results in heavily infested 

waters. It is best to treat such piles by the full-cell process to 

refusal; that is, to force in all the preservative the piles can 

hold without using treatments that cause serious damage to 

the wood. For highest retentions, it is necessary to air- or 

kiln-dry the piling before treatment. Details of treatments 

are discussed in Chapter 15.

The life of treated piles is influenced by the thoroughness of 

the treatment, the care and diligence used in avoiding damage to the treated shell during handling and installation, and 

the severity of borer attack. Differences in exposure conditions, such as water temperature, salinity, dissolved oxygen, 

water depth, and currents, tend to cause wide variations in 

the severity of borer attack even within limited areas. Service records show average-life figures of 22 to 48 years on 

well-treated Douglas-fir piles in San Francisco Bay waters. 

In South Atlantic and Gulf of Mexico waters, creosoted 

piles are estimated to last 10 to 12 years and frequently 

much longer. On the North Atlantic Coast, where exposure 

conditions are less severe, piles can last even longer than the 

22- to 48-year life recorded in the San Francisco Bay.

Metal armor and concrete or plastic jacketing have been 

used with various degrees of success for the protection of 

marine piles. The metal armor may be in the form of sheets, 

wire, or nails. Sheathing of piles with copper or muntz metal 

has been only partially successful, owing to difficulty in 

maintaining a continuous armor. Theft, mechanical damage 

from driving, damage by storm or driftwood, and corrosion 

of sheathing have sooner or later let in the borers, and in 

only a few cases has long pile life been reported. Attempts 

during World War II to electroplate wood piles with copper 

were not successful. Concrete casings are now in greater use 

than is metal armor, and they appear to provide better protection when high-quality materials are used and carefully 

applied. Unfortunately, they are readily damaged by ship 

impact. For this reason, concrete casings are less practical 

for fender piles than for foundation piles that are protected 

from mechanical damage.

Jacketing piles by wrapping them with heavy polyvinyl 

plastic is one form of supplementary protection. If properly 

applied, the jacketing will kill any borers that may have 

already become established by creating stagnant water, 

thereby decreasing oxygen levels in the water that is in contact with the piles. Like other materials, the plastic jacket is 

subject to mechanical damage.

Protection of Boats

Wood barges have been constructed with planking or 

sheathing pressure-treated with creosote to protect the hull 

from marine borers, and the results have been favorable. 

Although coal-tar creosote is an effective preservative for 

protecting wood against marine borers in areas of moderate 

borer hazard, it has disadvantages in many types of boats. 

Creosote adds considerably to the weight of the boat hull, 

and its odor is objectionable to boat crews. In addition, antifouling paints are difficult to apply over creosoted wood.

Antifouling paints that contain copper protect boat hulls 

against marine-borer attack, but the protection continues 

only while the coating remains unbroken. Because it is difficult to maintain an unbroken coating of antifouling paint, 

the U.S. Navy has found it desirable to impregnate the hull 

planking of some wood boats with certain copper-containing 

preservatives. Such preservatives, when applied with high 

retentions (40 kg m–3 (2.5 lb ft–3)), have some effectiveness 

against marine borers and should help to protect the hull of 

a boat during intervals between renewals of the antifouling 

coating. These copper preservatives do not provide protection equivalent to that furnished by coal-tar creosote; their 

effectiveness in protecting boats is therefore best assured if 

the boats are dry docked at regular and frequent intervals 

and the antifouling coating maintained. The leach-resistant 

wood preservatives containing copper arsenates have 

shown superior performance (at a retention of 40 kg m–3

(2.5 lb ft–3)) to creosote in tests conducted in areas of severe 

borer hazard.

Plywood as well as plank hulls can be protected against 

marine borers by preservative treatment. The plywood hull 

presents a surface that can be covered successfully with a 

protective membrane of reinforced plastic laminate. Such 

coverings should not be attempted on wood that has been 

treated with a preservative carried in oil, because the bond 

will be unsatisfactory.

References

Beal, R.H. 1967. Formosan invader. Pest Control. 35(2): 

13–17.

Beal, R.H.; Maulderi, J.K.; Jones, S.C. 1983. Subterranean 

termites, their prevention and control in buildings. Home & 

Garden Bull. 64 (rev.). Washington, DC: U.S. Department of 

Agriculture. 30 p.

Cassens, D.L.; Eslyn, W.E. 1981. Fungicides to prevent 

sapstain and mold on hardwood lumber. Forest Products 

Journal. 31: 39–42.

Ebeling, W. 1975. Wood destroying insects and fungi. In: 

Urban entomology. Berkeley, CA: University of California, 

Division of Agriculture Science: 128–216.

Esenther, G.R.; Beal, R.H. 1979. Termite control: decayed 

wood bait. Sociobiology. 4(2): 215–222.

Eslyn, W.E.; Clark, J.W. 1976. Appraising deterioration in 

submerged piling. Materials und Organismen. Supplement 

3: 43–52.

Eslyn, W.E.; Clark, J.W. 1979. Wood bridges—decay inspection and control. Agric. Handb. 557. Washington, DC: 

U.S. Department of Agriculture. 32 p.

Chapter 14 Biodeterioration of Wood

14–15
Morrell, J.J.; Corden, M.E. 1982. Controlling wood deterioration with fumigants: a review. Forest Products Journal. 

36(10): 26–34.

NPCA. 1963. Carpenter bees. Tech. Release 3–63. Dunn 

Loring, VA: National Pest Control Association.

NPCA. 1964. The horntails. Tech. Release 14–64. Dunn 

Loring, VA: National Pest Control Association.

NPCA. 1976. Carpenter ants. Tech. Release ESPC 052101. 

Dunn Loring, VA: National Pest Control Association.

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Washington, DC: U.S. Department of Agriculture. 63 p.

Roff, J.W.; Cserjesi, A.J.; Swann, G.W. 1980. Prevention 

of sap stain and mold in packaged lumber. Forintek Canada 

Corporate Technical Report. 14R. Vancouver, BC: Forintek 

Canada Corporation.

Scheffer, T.C. 1972. A climate index for estimating potential 

decay in wood structures above ground. Forest Products 

Journal. 21(10): 25–31.

Scheffer, T.C.; Eslyn, W.E. 1976. Winter treatments protect 

birch roundwood during storage. Forest Products Journal. 

26: 27–31.

Scheffer, T.C.; Verrall, A.F. 1973. Principles of protecting 

wood buildings from decay. Res. Pap. FPL 190. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 56 p.

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Department of Agriculture, Forest Service, Forest Products 

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70 p.

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252–257.

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Zabel, R.A.; Morrell, J.J. 1992. Wood microbiology: Decay 

and its prevention. San Diego, CA: Academic Press. 476 p.

14–16

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U.S. Department of Agriculture, Forest Service. 12 p.

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General Technical Report FPL–GTR–190
CHAPTER 15

Wood Preservation

Stan T. Lebow, Research Forest Products Technologist

15–1

Contents 

Wood Preservatives 15–1

 Waterborne Preservatives 15–3

 Preservatives with ICC–ES Evaluation 

 Reports 15–9

 Oil-Borne or Oil-Type Preservatives 15–10

 Treatments for Wood Composites 15–12

 Water-Repellent and Nonpressure 

 Treatments 15–12

Selecting Preservatives 15–12

Evaluating New Preservatives 15–13

Preservative Effectiveness 15–13

Effect of Species on Penetration 15–15

Preparation of Wood for Treatment 15–15

 Peeling 15–15

 Drying 15–16

 Conditioning of Green Products 15–17

 Incising 15–17

 Cutting and Framing 15–18

Application of Preservatives 15–18

 Pressure Processes 15–18

 Effect on Mechanical Properties 15–20

 Nonpressure Processes 15–20

 In-Place and Remedial Treatments 15–22

 Best Management Practices 15–23

Quality Assurance for Treated Wood 15–25

 Treating Conditions and Specifications 15–25

 Inspection of Treatment Quality 15–25

Effects on the Environment 15–26

Recycling and Disposal of Treated Wood 15–26

References 15–27

Many commonly used wood species can deteriorate if exposed to conditions that support growth of wood-degrading 

organisms (see Chap. 14). Wood products can be protected 

from the attack of decay fungi, harmful insects, or marine 

borers by applying chemical preservatives. Preservative 

treatments greatly increase the life of wood structures, thus 

reducing replacement costs and allowing more efficient 

use of forest resources. The degree of protection achieved 

depends on the preservative used and the proper penetration 

and retention of the chemicals. Some preservatives are more 

effective than others, and some are more adaptable to certain 

use requirements. To obtain long-term effectiveness, adequate penetration and retention are needed for each wood 

species, chemical preservative, and treatment method. Not 

only are different methods of treating wood available, but 

treatability varies among wood species—particularly their 

heartwood, which generally resists preservative treatment 

more than does sapwood. Although some tree species possess naturally occurring resistance to decay and insects (see 

Chap. 14), many are in short supply or are not grown 

in ready proximity to markets.

In considering preservative treatment processes and wood 

species, the combination must provide the required protection for the conditions of exposure and life of the structure. 

All these factors are considered by the consensus technical committees in setting reference levels required by the 

American Wood Protection Association (AWPA, formerly 

American Wood-Preservers’ Association)) and ASTM International (formerly American Society for Testing and Materials). Details are discussed later in this chapter. The characteristics, appropriate uses, and availability of preservative 

formulations may have changed after preparation of this 

chapter. For the most current information on preservative 

formulations, the reader is encouraged to contact the appropriate regulatory agencies, standardization organizations, or 

trade associations. Note that mention of a chemical in this 

chapter does not constitute a recommendation.

Wood Preservatives

Wood preservatives must meet two broad criteria: (1) They 

must provide the desired wood protection in the intended 

end use, and (2) they must do so without presenting unreasonable risks to people or the environment. Because wood 

preservatives are considered to be a type of pesticide, the 

U.S. Environmental Protection Agency (EPA) is responsible 

for their regulation. Federal law requires that before 

selling or distributing a preservative in the United States, 
15–2

a company must obtain registration 

from EPA. Before registering a new 

pesticide or new use for a registered 

preservative, EPA must first ensure 

that the preservative can be used with 

a reasonable certainty of no harm to 

human health and without posing unreasonable risks to the environment. 

To make such determinations, EPA 

requires more than 100 different scientific studies and tests from applicants. 

This chapter discusses only wood preservatives registered by the EPA.

Some preservatives are classified as 

“restricted use” by the EPA and these 

can be used only in certain applications and can be applied only by certified pesticide applicators. Restricted 

use refers to the chemical preservative 

and not to the treated wood product. The general consumer may buy 

and use wood products treated with 

restricted-use pesticides; EPA does 

not consider treated wood a toxic 

substance nor is it regulated as a pesticide. Although treated wood is not 

regulated as pesticide, there are limitations on how some types of treated 

wood should be used. Consumer Information Sheets (EPA-approved) are 

available from retailers of creosote-, 

pentachlorophenol-, and inorganicarsenical-treated wood products. The 

sheets provide information about the 

preservative and the use and disposal 

of treated-wood products (see Synopsis of EPA-Approved Consumer 

Information Sheets for Wood Treated 

with CCA, ACZA, Creosote, or Pentachlorophenol). The commercial 

wood treater is bound by the EPA 

regulation and can treat wood only 

for an end use that is allowed for that 

preservative. Some preservatives that 

are not classified as restricted by EPA 

are available to the general consumer 

for nonpressure treatments. It is the 

responsibility of the end user to apply 

these preservatives in a manner that is 

consistent with the EPA-approved labeling. Registration of preservatives is 

under constant review by the EPA, and 

a responsible State or Federal agency 

should be consulted as to the current 

status of any preservative.

Synopsis of EPA-approved consumer information sheets for wood treated with 

CCA, ACZA, creosote, or pentachlorophenol

NOTE: This is only a synopsis of information contained in consumer information 

sheets. For complete consumer information sheets, contact your treated wood supplier 

or the website of the Environmental Protection Agency.

Handling Precautions

Avoid frequent or prolonged inhalation of sawdust from treated wood. When sawing, 

sanding, and machining treated wood, wear a dust mask. Whenever possible, these 

operations should be performed outdoors to avoid indoor accumulations of airborne 

sawdust from treated wood. When power-sawing and machining, wear goggles to 

protect eyes from flying particles. Wear gloves when working with the wood. After 

working with the wood, and before eating, drinking, toileting, and use of tobacco 

products, wash exposed areas thoroughly. Avoid frequent or prolonged skin contact 

with creosote- or pentachlorophenol-treated wood. When handling creosote- or pentachlorophenol-treated wood, wear long-sleeved shirts and long pants and use gloves 

impervious to the chemicals (for example, gloves that are vinyl coated). Because 

preservatives or sawdust may accumulate on clothes, they should be laundered before 

reuse. Wash work clothes separately from other household clothing.

Treated wood should not be burned in open fires or in stoves, fireplaces, or residential 

boilers, because toxic chemicals may be produced as part of the smoke and ashes. 

Treated wood from commercial or industrial use (such as construction sites) may be 

burned only in commercial or industrial incinerators or boilers in accordance with 

state and Federal regulations. CCA-treated wood can be disposed of with regular 

municipal trash (municipal solid waste, not yard waste) in many areas. However, state 

or local laws may be stricter than federal requirements. For more information, please 

contact the waste management agency for your state.

Use Site Precautions

All sawdust and construction debris should be cleaned up and disposed of after construction. Do not use treated wood under circumstances where the preservative may 

become a component of food or animal feed. Examples of such sites would be use of 

mulch from recycled arsenic-treated wood, cutting boards, counter tops, animal bedding, and structures or containers for storing animal feed or human food. Only treated 

wood that is visibly clean and free of surface residue should be used for patios, decks, 

and walkways. Do not use treated wood for construction of those portions of beehives 

which may come into contact with honey. Treated wood should not be used where it 

may come into direct or indirect contact with drinking water, except for uses involving incidental contact such as docks and bridges.

Logs treated with pentachlorophenol should not be used for log homes. Wood treated 

with creosote or pentachlorophenol should not be used where it will be in frequent or 

prolonged contact with bare skin (for example, chairs and other outdoor furniture), 

unless an effective sealer has been applied. Creosote- and pentachlorophenol-treated 

wood should not be used in residential, industrial, or commercial interiors except for 

laminated beams or building components that are in ground contact and are subject to 

decay or insect infestation and where two coats of an appropriate sealer are applied. 

Do not use creosote- or pentachlorophenol-treated wood for farrowing or brooding 

facilities. Wood treated with pentachlorophenol or creosote should not be used in the 

interiors of farm buildings where there may be direct contact with domestic animals 

or livestock that may crib (bite) or lick the wood. In interiors of farm buildings where 

domestic animals or livestock are unlikely to crib (bite) or lick the wood, creosote- or 

pentachlorophenol-treated wood may be used for building components that are in 

ground contact and are subject to decay or insect infestation and where two coats of 

an appropriate sealer are applied. Sealers may be applied at the installation site. Urethane, shellac, latex epoxy enamel, and varnish are acceptable sealers for pentachlorophenol-treated wood. Coal-tar pitch and coal-tar pitch emulsion are effective sealers 

for creosote-treated wood-block flooring. Urethane, epoxy, and shellac are acceptable 

sealers for all creosote-treated wood.

General Technical Report FPL–GTR–190
15–3

Chapter 15 Wood Preservation

Before a wood preservative can be approved for pressure 

treatment of structural members, it must be evaluated to 

ensure that it provides the necessary durability and that it 

does not greatly reduce the strength properties of the wood. 

The EPA typically does not evaluate how well a wood preservative protects the wood. Traditionally this evaluation 

has been conducted through the standardization process of 

the AWPA. The AWPA Book of Standards lists a series of 

laboratory and field exposure tests that must be conducted 

when evaluating new wood preservatives. The durability of 

test products are compared with those of established durable 

products and nondurable controls. The results of those tests 

are then presented to the appropriate AWPA subcommittees 

for review. AWPA subcommittees are composed of representatives from industry, academia, and government agencies 

who have familiarity with conducting and interpreting durability evaluations. Preservative standardization by AWPA is 

a two-step process. If the performance of a new preservative 

is considered appropriate, it is first listed as a potential preservative. Secondary committee action is needed to have the 

new preservative listed for specific commodities and to set 

the required treatment level.

More recently the International Code Council–Evaluation 

Service (ICC–ES) has evolved as an additional route for 

gaining building code acceptance of new types of pressuretreated wood. In contrast to AWPA, the ICC–ES does not 

standardize preservatives. Instead, it issues Evaluation Reports that provide evidence that a building product complies 

with building codes. The data and other information needed 

to obtain an Evaluation Report are first established as Acceptance Criteria (AC). AC326, which sets the performance 

criteria used by ICC–ES to evaluate proprietary wood preservatives, requires submittal of documentation accredited 

third party agencies in accordance with AWPA , ASTM, and 

EN standard test methods. The results of those tests are then 

reviewed by an evaluation committee to determine if the 

preservative has met the appropriate acceptance criteria.

Wood preservatives have traditionally been divided into 

two general classes: (1) Oil-type or oil-borne preservatives, 

such as creosote and petroleum solutions of pentachlorophenol, and (2) waterborne preservatives that are applied as 

water solutions or with water as the carrier. Many different 

chemicals are in each of these classes, and each has different 

effectiveness in various exposure conditions. Some preservatives can be formulated so that they can be delivered with 

either water or oil-type carriers. In this chapter, both oilborne and waterborne preservative chemicals are described 

as to their potential end uses. Tables 15–1 and 15–2 summarize preservatives and their treatment levels for various 

wood products.

Waterborne Preservatives

Waterborne preservatives are often used when cleanliness 

and paintability of the treated wood are required. Formulations intended for use outdoors have shown high resistance 

to leaching and very good performance in service. Waterborne preservatives are included in specifications for items 

such as lumber, timber, posts, building foundations, poles, 

and piling (Table 15–1). Because water is added to the wood 

in the treatment process, some drying and shrinkage will 

occur after installation unless the wood is kiln-dried after 

treatment.

Copper is the primary biocide in many wood preservative 

formulations used in ground contact because of its excellent 

fungicidal properties and low mammalian toxicity (Table 

15–3). Because some types of fungi are copper tolerant, preservative formulations often include a co-biocide to provide 

further protection.

Inorganic arsenicals are a restricted-use pesticide. For use 

and handling precautions of pressure-treated wood containing inorganic arsenicals, refer to the EPA-approved Consumer Information Sheets.

Acid Copper Chromate (ACC)

Acid copper chromate (ACC) contains 31.8% copper oxide 

and 68.2% chromium trioxide (AWPA P5). The solid, paste, 

liquid concentrate, or treating solution can be made of copper sulfate, potassium dichromate, or sodium dichromate. 

Tests on stakes and posts exposed to decay and termite attack indicate that wood well impregnated with ACC generally provides acceptable service. However, some specimens 

placed in ground contact have shown vulnerability to attack 

by copper-tolerant fungi. ACC has often been used for treatment of wood in cooling towers. Its current uses are restricted to applications similar to those of chromated copper 

arsenate (CCA) (Table 15–4). ACC and CCA must be 

used at low treating temperatures (38 to 66 °C (100 to 

150 °F)) because they are unstable at higher temperatures. 

This restriction may involve some difficulty when higher 

temperatures are needed to obtain good treating results in 

woods such as Douglas-fir.

Ammoniacal Copper Zinc Arsenate (ACZA)

Ammoniacal copper zinc arsenate (ACZA) is commonly 

used on the West Coast of North America for the treatment 

of Douglas-fir. The penetration of Douglas-fir heartwood 

is improved with ACZA because of the chemical composition and stability of treating at elevated temperatures. Wood 

treated with ACZA performs and has characteristics similar 

to those of wood treated with CCA (Table 15–1).

ACZA should contain approximately 50% copper oxide, 

25% zinc oxide, and 25% arsenic pentoxide dissolved in a 

solution of ammonia in water (AWPA P5). The weight of 

ammonia is at least 1.38 times the weight of copper oxide. 

To aid in solution, ammonium bicarbonate is added (at least 

equal to 0.92 times the weight of copper oxide).

ACZA replaced an earlier formulation, ammoniacal copper 

arsenate (ACA) that was used for many years in the United 

States and Canada.
15–4

Chromated Copper Arsenate (CCA)

Wood treated with CCA (commonly called green treated) 

dominated the treated-wood market from the late 1970s 

until 2004. However, as the result of the voluntary label 

changes submitted by the CCA registrants, the EPA labeling 

of CCA currently permits the product to be used for primarily industrial applications (Table 15–4), and CCA-treated 

products are generally not available at retail lumber yards. 

CCA can no longer be used for treatment of lumber intended 

for use in residential decks or playground equipment. It is 

important to note that existing structures are not affected by 

this labeling change and that the EPA has not recommended 

removing structures built with CCA-treated lumber. These 

changes were made as part of the ongoing CCA re-registration process, and in light of the current and anticipated market demand for alternative preservatives for nonindustrial 

applications. Allowable uses for CCA are based on specific 

commodity standards listed in the 2001 edition of the AWPA 

standards. The most important of these allowable uses are 

based on the standards for poles, piles, and wood used in 

highway construction. A list of the most common allowable 

uses is shown in Table 15–4.

General Technical Report FPL–GTR–190

Table 15–1. Typical use categories and retentions for preservatives used in 

pressure treatment of Southern Pine speciesa

Retentions (kg m–3)

b

 for each type of exposure and AWPA 

use category designation 

Interior, 

dry or 

damp

Exterior 

above-ground 

Soil or 

fresh water 

Vertical,

coated Horizontal General 

Severe/

critical 

Very severe/ 

critical 

Preservative 1, 2 3A 3B 4A 4B 4C 4C (piles) 

Waterborne: Listed by the AWPA 

ACC NLc NLc 4.0 8 — — — 

ACZA 4.0 4.0 4.0 6.4 9.6 9.6 — 

ACQ–B 4.0 4.0 4.0 6.4 9.6 9.6 — 

ACQ–C 4.0 4.0 4.0 6.4 9.6 9.6 — 

ACQ–D 2.4 2.4 2.4 6.4 9.6 9.6 — 

CA–B 1.7 1.7 1.7 3.3 5.0 5.0 — 

CA–C 1.0 1.0 1.0 2.4 5.0 5.0 — 

CBA–A 3.3 3.3 3.3 6.5 9.8 9.8 — 

CCA NLc NLc 4 6.4 9.6 9.6 12.8 

CX–A 3.3 3.3 3.3 — — — — 

CuN (waterborne) 1.12 1.12 1.12 1.76 — — — 

EL2 0.30 0.30 0.30 — — — — 

KDS 3.0 3.0 3.0 7.5 — — — 

PTI 0.21 0.21 0.21/0.29d — — — — 

SBX 2.8/4.5e — — — — — — 

Oil-type: Listed by the AWPA 

Creosote 128/NRf 128.0 128.0 160 160 192 192 

Penta P9 Type A Oil 6.4/NRf

 6.4 6.4 8.0 8.0 8.0 9.6 

Penta P9 Type C Oil 6.4/NRf

 6.4 6.4 8.0 8.0 8.0 9.6 

CuN (oilborne) 0.64/NRf 0.64 0.64 0.96 1.2 1.2 1.6 

Cu8 0.32 0.32 0.32 — — — — 

Waterborne: Evaluation reports from ICC Evaluation Service, Inc. 

ESR–1721 0.8 0.8 0.8 2.2 3.6 5.3 5.3 

ESR–1980 2.4 2.4 2.4 5.4 9.6 9.6 — 

ESR–2067 0.3 0.3 0.3 — — — — 

ESR–2240 1.0 1.0 1.0 2.4 3.7 

ESR–2325 1.1 1.1 1.1 2.6 3.8 — — 

ESR–2711 2.1/2.7g 2.1/2.7g 2.1/2.7g

 4.5 6.9 — — a

Some exceptions exist for specific applications. See AWPA Standard U1 or ICC ES Evaluation Reports for 

details on specific applications. See Table 15–2 for seawater applications. b

To convert to retention expressed as lb ft–3, divide these values by 16.0. c

NL, not labeled. EPA labeling does not currently permit use of wood newly treated with these 

preservatives in most applications within these use categories. See Table 15–4 for more details. d

Higher retention specified if the preservative is used without a stabilizer in the treatment solution. e

Higher retention for areas with Formosan subterranean termites. 

f

NR, not recommended for interior use in inhabited structures. 

g

2.1 kg m–3 retention limited to decking and specialty use items. 
15–5

Although several formulations of CCA have been used in 

the past, CCA Type C has been the primary formulation and 

is currently the only formulation listed in AWPA standards. 

CCA–C was found to have the optimum combination of 

efficacy and resistance to leaching, but the earlier formulations (CCA–A and CCA–B) have also provided longterm protection for treated stakes exposed in Mississippi 

(Table 15–5). CCA–C has an actives composition of 47.5% 

chromium trioxide, 34.0% arsenic pentoxide, and 18.5% 

copper oxide. AWPA Standard P5 permits substitution of 

potassium or sodium dichromate for chromium trioxide; 

copper sulfate, basic copper carbonate, or copper hydroxide 

for copper oxide; and arsenic acid, sodium arsenate, or 

pyroarsenate for arsenic pentoxide.

High retention levels (40 kg m–3 (2.5 lb ft–3)) of CCA 

preservative provide good resistance to attack by the 

marine borers Limnoria and Teredo (Table 15–2).

Alkaline Copper Quat (ACQ)

Alkaline copper quat (ACQ) has an actives composition 

of 67% copper oxide and 33% quaternary ammonium 

compound (quat). Multiple variations of ACQ have been 

standardized. ACQ type B (ACQ–B) is an ammoniacal copper formulation, ACQ type D (ACQ–D) is an amine copper formulation, and ACQ type C (ACQ–C) is a combined 

ammoniacal-amine formulation with a slightly different 

quat compound. The multiple formulations of ACQ allow 

some flexibility in achieving compatibility with a specific 

wood species and application. When ammonia is used as the 

carrier, ACQ has improved ability to penetrate difficult-totreat wood species. However, if the wood species is readily 

treatable, such as Southern Pine sapwood, an amine carrier 

can be used to provide a more uniform surface appearance. 

Recently ACQ has been formulated using small particles 

of copper rather than copper solubilized in ethanolamine. 

These formulations are discussed in more detail in the Preservatives with ICC–ES Evaluation Reports section. Use of 

particulate copper formulations of ACQ is currently limited 

to permeable woods (such as species of pine with a high 

proportion of sapwood), but efforts continue to adapt the 

treatment to a broader range of wood species.

Alkaline Copper DCOI (ACD)

Alkaline copper DCOI (ACD) is a recently proposed formulation of alkaline copper ethanolamine that utilizes 4,5-dichloro-2-N-octyl-4-isothiazolin-3-one (DCOI) as co-biocide 

Chapter 15 Wood Preservation

Table 15–2. Preservative treatment and retention 

necessary to protect round timber piles from severe 

marine borer attacka

Retention (kg m–3)

b

Marine borers and preservatives 

Round

piles 

Sawn 

materials

Limnoria tripunctata only 

Ammoniacal copper zinc arsenate 40, 24c 40 

Chromated copper arsenate 40, 24c

Creosote 320, 256c

 400 

Limnoria tripunctata and Pholads 

(dual treatment) 

First treatment 

Ammoniacal copper zinc arsenate 16, (1.0) 24 

Chromated copper arsenate 16, (1.0) 24 

Second treatment 

Creosote 320, (20.0) 320 

Creosote solution 320, (20.0) 320 

a

See AWPA Commodity Specification G for more information. 

b

To convert to retention expressed as lb ft–3, divide these values by 16.0. c

Lower retention levels are for marine piling used in areas from New 

Jersey northward on the East Coast and north of San Francisco on the 

West Coast in the United States.

Table 15–3. Active ingredients in waterborne preservatives used for pressure treatments 

Active ingredient Preservative 

Inorganic actives 

Arsenic ACZA, CCA 

Boron CBA–A, CX–A, SBX, KDS 

Chromium ACC, CCA 

Copper ACC, ACZA, ACQ–B, ACQ–C, ACQ–D, CA–B, CA–C, CBA–A, CCA, CXA,

ESR–1721, ESR–1980, ESR–2240, ESR–2325, KDS, KDS–B, ESR–2711 

Zinc ACZA 

Organic actives 

Alkylbenzyldimethyl 

ammonium compound 

ACQ–C 

DCOI EL2, ESR–2711 

Didecyldimethyl 

ammonium compound 

ACQ–B, ACQ–D 

HDO: Bis-(N-cyclohexyldiazeniumdioxy)Cu CX–A

Imdiacloprid EL2, PTI, ESR–2067 

Propiconazole CA–C, PTI, ESR–1721 

Polymeric betaine KDS, KDS–B 

Tebuconazole PTI, ESR–1721, ESR–2067, ESR–2325 
to provide protection against copper-tolerant fungi. The 

ratio of alkaline copper to DCOI in the formulation ranges 

from 20:1 to 25:1. The ACD formulation is listed as a preservative in AWPA standards. It has been proposed for both 

above-ground and ground-contact applications, but at the 

time this chapter was finalized it had not yet been standardized for treatment of any commodities.

Copper bis(dimethyldithiocarbamate) (CDDC)

Copper bis(dimethyldithiocarbamate) (CDDC) is a reaction 

product formed in wood as a result of the dual treatment of 

two separate treating solutions. The first treating solution 

contains a maximum of 5% bivalent copper–ethanolamine 

(2-aminoethanol), and the second treating solution contains a minimum of 2.5% sodium dimethyldithiocarbamate 

(AWPA P5). Although this preservative is not currently 

commercially available, CDDC-treated wood products are 

included in the AWPA Commodity Standards for uses such 

as residential construction.

Copper Azole (CA–B, CA–C and CBA–A)

Copper azole (CA–B) is a formulation composed of amine 

copper (96%) and tebuconazole (4%). Copper azole (CA–C) 

is very similar to CA–B, but half the tebuconazole is replaced with propiconazole. The active ingredients in CA–C 

are in the ratio of 96% amine copper, 2% tebuconazole, and 

2% propiconazole. An earlier formulation (CBA–A) also 

contained boric acid. Although listed as an amine formulation, copper azole may also be formulated with an amine–

ammonia formulation. The ammonia may be included when 

the copper azole formulations are used to treat refractory 

species, and the ability of such a formulation to adequately 

treat Douglas-fir has been demonstrated. Inclusion of ammonia, however, is likely to have slight affects on the surface appearance and initial odor of the treated wood.

Copper HDO (CXA)

Copper HDO (CXA) is an amine copper water-based preservative that has been used in Europe and was recently 

standardized in the United States. The active ingredients are 

copper oxide, boric acid, and copper–HDO (bis-(N-cyclohexyldiazeniumdioxy copper). The appearance and handling 

characteristics of wood treated with copper HDO are similar 

to those of the other amine copper-based treatments. It is 

also referred to as copper xyligen. Currently, copper HDO is 

standardized only for applications that are not in direct contact with soil or water.

Copper Naphthenate (Waterborne)

Waterborne copper naphthenate (CuN–W) has an actives 

composition similar to oil-borne copper naphthenate, but the 

actives are carried in a solution of ethanolamine and water 

instead of petroleum solvent. Wood treated with the waterborne formulation has a drier surface and less odor than the 

oil-borne formulation. The waterborne formulation has been 

standardized for above-ground and some ground-contact 

applications (Table 15–1).

Inorganic Boron (Borax–Boric Acid)

Borate preservatives are readily soluble in water and highly 

leachable and should be used only above ground where the 

wood is protected from wetting. When used above ground 

and protected from wetting, this preservative is very effective against decay, termites, beetles, and carpenter ants. 

Inorganic boron (SBX) is listed in AWPA standards for 

protected applications such as framing lumber. The solid or 

treating solution for borate preservatives (borates) should be 

greater than 98% pure, on an anhydrous basis (AWPA P5). 

Acceptable borate compounds are sodium octaborate, sodium tetraborate, sodium pentaborate, and boric acid. These 

compounds are derived from the mineral sodium borate, 

which is the same material used in laundry additives.

15–6

General Technical Report FPL–GTR–190

Table 15–4. Generalized examples of products that may still be treated with CCA under 

conditions of current label languagea

Type of end use still allowed 

2001 AWPA 

standard 

Lumber and timbers used in seawater C2

Land, fresh-water, and marine piles C3

Utility poles C4

Plywood C9

Wood for highway construction C14

Round, half-round, and quarter-round fence posts C16

Poles, piles, and posts used as structural members on farms C16

Members immersed in or frequently splashed by seawater C18

Lumber and plywood for permanent wood foundations C22

Round poles and posts used in building construction C23

Sawn timbers (at least 5 in. thick) used to support residential and commercial structures C24

Sawn cross-arms C25

Structural glued-laminated members C28

Structural composite lumber (parallel strand or laminated veneer lumber) C33

Shakes and shingles C34

a

Refer to the EPA or a treated-wood supplier for the most recent definition of allowable uses. 
15–7

Chapter 15 Wood Preservation

Table 15–5. Results of Forest Products Laboratory studies on 38- by 89- 

by 457-mm (nominal 2- by 4- by 18-in.) Southern Pine sapwood stakes, 

pressure-treated with commonly used wood preservatives, installed at 

Harrison Experimental Forest, Mississippi 

Preservative 

Average retention

(kg m–3 (lb ft–3))a

Average life or condition 

at last inspection 

Controls (untreated stakes) 1.8 to 3.6 years 

Acid copper chromate 2.08 (0.13) 11.6 years 

2.24 (0.14) 6.1 years 

4.01 (0.25) 80% failed after 40 years 

4.17 (0.26) 80% failed after 60 years 

4.65 (0.29) 4.6 years 

5.93 (0.37) 60% failed after 60 years 

8.01 (0.50) 50% failed after 40 years 

12.18 (0.76) 22% failed after 40 years 

Ammoniacal copper arsenate 2.56 (0.16) 16.6 years 

3.52 (0.22) 80% failed after 30 years 

3.84 (0.24) 38.7 years 

4.01 (0.25) 60% failed after 40 years 

7.37 (0.45) 20% failed after 40 years 

8.17 (0.51) 10% failed after 60 years 

15.54 (0.97) No failures after 60 years 

20.02 (1.25) No failures after 60 years 

Chromated copper arsenate 2.40 (0.15) 28.7 years 

Type I (Type A) 3.52 (0.22) 45% failed after 40 years 

4.65 (0.29) 30% failed after 60 years 

7.05 (0.44) 10% failed after 40 years 

7.05 (0.44) 20% failed after 60 years 

Type II (Type B) 3.68 (0.23) 30% failed after 40 years 

4.17 (0.26) No failures after 46 years 

5.93 (0.37) No failures after 46 years 

8.33 (0.52) No failures after 46 years 

12.66 (0.79) No failures after 46 years 

16.66 (1.04) No failures after 46 years 

Type III (Type C) 2.24 (0.14) No failures after 25 years 

3.20 (0.20) No failures after 35 years 

4.00 (0.25) 20% failed after 20 years 

4.33 (0.27) 10% failed after 25 years 

6.41 (0.40) No failures after 35 years 

6.41 (0.40) No failures after 25 years 

9.61 (0.60) No failures after 35 years 

9.93 (0.62) No failures after 25 years 

12.66 (0.79) No failures after 25 years 

Oxine copper 0.22 (0.014) 26.9 years 

(Copper-8-quinolinolate) 0.48 (0.03) 27.3 years 

AWPA P9 heavy petroleum 0.95 (0.059) 31.3 years 

1.99 (0.124) No failures after 45 years 

Copper naphthenate 

0.11% copper in No. 2 fuel oil 0.19 (0.012) 15.9 years 

0.29% copper in No. 2 fuel oil 0.46 (0.029) 21.8 years 

0.57% copper in No. 2 fuel oil 0.98 (0.061) 27.1 years 

0.86% copper in No. 2 fuel oil 1.31 (0.082) 29.6 years 

Creosote, coal-tar 52.87 (3.3) 24.9 years 

65.68 (4.1) 14.2 years 

67.28 (4.2) 17.8 years 

73.69 (4.6) 21.3 years 

124.96 (7.8) 70% failed after 54-1/2 years 

128.24 (8.0) 90% failed after 60 years 

132.97 (8.3) 50% failed after 46 years 

160.20 (10.0) 90% failed after 55 years 

189.04 (11.8) 50% failed after 60 years 
In addition to pressure treatments, borates are commonly 

sprayed, brushed, or injected to treat wood in existing 

structures. They will diffuse into wood that is wet, so these 

preservatives are often used as a remedial treatment. Borates 

are widely used for log homes, natural wood finishes, and 

hardwood pallets.

EL2

EL2 is a waterborne preservative composed of the fungicide 

4,5-dichloro-2-N-octyl-4-isothiazolin-3-one (DCOI), the 

insecticide imidacloprid, and a moisture control stabilizer 

(MCS). The ratio of actives is 98% DCOI and 2% imidacloprid, but the MCS is also considered to be a necessary 

component to ensure preservative efficacy. EL2 is currently 

listed in AWPA standards for above-ground applications 

only (Table 15–1).

KDS

KDS and KDS Type B (KDS–B) utilize copper and polymeric betaine as the primary active ingredients. The KDS 

formulation also contains boron, and has an actives composition of 41% copper oxide, 33% polymeric betaine, and 

26% boric acid. KDS–B does not contain boron and has an 

actives composition of 56% copper oxide and 44% polymeric betaine. KDS is listed for treatment of commodities 

used above ground and for general use in contact with soil 

or fresh water. It is not listed for soil or fresh water contact 

in severe exposures. The listing includes treatment of 

common pine species as well as Douglas-fir and western 

hemlock. KDS–B is currently in the process of obtaining 

listings for specific commodities. The appearance of KDStreated wood is similar to that of wood treated with other 

15–8

General Technical Report FPL–GTR–190

Table 15–5. Results of Forest Products Laboratory studies on 38- by 89- 

by 457-mm (nominal 2- by 4- by 18-in.) Southern Pine sapwood stakes, 

pressure-treated with commonly used wood preservatives, installed at 

Harrison Experimental Forest, Mississippi—con.

Preservative 

Average retention

(kg m–3 (lb ft–3))a

Average life or condition 

at last inspection 

Creosote, coal-tar (con.) 211.46 (13.2) 20% failed after 54-1/2 years 

232.29 (14.5) No failures after 55 years 

264.33 (16.5) 10% failed after 60 years 

Pentachlorophenol 

Stoddard solvent 2.24 (0.14) 13.7 years 

(mineral spirits) 2.88 (0.18) 15.9 years 

3.20 (0.20) 9.5 years 

3.20 (0.20) 13.7 years 

6.09 (0.38) 80% failed after 39 years 

6.41 (0.40) 15.5 years 

10.73 (0.67) No failures after 39 years 

Heavy gas oil 3.20 (0.20) 89% failed after 50 years 

(Mid-United States) 6.41 (0.40) 80% failed after 50 years 

9.61 (0.60) 20% failed after 50 years 

No. 4 aromatic oil 3.36 (0.21) 21.0 years 

(West Coast) 6.57 (0.41) 70% failed after 50 years 

AWPA P9 (heavy petroleum) 1.76 (0.11) 90% failed after 39 years 

3.04 (0.19) 60% failed after 39 years 

4.65 (0.29) No failures after 39 years 

8.49 (0.53) No failures after 35 years 

10.73 (0.67) No failures after 39 years 

Petroleum solvent controls 64.08 (4.0) 7.6 years 

65.68 (4.1) 4.4 years 

75.29 (4.7) 12.9 years 

123.35 (7.7) 14.6 years 

126.56 (7.9) 90% failed after 50 years 

128.16 (8.0) 19.7 years 

128.16 (8.0) 23.3 years 

128.16 (8.0) 14.6 years 

129.76 (8.1) 3.4 years 

136.17 (8.5) 20.9 years 

157.00 (9.8) 6.3 years 

192.24 (12.0) 17.1 years 

193.84 (12.1) 80% failed after 50 years 

310.79 (19.4) 9.1 years 

a

Retention of active ingredients for preservatives and total solvent for petroleum solvent controls. 
alkaline copper formulations (light green–brown). It has 

some odor initially after treatment, but this odor dissipates 

as the wood dries.

Oligomeric Alkylphenol Polysulfide (PXTS)

PXTS is a recently developed and somewhat unusual preservative system. It is an oligomer formed by the reaction of 

cresylic acid and sulfur chlorides in the presence of excess 

sulfur. PXTS is a solid at room temperature but becomes 

a liquid when heated to above approximately 58 °C. It can 

also be dissolved and diluted in some aromatic and organic 

chlorinated solvents. PXTS is not currently listed for treatment of any commodities and is currently not commercially 

available.

Propiconazole and Tebuconazole

Propiconazole and tebuconazole are organic triazole biocides that are effective against wood decay fungi but not 

against insects (AWPA P5, P8). They are soluble in some 

organic solvents but have low solubility in water and are 

stable and leach resistant in wood. Propiconazole and tebuconazole are currently components of waterborne preservative treatments used for pressure-treatment of wood in the 

United States, Europe, and Canada. They are also used as 

components of formulations used to provide mold and sapstain protection. Propiconazole is also standardized for use 

with AWPA P9 Type C or Type F organic solvents.

Propiconazole–Tebuconazole–Imidacloprid (PTI)

PTI is a waterborne preservative solution composed of two 

fungicides (propiconazole and tebuconazole) and the insecticide imidacloprid. It is currently listed in AWPA standards 

for above-ground applications only. The efficacy of PTI is 

enhanced by the incorporation of a water-repellent stabilizer 

in the treatment solutions, and lower retentions are allowed 

with the stabilizer (Table 15–1).

Preservatives with ICC–ES Evaluation 

Reports

Some commercially available waterborne wood preservatives are not standardized by the AWPA. Instead, they have 

obtained ICC–ES evaluation reports. In this chapter we refer 

to these preservatives by their Evaluation Report number 

(Table 15–1).

ESR–1721

ESR–1721 recognizes three preservative formulations. 

Two are the same formulations of copper azole (CA–B and 

CA–C) also listed in AWPA standards. The other (referred to 

here as ESR–1721) uses particulate copper that is ground to 

sub-micron dimensions and dispersed in the treatment solution. Wood treated with ESR–1721 has a lighter green color 

than the CA–B or CA–C formulations because the copper 

is not dissolved in the treatment solution. All three formulations are listed for treatment of commodities used in a range 

of applications, including contact with soil or freshwater. 

Use of ESR–1721 (dispersed copper) is currently limited to 

easily treated pine species.

ESR–1980

ESR–1980 includes a listing for both the AWPA standardized formulation of ACQ–D and a waterborne, micronized 

copper version of alkaline copper quat (referred to here as 

ESR–1980). The formulation is similar to ACQ in that the 

active ingredients are 67% copper oxide and 33% quaternary ammonium compound. However, in ESR–1980 the 

copper is ground to sub-micron dimensions and suspended 

in the treatment solution instead of being dissolved in ethanolamine. The treated wood has little green color because 

the copper is not dissolved in the treatment solution. The 

use of the particulate form of copper is currently limited to 

the more easily penetrated pine species, but efforts are underway to adapt the formulation for treatment of a broader 

range of wood species. ESR–1980 is listed for treatment of 

commodities used in both above-ground and ground-contact 

applications.

ESR–2067

ESR–2067 is an organic waterborne preservative with an 

actives composition of 98% tebuconazole (fungicide) and 

2% imidacloprid (insecticide). The treatment does not impart any color to the wood. It is currently listed only for 

treatment of commodities that are not in direct contact 

with soil or standing water.

ESR–2240

ESR–2240 is a waterborne formulation that utilizes finely 

ground (micronized) copper in combination with tebuconazole in an actives ratio of 25:1. It is listed for above-ground 

and ground-contact applications. In addition to wood products cut from pine species, ESR–2240 can be used for treatment of hem–fir lumber and Douglas-fir plywood.

ESR–2325

ESR–2325 is another waterborne preservative that utilizes 

finely ground copper particles and tebuconazole as actives. 

The ratio of copper to tebuconazole in the treatment solution is 25:1. Its use is currently limited to more readily 

treated species such as the Southern Pine species group, but 

Douglas-fir plywood is also listed. ESR–2315 is listed for 

treatment of wood used above-ground and in contact with 

soil or fresh water.

ESR–2711

ESR–2711 combines copper solubilized in ethanolamine 

with the fungicide 4,5-dichloro-2-N-octyl-4-isothiazolin-3-

one (DCOI). The ratio of copper (as CuO) to DCOIT ranges 

from 10:1 to 25:1. The ESR listing provides for both aboveground and ground-contact applications. The appearance of 

the treated wood is similar to that of wood treated with other 

formulations utilizing soluble copper, such as ACQ. It is 

currently only listed for treatment of pine species.

15–9

Chapter 15 Wood Preservation
Oil-Borne or Oil-Type Preservatives

Oil-type wood preservatives are some of the oldest preservatives, and their use continues in many applications. Wood 

does not swell from treatment with preservative oils, but it 

may shrink if it loses moisture during the treating process. 

Creosote and solutions with heavy, less volatile petroleum 

oils often help protect wood from weathering but may adversely influence its cleanliness, odor, color, paintability, 

and fire performance. Volatile oils or solvents with oil-borne 

preservatives, if removed after treatment, leave the wood 

cleaner than do the heavy oils but may not provide as much 

protection. Wood treated with some preservative oils can be 

glued satisfactorily, although special processing or cleaning 

may be required to remove surplus oils from surfaces before 

spreading the adhesive.

Coal-Tar Creosote and Creosote Solutions

Coal-tar creosote (creosote) is a black or brownish oil made 

by distilling coal tar that is obtained after high-temperature 

carbonization of coal. Advantages of creosote are (a) high 

toxicity to wood-destroying organisms; (b) relative insolubility in water and low volatility, which impart to it a great 

degree of permanence under the most varied use conditions; 

(c) ease of application; (d) ease with which its depth of 

penetration can be determined; (e) relative low cost (when 

purchased in wholesale quantities); and (f) lengthy record of 

satisfactory use. Creosote is commonly used for heavy timbers, poles, piles, and railroad ties.

AWPA Standard P1/P13 provides specifications for coal-tar 

creosote used for preservative treatment of piles, poles, and 

timber for marine, land, and freshwater use. The character of 

the tar used, the method of distillation, and the temperature 

range in which the creosote fraction is collected all influence 

the composition of the creosote, and the composition may 

vary within the requirements of standard specifications. Under normal conditions, requirements of these standards can 

be met without difficulty by most creosote producers.

Coal tar or petroleum oil may also be mixed with coal-tar 

creosote, in various proportions, to lower preservative costs. 

AWPA Standard P2 provides specifications for coal-tar solutions. AWPA Standard P3 stipulates that creosote–petroleum 

oil solution shall consist solely of specified proportions 

of 50% coal-tar creosote by volume (which meets AWPA 

standard P1/P13) and 50% petroleum oil by volume (which 

meets AWPA standard P4). However, because no analytical standards exist to verify the compliance of P3 solutions 

after they have been mixed, the consumer assumes the risk 

of using these solutions. These creosote solutions have a 

satisfactory record of performance, particularly for railroad 

ties and posts where surface appearance of the treated wood 

is of minor importance. Compared with straight creosote, 

creosote solutions tend to reduce weathering and checking 

of the treated wood. These solutions have a greater tendency 

to accumulate on the surface of the treated wood (bleed) and 

penetrate the wood with greater difficulty because they are 

generally more viscous than is straight creosote. High temperatures and pressures during treatment, when they can be 

safely used, will often improve penetration of high-viscosity 

solutions.

Although coal-tar creosote or creosote solutions are well 

suited for general outdoor service in structural timbers, creosote has properties that are undesirable for some purposes. 

The color of creosote and the fact that creosote-treated wood 

usually cannot be painted satisfactorily make this preservative unsuitable where appearance and paintability are 

important.

The odor of creosote-treated wood is unpleasant to some 

people. Also, creosote vapors are harmful to growing plants, 

and foodstuffs that are sensitive to odors should not be 

stored where creosote odors are present. Workers sometimes object to creosote-treated wood because it soils their 

clothes, and creosote vapor photosensitizes exposed skin. 

With precautions to avoid direct skin contact with creosote, 

there appears to be minimal danger to the health of workers 

handling or working near the treated wood. The EPA or the 

wood treater should be contacted for specific information on 

this subject.

In 1986, creosote became a restricted-use pesticide, and its 

use is currently restricted to pressure-treatment facilities. 

For use and handling of creosote-treated wood, refer to the 

EPA-approved Consumer Information Sheet.

Freshly creosoted timber can be ignited and burns readily, 

producing a dense smoke. However, after the timber has 

seasoned for some months, the more volatile parts of the 

oil disappear from near the surface and the creosoted wood 

usually is little, if any, easier to ignite than untreated wood. 

Until this volatile oil has evaporated, ordinary precautions 

should be taken to prevent fires. Creosote adds fuel value, 

but it does not sustain ignition.

Other Creosotes

Creosotes distilled from tars other than coal tar have been 

used to some extent for wood preservation, although they 

are not included in current AWPA specifications. These 

include wood-tar creosote, oil-tar creosote, and water–gastar creosote. These creosotes provide some protection from 

decay and insect attack but are generally less effective than 

coal-tar creosote.

Pentachlorophenol Solutions

Water-repellent solutions containing chlorinated phenols, 

principally pentachlorophenol (penta), in solvents of the 

mineral spirits type, were first used in commercial dip treatments of wood by the millwork industry in about 1931. 

Commercial pressure treatment with pentachlorophenol in 

heavy petroleum oils on poles started in about 1941, and 

considerable quantities of various products soon were 

pressure treated. AWPA Standard P8 defines the properties 

of pentachlorophenol preservative, stating that pentachlorophenol solutions for wood preservation shall contain not less 

15–10

General Technical Report FPL–GTR–190
than 95% chlorinated phenols, as determined by titration of 

hydroxyl and calculated as pentachlorophenol.

AWPA standard P9 defines solvents and formulations for 

organic preservative systems. The performance of pentachlorophenol and the properties of the treated wood are influenced by the properties of the solvent used. A commercial 

process using pentachlorophenol dissolved in liquid petroleum gas (LPG) was introduced in 1961, but later research 

showed that field performance of penta–LPG systems was 

inferior to penta–P9 systems. Thus, penta–LPG systems 

are no longer used. The heavy petroleum solvent included 

in AWPA P9 Type A is preferable for maximum protection, 

particularly when wood treated with pentachlorophenol is 

used in contact with the ground. The heavy oils remain in 

the wood for a long time and do not usually provide a clean 

or paintable surface.

Because of the toxicity of pentachlorophenol, care is necessary when handling and using it to avoid excessive personal 

contact with the solution or vapor. Do not use indoors or 

where human, plant, or animal contact is likely. Pentachlorophenol became a restricted-use pesticide in November 

1986 and is currently only available for use in pressure 

treatment. For use and handling precautions, refer to the 

EPA-approved Consumer Information Sheet.

The results of pole service and field tests on wood treated 

with 5% pentachlorophenol in a heavy petroleum oil are 

similar to those with coal-tar creosote. This similarity has 

been recognized in the preservative retention requirements 

of treatment specifications. Pentachlorophenol is effective against many organisms, such as decay fungi, molds, 

stains, and insects. Because pentachlorophenol is ineffective 

against marine borers, it is not recommended for the treatment of marine piles or timbers used in coastal waters.

Copper Naphthenate

Copper naphthenate is an organometalic compound formed 

as a reaction product of copper salts and naphthenic acids 

that are usually obtained as byproducts in petroleum refining. It is a dark green liquid and imparts this color to the 

wood. Weathering turns the color of the treated wood to 

light brown after several months of exposure. The wood 

may vary from light brown to chocolate brown if heat is 

used in the treating process. AWPA P8 standard defines the 

properties of copper naphthenate, and AWPA P9 covers the 

solvents and formulations for organic preservative systems.

Copper naphthenate is effective against wood-destroying 

fungi and insects. It has been used commercially since the 

1940s and is currently standardized for a broad range of 

applications (Table 15–1). Copper naphthenate is not 

a restricted-use pesticide but should be handled as an 

industrial pesticide. It may be used for superficial treatment, 

such as by brushing with solutions with a copper content of 

1% to 2% (approximately 10% to 20% copper naphthenate). 

Water-based formulations of copper naphthenate may also 

be available.

Oxine Copper (copper-8-quinolinolate)

Oxine copper (copper-8-quinolinolate) is an organometalic 

compound, and the formulation consists of at least 10% copper-8-quinolinolate, 10% nickel-2-ethylhexanoate, and 80% 

inert ingredients (AWPA P8). It is accepted as a stand-alone 

preservative for aboveground use for sapstain and mold 

control and is also used for pressure treating (Table 15–1). 

A water-soluble form can be made with dodecylbenzene 

sulfonic acid, but the solution is corrosive to metals.

Oxine copper solutions are greenish brown, odorless, toxic 

to both wood decay fungi and insects, and have a low toxicity to humans and animals. Because of its low toxicity to humans and animals, oxine copper is the only EPA-registered 

preservative permitted by the U.S. Food and Drug Administration for treatment of wood used in direct contact with 

food. Some examples of its uses in wood are commercial 

refrigeration units, fruit and vegetable baskets and boxes, 

and water tanks. Oxine copper solutions have also been used 

on nonwood materials, such as webbing, cordage, cloth, 

leather, and plastics.

Zinc Naphthenate

Zinc naphthenate is similar to copper naphthenate but is less 

effective in preventing decay from wood-destroying fungi 

and mildew. It is light colored and does not impart the characteristic greenish color of copper naphthenate, but it does 

impart an odor. Waterborne and solventborne formulations 

are available. Zinc naphthenate is not widely used for pressure treating.

3-Iodo-2-Propynyl Butyl Carbamate

3-Iodo-2-propynyl butyl carbamate (IPBC) is a fungicide that is used as a component of sapstain and millwork 

preservatives. It is also included as a fungicide in several 

surface-applied water-repellent-preservative formulations. 

Waterborne and solvent-borne formulations are available. Some formulations yield an odorless, treated product 

that can be painted if dried after treatment. It is listed as a 

pressure-treatment preservative in the AWPA standards but 

is not currently standardized for pressure treatment of any 

wood products. IPBC also may be combined with other 

fungicides, such as didecyldimethylammonium chloride in 

formulations used to prevent mold and sapstain.

IPBC/Permethrin

IPBC is not an effective insecticide and has recently been 

standardized for use in combination with the insecticide permethrin (3-phenoxybenzyl-(1R,S)-cis, trans-2, 2-dimethyl3-(2,2-dichlorovinyl) cycloproanecarboxylate) under the 

designation IPBC/PER. Permethrin is a synthetic pyrethroid 

widely used for insect control in agricultural and structural 

applications. The ratio of IPBC to permethrin in the IPBC/

PER varies between 1.5:1 and 2.5:1. The formulation is 

15–11

Chapter 15 Wood Preservation
carried in a light solvent such as mineral spirits, making it 

compatible with composite wood products that might be 

negatively affected by the swelling associated with waterbased pressure treatments. The IPBC/PER formulation is 

intended only for use in above-ground applications. The 

formulation is listed as a preservative in AWPA standards, 

but at the time this chapter was finalized it had not yet been 

standardized for treatment of any commodities.

Alkyl Ammonium Compounds

Alkyl ammonium compounds such as didecyldimethylammonium chloride (DDAC) or didecyldimethylammonium 

carbonate (DDAC)/bicarbonate (DDABC) have some efficacy against both wood decay fungi and insects. They are 

soluble in both organic solvents and water and are stable in 

wood as a result of chemical fixation reactions. DDAC and 

DDABC are currently being used as a component of alkaline copper quat (ACQ) (see section on Waterborne Preservatives) for above-ground and ground-contact applications 

and as a component of formulations used for sapstain and 

mold control.

4,5-Dichloro-2-N-Octyl-4-Isothiazolin-3-One (DCOI)

4,5-dichloro-2-N-octyl-4-isothiazolin-3-one (DCOI) is a 

biocide that is primarily effective against wood decay fungi. 

It is soluble in organic solvents but not in water, and it is 

stable and leach resistant in wood. The solvent used in the 

formulation of the preservative is specified in AWPA P9 

Type C. DCOI can be formulated to be carried in a waterborne system, and it is currently used as a component in 

the waterborne preservative EL2. It has also recently been 

proposed for use as co-biocide in a copper ethanolamine 

formulation referred to as ACD.

Chlorpyrifos

Chlorpyrifos (CPF) is an organophosphate insecticide that 

has been widely used for agricultural purposes. It has been 

standardized by the AWPA as a preservative but is not currently used as a component of commercial pressure treatments. Chloropyrifos is not effective in preventing fungal 

attack and should be combined with an appropriate fungicidal preservative for most applications.

Treatments for Wood Composites

Many structural composite wood products, such as gluedlaminated beams, plywood, and parallel strand and laminated veneer lumber, can be pressure-treated with wood preservatives in a manner similar to lumber. However, flake- or 

fiber-based composites are often protected by adding preservative during manufacture. A commonly used preservative 

for these types of composites is zinc borate. Zinc borate is 

a white, odorless powder with low water solubility that is 

added directly to the furnish or wax during panel manufacture. Zinc borate has greater leach resistance than the more 

soluble forms of borate used for pressure treatment and 

thus can be used to treat composite siding products that are 

exposed outdoors but partially protected from the weather. 

Zinc borate is currently listed in AWPA Commodity Standard J for nonpressure treatment of laminated strand lumber, 

oriented strandboard, and engineered wood siding. The standard requires that these products have an exterior coating or 

laminate when used as siding. Another preservative that has 

been used to protect composites is ammoniacal copper acetate, which is applied by spraying the preservative onto the 

OSB flakes before drying.

Water-Repellent and Nonpressure 

Treatments

Effective water-repellent preservatives will retard the ingress of water when wood is exposed above ground. These 

preservatives help reduce dimensional changes in the wood 

as a result of moisture changes when the wood is exposed to 

rainwater or dampness for short periods. As with any wood 

preservative, the effectiveness in protecting wood against 

decay and insects depends upon the retention and penetration obtained in application. These preservatives are most 

often applied using nonpressure treatments such as vacuum 

impregnation, brushing, soaking, or dipping. Preservative 

systems containing water-repellent components are sold under various trade names, principally for the dip or equivalent 

treatment of window sash and other millwork. The National 

Wood Window and Door Association (NWWDA) standard, 

WDMA I.S. 4–07A, Water Repellent Preservative Treatment 

for Millwork, lists preservative formulations that have met 

certain requirements, including EPA registration and efficacy against decay fungi.

The AWPA Commodity Specification I for nonpressure 

treatment of millwork and other wood products provides requirements for these nonpressure preservatives but does not 

currently list any formulations. The preservative must also 

meet the Guidelines for Evaluating New Wood Preservatives 

for Consideration by the AWPA for nonpressure treatment.

Water-repellent preservatives containing oxine copper are 

used in nonpressure treatment of wood containers, pallets, 

and other products for use in contact with foods. When 

combined with volatile solvents, oxine copper is used to 

pressure-treat lumber intended for use in decking of trucks 

and cars or related uses involving harvesting, storage, and 

transportation of foods (AWPA P8).

Nonpressure preservatives sold to consumers for household 

and farm use typically contain copper naphthentate, zinc 

naphthenate, or oxine copper. Their formulations may also 

incorporate water repellents.

Selecting Preservatives

The type of preservative applied is often dependent on the 

requirements of the specific application. For example, direct 

contact with soil or water is considered a severe deterioration hazard, and preservatives used in these applications 

must have a high degree of leach resistance and efficacy 

against a broad spectrum of organisms. These same 

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General Technical Report FPL–GTR–190
preservatives may also be used at lower retentions to protect 

wood exposed in lower deterioration hazards, such as above 

the ground. The exposure is less severe for wood that is partially protected from the weather, and preservatives that lack 

the permanence or toxicity to withstand continued exposure 

to precipitation may be effective in those applications. Other 

formulations may be so readily leachable that they can be 

used only indoors.

To guide selection of the types of preservatives and loadings 

appropriate to a specific end use, the AWPA recently developed use category system (UCS) standards. The UCS standards simplify the process of finding appropriate preservatives and preservative retentions for specific end uses. They 

categorize treated wood applications by the severity of the 

deterioration hazard (Table 15–6). The lowest category, Use 

Category 1 (UC1), is for wood that is used in interior construction and kept dry; UC2 is for interior wood completely 

protected from the weather but occasionally damp. UC3 is 

for exterior wood used above ground; UC4 is for wood used 

in ground contact in exterior applications. UC5 includes applications that place treated wood in contact with seawater 

and marine borers. Individual commodity specifications then 

list all the preservatives that are standardized for a specific 

use category along with the appropriate preservative 

retention.

Although some preservatives are effective in almost all 

environments, they may not be well-suited for applications 

involving frequent human contact or for exposures that 

present only low to moderate biodeterioration hazards. Additional considerations include cost, potential odor, surface 

dryness, adhesive bonding, and ease of finish application.

Evaluating New Preservatives

Wood preservatives often need to provide protection from a 

wide range of wood-attacking organisms (fungi, insects, marine borers, and bacteria). Because they must protect wood 

in so many ways, and protect wood for a long time period, 

evaluating wood treatments requires numerous tests. Some 

of the most important tests are mentioned here, but they 

should be considered only as a minimum, and other tests are 

useful as well. Appendix A of the AWPA Standards provides 

detailed guidelines on the types of tests that may be needed 

to evaluate new wood preservatives.

The laboratory leaching test helps to evaluate how rapidly 

the treatment will be depleted. A treatment needs leach resistance to provide long-term protection. In this test small 

cubes of wood are immersed in water for 2 weeks.

The laboratory decay test is used to challenge the treated 

wood with certain fungal isolates that are known to aggressively degrade wood. It should be conducted with specimens 

that have been through the leaching test. The extent of decay 

in wood treated with the test preservative is compared to 

that of untreated wood and wood treated with an established 

preservative. This test can help to determine the treatment 

level needed to prevent decay.

Field stake evaluations are some of the most informative 

tests because they challenge the treated wood with a 

wide range of natural organisms under severe conditions 

(Fig. 15–1). Stakes are placed into the soil in regions with 

a warm, wet climate (usually either the southeastern United 

States or Hawaii). At least two different sites are used to 

account for differences in soil properties and types of organisms present. The extent of deterioration in wood treated 

with the test preservative is compared to that of untreated 

wood and wood treated with an established preservative.

Above-ground field exposures are useful for treatments that 

will be used to protect wood above ground. Although not 

as severe as field stake tests, above-ground tests do provide 

useful information on above-ground durability. Specimens 

are exposed to the weather in an area with a warm, wet 

climate (usually either the southeastern United States or 

Hawaii). The specimens are designed to trap moisture and 

create ideal conditions for above-ground decay. The extent 

of deterioration in wood treated with the test preservative is 

compared to that of untreated wood and wood treated with 

an established preservative.

Corrosion testing is used to determine the compatibility of 

the treatment with metal fasteners.

Treatability testing is used to evaluate the ability of a treatment to penetrate deeply into the wood. Shallow surface 

treatments rarely provide long-term protection because 

degrading organisms can still attack the interior of the wood. 

Strength testing compares the mechanical properties of 

treated wood with matched, untreated specimens. Treatment 

chemicals or processes have the potential to damage the 

wood, making it weak or brittle. 

Preservative Effectiveness

Preservative effectiveness is influenced not only by the protective value of the preservative chemical, but also by the 

15–13

Chapter 15 Wood Preservation

Figure 15–1. Field stake test plot at Harrison Experimental Forest in southern Mississippi.
method of application and extent of penetration and retention of the preservative in the treated wood. Even with an 

effective preservative, good protection cannot be expected 

with poor penetration or substandard retention levels. The 

species of wood, proportion of heartwood and sapwood, 

heartwood penetrability, and moisture content are among the 

important variables that influence the results of treatment. 

For various wood products, the preservatives and retention 

levels listed in the AWPA Commodity Standards or ICC–ES 

evaluation reports are given in Table 15–1.

Determining whether one preservative is more effective 

than another within a given use category is often difficult. 

Few service tests include a variety of preservatives under 

comparable conditions of exposure. Furthermore, service 

tests may not show a good comparison between different 

preservatives as a result of the difficulty in controlling for 

differences in treatment quality. Comparative data under 

similar exposure conditions, with various preservatives and 

retention levels, are included in the U.S. Forest Service, Forest Products Laboratory, stake test studies. A summary of 

these test results is included in Table 15–5. Note, however, 

that because the stakes used in these studies are treated under carefully controlled conditions, their performance may 

not reflect variability in performance exhibited by a broad 

range of commercially treated material.

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General Technical Report FPL–GTR–190

Table 15–6. Summary of use category system developed by the American Wood Protection Association 

Use category Service conditions Use environment 

Common agents of 

deterioration Typical applications 

UC1 Interior construction 

Above ground 

Dry 

Continuously protected 

from weather or other 

sources of moisture 

Insects only Interior construction and 

furnishings

UC2 Interior construction 

Above ground 

Damp 

Protected from weather, but 

may be subject to sources 

of moisture 

Decay fungi and insects Interior construction 

UC3A Exterior construction 

Above ground 

Coated and rapid water 

runoff

Exposed to all weather 

cycles, not exposed to 

prolonged wetting 

Decay fungi and insects Coated millwork, siding,

and trim 

UC3B Ground contact or fresh 

water

Non-critical components 

Exposed to all weather 

cycles, normal exposure 

conditions 

Decay fungi and insects Fence, deck, and 

guardrail posts, crossties 

and utility poles (low 

decay areas) 

UC4A Ground contact or fresh 

water

Non-critical components 

Exposed to all weather 

cycles, normal exposure 

conditions 

Decay fungi and insects Fence, deck, and 

guardrail posts, crossties 

and utility poles (low 

decay areas) 

UC4B Ground contact or fresh 

water

Critical components or 

difficult replacement 

Exposed to all weather 

cycles, high decay potential 

includes salt-water splash 

Decay fungi and insects 

with increased potential 

for biodeterioration 

Permanent wood 

foundations, building 

poles, horticultural 

posts, crossties and 

utility poles (high decay 

areas) 

UC4C Ground contact or fresh 

water

Critical structural 

components

Exposed to all weather 

cycles, severe environments, 

extreme decay potential 

Decay fungi and insects 

with extreme potential 

for biodeterioration 

Land and fresh-water 

piling, foundation piling, 

crossties and utility 

poles (severe decay 

areas) 

UC5A Salt or brackish water 

and adjacent mud zone 

Northern waters 

Continuous marine exposure 

(salt water) 

Salt-water organisms Piling, bulkheads, 

bracing 

UC5B Salt or brackish water 

and adjacent mud zone 

NJ to GA, south of San 

Francisco 

Continuous marine exposure 

(salt water) 

Salt-water organisms, 

including 

creosote-tolerant 

Limnoria tripunctata

Piling, bulkheads, 

bracing 

UC5C Salt or brackish water and 

adjacent mud zone 

South of GA, Gulf Coast, 

Hawaii, and Puerto Rico 

Continuous marine exposure 

(salt water) 

Salt-water organisms, 

including Martesia, 

Sphaeroma 

Piling, bulkheads, 

bracing 
Similar comparisons have been conducted for preservative 

treatments of small wood panels in marine exposure (Key 

West, Florida). These preservatives and treatments include 

creosotes with and without supplements, waterborne preservatives, waterborne preservative and creosote dual treatments, chemical modifications of wood, and various chemically modified polymers. In this study, untreated panels were 

badly damaged by marine borers after 6 to 18 months of 

exposure, whereas some treated panels have remained free 

of attack after 19 years in the sea.

Test results based on seawater exposure have shown that 

dual treatment (waterborne copper-containing preservatives 

followed by creosote) is possibly the most effective method 

of protecting wood against all types of marine borers. The 

AWPA standards have recognized this process as well as the 

treatment of marine piles with high retention levels of ammoniacal copper zinc arsenate (ACZA) or chromated copper 

arsenate (CCA). The recommended treatment and retention 

in kilograms per cubic meter (pounds per cubic foot) for 

round timber piles exposed to severe marine borer hazard 

are given in Table 15–2. Poorly treated or untreated heartwood faces of wood species containing “high sapwood” that 

do not require heartwood penetration (for example, southern 

pines, ponderosa pine, and red pine) have been found to 

perform inadequately in marine exposure. In marine applications, only sapwood faces should be allowed for waterborne-preservative-treated pine in direct seawater exposure.

Effect of Species on Penetration

The effectiveness of preservative treatment is influenced by 

the penetration and distribution of the preservative in the 

wood. For maximum protection, it is desirable to select 

species for which good penetration is assured.

In general, the sapwood of most softwood species is not difficult to treat under pressure (Fig. 15–2). Examples of species with sapwood that is easily penetrated when it is well 

dried and pressure treated are the pines, coastal Douglas-fir, 

western larch, Sitka spruce, western hemlock, western redcedar, northern white-cedar, and white fir (A. concolor). Examples of species with sapwood and heartwood somewhat 

resistant to penetration are the red and white spruces and 

Rocky Mountain Douglas-fir. Cedar poles are commonly 

incised to obtain satisfactory preservative penetration. With 

round members, such as poles, posts, and piles, the penetration of the sapwood is important in achieving a protective 

outer zone around the heartwood.

The proportion of sapwood varies greatly with wood species, and this becomes an important factor in obtaining adequate penetration. Species within the Southern Pine group 

are characterized by a large sapwood zone that is readily 

penetrated by most types of preservatives. In part because 

of their large proportion of treatable sapwood, these pine 

species are used for the vast majority of treated products 

in the United States. Other important lumber species, such 

as Douglas-fir, have a narrower sapwood band in the living 

tree, and as a result products manufactured from Douglas-fir 

have a lower proportion of treatable sapwood.

The heartwood of most species is difficult to treat. There 

may be variations in the resistance to preservative penetration of different wood species. Table 15–7 gives the relative 

resistance of the heartwood to treatment of various softwood and hardwood species. Although less treatable than 

sapwood, well-dried white fir, western hemlock, northern 

red oak, the ashes, and tupelo are examples of species with 

heartwood that is reasonably easy to penetrate. The southern 

pines, ponderosa pine, redwood, Sitka spruce, coastal Douglas-fir, beech, maples, and birches are examples of species 

with heartwood that is moderately resistant to penetration.

Preparation of Wood for Treatment

For satisfactory treatment and good performance, the wood 

product must be sound and suitably prepared. Except in specialized treating methods involving unpeeled or green material, the wood should be well peeled and either seasoned or 

conditioned in the cylinder before treatment. It is also highly 

desirable that all machining be completed before treatment, 

including incising (to improve the preservative penetration 

in woods that are resistant to treatment) and the operations 

of cutting or boring of holes.

Peeling

Peeling round or slabbed products is necessary to enable the 

wood to dry quickly enough to avoid decay and insect damage and to permit the preservative to penetrate satisfactorily. 

Even strips of the thin inner bark may prevent penetration. 

Patches of bark left on during treatment usually fall off in 

time and expose untreated wood, thus permitting decay to 

reach the interior of the member.

Careful peeling is especially important for wood that is to 

be treated by a nonpressure method. In the more thorough 

15–15

Chapter 15 Wood Preservation

Figure 15–2. During pressure treatment, preservative 

typically penetrates only the sapwood. Round members have a uniform treated sapwood shell, but sawn 

members may have less penetration on one or more 

faces.
processes, some penetration may take place both longitudinally and tangentially in the wood; consequently, small 

strips of bark are tolerated in some specifications. Processes 

in which a preservative is forced or permitted to diffuse 

through green wood lengthwise do not require peeling of the 

timber. Machines of various types have been developed for 

peeling round timbers, such as poles, piles, and posts (Fig. 

15–3).

Drying

Drying of wood before treatment is necessary to prevent 

decay and stain and to obtain preservative penetration. 

However, for treatment with waterborne preservatives by 

certain diffusion methods, high moisture content levels may 

be permitted. For treatment by other methods, however, drying before treatment is essential. Drying before treatment 

opens up the checks before the preservative is applied, thus 

increasing penetration, and reduces the risk of checks 

opening after treatment and exposing unpenetrated wood. 

Good penetration of heated organic-based preservatives may 

be possible in wood with a moisture content as high as 40% 

to 60%, but severe checking while drying after treatment 

can expose untreated wood.

For large timbers and railroad ties, air drying is a widely 

used method of conditioning. Despite the increased time, 

labor, and storage space required, air drying is generally the 

most inexpensive and effective method, even for pressure 

treatment. However, wet, warm climatic conditions make it 

difficult to air dry wood adequately without objectionable 

infection by stain, mold, and decay fungi. Such infected 

wood is often highly permeable; in rainy weather, infected 

wood can absorb a large quantity of water, which prevents 

satisfactory treatment.

How long the timber must be air dried before treatment 

depends on the climate, location, and condition of the 

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General Technical Report FPL–GTR–190

Table 15–7. Penetration of the heartwood of various softwood and hardwood speciesa

Ease of treatment Softwoods Hardwoods 

Least difficult Bristlecone pine (Pinus aristata) American basswood (Tilia americana)

Pinyon (P. edulis) Beech (white heartwood) (Fagus grandifolia)

Pondersosa pine (P. pondersosa) Black tupelo (blackgum) (Nyssa sylvatica)

Redwood (Sequoia sempervirens) Green ash (Fraxinus pennsylvanica var. lanceolata)

Pin cherry (Prunus pennsylvanica)

River birch (Betula nigra)

Red oak (Quercus spp.) 

Slippery elm (Ulmus fulva)

Sweet birch (Betula lenia)

Water tupelo (Nyssa aquatica)

White ash (Fraxinus americana)

Moderately

difficult 

Baldcypress (Taxodium distichum) Black willow (Salix nigra)

California red fir (Abies magnifica) Chestnut oak (Quercus montana)

Douglas-fir (coast) (Pseudotsuga taxifolia)) Cottonwood (Populus sp.) 

Eastern white pine (Pinus strobus) Bigtooth aspen (P. grandidentata)

Jack pine (P. banksiana) Mockernut hickory (Carya tomentosa)

Loblolly pine (P. taeda) Silver maple (Acer saccharinum)

Longleaf pine (P. palustris) Sugar maple (A. saccharum)

Red pine (P. resinosa) Yellow birch (Betula lutea)

Shortleaf pine (P. echinata)

Sugar pine (P. lambertiana)

Western hemlock (Tsuga heterophylla)

Difficult Eastern hemlock (Tsuga canadensis) American sycamore (Platanus occidentalis)

Engelmann spruce (Picea engelmanni) Hackberry (Celtis occidentalis)

Grand fir (Abies grandis) Rock elm (Ulmus thomoasi)

Lodgepole pine (Pinus contorta var. latifolia) Yellow-poplar (Liriodendron tulipifera)

Noble fir (Abies procera)

Sitka spruce (Picea sitchensis)

Western larch (Larix occidentalis)

White fir (Abies concolor)

White spruce (Picea glauca)

Very difficult Alpine fir (Abies lasiocarpa) American beech (red heartwood) (Fagus grandifolia)

Corkbark fir (A. lasiocarpa var. arizonica) American chestnut (Castanea dentata)

Douglas-fir (Rocky Mountain) (Pseudotsuga taxifolia) Black locust (Robinia pseudoacacia)

Northern white-cedar (Thuja occidentalis) Blackjack oak (Quercus marilandica)

Tamarack (Larix laricina) Sweetgum (redgum) (Liquidambar styraciflua)

Western redcedar (Thuja plicata) White oak (Quercus spp.) 

a

As covered in MacLean (1952). 
seasoning yard, methods of piling, season of the year, timber 

size, and species. The most satisfactory seasoning practice 

for any specific case will depend on the individual drying 

conditions and the preservative treatment to be used. Therefore, treating specifications are not always specific as to 

moisture content requirements.

To prevent decay and other forms of fungal infection during 

air drying, the wood should be cut and dried when conditions are less favorable for fungus development (Chap. 14). 

If this is impossible, chances for infection can be minimized 

by prompt conditioning of the green material, careful piling and roofing during air drying, and pretreating the green 

wood with preservatives to protect it during air drying.

Lumber of all species, including Southern Pine poles, is 

often kiln dried before treatment, particularly in the southern United States where proper air seasoning is difficult. 

Kiln drying has the important added advantage of quickly 

reducing moisture content, thereby reducing transportation 

charges on poles.

Conditioning of Green Products

Plants that treat wood by pressure processes can condition 

green material by means other than air and kiln drying. 

Thus, they avoid a long delay and possible deterioration 

of the timber before treatment.

When green wood is to be treated under pressure, one of 

several methods for conditioning may be selected. The 

steaming-and-vacuum process is used mainly for southern 

pines, and the Boulton or boiling-under-vacuum process is 

used for Douglas-fir and sometimes hardwoods.

In the steaming process, the green wood is steamed in the 

treating cylinder for several hours, usually at a maximum of 

118 °C (245 °F). When steaming is completed, a vacuum is 

immediately applied. During the steaming period, the outer 

part of the wood is heated to a temperature approaching 

that of the steam; the subsequent vacuum lowers the boiling 

point so that part of the water is evaporated or forced out 

of the wood by the steam produced when the vacuum is applied. The steaming and vacuum periods used depend upon 

the wood size, species, and moisture content. Steaming and 

vacuum usually reduce the moisture content of green wood 

slightly, and the heating assists greatly in getting the preservative to penetrate. A sufficiently long steaming period will 

also sterilize the wood.

In the Boulton or boiling-under-vacuum method of partial 

seasoning, the wood is heated in the oil preservative under 

vacuum, usually at about 82 to 104 °C (180 to 220 °F). This 

temperature range, lower than that of the steaming process, 

is a considerable advantage in treating woods that are especially susceptible to injury from high temperatures. The 

Boulton method removes much less moisture from heartwood than from sapwood.

Incising

Wood that is resistant to penetration by preservatives may 

be incised before treatment to permit deeper and more uniform penetration. To incise, lumber and timbers are passed 

through rollers equipped with teeth that sink into the wood 

to a predetermined depth, usually 13 to 19 mm (1/2 to 

3/4 in.). The teeth are spaced to give the desired distribution 

of preservative with the minimum number of incisions. A 

machine of different design is required for deeply incising 

the butts of poles, usually to a depth of 64 mm (2.5 in.) 

(Fig. 15–4).

Incising is effective because preservatives usually penetrate 

the wood much farther along the grain than across the grain. 

The incisions open cell lumens along the grain, which 

15–17

Chapter 15 Wood Preservation

Figure 15–3. Machine peeling of poles. The outer bark 

has been removed by hand, and the inner bark is being 

peeled by machine. Frequently, all the bark is removed 

by machine.

Figure 15–4. Deep incising permits better penetration 

of preservative.
greatly enhances penetration. Incising is especially effective 

in improving penetration in the heartwood areas of sawn 

surfaces.

Incising is practiced primarily on Douglas-fir, western hemlock, and western larch ties and timbers for pressure treatment and on cedar and Douglas-fir poles. Incising can result 

in significant reductions in strength (Chap. 5).

Cutting and Framing

All cutting and boring of holes should be done prior to preservative treatment. Cutting into the wood in any way after 

treatment will frequently expose the untreated interior of the 

timber and permit ready access to decay fungi or insects.

In some cases, wood structures can be designed so that 

all cutting and framing is done before treatment. Railroad 

companies have followed this practice and have found it 

not only practical but economical. Many wood-preserving 

plants are equipped to carry on such operations as the adzing and boring of crossties; gaining, roofing, and boring of 

poles; and framing of material for bridges and specialized 

structures, such as water tanks and barges.

Treatment of the wood with preservative oils results in little 

or no dimensional change. With waterborne preservatives, 

however, some change in the size and shape of the wood 

may occur even though the wood is redried to the moisture 

content it had before treatment. If precision fitting is necessary, the wood is cut and framed before treatment to its 

approximate final dimensions to allow for slight surfacing, 

trimming, and reaming of bolt holes. Grooves and bolt holes 

for timber connectors are cut before treatment and can be 

reamed out if necessary after treatment.

Application of Preservatives

Wood-preserving methods are of two general types: (a) pressure processes, in which the wood is impregnated in closed 

vessels under pressures considerably above atmospheric, 

and (b) nonpressure processes, which vary widely in the 

procedures and equipment used.

Pressure Processes

In commercial practice, wood is most often treated by immersing it in a preservative in a high-pressure apparatus and 

applying pressure to drive the preservative into the wood. 

Pressure processes differ in details, but the general principle 

is the same. The wood, on cars or trams, is run into a long 

steel cylinder, which is then closed and filled with preservative (Fig. 15–5). Pressure forces the preservative into the 

wood until the desired amount has been absorbed. Considerable preservative is absorbed, with relatively deep penetration. Three pressure processes are commonly used: full cell, 

modified full cell, and empty cell.

Full Cell

The full-cell (Bethel) process is used when the retention 

of a maximum quantity of preservative is desired. It is a 

standard procedure for timbers to be treated with creosote 

when protection against marine borers is required. Waterborne preservatives may be applied by the full-cell process 

if uniformity of penetration and retention is the primary concern. With waterborne preservatives, control over preservative retention is obtained by regulating the concentration of 

the treating solution.

Steps in the full-cell process are essentially the following:

1. The charge of wood is sealed in the treating cylinder, 

and a preliminary vacuum is applied for a half-hour or 

more to remove the air from the cylinder and as much 

as possible from the wood.

2. The preservative, at ambient or elevated temperature 

depending on the system, is admitted to the cylinder 

without breaking the vacuum.

3. After the cylinder is filled, pressure is applied until the 

wood will take no more preservative or until the required retention of preservative is obtained.

4. When the pressure period is completed, the preservative 

is withdrawn from the cylinder.

5. A short final vacuum may be applied to free the charge 

from dripping preservative.

When the wood is steamed before treatment, the preservative is admitted at the end of the vacuum period that follows 

steaming. When the timber has received preliminary conditioning by the Boulton or boiling-under-vacuum process, the 

cylinder can be filled and the pressure applied as soon as the 

conditioning period is completed.

Modified Full Cell

The modified full-cell process is basically the same as the 

full-cell process except for the amount of initial vacuum and 

the occasional use of an extended final vacuum. The modified full-cell process uses lower levels of initial vacuum; the 

actual amount is determined by the wood species, material 

size, and final retention desired. The modified full-cell process is commonly used for treatment of lumber with waterborne preservatives.

Empty Cell

The objective of the empty-cell process is to obtain deep 

penetration with a relatively low net retention of preservative. For treatment with oil preservatives, the empty-cell 

process should always be used if it will provide the desired 

retention. Two empty-cell processes, the Rueping and the 

Lowry, are commonly employed; both use the expansive 

force of compressed air to drive out part of the preservative 

absorbed during the pressure period.

The Rueping empty-cell process, often called the empty-cell 

process with initial air, has been widely used for many years 

in Europe and the United States. The following general procedure is employed:

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General Technical Report FPL–GTR–190
1. Air under pressure is forced into the treating cylinder, 

which contains the charge of wood. The air penetrates 

some species easily, requiring but a few minutes application of pressure. In treating the more resistant species, 

common practice is to maintain air pressure from 

1/2 to 1 h before admitting the preservative, but the 

necessity for lengthy air-pressure periods does not seem 

fully established. The air pressures employed generally 

range from 172 to 689 kPa (25 to 100 lb in–2), depending on the net retention of preservative desired and the 

resistance of the wood.

2. After the period of preliminary air pressure, preservative is forced into the cylinder. As the preservative is 

pumped in, the air escapes from the treating cylinder 

into an equalizing or Rueping tank, at a rate that keeps 

the pressure constant within the cylinder. When the 

treating cylinder is filled with preservative, the treating 

pressure is increased above that of the initial air and is 

maintained until the wood will absorb no more preservative, or until enough has been absorbed to leave the 

required retention of preservative in the wood after the 

treatment.

15–19

Chapter 15 Wood Preservation

Figure 15–5. Typical steps in pressure treating process: A, untreated wood is placed in cylinder; B, a vacuum is applied to pull air out 

of the wood; C, the wood is immersed in solution while still under 

vacuum; D, pressure is applied to force the preservative into the 

wood; E, preservative is pumped out, and a final vacuum is pulled 

to remove excess preservative; F, excess preservative is pumped 

away, and the wood is removed from the cylinder.
3. At the end of the pressure period, the preservative is 

drained from the cylinder, and surplus preservative 

is removed from the wood with a final vacuum. The 

amount of preservative recovered can be from 20% to 

60% of the gross amount injected.

The Lowry is often called the empty-cell process without 

initial air pressure. Preservative is admitted to the cylinder 

without either an initial air pressure or a vacuum, and the air 

originally in the wood at atmospheric pressure is imprisoned 

during the filling period. After the cylinder is filled with the 

preservative, pressure is applied, and the remainder of 

the treatment is the same as described for the Rueping 

treatment.

The Lowry process has the advantage that equipment for the 

full-cell process can be used without other accessories that 

the Rueping process usually requires, such as an air compressor, an extra cylinder or Rueping tank for the preservative, or a suitable pump to force the preservative into the 

cylinder against the air pressure. However, both processes 

have advantages and are widely and successfully used.

With poles and other products where bleeding of preservative oil is objectionable, the empty-cell process is followed 

by either heating in the preservative (expansion bath) at a 

maximum of 104 °C (220 °F) or a final steaming for a specified time limit at a maximum of 116 °C (240 °F) prior to the 

final vacuum.

Treating Pressures and Preservative Temperatures

The pressures used in treatments vary from about 345 to 

1,723 kPa (50 to 250 lb in–2), depending on the species and 

the ease with which the wood takes the treatment. Most 

commonly, pressures range from about 862 to 1,207 kPa 

(125 to 175 lb in–2). Many woods are sensitive to high 

treating pressures, especially when hot. For example, 

AWPA standards permit a maximum pressure of 1,050 kPa 

(150 lb in–2) in the treatment of redwood, eastern hemlock, 

and eastern white pine, while the limitation for oak is 

1,723 kPa (250 lb in–2).

AWPA T1 standard requires that the temperature of creosote and creosote solutions, as well as that of the oil-type 

preservatives, during the pressure period not be greater 

than 100 °C (212 °F). For the waterborne preservatives that 

contain chromium (ACC and CCA), the maximum solution 

temperature is limited to 50 °C (120 °F) to avoid premature 

precipitation of the preservative. For most other waterborne 

preservatives, the maximum solution temperature is 65 °C 

(150 °F), although a higher limit 93 °C (200 °F) is permitted 

for inorganic boron solutions.

Effect on Mechanical Properties

Coal-tar creosote, creosote solutions, and pentachlorophenol 

dissolved in petroleum oils are practically inert to wood and 

have no chemical influence that would affect its strength. 

Chemicals commonly used in waterborne salt preservatives, 

including chromium, copper, arsenic, and ammonia, are 

reactive with wood. Thus, these chemicals are potentially 

damaging to mechanical properties and may also promote 

corrosion of mechanical fasteners.

Significant reductions in mechanical properties may be observed if the treating and subsequent drying processes are 

not controlled within acceptable limits. Factors that influence the effect of the treating process on strength include 

(a) species of wood, (b) size and moisture content of the 

timbers treated, (c) type and temperature of heating medium, 

(d) length of the heating period in conditioning the wood for 

treatment and time the wood is in the hot preservative, 

(e) post-treatment drying temperatures, and (f) amount of 

pressure used. Most important of those factors are the severity and duration of the in-retort heating or post-treatment 

redrying conditions used. The effect of wood preservatives 

on the mechanical properties of wood is covered in 

Chapter 5.

Nonpressure Processes

The numerous nonpressure processes differ widely in the 

penetration and retention levels of preservative attained, and 

consequently in the degree of protection they provide to the 

treated wood. When similar retention and penetration levels 

are achieved, wood treated by a nonpressure method should 

have a service life comparable to that of wood treated by 

pressure. Nevertheless, results of nonpressure treatments, 

particularly those involving surface applications, are not 

generally as satisfactory as those of pressure treatment. The 

superficial processes do serve a useful purpose when more 

thorough treatments are impractical or exposure conditions 

are such that little preservative protection is required.

Nonpressure methods, in general, consist of (a) surface 

application of preservatives by brief dipping, (b) soaking 

in preservative oils or steeping in solutions of waterborne 

preservatives, (c) diffusion processes with waterborne preservatives, (d) vacuum treatment, and (e) a variety of miscellaneous processes.

Brief Dipping

It is a common practice to treat window sash, frames, and 

other millwork, either before or after assembly, by dipping 

the item in a water-repellent preservative.

In some cases, preservative oil penetrates the end surfaces 

of ponderosa pine sapwood as much as 25 to 76 mm 

(1 to 3 in.). However, end penetration in such woods as the 

heartwood of southern pines and Douglas-fir is much less. 

Transverse penetration of the preservative applied by brief 

dipping is very shallow, usually less than a millimeter (a few 

hundredths of an inch). The exposed end surfaces at joints 

are the most vulnerable to decay in millwork products; 

therefore, good end penetration is especially advantageous. 

Dip applications provide very limited protection to wood 

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General Technical Report FPL–GTR–190
used in contact with the ground or under very moist conditions, and they provide very limited protection against attack 

by termites. However, they do have value for exterior woodwork and millwork that is painted, not in contact with the 

ground, and exposed to moisture only for brief periods.

Cold Soaking and Steeping

The methods of cold soaking well-seasoned wood for several hours or days in low-viscosity preservative oils or steeping green or seasoned wood for several days in waterborne 

preservatives have provided a range of success on fence 

posts, lumber, and timbers.

Pine posts treated by cold soaking for 24 to 48 h or longer in 

a solution containing 5% of pentachlorophenol in No. 2 fuel 

oil have shown an average life of 16 to 20 years or longer. 

The sapwood in these posts was well penetrated, and preservative solution retention levels ranged from 32 to 96 kg m–3 

(2 to 6 lb in–3). Most species do not treat as satisfactorily as 

do the pines by cold soaking, and test posts of such woods 

as birch, aspen, and sweetgum treated by this method have 

failed in much shorter times.

Preservative penetration and retention levels obtained by 

cold soaking lumber for several hours are considerably better than those obtained by brief dipping of similar species. 

However, preservative retention levels seldom equal those 

obtained in pressure treatment except in cases such as sapwood of pines that has become highly absorptive through 

mold and stain infection.

Steeping with waterborne preservatives has very limited use 

in the United States but it has been used for many years in 

Europe. In treating seasoned wood, both the water and the 

preservative salt in the solution soak into the wood. With 

green wood, the preservative enters the water-saturated 

wood by diffusion. Preservative retention and penetration 

levels vary over a wide range, and the process is not 

generally recommended when more reliable treatments 

are practical.

Diffusion Processes

In addition to the steeping process, diffusion processes are 

used with green or wet wood. These processes employ 

waterborne preservatives that will diffuse out of the water of 

the treating solution or paste into the water of the wood.

The double-diffusion process developed by the Forest Products Laboratory has shown very good results in fence post 

tests and standard 38- by 89-mm (nominal 2- by 4-in.) stake 

tests, particularly for full-length immersion treatments. This 

process consists of steeping green or partially seasoned 

wood first in one chemical solution, then in another. The 

two chemicals then react in the wood to form a precipitate 

with low solubility. However, the preservatives evaluated in 

this process do not currently have EPA registration for use in 

nonpressure treatments.

Vacuum Process

The vacuum process, or “VAC–VAC” as referred to in Europe, has been used to treat millwork with water-repellent 

preservatives and construction lumber with waterborne and 

water-repellent preservatives.

In treating millwork, the objective is to use a limited quantity of water-repellent preservative and obtain retention 

and penetration levels similar to those obtained by dipping 

for 3 min. In this treatment, a quick, low initial vacuum is 

followed by filling the cylinder under vacuum, releasing 

the vacuum and soaking, followed by a final vacuum. This 

treatment provides better penetration and retention than the 

3-min dip treatment, and the surface of the wood is quickly 

dried, thus expediting glazing, priming, and painting. The 

vacuum treatment is also reported to be less likely than dip 

treatment to leave objectionably high retention levels in 

bacteria-infected wood referred to as “sinker stock.”

Lumber intended for buildings has been treated by the 

vacuum process, either with a waterborne preservative or 

a water-repellent/preservative solution, with preservative 

retention levels usually less than those required for pressure 

treatment. The process differs from that used in treating 

millwork in employing a higher initial vacuum and a longer 

immersion or soaking period.

In a study by the Forest Products Laboratory, an initial 

vacuum of -93 kPa (27.5 inHg) was applied for 30 min, followed by a soaking for 8 h, and a final or recovery vacuum 

of -93 kPa (27.5 inHg) for 2 h. Results of the study showed 

good penetration of preservative in the sapwood of dry 

lumber of easily penetrated species such as the pines. However, in heartwood and unseasoned sapwood of pine and 

heartwood of seasoned and unseasoned coastal Douglas-fir, 

penetration was much less than that obtained by pressure 

treatment. Preservative retention was less controllable in 

vacuum than in empty-cell pressure treatment. Good control 

over retention levels is possible in vacuum treatment with 

a waterborne preservative by adjusting concentration of the 

treating solution.

Miscellaneous Nonpressure Processes

Several other nonpressure methods of various types have 

been used to a limited extent. Many of these involve the 

application of waterborne preservatives to living trees. The 

Boucherie process for the treatment of green, unpeeled 

poles has been used for many years in Europe. This process 

involves attaching liquid-tight caps to the butt ends of the 

poles. Then, through a pipeline or hose leading to the cap, a 

waterborne preservative is forced under hydrostatic pressure 

into the pole.

A tire-tube process is a simple adaptation of the Boucherie 

process used for treating green, unpeeled fence posts. In 

this treatment, a section of used inner tube is fastened tight 

around the butt end of the post to make a bag that holds a 

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Chapter 15 Wood Preservation
solution of waterborne preservative. There are now limitations for application of these processes because of the potential loss of preservative to the soil around the treatment site.

In-Place and Remedial Treatments

In-place treatments may be beneficial both during construction and as part of an inspection and maintenance program. 

Although cutting or drilling pressure-treated wood during 

construction is undesirable, it cannot always be avoided. 

When cutting is necessary, the damage can be partly overcome by a thorough application of copper naphthenate (1% 

to 2% copper) to the cut surface. This provides a protective 

coating of preservative on the surface that may slowly migrate into the end grain of the wood. The exposed end-grain 

in joints, which is more susceptible to moisture absorption, 

and the immediate area around all fasteners, including drill 

holes, will require supplemental on-site treatment. A special 

device is available for pressure-treating bolt holes that are 

bored after treatment. For treating the end surfaces of piles 

where they are cut off after driving, at least two generous 

coats of copper naphthenate should be applied. A coat of asphalt or similar material may be thoroughly applied over the 

copper naphthenate, followed by some protective sheet material, such as metal, roofing felt, or saturated fabric, fitted 

over the pile head and brought down the sides far enough to 

protect against damage to the treatment and against the entrance of storm water. AWPA Standard M4 contains instructions for the care of pressure-treated wood after treatment.

Surface Applications

The simplest treatment is to apply the preservative to the 

wood with a brush or by spraying. Preservatives that are 

thoroughly liquid when cold should be selected, unless it is 

possible to heat the preservative. When practical, the preservative should be flooded over the wood rather than merely 

painted. Every check and depression in the wood should 

be thoroughly filled with the preservative, because any untreated wood left exposed provides ready access for fungi. 

Rough lumber may require as much as 40 L of preservative 

per 100 m2 (10 gallons per 1,000 ft2) of surface, but 

surfaced lumber requires considerably less. The transverse 

penetration obtained will usually be less than 2.5 mm (0.1 

in.), although in easily penetrated species, end-grain (longitudinal) penetration is considerably greater. The additional 

life obtained by such treatments over that of untreated wood 

will be affected greatly by the conditions of service. For 

wood in contact with the ground, service life may be from 

1 to 5 years.

For brush or spray applications, copper naphthenate in oil is 

the preservative that is most often used. The solution should 

contain 1% to 2% elemental copper. Copper naphthenate is 

available as a concentrate or in a ready-to-use solution in 

gallon and drum containers. Borate solutions can also be 

sprayed or brushed into checks or splits. However, because 

they are not fixed to the wood they can be leached during 

subsequent precipitation. Borates are sold either as concentrated liquids (typically formulated with glycol) or as powders that can be diluted with water.

Another type of surface treatment is the application of water-soluble pastes containing combinations of copper naphthenate, copper quinolinolate, copper hydroxide, or borates. 

The theory with these treatments is that the diffusible components (such as boron) will move through the wood, while 

the copper component remains near the surface of a void or 

check. These pastes are most commonly used to help protect 

the ground-line area of poles. After the paste is applied, it is 

a covered with a wrap to hold the paste against the pole and 

prevent loss into the soil. In bridge piles this type of paste 

application should be limited to terrestrial piles that will 

not be continually or frequently exposed to standing water. 

These pastes may also be effective if used under cap beams 

or covers to protect exposed end-grain. Reapplication schedules will vary based on the manufacturers recommendations 

as well as the method and area of application.

Internal Diffusible Treatments

Surface-applied treatments often do not penetrate deeply 

enough to protect the inner portions of large wooden members. An alternative to surface-applied treatments is installation of internal diffusible chemicals. These diffusible 

treatments are available in liquid, solid, or paste form and 

are applied into treatment holes that are drilled deeply into 

the wood. They are similar (and in some cases identical) to 

the surface-applied treatments or pastes. Boron is the most 

common active ingredient, but fluoride and copper have also 

been used. In timbers, deep holes are drilled perpendicular 

to the upper face on either side of checks. In round piles, 

steeply sloping holes are drilled across the grain to 

maximize the chemical diffusion and minimize the number 

of holes needed. The treatment holes are plugged with tight 

fitting treated wooden plugs or removable plastic plugs. 

Plugs with grease fittings are also available so that the paste 

can be reapplied without removing the plug.

Solid rod treatments have advantages in environmentally 

sensitive areas or in applications where the treatment hole 

can only be drilled at an upward angle. However, the chemical may not diffuse as rapidly or for as great a distance 

as compared to a liquid form. Solid forms may be less 

mobile because diffusible treatments require moisture to 

move through wood. Concentrated liquid borates may also 

be poured into treatment holes and are sometimes used in 

conjunction with the rods to provide an initial supply of 

moisture. When the moisture content falls below 20%, little 

chemical movement occurs, but fortunately growth of decay 

fungi is substantially arrested below 30% moisture. Because 

there is some risk that rods installed in a dry section of a 

timber would not diffuse to an adjacent wet section, some 

experience in proper placement of the treatment holes is 

necessary. The diffusible treatments do not move as far in 

the wood as do fumigants, and thus the treatment holes must 

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General Technical Report FPL–GTR–190
be spaced more closely. A study of borate diffusion in timbers of several wood species reported that diffusion along 

the grain was generally less than 12 cm (5 in.), and diffusion 

across the grain was typically less than 5 cm (2 in.).

Internal Fumigant Treatments

As with diffusibles, fumigants are applied in liquid or solid 

form in predrilled holes. However, they then volatilize into 

a gas that moves through the wood. To be most effective, 

a fumigant should be applied at locations where it will not 

readily volatilize out of the wood to the atmosphere. When 

fumigants are applied, the timbers should be inspected thoroughly to determine an optimal drilling pattern that avoids 

metal fasteners, seasoning checks, and severely rotted wood. 

In vertical members such as piles, holes to receive liquid fumigant should be drilled at a steep angle (45° to 60°) downward toward the center of the member, avoiding seasoning 

checks. The holes should be no more than 1.2 m (4 ft) apart 

and arranged in a spiral pattern. With horizontal timbers, the 

holes can be drilled straight down or slanted. As a rule, the 

holes should be extended to within about 5 cm (2 in.) of the 

bottom of the timber. If strength is not jeopardized, holes 

can be drilled in a cluster or in pairs to accommodate the required amount of preservative. If large seasoning checks are 

present, the holes should be drilled on each side of the member to provide better distribution. As soon as the fumigant 

is injected, the hole should be plugged with a tight-fitting 

treated wood dowel or removable plastic plug. For liquid 

fumigants, sufficient room must remain in the treating hole 

so the plug can be driven without displacing the chemical 

out of the hole. The amount of fumigant needed and the size 

and number of treating holes required depends upon the timber size. Fumigants will eventually diffuse out of the wood, 

allowing decay fungi to recolonize. Fortunately, additional 

fumigant can be applied to the same treatment hole. Fumigant treatments are generally more toxic and more difficult 

to handle than are diffusible treatments. Some are classified 

as restricted-use pesticides by the U.S. EPA.

One of the oldest and most effective fumigants is chloropicrin (trichloronitromethane). Chloropicrin is a liquid and has 

been found to remain in wood for up to 20 years; however, 

a 10-year retreatment cycle is recommended, with regular 

inspection. Chloropicrin is a strong eye irritant and has high 

volatility. Due to chloropicrin’s hazardous nature, it should 

be used in areas away from buildings permanently inhabited 

by humans or animals. During application, workers must 

wear protective gear, including a full face respirator. Methylisothiocyante (MITC) is the active ingredient in several 

fumigants, but is also available in a solid-melt form that is 

97% actives. The solid-melt MITC is supplied in aluminum 

tubes. After the treatment hole is drilled the cap is removed 

from the tube, and the entire tube is placed into the whole. 

This formulation provides ease of handling and application to upward drilled sloping treatment holes. Metham 

sodium (sodium N-methldithiocarbamate) is a widely used 

liquid fumigant that decomposes in the wood to form the 

active ingredient MITC. Granular dazomet (tetrahydro-3, 

5-dimethyl-2-H-1,3,5, thiodazine-6-thione) is applied in a 

solid granular form that decomposes to a MITC content of 

approximately 45%. Dazomet is easy to handle but slower 

to decompose and release MITC than the solid-melt MITC 

or liquid fumigants. Some suppliers recommend the addition 

of a catalyst such as copper naphthenate to accelerate the 

breakdown process.

Best Management Practices

The active ingredients of various waterborne wood preservatives (copper, chromium, arsenic, and zinc) are water 

soluble in the treating solution but resist leaching when 

placed into the wood. This resistance to leaching is a result 

of chemical stabilization (or fixation) reactions that render 

the toxic ingredients insoluble in water. The mechanism and 

requirements for the stabilization reactions differ, depending 

on the type of wood preservative.

For each type of preservative, some reactions occur very 

rapidly during pressure treatment, while others may take 

days or even weeks, depending on storage and processing 

after treatment. If the treated wood is placed in service before these fixation reactions have been completed, the initial 

release of preservative into the environment may be much 

greater than if the wood has been conditioned properly.

With oil-type preservatives, preservative bleeding or oozing out of the treated wood is a particular concern. This 

problem may be apparent immediately after treatment. Such 

members should not be used in bridges over water or other 

aquatic applications. In other cases, the problem may not 

become obvious until after the product has been exposed to 

heating by direct sunlight. This problem can be minimized 

by using treatment practices that remove excess preservative 

from the wood.

Best management practice (BMP) standards have been developed to ensure that treated wood is produced in a way 

that will minimize environmental concerns. The Western 

Wood Preservers Institute (WWPI) has developed guidelines 

for treated wood used in aquatic environments. Although 

these practices have not yet been adopted by the industry 

in all areas of the United States, purchasers can require that 

these practices be followed. Commercial wood treatment 

firms are responsible for meeting conditions that ensure 

stabilization and minimize bleeding of preservatives, but 

persons buying treated wood should make sure that the firms 

have done so.

Consumers can take steps to ensure that wood will be treated according to the BMPs. Proper stabilization may take 

time, and material should be ordered well before it is needed 

so that the treater can hold the wood while it stabilizes. If 

consumers order wood in advance, they may also be able to 

store it under cover, allowing further drying and fixation. In 

general, allowing the material to air dry before it is used is 

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Chapter 15 Wood Preservation
a good practice for ensuring fixation, minimizing leaching, 

and reducing risk to construction personnel. With all preservatives, the wood should be inspected for surface residue, 

and wood with excessive residue should not be placed in 

service.

CCA

The risk of chemical exposure from wood treated with 

CCA is minimized after chemical fixation reactions lock 

the chemical in the wood. The treating solution contains 

hexavalent chromium, but the chromium reduces to the 

less toxic trivalent state within the wood. This process of 

chromium reduction also is critical in fixing the arsenic and 

copper in the wood. Wood treated with CCA should not be 

immersed or exposed to prolonged wetting until the fixation 

process is complete or nearly complete. The rate of fixation 

depends on temperature, taking only a few hours at 66 °C 

(150 °F) but weeks or even months at temperatures below 

16 °C (60 °F). Some treatment facilities use kilns, steam, or 

hot-water baths to accelerate fixation.

The BMP guideline for CCA stipulates that the wood should 

be air seasoned, kiln dried, steamed, or subjected to a hotwater bath after treatment. It can be evaluated with the 

AWPA chromotropic acid test to determine whether fixation 

is complete.

ACZA and ACQ–B

The key to achieving stabilization with ACZA and ACQ–B 

is to allow ammonia to volatilize. This can be accomplished 

by air or kiln drying. The BMPs require a minimum of 

3 weeks of air drying at temperatures higher than 16 °C 

(60 °F). Drying time can be reduced to 1 week if the material is conditioned in the treatment cylinder. At lower 

temperatures, kiln drying or heat is required to complete 

fixation. There is no commonly used method to determine 

the degree of stabilization in wood treated with ACZA or 

ACQ–B, although wood that has been thoroughly dried is 

acceptable. If the wood has a strong ammonia odor, fixation 

is not complete.

ACQ–C, ACQ–D, and Copper Azole

Proper handling and conditioning of the wood after treatment helps minimize leaching and potential environmental 

impacts for these preservatives. Amine (and ammonia in 

some cases) keeps copper soluble in these treatment solutions. The mechanism of copper’s reaction in the wood is 

not completely understood but appears to be strongly influenced by time, temperature, and retention levels. As a general rule, wood that has been thoroughly dried after treatment 

is properly stabilized.

Copper stabilization in the copper azole CA–B formulation 

is extremely rapid (within 24 h) at the UC3B retention of 

1.7 kg m–3 (0.10 lb ft–3) but slows considerably at higher 

retentions unless the material is heated to accelerate fixation.

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General Technical Report FPL–GTR–190

Pentachlorophenol, Creosote, and Copper 

Naphthenate

For creosote, the BMPs stipulate use of an expansion bath 

and final steaming period at the end of the charge.

Expansion Bath—Following the pressure period, the creosote should be heated to a temperature 6 to 12 °C (10 to 

20 °F) above the press temperatures for at least 1 h. Creosote should be pumped back to storage and a minimum 

gauge vacuum of –81 kPa (24 inHg) should be applied for at 

least 2 h.

Steaming—After the pressure period and once the creosote 

has been pumped back to the storage tank, a vacuum of not 

less than –74 kPa (22 inHg) is applied for at least 2 h to recover excess preservative. The vacuum is then released back 

to atmospheric pressure and the charge is steamed for 2 to 

3 h. The maximum temperature during this process should 

not exceed 116 °C (240 °F). A second vacuum of not less 

than –74 kPa (22 inHg) is then applied for a minimum of 

4 h.

The BMPs for copper naphthenate are similar to those for 

creosote and pentachlorophenol. The recommended treatment practices for treatment in heavy oil include using an 

expansion bath, or final steaming, or both, similar to that 

described for creosote. When No. 2 fuel oil is used as the 

solvent, the BMPs recommend using a final vacuum for at 

least 1 h.

Handling and Seasoning of Timber after Treatment

Treated timber should be handled with sufficient care to 

avoid breaking through the treated shell. The use of pikes, 

cant hooks, picks, tongs, or other pointed tools that dig 

deeply into the wood should be prohibited. Handling heavy 

loads of lumber or sawn timber in rope or cable slings can 

crush the corners or edges of the outside pieces. Breakage 

or deep abrasions can also result from throwing or dropping 

the lumber. If damage results, the exposed areas should be 

retreated, if possible.

Wood treated with preservative oils should generally be 

installed as soon as practicable after treatment to minimize 

lateral movement of the preservative, but sometimes cleanliness of the surface can be improved by exposing the treated 

wood to the weather for a limited time before installation. 

Lengthy, unsheltered exterior storage of treated wood before 

installation should be avoided. Treated wood that must be 

stored before use should be covered for protection from the 

sun and weather.

With waterborne preservatives, seasoning after treatment is 

important for wood that will be used in buildings or other 

places where shrinkage after placement in the structure 

would be undesirable. Injecting waterborne preservatives 

puts large amounts of water into the wood, and considerable 

shrinkage is to be expected as subsequent seasoning 

takes place. For best results, the wood should be dried to 
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Chapter 15 Wood Preservation

approximately the moisture content it will ultimately 

reach in service. During drying, the wood should be carefully piled and, whenever possible, restrained by sufficient 

weight on the top of the pile to prevent warping.

Quality Assurance for Treated Wood

Treating Conditions and Specifications

Specifications on the treatment of various wood products by 

pressure processes have been developed by AWPA. These 

specifications limit pressures, temperatures, and time of conditioning and treatment to avoid conditions that will cause 

serious injury to the wood. The specifications also contain 

minimum requirements for preservative penetration and 

retention levels and recommendations for handling wood 

after treatment to provide a quality product. Specifications 

are broad in some respects, allowing the purchaser some 

latitude in specifying the details of individual requirements. 

However, the purchaser should exercise great care so as not 

to hinder the treating plant operator from doing a good treating job and not to require treating conditions so severe that 

they will damage the wood.

Penetration and Retention

Penetration and retention requirements are equally important in determining the quality of preservative treatment. 

Penetration levels vary widely, even in pressure-treated material. In most species, heartwood is more difficult to penetrate than sapwood. In addition, species differ greatly in the 

degree to which their heartwood may be penetrated. Incising 

tends to improve penetration of preservative in many refractory species, but those highly resistant to penetration will 

not have deep or uniform penetration even when incised. 

Penetration in unincised heartwood faces of these species 

may occasionally be as deep as 6 mm (1/4 in.) but is often 

not more than 1.6 mm (1/16 in.).

Experience has shown that even slight penetration has some 

value, although deeper penetration is highly desirable to 

avoid exposing untreated wood when checks occur, particularly for important members that are costly to replace. 

The heartwood of coastal Douglas-fir, southern pines, and 

various hardwoods, although resistant, will frequently show 

transverse penetrations of 6 to 12 mm (1/4 to 1/2 in.) and 

sometimes considerably more.

Complete penetration of the sapwood should be the goal 

in all pressure treatments. It can often be accomplished in 

small-size timbers of various commercial woods, and with 

skillful treatment, it may often be obtained in piles, ties, and 

structural timbers. Practically, however, the operator cannot 

always ensure complete penetration of sapwood in every 

piece when treating large pieces of round material with thick 

sapwood (such as poles and piles). Therefore, specifications 

permit some tolerance. For instance, AWPA Processing and 

Treatment Standard T1 for Southern Pine poles requires 

that 89 mm (3.5 in.) or 90% of the sapwood thickness be 

penetrated for waterborne preservatives. The requirements 

vary, depending on the species, size, class, and specified 

retention levels.

Preservative retentions are typically expressed on the basis 

of the mass of preservative per unit volume of wood within 

a prescribed assay zone. The retention calculation is not 

based on the volume of the entire pole or piece of lumber. 

For example, the assay zone for Southern Pine poles is between 13 and 51 mm (0.5 and 2.0 in.) from the surface. To 

determine the retention, a boring is removed from the assay 

zone and analyzed for preservative concentration. The preservatives and retention levels listed in the AWPA Commodity Standards and ICC–ES evaluation reports are shown in 

Table 15–1. The current issues of these specifications should 

be referenced for up-to-date recommendations and other details. In many cases, the retention level is different depending on species and assay zone. Higher preservative retention 

levels are specified for products to be installed under severe 

climatic or exposure conditions. Heavy-duty transmission poles and items with a high replacement cost, such as 

structural timbers and house foundations, are required to be 

treated to higher retention levels. Correspondingly, deeper 

penetration or heartwood limitations are also necessary for 

the same reasons. It may be necessary to increase retention 

levels to ensure satisfactory penetration, particularly when 

the sapwood is either unusually thick or is somewhat resistant to treatment. To reduce bleeding of the preservative, 

however, it may be desirable to use preservative-oil retention levels less than the stipulated minimum. Older specifications based on treatment to refusal do not ensure adequate 

penetration or retention of preservative, should be avoided, 

and must not be considered as a substitute for results-type 

specification in treatment.

Inspection of Treatment Quality

AWPA standards specify how charges of treated wood 

should be inspected to ensure conformance to treatment 

standards. Inspections are conducted by the treating company and also should be routinely conducted by independent 

third-party inspection agencies. These third-party agencies 

verify for customers that the wood was properly treated in 

accordance with AWPA standards. The U.S. Department of 

Commerce American Lumber Standard Committee (ALSC) 

accredits third-party inspection agencies for treated-wood 

products. Quality control overview by ALSC-accredited 

agencies is preferable to simple treating plant certificates or 

other claims of conformance made by the producer without 

inspection by an independent agency. Updated lists of accredited agencies can be obtained from the ALSC website at 

www.alsc.org. Each piece of treated wood should be marked 

with brand, ink stamp, or end-tag that shows the logo of an 

accredited inspection agency and other information required 

by AWPA standards (Fig. 15–6). Other important information that should be shown includes the type of preservative, 

preservative retention, and the intended use category 
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General Technical Report FPL–GTR–190

(exposure condition). Purchasers may also elect to have an 

independent inspector inspect and analyze treated products 

to ensure compliance with the specifications—recommended for treated-wood products used for critical structures. Railroad companies, utilities, and other entities that 

purchase large quantities of treated timber usually maintain 

their own inspection services.

Effects on the Environment

Preservatives intended for use outdoors have mechanisms 

that are intended to keep the active ingredients in the wood 

and minimize leaching. Past studies indicate that a small 

percentage of the active ingredients of all types of wood 

preservatives leach out of the wood. The amount of leaching 

depends on factors such as fixation conditions, preservative retention in the wood, product size and shape, type of 

exposure, and years in service. Ingredients in all preservatives are potentially toxic to a variety of organisms at high 

concentrations, but laboratory studies indicate that the levels 

of preservatives leached from treated wood generally are too 

low to create a biological hazard.

In recent years, several studies have been conducted on preservative releases from structures and on the environmental 

consequences of those releases. These recent studies of the 

environmental impact of treated wood reveal several key 

points. All types of treated wood evaluated release small 

amounts of preservative components into the environment. 

These components can sometimes be detected in soil or 

sediment samples. Shortly after construction, elevated levels 

of preservative components can sometimes be detected in 

the water column. Detectable increases in soil and sediment 

concentrations of preservative components generally are 

limited to areas close to the structure. Leached preservative 

components either have low water solubility or react 

with components of the soil or sediment, limiting their 

mobility and limiting the range of environmental contamination. Levels of these components in the soil immediately 

adjacent to treated structures can increase gradually over the 

years, whereas levels in sediments tended to decline over 

time. Research indicates that environmental releases from 

treated wood do not cause measurable impacts on the abundance or diversity of aquatic invertebrates adjacent to the 

structures. In most cases, levels of preservative components 

were below concentrations that might be expected to affect 

aquatic life. Samples with elevated levels of preservative 

components tended to be limited to fine sediments beneath 

stagnant or slow-moving water where the invertebrate community is not particularly intolerant to pollutants.

Conditions with a high potential for leaching and a high 

potential for metals to accumulate are the most likely to affect the environment (Fig. 15–7). These conditions are most 

likely to be found in boggy or marshy areas with little water 

exchange. Water at these sites has low pH and high organic 

acid content, increasing the likelihood that preservatives 

will be leached from the wood. In addition, the stagnant water prevents dispersal of any leached components of preservatives, allowing them to accumulate in soil, sediments, and 

organisms near the treated wood. Note that all construction 

materials, including alternatives to treated wood, have some 

type of environmental impact. In addition to environmental 

releases from leaching and maintenance activities, the alternatives may have greater impacts and require greater energy 

consumption during production.

Recycling and Disposal of Treated 

Wood

Treated wood is not listed as a hazardous waste under Federal law, and it can be disposed of in any waste management 

facility authorized under State and local law to manage such 

material. State and local jurisdictions may have additional 

regulations that impact the use, reuse, and disposal of treated wood and treated-wood construction waste, and users 

should check with State and local authorities for any special 

regulations relating to treated wood. Treated wood must not 

be burned in open fires or in stoves, fireplaces, or residential boilers, because the smoke and ashes may contain toxic 

chemicals.

Treated wood from commercial and industrial uses (construction sites, for example) may be burned only in commercial or industrial incinerators or boilers in accordance with 

State and Federal regulations. Spent railroad ties treated 

with creosote and utility poles treated with pentachlorophenol can be burned in properly equipped facilities to generate 

electricity (cogeneration). As fuel costs and energy demands 

increase, disposal of treated wood in this manner becomes 

more attractive. Cogeneration poses more challenges for 

wood treated with heavy metals, and particularly for wood 

treated with arsenic. In addition to concerns with emissions, 

the concentration of metals in the ash requires further 

processing.

Figure 15–6. Typical end tag for preservative-treated 

lumber conforming to the ALSC accreditation 

program.
15–27

Chapter 15 Wood Preservation

As with many materials, reuse of treated wood may be a viable alternative to disposal. In many situations treated wood 

removed from its original application retains sufficient durability and structural integrity to be reused in a similar application. Generally, regulatory agencies also recognize that 

treated wood can be reused in a manner that is consistent 

with its original intended end use.

The potential for recycling preservative-treated wood depends on several factors, including the type of preservative 

treatment and the original use. Researchers have demonstrated that wood treated with heavy metals can be chipped 

or flaked and reused to form durable panel products or 

wood–cement composites. However, this type of reuse has 

not yet gained commercial acceptance. Techniques for extraction and reuse of the metals from treated wood have also 

been proposed. These include acid extraction, fungal degradation, bacterial degradation, digestion, steam explosion, or 

some combination of these techniques. All these approaches 

show some potential, but none is currently economical. In 

most situations landfill disposal remains the least expensive 

option. For treated wood used in residential construction, 

one of the greatest obstacles is the lack of an efficient process for collecting and sorting treated wood. This is less of a 

problem for products such as railroad ties and utility poles.

References

AWPA. [Current edition]. Annual proceedings. (Reports of 

preservations and treatment committees containing information on new wood preservatives considered in the development of standards). Birmingham, AL: American Wood Protection Association.

AWPA. 2008. Book of standards. (Includes standards on 

preservatives, treatments, methods of analysis, and inspection.) Birmingham, AL: American Wood Protection Association.

Archer, K.; Lebow, S.T. 2006. Wood preservation. Second 

edition. Primary wood processing, principals and practice, 

Walker, J.C.F. ed. The Netherlands: Springer. 596 p.

Baechler, R.H.; Roth, H.G. 1964. The double-diffusion 

method of treating wood: a review of studies. Forest Products Journal. 14(4): 171–178.

Baechler, R.H.; Blew, J.O.; Roth, H.G. 1962. Studies on the 

assay of pressure-treated lumber. Proceedings of American 

Wood Preservers’ Association. 58: 21–34.

Baechler, R.H.; Gjovik, L.R.; Roth, H.G. 1969. Assay zones 

for specifying preservative-treated Douglas-fir and Southern 

Pine timbers. Proceedings of American Wood Preservers’ 

Association. 65: 114–123.

Baechler, R.H.; Gjovik, L.R.; Roth, H.G. 1970. Marine 

tests on combination-treated round and sawed specimens. 

Proceedings of American Wood Preservers’ Association. 

66: 249–257.

Barnes, M.H., ed. 2007. Wood protection 2006. Publication 

No. 7229. Madison, WI: Forest Products Society. 388 p.

Blew, J.O.; Davidson, H.L. 1971. Preservative retentions 

and penetration in the treatment of white fir. Proceedings of 

American Wood Preservers’ Association. 67: 204–221.

Boone, R.S.; Gjovik, L.R.; Davidson, H.L. 1976. Treatment 

of sawn hardwood stock with double-diffusion and modified 

double-diffusion methods. Res. Pap. FPL–RP–265. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory.

Brooks, K.M. 2000. Assessment of the environmental 

effects associated with wooden bridges preserved with 

creosote, pentachlorophenol or chromated-copper-arsenate. 

Res. Pap. FPL–RP–587. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

100 p.

Cassens, D.L.; Johnson, B.R.; Feist, W.C.; De Groot, R.C. 

1995. Selection and use of preservative-treated wood. 

Publication N. 7299. Madison, WI: Forest Products Society.

Crawford, D.M; Woodward, B.M.; Hatfield, C.A. 2002. 

Comparison of wood preservative in stake tests. 2000 Progress Report. Res. Note FPL–RN–02. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Eaton, R.A.; Hale, M.D.C. 1993. Wood: decay, pests and 

protection. New York, NY: Chapman & Hall.

Forest Products Laboratory. Environmental impact of preservative treated wood in a wetland boardwalk. Res. Pap. 

FPL–RP–582. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 126 p.

Figure 15–7. Wood preservative leaching, environmental mobility, and effects on aquatic insects were evaluated at this wetland boardwalk in western Oregon.
15–28

General Technical Report FPL–GTR–190

Gaby, L.I.; Gjovik, L.R. 1984. Treating and drying composite lumber with waterborne preservatives: Part I. Short 

specimen testing. Forest Products Journal. 34(2): 23–26.

Gjovik, L.R.; Baechler, R.H. 1970. Treated wood foundations for buildings. Forest Products Journal. 20(5): 45–48.

Gjovik, L.R.; Davidson, H.L. 1975. Service records on 

treated and untreated posts. Res. Note FPL–068. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

Gjovik, L.R.; Johnson, D.B.; Kozak, V.; [and others]. 1980. 

Biologic and economic assessment of pentachlorophenol, 

inorganic arsenicals, and creosote. Vol. I: Wood preservatives. Tech. Bull 1658–1. Washington, DC: U.S. Department 

of Agriculture, in cooperation with State Agricultural Experimental Stations, Cooperative Extension Service, other state 

agencies and the Environmental Protection Agency.

Groenier, J.S.; Lebow, S. 2006. Preservative-treated wood 

and alternative products in the Forest Service. Tech. Rep. 

0677–2809–MTDC. Missoula, MT: U.S. Department of 

Agriculture, Forest Service, Missoula Technology and Development Center. 44 p.

Hunt, G.M.; Garratt, G.A. 1967. Wood preservation. Third 

edition. The American Forestry Series. New York, NY: McGraw–Hill.

ICC–ES. Evaluation Reports, Section 06070–wood 

treatment. Whittler, CA: ICC Evaluation Service, Inc. 

www.icc-es.org.

Johnson, B.R.; Gutzmer, D.I. 1990. Comparison of preservative treatments in marine exposure of small wood panels. 

Res. Note FPL–RN–0258. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory.

Lebow, S. 1996. Leaching of wood preservative components 

and their mobility in the environment—summary of pertinent literature. Gen. Tech. Rep. FPL–GTR–93. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

Lebow, S.T.; Foster, D.O. 2005. Environmental concentrations of copper, chromium and arsenic released from a chromated-copper-arsenate (CCA-C) treated wetland boardwalk. 

Forest Products Journal. 55(2): 62–70.

MacLean, J.D. 1952. Preservation of wood by pressure 

methods. Agric. Handb. 40. Washington, DC: U.S.

Department of Agriculture, Forest Service.

NFPA. 1982. The all-weather wood foundation. NFPA 

Tech. Rep. 7. Washington, DC: National Forest Products 

Association.

NFPA. 1982. All-weather wood foundation system, design 

fabrication installation manual. NFPA report. Washington 

DC: National Forest Products Association.

Richardson, B.A. 1993. Wood preservation. Second edition. 

London: Chapman and Hall. 226 p.

Schlultz, T.P.; Militz, H.; Freeman, M.H.; Goodell, B.; Nicholas, D.D.; eds. 2008. Development of commercial wood 

preservatives. ACS Symposium Series 982. Washington, 

DC: American Chemical Society. 655 p.

Western Wood Preservers Institute. 2006. Best management 

practices for the use of treated wood in aquatic environments. Vancouver, WA: Western Wood Preservers Institute. 

34 p.

Western Wood Preservers Institute. 2006. Treated wood in 

aquatic environments. Vancouver, WA: Western Wood Preservers Institute. 32 p.
CHAPTER 16

Finishing of Wood

R. Sam Williams, Supervisory Research Chemist (Retired)

16–1

Contents 

Factors Affecting Finish Performance 16–2

 Wood Surface Properties 16–2

 Wood Products 16–8

 Weathering 16–11

 Decay and Insects 16–13

 Control of Water and Water Vapor 16–13

 Compliance of VOC Finishes with Pollution 

 Regulations 16–15

Exterior Wood Finishes 16–16

 Weathered Wood as Natural Finish 16–16

 Penetrating Wood Finishes 16–16

 Film-Forming Finishes 16–21

Application of Finishes, Special Uses 16–24

 Porches, Decks, Deck Railings, and Fences 16–24

 Treated Wood 16–24

 Marine Uses 16–24

 Boats 16–25

 Panel Products 16–25

 Fire-Retardant Coatings 16–25

 Back-Priming 16–25

 Factory Finishing 16–25

Finish Failure or Discoloration 16–25

 Paint Cracking (parallel to grain) 16–26

 Peeling and Flaking 16–26

 Cross-Grain Cracking 16–26

 Water Blisters 16–26

 Mill Glaze 16–26

 Intercoat Peeling 16–27

 Chalking 16–28

 Mildew 16–28

 Loss of Gloss and Fading 16–29

 Water Soluble Extractives 16–29

 Blue Stain 16–30

 Iron Stain 16–30

 Knots 16–31

 Pitch 16–31

Finishing Interior Wood 16–32

 Type of Finish and Wood Fillers 16–32

 Finishes for Wood Floors 16–33

 Finishes for Items Used for Food 16–34

Wood Cleaners and Brighteners 16–34

Paint Strippers 16–35

 Mechanical Methods 16–35

 Chemical Methods 16–36

Lead-Based Paint 16–36

Literature Cited 16–37

Additional References 16–37

Wood finishes (paint, varnish, and stain, for example) give 

a desired appearance, protect wood surfaces, and provide 

a cleanable surface. Many people consider appearance

most important when choosing finishes for wood (lumber 

and wood composites). However, from a technical aspect, 

protection is most important for wood used outdoors, and 

providing a cleanable surface is most important for wood 

used indoors. When selecting a finish, one should consider 

appearance, protection, and cleanability and also how bulk 

and surface properties of wood affect finish application and 

performance (how long a finish lasts—its service life).

Wood properties such as density (specific gravity), growth 

rate, heartwood–sapwood, earlywood–latewood, grain 

angle, vessels, and texture vary within and across wood species. Wood composites, such as plywood, fiberboard, and 

oriented strandboard (OSB), have different properties. Of 

the 18,000 to 25,000 known wood species (exact number 

varies depending on the grouping of species), approximately 

50 are commercial species used in the United States and 

Canada. Chapters 2–4 give their properties. Of these commercial species, researchers report finishing characteristics 

for only a few species common to North America, Europe, 

Japan, and the tropics. However, if one understands how 

wood properties, finish, and environmental conditions interact, it should be possible to estimate finish performance for 

most wood species.

Performance depends on choosing an appropriate finish for 

wood, considering the use conditions, and applying finishes 

correctly in sufficient amount. For long service life, choose 

wood products and finishes appropriate for environments 

where they are used. Indoor use places less stress on finishes 

than outdoor use. A climate having severe seasonal changes 

(U.S. Upper Midwest, for example) places greater stress on 

finishes then does a mild climate (such as the Pacific Northwest).

Guidelines in this chapter explain how to obtain long service 

life for contemporary finishes on lumber and wood composites used in the United States and Canada. The chapter 

begins with a review of wood properties important for wood 

finishing and describes effects of water and weathering on 

wood and finishes. This background establishes a basis for 

describing finishes for wood, their application, and common 

types of finish failures (and ways to avoid them). Publications listed at the end of this chapter provide additional 

information.
16–2

Factors Affecting Finish 

Performance

Wood surface properties, type of wood product, and weather 

affect finish performance.

Wood Surface Properties

Wood anatomy, manufacturing processes, moisture content 

(MC), dimensional change, extractives, and changes as 

wood ages determine wood surface properties.

Anatomy

Wood species (thus its anatomy) is the primary factor that 

determines surface properties of wood—properties that affect adhesion and performance of finishes. Wood anatomy 

determines whether a wood species is a hardwood or softwood, not the density (specific gravity) or its hardness. 

Finish performance is affected by

•	 density (overall density, earlywood (EW)–latewood 

(LW) density difference, and how abruptly density 

changes at the EW–LW boundary),

•	 thickness of LW bands,

•	 ray cells (number and placement),

•	 vessels (size and location),

•	 extractives content, and

•	 growth rate (some species grow faster than others, 

and environment affects growth rate within a specific 

species).

Most wood cells (called tracheids in softwoods, fibers in 

hardwoods) align parallel (axial) to the stem or branch. 

Softwood tracheids support the tree and transport water 

and nutrients. Hardwood fibers just support the tree; hardwoods have special cells (vessels) for transporting water 

and nutrients. Vessel cells are open at each end and stacked 

to form “pipes.” Axial tracheids and fibers are hollow tubes 

closed at each end. In softwoods, liquids move in the axial 

direction by flowing from one tracheid to another through 

openings called pits. Liquid transport between the bark and 

center of the stem or branch in hardwoods and softwoods is 

by ray cells. Figures 16–1 to 16–3 are micrographs showing the orientation of axial and ray cells for white spruce, 

red oak, and red maple, respectively. Note that the softwood (Fig. 16–1) has no vessels. The large openings are 

resin canal complexes (common to spruce, pine, larch, and 

Douglas-fir). Figure 16–2 shows red oak, a ring-porous 

hardwood. Large-diameter vessels in ring-porous species 

form along with EW; later in the growing season, the vessels have smaller diameters. Figure 16–3 shows red maple, a 

diffuse-porous hardwood; small vessels having similar size 

form throughout the EW and LW. Hardwoods can also be 

semi-ring porous.

Axial and ray cells form in the cambium, a layer of cells just 

under the bark. In the early part of the growing season 

General Technical Report FPL–GTR–190

Figure 16–1. Micrograph of white spruce showing 

gradual transition of cell wall thickness and resin canal 

complexes. (Micrographs prepared by H.A. Core, W.A. 

Côté, and A.C. Day. Copyright by N.C. Brown Center for 

Ultrastructure Studies, College of Environmental Science and Forestry, State University of New York, Syracuse, New York. Used with permission.)

Figure16–2. Micrograph of red oak showing ringporous vessels. (Micrographs prepared by H.A. Core, 

W.A. Côté, and A.C. Day. Copyright by N.C. Brown Center for Ultrastructure Studies, College of Environmental 

Science and Forestry, State University of New York, 

Syracuse, New York. Used with permission.)
16–3

Chapter 16 Finishing of Wood

(temperate species), the cells have large open centers (lumens) and thin cell walls. This is earlywood (also called 

springwood). As the growing season progresses, cell walls 

become thicker, forming latewood (also called summerwood). The combination of EW–LW (and vessels in hardwoods) gives annual growth rings. The properties of these 

growth rings affect the ease with which finishes can be applied (paintability) and how long finishes last (service life).

Cross-section micrographs of three softwoods and hardwoods (Fig. 16–4) show three types of growth characteristics. Softwoods may show “no transition” (no EW–LW 

boundary, Fig. 16–4a), gradual transition (Fig. 16–4b), or 

abrupt transition (Fig. 16–4c). Note: the “no transition” 

softwood is a tropical species (that is, no seasons, therefore 

no EW–LW transition). Hardwoods may be diffuse porous 

(Fig. 16–4d), semi-ring porous (Fig. 16–4e), or ring porous 

(Fig. 16–4f). As a first approximation for explaining finishing characteristics of wood, the various wood species can be 

grouped into three categories:

•	 Easy to finish (“no transition” or gradual-transition softwoods and diffuse-porous hardwoods)

•	 Moderately easy to finish (abrupt-transition softwoods 

having narrow LW bands and semi-ring-porous 

hardwoods)

•	 Difficult to finish (abrupt-transition softwoods having 

wide LW bands and ring-porous hardwoods)

The important message from wood anatomy is to look at 

the wood. The six micrographs showing end-grain woodcell structure do not include all possible combinations of 

growth rate, grain, and surface texture. When determining 

paintability, look at grain angles. Look at the width of the 

LW bands and the transition between them (Fig. 16–5). The 

blocks show radial and tangential surfaces (that is, verticaland flat-grain surfaces for six softwoods and quarter-sawn 

and flat-sawn for two hardwoods). Note the abrupt transitions on the southern yellow pine and Douglas-fir and the 

gradual transitions on the western redcedar and white pine. 

Also, note the growth rate and width of the LW bands. Surfaces having abrupt transition, rapid growth rate, and wide 

LW bands are difficult to finish, particularly on flat-grain 

wood. Moisture-induced dimensional change increases as 

wood density increases. Changes are greater for LW than 

EW. Different dimensional change for abrupt-transition (or 

ring-porous) species at the EW–LW boundary places stress 

on coatings.

Shrinkage values given in Table 16–1 were obtained from 

drying wood from its green state (fiber saturation) to ovendry (0% MC); swelling rates would be approximately the 

same. Some species have wide bands of EW and LW. These 

distinct bands often lead to early paint failure. Wide, prominent bands of LW are characteristic of the southern yellow 

pines, radiata pine, and Douglas-fir (Fig. 16–5a,b,c), and 

getting good paint performance is more difficult on these 

species. In contrast, white pine, redwood, and western 

redcedar (Fig. 16–5d,e,f) do not have wide LW bands, and 

these species give excellent paint performance. Diffuseporous hardwoods such as aspen (Fig. 16–5g) have a fine 

surface texture and are easy to finish, whereas red oak 

(Fig. 16–5h) has a highly textured surface and requires 

surface preparation prior to finishing.

Figure 16–3. Micrograph of red maple showing diffuseporous vessels. (Micrographs prepared by H.A. Core, 

W.A. Côté, and A.C. Day. Copyright by N.C. Brown 

Center for Ultrastructure Studies, College of Environmental Science and Forestry, State University of New 

York, Syracuse, New York. Used with permission.)

Figure 16–4. Cross-section micrographs of (A) a tropical softwood (Podocarpus imbricate), (B) white spruce 

(Picea glauca), (C) Douglas-fir, Pseudotsuga menziesii 

(D) sugar maple (Acer saccharum), (E) persimmon 

(Diospyros virginiana), and (F) white ash (Fraxinus 

americana). The arrows show a single growth year for 

the temperate species.
16–4

Fifty years ago, most exterior siding and trim were verticalgrain heartwood of western redcedar or redwood. All-heartwood vertical-grain grades of these species are still available as resawn bevel siding and lumber and are excellent 

wood substrates for painting (Table 16–1). Other species are 

generally available only as flat-grain or a mix of flat- and 

vertical-grain lumber (for example, western hemlock, eastern white pine, lodgepole pine, eastern white cedar, radiata 

pine, and southern yellow pine). Finishing characteristics of 

flat-grain western redcedar and redwood are similar to other 

low-density wood species having moderate EW–LW transition (such as eastern white pine, eastern white cedar, and 

yellow poplar) Sawing to yield vertical grain is only practical with fairly large-diameter logs. Species available 

in small-diameter logs yield mostly flat-grain lumber.

Other wood properties, such as knots, juvenile wood, and 

extractives, affect wood finishing. Extractives include many 

chemicals with different solubilities in water, organic solvents, and paint resins (also called binders).

Manufacturing

The axial EW and LW cells in a log yield lumber of various grain angles (Fig. 16–6). At one extreme (board a), the 

growth rings are perpendicular to the plane of the board; at 

the other extreme (board c), growth rings are parallel to the 

plane of the board (although they have an arc). Grain varies between these two extremes. Vertical-grain lumber has a 

grain angle from 90° (growth rings perpendicular to surface) 

to approximately 45°. From 45° to the other extreme (board 

c), lumber is considered flat grain. Board b is different. 

Lumber cut close to the pith (the center of the log) contains 

abnormal wood cells. These abnormal cells are juvenile 

wood and have extremely high longitudinal dimensional 

change (2%) compared with normal wood (0.1–0.2%). The 

values are the change from green to ovendry (see Chap. 4). 

A 10-ft (3-m) board could shrink 2.4 in. (61 mm). This 

dimensional instability leads to severe warping and crossgrain checking in lumber containing juvenile wood (see 

Chap. 5).

The bark side and pith side of flat-grain or flat-sawn lumber have slightly different properties. The pith side is more 

prone to have raised grain than the bark side, particularly 

with abrupt-transition wood species (southern yellow pine, 

Douglas-fir, and oak (Table 16–1)). The bark side tends to 

check more, and the checking is more pronounced in the 

LW bands.

General Technical Report FPL–GTR–190

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 16–5. Wide LW bands characteristic 

of (a) the southern yellow pines, (b) radiata 

pine, and (c) Douglas-fir and narrow LW 

bands characteristic of (d) white pine, (e) 

redwood, and (f) western redcedar; (g) and 

(h) are examples of the difference in surface 

texture between diffuse-porous and ring 

porous hardwoods, respectively; (e) shows 

examples of second or third growth (left) 

and old-growth (right) redwood.
16–5

Chapter 16 Finishing of Wood

Table 16–1. Painting characteristics of common wood species

Wood species 

Specific 

gravitya

(green/dry) 

Shrinkage (%)b

Paintabilityc

(latex paint) 

EW/LW 

transitiond

Is LW 

greater than 

about 1/3 of 

GRe

Color of 

Tangential Radial heartwood 

Softwoods 

Baldcypress 0.42/0.46 6.2 3.8 II A No Light brown 

Cedars I 

Incense 0.35/0.37 5.2 3.3 I G No Brown 

Northern white 0.29/0.31 4.9 2.2 I G No Light brown 

Port-Orford 0.39/0.43 6.9 4.6 I G No Cream 

Western red 0.31/0.32 5 2.4 I G No Brown 

Alaska yellow 0.42/0.44 6 2.8 I G No Yellow 

Douglas-firf,g 0.45/0.48 7.6 4.8 III A Yes Pale red 

Pines 

Eastern white 0.34/0.35 6.1 2.1 I G No Cream 

Ponderosa 0.38/0.42 6.2 3.9 II A Yes/No Cream 

Southernh 0.47/0.51h 8 5 III A Yes Light brown 

Western white 0.36/0.38 7.4 4.1 I G No Cream 

Radiata 0.45/0.53 7.0 4.2 III A Yes/No Cream 

Redwoodi 0.38/0.40 4.4 2.6 I A No Dark brown 

Sprucej 0.33/0.35 7.1 3.8 I G No White 

Tamarack/larch 0.49/0.53 7.4–9.1 3.7–4.5 II A Yes/No Brown 

True fir 0.37/0.39 7.0 3.3 I G No White 

Western hemlock 0.42/0.45 7.8 4.2 II G/A Yes/No Pale brown 

Hardwoods 

Red alder 0.37/0.41 7.3 4.4 I D NA Pale brown 

Ash 0.55/0.60 8 5 III R Yes Light brown 

Aspen/cottonwood 0.36/0.40 7.0–9.2 3.5–3.9 I D NA Pale brown 

Basswood 0.32/0.37 7.8 5.9 I D NA Cream 

Beech 0.56/0.64 11.9 5.5 I D NA Pale brown 

Birch 0.55/0.62 9.5 7.3 I D NA Light brown 

Butternut 0.36/0.38 6.4 3.4 II SR Yes Light brown 

Cherry 0.47/0.50 7.1 3.7 I D NA Brown 

Chestnut 0.40/0.43 6.7 3.4 III R Yes Light brown 

Elm, American 0.46/0.50 9.5 4.2 III R Yes Brown 

Hickory 0.64/0.72 11 7 III R Yes Light brown 

Maple, sugar 0.56/0.63 9.9 4.8 I D NA Light brown 

Oaks 

White oak group 0.60/0.68 8.8 4.4 III R Yes Brown 

Red oak group 0.56/0.63 8.6 4.0 III R Yes Brown 

Sweetgum 0.46/0.52 10.2 5.3 I D NA Brown 

Sycamore 0.46/0.49 8.4 5 I D NA Pale brown 

Walnut 0.51/0.55 7.8 5.5 II SR Yes Dark brown 

Yellow-poplar 0.40/0.42 8.2 4.6 I D NA Pale brown 

a

Specific gravity based on weight ovendry and volume at green or 12% moisture content. b

Dimensional change obtained by drying from green to ovendry. Values reported here are averages from a variety of sources and are provided 

for comparative purposes. For more specific values, see Chapter 4. c

I, easy to finish; III, difficult to finish. 

d

A, abrupt-transition softwood; G, gradual-transition softwood; R, ring-porous hardwood; D, diffuse-porous hardwood; SR, semi-ring-porous 

hardwood. 

e

GR, growth ring; NA, not applicable; yes/no, depends on the specimen. In ring-porous hardwoods, the growth rate (number of rings per 

centimeter or inch) will determine the relative proportions of earlywood and latewood. f

Lumber and plywood. 

g

Coastal Douglas-fir. 

h

Loblolly, shortleaf; specific gravity of 0.54/0.59 for longleaf and slash. 

i

Redwood is listed as paintability “I” because its LW band is very narrow. 

j

Spruce. Values are for Engelmann spruce; other species are similar.
Moisture content

Moisture content (MC) is the amount of water (in any of 

its forms) contained in wood (see Chap. 4). MC includes 

water or water vapor absorbed into cell walls and free water 

within the hollow center of the cells (lumina); it is expressed 

as weight percentage. The amount of water vapor wood can 

absorb, depends on wood species; most species can absorb 

water vapor to increase their mass approximately 30% 

above an ovendry MC condition. This water is hydrogen 

bound within the cell wall matrix of hemicelluloses and, 

to some extent, cellulose. The limit to the amount of water 

bound in the wood cell wall is the fiber saturation point.

The amount of water vapor wood absorbs depends on the 

relative humidity (RH) of the surrounding air. If wood is 

stored at 0% RH, the MC will eventually approach 0%. If 

wood is stored at 100% RH, the MC will eventually reach 

fiber saturation (approximately 30% moisture). Of course, 

if kept at a constant RH between these two extremes, wood 

will stabilize at a MC between 0% and 30%. The RH controls the MC, and when the MC is in balance with the RH, 

the wood is at its equilibrium moisture content (EMC). This 

rarely happens because as the RH changes, so does the MC 

of the wood, and atmospheric RH is continually changing. 

It varies through daily and seasonal cycles, thus driving the 

MC of wood through daily and seasonal cycles. See Chapter 

4 for more information on MC and EMC.

Finishes cannot change EMC; they affect only the rate at 

which absorption and desorption occur (see MoistureExcluding Effectiveness).

Wood outdoors in most areas of the United States cycles 

around a MC of approximately 12% to 14%. In the Pacific Northwest, average MC can be slightly higher (12% 

to 16%), and in the Southwest, slightly lower (6% to 9%) 

(Chap. 13, Tables 13–1 and 13–2). Daily and annual MC 

may vary from these averages. In general, wood outdoors 

decreases MC during the summer and increases MC during 

the winter. (Wood indoors in northern climates increases 

MC during the summer and decreases MC during the winter. 

In the south, this distinction is not clear because air conditioning affects indoor RH and thus MC.) Even in humid 

areas, RH is rarely high enough for a long enough period 

to bring the MC of wood above 20%. Wood warmed by the 

sun experiences a virtual RH far below the ambient RH. The 

surface dries faster than the rest of the lumber. This is why 

cupping and checking often occur on decking boards; the 

top surface is much drier than the rest of the board. Shrinkage of the top surface commensurate with this dryness 

causes cupping and checking parallel to the grain. (Juvenile 

wood often checks perpendicular to the grain.)

As mentioned, fiber saturation is the limit to the amount of 

water vapor that wood absorbs. Water vapor absorbs slowly 

compared with liquid water. Liquid water can quickly bring 

General Technical Report FPL–GTR–190

16–6

General Technical Report FPL–GTR–190

Figure 16–6. Lumber grain affects finish performance: (a) edge-grain (vertical-grain or quartersawn) board; (b) edge-grain board containing pith; 

(c) flat-grain (slash-grain or plain-sawn) board. 

Arrows show radial (R), tangential (T), and 

longitudinal (L) orientation of wood grain.

Moisture

The chemical commonly called water (H2O) has three 

states according to temperature and pressure conditions: 

gas (water vapor or steam), liquid (water), or solid (ice). 

When water interacts with wood, it can occur in a fourth 

state (bound water). Moisture is not one of the states of 

water; it is a term with the power to indicate uncertainty 

about the water’s state, or to refer collectively to water in 

all its states in wood. For example, some of the moisture in 

a board at 50% moisture content will occur as liquid water 

(or ice, depending on the temperature) within cell cavities 

of the wood, some will occur as water vapor, and some will 

be bound water (bound within cell walls). Moisture thus accounts for any or all of these states in a single word. In this 

chapter, the term water designates water in its liquid state.
wood to fiber saturation, and it is the only way to bring the 

MC of wood above fiber saturation. As wood continues to 

absorb liquid water above its fiber saturation point, the water is stored in the lumen; when water replaces all the air in 

the lumen, the wood is waterlogged and its MC can be as 

high as 200%.

Wood can get wet many ways (such as windblown rain, 

leaks, condensation, dew, and melting ice and snow). The 

result is always the same—poor performance of wood and 

finish. Water is usually involved if finishes perform poorly 

on wood. Even if other factors initially cause poor performance, water accelerates degradation. Fortunately, the MC 

of lumber can be controlled. However, all too often, this 

critical factor is neglected during construction and finishing.

Paint wood when its average MC is about that expected to 

prevail during its service life (approximately 12% for most 

of the United States and Canada). Painting wood after it 

acclimates to a MC commensurate with the environment 

minimizes stress on film-forming finishes. The MC and thus 

the dimensions of the piece will still fluctuate somewhat, 

depending on the cyclic changes in atmospheric RH, but 

the dimensional change should not be excessive. Therefore, 

film-forming finishes (such as paints) are not stressed and 

should not fail by cracking.

Most siding and trim is kiln dried to less than 20% MC 

before shipment, and if it has been kept dry during shipment and storage at the construction site, it should be close 

to EMC by the time it is finished. If wood gets wet during 

shipping or storage or at the construction site, a MC of less 

than 20% is not likely. If wet wood is used, it will dry in 

service and shrinkage may cause warping, twisting, and 

checking. If the MC of wood exceeds 20% when the wood 

is painted, the risk of blistering and peeling is increased. 

Moreover, water-soluble extractives in species such as 

redwood and western redcedar may discolor paint.

Plywood, particleboard, hardboard, and other wood composites change MC during manufacture. Frequently, the MC of 

these materials is not known and may vary depending on the 

manufacturing process. As with other wood products, condition wood composites prior to finishing.

Dimensional Change

Dimensional change depends on wood species and varies 

within a particular species. Average shrinkage values obtained by drying wood from its green state to ovendry vary 

from 2.4% for radial western redcedar to 11.9% for tangential beech (Table 16–1). Dimension in service does not vary 

to this extent because the MC seldom goes below 6% (Chap. 

13, Table 13–1). A film-forming finish would likely decrease 

this range, but only if the end grain is sealed; unsealed end 

grain increases MC of painted wood (see Moisture Excluding Effectiveness).

Wood having little tendency to shrink and swell gives a 

stable surface for painting. Vertical-grain surfaces are more 

stable than flat-grain surfaces (Table 16–1, Fig. 16–6), especially outdoors where periodic wetting may produce rapid 

dimensional change. Wood species having low specific 

gravity tend to be more dimensionally stable than those having high specific gravity (Fig. 16–7). Low-specific-gravity 

wood species (that is, those that are more dimensionally 

stable) hold paint better than high-specific-gravity wood 

species; however, other factors, such as wood anatomy and 

manufacturing, also affect paint adhesion.

Wood Extractives

Highly colored extractives occur in heartwood of softwoods 

such as western redcedar and redwood and hardwoods such 

as walnut and mahogany. Extractives give heartwood its 

color, and many extractives are soluble in water. Discoloration of painted or unpainted wood may occur when rain 

leaches water-soluble extractives from wood. (If indoors, 

plumbing leaks or high RH can also cause it.) The water 

carries extractives to wood or paint surfaces and evaporates, 

leaving extractives as a yellow to reddish brown stain on the 

surface. Some paints, such as oil-alkyd stain-blocking primers, block leaching of water-soluble extractives.

Wood also contains compounds (resins and oils) that are insoluble in water. Species and growing conditions determine 

the type and amount of these compounds. For example, 

many pines contain pitch, and knots of almost all species 

contain sufficient oils and resins to discolor light-colored 

paint. These oils and resins are similar chemically to oilalkyd paints; therefore, oil-alkyd stain-blocking primers 

cannot block them. Latex-based formulations are also ineffective (see Knots and Pitch).

16–7

Chapter 16 Finishing of Wood

0

2

4

6

8

10

12

14

0.00 0.20 0.40 0.60 0.80

Dry specific gravity

Shrinkage vs dry specific gravity

Shrinkage fro

m

Green to

OD (%)

Figure 16–7. Plots of radial (�) and tangential (♦) 

shrinkage from green to oven dry (OD) as a function of 

specific gravity for various hardwoods and softwoods 

from Table 16–1. Lines show least-squares fit.
Shellac (a natural product made from the secretion of 

lac-producing insects such as Kerria lacca) and specially 

formulated synthetic finishes block extractives bleed from 

knots. Use shellac or synthetic knot sealers only over knots 

and paint over them to protect them from water. Blocking 

diffusion of extractives from knots is difficult, and no easy 

fix is available other than the extra step of sealing knots 

before priming. By doing this extra step, you can minimize 

discoloration of white paint on knotty pine—but it is not 

easy. If you want white, use knot-free wood. Difficulty sealing knots is the main reason manufacturers cut out the knots 

to make fingerjointed/edge-glued lumber.

Another option for knots is to use them to accentuate the 

wood. Use a stain to bring out the color and make the knots 

a part of the desired appearance.

Wood Products

Six types of wood products are commonly used on the exterior of structures: (1) lumber, (2) plywood, (3) fingerjointed 

wood, (4) reconstituted wood products (such as hardboard 

and oriented strandboard (OSB)), (5) wood–plastic composites, and (6) preservative- or fire-retardant-treated wood. 

Each product has unique characteristics that affect application and performance of finishes.

Lumber

Lumber (such as siding, trim, and decking) for exterior use 

is available in many species and products, and several publications describe grades:

•	 “Standard Grading Rules for West Coast Lumber,” West 

Coast Lumber Inspection Bureau, Portland, Oregon

•	 “Standard Grading Rules for Canadian Lumber,” National Lumber Grades Authority, New Westminster, 

British Columbia

•	 “Western Lumber Grading Rules,” Western Wood Products Association, Portland Oregon

•	 “Standard Grading Rules for Northeastern Lumber,” 

Northeastern Lumber Manufacturers Association, Cumberland Maine

•	 “Standard Grading Rules,” Northern Softwood Lumber 

Bureau, Cumberland Maine

•	 “Standard Specifications for Grades of California Redwood Lumber,” Redwood Inspection Service, Pleasant 

Hill, California

•	 “Standard Grading Rules for Southern Pine Lumber,” 

Southern Pine Inspection Bureau, Pensacola Florida

These publications are the basis for selecting wood to meet 

codes. They give specifications for appearance grades (such 

as siding and trim) and for structural lumber (such as framing and decking). Western redcedar and redwood are the 

only species available in vertical-grain grades and sawtextured surfaces (Table 16–1). Southern yellow pine and 

Douglas-fir plywood are available in saw-textured surfaces. 

Unless specified as vertical grain, the grade contains mostly 

flat-grain lumber. Lumber used for board and batten, drop, 

or shiplap siding is frequently flat grain. Bevel siding is 

commonly produced in several grades. The highest grade of 

redwood and western redcedar bevel siding is vertical grain 

and all heartwood. Other grades of redwood and western 

redcedar may be flat, vertical, or mixed grain and may not 

be all heartwood. Grade is important because species, grain 

orientation, and surface texture affect paint-holding characteristics.

Descriptions of grades and pictures of many wood species 

are contained in brochures published by trade associations 

(such as Western Red Cedar Lumber Association, California 

Redwood Association, Western Wood Products Association, Southern Forest Products Association, and Northeast 

Lumber Manufacturing Association) (Fig. 16–8), and these 

brochures reference the grade rules. When specifying lumber, refer to the grade rules for the product to ensure that the 

product meets code requirements and use the association 

brochures to get an idea of appearance.

Textures (roughness or smoothness) of wood surfaces affect selection, application, and service life of finishes. Until 

recently, a general rule of thumb for matching substrates to 

finishes was to paint smooth wood and stain saw-textured 

wood. This easy rule of thumb no longer applies. Although 

General Technical Report FPL–GTR–190

16–8

General Technical Report FPL–GTR–190

Figure 16–8. Examples of trade association brochures 

describing wood products.
penetrating finishes such as solvent-borne oil-based semitransparent stains last longer on saw-textured wood than on 

smooth-planed wood, many film-forming finishes such as 

opaque stains and paints also last longer on saw-textured 

wood than on smooth-planed wood. Finishes adhere better, 

film buildup is thicker, and service life of the finish is longer 

on saw-textured surfaces than smooth-planed surfaces, particularly for flat-grain lumber.

Plywood

As with lumber, species, grain orientation, and surface texture affect finishing of plywood. Manufacturers of softwood 

plywood use a lathe to peel logs to give flat-grain veneer. 

Peeling causes small checks parallel to grain. When the 

face veneer is laid up to form the plywood panel, the side 

of the veneer having lathe checks is placed interior to give 

a surface free of checks. However, after plywood is placed 

outdoors, wet–dry cycles (swelling and shrinking) cause 

the checks to propagate to the surface (face checking). Face 

checking sometimes extends through paint coatings to 

detract from the appearance and durability of the paint 

(Fig. 16–9).

Veneer produced by peeling gives flat-grain plywood and 

it is commonly available with a saw-textured, abrasively 

planed (smooth), or paper overlay surface. Douglas-fir and 

southern yellow pine are available saw-textured (Table 

16–1). Saw-textured plywood holds paint much better than 

does smooth plywood. If smooth plywood is to be painted, 

scuff-sand it with 50-grit sandpaper and use high-quality 

latex paint. Latex primer and top-coat generally perform 

better than oil-alkyd paint. Paint performs poorly on smooth 

plywood if used as siding but reasonably well on smooth 

plywood in protected areas such as soffits. Resin-treated 

paper bonded to plywood forms a medium-density overlay 

(MDO); MDO eliminates cracks caused by lathe checking 

and provides plywood with excellent paintability, but the 

edges are still vulnerable to water. Seal the edges with oilalkyd primer or an edge sealer formulated for this use. Paper 

over-laid products should not be finished with semitransparent stain or other penetrating finishes. Use film-forming 

finishes such as paints or solid-color stains and ensure sufficient film thickness (0.004–0.005 in. (0.10–0.13 mm), or 

4–5 mils).

APA–The Engineered Wood Association (Tacoma, Washington) provides information on plywood grades and standards 

(see Chap. 11).

Fingerjointed Lumber

To obtain “knot free” lumber, mills produce lumber that 

consists of many small pieces of wood edge-glued and fingerjointed at the end-grain (see Chaps. 10 and 12). Although 

fingerjointed lumber contains no knots or other obvious defects, most mills do not sort wood pieces prior to gluing to 

give lumber with similar grain orientation and heartwood–

sapwood content. A particular board may contain pieces 

from different trees, and each piece may have different 

finishing characteristics; therefore, finishing requirements 

are determined by the most difficult-to-paint component in a 

fingerjointed board. Fingerjointed lumber is commonly used 

for fascia boards, interior and exterior trim, siding, windows, and doors. Paint often fails in a “patchwork” manner 

according to the paintability of various pieces. The board 

pictured in Figure 16–10 shows extractive bleed on the component to the right, but not on the component to the left.

Some manufacturers decrease variability in fingerjointed 

lumber. For example, fingerjointed redwood siding is available in clear all-heart vertical grain and clear flat grain.

Finishing fingerjointed lumber requires care to ensure consistent finish performance on the whole board. To hide color 

16–9

Chapter 16 Finishing of Wood

Figure 16–9. Early paint failure on plywood caused by 

penetration of moisture into surface face-checks.

Figure 16–10. Differences in stain from extractives on 

fingerjointed wood from the white pine group (either 

eastern or western species) painted with acrylic solidcolor stain.
differences of the various pieces, use opaque finishes rather 

than natural finishes (such as semitransparent stain). As with 

other wood products, planed surfaces should be scuff-sanded with 50-grit sandpaper prior to priming. Saw-textured 

lumber should hold paint better than planed lumber.

Particleboard and Similar Reconstituted Wood 

Products

Reconstituted wood products are made by forming small 

pieces of wood into large sheets; sheets are cut into 1.2- by 

2.4-m (4- by 8-ft) panel products or other sizes such as siding. These products are classified as particleboard or fiberboard, depending upon the nature of the wood component 

(see Chap. 11).

Particleboard is made from splinters, chips, flakes, strands, 

or shavings. Flakeboard is a type of particleboard made 

from large flakes or shavings. Oriented strandboard (OSB) 

is a refinement of flakeboard; the flakes have a large lengthto-width aspect ratio and are laid down in three layers, with 

the flakes in each layer oriented 90° to each other as are 

veneers in plywood (see Chap. 11). Most OSB is used inside 

the external envelope of structures for sheathing and underlayment, however it contains “exterior” adhesives and water 

repellent. The water repellent gives OSB water resistance 

while in transit and storage prior to construction. The water 

repellent does not decrease paint adhesion.

Lumber characteristics, such as grain orientation, specific 

gravity, grain boundary transition, warping, and splitting, 

are not considerations with particleboard, but paint applied 

directly to particleboard performs poorly. Differential dimensional change of surface flakes causes telegraphing, and 

paint usually cracks and peels (Fig. 16–11). Telegraphing is 

the formation of an uneven paint surface caused by swelling 

of flakes and particles under the paint. Telegraphing occurs 

on all types of particleboard, but not on fiberboard. Adhesive failure leads to loss of flakes from the surface. Figure 

16–11 shows painted flakeboard after 3 years outdoors. The 

area on the left has one coat of acrylic-latex top-coat and 

the area on the right has one coat of oil-alkyd primer and 

acrylic-latex top-coat. The single coat (top-coat only) has 

failed, and the area having two-coats (primer and top-coat) 

is starting to fail, particularly over large flakes. Products 

intended for outdoor use, such as siding, are overlaid with 

MDO or wood veneer to improve paint performance. Products having MDO can be finished in the same way as other 

paper-overlaid products. Seal edges with a product specifically formulated for this use, and apply an oil-alkyd primer 

to give additional water resistance (see Plywood).

When finishing particleboard that does not have a paper 

overlay, use a three-coat latex paint system on the surface 

and seal edges as described above. However, do not expect 

long-term paint performance.

When particleboard or OSB, without an overlay, is used 

outdoors, it requires a rigorous maintenance schedule (often 

every 6 to 12 months).

Mechanical pulping produces wood fibers that are dry- or 

wet-formed into fiberboard (Chap. 11). Hardboard is a dense 

fiberboard often used for exterior siding. Hardboard is available in 152- to 203-mm (6- to 8-in.) widths as a substitute 

for solid-wood beveled siding. The surface of fiberboard accepts and holds paint well, and MDO improves paintability. 

As with particleboard, seal edges with oil-alkyd primer or 

other suitable sealer.

Wood–Plastic Composites

Wood-plastic composites (WPCs) account for approximately one-fourth of wood decking. Manufacturers combine 

wood flour, fibers, particles, or a combination, with polyethylene, polyvinyl chloride, or polypropylene and extrude 

“boards” in various profiles. Wood content and particle size 

in the boards vary and thus their ability to accept a finish 

varies. Boards high in wood content with large particle size 

may accept a finish; boards high in plastic content may not. 

Finish a small area to ensure the finish will wet the surface. 

After the finish cures, check adhesion using the tape pull-off 

test (see Chalking). Plastics are routinely finished in industrial applications, such as car parts, by activating the plastic 

surface using flame or plasma. This technology is not used 

on WPCs for the construction industry, because most manufacturers do not expect their products to be finished.

Treated Wood

Wood used in structures fully exposed to the weather, such 

as in decks and fences (particularly those portions of the 

structure in ground contact), needs preservative treatment to 

protect it from decay (rot) and termites. Wood used in marine exposure also requires preservative treatment to protect 

it from decay and marine borers. For some uses, building 

codes may require treatment of wood with either preservative or fire-retardant, or both.

Wood is pressure-impregnated with three types of preservatives: (a) preservative oils (such as coal-tar creosote), (b) 

organic solvent solution (such as pentachlorophenol), and 

16–10

General Technical Report FPL–GTR–190

Figure 16–11. Absorption of water causes differential 

dimensional change of surface flakes to give an uneven surface (telegraphing).
(c) waterborne salts (such as copper quaternary ammonium 

complexes, copper azole, and chromated copper arsenate 

(CCA)) (Chap. 15). Note: Except for the all-wood 

foundation, CCA-treated wood is not used in residential 

construction.

Components for doors and windows are usually dip-treated 

with a water-repellent preservative (WRP). The American 

Wood Protection Association sets standards for pressureimpregnated and dip-preservative treatment of wood 

(AWPA 2008).

Wood treated with waterborne preservatives, such as copper-based systems, can be painted or stained if the wood 

is clean and dry. Bleed of preservative through finishes, 

particularly latex-based paints and solid-color stains, can 

occur if wood is still wet from the preservative treatment. 

Allow wood to dry before painting; 1 week should be sufficient. Wood treated with coal-tar creosote or other dark oily 

preservatives is not paintable, except with specially formulated finishes such as two-component epoxy paints; even if 

the paint adheres to the treated wood, the dark oils tend to 

discolor paint, especially light-colored paint. Wood treated 

with a water-repellent preservative, by vacuum-pressure or 

dipping, is paintable.

Fire-retardant- (FR-) treated wood is generally painted rather than left unfinished because the FR treatment may darken 

or discolor wood. FR treatment does not generally interfere 

with adhesion of finishes; however, you should contact the 

paint manufacturer, the FR manufacturer, and the treating 

company to ensure that the products are compatible. Some 

fire retardants may be hydroscopic and cause wood to have 

high MC. FRs for wood used outdoors are formulated to 

resist leaching.

Weathering

Weathering is the general term describing outdoor degradation of materials and manifests itself physically and 

chemically (for example, cracking and exfoliation of rock, 

corrosion of metals, and photodegradation of organic materials). Ultraviolet (UV) radiation in sunlight catalyzes 

photodegradation of organic materials exacerbated by moisture, temperature change, freeze–thaw cycles, abrasion by 

windblown particles, and growth of microorganisms. Degradation occurs near the surface of wood, wood products, and 

finishes.

Effect on Wood

Weathering takes many forms depending on the material; 

wood and wood products initially show color change and 

slight checking. Leaching of water-soluble extractives, 

chemical changes, and discoloration of the surface by microorganisms cause color change. As weathering continues, 

wood develops checks on lateral surfaces and checks and 

cracks near the ends of boards, and wood fibers slowly 

erode from the surface. Wood consists of three types of 

organic components: carbohydrates (cellulose and hemicelluloses), lignin, and extractives. Weathering affects each of 

these components differently, and physical and chemical 

changes affect paintability.

Carbohydrates

Carbohydrates (cellulose and hemicelluloses) are polymers 

of sugars and make up 55% to 65% of wood (Chap. 3). 

Carbohydrates do not absorb UV radiation and are therefore 

resistant to UV degradation. However, hemicelluloses and 

amorphous cellulose readily absorb–desorb moisture; this 

cyclic wetting and drying may cause different dimensional 

change for EW/LW bands. Differential dimensional change 

roughens wood, raises grain, and causes checks, cracks, 

warping, and cupping. Fewer checks develop in woods with 

moderate to low specific gravity than in those with high 

specific gravity; vertical-grain boards develop fewer checks 

than do flat-grain boards; and vertical-grain boards warp and 

cup less than do flat-grain boards. To minimize cupping, the 

width of a board should not exceed eight times its thickness. 

The tendency to cup increases with the specific gravity and 

width/thickness ratio.

Lignin

Approximately 20% to 30% of wood is composed of lignin, 

a polymer that helps bond cellulose and hemicelluloses 

within cell walls and bonds cells together. The volume between adjacent wood cells (middle lamella) is rich in lignin. 

If exposed to UV radiation, lignin in the middle lamella, at 

the surface of wood, begins to degrade within a few hours. 

The changes are not obvious visually, but they affect the surface chemistry of wood and thus adhesion of finishes. Lignin photodegrades, leaving cellulose fibers loosely attached 

to the surface. Further weathering causes fibers to be lost 

from the surface (a process called erosion), but this process 

is slow. Approximately 6 mm (1/4 in.) of wood is lost in a 

century (Fig. 16–12). Erosion is slower for most hardwoods 

and faster for low-density softwoods. Other factors such as 

16–11

Chapter 16 Finishing of Wood

Figure 16–12. Artist’s rendition of weathering process 

of round and square timbers. As cutaway shows, interior wood below surface is relatively unchanged.
growth rate, degree of exposure, grain orientation, temperature, and wetting and drying cycles affect erosion rate. 

Table 16–2 shows erosion rates for several wood species 

measured over 16 years.

Extractives

Extractives (chemicals in heartwood that give each species 

its distinctive color) change color when exposed to UV radiation or visible light, and this color change indicates degradation of extractives near the surface. The color change 

causes wood to lighten or darken. Some wood species 

change color within minutes of outdoor exposure. Wood 

also changes color indoors. Ordinary window glass blocks 

most UV radiation, therefore visible light causes indoor 

color change. UV stabilizers in finishes do not prevent color 

change.

Biological Factors

The most common biological factor is mildew, a microorganism that contributes to color change. Mildew does not 

cause degradation, but it may cause initial graying or an unsightly dark gray or black blotchy appearance. Dark-colored 

fungal spores and mycelia on the wood surface cause this 

color. In advanced stages of weathering, after extractives 

and lignin have been removed leaving a cellulose surface, 

wood may develop a bright silvery-gray sheen. This sheen 

on weathered wood occurs most frequently in arid climates 

or coastal regions (see Mildew).

Algae can also grow on wood, particularly in damp locations; algae is usually green, and it often grows in combination with mildew.

Effect on Paint Adhesion

Wood erosion is slow, but chemical changes occur within a 

few weeks of outdoor exposure. Badly weathered wood having loosely attached fibers on the surface cannot hold paint. 

This is not obvious on wood that has weathered for only 

2 to 3 weeks. The wood appears unchanged. Research has 

shown that surface degradation of wood exposed to sunlight 

for 1, 2, 4, 8, or 16 weeks prior to painting (preweathering) 

affects service life of subsequently applied paint. The longer 

the wood preweathered, the shorter the time until the paint 

began to peel. For boards preweathered 16 weeks, the paint 

peeled within 3 years; for boards preweathered only 1 week, 

the paint peeled after 13 years. Panels that were not preweathered showed no sign of peeling after 20 years. Paints 

were commercial oil-alkyd or acrylic-latex primer with one 

acrylic-latex top-coat over planed all-heartwood verticalgrain western redcedar. For species with low specific gravity, finish the wood as soon as possible after installation, or 

better yet, prime it before installation. In other tests using 

wood species having higher specific gravity (such as 

Douglas-fir and southern yellow pine), little loss of paint 

adhesion occurred until boards had been preweathered for 

3 to 4 weeks.

16–12

General Technical Report FPL–GTR–190

Table 16–2. Erosion of earlywood and latewood on smooth planed surfaces of various wood species 

after outdoor exposurea

Erosion (μm) after various exposure timesc

Avg.

SGb

4 years 8 years 10 years 12 years 14 years 16 years 

Wood species LW EW LW EW LW EW LW EW LW EW LW EW 

Western redcedar 

plywood 

— 170 580 290 920 455 1,095 615 1,165 805 1,355 910 1,475

Redwood plywood — 125 440 295 670 475 800 575 965 695 1,070 845 1,250

Douglas-fir plywood — 110 270 190 390 255 500 345 555 425 770 515 905

Douglas-fir 0.46 105 270 210 720 285 905 380 980 520 1,300 500 1,405

Southern Pine 0.45 135 320 275 605 315 710 335 710 445 1,180 525 1,355

Western redcedar 0.31 200 500 595 1,090 765 1,325 970 1,565 1,160 1,800 1,380 1,945

Redwood 0.36 165 405 315 650 440 835 555 965 670 1,180 835 1,385

Loblolly pine 0.66 80 205 160 345 220 490 — — — — — — 

Western redcedar 0.35 115 495 240 1,010 370 1,225 — — — — — — 

Southern Pine 0.57 95 330 180 640 195 670 — — — — — — 

Yellow-poplar 0.47 — 220 — 530 — 640 — — — — — — 

Douglas-fir 0.48 75 255 175 605 225 590 — — — — — — 

Red oak 0.57 180 245 340 555 440 750 — — — — — — 

Ponderosa pine 0.35 130 270 315 445 430 570 Decay Decay Decay Decay — — 

Lodgepole pine 0.38 105 255 265 465 320 580 475 745 560 810 — — 

Engelmann spruce 0.36 125 320 310 545 390 650 505 795 590 950 — — 

Western hemlock 0.34 145 320 310 575 415 680 515 1,255 600 1,470 — — 

Red alder 0.39 — 295 — 545 — 620 — 920 — 955 — — 

a

Data from three studies are shown. Specimens were exposed vertically facing south. Radial surfaces were exposed with the grain vertical. 

EW denotes earlywood; LW, latewood. 

b

SG is specific gravity. 

c

All erosion values are averages of nine observations (three measurements of three specimens). 
Effect on Wood Finishes

Finish resins (ingredients that form films or penetrate wood) 

are organic polymers, and as with lignin in wood, UV radiation degrades the polymer, causing slow erosion. Erosion 

rate depends on the resistance of the polymer to UV radiation. Paints and stains based on latex polymers are more 

resistant to UV radiation than those based on oil-alkyds. UV 

radiation does not usually degrade paint pigments; therefore, 

as resin degrades, pigments loosen and erode from the surface. Degraded resin and loose pigments give film-forming 

finishes a chalky appearance. Pigment erodes from oil-based 

semitransparent stains to expose wood.

Decay and Insects

Decayed wood does not hold paint. One expects wood used 

for new construction to be free of decay; contractors can 

do several things to keep it that way. If possible, paint all 

end grain surfaces with an oil-alkyd primer (such as ends of 

siding and trim, brick molding, railings, balustrade, posts, 

beams, and edges of panel products (plywood, T1-11 siding, 

medium-density fiberboard, and OSB).

When repainting, inspect wood for decay. Problematic areas 

include end grain of balustrade, brick molding, siding that 

butts against a roof, and bottoms of posts on porches. Decay 

often occurs in the center of wood and the surface can appear sound; probe several areas with an ice pick to ensure 

the wood is sound. Replace boards having decay. Siding 

intersecting a sloping roof should have a 2-in. (50-mm) gap 

between the end grain of the siding and the roof shingles. 

Check for a finish on the end grain; if there is no finish, treat 

end grain with a WRP, prime, and top-coat. If there is already a coating on the end grain, keep it painted. End grain 

of siding that butts directly against roof shingles (a bad 

practice—see Structure Design and Construction Practices) 

is not accessible for painting, however you can try to wick 

WRP into the end grain from a wet brush.

Insects seldom cause problems with finishes. However, 

when repainting a structure, inspect it for termite tunnels 

and carpenter ants. A termite tunnel is a sure sign of infestation. Presence of carpenter ants may indicate decay in the 

structure. Carpenter ants do not eat wood, but they often 

tunnel out decayed areas to build their nests. Note that 

woodpecker holes often indicate insect infestation.

Control of Water and Water Vapor

Control of liquid water and water vapor requires different 

types of finishes.

Water Repellents

Water repellents and WRPs retard the absorption of liquid 

water into wood, particularly at the end grain. They are an 

excellent treatment for wood used outdoors because they 

inhibit absorption of rain yet allow wood to dry after rain. 

WRPs and similar penetrating finishes (tinted clear finishes 

and oil-based semitransparent stains) have almost no effect 

on diffusion of water vapor; that is, they have little effect on 

the change in wood moisture content caused by changes in 

RH.

Moisture-Excluding Effectiveness

Moisture-excluding effectiveness (MEE) of a finish is a 

measure of its resistance to diffusion of water vapor (that is, 

a measure of the permeability of a coating to water vapor); 

it is not a measure of water repellency. A coating that blocks 

all water vapor is 100% effective; however, no coating is 

impermeable. A coating that excludes water vapor merely 

slows its absorption or desorption; it cannot change the 

EMC (Chap. 4). MEE depends on a number of variables: 

coating film thickness, defects and voids in the film, type 

and amount of pigment, chemical composition and amount 

of resin, vapor-pressure gradient across the film, and length 

of exposure.

Table 16–3 lists coatings and their MEE. Note that maleicalkyds, two-part polyurethane, and paraffin wax have high 

MEE. Coatings that retard water vapor diffusion also repel 

liquid water. Porous paints, such as latex and low-luster 

(flat) paints, afford little protection against water vapor 

transmission. They may not repel liquid water, either. In 

general, a low MEE value also indicates low resistance to 

absorption to liquid water. These finishes permit entry of 

water vapor and water from dew and rain unless applied 

over a nonporous primer (such as oil-alkyd primer). Latex 

finishes contain surfactants that can encourage absorption of 

water into the coating and wood, particularly just after the 

coating has been applied. Most of these surfactants wash out 

of the coating after a short time. MEE also gives a measure 

of vapor transmission out of wood. Paint film can inhibit 

drying (Fig. 16–13). Retardation of drying after periodic 

wetting of wood causes it to reach a MC where decay can 

occur. This type of wood paint failure usually occurs on 

painted fences and porch railings that are fully exposed to 

weather (Fig. 16–14). Paint coatings usually crack at the 

joint between two pieces of wood, water enters the wood 

through these cracks, and the coating slows drying. Priming the end grain of wood used in these applications inhibits 

water absorption; thus, end-grain priming works with the 

coating on the lateral surface to keep the wood dry.

Structure Design and Construction Practices

Structure design and construction practices affect finish 

performance. Design and construct structures to keep water 

out and to remove it when water gets through the structure 

envelope. This section summarizes recommendations for 

improving finish performance.

Large roof overhangs protect siding from rain and dew; gutters and downspouts greatly decrease the amount of water 

draining down the siding.

Flash all wall and roof penetrations. Shingle the flashing to 

keep water moving out of the structure. Sealants, caulking 

compounds, and similar compounds that come in a tube 

16–13

Chapter 16 Finishing of Wood
need to work in concert with flashing; they are not a substitute for flashing.

Vent clothes dryers, showers, and cooking areas to outside, 

not to the crawl space or attic. Place an air barrier in exterior 

walls and top-floor ceilings, and flash penetration through 

exterior walls (doors, windows, and vents). Vent to soffits if 

possible. Avoid using humidifiers. If the structure contains 

a crawl space, cover the soil with a vapor-retarding material 

such as black plastic or tar paper.

Do not seal the overlap of lap siding.

In northern climates, use an air barrier–vapor retarder on the 

interior side of all exterior walls and top-floor ceiling to 

prevent condensation in the walls and attic. In buildings 

that are air-conditioned most of the year, place the air 

barrier–vapor retarder on the exterior side.

16–14

General Technical Report FPL–GTR–190

Table 16–3. Moisture-excluding effectiveness of 

various finishes on ponderosa pinea

Finish

No. 

of coats

Moisture-excluding

effectiveness (%) 

1

day 

7

days 

14

days

Linseed oil 1 12 0 0 

2 22 0 0 

3 33 2 0 

Water repellentb 1 12 0 0 

2 46 2 0 

3 78 27 11 

Latex flat wall paint 1 5 0 0 

(vinyl acrylic resin) 2 11 0 0 

3 22 0 0 

Latex primer wall paint 1 78 37 20 

(butadiene–styrene resin) 2 86 47 27 

3 88 55 33 

Alkyd flat wall paint 1 9 1 0 

(soya alkyd) 2 21 2 0 

3 37 5 0 

Acrylic latex house 1 43 6 1 

primer paint 2 66 14 2 

3 72 20 4 

Acrylic latex flat 1 52 12 5 

house paint 2 77 28 11 

3 84 39 16 

Solid-color latex stain 1 5 0 0 

(acrylic resin) 2 38 4 0 

3 50 6 0 

Solid-color oil-based stain 1 45 7 1 

(linseed oil) 2 84 48 26 

3 90 64 42 

Semitransparent oil-based 1 7 0 0 

stain (commercial) 2 13 0 0 

3 21 1 0 

Alkyd house primer paint 1 85 46 24 

(maleic-alkyd resin) 2 93 70 49 

3 95 78 60 

Urethane varnish 1 55 10 2 

(oil-modified) 2 83 43 23 

3 90 64 44 

4 91 68 51 

5 93 72 57 

6 93 76 62 

Polyurethane paint, gloss 1 91 66 44 

(two components) 2 94 79 62 

3 96 86 74 

Aluminum flake pigmented 3 98 91 84 

varnish (oil-modified) 4 98 93 87 

Paraffin wax, brushed 1 97 82 69 

Paraffin wax, dipped 1 100 97 95 

a

Sapwood was initially finished and conditioned to 26 °C (80 °F) 

and 30% RH, then exposed to the same temperature and 90% RH.

b

WRP would be about the same. 

Figure 16–14. Decay and paint failure in wood railing 

fully exposed to weather.

Figure 16–13. Change in moisture content of ponderosa pine sapwood finished with three coats of 

aliuminum-pigmented alkyd paint and exposed to 

90% and 30% RH at 26 °C (80 °F), compared with 

moisture content of unfinished wood.
Prevent moisture-related problems in siding by using rainscreen design (that is, by furring out the siding 9 to 19 mm 

(3/8 to 3/4 in.) from the sheathing–house wrap) 

(Fig. 16–15).

Seal all end-grain surfaces with oil-alkyd primer or WRP. 

Ensure adequate space (approximately 50 mm (2 in.)) 

between siding and roof shingles in places where a side-wall 

intersects a roof. Siding and panel products above z-flashing 

need to be placed approximately 9 to 12 mm (3/8 to 1/2 in.) 

above the flashing to form a drip edge (Fig. 16–16).

When installing siding or shingles, ensure that the spacing is 

commensurate with the MC of the wood and the anticipated 

MC during the service life. Figure 16–17 shows shingles 

that were spaced too closely and buckled in service.

Avoid inside–outside beams and joists. For example, a 

second-floor floor joist that penetrates a wall to form a porch 

rafter is destined to have moisture problems and subsequent 

decay and finish failure. This type of wall penetration is difficult to seal to avoid air movement. Air carries water vapor 

that condenses in the space between floors or the porch 

ceiling.

Compliance of VOC Finishes with Pollution 

Regulations

Volatile organic compounds (VOCs) are organic chemicals 

in finishes that evaporate as finishes dry and cure. VOCs are 

air pollutants, and the amount that evaporates for a given 

amount of solids (such as binder and pigments) in finishes is 

regulated. Under the 1990 New Clean Air Act, the U.S. Environmental Protection Agency (EPA) required paint companies to decrease the amount of VOCs in their finishes.

Traditional solvent-borne wood finishes containing mineral 

spirits are no longer available, including oil-based semitransparent stains, oil- and oil-alkyd-based primers and 

top-coats, solvent-borne water repellents, and solvent-borne 

water-repellent preservatives. Solvent-borne finishes are still 

available, but the solvent systems are more complex than 

mineral spirits. Prior to VOC regulations, penetrating finishes, such as semitransparent stains, had low solids content 

16–15

Chapter 16 Finishing of Wood

Figure 16–15. Demonstration of siding installation over a secondary 

drainage plane (rain screen) showing 

wall studs, sheathing, water-resistive 

barrier (WRB), furring strips, and interleaved WRB at the butt joint. Note 

that the butt joint is centered directly 

over the furring strip and the underlying stud and the end grain has been 

sealed.

Figure 16–16. Demonstration of proper and improper 

z-flashing installation: (top) siding installed with a 9-mm 

(3/8-in.) gap between the z-flashing and siding to allow 

water to drain off the siding; (bottom) siding installed 

without a gap, which gives easy access for water absorption into the siding and thus shows extractives 

staining.

Figure 16–17. Shingles installed with insufficient 

gap later warped when they expanded after getting 

wet.
(pigment, oils, and polymers). Reformulated finishes may 

contain more solids, new types of solvents and co-solvents, 

or other nontraditional additives. These high-solids formulations are prone to form films rather than penetrate wood.

The paint industry also reformulated latex-based finishes 

to meet stringent requirements for water-based paints.

Exterior Wood Finishes

Exterior finishes either penetrate wood cell walls or form 

films on the surface. Penetrating finishes give a more 

“natural” look to the wood than film-forming finishes—

that is, they allow some of the character of wood to show 

through the finish. In general, the more natural a finish, 

the less durable it is. This section also discusses weathered 

wood as a “finish.”

Weathered Wood as Natural Finish

Leaving wood to weather to a natural finish may seem like 

an inexpensive low-maintenance alternative to finishing, 

but this approach leads to problems. Wood surfaces erode, 

some wood species decay, lumber is more prone to split 

and check, and in most climates in North America, exterior 

wood develops blotchy mildew growth. To avoid decay, 

wood must be all heartwood from a decay-resistant species such as redwood or western redcedar and be vertical 

grain to decrease the potential for splitting, raised grain, and 

cupping. Only limited areas have a climate conducive to 

achieving a driftwood-gray appearance as wood weathers 

naturally; the climate along the coast of New England seems 

conducive to developing the silvery-gray weathered patina 

that some people desire. Even when the climatic conditions 

favor the development of silvery-gray patina, it takes several 

years to achieve this appearance. Protected areas under the 

eaves will not weather as fast as areas that are not protected, 

which leads to a different appearance at the top and bottom 

of a wall.

Do not leave composite wood products, such as plywood, 

unprotected. The surface veneer of plywood can be completely destroyed within 10 years if not protected from 

weathering. Figure 16–18 shows weathering of unfinished 

plywood (right); the intact portion of the plywood (left) had 

been covered with a board to give a board-and-batten appearance.

Penetrating Wood Finishes

Penetrating finishes such as transparent or clear WRPs, 

lightly colored WRPs, oil-based semitransparent stains, and 

oils do not form a film on wood. However, semitransparent 

stains having high-solids content may form a thin film.

Penetration into Wood

Finishes penetrate wood in two ways: flow of liquid into cut 

cells at the surface and absorption into cell walls.

Lumber is almost never cut aligned with axial wood cells; 

therefore, the surface has cut axial cells (and of course, ray 

cells) and, if it is a hardwood, cut vessels. Cut cells and vessels give macroscopic porosity. The diameter of lumina and 

vessels varies depending on the wood species, but in all species, the hollow spaces formed by cut lumina and vessels are 

quite large compared with pigment particles and binders in 

finishes (that is, a high-molecular-weight (MW) latex molecule is small compared with these openings). Any finish 

can easily flow into cut lumina and vessels.

Penetration of a finish into the cell wall takes place at the 

molecular scale. The finish or components of the finish absorb into void space of hemicelluloses, amorphous cellulose, 

and lignin polymers contained in the cell wall. Penetration 

is excellent for resins having a MW less than 1,000 Daltons. 

The limit to penetration into these void spaces is a MW of 

approximately 3,000 Daltons. Natural oils (such as linseed 

oil and tung oil), solvents, oil-alkyds, and low-MW polymer 

precursors can penetrate the cell wall and thus modify the 

properties of cells located near the surface. Cell walls modified with finish typically absorb less water and swell less 

than do unmodified cell walls.

Traditional solvent-borne finishes such as water-repellent 

preservatives and solvent-borne oil-based stains can penetrate cell walls. To some extent, some of the excess oil 

in a long-oil-alkyd primer can penetrate cell walls. Highmolecular-weight polymers such as acrylics and vinyl acrylics and pigments are too large to penetrate cell walls and 

therefore cannot modify cell wall properties. Water in these 

formulations penetrates the cell wall, but the polymer does 

not. As water absorbs into wood, it enters the cell wall and 

hydrogen-bonds to the hemicelluloses and amorphous cellulose to cause swelling. Water absorption causes raised grain, 

and as a latex finish coalesces, the finish deforms around the 

raised grain while it is still flexible. Thus, latex finishes are 

less likely to crack if the surface develops raised grain.

16–16

General Technical Report FPL–GTR–190

Figure 16–18. Front view of exterior grade of plywood siding after 10 years of exposure. The righthand portion was exposed to the weather, whereas 

the left-hand side was covered with a board to give 

a board-and-batten appearance.
Penetrating Clear and Lightly Colored (Tinted) 

Finishes

Penetrating transparent clear finishes have no pigments and 

the generic names for them are water repellents (WRs) or 

water-repellent preservatives (WRPs). A typical WR formulation contains 10% resin or drying oil, 1% to 3% wax or 

other water repellent, and solvent. WRPs contain a fungicide 

such as 3-iodo-2-propynyl butyl carbamate (IPBC). They 

were traditionally formulated using turpentine or mineral 

spirits, but now paint companies formulate them using 

VOC-compliant solvent and waterborne systems to comply 

with VOC regulations.

WRPs give wood a bright, golden-tan color close to the 

original appearance of the wood and are the first step in 

protection from weathered wood as a finish. WRPs decrease 

checking, prevent water staining, and help control mildew 

growth. The first application of these finishes to smoothplaned lumber lasts approximately one year on exposed lateral wood surfaces; subsequent applications may last longer 

because weathered boards absorb more finish. WRPs absorb 

readily into end grain and can last for years to retard water 

absorption into end grain. WRPs last longer if applied to 

saw-textured wood.

Few companies manufacture traditional clear WRs and 

WRPs; almost all WR and WRP formulations are lightly 

pigmented and contain other additives to extend their service life (Table 16–4). Lightly pigmented finishes perform 

well on decks. Water- and solvent-borne formulations are 

available; waterborne formulations may be a water emulsion 

16–17

Chapter 16 Finishing of Wood

Table 16–4. Suitability and expected service life of finishes for exterior wood surfacesa

 Paint and solid-color stain 

Tinted finishes such as

deck finishes 

Semitransparent 

stain 

Expected service

lifed

 (years) 

Type of exterior wood surface 

Suitability 

Expected

service lifeb

(years) 

Suitability 

Expected

service lifec

(years) 

Suitability Paint 

Solidcolor

stain 

Siding 

Cedar and redwood 

Smooth (vertical grain) Low 1–2 Moderate 2–4 High 10–15 8–12 

Smooth (flat grain) Low 1–2 Moderate 2–4 Moderate 8–12 6–10 

Saw-textured High 2–3 High 4–8 Excellent 15–20 10–15 

Pine, fir, spruce 

Smooth (flat grain) Low 1–2 Low 2–3 Moderate 6–10 6–8 

Saw-textured (flat grain) High 2–3 High 4–7 Moderate 8–12 8–10 

Shingles (sawn shingles used 

on side-walls) 

High 2–3 High 4–8 Moderate 6–10 6–8 

Plywood 

Douglas-fir and Southern Pine 

Sanded Low 1–2 Moderate 2–4 Moderate 4–8 4–6 

Saw-textured Low 2–3 High 4–8 Moderate 8–12 6–10 

MDO plywoode — — — — Excellentf

 12–15 10–15 

Hardboard, medium densityg

 

Unfinished — — — — High 8–12 6–10 

Preprimed — — — — High 8–12 6–10 

MDO overlay — — — — Excellentf

 10–15 10–15 

Decking 

New (smooth-sawn) High 1–2 Moderate 2–3 Low — — 

Weathered or saw-textured High 2–3 High 3–6 Low — — 

Oriented strandboard — — Low 1–3 Moderate 4–5 4–5 a

Estimates were compiled from observations of many researchers. Expected life predictions are for average location in the 

contiguous USA; expected life depends on climate and exposure (such as desert, seashore, and deep woods). b

The higher the pigment concentration, the longer the service life. Mildew growth on surface usually indicates the need for 

refinishing. 

c

Smooth unweathered surfaces are generally finished with only one coat of stain. Saw-textured or weathered surfaces, which are 

more adsorptive, can be finished with two coats; second coat is applied while first coat is still wet. d

Expected service life of an ideal paint system: three coats (one primer and two top-coats). Applying only a two-coat paint system 

(primer and one top-coat) will decrease the service life to about half the values shown in the table. Top-quality latex top-coat paints 

have excellent resistance to weathering. Dark colors may fade within a few years. e

Medium-density overlay (MDO) is painted. f

Edges are vulnerable to water absorption and need to be sealed. 

g

Water-repellent preservatives and semitransparent stains are not suitable for hardboard. Solid-color stains (latex or alkyd) will

perform like paints. Paints give slightly better performance because the solids content of paint is higher than that for solid-color 

stains and thus paints give greater film build for the same volume of finish used. 
of synthetic polymers. Synthetic polymers do not penetrate 

the cell wall, but form a thin film, seal the surface, and provide water repellency. Finely ground pigment gives color 

and partially blocks UV radiation. Pigment, UV stabilizers, 

and other additives give these finishes a service life of 2 

to 3 years, but they lack sufficient pigment to inhibit UV 

degradation of the wood. As with clear WRPs, they usually 

contain a preservative to retard mildew growth.

Caution: Fungicides in WRPs and semitransparent stains 

are toxic and may be herbicides; use caution to avoid skin 

contact and breathing vapors, and protect plants and the 

soil around them from accidental contamination.

Prior to changes in finish formulation because of VOC regulation, paint companies formulated solvent-borne WRPs for 

use as a pretreatment prior to priming. At this time, finding a 

WRP formulated for this use is difficult. In fact, paint manufactures seldom honor a finish warranty, if customers apply 

a WRP prior to using their finish, particularly if a different 

paint company made the WRP. In spite of decades of research showing the benefits of WRP treatment of wood prior 

to priming, they are seldom used. Information on WRPs as 

a wood treatment, prior to priming, is included in this chapter in case a paint manufacturer markets a WRP specially 

formulated for this use in the future. They are particularly 

effective for improving the service life of paint on difficultto-paint wood species and decay-prone areas (Fig. 16–19). 

Currently available WRPs can be used for sealing end grain 

Protect wood and wood-based products from water and 

sunlight prior to delivery and while stored at the construction site. Avoid contaminating them with dirt, oil, or other 

contaminants. Finish wood as soon as possible after 

installing it.

of lumber, edges of plywood, and back-priming and are discussed in several sections of this chapter.

Penetrating finishes that use paraffin oil as the solvent are 

also available. These formulations penetrate wood, and the 

oil helps improve water repellency. Paraffin oil is not a volatile solvent; therefore, these finishes comply with air quality requirements. They are usually a good value, because 

virtually all of what comes in the can ends up in the wood. 

The service life is approximately 1 year, but they are easy to 

apply. If an excessive amount is applied, the wood surface 

may remain oily for a few weeks. Do not use them as a pretreatment prior to applying other finishes.

Application, New Construction

For new construction, the most effective method for applying a WR or WRP is to dip the entire board into the finish 

(Table 16–5). If finish is roller or spray applied, back brush 

following application to work the finish into the wood. Finish the back side of siding, particularly for highly colored 

wood species (see Back-Priming).

When wood is finished following installation, apply liberal amounts of WRP to all end grain areas, edges of panel 

products, and other areas vulnerable to water, such as the 

bottoms of doors and window frames. Coverage is approximately 6.1 m2 L–1 (250 ft2 gal–1) on a smooth surface or 3.7 

m2 L–1 (150 ft2 gal–1) on a saw-textured surface. Smooth 

wood will usually accept only a single coat; a second coat 

will not penetrate the wood. WRP treatment lasts longer 

on saw-textured surfaces than on smooth surfaces because 

more finish penetrates the wood. As a natural finish, the life 

expectancy of a WRP is only 1 to 2 years, depending upon 

the wood and exposure. However, reapplication is easy, 

particularly on decks and fences.

Refinishing

Clear and lightly colored finishes (penetrating natural finishes such as WRPs and lightly pigmented deck finishes) do 

not peel; they fade, and if pigmented, the pigments erode. 

As clear finishes weather, they lose their water repellency, 

turn gray, and develop mildew. Lightly pigmented finishes 

lose color. If not blackened by mildew, they can often be 

prepared for refinishing by removing dirt with a stiff-bristle 

brush. If discolored by mildew, wash the wood with commercial mildew cleaner or dilute liquid household bleach 

and detergent prior to refinishing (see Mildew).

16–18

General Technical Report FPL–GTR–190

Figure 16–19. Effect of water-repellent preservative 

treatment after 5 years of outdoor exposure. A, window sash and frame treated with a water-repellent preservative and then painted; B, window sash and frame 

not treated before painting. 
Refinish exterior wood when the old finish has worn thin 

and no longer protects the wood. If all factors are working 

in concert (good structure design to shed water, effective 

flashing, paintable wood surface, and end grain sealed), 

paint degradation is benign weathering of paint to expose 

the primer or in the case of a penetrating finish, to expose 

the wood surface. In these cases, there is rarely much 

surface preparation other than mild washing prior to refinishing. Mildew growth is not paint degradation, but an 

appearance problem; remove it with a commercial cleaner 

or bleach–detergent solution. If factors are not working in 

concert, paint may crack and peel.

Oil-Based Semitransparent Stains

Oil-based semitransparent stains have more pigment than 

tinted WRPs, and the pigment gives more protection to 

wood. Stains usually contain a WR and fungicide. Additional pigment maintains color and increases finish service 

life, but pigments give stain a less natural appearance than 

lightly colored finishes because they partially hide wood 

grain and color. Pigment content in semitransparent stains 

can vary, thus providing a range of UV protection and color. 

Most people prefer colors that accentuate the natural color 

of the wood.

Oil or oil-alkyd resin in oil-based semitransparent stains can 

flow into cut lumina at the wood surface carrying pigment 

with it. Some resin penetrates the cell wall; the rest remains on the surface and bonds the pigments to the surface. 

Semitransparent stains are porous and do not form surface 

films like paints and solid-color stains; therefore, they will 

not blister or peel even in the presence of excessive water. 

Service life varies considerably depending on substrate and 

amount of pigment (Table 16–4).

Resin and paint manufacturers have tried to achieve the 

properties of solvent-borne semitransparent stains using waterborne formulations. These finishes achieve a semitransparent appearance by forming a thin coating on the wood.

Recently, paint companies have developed “semipenetrating” stains. Semipenetrating stains partially penetrate the 

cell wall and form a surface film. This finish is similar to a 

high-solids oil-based semitransparent stain.

Application, New Construction

Semitransparent stains perform well on saw-textured surfaces. If used on smooth wood, expect approximately half 

the service life compared with saw-textured surfaces (Table 

16–4). They are an excellent finish for weathered wood.

To get consistent application and good penetration of stain, 

brush-apply oil-based semitransparent penetrating stains. 

The finish is too fluid to use a roller and spraying leads to 

an uneven appearance and lap-marks. Brushing works the 

finish into the wood and evens out the application to minimize lap marks. Lap-marks form when application of a stain 

overlaps a previously stained area (Fig. 16–20). Prevent lapmarks by staining two or three boards at a time and keeping a wet edge. This method prevents the front edge of the 

stained area from drying before reaching a logical stopping 

16–19

Chapter 16 Finishing of Wood

Table 16–5. Initial application and maintenance of exterior wood finishesa

Maintenance 

Finish Application process Appearance of wood Process Service lifeb

Water-repellent 

preservative (WRP) 

Brush-apply 1 coat or 

dip. Apply a second coat 

only if it will absorb. 

Grain visible; wood tan 

to brown, fades to gray 

with age 

Brush to remove 

surface dirt; wash 

to remove mildew 

1–3 years 

Tinted clear finish 

(slightly pigmented 

deck finish) 

Brush-apply 1 coat or 

dip. Apply a second coat 

only if it will absorb. 

Grain and natural color 

slightly changed 

Same as with WRP 2–3 years 

Semitransparent stain Brush-apply 1 coat or 

dip. Apply a second coat 

only if it will absorb. 

Grain visible; color as 

desired 

Same as with WRP 4–8 years (on 

saw-textured or 

weathered 

wood)

Paint and solid-color 

stain 

Brush-, roller-, or sprayapply primer and 2 

top-coats

Grain and natural color 

obscured 

Clean and apply 

topcoat if old finish 

is sound; if not 

sound, remove peeled 

finish, prime, and 

apply topcoatsd

10–20 years 

for paintc

; 6–15

years for solidcolor staine

a

Compilation of data from observations of many researchers. 

b

Vertical exposure; service life depends on surface preparation, climate and exposure, amount and quality of finish, and the wood

species and its surface texture. 

c

Service life of 20 years if primer and two coats of top-quality latex top-coats are used on gradual transition wood species having a 

saw-textured surface. Dark colors may fade within a few years. d

If old finish does not contain lead, sand to feather rough edges of paint surrounding bare areas and areas of weathered wood (see

Lead-Based Paint). 

e

Service life of 15 years if primer and two top-coats are used on saw-textured wood.
place (corner, door, or window). If possible, work in the 

shade to slow drying. Coverage is approximately 

4.9 to 9.8 m2 L–1 (200 to 400 ft2 gal–1) on smooth wood and 

from 2.4 to 4.9 m2 L–1 (100 to 200 ft2 gal–1) on saw-textured 

or weathered wood.

To increase service life of oil-based semitransparent stains 

on saw-textured or weathered lumber, apply two coats. Apply the first coat keeping a wet edge to prevent lap marks. 

Then, work on another area so that the first coat can soak 

into the wood for 20 to 60 min. Apply the second coat before the first dries (wet on wet application). (Again, apply 

stain keeping a wet edge to prevent lap-marks.) If the first 

coat dries completely, it seals the wood surface so that the 

second coat cannot penetrate. About an hour after applying 

the second coat, use a cloth, sponge, or brush lightly wetted 

with stain to wipe off excess stain that has not penetrated 

into the wood. Where stain failed to penetrate, it forms an 

unsightly shiny surface film. Stir the stain occasionally and 

thoroughly during application to prevent settling of pigment.

Two coats of semitransparent penetrating stain may last 10 

years on saw-textured wood. By comparison, the life expectancy of one coat of stain on new smooth wood is only 2 to 

4 years; however, as the stained wood ages, it becomes more 

porous and subsequent staining lasts longer (Table 16–5).

Semitransparent stain formulations have changed because 

of VOC regulations. Solvent systems have changed, and the 

amount of solids has increased. Formulations having high 

solids may leave excess resin on the surface (particularly the 

LW) even if the resin has a low MW. If the finish appears 

shiny an hour after application, the finish has not penetrated 

the wood. Remove the excess finish on the surface to avoid 

forming a thin film; thin films crack and peel within a year 

or two. Even if the wood surface has weathered or is sawtextured, it may not be possible for a second coat of these 

finishes to absorb into wood.

Caution: Sponges, cloths, and paper towels that are wet 

with oil-based stain, any other oil or oil-alkyd, or urethane 

finish are particularly susceptible to spontaneous combustion. To prevent fires, immerse such materials in water and 

seal in a water-filled air-tight metal container immediately 

after use.

Refinishing

Oil-based semitransparent penetrating stains degrade by 

slow erosion of pigments to give a gray slightly weathered 

appearance. Refinish when wood begins to show before all 

pigment is lost. Stains do not crack or peel unless excessive 

stain formed a film. Simply use a dry stiff-bristle brush to 

remove surface dirt, dust, and loose wood fibers and restain. As with clear finishes, remove mildew prior to refinishing. The subsequent application of penetrating stain often 

lasts longer than the first because it penetrates the porous 

weathered surface.

If oil-based semitransparent stain did not penetrate properly and formed a film, it may fail by cracking and flaking. 

In this case, surface preparation may involve scraping and 

sanding. For wood having a thick film, it may be necessary 

to remove all the old finish with a paint stripper prior to restaining. This is a difficult situation; parts of the structure 

may have areas where the old finish eroded and the surface 

is weathered; parts may have an intact or peeling film. 

Oil-based stains do not penetrate areas having a film; filmforming finishes (paint or solid color stain) do not bond to 

weathered areas. Either remove the finish in places having a 

film and re-stain or scuff sand the weathered area, scrap and 

scuff sand the area having a film, and refinish with solidcolor stain or paint.

When refinishing semitransparent stains, the stain must 

penetrate wood. As mentioned above, stain service life 

varies with exposure (that is, the weathering of the stain); 

therefore, stain may not penetrate well in some areas. For 

example, an area under the eaves, even on the south side of 

a structure, may be relatively unweathered compared with 

the lower part of the wall. When applying stain to such an 

area, feather the new stain into the old. If the stain does not 

penetrate the wood within an hour, remove excess stain to 

avoid forming shiny spots, which indicate a film. The north 

side of a structure may not need to be re-stained nearly as 

often as the south side (northern hemisphere).

Do not apply oil-based semitransparent stains over solidcolor stain or paint.

Note: Do not use steel wool or wire brushes to clean wood 

or to prepare a surface for refinishing because they contaminate the wood with iron. Minute amounts of iron react 

with tannins in woods like western redcedar, redwood, and 

oak to yield dark blue–black stains (see Finish Failure or 

Discoloration).

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General Technical Report FPL–GTR–190

Figure 16–20. Lap marks on wood finished with semitransparent stain.
Oils

Drying oils, such as linseed and tung, are appropriate natural 

finishes for indoor use and are fine for indoor furniture and 

other interior uses not subjected to water or high humidity. 

Oils perform poorly outdoors because they are natural products and therefore provide food for mildew. When used on 

highly colored woods such as redwood or the cedars, they 

tend to increase mildew growth. Even if formulated with a 

mildewcide, they may not give adequate performance outdoors. The original “Madison Formula” for a semitransparent stain could be formulated with up to 60% linseed oil and 

it contained 5% pentachlorophenol as a mildewcide. Even 

with this mildewcide, it was prone to develop mildew.

Film-Forming Finishes

In a range of least to most protection from UV radiation and 

photochemical degradation of wood, film-forming finishes 

are ranked as follows: clear varnish, pigmented varnish, 

waterborne latex semitransparent stains, solid-color stains, 

and paints.

Clear Varnish

Clear varnish is a transparent film-forming finish that enhances the natural beauty and figure of wood. In a book 

originally published in 1904, A.H. Sabin listed 16 types of 

varnish (architectural, cabinet, carriage, marine, and piano, 

to name just a few) (Sabin 1927). These varnishes were a 

solution of natural resins, linseed or tung oil, or both, and 

turpentine. In a recent publication, Wicks and others (2007) 

describe modern varnishes as urethane-modified alkyds. 

Spar varnish (a combination of novolac phenolics resin and 

tung and linseed oils) is also available. Urethane-based varnishes have good abrasion resistance and perform well on 

furniture, floors, and interior woodwork. However, varnish 

lacks exterior permanence unless protected from direct sunlight; varnishes in direct sunlight generally require refinishing every l to 2 years. Varnishes embrittle by exposure to 

sunlight and develop severe cracking and peeling. They last 

longer in protected areas, such as soffits, doors protected by 

porches, or the north side of structures; however, even in 

protected areas, apply a minimum of three coats. Staining 

the wood (oil-based semitransparent stain) prior to applying 

varnish improves its service life; the pigments in the stain 

decrease the photodegradation of the wood, thus maintaining varnish adhesion. Varnish is a high-maintenance finish 

and is not generally used on the exterior of structures.

Clear varnish usually fails by a combination of cracking and 

UV degradation of the wood at the wood–varnish interface. 

This can be identified by examining the back of a chip of 

varnish and finding wood fiber attached. Refinishing usually 

requires scraping, sanding, or power-washing the finish off 

and then reapplying the finish. 

Pigmented Varnish

Finish manufacturers have modified clear varnish to improve exterior performance by adding finely ground inorganic pigments (nanopigments). These pigments partially 

block UV radiation yet allow much of the visible light to 

pass through the finish—that is, they appear transparent. The 

particle size of these pigments is similar to the wavelength 

of UV radiation (300–400 nm), and much like dust in the atmosphere that blocks UV radiation and blue wavelengths of 

visible light to make the sun appear red during a sunset, pigments block UV radiation to protect wood. These products 

perform better than traditional clear varnishes. However, 

as with clear varnishes, pigmented varnish gives excellent 

performance in protected areas. The varnish is less prone 

to peel; degradation initially occurs on the film surface as 

crazing. Refinishing before the crazing develops into cracks 

restores the appearance. Eventually, however, the buildup of 

coats will block visible light and the wood will appear dark.

Varnish can give years of service on outdoor furniture if 

the furniture is covered with an opaque waterproof cover 

when not in use. The cover protects the varnished wood 

from UV degradation and discourages birds from roosting 

on the furniture. Several coats of varnish eliminate splinters, allow the beauty of the wood to show, and give 

a cleanable surface.

Waterborne Latex Semitransparent Stains

Waterborne latex semitransparent stains (introduced in the 

section on Oil-Based Semitransparent Stains) are discussed 

here because they form films. These finishes are usually an 

acrylic or modified acrylic and have high MW; the polymers 

are too large to penetrate the cell wall. Considerable confusion remains concerning penetration of these finishes. As 

mentioned previously, penetration of a finish into cut lumina 

on the wood surface is not penetration into wood. Filling 

the lumen does not modify the wood cells near the surface. 

Latex semitransparent stains give the look of an oil-based 

semitransparent stain by forming a thin film.

Whereas oil-based semitransparent stains slowly erode, latex semitransparent stains tend to crack and flake. The film 

buildup is not sufficient to give performance needed for a 

film-forming finish. If applied in sufficient coats to give 

more than a few years performance, they give the appearance of a solid-color stain. Some formulations are modified 

with oil-alkyds. The oil penetrates the surface, thus improving the performance of the finish. Paint companies continue 

to improve these formulations; check with paint suppliers 

for the latest information on new products.

Application, New Construction

Latex-based semitransparent stains should be brush-applied. 

As with oil-based semitransparent stains, they are susceptible to forming lap marks. Apply the second coat within 

2 weeks after the first has dried. Latex-based stains last 

longer on saw-textured wood.

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Chapter 16 Finishing of Wood
Refinishing

Scrape areas where the stain has flaked, wash, if necessary, 

and refinish. As with oil-based semitransparent stains, to 

avoid an uneven appearance, it may be necessary to feather 

the new finish into the old in areas where the old stain is still 

in good condition. Waterborne latex stains form a thin film 

and may not adhere well to weathered wood.

Solid-Color Stains

Solid-color stains are opaque finishes (also called hiding, 

heavy-bodied, or blocking stains) that come in many colors 

and are made with a higher concentration of resin and pigment than are semitransparent penetrating stains; therefore, 

solid-color stains obscure the natural color and grain of 

wood. They are available in latex-based (usually acrylic or 

modified acrylic polymers) and oil-based formulations. Oil 

and latex solid-color stains are similar to paints; they form 

a film.

Application

Apply solid-color stains by brush, sprayer, or roller. If using 

a sprayer or roller, back-brush to even out the application 

and work the finish into the surface, particularly on sawtextured wood. One coat of solid-color stain is not adequate 

for smooth wood; apply a sufficient number of coats to give 

a 0.10–0.13-mm (0.004–0.005-in., or 4–5-mil) dry film 

thickness. If applied in a single coat to smooth wood, they 

tend to crack and flake; the film lacks sufficient cohesive 

strength to accommodate moisture-driven changes in dimension of the substrate. Two coats of solid-color stain applied 

over a quality latex or oil primer should give service life 

similar to that of a good paint system on smooth-planed 

wood. Some manufacturers recommend using the first coat 

of a solid-color stain as a primer, but primer paint might be 

better, particularly for wood containing extractives (such as 

cedar and redwood). On saw-textured wood, sufficient film 

thickness may be possible with a single coat, but primer and 

one top-coat will usually give 15 to 20 years service life. 

Solid-color stains lack abrasion resistance and manufacturers do not generally recommended them for horizontal wood 

surfaces such as decks.

Refinishing

Solid-color stains can usually be applied over paint. See the 

following section (Paint) for additional information on refinishing. If the old finish has cracked or peeled, remove it and 

scuff-sand the wood prior to refinishing.

Paint

Paint appears somewhere on almost all buildings. For example, brick-, vinyl-, and aluminum-sided buildings often 

have painted wood trim. Paints are highly pigmented filmforming coatings and give the most protection against UV 

radiation. Paints protect wood surfaces from weathering, 

conceal some surface defects, provide a cleanable surface, 

offer many colors, and give high gloss (high gloss is not 

possible with stains). Paint is the only finish that can give 

a bright white appearance. Paint retards penetration of 

moisture, decreases discoloration by wood extractives, and 

retards checking and warping of wood. However, paint is 

not a preservative. It will not prevent decay if conditions are 

favorable for fungal growth.

Paint is available in two general types: solvent-borne oilalkyds and waterborne latexes (usually acrylic or vinyl 

acrylic polymers).

Oil-based paint is a mixture of finely ground inorganic pigment in a resin (binder) with additives to speed curing, improve application, and give mildew resistance. The simplest 

resin is a drying oil, such as linseed oil. Modern oil-based 

paints have the drying oil combined with a poly functional 

alcohol to form an oil-alkyd. Oil-alkyds for wood have excess oil (that is, long-oil-alkyds), making them more flexible 

than short-oil-alkyds (that is, having a shortage of oil). Oilalkyds form a film by reacting with oxygen in the air to give 

a cross-linked polymeric network. Prior to regulation of the 

amount of organic solvent in oil-alkyds, they contained turpentine or mineral spirits. Modern oil-alkyds have complex 

solvent systems to meet VOC requirements.

Latex-based paint is also a mixture of finely ground pigment 

in a resin. The resin is a synthetic polymer, and it coalesces 

to form a film; these polymers do not react with oxygen. The 

main solvent is water, with other solvents to keep the polymer flexible while it coalesces. Acrylics and vinyl acrylics 

are typical resins in wood finishes.

Oil-alkyd or latex primers link wood to top-coats and provide a base for all succeeding top-coats (initial top-coats and 

refinishing). Primers seal the surface to prevent extractives 

bleed, provide adhesion between the wood and top-coats, 

and give color base to even out differences in wood color 

and top-coat color. Primers flow into void spaces at the 

wood surface to improve top-coat adhesion and block extractives in species such as redwood and western redcedar. 

At this time, oil-alkyd primers block extractives better than 

do latex primers, but paint manufacturers continue to improve latex primers. Oil-alkyd primers block water absorption into end grain and, to a limited extent, can penetrate 

wood cell walls, thus modifying the surface and improving 

its dimensional stability. Latex primers do not penetrate cell 

walls but merely flow into cut cells and vessels. Latex primers do not seal the end grain as well as oil-alkyd primers 

do. Latex primers are more flexible and stay more flexible; 

thus, they are less likely to crack as they age. Latex primers 

are porous and thus permeable to water and water vapor; 

oil-alkyd paints are less permeable to water and water vapor 

(Table 16–3).

Latex top-coats can be applied over oil-alkyd primers. Latex 

paints formulated with acrylic resins are resistant to weathering; they maintain their gloss better than oil-alkyd paints. 

Oil-alkyd top-coats tend to lose gloss within a year or two 

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General Technical Report FPL–GTR–190
and are prone to embrittle over time. Latex paints (primers 

and top-coats) permit water cleanup; oil-alkyd paints require 

organic solvents for cleanup. Sufficient dry film thickness 

on smooth-planed surfaces obscures wood grain and texture; 

on saw-textured surfaces, some surface texture remains.

Application, New Construction

On smooth-planed wood, apply a primer and two top-coats 

to achieve a 0.10–0.13-mm (4–5-mil) dry film thickness; on 

saw-textured wood, primer and one top-coat may suffice. 

As with solid-color stains, apply paints with brush, roller, 

or sprayer. If using a roller or sprayer, back-brush to get an 

even coating and ensure the finish wets the surface. Apply 

the first coat of film-forming finishes (paint, latex semitransparent stains, and solid-color stains) within 2 weeks after 

installing smooth-planed exterior wood products; timely 

application ensures good paint adhesion. Improve film adhesion to smooth-planed flat-grain products, particularly those 

species having abrupt grain transition, by wetting the wood 

to raise the grain and scuff sanding (lightly sanding with 

50–80 grit sandpaper) after it dries.

For woods with water-soluble extractives, such as redwood 

and western redcedar, primers block extractives bleed into 

the top-coat. Use a primer that is labeled to “block extractives bleed,” usually an oil-alkyd-based paint. Some manufacturers also formulate stain-blocking acrylic-latex primers. 

Allow latex stain-blocking primer to dry for at least 24 to 

48 h before applying the first top-coat. If the primer has not 

fully coalesced, extractives may bleed into the top-coat. For 

species, such as pine, that do not tend to have extractives 

bleed, a quality primer is still necessary to give a good base 

for top-coats. Follow the application rates recommended by 

the manufacturer to achieve sufficient film thickness. A uniform primer coating having sufficient thickness distributes 

wood swelling stresses and thus helps prevent premature 

paint failure. Primer should cover approximately 6.1 to 

7.4 m2 L–1 (250 to 300 ft2 gal–1) on smooth unfinished 

wood; coverage is considerably less on saw-textured wood.

Apply two coats of acrylic latex paint over the primer. If applying two top-coats to the entire structure is not practical, 

consider two top-coats for fully exposed areas on the south 

and west sides and a single top-coat on other areas. Two topcoats over a properly applied primer should last more than 

10 years on smooth wood (Tables 16–4 and 16–5) and many 

three-coat paint systems in test at FPL have lasted 20 years. 

To avoid peeling between paint coats, paint manufacturers 

recommend applying the first top-coat within 2 weeks after 

the primer and the second top-coat within 2 weeks of the 

first. If more than 2 weeks elapse between paint coats, it 

may be necessary to wash the paint with mild detergent and 

rinse thoroughly. If the primer has been exposed for several 

months, it may need to be primed again prior to applying the 

top-coats. However, some primer may not weather as quickly and some top-coats may adhere well to weathered primer; 

check with manufacturers for information on their products.

Avoid applying oil-alkyd paint to a hot surface in direct 

sunlight and to a cool surface that the sun will heat within 

a few hours. The heat causes the surface of the coating to 

dry, trapping solvent in the film. The trapped solvent forms 

a “temperature blister,” which usually occurs within a day or 

two after painting. They do not contain water. Do not cool 

the surface by spraying with water.

Apply latex-based waterborne paints when the temperature 

is at least 10 °C (50 °F) and expected to remain above this 

temperature for 24 h. (The dew point is a good estimate of 

nighttime low temperature.) Most latex paints do not coalesce properly if the temperature drops below 10 °C 

(50 °F). Oil-alkyd paint may be applied when the temperature is at least 4 °C (40 °F). Check with paint manufacturers 

on the temperature requirements because some paints can be 

applied at lower temperatures than these. As with oil-alkyd 

paints, avoid painting hot surfaces in direct sunlight. Prior 

to applying latex paints, the surface can be cooled with water spray and allowed to dry.

Avoid painting late in the afternoon if heavy dew is expected during the night. Water absorption into partially cured 

oil-alkyds or partially coalesced latexes can cause wrinkling, 

fading, loss of gloss, and streaking.

Refinishing

In the absence of catastrophic failure such as cracking, flaking, and peeling, solid-color stains and paints slowly erode. 

A three-coat finish system (0.10–0.13 mm thick) may last 

20 years on saw-textured wood. When the top-coats begin 

to wear thin exposing the primer, reapply one or two new 

top-coats. One coat may be adequate if the old paint surface 

is in good condition. Surface preparation merely involves 

washing the surface to remove mildew, dirt, and chalk. 

Paint erodes at different rates, depending on the exposure 

to sunlight; therefore, different sides of a structure do not 

need to be painted on the same schedule. Paint on the north 

side lasts twice as long as that on the south side (northern 

hemisphere). When repainting, coverage should be approximately 9.8 m2 L–1 (400 ft2 gal–1).

Clean areas that are protected from sun and rain, such as 

porches, soffits, and walls protected by overhangs. These areas tend to collect dirt that decreases adhesion of new paint. 

Repainting protected areas every other time the structure is 

painted usually gives adequate performance.

Do not paint too often. If paint is sound, but discolored 

with mildew, wash it. It does not need repainting. Frequent 

repainting may form an excessively thick film; thick oilbased paint is likely to crack across the grain of the wood 

(see Cross-Grain Cracking). Latex paints seldom develop 

cross-grain cracking because they are more flexible than are 

oil-based paints. Since latex paints have replaced oil-based 

top-coats for most exterior applications, cross-grain cracking is rare except for latex paint applied over thick oil-based 

16–23

Chapter 16 Finishing of Wood
paint. However, too many coats of latex paint can eventually 

lead to adhesion failure of the primer.

In situations where catastrophic failure has occurred, refinishing paint and solid-color stains may require extensive 

surface preparation. First, scrape off all loose paint. In the 

absence of lead-based paint, sand areas of exposed wood 

with 50- to 80-grit sandpaper to remove the weathered surface and to feather the abrupt paint edge. Wash the remaining old paint using a commercial cleaner or a dilute household bleach and detergent solution to remove dirt and mildew and rinse thoroughly (see Mildew). Prime the areas of 

exposed wood, then top-coat. If the old paint has excessive 

chalking, it may be necessary to re-prime (see Chalking).

Note: Do not sand lead-based paint. Use special precautions if the old paint contains lead (see Lead-Based Paint).

Table 16–4 summarizes the suitability and expected life of 

commonly used exterior finishes on several wood species 

and wood-based products. The information in these tables 

gives general guidelines. Many factors affect paintability of 

wood and service life of wood finishes. Table 16–5 summarizes the properties, treatment, and maintenance of exterior 

finishes.

Application of Finishes, Special 

Uses

Porches, Decks, Deck Railings, and Fences

Porches get wet from windblown rain; therefore, apply a 

WRP or primer to end grain of flooring, railings, posts, and 

balustrade prior to or during construction. Primers and topcoats for porch floors are formulated to resist abrasion.

Decks are usually finished with penetrating clears, lightly 

pigmented clears, or semitransparent stains. These finishes 

need more frequent application than does paint but do not 

need extensive surface preparation, because they seldom fail 

by cracking and peeling. Limit the application of semitransparent stain to what the surface can absorb. The best application method is by brush; roller and spray application may 

put too much stain on horizontal surfaces. Unless specially 

formulated for use on decks, solid-color stains should not be 

used on decks or porches because they lack abrasion resistance and they tend to fail by peeling.

Like decks, fences are fully exposed to the weather, and 

some parts (such as posts) are in contact with the ground; 

therefore, wood decay and termite attack are potential problems. Use lumber pressure-treated with preservatives or 

naturally durable wood species for all posts and other fence 

components that are in ground contact. When designing and 

constructing fences and railings for decks and porches, architects and contractors need to consider protecting exposed 

end-grain of components to resist water absorption.

Film-forming finishes on fences and railings trap moisture 

if the end grain is not sealed during construction. Figure 

16–14 shows a railing 8 years after construction. Water 

flowed down the railing and absorbed into the end grain, 

and the paint kept the wood from drying. If railings are to 

be painted, seal the end grain or use pressure-treated wood, 

particularly where decay of wood is a safety hazard (railings 

on decks and porches high off the ground).

Concerning the service life of naturally durable wood species compared with wood pressure-treated with preservatives, there are no absolute “rules.” However, for in-ground 

contact uses and structural components of decks and porches 

(beams, joist, and railings), pressure-treated wood is probably better and may be a code requirement in some areas. The 

service lives of naturally durable and preservative-treated 

woods are quite comparable in aboveground exposures, such 

as decking boards. In selecting wood for porches, decks, and 

fences, whether preservative treated or a naturally durable 

species, consider the exposure conditions, design of the 

structure, properties of the wood, and the finish to be used.

Wood weathering can be as much a factor in long-term 

service life of decks and fences as decay. Protect naturally 

durable wood species and preservative-treated wood with a 

finish. Periodic treatment with a penetrating sealer, such as a 

WRP or lightly pigmented deck finish will decrease checking and splitting. Pigmented finishes retard weathering.

Treated Wood

Copper-based preservatives (copper azole, ammoniacal copper quat (ACQ), ammoniacal copper zinc arsenate (ACZA), 

chromated copper arsenate (CCA)), creosote, and pentachlorophenol are common factory-applied preservatives. 

Of these, wood treated with copper azole and ACQ is often 

used to construct porches, decks, and fences. The treatment 

has little effect on finishing once the wood has dried; species 

and grain orientation affect finishing more than preservative 

treatment does. Waterborne treatments containing copper 

may maintain a brown color for approximately 2 years. 

Some copper-based preservatives may have a water repellent included in the treatment to give the treated wood better 

resistance to weathering. Even if the manufacturer treated 

the wood with water repellent, maintain it with a finish to 

extend its service life. People often replace decking because 

of weathering, not decay.

Creosote and pentachlorophenol are generally used for 

industrial and commercial applications where applying a 

finish is not considered practical. Creosote is oily, and wood 

treated with creosote does not accept a finish. Pentachlorophenol is often formulated in heavy oil. Wood treated with 

preservatives formulated in oil will not accept a finish.

Marine Uses

The marine environment is particularly harsh on wood 

because of wind-blown salt spray, abrasion by sand, and 

direct and reflected UV radiation. Any of the types of finish 

discussed previously can be used in marine environments. 

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General Technical Report FPL–GTR–190
WRPs, tinted clears, and oil-based semitransparent stains 

give some protection; however, a paint system gives the best 

protection against photochemical degradation. If possible, 

finish wood with a WRP prior to painting. Consult paint 

manufacturers for products formulated for marine use.

Note: Any wood in contact with water must be pressure 

treated to specifications for marine use. Chromated copper arsenate (CCA) is still used in marine environments, 

and the chromium in the formulation improves the performance of stains and paints.

Boats

Varnish enhances the appearance and protects wood trim 

on boats (hence the name spar varnish), but it is exposed 

to more sunlight and water than on structures. Therefore, it 

needs regular and frequent refinishing. Paint manufacturers recommend three to six coats for best performance. 

Applying oil-based semitransparent stain to wood prior to 

varnishing increases the service life of the varnish, but the 

stain obscures some of the color of the wood. Keeping the 

appearance of wood trim bright and new is labor intensive 

but often well worth the effort.

Finish hulls with marine paint (two-part epoxy- or urethanepaint). Protect areas below the water line with antifouling 

paint. Consult manufacturers for information on these 

products.

Panel Products

The edges of panel products such as plywood, OSB, and fiberboard are vulnerable to absorption of water. To minimize 

edge swelling and subsequent finish peeling, seal the edges 

of these products with a WRP, oil-alkyd primer, or sealer 

formulated for this use. The type of edge sealer depends on 

the surface finish. Prior to staining (oil-based semitransparent stain), seal with the stain or a WRP; prior to painting 

(paint or solid-color stain), seal with an oil-alkyd primer.

Plywood siding products may have a saw-textured surface 

(such as T1–11 siding) or a paper overlay (MDO). Saw-textured surfaces may be finished with oil-based semitransparent stain, solid-color stain, or paint. Paint gives the longest 

service. Paper overlay products will not accept a penetrating 

finish (such as oil-based semitransparent stain); finish with 

paint or solid-color stain.

During pressing of OSB and fiberboard panels for exterior 

use, manufactures usually include MDO. The panels are 

cut to give lap siding. The MDO protects the surface from 

moisture and gives a good surface for film-forming finishes. 

However, as with plywood, the edges and areas around fasteners are vulnerable to water absorption and need to 

be sealed.

Fire-Retardant Coatings

Fire-retardant finishes have low surface flammability, and 

when exposed to fire, they “intumesce” to form an expanded 

low-density film. The expanded film insulates the wood 

from heat and retards combustion. The finishes have additives to promote wood decomposition to charcoal and water 

rather than flammable vapors.

Back-Priming

Back-priming is applying primer or WRP to the back side 

of wood (usually siding) before installing it. Back-priming 

with stain-blocking primer retards extractives staining, particularly run-down extractives bleed. It decreases absorption 

of water, thus improving dimensional stability. Siding is less 

likely to cup, an important consideration for flat-grain wood. 

Improved dimensional stability decreases stress on the finish, thus decreasing paint cracking.

At the time siding is back-primed, seal end grain with oilalkyd primer. This process has an even greater effect in stopping water absorption than back-priming. Primed end-grain 

eliminates paint failure near the ends of boards. Prime ends 

cut during installation.

Factory Finishing

Factory priming hardboard siding has been a standard industry practice for many years, and recently, factory-finished 

(primer and top-coats) siding, trim, and decking have 

become common. Factory finishing offers several advantages: avoids finishing during inappropriate weather, gives 

consistent film thickness, contributes to timely completion 

of structures, and decreases overall cost. Factory finishing 

is advantageous in northern climates where exterior finishing is impossible during the winter. Controlled application 

ensures consistent 0.10 to 0.13 mm (4 to 5 mil) dry film 

thickness. Siding is normally primed on all sides, including 

the end grain. When installing factory-finished siding, prime 

following cross-cuts. Controlled conditions enable many 

factory finishers to guarantee their products against cracking, peeling, and blistering for 15 years.

Finish Failure or Discoloration

Properly applied to a compatible substrate on a well-designed and constructed structure, finishes rarely fail prematurely. In the absence of finish failure (cracking and peeling) 

or discoloration (extractives bleed, iron stain, and mildew 

growth), finishes undergo a slow erosion lasting several 

years—even decades. This section is about “when things go 

wrong”. 

The most common causes of premature failure of film-forming finish (paint and solid-color stains) are water, weathering 

of wood prior to painting, inadequate surface preparation, 

and insufficient film thickness. Structure design, wood species, and grain angle can also affect performance. Topics 

covered in this section are paint cracking (parallel to grain), 

cross grain cracking, peeling, intercoat peeling, chalking, 

mill-glaze, mildew, blue stain, iron stain, and brown stain 

over knots.

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Chapter 16 Finishing of Wood
Exterior paint is subject to wetting from rain, dew, and frost. 

Equally serious is “unseen moisture” (water vapor) that 

moves from inside to outside structures in cold climates and 

from outside to inside of air-conditioned buildings in hot 

climates. Effective air and vapor barriers can minimize water vapor movement (see Chapter 13).

Paint Cracking (parallel to grain)

Cracking parallel to grain occurs on smooth flat-grain lumber, particularly with wood species having abrupt transition 

between EW and LW bands (such as southern yellow pine, 

Douglas-fir, and oak). LW bands are compressed into EW 

during planing. Normal rebound of LW bands after wood is 

in service causes films to crack along the EW–LW boundary. Other contributing factors are coatings having insufficient thickness and lacking flexibility. If the cracking is not 

too severe, scuff sand and apply one or two top-coats to give 

additional film-build.

Peeling and Flaking

Peeling and flaking (adhesion failure between wood and the 

primer) can have several causes: water, wood weathering, 

and dimensional change of thick LW bands on flat grain of 

high-density wood species. Flaking often follows cracking; 

small cracks in paint caused by raised grain allow water to 

enter. Flaking is similar to peeling; small pieces of finish 

peel from the surface usually along an EW–LW boundary. 

Flaking often occurs with cracking parallel to grain and is 

attributed to thin films. It can occur with thinly applied filmforming finishes and with oil-based semitransparent stains 

if they do not absorb properly. Water is the main cause, but 

other factors can also cause it. 

Water speeds the failure by other causes. One cause is 

weathering of wood prior to primer application. Protect 

wood from the weather prior to installation and paint it as 

soon as possible after installing it. Leaving smooth-planed 

lumber exposed to the weather for as little as 2 weeks decreases its paint-holding properties. If wood was exposed 

more than 2 weeks, scuff sand it prior to painting. In fact, 

scuff sanding is always a good idea on planed lumber. The 

wide bands of LW on flat-grain surfaces hold paint poorly. 

If possible, flat-grain boards should be installed “bark-side” 

out to minimize raised grain particularly with wood species 

having abrupt EW/LW transition. Paint applied to weathered 

wood often fails over large areas and can be easily diagnosed by inspecting the back side of the peeled paint. Wood 

fibers are attached to the film clearly showing the grain of 

the wood.

Priming end grain with oil-alkyd paint eliminates peeling 

at the ends of boards. Saw-texture greatly improves finish 

adhesion to all species and grain angles. Paint and solidcolor stains adhere quite well to difficult-to-paint wood 

species such as flat-grain southern yellow pine, Douglasfir, and radiata pine, if applied to saw-textured surfaces.

Cross-Grain Cracking

Modern waterborne latex finishes seldom fail by cross-grain 

cracking. If latex finishes crack across the grain, dimensional instability of wood under the finish causes it. For example, cross-grain checking of juvenile wood causes paint 

to crack. In this case, replace the board and repaint.

If juvenile wood is not to blame, cross-grain cracking usually occurs on structures having thick layers of oil-alkyd 

paint. If the wood is not the cause of paint failure, remove 

the old paint and apply new finish to the bare wood. Old 

paint probably contains lead (see Lead-Based Paint).

Water Blisters

Water Blisters (also called moisture blisters) are bubble-like 

deformation of paint films (Fig. 16–21). As the name implies, these blisters usually contain water when they form. 

Water blisters form between the wood substrate and the first 

coat of paint. After the blisters appear, they may dry out and 

collapse. Small blisters may disappear completely and large 

ones may leave rough spots; in severe cases, the paint peels. 

Oil-alkyd paint recently applied to wet wood is most likely 

to blister. Old paint films are too rigid to swell and form 

blisters; they usually crack and peel. Water blisters are not 

common on latex paint systems.

Minimizing water absorption into wood is the only way 

to prevent water blisters. Water blisters may occur on siding and trim where rain enters through improperly flashed 

doors, windows, and vents; they are common near unsealed 

end grain of siding and trim. Water from ice dams and 

overflow from blocked gutters can also cause water blisters. 

Movement of water vapor from the inside of a structure to 

siding and trim may also cause water blisters. Plumbing 

leaks, humidifiers, and shower spray are sources of inside 

water. Minimizing water absorption also prevents decay 

(rot), warping, and checking of wood.

Mill Glaze

Since the mid-1980s, a condition known as “mill glaze” 

(also called planer’s glaze) has been reported to cause paint 

16–26

General Technical Report FPL–GTR–190

Figure 16–21. Water blisters (also called moisture blisters) caused bubble-like deformation of paint film.
failure. Controversy exists over the exact cause of this condition, and many people use it as a catch all for unexplained 

paint failures. They attributed the paint failure to dull planer 

blades or excessive heat during planing. However, investigations of reported mill glaze by FPL scientists showed that 

other factors caused finish failure; scientists were unable to 

duplicate mill glaze in the laboratory. FPL scientists found 

three causes for paint failures that others had attributed to 

“mill glaze”: (1) raised grain under a thin film, particularly 

on smooth flat-grain lumber, (2) wood weathering prior to 

application of film-forming finishes, and (3) moisture (usually water). These factors often occurred together.

Paint failure occurred because of raised grain on flat-grain 

boards, particularly on species having abrupt EW–LW transitions. Planer blades tend to crush dense LW bands into less 

dense EW that lie directly beneath them on flat-grain surfaces (Fig. 16–22a). Later, when these boards are exposed 

to moisture, crushed EW absorbs moisture and rebounds, 

which causes the surface LW bands to protrude from the 

surface (Fig. 16–22b). A thin coat of film-forming finish 

applied over a stressed flat-grain surface will crack as the 

wood rebounds (Fig. 16–22c; see Paint Cracking). Failure is 

most common on flat-grain siding finished with insufficient 

film build of oil-based solid-color stain. Thin coatings of oilbased solid-color stain and to some extent waterborne latex 

stains are weak and do not withstand the stresses caused by 

raised grain. These low-solids coatings provide only 0.03 to 

0.05 mm (1 to 2 mil) of dry-film thickness, whereas a brushapplied three-coat paint system (primer and two top-coats) 

provides 0.10 to 0.13 mm (4 to 5 mil) of dry-film thickness. 

Raised grain is less likely to occur with vertical-grain wood 

because the EW–LW bands are perpendicular to the surface 

and the EW is not crushed during planing.

Install flat-grain bevel siding saw-textured side out. Sawtextured surfaces do not have LW bands compressed into the 

EW. The saw-textured side is the side of choice for application of penetrating semitransparent stains and film-forming 

finishes. The film buildup on the saw-textured side will 

be greater than on a planed surface, and the film will have 

greater mechanical adhesion or “bite.”

If flat-grain siding must be installed smooth-side out, remove the planing stresses by wetting the surface, then allow 

2 to 3 days for the surface to dry. Scuff-sand the surface 

with 50- to 80-grit sandpaper and apply primer and two 

top-coats.

Another paint failure that has been attributed to “mill glaze” 

is peeling caused by wood weathering prior to applying 

film-forming finishes (see Weathering, Effect on Paint Adhesion). Water causes paint to peel (see Peeling and Flaking 

and Water Blisters).

Intercoat Peeling

As the name implies, intercoat peeling is loss of adhesion 

between coats of finish, usually peeling of a new paint from 

old paint (Fig. 16–23). It usually occurs within a year of repainting. Prevent intercoat peeling by ensuring that old paint 

is free of dirt, mildew, and chalk prior to repainting.

Intercoat peeling can also result from allowing too much 

time between applying primer and top-coat. If more than 

2 weeks elapse between applying an oil-based primer and a 

top-coat, clean the surface before applying the second coat. 

If the primer (particularly oil-alkyd primers) has weathered 

for several months, it may be necessary to re-prime prior to 

applying the top-coats (see Testing for Adhesion).

16–27

Chapter 16 Finishing of Wood

(a)

(b)

(c)

Figure 16–22. (a) Cross-section view of flat-grain 

southern yellow pine showing dense LW bands 

crushed into less dense EW directly beneath them; (b) 

raised grain caused by rebound of LW bands following 

wetting; (c) a thin coat of film-forming finish applied 

over a stressed flat-grain surface will crack as the 

wood rebounds.

Figure 16–23. Intercoat peeling of paint, usually 

caused by poor preparation of old paint surface or 

excessive weathering of primer prior to application of 

top coat.
Chalking

Weathering of paint causes chalking; chalk is a residue of 

degraded resin and pigments. These degradation products 

form a fine powder. Some chalking is desirable because 

it allows the paint to self-clean. However, chalking is objectionable when the degradation products (especially the 

pigments) wash down a surface having a different color or 

when it causes premature paint failure through excessive 

erosion. Most paints chalk to some extent, but chalking is 

minimal with modern latex paints.

Latex paint or solid-color stain can be applied over existing 

paint if the old paint is clean and sound (chalk free). Prior to 

refinishing a chalky surface, scrub it thoroughly with a detergent solution to remove degraded finish residue and dirt. 

Rinse thoroughly before repainting. To check for excessive 

chalking, lightly rub the paint surface with a dark (for lightcolored paint) or white (for dark-colored paint) cloth. The 

amount of pigment removed by the cloth is a good indication of chalking. If the surface is still chalky after cleaning, 

it may need to be primed prior to repainting. Otherwise, the 

new paint may peel. Before repainting surfaces, conduct a 

simple test (see Testing for Adhesion).

Testing for Adhesion

After preparing old paint for repainting, repaint a small inconspicuous area and allow it to dry at least overnight. To 

test for adhesion, firmly press one end of an adhesive bandage onto the repainted surface. Remove the bandage with 

a snapping action. If the tape is free of paint, the new paint 

is well-bonded to the old surface and does not need priming or additional cleaning. If the new latex paint adheres to 

the tape, the old surface is too chalky and needs additional 

cleaning or priming with an oil-alkyd primer. If both the 

new latex paint and the old paint coat adhere to the tape, 

the old paint is not well bonded to the wood and must be 

removed before repainting. You should test several areas 

of the structure to determine the extent of poor paint bonds 

before stripping all the paint.

Mildew

In the absence of catastrophic paint failures described 

above, mildew is probably the most common problem 

with finishes. Mildew is the term for fungi that infect wood 

(Fig. 16–24a) and painted wood (Fig. 16–24b). These microorganisms can live on any surface that supplies a food 

source from either within the material or from air or liquids 

that contact the surface. Although the organisms cannot 

decay wood, they can metabolize some of the extractives in 

wood and natural oils (such as linseed oil) in finishes. They 

usually discolor wood or finishes with black deposits and 

often grow in combination with algae (usually green discoloration).

Mildew may be found anywhere on a building and is common on walls behind trees or shrubs where air movement is 

restricted and walls stay damp. Mildew may also be associated with dew patterns of structures. Dew forms on parts of 

structures that cool rapidly, such as eaves, soffits, and ceilings of carports and porches. The dew provides a source of 

water for mildew.

Mildew can be distinguished from dirt by examining it with 

a 10× magnifying glass (such as a jeweler’s eye loupe). In 

the growing stage, when the surface is damp or wet, the fungus has threadlike growth. In the dormant stage, when the 

surface is dry, the fungus has numerous egg-shaped spores; 

by contrast, granular particles of dirt appear irregular in 

size and shape. A simple test for the presence of mildew on 

wood or paint is to apply a drop or two of liquid household 

bleach (5% sodium hypochlorite) to the discolored area. The 

dark color of mildew will usually bleach out in 1 to 2 min. 

Surface discoloration that does not bleach is probably dirt, 

extractives bleed, or iron stain. Mildew can grow through a 

surface coating or under a clear finish. In these cases, it may 

be difficult to test for or to clean the mildew; the finish protects the mildew from the cleaning solution.

To remove mildew, use a commercial cleaner or a dilute 

solution of household bleach with detergent. If using household bleach, use as dilute a solution as possible. One part 

16–28

General Technical Report FPL–GTR–190

(a)

(b)

Figure 16–24. Mildew is most common in shaded, 

moist, or protected areas (a) on wood and (b) on painted wood.
bleach to five parts water should be adequate. In no case 

should a mixture stronger than one part bleach to three 

parts water be necessary. Add a little powdered detergent to 

help remove the dirt. Do not use liquid detergent because 

it may contain ingredients that react with bleach to give 

toxic fumes. Gently scrub the surface with a bristle brush 

or sponge and rinse thoroughly. Rinse using a garden hose, 

keeping the water stream pointed down to avoid flooding 

the back side of siding with water. If using a power-washer, 

keep the pressure low to avoid damaging the wood and, as 

with the garden hose, keep the water stream pointed down. 

Refinish the cleaned surface as soon as it has dried using a 

finish containing a mildewcide.

Household bleach mildew remover

• 1 part (5%) sodium hypochlorite (household bleach) 

 (1 gallon)

• 3 to 5 parts warm water (3–5 gallons)

• A little powdered household detergent (1/2 cup) 

Warning: Do not mix bleach with ammonia or with any 

detergents or cleansers that contain ammonia. Mixed 

together, bleach and ammonia form a toxic combination, 

similar to mustard gas. Many household cleaners contain 

ammonia, so be careful in selecting the type of cleaner to 

mix with bleach. Avoid splashing the cleaning solution on 

yourself or plants.

Loss of Gloss and Fading

Loss of gloss and fading typically occurred with traditional 

oil-alkyd finishes. Although modern acrylic-based latex 

finishes do not give the high gloss of an oil-alkyd, they 

maintain gloss much longer. Some pigments fade more than 

others; check with the paint manufacturer to ensure that the 

colors will last. White is always a safe choice. The paint 

and solid-color service-life estimates given in Tables 16–4 

and 16–5 do not take into account loss of gloss and fading. 

Many dark-colored finishes will fade to give unacceptable 

performance long before the finish fails.

Water-Soluble Extractives

In many hardwoods and softwoods, the heartwood contains 

water-soluble extractives. (Sapwood does not contain watersoluble extractives.) Western redcedar and redwood are 

two common softwoods that contain highly colored watersoluble extractives; extractives give these species their attractive color, but they can also discolor paint. When wood 

gets wet, water dissolves some extractives; then as the wood 

dries, water carries water-soluble extractives to the surface. 

The water evaporates leaving extractives behind as a reddish 

brown stain. Discoloration shows in two ways: diffused and 

run-down extractives bleed.

Diffused extractives bleed is caused by (1) water from rain 

and dew that penetrates a porous or thin paint coating, (2) 

water that penetrates joints in the siding, railings, or trim, 

and (3) absorption of water vapor in high humidity areas 

such as bathrooms, swimming pools, and greenhouses 

(Fig. 16–25).

Good painting practices prevent diffused extractives bleed. 

Use an oil-alkyd stain-blocking primer or a latex primer 

formulated for use over woods like redwood. Do not use 

porous paints such as flat alkyds or latexes directly over 

extractive-rich woods. If the wood is already painted and 

is discolored by extractives, clean the surface and apply a 

stain-blocking primer. Allow sufficient time for the primer 

to cure so that it blocks the extractives, and then apply topcoat.

Run-down extractives bleed is caused by (1) water draining 

behind siding from roof leaks, faulty gutters, or ice dams, 

(2) condensation of water vapor, originating inside the structure, on the back side of siding, and (3) wind-blown water 

that wets the back side of siding. The water on the back side 

of the siding dissolves extractives and runs off of the back 

16–29

Chapter 16 Finishing of Wood

Figure 16–25. High moisture content of wood can 

cause diffuse extractives bleed, particularly if a stainblocking primer is not used.

Figure 16–26. Water-soluble extractive discoloration 

can result from water wetting the back of the siding 

and then running down the front of the board.
side of the siding onto the front side of the siding below it, 

where it evaporates leaving red streaks (Fig. 16–26).

Prevent run-down extractives bleed by (1) fixing roof leaks, 

maintaining gutters, and preventing ice dams, (2) decreasing condensation or the accumulation of moisture in wall by 

lowering indoor humidity and installing effective air barriers 

in wall systems, (3) designing structures having adequate 

roof overhang to minimize wetting by dew and wind-blown 

rain, (4) back-priming siding prior to installation with a 

stain-blocking primer, and (5) using rain-screen construction 

to vent the back side of siding (see Back-Priming).

By eliminating the cause of extractives bleed, the discoloration will usually weather away in a few months. However, 

extractives in protected areas (under the eaves, soffits, and 

porch ceilings) become darker and more difficult to remove 

with time. In these cases, wash the discolored areas with a 

mild detergent soon after the problem develops. Paint 

cleaners containing oxalic acid may remove stains.

Blue Stain

Blue stain is a fungus that can infect sapwood of trees and 

logs (Fig. 16–27). Insects, such as the pine beetle, may carry 

it into a living tree. Pine beetle infestation often disrupts the 

flow of nutrients, thus killing the tree. Sapwood of lumber 

from beetle-killed trees usually contains blue stain. Blue 

stain may also infect logs after harvest while the MC is still 

high. The fungus causes a blue discoloration of the wood, 

but the organism does not weaken wood structurally. The 

fungus lacks the enzymes necessary to digest wood polymers; it lives off the unpolymerized sugars in the sapwood 

(see Chap. 14). Neither commercial mildew cleaners nor 

household bleach with detergent can remove it. If the color 

is objectionable, use a pigmented finish to hide it (see Mildew).

Effective control of blue stain takes place prior to using 

lumber at the construction site: maintain healthy forests,

apply fungicides to logs while in storage prior to cutting 

lumber, use kiln dry lumber, and keep lumber dry.

Iron Stain

Iron stains occur from rusting of fasteners or by the reaction 

of iron with tannins in wood. The appearance is different for 

each of these reactions.

In wood species that lack tannins, iron merely rusts, giving 

a brown stain to the wood surrounding the fastener. The iron 

also causes slight degradation of the wood near it (often 

referred to as “wood sickness”). This discoloration develops 

over many months or years of exposure.

In those wood species that have tannins, a chemical reaction 

takes place between the iron and the tannins. Tannins are 

just one of the many chemicals (extractives) in wood. Species such as the cedars, the oaks, and redwood are rich in 

tannins. Iron reacts immediately with the tannins to give a 

blue-black discoloration.

Steel fasteners are the most common source of iron 

(Fig. 16–28), but traces of iron left from cleaning wood 

with steel wool or wire brushes cause iron stain. Poor quality galvanized nails corrode easily and, like uncoated steel 

nails, usually cause unsightly staining of the wood.

If iron stain is a serious problem on a painted surface, countersink the fastener, caulk, spot prime, and top-coat. This 

costly and time-consuming process is only possible with 

opaque finishes. Little can be done to give a permanent fix 

to iron stains on wood having a natural finish. Removing 

fasteners, cleaning the affected areas with oxalic acid solution, and replacing the fasteners may not give a permanent 

fix because residual iron left behind continues to cause 

staining. Removing the fasteners often splits the siding. Using the wrong fastener can be costly—it may become necessary to replace all the siding (Fig. 16–28). Use corrosionresistant fasteners such as stainless steel rather than risk iron 

stain, particularly when using natural finishes on wood containing high amounts of tannin (such as western redcedar, 

redwood, and oak). If using galvanized fasteners, they must 

be hot-dipped galvanized fasteners meeting ASTM A 153/A 

specification. Other galvanized fasteners fail. Unfortunately, 

contractors and their employees may have difficulty 

recognizing the difference among galvanized fasteners 

(Fig. 16–28).

Iron stain occurring beneath a finish is extremely difficult to 

fix. The coating must be removed before the iron stain can 

be removed. Oxalic acid will remove the blue–black discoloration. Apply a saturated solution (0.5 kg of oxalic acid per 

4 L (1 lb gal–1) of hot water) to the stained surface. Many 

commercial brighteners contain oxalic acid, and these are 

usually effective for removing iron stains. After removing 

the stain, wash the surface thoroughly with warm water to 

remove the oxalic acid. If even minute traces of iron remain, 

the discoloration will recur.

16–30

General Technical Report FPL–GTR–190

Figure 16–27. Blue stain may infect sapwood.
Caution: Oxalic acid is toxic; take care when using it. 

(It is the poison in rhubarb leaves.)

Knots

Knots in many species contain an abundance of resins and 

other highly colored compounds. These compounds can 

sometimes cause paint to peel or turn brown. Eliminating 

paint discoloration caused by extractives in knots is difficult 

because some of the extractives are soluble in oil-based 

primers and diffuse through them. Latex-based formulations do not block them either. Coat the knot with shellac or 

specially formulated knot sealer. Do not use varnish to seal 

knots; varnish is not formulated for this use. After sealing 

knots, apply primer and two top-coats. Knots usually check 

as wood dries; if the checks form after the wood has been 

painted, the checks cause the paint to crack (see Wood

Extractives).

Pitch

Pitch and other resins are one of the defense mechanisms 

that a tree uses to protect itself from harmful pathogens and 

insects following injury. When a tree’s bark is damaged, 

pitch flows into these areas to protect the wound 

(Fig. 16–29). Pitch exists as a normal part of the wood of 

pines (Pinus spp.), spruces (Picea spp.), larches (Larix

spp.), and Douglas-firs (Pseudotsuga spp.), and it can be 

found in specialized wound structures called pitch pockets 

in the wood of most softwood species. Pitch is a solution of 

natural rosins and turpentine in species such as spruce, pine, 

and fir. It remains in the lumber from these species. The ease 

with which it exudes to the surface of lumber depends on 

the amount of turpentine in which the pitch is dissolved and 

the temperature (that is, the more turpentine, the more fluid 

the pitch; the higher the temperature, the more fluid 

the pitch). Pitch exudation can occur in isolated spots 

(Fig. 16–30a) or in large pockets or seams (Fig. 16–30b). 

When pitch bleed occurs, high temperature is the cause. If 

the temperature at the surface of the wood increases, usually 

from being exposed to direct sunlight, the pitch oozes to the 

surface. If the wood is finished, the pitch may exude through 

the coating or cause the finish to discolor or blister.

The only way to prevent pitch bleed is to remove the turpentine from the wood during lumber processing. Depending on 

the species, specific kiln schedules can be used to drive off 

most of the turpentine, thus “fixing” or “setting” the pitch 

(making it less fluid). However, not all end uses of lumber 

require pitch to be set; construction grades of lumber, even 

if kiln-dried, seldom have the pitch set. This is usually not a 

problem for construction grades because the wood surface is 

seldom visible. The difficulty occurs with appearance grades 

of lumber, such as for siding and trim.

Kiln schedules for setting pitch involve higher temperatures 

and last longer than normal drying schedules. For a complete guide to drying schedules, refer to publications such 

as the Dry Kiln Operator’s Manual.

Pitch can be removed in several ways, depending on how 

fluid it is. If the pitch has not hardened (it still contains a lot 

of turpentine), remove it with turpentine or mineral spirits. 

Once it has hardened, scrape it off with a putty knife or 

paint scraper. However, if the pitch is still soft, such procedures smear it over the surface of wood or paint. Let it 

harden, and then scrape it off. After removing pitch, sand to 

bare wood, spot prime, and top-coat. Shellac seals extractives but not pitch. Paint will not prevent future bleeding of 

pitch during periods of high temperature. If pitch is a recurring problem, it may be necessary to replace the board. One 

should note that many paints, particularly oil-alkyds, fade as 

they age and repainting the spots where pitch was removed 

may show color differences.

16–31

Chapter 16 Finishing of Wood

Figure 16–28. Iron stain on newly installed wood siding. Poor quality galvanized nails corrode easily and, 

like uncoated steel nails, usually cause unsightly staining of the wood.

Figure 16–29. Pitch flow from wound.
Finishing Interior Wood

Many finishes and finishing methods are used indoors 

because of the breadth of wood products and uses—from 

wood floors to cutting boards. This section includes general 

information on a few common products used for interior 

wood finishing and brief subsections on finishing wood 

floors and kitchen utensils. Many finishing methods exist for 

just furniture. Factory finishing of furniture is often proprietary and may involve more than a dozen steps. Methods for 

furniture finishing are not included in this chapter, but most 

public libraries contain books on furniture finishing. Product 

literature for furniture finishes often contains recommendations for application. Interior wood products require less 

protection against water and UV radiation than do exterior 

wood products, and finishes usually last for decades. However, interior wood products have more exacting standards 

for appearance and cleanability than do exterior wood 

products.

As with wood used outdoors, wood changes color as it ages 

indoors, whether unfinished or finished. In general, dark 

wood gets lighter and light wood gets darker. Color change 

is natural aging of newly cut wood and is caused by visible 

light, not UV radiation associated with outdoor weathering. If removing a picture from paneling shows a color 

difference (shadowing by the picture), correct it by leaving 

the wood exposed to light. The color will usually even out 

within several months. To avoid shadowing, keep all paintings and other wall coverings off paneling until most color 

change has occurred (usually 2 to 3 months, depending on 

the light intensity).

Fingerjointed lumber has become common for interior trim. 

Pieces of wood for fingerjointed lumber often come from 

different trees having different amounts of extractives. 

These extractives can discolor finishes, particularly in humid 

environments such bathrooms and kitchens (Fig. 16–10). 

When painting fingerjointed lumber, use a stain-blocking 

primer to minimize discoloration. In new buildings, allow 

wood adequate time to reach EMC before finishing.

Types of Finish and Wood Fillers

Opaque Finishes

Interior woodwork, especially wood trim, requires smooth 

surfaces, consistent color, and a lasting sheen. Therefore, 

enamels, high-gloss or semi-gloss, are preferable to flat 

paints. However, the higher the gloss, the more the finish 

accentuates imperfections such as planer marks, hammer 

marks, and raised grain. Raised grain is troublesome on flatgrain surfaces having abrupt EW–LW transitions, because 

planing crushes LW bands into the EW; later, when the MC 

changes, the EW swells causing raised grain. To obtain a 

smooth finish, sponge unfinished wood with water to raise 

the grain, allow it to dry thoroughly, sand, remove surface 

dust with a tack cloth, and finish.

Stains

Stains accentuate wood grain by absorbing differently into 

EW, LW, knots, vessels, and flaws. Stains color EW more 

than LW, reversing the natural color gradation. For uniform 

color, apply a penetrating sealer (“wash coat”) before applying stain. It impedes stain absorption into the EW. Interior 

stains are often natural or synthetic dyes dissolved in water 

or organic solvent. Water-soluble stains give depth to a finish, dry slowly, raise the grain, and require sanding. Solventborne stains dry quickly, do not raise the grain, and need 

little or no sanding. A combination of solvent- and waterborne stains or dyes can give the finish color “depth.”

If stain absorbs into wood unevenly, causing a blotchy appearance, blue-stain fungi or bacteria probably infected 

the tree prior to cutting for lumber. Blue stain on lumber is 

easy to see. However, bacteria-infected areas have no color 

and wood appears normal. Infected areas absorb excessive 

amounts of stain quickly, giving wood an uneven blotchy 

appearance. The infection occurs across grain boundaries. 

This problem is not very common, but should it occur, it 

cannot be fixed once the stain is applied. If wood is to be 

used for furniture or fine woodwork, it might be a good idea 

to check lumber before using it by applying a stain or denatured alcohol to identify infected areas. (Schofield (2008) 

describes diagnosing blotching and treating boards prior to 

staining.) Discard pieces on which stain appears blotchy, 

apply a wash coat to decrease absorption, or use them where 

they will not show. Sealing the lumber with dewaxed shellac prior to staining may help; commercial sealers are also 

available.

16–32

General Technical Report FPL–GTR–190

(a)

(b)

Figure 16–30. (a) Pitch exudation from an isolated spot; 

(b) pitch exudation from a large pocket or seam.
Fillers

Hardwoods are ring porous, semi-ring porous, or diffuse porous according to size and location of vessels (see 

Anatomy). Diffuse-porous and semi-ring-porous hardwoods 

with small vessels may be finished with paints, enamels, and 

varnishes in the same way as softwoods. Vessels in most 

ring-porous hardwoods need to be filled to obtain a smooth 

finished surface. Filler may be a paste or liquid, natural or 

colored. Wipe the filler across wood grain to pack it into the 

vessels; then, wipe with a few light strokes with the grain. 

Remove surplus filler immediately after the glossy wet appearance disappears. After the filler dries thoroughly, lightly 

sand it before finishing the wood.

Use slightly different methods for opaque and clear coatings. For opaque finishes, fill vessels, sand, and apply 

primer/sealer and top-coats. For clear finishes, stain prior to 

filling to bring out the color of the vessels. Transparent fillers do not affect finish or wood color; colored fillers match 

or contrast with wood color.

Sealers

Sealers are thinned varnish, shellac, or lacquer used to prevent absorption of finish and prevent bleeding of stains into 

surface coatings, especially lacquer coatings. Lacquer and 

shellac sealers dry quickly.

Transparent Finishes

Transparent film-forming finishes such as varnish give 

excellent performance on wood indoors. However, as with 

high-gloss finishes, transparent finishes accentuate surface 

blemishes. Remove all blemishes, such as planer marks and 

raised grain before finishing. Transparent finishing consists 

of sanding, staining, filling, sealing, finishing, and sometimes waxing.

Transparent coatings may be gloss varnish, semi-gloss varnish, shellac, nitrocellulose lacquer, natural oils, or wax. 

Wax provides protection without forming a thick coating 

and enhances the natural luster of wood. Other coatings, 

such as shellac, linseed or tung oil, lacquer, and varnish accentuate the natural luster of some hardwoods and seem to 

give the surface “depth.” Shellac applied by the laborious 

process of French polishing probably achieves this impression of depth most fully, but the coating is expensive and 

easily marred by water. Rubbing varnishes give almost as 

much depth. Lacquers have the advantages of drying rapidly 

and forming a hard surface, but lacquer requires more coats 

than varnish to obtain a lustrous appearance. Sufficient film 

thickness is needed for long service life, particularly for 

products that are cleaned often, such as kitchen cabinets 

and tabletops. Varnishes are usually alkyd-modified polyurethane and are available in solvent-borne and waterborne 

formulations. Waterborne finishes are more likely to raise 

grain than are solvent-borne finishes and may appear like a 

plastic film, rather than bringing out the “depth” of the wood 

16–33

Chapter 16 Finishing of Wood

Figure 16–31. Number 2 grade of hickory finished to 

accentuate the beauty of the various colors, knots, and 

grain pattern of this species.

substrate. Apply varnish directly to wood or stain prior to 

varnishing.

Varnish and lacquer usually dry to a high gloss. To decrease 

gloss, rub finish surface with polishing compound (waterproof sandpaper or powdered pumice stone and water or 

polishing oil). The final sheen varies with the fineness of 

the polishing compound; coarse powders make a dull surface and fine powders produce a bright sheen. For a smooth 

surface with high polish, use rottenstone and oil for final 

polishing. Varnish and lacquer that give a semi-gloss or satin 

finish are also available. Do not use steel wool (see Iron 

Stain).

Natural oils such as linseed oil or teak oil and commercial 

formulations such as Danish oil are popular. These finishes 

penetrate wood and do not form a film. Apply two or more 

coats of oil followed by a paste wax. Oil finishes are easy 

to apply and maintain, but they soil more easily than filmforming finishes.

Finishes for Wood Floors

Wood is highly desirable flooring for homes, factories, and 

public buildings and is available in many wood species. 

Natural color and grain accentuate many architectural styles. 

Finishes enhance the natural beauty of wood floors, protect 

them from excessive wear, and make them easier to clean 

(Fig. 16–31). Detailed procedures and specific products depend largely on the species of wood used and finish preference. Obtain additional information specific to your needs 

from flooring associations or individual flooring manufacturers.

Finishing floors consists of four steps: sanding the surface, 

applying filler, staining to achieve a desired color, and 

finishing with a clear coat. Careful sanding to provide a 
smooth surface is essential for a good appearance because 

the finish accentuates any irregularities or roughness in the 

surface. A smooth surface requires sanding in several steps 

with progressively finer sandpaper, usually with a machine 

unless the area is small. After sanding, remove all dust. 

Never use steel wool on floors because minute steel particles 

left in wood cause iron stains. Filler is necessary for wood 

with large pores, such as red oak, to obtain a smooth glossy 

appearance (Table 16–1). Stain to obtain a uniform color or 

to accent the grain pattern. Stain should be an oil-based or 

non-grain-raising type. Stains penetrate wood only slightly; 

therefore, protect the stained surface with a clear coating. 

Refinish the clear top-coats as needed to prevent wearing 

through to the stained wood. Staining worn spots in a way 

that will match the color of the surrounding area is difficult.

Whether the wood is stained or not, sealers or varnishes give 

a clear finish for wood floors. Floor varnish is usually alkydmodified polyurethane. Sealers are usually thinned varnish 

and penetrate the surface without forming a coating of appreciable thickness. Prolong the service life of floor finishes 

by keeping them waxed. Paste wax generally provides better 

appearance and lasts longer than liquid wax. Re-waxing or 

resealing and waxing of high traffic areas are relatively simple maintenance procedures, as long as the stained surface 

of the wood has not been worn.

Finishes for Items Used for Food

The durability and beauty of wood make it an attractive 

material for bowls, butcher blocks, and other items used to 

serve or prepare food. A finish helps keep wood dry, which 

makes it less prone to harbor bacteria, check, or crack. Finishes that repel water decrease the effects of brief periods 

of wetting (washing). Finished wood is easier to clean than 

unfinished wood.

Types of Finish

Sealers and Drying Oils

Sealers and drying oils penetrate wood and cure (dry) to 

form a barrier to liquid water. Many commercial sealers are 

similar to thinned varnish (e.g., polyurethane or alkyd-modified polyurethane). Drying oils such as tung, linseed, and 

walnut can also be used as sealers. Sealers and drying oils 

give a surface that is easy to clean and resistant to scratching. Sealers are easy to apply and cure quickly. Drying oils 

may require several weeks to cure.

Nondrying Oils

Nondrying oils (vegetable and mineral oils) penetrate wood 

but do not cure. As with sealers and drying oils, they improve water resistance. Vegetable oils (such as olive, corn, 

peanut, and safflower) are food for microorganisms such as 

mildew or bacteria. Vegetable oils can become rancid and 

may impart undesirable odors or flavors to food. Mineral (or 

paraffin) oil is a nondrying oil from petroleum. Mineral oil 

is not a natural product; therefore, it is not prone to mildew 

or to harbor bacteria.

16–34

General Technical Report FPL–GTR–190

Varnish and Lacquer

Finishes that form a film, such as varnish or lacquer, give a 

smooth cleanable surface. These finishes resist staining and 

should perform well if you minimize their exposure to water; avoid placing them in a dishwasher. However, eventually the finish may crack, chip, and peel.

Paraffin Wax

Paraffin wax is similar to paraffin oil but is solid at room 

temperature. Paraffin wax is one of the simplest ways to finish wood utensils, especially countertops, butcher blocks, 

and cutting boards.

Food Service Items

Food service items such as salad bowls and eating utensils 

need a finish that is easy to clean and resistant to abrasion, 

water, acids, and stains. Varnishes, lacquers, penetrating 

wood sealers, and drying oils can be used; however, varnishes and lacquers are easiest to keep clean and most resistant 

to absorption of stains.

Note: Whatever finish is chosen for wood utensils used to 

store, handle, or eat food, be sure the finish is safe and not 

toxic. Also, be sure the finish you select is recommended for 

use with food or is described as food grade. For information 

on the safety and toxicity of any finish, check the label, contact the manufacturer or the Food and Drug Administration, 

or check with your local extension home economics expert 

or county agent.

Butcher Blocks and Cutting Boards

The simplest finish for wood butcher blocks and cutting 

boards is melted paraffin wax (the type used for home canning). Melt wax using hot plate or other low-temperature 

heat source—do not use an open flame. Brush melted wax 

on the wood. Use an iron to melt excess wax that has solidified on the surface so that it absorbs into the wood, or just 

scrape off the excess wax. Refinishing is simple and easy. 

Other penetrating finishes (sealers, drying and nondrying 

oils) may be used for butcher blocks and cutting boards, 

but as mentioned in the subsection on eating utensils, vegetable oils may become rancid. Film-forming finishes such 

as varnish or lacquer perform poorly on butcher blocks and 

cutting boards.

Wood Cleaners and Brighteners

The popularity of wood decks and the desire to keep them 

looking bright and new has led to a proliferation of commercial cleaners and brighteners. The active ingredient in many 

of these products is sodium percarbonate (2Na2CO3·3H2O2). 

Sodium percarbonate is bleach; however, it is oxygen bleach 

rather than chlorine bleach such as laundry bleach—sodium 

hypochlorite and calcium hypochlorite. Oxygen bleaches 

remove mildew and have been reported to be less likely 

to damage wood surfaces than “chlorine” bleaches, particularly with low-density woods like western redcedar, Alaska 
yellow-cedar, and redwood. However, it is difficult to compare the advantages and disadvantages of the two types of 

cleaner (oxygen versus chlorine) because of the wide range 

of active ingredient concentrations in the cleaners, additives 

in the cleaners, and various wood substrates that have been 

used for evaluating the cleaners. Some commercial products 

contain household bleach. Commercial cleaners usually 

have a surfactant or detergent to enhance the cleansing 

action.

At the other extreme from the reported gentle bleaching action of sodium percarbonate are those cleaners containing 

sodium hydroxide. Sodium hydroxide is a strongly alkaline 

chemical that pulps wood and is used in some paint strippers. These cleaners may be necessary where mildew is 

imbedded in a surface finish; however, they should be used 

only as a last resort.

Manufacturers of some cleaners and brighteners report that 

their products restore color to wood. Cleaning wood does 

not add color. Removing mildew reveals the original color. 

Brightening the wood may make it appear as if it has more 

color. Weathered wood has a silvery gray appearance because weathering removes colored components from the 

surface. If you want to restore color, stain the wood. Some 

commercial cleaners pulp the wood surface and subsequent 

power washing removes the pulped surface. In this case, the 

color is “restored” because the surface of the wood was removed. Sanding would give the same result.

Some brighteners contain oxalic acid. Oxalic acid removes 

extractives bleed and iron stains, but it is not effective for 

removing mildew.

Paint Strippers

Removing paint and other film-forming finishes from wood 

is a time-consuming and often difficult process. Finish 

removal is necessary if a finish has extensive cracking or 

peeling (see Finish Failure or Discoloration). It may be necessary to remove paint containing lead; however, if the paint 

is still sound and it is not illegal to leave it on the structure, 

paint over the lead-based paint to seal in the lead (see LeadBased Paint).

Methods described here can remove finishes from furniture; 

however, companies that specialize in stripping furniture 

usually immerse the furniture in a vat of paint stripper, then 

clean and brighten the wood.

Mechanical and chemical are general types of stripping 

methods. Consult product literature for additional information on appropriate uses and safety precautions. Regardless 

of the method used to strip paint, sand the wood prior to 

applying new finish.

Note: Dust caused by mechanical stripping methods and 

fumes given off by chemical strippers are usually toxic. 

Use effective safety equipment, including a respirator, 

even if the paint does not contain lead (see Lead-Based 

Paint). Dust masks sold in hardware stores do not block 

chemical fumes and are not very effective against dust.

Mechanical Methods

Scraping, sanding, wet or dry sandblasting, spraying with 

pressurized water (power washing), and using electrically 

heated pads, hot air guns, and blowtorches are mechanical 

methods for removing finishes.

Scraping is effective for removing loosely bonded paint 

or paint that has already partially peeled from small areas 

of the structure. If possible, sand weathered surfaces and 

feather edges of paint still bonded to wood. Do not sand if 

the old paint contains lead (see Lead-Based Paint).

If paint has partially debonded on large areas of a structure, 

contractors usually remove the finish by power washing. 

This methods work well for paint that is loosely bonded. If 

paint is tightly bonded, complete removal can be difficult 

without severely damaging wood. The pressure needed to 

debond tightly bound paint from wood can easily cause deep 

erosion of wood. If high pressure is necessary to remove 

paint, the paint probably does not need to be removed prior 

to refinishing. Power washing erodes less dense EW more 

than dense LW, leaving behind ridges of LW, which are difficult to repaint. Power washing is less damaging to wood 

than is wet or dry sandblasting, particularly if low pressure is used. If more aggressive mechanical methods are 

required, wet sandblasting can remove even tightly-bonded 

paint. Dry sandblasting is not suitable for removing paint 

from wood because it severely erodes wood along with the 

paint and it tends to glaze the surface. Power washing and 

wet and dry sandblasting are not suitable for paint containing lead.

Power sanders and similar devices are available for complete paint removal. Some devices are suitable for removing 

paint that contains lead; they have attachments for containing the dust. Equipment that has a series of blades similar 

to a power hand-planer is less likely to “gum up” with paint 

than equipment that merely sands the surface. Planers and 

sanders cannot be used unless the fasteners are countersunk. 

Consult the manufacturers’ technical data sheets for detailed 

information to determine the suitability of their equipment 

for your needs and to meet government regulations on leadcontaining paint.

Paint can be softened using electrically heated pads, hot air 

guns, or blow torches, then removed by scraping it from the 

wood. Heated pads and hot air guns are slow methods and 

cause little damage to the wood. Blowtorches have been 

16–35

Chapter 16 Finishing of Wood
used to remove paint, but they are extremely hazardous; 

the flame can easily ignite flammable materials beneath 

the siding through gaps in the siding. These materials may 

smolder, undetected, for hours before bursting into flame 

and causing loss of the structure. Heated pads, hot air guns, 

and blowtorches are not suitable for paint containing lead.

These methods volatilize lead at their operating temperatures. Lead fumes are released at approximately 371 °C 

(700 °F).

Note: Removing paint from wood with a blowtorch is not 

recommended.

Chemical Methods

Efficient paint removal may involve mechanical and chemical methods. Stripping paint chemically has the following 

steps: apply paint stripper, wait, scrap off the softened paint, 

neutralize the stripper (if necessary), wash the wood, and 

sand the surface to remove wood damaged by the stripper and raised grain caused by washing. Chemical paint 

strippers, although tedious to use, are sometimes the most 

reasonable choice. Some are extremely strong chemicals 

that quickly remove paint but are dangerous to use. Others 

remove the paint slowly but are safer. With the exception of 

alkali paint stripper, how safe a product is and how fast it 

removes paint seem to be inversely correlated.

Solvent-Based Strippers

Fast-working paint strippers usually contain methylene 

chloride, a possible carcinogen that can burn eyes and skin. 

Eye and skin protection and a supplied-air respirator are 

essential when using this paint stripper. Paint strippers having methylene chloride can remove paint in as little as 10 

min. Some paint strippers are formulated using other strong 

solvents because of concerns with methylene chloride; the 

same safety precautions should be used with these formulations as with those containing methylene chloride. Consult 

product literature and strictly observe safety precautions.

Alkali-Based Strippers

As an alternative to strong solvents, some paint strippers 

contain strong bases (alkali). As with solvent-based paint 

strippers, alkali-based strippers require eye and skin protection. Follow manufacturers’ recommendations concerning 

use of a respirator. Although alkali-based paint strippers 

soften paint rather slowly, they are strong chemicals and can 

severely damage wood. Strong alkali pulps the wood surface. After paint removal, neutralize the surface with mild 

acid. Unfortunately, balancing the acid and base concentrations is difficult. If excess alkali remains in the wood, it may 

degrade the wood and subsequent paint coating. Excess acid 

can also damage wood. Alkali strippers are often left on 

painted wood a full day or overnight and are usually covered to slow evaporation. These covered types of products 

have the advantage of containing the paint stripper and paint 

quite well, an important consideration when removing paint 

containing lead. Do not let alkali chemicals dry on the 

surface, particularly on those finishes containing lead. The 

dry chemicals contain lead dust.

Note: Alkali-based strippers require extra care to ensure 

that the wood is neutralized and that residual salts are 

washed from the wood. The surface usually needs to be 

sanded before repainting to remove raised grain.

“Safe” Paint Strippers

Several manufactures have marketed “safe” paint strippers. 

These strippers work slower than those having methylene 

chloride. The active ingredient in such paint strippers is 

usually proprietary. Concerning safety, follow the manufacturer’s recommendations.

Avoidance of Problems

Avoid finish failure subsequent to removing the old finish 

by using methods that do not damage wood. The best way 

to remove paint may involve a combination of methods. For 

example, use power washing to remove as much loosely 

bound paint as possible. Then, use a chemical paint stripper 

on tightly-bonded paint. Avoid using excessive amounts of 

chemical stripper. Applying too much stripper or leaving it 

on painted wood too long can damage wood. Use less paint 

stripper and reapply it rather than trying to remove all the 

paint with one application and risk damaging wood.

The range of wood species and finishes and the possibility of finishes containing lead complicates paint removal. 

Companies may optimize paint stripper formulation without considering the effects on wood. Removing paint from 

wood is only half the task. Getting a paintable surface is the 

other half. Companies that formulate paint strippers should 

consider this other half. Those who use paint strippers need 

to understand the added burden of surface preparation.

Disposal of Old Paint

No matter what method you use to remove paint, be careful in disposing of old paint, particularly paint that contains 

lead. Lead paint is hazardous waste; follow all regulations, 

national and local, during the removal, storage, and disposal 

of all paint, especially paint containing lead (see LeadBased Paint).

Lead-Based Paint

Lead-based paint was widely used in residential structures 

in the United States until the early 1940s, and its use continued to some extent, for the exterior of dwellings, until 1976. 

In 1971, Congress passed the Lead-Based Paint Poisoning 

Prevention Act, and in 1976, the Consumer Product Safety 

Commission (CPSC) issued a ruling under this Act that limited the lead content of paint used in residential dwellings, 

toys, and furniture to 0.06%. Prior to any paint restoration 

on structures built prior to 1976 (and probably a good idea 

on any structure), check paint for lead. Check for lead using 

a solution of 6% to 8% sodium sulfide in water or using a 

16–36

General Technical Report FPL–GTR–190
test kit. Test kits should be available in most paint and hardware stores. Be certain to check all paint layers, because the 

older ones are more likely to contain lead.

Lead-based paint is still manufactured for applications not 

covered by the CPSC ruling, such as paint for metal products, particularly those made of steel. Occasionally, such 

lead-based paint inadvertently gets into the hands of consumers. Imported products may also contain lead paint.

Studies have shown that ingestion of even minute amounts 

of lead can have serious effects on health; lead causes hypertension, fetal injury, damage to the brain, kidneys, and 

red blood cells, partial loss of hearing, impairment of mental 

development, growth retardation, and inhibited metabolism 

of vitamin D. The American Academy of Pediatrics regards 

lead as one of the foremost toxicological dangers to 

children.

Lead-based paint on the exterior of structures weathers to 

give flakes and powder. The degraded paint particles accumulate in the soil near the structure. Lead-based paint used 

on interior surfaces can also degrade to produce lead-containing dust. Sanding coatings prior to repainting generates 

lead dust. Sanding the exterior of a structure without proper 

equipment can cause lead contamination inside the structure.

Methods used to remove lead paint can themselves generate lead dust. This is particularly true when unacceptable 

methods and work practices are used. Poorly performed 

abatement can be worse than no abatement. Micron-sized 

lead dust particles can remain airborne for substantial periods and cannot be completely removed by standard cleaning 

methods. When working on old painted surfaces, assume 

that one or more of the paint coats contain lead. Take precautions accordingly.

Check with the U.S. Department of Health and Urban Development (HUD), U.S. Environmental Protection Agency 

(EPA), and American Coatings Association for the latest 

regulations and guidelines for remediating lead-based paint 

(www.hud.gov/offices/lead) (www.epa.gov/lead) 

(www.paint.org/issues/lead.cfm).

Caution: Remodeling or refinishing projects that require 

disturbing, removing, or demolishing portions of structures coated with lead-based paint pose serious health 

risk. The consumer should seek information, advice, and 

perhaps professional assistance for addressing these risks. 

Contact HUD for the latest information on the removal of 

lead-based paints. Debris coated with lead-based paint is 

hazardous waste and must be disposed of in accordance 

with federal and local regulations.

Literature Cited

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Wicks, Z.W., Jr.; Jones, F.N.; Pappas, S.P.; Wicks, D.A. 

2007. Organic Coatings: Science and Technology. 3rd ed. 

Hoboken, NJ: John Wiley & Sons.

Additional References

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Arnold, M.; Feist, W.C.; Williams, R.S. 1992. Effect of 

weathering of new wood on the subsequent performance 

of semitransparent stains. Forest Products Journal. 42(3): 

10–14.

ASTM. 2003. Standard specification for zinc coating (hotdip) on iron and steel hardware. A 153/A 153M–01a. West 

Conshohocken, PA: ASTM International. 1(6).

Black, J.M.; Mraz, E.A. 1974. Inorganic surface treatments 

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Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 44 p.

Bussjaeger, S.; Daisey, G.; Simmons, R.; Spindel, S.; 

Williams, S. 1999. Mildew and mildew control for wood 

surfaces. JCT Coatings Tech. 71(890): 67–69.

Cassens, D.L.; Feist, W.C. 1980a. Wood finishing: finishing 

exterior plywood, hardboard and particle board. North Central Region Extension Pub. 132. West Lafayette, IN: Purdue 

University, Cooperative Extension Service.

Cassens, D.L.; Feist, W.C. 1980b. Wood finishing: paint 

failure problems and their cure. North Central Region Extension Pub. 133. West Lafayette, IN: Purdue University, 

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Cassens, D.L.; Feist, W.C. 1980c. Wood finishing: discoloration of house paint—causes and cures. North Central 

Region Extension Pub. 134. West Lafayette, IN: Purdue 

University, Cooperative Extension Service.

Cassens, D.L.; Feist, W.C. 1980d. Wood finishing: selection 

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Region Extension Pub. 135. West Lafayette, IN: Purdue 

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Cassens, D.L.; Feist, W.C. 1980e. Wood finishing: finishing 

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Daniels, T.; Hirsch, M.; McClelland, K.; Ross, A.; Williams, 

R.S. 2004. Clear exterior finishes: finding the balance 

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1(9): 42–48.

Feist, W.C. 1979. Protection of wood surfaces with chromium trioxide. Res. Pap. FPL–339. Madison, WI: U.S. 

16–37

Chapter 16 Finishing of Wood
Department of Agriculture, Forest Service, Forest Products 

Laboratory. 11 p.

Feist, W.C. 1982a. Weathering characteristics of finished 

wood-based panel products. JCT Coatings Tech. 

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Feist, W.C. 1982b. Weathering of wood in structural uses. 

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Feist, W.C. 1990. Outdoor wood weathering and protection. In: Rowell, R., ed. Archaeological wood, properties, 

chemistry, and preservation. Advanced in Chemistry Series 

No. 225. Washington, DC: American Chemical Society: 

263–298. Chapter 11.

Feist, W.C. 1996. Finishing exterior wood. Federation Series 

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wood. Advances in Chemistry Series No. 207. Washington, 

DC: American Chemical Society: 401–451. Chapter 11.

Feist, W.C.; Mraz, E.A. 1980. Performance of mildewcides 

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Feist, W.C.; Ross, A.S. 1995. Performance and durability 

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Products Journal. 45(9): 29–36.

Feist, W.C.; Williams, R.S. 1991. Weathering durability of 

chromium treated Southern Pine. Forest Products Journal. 

41(1): 8 14.

Gorman, T.M.; Feist, W.C. 1989. Chronicle of 65 years of 

wood finishing research of the Forest Products Laboratory. 

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R.; Williams, S. 1999. Changing nature of wood products—

what does it mean for coatings and finish performance? JCT 

Coatings Tech. 71(890): 61–66.

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wood with weathering. Forest Products Journal. 

43(2): 55–57.

Kiguchi, M.; Evans, P.D.; Ekstedt, J.; Williams, R.S.; Katoaka, Y. 2001. Improvement of the durability of clear coatings by grafting of UV-absorbers on to wood. Surface 

Coatings International Part B: Coatings Transactions. 

84(B4): 263–270.

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coatings applications. JCT Coatings Tech. 1(3): 36–49.

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1996. Wood decks: materials, construction, and finishing. 

Madison, WI: Forest Products Society. 94 p.

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wood shingle and shake roofs. Corvallis, OR: Oregon State 

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Cleaners and restorers for wood decks. The Paint Dealer. 

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wood on the performance of surface finishes. American 

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Society Spring Meeting. 1993; March 29 April 2.

Williams, R.S.; Feist, W.C. 1994. Effect of preweathering, 

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repellent preservative finishes for wood. Gen. Tech. Rep. 

FPL–GTR–109. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory: 1–12.

Williams, R.S.; Feist, W.C. 2001. Duration of wood preweathering: effect on the service life of subsequently applied paint. JCT Coatings Tech. 73(930): 65–72.

Williams, R.S.; Feist, W.C. 2004. The service life of 

semitransparent stain, solid color stain, and latex paint on 

yellow-poplar and sweetgum plywood and the effect of 

these finishes on wood durability. Forest Products Journal. 

54(7/8): 96–101.

Williams, R.S.; Knaebe, M. 2000. Restoration of severely 

weathered wood. JCT Coatings Tech. 72(902): 43–51.

Williams, R.S.; Knaebe, M.T.; Feist, W.C. 1996. Finishes 

for exterior wood. Madison, WI: Forest Products Society.

Williams, R.S.; Miller, R.B.; Gangstad, J. 2001. Characteristics of ten tropical hardwoods from certified forests in 

Bolivia: part I, weathering characteristics and dimensional 

stability. Wood and Fiber Science. 33(4): 618–626.

Williams, R.S.; Plantinga, P.L.; Feist, W.C. 1990. Photodegradation of wood affects paint adhesion. Forest Products 

Journal. 40(1): 45–49.

Williams, R.S.; Sotos, P.; Feist, W.C. 1999. Evaluation of 

several paint systems on severely weathered wood. JCT 

Coatings Tech. 71(895): 97–102.

Williams, R.S.; Winandy, J.E.; Feist, W.C. 1987a. Adhesion of paint to weathered wood. Forest Products Journal. 

37(11/12): 29–31.

Williams, R.S.; Winandy, J.E.; Feist, W.C. 1987b. 

Paint adhesion to weathered wood. JCT Coatings Tech. 

59(749): 43–49.

Williams, R.S.; Winandy, J.E.; Feist, W.C. 2002. Correlation of adhesive strength with service life of paint applied to 

weathered wood. Paper 161. In: Proceedings, 9th durability 

of building materials and components conference. Brisbane, 

Australia: 17–20 March.

Williams, R.S.; Jourdain, C.; Daisey, G.; Springate, R.W. 

2000. Wood properties affecting finish service life. JCT 

Coatings Tech. 72(902): 35–42.

Williams, R.S.; Knaebe, M.T.; Evans, J.W.; Feist, W.C. 

2001. Erosion rates of wood during natural weathering: part 

III, effect of exposure angle on erosion rates. Wood and Fiber Science. 33(1): 50–57.

Williams, R.S.; Knaebe, M.T.; Sotos, P.G.; Feist, W.C. 2001. 

Erosion rates of wood during natural weathering: part I, effect of grain angle and surface texture. Wood and Fiber Science. 33(1): 33–42.

Williams, R.S.; Lacher, S.; Halpin, C.; White, C. 2005. 

Evaluating weather factors and material response during 

outdoor exposure to determine accelerated test protocols for 

predicting service life. In: Martin, J.W.; Ryntz, R.A.; Dickie, 

R.A., eds. Service Life Prediction, Challenging the Status 

Quo, Federation of Societies for Coatings Technology. Proceedings, 3rd international symposium on service life prediction. Sedona, AZ. 2004 February 1–6.

Williams, R.S.; Ross, A.S.; Sotos, P.; Cheeks, C.N. 2001. 

Water-repellent preservative treatment of brick molding 

prior to factory priming improves paint service life. In: 

Proceedings of the annual meeting of the American Wood 

Preserver’s Association. (Vol. 97): 65–67.

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Chapter 16 Finishing of Wood

CHAPTER 17

Use of Wood in Buildings and Bridges

James P. Wacker, Research General Engineer

17–1

Contents 

Light-Frame Buildings 17–1

 Foundations 17–2

 Floors 17–2

 Exterior Walls 17–3

 Ceiling and Roof 17–4

 Wood Decks 17–4

Post-Frame and Pole Buildings 17–4

Log Buildings 17–6

Heavy Timber Buildings 17–6

 Timber Frame 17–6

 Mill Type 17–7

 Glulam Beam 17–7 

 Arch Structure 17–8

 Dome 17–8

 Timber Bridges 17–8

 Log Stinger 17–8

 Sawn Lumber 17–9

 Glulam 17–9

 Structural Composite Lumber 17–9

Considerations for Wood Buildings 17–9

 Structural 17–9

 Thermal Insulation and Air Infiltration 

 Control 17–9

 Moisture Control 17–10

 Sound Control 17–11

Literature Cited 17–12

In North America, most housing and commercial structures 

built prior to the 20th century used wood as the major structural material. The abundant wood resource formed the basic structure for most houses, commercial buildings, bridges, 

and utility poles. Today, houses and many light commercial 

and industrial buildings are made using modern wood structural materials. Recently, there has been increased interest 

in using wood for various types of transportation structures, 

including highway bridges.

In this chapter, the features of various types of building 

systems are described. Emphasis is placed on how these 

systems have adapted to the use of modern materials and 

techniques. For example, where floor, wall, and roof sheathing for light-frame construction were once commonly 

made from wood boards, sheathing is now commonly 

made from structural panel products, such as plywood and 

oriented strandboard (OSB). Compared with boards, these 

panel products are quicker to install and provide improved 

structural resistance to wind and earthquake loadings. Furthermore, prefabricated floor and wall panels along with 

prefabricated roof and floor trusses or I-joists are replacing 

piece-by-piece on-site construction with dimension lumber. 

A structure can be enclosed within a short time on site using 

factory-made panelized systems.

Engineered wood products are being used increasingly for 

transportation structures. A brief description of the uses of 

wood in railroad and highway bridges and other transportation structures is included.

Light-Frame Buildings

Historically, two general types of light-frame construction 

have been used—balloon and platform framing. Balloon 

framing, which was used in the early part of the 20th century, consists of full-height wall framing members for two-story construction. Additional information on balloon framing 

is available from older construction manuals. Since the latter 

part of the 20th century, platform framing has dominated the 

housing market and is widely used in commercial and light 

industrial applications. Platform framing features the construction of each floor on top of the one beneath. Platform 

framing construction differs from that of 60 years ago in 

the use of new and innovative materials, panel products for 

floor and roof sheathing, and prefabricated components and 

modules as opposed to “stick built” or on-site construction. 

A detailed description of the platform-type of construction 
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General Technical Report FPL–GTR–190

is given in Wood Frame House Construction (Sherwood and 

Stroh 1989); additional information is given in the Wood 

Frame Construction Manual for One- and Two-Family 

Dwellings, 2001 (AF&PA 2001).

Foundations

Light-frame buildings with basements are typically supported on cast-in-place concrete walls or concrete block 

walls supported by footings. This type of construction with a 

basement is common in northern climates. Another practice 

is to have concrete block foundations extend a short distance above ground to support a floor system over a “crawl 

space.” In southern and western climates, some buildings 

have no foundation; the walls are supported by a concrete 

slab, thus having no basement or crawl space.

Treated wood is also used for basement foundation walls. 

Basically, such foundations consist of wood-frame wall 

sections with studs and plywood sheathing supported on 

treated wood plates, all of which are preservatively treated 

to a specified level of protection. To distribute the load, the 

plates are laid on a layer of crushed stone or gravel. Walls, 

which must be designed to resist the lateral loads of the 

backfill, are built using the same techniques as conventional 

walls. The exterior surface of the foundation wall below 

grade is draped with a continuous moisture barrier to prevent direct water contact with the wall panels. The backfill 

must be designed to permit easy drainage and provide drainage from the lowest level of the foundation.

Because a foundation wall needs to be permanent, the preservative treatment of the plywood and framing and the 

type of fasteners used for connections are very important. 

A special foundation (FDN) treatment has been established 

for the plywood and framing, with strict requirements for 

depth of chemical penetration and amount of chemical retention. Corrosion-resistant fasteners (for example, stainless 

steel) are recommended for all preservatively treated wood. 

Additional information and materials and construction procedures are given in Permanent Wood Foundation Basic 

Requirements (AF&PA 2007).

Floors

For houses with basements, the central supporting structure 

may consist of wood posts on suitable footings that carry 

a built-up girder, which is frequently composed of planks 

the same width as the joists (standard 38 by 184 mm to 38 

by 286 mm (nominal 2 by 8 in. to 2 by 12 in.)), face-nailed 

together, and set on edge. Because planks are seldom sufficiently long enough to span the full length of the beam, butt 

joints are required in the layers. The joints are staggered in 

the individual layers near the column supports. The girder 

may also be a glulam beam or steel I-beam, often supported 

on adjustable steel pipe columns. Similar details may be 

applied to a house over a crawl space. The floor framing in 

residential structures typically consists of wood joists on 

400- or 600-mm (16- or 24-in.) centers supported by the 

foundation walls and the center girder (Fig. 17–1).

Joist size depends on the anticipated loading, spacing between joists, distance between supports (span), species, and 

grade of lumber. Commonly used joists are standard 38- by 

184-mm or 38- by 235-mm (nominal 2- by 8-in. or 2- by 

10-in.) lumber, prefabricated wood I-joists, or parallel chord 

trusses. Lumber joists typically span from 3.6 to 4.8 m 

(12 to 16 ft). Span tables are available from the American 

Forest & Paper Association (AF&PA 2005b). Span capabilities of prefabricated wood I-joists or parallel chord trusses 

are recommended by the manufacturer.

Floor openings for stairways, fireplaces, and chimneys may 

interrupt one or more joists. Preferably, such openings are 

parallel to the length of the joists to reduce the number of 

joists that will be interrupted. At the interruption, a support (header) is placed between the uninterrupted joists 

and attached to them. A single header is usually adequate 

for openings up to about 1.2 m (4 ft) in width, but double 

headers are required for wider openings. Special care must 

be taken to provide adequate support at headers (using joist 

hangers, for example).

Cutting of framing members to install items such as plumbing lines and heating ducts should be minimized. Cut members may require a reinforcing scab, or a supplementary 

member may be needed. Areas of highly concentrated loads, 

such as under bathtubs, require doubling of joists or other 

measures to provide adequate support. One advantage of 

framing floors with parallel-chord trusses or prefabricated 

I-joists is that their longer span capabilities may eliminate 

the need for interior supports. An additional advantage is 

that the web areas of these components are designed for 

easy passing of plumbing, electrical, and heating ducts.

Floor sheathing, or subflooring, is used over the floor framing to provide a working platform and a base for the finish flooring. Older homes have board sheathing but newer 

homes generally use panel products. Common sheathing 

materials include plywood and OSB, which are available in 

a number of types to meet various sheathing requirements. 

Exterior-type panels with water-resistant adhesive are desirable in locations where moisture may be a problem, such as 

floors near plumbing fixtures or situations where the subfloor may be exposed to the weather for some time during 

construction.

Plywood should be installed with the grain direction of the 

face plies at right angles to the joists. Oriented strandboard 

also has a preferred direction of installation. Nailing patterns are either prescribed by code or recommended by the 

manufacturer. About 3 mm (1/8 in.) of space should be left 

between the edges and ends of abutting panels to provide for 

dimensional changes associated with moisture content.
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Chapter 17 Use of Wood in Buildings and Bridges

Literature from APA–The Engineered Wood Association 

includes information on the selection and installation of the 

types of structural panels suitable for subfloors (APA 2007).

Exterior Walls

Exterior walls of light-frame structures are generally load 

bearing; they support upper floors and the roof. An exception is the gable ends of a one- or two-story building. Basically, wall framing consists of vertical studs and horizontal 

members, including top and bottom plates and headers (or 

lintels) over window and door openings. The studs are generally standard 38- by 89-mm, 38- by 114-mm, or 38- by 

140-mm (nominal 2- by 4-in., 2- by 5-in., or 2- by 6-in.) 

members spaced between 300 and 600 mm (12 and 24 in.) 

on center. Selection of the stud size depends on the load the 

wall will carry, the need for support of wall-covering materials, and the need for insulation thickness in the walls. 

Headers over openings up to 1.2 m (4 ft) are often 38 by 

140 mm (2 by 6 in.), nailed together face to face with spacers to bring the headers flush with the faces of the studs. 

Special headers that match the wall thickness are also available in the form of either prefabricated I-joists or structural 

composite lumber. Wall framing is erected over the platform 

formed by the first-floor joists and subfloor. In most cases, 

an entire wall is framed in a horizontal position on the subfloor, then tilted into place. If a wall is too long to make this 

procedure practical, sections of the wall can be formed horizontally and tilted up, then joined to adjacent sections.

Corner studs are usually prefabricated in such a configuration as to provide a nailing edge for the interior finish 

(Fig. 17–2). Studs are sometimes doubled at the points of 

intersection with an interior partition to provide backup support for the interior wall finish. Alternatively, a horizontal 

block is placed midheight between exterior studs to support 

the partition wall. In such a case, backup clips on the partition stud are needed to accommodate the interior finish.

Upper plates are usually doubled, especially when rafters 

or floor joists will bear on the top plate between studs. The 

second top plate is added in such a way that it overlaps the 

first plate at corners and interior wall intersections. This provides a tie and additional rigidity to the walls. In areas subject to high winds or earthquakes, ties should be provided 

between the wall, floor framing, and sill plate that should be 

anchored to the foundation. If a second story is added to the 

structure, the edge floor joist is nailed to the top wall plate, 

and subfloor and wall framing are added in the same way as 

the first floor.

Sheathing for exterior walls is commonly some type of panel product. Here again, plywood or OSB may be used. Fiberboard that has been treated to impart some degree of water resistance is another option. Several types of fiberboard 

are available. Regular-density board sometimes requires 

additional bracing to provide necessary resistance to lateral 

loads. Intermediate-density board is used where structural 

support is needed. Numerous foam-type panels can also be 

used to impart greater thermal resistance to the walls.

In cases where the sheathing cannot provide the required 

racking resistance, diagonal bracing must be used. Many 

foam sheathings cannot provide adequate racking resistance, 

Figure 17–1. Typical floor details for platform construction with joists spliced on center beam.
17–4

General Technical Report FPL–GTR–190

so either diagonal braces must be placed at the corners or 

structural panels must be applied over the first 1.2 m (4 ft) 

of the wall from the corner. When light-weight insulating 

foam sheathings are used, bracing is commonly provided by 

standard 19- by 89-mm (nominal 1- by 4-in.) lumber or steel 

strapping.

Ceiling and Roof

Roof systems are generally made of either the joists-andrafter systems or with trusses. Engineered trusses reduce 

on-site labor and can span greater distances without intermediate support, thus eliminating the need for interior loadcarrying partitions. This provides greater flexibility in the 

layout of interior walls. Prefabricated roof trusses are used 

to form the ceiling and sloped roof of more than two-thirds 

of current light-frame buildings. For residential buildings, 

the trusses are generally made using standard 38- by 89-

mm (nominal 2- by 4-in.) lumber and metal plate connectors with teeth that are pressed into the pieces that form the 

joints (TPI 2007).

Joists and rafter systems are found in most buildings constructed prior to 1950. Rafters are generally supported on 

the top plate of the wall and attached to a ridge board at the 

roof peak. However, because the rafters slope, they tend to 

push out the tops of the walls. This is prevented by nailing 

the rafters to the ceiling joists and nailing the ceiling joists 

to the top wall plates (Fig. 17–3a).

A valley or hip is formed where two roof sections meet perpendicular to each other. A valley rafter is used to support 

short-length jack rafters that are nailed to the valley rafter 

and the ridge board (Fig. 17–3b). In some cases, the roof 

does not extend to a gable end but is sloped from some point 

down to the end wall to form a “hip” roof. A hip rafter supports the jack rafters, and the other ends of the jack rafters 

are attached to the top plates (Fig. 17–3c). In general, the 

same materials used for wall sheathing and subflooring are 

used for roof sheathing.

Wood Decks

A popular method of expanding the living area of a home is 

to build a wood deck adjacent to one of the exterior walls. 

Decks are made of preservatively treated lumber, which is 

generally available from local building supply dealers and, 

depending upon the complexity, may be built by the “doit-yourselfer.” To ensure long life, acceptable appearance, 

and structural safety, several important guidelines should be 

followed. Proper material selection is the first step. Then, 

proper design and construction techniques are necessary. Finally, proper maintenance practices are necessary. Detailed 

recommendations for all these areas are included in Wood 

Decks: Materials, Construction, and Finishing (McDonald 

and others 1996) and Prescriptive Residential Wood Deck 

Construction Guide (AWC 2009).

Post-Frame and Pole Buildings

In post-frame and pole buildings, round poles or rectangular 

posts serve both as the foundation and the principal vertical 

framing element. This type of construction was known as 

“pole buildings” but today, with the extensive use of posts, 

Figure 17–2. Corner details for wood stud walls 

that provide support for interior sheathing: (a) traditional three-stud corner with blocking; (b) threestud corner without blocking; (c) two-stud corner 

with wallboard backup clips.
17–5

Chapter 17 Use of Wood in Buildings and Bridges

is commonly referred to as “post-frame” construction. For 

relatively low structures, light wall and roof framing are 

nailed to poles or posts set at fairly frequent centers, commonly 2.4 to 3.6 m (8 to 12 ft). This type of construction 

was originally used with round poles for agricultural buildings, but the structural principle has been extended to commercial and residential buildings (Fig. 17–4).

Round poles present some problems for connecting framing 

members; these problems can be eased by slabbing the outer 

face of the pole. For corner poles, two faces may be slabbed 

at right angles. This permits better attachment of both light 

and heavy framing by nails or timber connectors. When the 

pole is left round, the outer face may be notched to provide 

seats for beams.

Rectangular posts are the most commonly used and may be 

solid sawn, glulam, or built-up by nail laminating. Built-up 

posts are advantageous because only the base of the post 

must be preservatively treated. The treated portion in the 

ground may have laminations of various lengths that are 

matched with the lengths of untreated laminations in the upper part of the post. The design of these types of posts must 

consider the integrity of the splice between the treated and 

untreated lumber. The wall system consists of horizontal 

girts often covered by light-gauge metal that provides some 

degree of racking resistance. Roof trusses made with metal 

plate connectors are attached to each pole, or post, and roof 

purlins are installed perpendicular to the trusses at spacings 

from 1.2 to 3.7 m (4 to 12 ft), with 2.4 m (8 ft) as a common spacing. For 2.4-m (8-ft) truss spacing, these purlins 

are often standard 38 by 89 mm (nominal 2 by 4 in.) spaced 

on 0.6-m (2-ft) centers and attached to either the top of the 

trusses or between the trusses using joists hangers. The roofing is often light-gauge metal that provides some diaphragm 

Figure 17–3. (a) A rafter-type roof with typical framing details for (b) a valley and (c) a hip corner.
stiffness to the roof and transmits a portion of the lateral 

loading to the walls parallel to the direction of the load. Detailed information on the design of post-frame buildings is 

provided by the National Frame Builders Association (1999) 

or Walker and Woeste (1992).

Log Buildings

Interest is growing in log houses—from small, simple 

houses for vacation use to large, permanent residences 

(Fig. 17–5). Many U.S. firms specialize in the design and 

materials for log houses. Log houses nearly always feature 

wall systems built from natural or manufactured logs rather 

than from dimension lumber. Roof and floor systems may 

also be built with logs or conventional framing. Log house 

companies tend to categorize log types into two systems: 

round and shaped. In the round log system, the logs are machined to a smooth, fully rounded surface, and they are generally all the same diameter. In the shaped system, the logs 

are machined to specific shapes, generally not fully round. 

The exterior surfaces of the logs are generally rounded, but 

the interior surfaces may be either flat or round. The interface between logs is machined to form an interlocking joint.

Consensus standards have been developed for log 

grading and the assignment of allowable properties, and 

these standards are being adopted by building codes 

(ASTM 2009). Builders and designers need to realize that 

logs can reach the building site at moisture content levels 

greater than ideal. The effects of seasoning and the 

consequences of associated shrinkage and checking must be 

considered. Additional information on log homes is available from The Log Home Council, National Association of 

Home Builders, Washington, D.C., or in Standard on the 

Design and Construction of Log Structures (ICC 2007).

Heavy Timber Buildings

Timber Frame

Timber frame houses were common in early America and 

are enjoying some renewed popularity today. Most barns 

and factory buildings dating prior to the middle of the 20th 

century were heavy timber frame. The traditional timber 

frame is made of large sawn timbers (larger than 114 by 

114 mm (5 by 5 in.)) connected to one another by handfabricated joints, such as mortise and tenon. Construction 

of such a frame involves rather sophisticated joinery, as illustrated in Figure 17–6.

In today’s timber frame home, a prefabricated, composite 

sheathing panel (1.2 by 2.4 m (4 by 8 ft)) is frequently applied directly to the frame. This panel may consist of an 

inside layer of 13-mm (1/2-in.) gypsum, a core layer of rigid 

foam insulation, and an outside layer of exterior plywood 

or OSB. Finish siding is applied over the composite panel. 

In some cases, a layer of standard 19-mm (nominal l-in.) 

tongue-and-groove, solid-wood boards is applied to the 

frame, and a rigid, foam-exterior, plywood composite panel 

is then applied over the boards to form the building exterior. 

Figure 17–4. Pole and post-frame buildings: (left) pole or post forms both foundation and wall; 

(right) pole or post forms only the foundation for conventional platform-framed structure.

17–6

General Technical Report FPL–GTR–190
Local fire regulations should be consulted about the acceptance of various foam insulations.

Framing members are cut in large cross sections; therefore, 

seasoning them before installation is difficult, if not impossible. Thus, the builder (and the owner) should recognize the 

dimensional changes that may occur as the members dry in 

place. The structure must be designed to accommodate these 

dimensional changes as well as seasoning checks, which are 

almost inevitable.

Mill Type

Mill-type construction has been widely used for warehouse 

and manufacturing structures, particularly in the eastern 

United States. This type of construction uses timbers of 

large cross sections with columns spaced in a grid according to the available lengths of beam and girder timbers. The 

size of the timbers makes this type of construction resistant 

to fire. The good insulating qualities of wood as well as the 

char that develops during fire result in slow penetration of 

fire into the large members. Thus, the members retain a large 

proportion of their original load-carrying capacity and stiffness for a relatively lengthy period after the onset of fire. 

To be recognized as mill-type construction, the structural 

elements must meet specific sizes—columns cannot be 

less than standard 184 mm (nominal 8 in.) in dimension, 

and beams and girders cannot be less than standard 140 by 

235 mm (nominal 6 by 10 in.) in cross section. Other limitations must be observed as well. For example, walls must be 

made of masonry, and concealed spaces must be avoided. 

The structural frame has typically been constructed of solidsawn timbers, which should be stress graded. These timbers 

can now be supplanted with glulam timbers, and longer 

spans are permitted.

Glulam Beam

A panelized roof system using glulam roof framing is 

widely used for single-story commercial buildings in the 

southwestern United States. This system is based on supporting columns located at the corners of pre-established 

grids. The main glulam beams support purlins, which may 

be sawn timbers, glulam, parallel chord trusses, or prefabricated wood I-joists. These purlins, which are normally on 

2.4-m (8-ft) centers, support preframed structural panels. 

The basic unit of the preframed system is a 1.2- by 2.4-m 

(4- by 8-ft) structural panel nailed to standard 38- by 89-mm 

or 38- by 140-mm (nominal 2- by 4-in. or 2- by 6-in.) stiffeners (subpurlins). The stiffeners run parallel to the 2.4-m 

(8-ft) dimension of the structural panel. One stiffener is located at the centerline of the panel; the other is located at an 

edge, with the plywood edge at the stiffener centerline. The 

stiffeners are precut to a length equal to the long dimension 

of the plywood less the thickness of the purlin, with a small 

allowance for the hanger.

In some cases, the purlins are erected with the hangers in 

place. The prefabricated panels are lifted and set into place 

in the hangers, and the adjoining basic panels are then attached to each other. In other cases, the basic panels are attached to one purlin on the ground. An entire panel is lifted 

into place to support the loose ends of the stiffeners. Additional details on this system and other glulam details are 

available from the American Institute of Timber Construction (www.aitc-glulam.org).

Figure 17–5. Modern log homes are available in a variety of designs.

Figure 17–6. Timber frame structure with typical 

joint details.

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Chapter 17 Use of Wood in Buildings and Bridges
Arch Structure

Arch structures are particularly suited to applications in 

which large, unobstructed areas are needed, such as churches, recreational buildings, and aircraft hangars. Many arch 

forms are possible with the variety limited only by the imagination of the architect. Churches have used arches from the 

beginning of glulam manufacture in the United States. Additional information on the use and design of arches is given 

in The Timber Construction Manual (AITC 2004).

Dome

Radial-rib domes consist of curved members extending from 

the base ring (tension ring) to a compression ring at the top 

of the dome along with other ring members at various 

elevations between the tension and compression rings 

(Fig. 17–7). The ring members may be curved or straight. 

If they are curved to the same radius as the rib and have 

their centers at the center of the sphere, the dome will have 

a spherical surface. If the ring members are straight, the 

dome will have an umbrella look. Connections between the 

ribs and the ring members are critical because of the high 

compressive loads in the ring members. During construction, care must be taken to stabilize the structure because the 

dome has a tendency to rotate about the central vertical axis.

Other dome patterns called Varax and Triax are also used. 

Their geometries are quite complex, and specialized computer programs are used in their design. Steel hubs used at 

the joints and supports are critical. An example of a Triax 

dome is shown in Figure 17–8.

Timber Bridges

Prior to the 20th century, timber was the major material used 

for both highway and railroad bridges. The development of 

steel and reinforced concrete provided other options, and 

these have become major bridge building materials. However, the U.S. inventory does contain a substantial number 

of timber bridges, many of which continue to carry loads 

beyond their design life. A recent initiative in the United 

States has focused research and technology transfer efforts 

on improving the design and performance of timber bridges. 

As a result, hundreds of timber highway bridges were built 

across the United States during the past several years, many 

using innovative designs and materials.

Bridges consist of a substructure and a superstructure. The 

substructure consists of abutments, piers, or piling, and it 

supports the superstructure that consists of stringers and/

or a deck. The deck is often covered with a wearing surface 

of asphalt. Timber may be combined with other materials 

to form the superstructure, for example, timber deck over 

steel stringers. Several bridge railing systems were recently 

crash-tested and approved for use by the Federal Highway 

Administartion (Faller and others 1999). Covered bridges 

are also undergoing a resurgence of interest, with a recent 

national program for the rehabilitation and restoration of 

numerous historic structures. The various types of timber 

bridge superstructures are described in the following sections. Detailed information on modern timber bridges is 

given in Timber Bridges: Design, Construction, Inspection, 

and Maintenance (Ritter 1992).

Log Stringer

A simple bridge type that has been used for centuries consists of one or more logs used to span the opening. Several 

logs may be laid side-by-side and fastened together. The log 

stringer bridge has been used to access logging areas and is 

advantageous when adequate-sized logs are available and 

the bridge is needed for only a short time. Unless built with 

a durable species, the life span of log stringer bridges is usually limited to less than 10 years.

Figure 17–7. Member layout for a radial-rib dome.

Figure 17–8. This 161.5-m- (530-ft-) diameter Tacoma 

dome (Tacoma, Washington), built in 1982–1983, 

is one of the longest clear roof spans in the world. 

(Photo courtesy of Western Wood Structures, Inc., 

Tualatin, Oregon.)

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General Technical Report FPL–GTR–190
Sawn Lumber

Several types of bridges can be built with sawn lumber. 

Even though the span is usually limited to about 9 m (30 ft) 

because of the limited size of lumber available, this span 

length entails the majority of timber bridges in the United 

States.

Several timbers can be used to span the opening, and a 

transverse lumber deck can be placed over them to form a 

stringer and deck bridge. Lumber can be placed (on-edge) 

side-by-side and used to span the entire opening, forming 

a longitudinal deck bridge. The lumber can be fastened together with nails or large spikes in partial-width panelized 

bridge systems or compressed together with high-strength 

tension rods to form a “stress-laminated” slab-type deck.

Glulam

Structural glued-laminated (glulam) timber greatly extends 

the span capabilities of the same types of bridges described 

in the previous paragraph. Glulam stringers placed 0.6 to 

1.8 m (2 to 6 ft) on center can support a glulam deck system 

and result in spans of 12 to 30 m (40 to 100 ft) or more 

(Fig. 17–9). Using glulam panels to span the opening results 

in a longitudinal deck system, but this is usually limited to 

about 9-m (30-ft) spans. These panels are either interconnected or supported at one or more locations with transverse 

distributor beams. Glulam beams can be used to form a solid 

deck and are held together with high-stress tension rods 

to form a stress-laminated slab-type deck. Curved glulam 

members can be used to produce various aesthetic effects 

and long-span bridges (Fig. 17–10).

Structural Composite Lumber

Two types of structural composite lumber (SCL)—laminated veneer and oriented strand—are beginning to be used to 

build timber bridges. Most of the same type of bridges built 

with either solid-sawn or glulam timber can be built with 

SCL (Chap. 11).

Considerations for Wood Buildings

Many factors must be considered when designing and constructing wood buildings, including structural, insulation, 

moisture, and sound control. The following sections provide 

a brief description of the design considerations for these 

factors. Fire safety, another important consideration, is addressed in Chapter 18.

Structural

The structural design of any building consists of combining 

the prescribed performance requirements with the anticipated loading. One major performance requirement is that there 

be an adequate margin of safety between the structure’s 

ultimate capacity and the maximum anticipated loading. The 

probability that the building will ever collapse is minimized 

using material property information recommended by the 

material manufacturers along with code-recommended design loads.

Another structural performance requirement relates to serviceability. These requirements are directed at ensuring that 

the structure is functional, and the most notable one is that 

deformations are limited. It is important to limit deformations so that floors are not too “bouncy” or that doors do not 

bind under certain loadings. Building codes often include 

recommended limits on deformation, but the designer may 

be provided some latitude in selecting the limits. The basic 

reference for structural design of wood in all building systems is the National Design Specification for Wood Construction (AF&PA 2005a).

Thermal Insulation and Air Infiltration Control

For most U.S. climates, the exterior envelope of a building needs to be insulated either to keep heat in the building 

or prevent heat from entering. Wood frame construction is 

well-suited to application of both cavity insulation and surface-applied insulation. The most common materials used 

for cavity insulation are glass fiber, mineral fiber, cellulose 

insulation, and spray-applied foams. For surface applications, a wide variety of sheathing insulations exist, such as 

rigid foam panels. Insulating sheathing placed on exterior 

walls may also have sufficient structural properties to provide required lateral bracing. Prefinished insulating paneling 

can be used as an inside finish on exterior walls or one or 

both sides of the interior partitions. In addition, prefinished 

insulation can underlay other finishes.

Attic construction with conventional rafters and ceiling 

joists or roof trusses can be insulated between framing 

members with batt, blanket, or loose-fill insulation. In some 

warm climates, radiant barriers and reflective insulations 

can provide an additional reduction in cooling loads. The 

“Radiant Barrier Attic Fact Sheet” from the U.S. Department of Energy (1991) provides information on climatic 

areas that are best suited for radiant barrier applications. 

This document also provides comparative information on 

Figure 17–9. Glulam beam bridge over the Dangerous 

River, near Yukatat, Alaska, consists of three 43.5-m 

(143-ft) spans. Each span is supported by four 2.3-m- 

(91.5-in.-) deep glulam beams.

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Chapter 17 Use of Wood in Buildings and Bridges
the relative performance of these products and conventional 

fibrous insulations.

Existing frame construction can be insulated pneumatically 

using suitable loose-fill insulating material. When loose-fill 

materials are used in wall retrofit applications, extra care 

must be taken during the installation to eliminate the existence of voids within the wall cavity. All cavities should be 

checked prior to installation for obstructions, such as fire 

stop headers and wiring, that would prevent the cavity from 

being completely filled. Care must also be taken to install 

the material at the manufacturer’s recommended density to 

ensure that the desired thermal performance is obtained. Accessible space can be insulated by manual placement of batt, 

blanket, or loose-fill material.

In addition to being properly insulated, the exterior envelope 

of all buildings should be constructed to minimize air flow 

into or through the building envelope. Air flow can degrade 

the thermal performance of insulation and cause excessive 

moisture accumulation in the building envelope.

More information on insulation and air flow retarders can be 

found in the ASHRAE Handbook of Fundamentals, chapters 

22 to 24 (ASHRAE 2005).

Moisture Control

Moisture sources for buildings can be broadly classified as 

follows: (1) surface runoff of precipitation from land areas, 

(2) ground water or wet soil, (3) precipitation or irrigation 

water that falls on the building, (4) indoor humidity, 

(5) outdoor humidity, (6) moisture from use of wet building 

materials or construction under wet conditions, and (7) errors, accidents, and maintenance problems associated with 

indoor plumbing. At a given instant of time, the categories 

are distinct from each other. Water can change phase and 

can be transported over space by various mechanisms. Water 

may therefore be expected to move between categories over 

time, blurring the distinctions between categories. Christian 

(1994) provides quantitative estimates of potential moisture 

loads from various sources.

Moisture accumulation within a building or within parts of 

a building can affect human comfort and health, influence 

building durability, and necessitate maintenance and repair 

activities (or can require that these activities be undertaken 

more frequently). Moisture accumulation in the building’s 

thermal envelope is also likely to influence the building’s 

energy performance. Some problems associated with moisture accumulation are easily observed. Examples include: 

(a) mold and mildew, (b) decay of wood-based materials, 

(c) corrosion of metals, (d) damage caused by expansion of 

materials from moisture (such as buckling of wood floors), 

and (e) decline in visual appearance (such as paint peeling, 

distortion of wood-based siding, or efflorescence on masonry surfaces). Some problems associated with moisture accumulation may not be readily apparent but are nonetheless 

real; an example is reduced performance of insulated assemblies (resulting in increased energy consumption). Detailed 

discussions on the effects and the control of moisture in 

buildings can be found in an ASTM standard (ASTM 2008), 

the ASHRAE Handbook of Fundamentals, chapters 23 and 

24 (ASHRAE 2005), and Lstiburek and Carmody (1999).

Mold, Mildew, Dust Mites, and Human Health

Mold and mildew in buildings are offensive, and the spores 

can cause respiratory problems and allergic reactions in 

humans. Mold and mildew will grow on most surfaces if 

the relative humidity at the surface is above a critical value 

and the surface temperatures are conducive to growth. The 

longer the surface remains above this critical relative humidity level, the more likely mold will appear; the higher 

the humidity or temperature, the shorter the time needed 

for germination. The surface relative humidity is a complex 

function of material moisture content, material properties, 

local temperature, and humidity conditions. In addition, 

mold growth depends on the type of surface. Mildew and 

mold can usually be avoided by limiting surface relative 

humidity conditions >80% to short periods. Only for nonporous surfaces that are regularly cleaned should this criterion 

be relaxed. Most molds grow at temperatures approximately 

above 4 °C (40 °F). Moisture accumulation at temperatures 

below 4 °C (40 °F) may not cause mold and mildew if the 

material is allowed to dry out below the critical moisture 

content before the temperature increases above 4 °C (40 °F).

Dust mites can trigger allergies and are an important cause 

of asthma. They thrive at high relative humidity levels 

(>70%) at room temperature, but will not survive at sustained relative humidity levels less than 50%. However, 

these relative humidity levels relate to local conditions in 

the typical places that mites tend to inhabit (for example, 

mattresses, carpets, soft furniture).

Figure 17–10. The Alton Sylor Memorial Bridge is a 

glulam deck arch bridge crossing Joncy Gorge in Angelica, New York. The center three-hinged arch spans 

52 m (171 ft) and is the longest clear span in the United States. (Photo courtesy of Laminated Concepts, 

Inc., Big Flats, New York.)

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General Technical Report FPL–GTR–190
Paint Failure and Other Appearance Problems

Moisture trapped behind paint films may cause failure of 

the paint (Chap. 16). Water or condensation may also cause 

streaking or staining. Excessive swings in moisture content of wood-based panels or boards may cause buckling 

or warp. Excessive moisture in masonry and concrete can 

produce efflorescence, a white powdery area or lines. When 

combined with low temperatures, excessive moisture can 

cause freeze–thaw damage and spalling (chipping).

Structural Failures

Structural failures caused by decay of wood are rare but 

have occurred. Decay generally requires a wood moisture 

content equal to or greater than fiber saturation (usually 

about 30%) and temperatures between 10 and 43 °C (50 and 

100 °F). Wood moisture content levels above fiber saturation 

are only possible in green lumber or by absorption of liquid 

water from condensation, leaks, ground water, or other saturated materials in contact with the wood. To maintain a safety margin, a 20% moisture content is sometimes used during 

field inspections as the maximum allowable level. Once 

established, decay fungi produce water that enables them to 

maintain moisture conditions conducive to their growth. See 

Chapter 14 for more information on wood decay.

Rusting or corrosion of nails, nail plates, or other metal 

building products is also a potential cause of structural 

failure. In the rare cases of catastrophic structural failure 

of wood buildings (almost always under the influence of a 

large seismic load or an abnormally high wind load), failure 

of mechanical connections usually plays a critical role. Corrosion may occur at high relative humidity levels near the 

metal surface or as a result of liquid water from elsewhere. 

Wood moisture content levels >20% encourage corrosion 

of steel fasteners in wood, especially if the wood is treated 

with preservatives. In buildings, metal fasteners are often 

the coldest surfaces, which encourages condensation on, 

and corrosion of, fasteners.

Effect on Heat Flow

Moisture in the building envelope can significantly degrade 

the thermal performance of most insulation materials but 

especially the thermal resistance of fibrous insulations and 

open cell foams. The degradation is most pronounced when 

daily temperature reversals across the insulation drive moisture back and forth through the insulation.

Moisture Control Strategies

Strategies to control moisture accumulation fall into two 

general categories: (1) minimize moisture entry into the 

building envelope and (2) provide for removal (dissipation) of moisture from the building envelope. Inasmuch as 

building materials and assemblies are often wetted during 

construction, design strategies that encourage dissipation 

of moisture from the assemblies are highly recommended. 

Such strategies will also allow the building to better withstand wetting events that occur rarely, and thus are largely 

unanticipated, but which nonetheless occur at least once 

during the building’s lifespan. Effective moisture dissipation 

strategies typically involve drainage and ventilation.

The transport mechanisms that can move moisture into or 

out of building envelopes have various transport capabilities. The mechanisms, in order of the quantities of moisture that they can move, are as follows: (a) liquid water 

movement, including capillary movement; (b) water vapor 

transport by air movement; and (c) water vapor diffusion. 

Trechsel (2001) discusses these in detail. In design for control of moisture entry, it is logical to prioritize control of the 

transport mechanisms in the order of their transport capabilities. A logical prioritization is thus as follows: (a) control 

of liquid entry by proper site grading and installing gutters 

and downspouts and appropriate flashing around windows, 

doors, and chimneys; (b) control of air leakage by installing 

air flow retarders or careful sealing by taping and caulking; 

and (c) control of vapor diffusion by placing vapor retarders 

on the “warm” side of the insulation. Trechsel (2001) makes 

the point that although air leakage can potentially move 

much greater amounts of moisture than diffusion, the potential for moisture damage is not necessarily proportional to 

the amount of moisture movement, and thus that moisture 

diffusion in design of building envelopes should not be 

ignored.

In inhibiting vapor diffusion at the interior surfaces of building assemblies in heating climates (or alternatively, encouraging such diffusion at interior surfaces of building assemblies in cooling climates), control of indoor humidity levels 

is usually important. In heating climates, ventilation of the 

living space with outdoor air and limiting indoor sources 

of moisture (wet firewood, unvented dryers, humidifiers) 

will lower indoor humidity levels. This is very effective at 

lowering the rate of moisture diffusion into the building’s 

thermal envelope. In cooling climates, the lower indoor humidity levels afforded by mechanical dehumidification will 

encourage dissipation of moisture (to the interior) from the 

building’s thermal envelope. More information on the definition of heating and cooling climates and specific moisture 

control strategies can be found in the ASHRAE Handbook 

of Fundamentals, chapter 24 (ASHRAE 2005).

Sound Control

An important design consideration for residential and office 

buildings is the control of sound that either enters the structure from outside or is transmitted from one room to another. Wood frame construction can achieve levels of sound 

control equal to or greater than more massive construction, 

such as concrete. However, to do so requires designing for 

both airborne and impact noise insulation.

Airborne noise insulation is the resistance to transmission of 

airborne noises, such as traffic or speech, either through or 

around an assembly such as a wall. Noises create vibrations 

on the structural surfaces that they contact, and the design 

challenge is to prevent this vibration from reaching and 

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Chapter 17 Use of Wood in Buildings and Bridges
leaving the opposite side of the structural surface. Sound 

transmission class (STC) is the rating used to characterize 

airborne noise insulation. A wall system with a high STC 

rating is effective in preventing the transmission of sound. 

Table 17–1 lists the STC ratings for several types of wall 

systems; detailed information for both wall and floor are 

given in FPL–GTR–43 (Rudder 1985).

Impact noise insulation is the resistance to noise generated 

by footsteps or dropping objects, generally addressed at 

floor–ceiling assemblies in multi-family dwellings. Impact 

insulation class (IIC) is the rating used to characterize the 

impact noise insulation of an assembly. Both the character 

of the flooring material and the structural details of the floor 

influence the IIC rating. Additional information on IIC ratings for wood construction is given in FPL–GTR–59 (Sherwood and Moody 1989).

Literature Cited

AF&PA. 2001. Wood frame construction manual for 

one- and two-family dwellings, 2001 edition. Washington, 

DC: American Forest & Paper Association.

AF&PA. 2005a. National design specification for wood 

construction. Washington, DC: American Forest & Paper 

Association.

AF&PA. 2005b. Span tables for joists and rafters. 

Washington, DC: American Forest & Paper Association.

AF&PA. 2007. Permanent wood foundation design 

specification. Washington, DC: American Forest & Paper 

Association.

AITC. 2004. Timber construction manual, 5th edition. 

American Institute of Timber Construction. New York: 

John Wiley & Sons, Inc.

APA. 2007. Engineered wood construction guide: floor construction. Section 2, Form E30. September 2007. Tacoma, 

WA: APA–The Engineered Wood Association.

ASHRAE. 2005. Handbook of fundamentals. Atlanta, 

GA: American Society of Heating, Refrigerating and 

Air-Conditioning Engineers.

ASTM. 2008. Standard guide for limiting water-induced 

damage to buildings. ASTM E241-08. West Conshohocken, 

PA: American Society for Testing and Materials.

ASTM. 2009. Standard methods for establishing stress 

grades for structural members used in log buildings. ASTM 

D3957-09. West Conshohocken, PA: American Society for 

Testing and Materials.

AWC. 2009. Prescriptive wood deck construction guide. 

Washington, DC: American Wood Council.

Christian, J.E. 1994. Moisture sources. Chapter 8 in moisture control in buildings, ASTM MNL18 (ISBN 0-8031-

2051-6). Philadelphia, PA. American Society for Testing 

and Materials.

Faller, R.K.; Ritter, M.A.; Rosson, B.T.; Duwadi, S.R. 1999. 

Railing systems for use on timber deck bridges. Underground and other structural design issues. Washington, 

D.C.: National Academy Press. Transportation research 

record 1656. pp. 110–119.

Table 17–1. Sound transmission class (STC) ratings for typical wood-frame walls

STC 

rating Privacy afforded Wall structure 

25 Normal speech easily understood 6-mm (1/4-in.) wood panels nailed on each side of standard 38- by 89-mm 

(nominal 2- by 4-in.) studs. 

30 Normal speech audible but not 

intelligible 

9.5-mm (3/8-in.) gypsum wallboard nailed to one side of standard 38- by 

89-mm (nominal 2- by 4-in.) studs. 

35 Loud speech audible and fairly 

understandable 

20-mm (5/8-in.) gypsum wallboard nailed to both sides of standard 38- by 

89-mm (nominal 2- by 4-in.) studs. 

40 Loud speech audible but not 

intelligible 

Two layers of 20-mm (5/8-in.) gypsum wallboard nailed to both sides of 

standard 38- by 89-mm (nominal 2- by 4-in.) studs. 

45 Loud speech barely audible Two sets of standard 38- by 64-mm (nominal 2- by 3-in.) studs staggered 0.2 m 

(8 in.) on centers fastened by standard 38- by 89-mm (nominal 2- by 4-in.) base 

and head plates with two layers of 20-mm (5/8-in.) gypsum wallboard nailed on 

the outer edge of each set of studs. 

50 Shouting barely audible Standard 38- by 89-mm (nominal 2- by 4-in.) wood studs with resilient channels 

nailed horizontally to both sides with 20-mm (5/8-in.) gypsum wallboard 

screwed to channels on each side. 

55 Shouting not audible Double row of standard 38- by 89-mm (nominal 2- by 4-in.) studs 0.4 m (16 in.) 

on centers fastened to separate plates spaced 25 mm (1 in.) apart. Two layers of 

20-mm (5/8-in.) gypsum wallboard screwed 0.3 m (12 in.) on center to the 

studs. An 89-mm- (3.5-in.-) thick sound-attenuation blanket installed in one stud 

cavity. 

17–12

General Technical Report FPL–GTR–190
ICC. 2007. Standard on the design and construction of log 

structures. ICC400-2007. Washington, DC: International 

Code Council.

Lstiburek, J.; Carmody, J. 1999. Moisture control handbook: 

principles and practices for residential and small commer-

cial buildings. New York: Wiley.

McDonald, K.A.; Falk, R.H.; Williams, R.S.; Winandy, J.E. 

1996. Wood decks: materials, construction, and finishing. 

Madison, WI: Forest Products Society.

National Frame Builders Association. 1999. Post frame 

building design manual. Glenview, IL: National Frame 

Builders Association.

Ritter, M.A. 1992. Timber bridges—design, construction, 

inspection and maintenance. EM 7700–8. Washington, DC: 

U.S. Department of Agriculture, Forest Service, Engineering 

Staff.

Rudder, F.F. Jr. 1985. Airborne sound transmission loss 

characteristics of wood-frame construction. Gen. Tech. Rep. 

FPL–GTR–43. Madison, WI: U.S. Department of Agricul-

ture, Forest Service, Forest Products Laboratory.

Sherwood, G.E.; Moody, R.C. 1989. Light-frame wall and 

floor systems—analysis and performance. Gen. Tech. Rep.

FPL–GTR–59. Madison, WI: U.S. Department of Agricul-

ture, Forest Service, Forest Products Laboratory.

Sherwood, G.E.; Stroh, R.C. 1989. Wood frame house con-

struction. Agric. Handb. 73. Washington, DC: U.S. Govern-

ment Printing Office. (Also available from Armonk Press, 

Armonk, NY.)

TPI. 2007. National design standards for metal plate con-

nected wood truss construction. ANSI/TPI 1–2007. Alexan-

dria, VA: Truss Plate Institute.

Trechsel, H.R. 2001. Moisture primer. Chapter 1 in mois-

ture analysis and condensation control in buildings. ASTM 

MNL40 (ISBN 0-8031-2089-3). West Conshohocken, PA. 

ASTM International.

U.S. Department of Energy. 1991. Radiant barrier attic fact 

sheet. Oak Ridge, TN: Oak Ridge National Laboratory.

Walker, J.N.; Woeste, F.E. 1992. Post-frame building design. 

ASAE Monograph No. 11. St. Joseph, MI: American Soci-

ety of Agricultural and Biological Engineers. 

17–13

Chapter 17 Use of Wood in Buildings and Bridges

CHAPTER 18

Fire Safety of Wood Construction

Robert H. White, Research Forest Products Technologist

Mark A. Dietenberger, Research General Engineer

18–1

Contents 

Fire Safety Design and Evaluation 18–1

 Types of Construction 18–2

 Ignition 18–2

 Exterior Fire Exposure in the Wildland–Urban 

 Interface 18–3

 Fire Growth Within Compartment 18–3

 Containment to Compartment of Origin 18–6

Fire-Performance Characteristics of Wood 18–8

 Thermal Degradation of Wood 18–8

 Ignition 18–9

 Heat Release and Smoke 18–11

 Flame Spread 18–12

 Charring and Fire Resistance 18–13

Fire-Retardant-Treated Wood 18–15

 Pressure Treatments 18–15

 Performance Requirements 18–16

 Fire-Retardant Coatings 18–17

Literature Cited 18–18

Additional References 18–18

 General 18–18

 Fire Test Standards 18–19

 Ignition 18–19

 Flame Spread 18–20

 Flashover and Room/Corner Tests 18–20

 Heat Release and Heat of Combustion 18–20

 Combustion Products 18–21

 Fire Resistance 18–21

 Fire-Retardant-Treated Wood 18–22

Fire safety is an important concern in all types of construction. The high level of national concern for fire safety is 

reflected in limitations and design requirements in building 

codes. These code requirements and related fire performance 

data are discussed in the context of fire safety design and 

evaluation in the initial section of this chapter. Because 

basic data on fire behavior of wood products are needed to 

evaluate fire safety for wood construction, the second major 

section of this chapter provides additional information on 

fire behavior and fire performance characteristics of wood 

products. The chapter concludes with a discussion of fireretardant treatments that can be used to reduce the combustibility of wood.

Fire Safety Design and Evaluation

Fire safety involves prevention, detection, evacuation, containment, and extinguishment. Fire prevention basically 

means preventing the sustained ignition of combustible 

materials by controlling either the source of heat or the 

combustible materials. This involves proper design, installation or construction, and maintenance of the building and 

its contents. Proper fire safety measures depend upon the 

occupancy or processes taking place in the building. Smoke 

and heat detectors can be installed to provide early detection 

of a fire. Early detection is essential for ensuring adequate 

time for egress. Egress, or the ability to escape from a fire, 

often is a critical factor in life safety. Statutory requirements 

pertaining to fire safety are specified in building codes or 

fire codes. Design deficiencies are often responsible for 

spread of heat and smoke in a fire. Spread of a fire can be 

prevented with designs that limit fire growth and spread 

within a compartment and contain fire to the compartment 

of origin. Sprinklers provide improved capabilities to extinguish a fire in its initial stages. These requirements fall into 

two broad categories: material requirements and building 

requirements. Material requirements include such things as 

combustibility, flame spread, and fire resistance. Building 

requirements include area and height limitations, firestops 

and draftstops, doors and other exits, automatic sprinklers, 

and fire detectors.

Adherence to codes will result in improved fire safety. Code 

officials should be consulted early in the design of a building because the codes offer alternatives. For example, floor 

areas can be increased if automatic sprinkler systems are 

added. Code officials have the option to approve alternative 

materials and methods of construction and to modify 
18–2

General Technical Report FPL–GTR–190

provisions of the codes when equivalent fire protection and 

structural integrity are documented.

Most current building codes in the United States are based 

on the model building code produced by the International 

Code Council (ICC) (International Building Code® (IBC)) 

and related International Code® (I-Codes®) documents). 

In addition to the documents of the ICC, the National Fire 

Protection Association’s (NFPA’s) Life Safety Code (NFPA 

101) provides guidelines for life safety from fire in buildings 

and structures. NFPA also has a model building code known 

as NFPA 5000. The provisions of the ICC and NFPA documents become statutory requirements when adopted by local 

or state authorities having jurisdiction.

Information on fire ratings for different products and assemblies can be obtained from industry literature, evaluation 

reports issued by ICC Evaluation Service, Inc. (ICC-ES) 

and other organizations, and listings published by testing 

laboratories or quality assurance agencies. Products listed 

by Underwriters Laboratories, Inc. (UL), Intertek, and other 

such organizations are stamped with the rating information.

The field of fire safety engineering is undergoing rapid 

changes because of the development of more engineering 

and scientific approaches to fire safety. This development 

is evidenced by the publication of the fourth edition of The 

Society of Fire Protection Engineers (SFPE) Handbook of 

Fire Protection Engineering. Steady advances are being 

made in the fields of fire dynamics, fire hazard calculations, 

fire design calculations, and fire risk analysis. Such efforts 

support the worldwide trend to develop alternative building 

codes based on performance criteria rather than prescriptive 

requirements. Additional information on fire protection can 

be found in various publications of the NFPA and SFPE.

In the following sections, various aspects of building code 

provisions pertaining to fire safety of building materials are 

discussed under the broad categories of (a) types of construction, (b) ignition, (c) fire growth within compartment, 

(d) containment to compartment of origin, and (e) exterior 

fires. These are largely requirements for materials. Information on prevention and building requirements not related to 

materials (for example, detection) can be found in NFPA 

publications.

Types of Construction

A central aspect of the fire safety provisions of building 

codes is the classification of buildings by types of construction and use or occupancy. Based on classifications of building type and occupancy, the codes set limits on areas and 

heights of buildings. Building codes generally recognize 

five classifications of construction based on types of materials and required fire resistance ratings. The two classifications known as Type I (fire-resistant construction) and Type 

II (noncombustible construction) basically restrict 

the building elements to noncombustible materials. Wood 

is permitted to be used more liberally in the other three 

classifications, which are Type III (ordinary), Type IV 

(heavy timber), and Type V (light-frame). Type III construction allows smaller wood members to be used for interior 

walls, floors, and roofs including wood studs, joists, trusses, 

and I-joists. For Type IV (heavy timber) construction, interior wood columns, beams, floors, and roofs are required 

to satisfy certain minimum dimensions and no concealed 

spaces are permitted. In both Types III and IV construction, exterior walls must be of noncombustible materials, 

except that fire-retardant-treated (FRT) wood is permitted 

within exterior wall assemblies of Type III construction 

when the requirements for fire resistance ratings are 2-h or 

less. In Type V construction, walls, floors, and roofs may be 

of any dimension lumber and the exterior walls may be of 

combustible materials. Types I, II, III, and V constructions 

are further subdivided into two parts—A (protected) and B 

(unprotected), depending on the required fire resistance ratings. In Type V-A (protected light-frame) construction, most 

of the structural elements have a 1-h fire resistance rating. 

No general fire resistance requirements are specified for 

buildings of Type V-B (unprotected light-frame) construction. The required fire resistance ratings for exterior walls 

also depend on the fire separation distance from the lot line, 

centerline of the street, or another building. Such property 

line setback requirements are intended to mitigate the risk of 

exterior fire exposure.

Based on their performance in the ASTM E 136 test (see list 

of fire test standards at end of chapter), both untreated and 

FRT wood are combustible materials. However, building 

codes permit substitution of FRT wood for noncombustible 

materials in some specific applications otherwise limited to 

noncombustible materials. Specific performance and treatment requirements are defined for FRT wood used in such 

applications.

In addition to type of construction, height and area limitations also depend on the use or occupancy of a structure. 

Fire safety is improved by automatic sprinklers, property 

line setbacks, or more fire-resistant construction. Building 

codes recognize the improved fire safety resulting from 

application of these factors by increasing allowable areas 

and heights beyond that designated for a particular type of 

construction and occupancy. Thus, proper site planning and 

building design may result in a desired building area classification being achieved with wood construction.

Ignition

The most effective ways to improve fire safety are preventive actions that will reduce or eliminate the risks of 

ignition. Some code provisions, such as those in electrical 

codes, are designed to address this issue. Other such provisions are those pertaining to separations between heated 

pipes, stoves, and similar items and any combustible material. In situations of prolonged exposures and confined 

spaces, wood has been known to ignite at temperatures 

much lower than the temperatures normally associated with 
18–3

Chapter 18 Fire Safety of Wood Construction

wood ignition. To address this concern, a safe margin of fire 

safety from ignition even in cases of prolonged exposures 

can be obtained if surface temperatures of heated wood are 

maintained below about 80 °C, which avoids the incipient 

wood degradation associated with reduction in the ignition 

temperature.

Other examples of regulations addressing ignition are requirements for the proper installation and treatment of cellulosic installation. Proper chemical treatments of cellulosic 

insulation are required to reduce its tendency for smoldering 

combustion and to reduce flame spread. Cellulosic insulation is regulated by a product safety standard of the U.S. 

Consumer Product Safety Commission. One of the required 

tests is a smoldering combustion test. Proper installation 

around recessed light fixtures and other electrical devices is 

necessary.

Exterior Fire Exposure in the Wildland–Urban 

Interface

In areas subjected to wildfires, actions to remove ignition 

sources around the home or other structures and prevent 

easy fire penetration into such buildings can significantly 

improve the chances that a structure will survive a wildfire. 

This includes appropriate landscaping to create a defensible 

space around the structure. Particular attention should be 

paid to the removal of vegetation and other combustible 

exterior items (such as firewood, fence, landscape mulch) 

that are close to openings (vents, windows, and doors), 

combustible surfaces of the building, and soffits. Openings 

in building exteriors can allow the fire to penetrate into the 

building and cause interior ignitions. Building design and 

maintenance should be done to limit the accumulation of 

combustible debris that could be ignited by firebrands that 

originate from burning trees and buildings, with particular 

attention paid to nooks and crannies that allow accumulation 

of debris. The firebrands’ distribution is such that they can 

cause destruction of unprotected structures that are some 

distance from the actual flames of the wildfire. Regardless 

of the type of material used for the exterior membrane, the 

type and placement of the joints of the membrane can affect 

the likelihood that a fire will penetrate the exterior membrane. For example, birdstops should be installed at the ends 

of clay tile barrel roof coverings to prevent firebrands from 

igniting the underlining substrate.

Rated roof covering materials are designated Class A, B, 

or C according to their performance in the tests described 

in ASTM E 108, Fire Tests of Roof Coverings. This test 

standard includes intermittent flame exposure, spread of 

flame, burning brand, flying brand, and rain tests. Each of 

the three classes has a different version of the pass–fail test. 

The Class A test is the most severe, Class C the least. In the 

case of the burning brand tests, the brand for the Class B test 

is larger than that for the Class C test. FRT wood shingles 

and shakes are available that carry a Class B or C fire rating. 

A Class A rated wood roof system can be achieved by using 

Class B wood shingles with specified roof deck and 

underlayment.

For other exterior applications, FRT wood is tested in accordance with ASTM E 84. An exterior treatment is required to 

have no increase in the listed flame spread index after being 

subjected to the rain test of ASTM D 2898. At the present 

time, a commercial treated-wood product for exterior applications is either treated to improve fire retardancy or treated 

to improve resistance to decay and insects, not both.

Various websites (such as www.firewise.org) provide additional information addressing the protection of homes in the 

wildland–urban interface. The national Firewise Communities program is a multi-agency effort designed to reach beyond the fire service by involving homeowners, community 

leaders, planners, developers, and others in the effort to protect people, property, and natural resources from the risk of 

wildland fire, before a fire starts. The Firewise Communities 

approach emphasizes community responsibility for planning 

in the design of a safe community and effective emergency 

response, along with individual responsibility for safer home 

construction and design, landscaping, and maintenance.

The ICC’s International Wildland–Urban Interface Code 

provides model code regulations that specifically address 

structures and related land use in areas subjected to wildfires. NFPA 1144 is a standard that focuses on individual 

structure hazards from wildland fires. In response to losses 

due to wildfires, the California State Fire Marshal’s Office (www.fire.ca.gov) has implemented ignition-resistant 

construction standards for structures in the wildland–urban 

interface. These test requirements intended to address ignitability of the structure are based on tests developed at the 

University of California for exterior wall siding and sheathing, exterior windows, under eave, and exterior decking.

Fire Growth within Compartment

Flame Spread

Important provisions in the building codes are those that 

regulate the exposed interior surface of walls, floors, and 

ceilings (that is, the interior finish). Codes typically exclude 

trim and incidental finish, as well as decorations and furnishings that are not affixed to the structure, from the more 

rigid requirements for walls and ceilings. For regulatory 

purposes, interior finish materials are classified according 

to their flame spread index. Thus, flame spread is one of 

the most tested fire performance properties of a material. 

Numerous flame spread tests are used, but the one cited by 

building codes is ASTM E 84 (also known as NFPA 255 

and UL 723), the “25-ft tunnel” test. In this test method, the 

508-mm-wide, 7.32-m-long specimen completes the top of 

the tunnel furnace. Flames from a burner at one end of the 

tunnel provide the fire exposure, which includes forced draft 

conditions. The furnace operator records the flame front 

position as a function of time and the time of maximum 

flame front travel during a 10-min period. The standard 
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General Technical Report FPL–GTR–190

prescribes a formula to convert these data to a flame spread 

index (FSI), which is a measure of the overall rate of flame 

spreading in the direction of air flow. In the building codes, 

the classes for flame spread index are A (FSI of 0 to 25), B 

(FSI of 26 to 75), and C (FSI of 76 to 200). Generally, codes 

specify FSI for interior finish based on building occupancy, 

location within the building, and availability of automatic 

sprinkler protection. The more restrictive classes, Classes A 

and B, are generally prescribed for stairways and corridors 

that provide access to exits. In general, the more flammable 

classification (Class C) is permitted for the interior finish 

of other areas of the building that are not considered exit 

ways or where the area in question is protected by automatic 

sprinklers. In other areas, no flammability restrictions are 

specified on the interior finish, and unclassified materials 

(that is, more than 200 FSI) can be used. The classification 

labels of I, II, and III have been used instead of A, B, and C.

The FSI for most domestic wood species is between 90 

and 160 (Table 18–1). Thus, unfinished lumber, 10 mm or 

thicker, is generally acceptable for interior finish applications requiring a Class C rating. Fire-retardant treatments 

are necessary when a Class A flame spread index is required 

for a wood product. Some domestic softwood species meet 

the Class B flame spread index without treatment. Other 

domestic softwood species have FSIs near the upper limit of 

200 for Class C. All available data for domestic hardwoods 

Table 18–1. ASTM E 84 flame spread indexes for 19-mm-thick solid lumber of 

various wood species as reported in the literaturea

Speciesb

Flame spread 

indexc

Smoke

developed 

indexc Sourced

Softwoods 

Yellow-cedar (Pacific Coast yellow cedar) 78 90 CWC 

Baldcypress (cypress) 145–150 — UL 

Douglas-fir 70–100 — UL 

Fir, Pacific silver 69 58 CWC 

Hemlock, western (West Coast) 60–75 — UL 

Pine, eastern white (eastern white, northern white) 85, 120–215f

 122, — CWC, UL 

Pine, lodgepole 93 210 CWC 

Pine, ponderosa 105–230e — UL 

Pine, red 142 229 CWC 

Pine, Southern (southern) 130–195f — UL 

Pine, western white 75f

 — UL 

Redcedar, western 70 213 HPVA 

Redwood 70 — UL 

Spruce, eastern (northern, white) 65 — UL, CWC 

Spruce, Sitka (western, Sitka) 100, 74 —, 74 UL, CWC 

Hardwoods

Birch, yellow 105–110 — UL 

Cottonwood 115 — UL 

Maple (maple flooring) 104 — CWC 

Oak (red, white) 100 100 UL 

Sweetgum (gum, red) 140–155 — UL 

Walnut 130–140 — UL 

Yellow-poplar (poplar) 170–185 — UL 

a

Additional data for domestic solid-sawn and panel products are provided in the AF&PA–AWC DCA 

No. 1, “Flame Spread Performance of Wood Products.” 

b

In cases where the name given in the source did not conform to the official nomenclature of the Forest 

Service, the probable official nomenclature name is given and the name given by the source is given in 

parentheses. 

c

Data are as reported in the literature (dash where data do not exist). Changes in the ASTM E 84 test 

method have occurred over the years. However, data indicate that the changes have not significantly 

changed earlier data reported in this table. The change in the calculation procedure has usually resulted in 

slightly lower flame spread results for untreated wood. Smoke developed index is not known to exceed 

450, the limiting value often cited in the building codes. 

d

CWC, Canadian Wood Council (CWC 1996); HPVA, Hardwood Plywood Manufacturers Association 

(Tests) (now Hardwood Plywood & Veneer Assoc.); UL, Underwriters Laboratories, Inc. (Wood-fire 

hazard classification. Card Data Service, Serial No. UL 527, 1971). 

e

Footnote of UL: In 18 tests of ponderosa pine, three had values over 200 and the average of all tests is 

154. 

f

Footnote of UL: Due to wide variations in the different species of the pine family and local connotations 

of their popular names, exact identification of the types of pine tested was not possible. The effects of 

differing climatic and soil conditions on the burning characteristics of given species have not been 

determined. 
18–5

Chapter 18 Fire Safety of Wood Construction

are for Class C. Some high-density imported hardwood species have FSIs in Class B. Additional FSI data for domestic 

solid-sawn and panel products are provided in the American 

Forest and Paper Association (AF&PA)–American Wood 

Council (AWC) design for code acceptance (DCA) 

No. 1 (see list of references at end of chapter). Report 128 

of APA–The Engineered Wood Association (APA) discusses 

the flame spread indexes of construction plywood panels.

Code provisions pertaining to floors and floor coverings 

include those based on the critical radiant flux test (ASTM 

E 648). In the critical radiant flux test, the placement of the 

radiant panel is such that the radiant heat being imposed on 

the surface has a gradient in intensity down the length of 

the horizontal specimen. Flames spread from the ignition 

source at the end of high heat flux (or intensity) to the other 

end until they reach a location where the heat flux is not sufficient for further propagation. This is reported as the critical 

radiant flux (CRF). Thus, low CRF reflects materials with 

high flammability.

Depending on location and occupancy, building code requirements are for a minimum critical radiant flux level 

of 2.2 kW m–2 (0.22 W cm–2) for Class II or 4.5 kW m–2

(0.45 W cm–2) for Class I. These provisions are mainly 

intended to address the fire safety of some carpets. One 

section in the International Building Code (IBC) (Sec. 804) 

where this method is cited exempts wood floors and other 

floor finishes of a traditional type from the requirements. 

This method is also cited in standards of the National Fire 

Protection Association (NFPA) such as the Life Safety 

Code. Very little generic data is published on wood products tested in accordance with ASTM E 648. In one report 

published during the development of the test, a CRF of approximately 3.5 to 4.0 kW m–2 was cited for oak flooring 

(Benjamin and Davis 1979). Company literature for proprietary wood floor products indicates that such products can 

achieve CRF in excess of the 4.5 kW m–2 for Class I. For 

wood products tested in accordance with the similar European radiant panel test standard (EN ISO 9239-1 (2002)) 

(Östman and Mikkola 2006, Tsantaridis and Östman 2004), 

critical heat flux (CHF) ranged from 2.6 to 5.4 kW m–2 for 

25 wood floorings tested without a surface coating. Most 

densities ranged from 400 to 600 kg m–3. One additional 

wood flooring product had a CHF of 6.7 kW m–2. Additional 

results for the wood flooring products tested with a wide 

range of coating systems indicated that the non-fireretardant coatings may significantly improve the CHF 

to levels above 4.5 kW m–2.

The critical radiant flux apparatus is also used to test the 

flammability of cellulosic insulation (ASTM E 970). There 

are many other test methods for flame spread or flammability. Most are used only for research and development or 

quality control, but some are used in product specifications 

and regulations of materials in a variety of applications. 

Other tests for flammability include those that measure heat 

release.

Flashover

With sufficient heat generation, the initial growth of a fire 

in a compartment leads to the condition known as flashover. 

The visual criteria for flashover are full involvement of 

the compartment and flames out the door or window 

(Figure 18–1). The intensity over time of a fire starting in 

one room or compartment of a building depends on the 

amount and distribution of combustible contents in the 

room and the amount of ventilation.

The standard full-scale test for pre-flashover fire growth 

is the room-corner test (ASTM E 2257). In this test, a gas 

burner is placed in the corner of the room, which has a single door for ventilation. Three of the walls are lined with the 

test material, and the ceiling may also be lined with the test 

material. Other room-corner tests use a wood crib or similar 

item as the ignition source. Such a room-corner test is used 

to regulate foam plastic insulation, a material that is not 

properly evaluated in the ASTM E 84 test. Observations are 

made of the growth of the fire and the duration of the test 

until flashover occurs. Instruments record the heat generation, temperature development within the room, and the 

heat flux to the floor. Results of full-scale room-corner 

tests are used to validate fire growth models and benchscale test results. In a series of room-corner tests using a 

100/300-kW burner and no test material on the ceiling, the 

ranking of the different wood products was consistent with 

their flame spread index in the ASTM E 84 test (White and 

others 1999). Another room-corner test standard (NFPA 

286) is cited in codes as an alternative to ASTM E 84 for 

evaluating interior wall or ceiling finishes for Class A 

applications.

Figure 18–1. 

Flashover in 

standard room 

test.
Smoke and Toxic Gases

One of the most important problems associated with evacuation during a fire is the smoke produced. The term smoke 

is frequently used in an all-inclusive sense to mean the 

mixture of pyrolysis products and air that is present near 

the fire site. In this context, smoke contains gases, solid 

particles, and droplets of liquid. Smoke presents potential 

hazards because it interacts with light to obscure vision and 

because it contains noxious and toxic substances. Generally, 

two approaches are used to deal with the smoke problem: 

limit smoke production and control the smoke that has been 

produced. The control of smoke flow is most often a factor 

in the design and construction of large or tall buildings. In 

these buildings, combustion products may have serious effects in areas remote from the actual fire site.

The smoke yield restrictions in building codes are also 

based on data from the ASTM E 84 standard. Smoke measurement is based on a percentage attenuation of white light 

passing through the tunnel exhaust stream and detected 

by a photocell. This is converted to the smoke developed 

index (SDI), with red oak flooring set at 100. Flame spread 

requirements for interior finish generally are linked to an 

added requirement that the SDI be less than 450. Available 

SDI data for wood products are less than 450 (Table 18–1). 

In the 1970s, the apparatus known as the NBS smoke chamber was developed and approved as an ASTM standard for 

research and development (ASTM E 662). This test is a 

static smoke test because the specimen is tested in a closed 

chamber of fixed volume and the light attenuation is recorded over a known optical path length. The corresponding 

light transmission is reported as specific optical density as a 

function of time. Samples are normally tested in both flaming (pilot flame) and nonflaming conditions using a radiant 

flux of 25 kW m–2. Some restrictions in product specifications are based on the smoke box test (ASTM E 662). As 

discussed in a later section, dynamic measurements of 

smoke can be obtained with the cone calorimeter 

(ASTM E 1354) and the room-corner test (ASTM E 2257).

Toxicity of combustion products is a concern. Fire victims 

are often not touched by flames but die as a result of exposure to smoke, toxic gases, or oxygen depletion. These 

life-threatening conditions can result from burning contents, 

such as furnishings, as well as from the structural materials 

involved. The toxicity resulting from the thermal decomposition of wood and cellulosic substances is complex because 

of the wide variety of types of wood smoke. Composition 

and the concentration of individual constituents depend 

on such factors as the fire exposure, oxygen and moisture 

present, species of wood, any treatments or finishes that 

may have been applied, and other considerations. The vast 

majority of fires that attain flashover do generate dangerous 

levels of carbon monoxide, independent of what is burning. 

Carbon monoxide is a particularly insidious toxic gas and is 

often generated in significant amounts in wood fires. Small 

amounts of carbon monoxide are particularly toxic because 

the hemoglobin in the blood is much more likely to combine 

with carbon monoxide than with oxygen, even with plenty 

of breathable oxygen (carboxyhemoglobin) present. 

Containment to Compartment of Origin

The growth, intensity, and duration of the fire is the “load” 

that determines whether a fire is confined to the room of origin. Whether a given fire will be contained to the compartment depends on the fire resistance of the walls, doors, ceilings, and floors of the compartment. Requirements for fire 

resistance or fire resistance ratings of structural members 

and assemblies are another major component of the building 

code provisions. In this context, fire resistance is the ability 

of materials or their assemblies to prevent or retard the passage of excessive heat, hot gases, or flames while continuing to support their structural loads. Fire resistance ratings 

are usually obtained by conducting standard fire tests. The 

standard fire resistance test (ASTM E 119) has three failure 

criteria: element collapse, passage of flames, or excessive 

temperature rise on the non-fire-exposed surface (average 

increase of several locations exceeding 139 or 181 °C at 

a single location).

Doors can be critical in preventing the spread of fires. Doors 

left open or doors with little fire resistance can easily defeat 

the purpose of a fire-rated wall or partition. Listings of firerated doors, frames, and accessories are provided by various fire testing agencies. When a fire-rated door is selected, 

details about which type of door, mounting, hardware, and 

closing mechanism need to be considered.

Fires in buildings can spread by the movement of hot fire 

gases through open channels in concealed spaces. Codes 

specify where fireblocking and draftstops are required in 

concealed spaces, and they must be designed to interfere 

with the passage of the fire up or across a building. In addition to going along halls, stairways, and other large spaces, 

heated gases also follow the concealed spaces between floor 

joists and between studs in partitions and walls of frame 

construction. Obstruction of these hidden channels provides 

an effective means of restricting fire from spreading to 

other parts of the structure. Fireblockings are materials used 

to resist the spread of flames via concealed spaces within 

building components such as floors and walls. They are generally used in vertical spaces such as stud cavities to block 

upward spread of a fire. Draftstops are barriers intended 

to restrict the movement of air within concealed areas of a 

building. They are typically used to restrict horizontal dispersion of hot gases and smoke in larger concealed spaces 

such as those found within wood joist floor assemblies with 

suspended dropped ceilings or within an attic space with 

pitched chord trusses.

Exposed Wood Members

The self-insulating quality of wood, particularly in the large 

wood sections of heavy timber construction, is an important 

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General Technical Report FPL–GTR–190
factor in providing a degree of fire resistance. In Type IV or 

heavy timber construction, the need for fire resistance requirements is achieved in the codes by specifying minimum 

sizes for the various members or portions of a building and 

other prescriptive requirements. In this type of construction, 

the wood members are not required to have specific fire 

resistance ratings. The acceptance of heavy timber construction is based on historical experience with its performance 

in actual fires. Proper heavy timber construction includes 

using approved fastenings, avoiding concealed spaces under 

floors or roofs, and providing required fire resistance in the 

interior and exterior walls.

The availability and code acceptance of a procedure to 

calculate the fire resistance ratings for large timber beams 

and columns have allowed their use in fire-rated buildings 

not classified as Type IV (heavy timber) construction. In 

the other types of construction, the structural members and 

assemblies are required to have specified fire resistance 

ratings. There are two accepted procedures for calculating 

the fire ratings of exposed wood members. In the first such 

procedure, the equations are simple algebraic equations that 

only need the dimensions of the beam or column and a load 

factor. Determination of the load factor requires the minimum dimension of column, the applied load as a percentage 

of the full allowable design load, and the effective column 

length. The acceptance of this procedure is normally limited 

to beams and column with nominal dimensions of 152 mm 

(6 in.) or greater and for fire ratings of 1 h or less. This procedure is applicable to glued-laminated timbers that utilize 

standard laminating combinations. Because the outer tension 

laminate of a glued-laminated beam is charred in a 1-h fire 

exposure, a core lamination of a beam needs to be removed 

and the equivalent of an extra nominal 51-mm- (2-in.-) thick 

outer tension lamination added to the bottom of the beam. 

Details on this procedure can be found in various industry 

publications (American Institute of Timber Construction 

(AITC) Technical Note 7, AF&PA-AWC DCA #2, APA 

Publication EWS Y245A) and the IBC.

A second more flexible mechanistic procedure was incorporated within the National Design Specification for Wood 

Construction (NDS®) in 2001 and is referred to as the NDS 

Method. As an explicit engineering method, it is applicable 

to all wood structural members covered under the NDS, 

including structural composite lumber wood members. Normal engineering calculations of the ultimate load capacity 

of the structural wood element are adjusted for reductions in 

dimensions with time as the result of charring. As discussed 

more in a later section, a char depth of 38 mm (1.5 in.) at 

1 h is generally used for solid-sawn and structural gluedlaminated softwood members. The char depth is adjusted 

upward by 20% to account for the effect of elevated temperatures on the mechanical properties of the wood near the 

wood–char interface. This procedure also requires that core 

lamination(s) of glued-laminated beams be replaced by extra 

outer tension laminate(s). A provision of the NDS procedure 

addresses the structural integrity performance criteria for 

timber decks, but the thermal separation criteria are not addressed. This second procedure was developed by the American Wood Council and is fully discussed in their Technical 

Report No. 10. Fire resistance tests on glued-laminated 

specimens and structural composite lumber products loaded 

in tension are discussed in FPL publications.

The fire resistance of glued-laminated structural members, 

such as arches, beams, and columns, is approximately 

equivalent to the fire resistance of solid members of similar 

size. Laminated members glued with traditional phenol, 

resorcinol, or melamine adhesives are generally considered 

to be at least equal in their fire resistance to a one-piece 

member of the same size. In recent years, the fire resistance 

performance of structural wood members manufactured 

with adhesives has been of intense interest. As a result of 

concerns about some adhesives that were being used in 

fingerjointed lumber, industry test protocols and acceptance criteria were developed to address this issue. When a 

wood-frame assembly is required to have a fire resistance 

rating, any finger-jointed lumber within the assembly must 

include the HRA designation for heat-resistant adhesives in 

the grademark. The designation is part of the Glued Lumber 

Policy of the American Lumber Standard Committee, Inc. 

The activities to address questions concerning the adhesives 

have included the development of ASTM standard test 

methods and revisions to the ASTM standard specifications 

for the applicable wood products.

Light-Frame Assemblies

Light-frame wood construction can provide a high degree 

of fire containment through use of gypsum board as the interior finish. This effective protective membrane provides 

the initial fire resistance rating. Many recognized assemblies 

involving wood-frame walls, floors, and roofs provide a 

1- or 2-h fire resistance rating. Fire-rated gypsum board 

(Type X or C) is used in rated assemblies. Type X and the 

higher grade Type C gypsum boards have textile glass filaments and other ingredients that help to keep the gypsum 

core intact during a fire. Fire resistance ratings of various 

assemblies are listed in the IBC and other publications such 

as the Gypsum Association Fire Resistance Design Manual, 

AF&PA-AWC DCA #3, and product directories of listing 

organizations, such as UL and Intertek. Traditional constructions of regular gypsum wallboard (that is, not fire rated) 

or lath and plaster over wood joists and studs have fire 

resistance ratings of 15 to 30 min. In addition to fire-rated 

assemblies constructed of sawn lumber, there are rated assemblies for I-joists and wood trusses.

Fire-rated assemblies are generally tested in accordance 

with ASTM E 119 while loaded to 100% of the allowable 

design load calculated using the NDS. The calculation of the 

allowable design load of a wood stud wall is described in 

ASTM D 6513. Some wood stud wall assemblies were 

tested with a load equivalent to 78% of the current design 

18–7

Chapter 18 Fire Safety of Wood Construction
load (NDS dated 2005) calculated using a le/d of 33. Less 

than full design load in the fire test imposes a load restriction on the rated assembly.

While fire resistance ratings are for the entire wall, floor, or 

roof assembly, the fire resistance of a wall or floor can be 

viewed as the sum of the resistance of the interior finish and 

the resistance of the framing members. In a code-accepted 

procedure, the fire rating of a light-frame assembly is calculated by adding the tabulated times for the fire-exposed 

membrane to the tabulated times for the framing. For example, the fire resistance rating of a wood stud wall with 

16-mm-thick Type X gypsum board and rock wool insulation is computed by adding the 20 min listed for the stud 

wall, the 40 min listed for the gypsum board, and the 15 min 

listed for the rock wool insulation to obtain a rating for the 

assembly of 75 min. Additional information on this component additive method (CAM) can be found in the IBC and 

AF&PA DCA No. 4. More sophisticated mechanistic models 

have been developed.

The relatively good structural behavior of a traditional wood 

member in a fire test results from the fact that its strength is 

generally uniform through the mass of the piece. Thus, the 

unburned fraction of the member retains high strength, and 

its load-carrying capacity is diminished only in proportion 

to its loss of cross section. Innovative designs for structural 

wood members may reduce the mass of the member and 

locate the principal load-carrying components at the outer 

edges where they are most vulnerable to fire, as in structural 

sandwich panels. With high strength facings attached to a 

low-strength core, unprotected load-bearing sandwich panels have failed to support their load in less than 6 min when 

tested in the standard test. If a sandwich panel is to be used 

as a load-bearing assembly, it should be protected with gypsum wallboard or some other thermal barrier. In any protected assembly, the performance of the protective membrane is 

the critical factor in the performance of the assembly.

Unprotected light-frame wood buildings do not have the 

natural fire resistance achieved with heavier wood members. 

In these, as in all buildings, attention to good construction 

details is important to minimize fire hazards. Quality of 

workmanship is important in achieving adequate fire resistance. Inadequate nailing and less than required thickness of 

the interior finish can reduce the fire resistance of an assembly. The method of fastening the interior finish to the framing members and the treatment of the joints are significant 

factors in the fire resistance of an assembly. The type and 

quantity of any insulation installed within the assembly may 

also affect the fire resistance of an assembly.

Any penetration in the membrane must be addressed with 

the appropriate fire protection measures. This includes the 

junction of fire-rated assemblies with unrated assemblies. 

Fire stop systems are used to properly seal the penetration 

of fire-rated assemblies by pipes and other utilities. 

Through-penetration fire stops are tested in accordance with 

ASTM E 814. Electrical receptacle outlets, pipe chases, and 

other through openings that are not adequately firestopped 

can affect the fire resistance. In addition to the design of 

walls, ceilings, floors, and roofs for fire resistance, stairways, doors, and firestops are of particular importance.

Fire-Performance Characteristics 

of Wood

Several characteristics are used to quantify the burning 

behavior of wood when exposed to heat and air, including 

thermal degradation of wood, ignition from heat sources, 

heat and smoke release, flame spread in heated environments, and charring rates in a contained room.

Thermal Degradation of Wood

As wood reaches elevated temperatures, the different chemical components undergo thermal degradation that affect 

wood performance. The extent of the changes depends on 

the temperature level and length of time under exposure 

conditions. At temperatures below 100 °C, permanent reductions in strength can occur, and its magnitude depends 

on moisture content, heating medium, exposure period, and 

species. The strength degradation is probably due to depolymerization reactions (involving no carbohydrate weight 

loss). The little research done on the chemical mechanism 

has found a kinetic basis (involving activation energy, preexponential factor, and order of reaction) of relating strength 

reduction to temperature. Chemical bonds begin to break at 

temperatures above 100 °C and are manifested as carbohydrate weight losses of various types that increases with the 

temperature. Literature reviews by Bryan (1998), Shafizadeh (1984), Atreya (1983), and Browne (1958) reveal the 

following four temperature regimes of wood pyrolysis and 

corresponding pyrolysis kinetics.

Between 100 and 200 °C, wood becomes dehydrated and 

generates water vapor and other noncombustible gases 

including CO2, formic acid, acetic acid, and H2O. With prolonged exposures at higher temperatures, wood can become 

charred. Exothermic oxidation reactions can occur because 

ambient air can diffuse into and react with the developing 

porous char residue.

From 200 to 300 °C, some wood components begin to undergo significant pyrolysis and, in addition to gases listed 

above, significant amounts of CO and high-boiling-point tar 

are given off. The hemicelluloses and lignin components are 

pyrolyzed in the range of 200 to 300 °C and 225 to 450 °C, 

respectively. Much of the acetic acid liberated from wood 

pyrolysis is attributed to deactylation of hemicellulose. Dehydration reactions beginning around 200 °C are primarily 

responsible for pyrolysis of lignin and result in a high char 

yield for wood. Although the cellulose remains mostly unpyrolyzed, its thermal degradation can be accelerated in the 

presence of water, acids, and oxygen. As the temperature 

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General Technical Report FPL–GTR–190
increases, the degree of polymerization of cellulose decreases further, free radicals appear and carbonyl, carboxyl, 

and hydroperoxide groups are formed. Overall pyrolysis 

reactions are endothermic due to decreasing dehydration 

and increasing CO formation from porous char reactions 

with H2O and CO2 with increasing temperature. During this 

“low-temperature pathway” of pyrolysis, the exothermic 

reactions of exposed char and volatiles with atmospheric 

oxygen are manifested as glowing combustion.

The third temperature regime is from 300 to 450 °C because 

of the vigorous production of flammable volatiles. This begins with the significant depolymerization of cellulose in the 

range of 300 to 350 °C. Also around 300 °C, aliphatic side 

chains start splitting off from the aromatic ring in the lignin. 

Finally, the carbon–carbon linkage between lignin structural 

units is cleaved at 370 to 400 °C. The degradation reaction 

of lignin is an exothermic reaction, with peaks occurring between 225 and 450 °C; temperatures and amplitudes of these 

peaks depend on whether the samples were pyrolyzed under nitrogen or air. All wood components end their volatile 

emissions at around 450 °C. The presence of minerals and 

moisture within the wood tend to smear the separate pyrolysis processes of the major wood components. In this “hightemperature pathway,” pyrolysis of wood results in overall 

low char residues of around 25% or less of the original dry 

weight. Many fire retardants work by shifting wood degradation to the “low-temperature pathway,” which reduces the 

volatiles available for flaming combustion.

Above 450 °C, the remaining wood residue is an activated 

char that undergoes further degradation by being oxidized to 

CO2, CO, and H2O until only ashes remain. This is referred 

to as afterglow.

The complex nature of wood pyrolysis often leads to selecting empirical kinetic parameters of wood pyrolysis applicable to specific cases. Considering the degrading wood to 

be at low elevated temperature over a long time period and 

ignoring volatile emissions, a simple first-order reaction following the Arrhenius equation, dm/dt = –mA exp(–E/RT), 

was found practical. In this equation, m is mass of specimen, 

t is time, A is the preexponential factor, E is activation energy, R is the universal gas constant, and T is temperature in 

kelvins. The simplest heating environment for determination 

of these kinetic parameters is isothermal, constant pressure, 

and uniform flow gas exposures on a nominally thick specimen. As an example, Stamm (1955) reported on mass loss 

of three coniferous wood sticks (1 by 1 by 6 in.)—Southern 

and white pine, Sitka spruce, and Douglas-fir—that were 

heated in a drying oven in a temperature range of 93.5 to 

250 °C. The fit of the Arrhenius equation to the data resulted in the values of A = 6.23 × 107 s–1 and E = 124 kJ 

mol–1. If these same woods were exposed to steam instead 

of being oven dried, degradation was much faster. With the 

corresponding kinetic parameters, A = 82.9 s–1 and E = 66 

kJ mol–1, Stamm concluded that steam seemed to act as a 

catalyst because of significant reduction in the value of activation energy. Shafizadeh (1984) showed that pyrolysis proceeds faster in air than in an inert atmosphere and that this 

difference gradually diminishes around 310 °C. The value of 

activation energy reported at large for pyrolysis in air varied 

from 96 to 147 kJ mol–1.

In another special case, a simple dual reaction model could 

distinguish between the low- and high- temperature pathways for quantifying the effect of fire retardant on wood 

pyrolysis. The reaction equation, dm/dt = (mend – m)[A1

exp(–E1/RT) + A2 exp(–E2/RT)] , was found suitable by 

Tang (1967). In this equation, mend is the ending char mass, 

and subscripts 1 and 2 represent low- and high-temperature 

pathways, respectively. A dynamic thermogravimetry was 

used to span the temperature to 500 °C at a rate of 3 °C per 

minute using tiny wood particles. The runs were made in 

triplicate for ponderosa pine sapwood, lignin, and alphacellulose samples with five different inorganic salt treatments. Tang’s derived values for the untreated wood are 

mend = 0.21 of initial weight, A1 = 3.2 × 105 s–1, E1 = 96 

kJ mol–1, A2 = 6.5e+16 s–1, and E2 = 226 kJ mol–1. A wellknown fire-retardant-treatment chemical, monobasic ammonium phosphate, was the most effective chemical tested 

in that char yield was increased to 40% and E1 decreased to 

80 kJ mol–1, thereby promoting most volatile loss through 

the low-temperature pathway. The alpha-cellulose reacted 

to the chemicals similarly as the wood, while the lignin did 

not seem to be affected much by the chemicals. From this 

we conclude that flammable volatiles generated by the cellulose component of wood are significantly reduced with fire 

retardant treatment. For applications to biomass energy and 

fire growth phenomology, the kinetic parameters become 

essential to describe flammable volatiles and their heat of 

combustion but are very complicated (Dietenberger 2002). 

Modern pyrolysis models now include competing reactions 

to produce char, tar, and noncondensing gases from wood as 

well as the secondary reaction of tar decomposition.

Ignition

Ignition of wood is the start of a visual and sustained combustion (smoldering, glow, or flame) fueled by wood pyrolysis. Therefore the flow of energy or heat flux from a fire or 

other heated objects to the wood material to induce pyrolysis is a necessary condition of ignition. A sufficient condition of flaming ignition is the mixing together of volatiles 

and air with the right composition in a temperature range 

of about 400 to 500 °C. An ignition source (pilot or spark 

plug) is therefore usually placed where optimum mixing of 

volatiles and air can occur for a given ignition test. In many 

such tests the surface temperature of wood materials has 

been measured in the range of 300 to 400 °C prior to piloted 

ignition. This also coincides with the third regime of wood 

pyrolysis in which there is a significant production of flammable volatiles. However, it is possible for smoldering or 

18–9

Chapter 18 Fire Safety of Wood Construction
glow to exist prior to flaming ignition if the imposed radiative or convective heating causes the wood surface to reach 

200 °C or higher for the second regime of wood pyrolysis. 

Indeed, unpiloted ignition is ignition that occurs where no 

pilot source is available. Ignition associated with smoldering 

is another important mechanism by which fires are initiated.

Therefore, to study flaming or piloted ignition, a high heat 

flux (from radiant heater) causes surface temperature to rapidly reach at least 300 °C to minimize influence of unwanted 

smoldering or glow at lower surface temperatures. Surface 

temperature at ignition has been an elusive quantity that 

was experimentally difficult to obtain, but relatively recent 

studies show some consistency. For various horizontally 

orientated woods with specific gravities ranging from 0.33 

to 0.69, the average surface temperature at ignition increases 

from 347 °C at imposed heat flux of 36 kW m–2 to 377 °C 

at imposed heat flux of 18 kW m–2. This increase in the ignition temperature is due to the slow decomposition of the 

material at the surface and the resulting buildup of the char 

layer at low heat fluxes (Atreya 1983). In the case of naturally high charring material such as redwood that has high 

lignin and low extractives, the measured averaged ignition 

temperatures were 353, 364, and 367 °C for material thicknesses of 19, 1.8, and 0.9 mm, respectively, for various

heat flux values as measured in the cone calorimeter 

(ASTM E 1354) (Dietenberger 2004). This equipment 

along with the lateral ignition and flame spread test (LIFT) 

apparatus (ASTM E 1321) are used to obtain data on time to 

piloted ignition as a function of heater irradiance. From such 

tests, values of ignition temperature, critical ignition flux 

(heat flux below which ignition would not occur), and thermophysical properties have been derived using a transient 

heat conduction theory (Table 18–2). In the case of redwood, the overall piloted ignition temperature was derived 

to be 365 °C (638 K) in agreement with measured values, 

regardless of heat flux, thickness, moisture content, surface 

orientation, and thin reflective paint coating. The critical 

heat flux was derived to be higher on the LIFT apparatus 

than on the cone calorimeter primarily due to the different 

convective coefficients (Dietenberger 1996). However, the 

heat properties of heat capacity and thermal conductivity 

were found to be strongly dependent on density, moisture content, and internal elevated temperatures. Thermal 

conductivity has an adjustment factor for composite, engineered, or treated wood products. Critical heat fluxes 

for ignition have been calculated to be between 10 and 

13 kW m–2 for a range of wood products. For exposure to 

a constant heat flux, ignition times for solid wood typically 

ranged from 3 s for heat flux of 55 kW m–2 to 930 s for heat 

flux of 18 kW m–2. Estimates of piloted ignition in various 

scenarios can be obtained using the derived thermal properties listed in Table 18–2 and an applicable heat conduction 

theory (Dietenberger 2004).

18–10

General Technical Report FPL–GTR–190

Table 18–2. Derived wood-based thermophysical parameters of ignitability

Material 

Thickness 

(mm) 

Density 

(kg m–3) 

ρ

Moisture 

content (%)

M 

Material 

emissivity r

a

Tig

(K) 

k/c

a 

(m2/s) 

x107

kc

a

(kJ2

 m–4 K–2 s

–1) 

Gypsum board, Type X 16.5 662 — 0.9 N/A 608.5 3.74 0.451 

FRT Douglas-fir plywood 11.8 563 9.48 0.9 0.86 646.8 1.37 0.261 

Oak veneer plywood 13 479 6.85 0.9 1.11 563 1.77 0.413 

FRT plywood (Forintek) 11.5 599 11.17 0.9 0.86 650 1.31 0.346 

Douglas-fir plywood (ASTM) 11.5 537 9.88 0.85 0.863 604.6 1.37 0.221 

FRT Southern Pine plywood 11 606 8.38 0.9 1.43 672 2.26 0.547 

Douglas-fir plywood (MB) 12 549 6.74 0.89 0.86 619 1.38 0.233 

Southern Pine plywood 11 605 7.45 0.88 0.86 620 1.38 0.29 

Particleboard 13 794 6.69 0.88 1.72 563 2.72 0.763 

Oriented strandboard 11 643 5.88 0.88 0.985 599 1.54 0.342 

Hardboard 6 1,026 5.21 0.88 0.604 593 0.904 0.504 

Redwood lumber 19 421 7.05 0.86 1.0 638 1.67 0.173 

White spruce lumber 17 479 7.68 0.82 1.0 621 1.67 0.201 

Southern Pine boards 18 537 7.82 0.88 1.0 644 1.63 0.26 

Waferboard 13 631 5.14 0.88 1.62 563 2.69 0.442 a

Formulas for wood thermal conductivity k, heat capacity c, and density ρ, at elevated temperatures used to calculate thermal inertia kρc and

thermal diffusivity k/ρc are as follows: 

     3 1 1 m 3 0.1941 0.004064 od 10 0.01864 297 10 kWm K     k  r  M    T 

   1 1 1.25 1 0.025 m 297 kJkg K  c   M T

  3

od 1 0.01 kgm     M

where Tig is ignition temperature, ambient temperature Ta = 297 K, mean temperature Tm = (Ta + Tig)/2, and the parameter r is an adjustment factor 

used in the calculation of the thermal conductivity for composite, engineered, or treated wood products (Dietenberger 2004). 
Some, typically old, apparatuses for testing piloted ignition 

measured the temperature of the air flow rather than the 

imposed heat flux with the time to ignition measurement. 

These results were often reported as the ignition temperature 

and as varying with time to ignition, which is misleading. 

When the imposed heat flux is due to a radiant source, such 

reported air flow ignition temperature can be as much as 

100 °C lower than the ignition surface temperature. For a 

proper heat conduction analysis in deriving thermal properties, measurements of the radiant source flux and air flow 

rate are also required. Because imposed heat flux to the surface and the surface ignition temperature are the factors that 

directly determine ignition, some data of piloted ignition are 

inadequate or misleading.

Unpiloted ignition depends on special circumstances that result in different ranges of ignition temperatures. At this time, 

it is not possible to give specific ignition data that apply to 

a broad range of cases. For radiant heating of cellulosic 

solids, unpiloted transient ignition has been reported at 

600 °C. With convective heating of wood, unpiloted 

ignition has been reported as low as 270 °C and as high as 

470 °C. Unpiloted spontaneous ignition can occur when a 

heat source within the wood product is located such that the 

heat is not readily dissipated. This kind of ignition involves 

smoldering and generally occurs over a longer period of 

time. Continuous smoking is visual evidence of smoldering, 

which is sustained combustion within the pyrolyzing material. Although smoldering can be initiated by an external 

ignition source, a particularly dangerous smoldering is that 

initiated by internal heat generation. Examples of such fires 

are (a) panels or paper removed from the press or dryer and 

stacked in large piles without adequate cooling and (b) very 

large piles of chips or sawdust with internal exothermic reactions such as biological activities. Potential mechanisms 

of internal heat generation include respiration, metabolism 

of microorganisms, heat of pyrolysis, abiotic oxidation, and 

adsorptive heat. These mechanisms, often in combination, 

may proceed to smoldering or flaming ignition through a 

thermal runaway effect within the pile if sufficient heat is 

generated and is not dissipated. The minimum environmental temperature to achieve smoldering ignition decreases 

with the increases in specimen mass and air ventilation, and 

can be as low as air temperatures for large ventilating piles. 

Therefore, safe shipping or storage with wood chips, dust, 

or pellets often depends on anecdotal knowledge that advises maximum pile size or ventilation constraints, or both 

(Babrauskas 2003).

Unpiloted ignitions that involve wood exposed to low-level 

external heat sources over very long periods are an area of 

dispute. This kind of ignition, which involves considerable 

charring, does appear to occur, based on fire investigations. 

However, these circumstances do not lend themselves easily 

to experimentation and observation. There is some evidence 

that the char produced under low heating temperatures can 

have a different chemical composition, which results in a 

somewhat lower ignition temperature than normally recorded. Thus, a major issue is the question of safe working 

temperature for wood exposed for long periods. Temperatures between 80 and 100 °C have been recommended as 

safe surface temperatures for wood. As noted earlier, to address this concern, a safe margin of fire safety from ignition 

can be obtained if surface temperatures of heated wood are 

maintained below about 80 °C, which avoids the incipient 

wood degradation associated with reduction in ignition 

temperature.

Heat Release and Smoke

Heat release rates are important because they indicate the 

potential fire hazard of a material and also the combustibility of a material. Materials that release their potential chemical energy (and also the smoke and toxic gases) relatively 

quickly are more hazardous than those that release it more 

slowly. There are materials that will not pass the current 

definition of noncombustible in the model codes but will 

release only limited amounts of heat during the initial and 

critical periods of fire exposure. There is also some criticism 

of using limited flammability to partially define noncombustibility. One early attempt was to define combustibility 

in terms of heat release in a potential heat method (NFPA 

259), with the low levels used to define low combustibility or noncombustibility. This test method is being used to 

regulate materials under some codes. The ground-up wood 

sample in this method is completely consumed during the 

exposure to 750 °C for 2 h, which makes the potential heat 

for wood identical to the gross heat of combustion from the 

oxygen bomb calorimeter. The typical gross heat of combustion averaged around 20 MJ kg–1 for ovendried wood, depending on the lignin and extractive content of the wood.

A better or a supplementary measure of degrees of combustibility is a determination of the rate of heat release (RHR) 

or heat release rate (HRR). This measurement efficiently 

assesses the relative heat contribution of materials—thick, 

thin, untreated, or treated—under fire exposure. The cone 

calorimeter (ASTM E 1354) is currently the most commonly used bench-scale HRR apparatus and is based on 

the oxygen consumption method. An average value of 

13.1 kJ g–1 of oxygen consumed was the constant found for 

organic solids and is accurate with very few exceptions to 

within 5%. In the specific case of wood volatiles flaming 

and wood char glowing, this oxygen consumption constant 

was reconfirmed at the value of 13.23 kJ g–1 (Dietenberger 

2002). Thus, it is sufficient to measure the mass flow rate 

of oxygen consumed in a combustion system to determine 

the net HRR. The intermediate-scale apparatus (ASTM E 

1623) for testing 1- by 1-m assemblies or composites and 

the room full-scale test (ASTM E 2257) also use the oxygen 

consumption technique to measure the HRR of fires at larger 

scales.

18–11

Chapter 18 Fire Safety of Wood Construction
The cone calorimeter is ideal for product development with 

its small specimen size of 100 by 100 mm. The specimen is 

continuously weighed by use of a load cell. In conjunction 

with HRR measurements, the effective heat of combustion 

as a function of time is calculated by the ASTM E 1354 

method. Basically, the effective heat of combustion is the 

HRR divided by the mass loss rate as determined from the 

cone calorimeter test as a function of time. Typical HRR 

profiles, as shown in Figure 18–2, begin with a sharp peak 

upon ignition, and as the surface chars, the HRR drops to 

some minimum value. After the thermal wave travels completely through the wood thickness, the back side of a wood 

sample reaches pyrolysis temperature, thus giving rise to a 

second, broader, and even higher HRR peak. For FRT wood 

products, the first HRR peak may be reduced or eliminated.

Heat release rate depends upon the intensity of the imposed 

heat flux. Generally, the averaged effective heat of combustion is about 65% of the oxygen bomb heat of combustion 

(higher heating value), with a small linear increase with irradiance. The HRR itself has a large linear increase with the 

heat flux. This information along with a representation of 

the heat release profile shown in Figure 18–2 has been used 

to model or correlate with large scale fire growth such as 

the Steiner tunnel test and the room-corner fire test (Dietenberger and White 2001)

The cone calorimeter is also used to obtain dynamic measurements of smoke consisting principally of soot and CO in 

the overventilated fires and of white smoke during unignited 

pyrolysis and smoldering. The measurements are dynamic 

in that smoke continuously flows out the exhaust pipe where 

optical density and CO are measured continuously. This 

contrasts with a static smoke test in which the specimen is 

tested in a closed chamber of fixed volume and the light attenuation is recorded over a known optical path length. In 

the dynamic measurements of smoke, the appropriate smoke 

parameter is the smoke release rate (SRR), which is the optical density multiplied by the volume flow rate of air into the 

exhaust pipe and divided by the product of exposed surface 

area of the specimen and the light path length. Often the 

smoke extinction area, which is the product of SRR and the 

specimen area, is preferred because it can be correlated linearly with HRR in many cases. This also permits comparison with the smoke measured in the room-corner fire test 

because HRR is a readily available test result (Dietenberger 

and Grexa 2000). Although SRR can be integrated with time 

to get the same units as the specific optical density, they 

are not equivalent because static tests involve the direct accumulation of smoke in a volume, whereas SRR involves 

accumulation of freshly entrained air volume flow for each 

unit of smoke. Methods investigated to correlate smoke between different tests included alternative parameters such as 

particulate mass emitted per area of exposed sample. As pertaining to CO production, some amount of correlation has 

been obtained between the cone calorimeter’s CO mass flow 

rate as normalized by HRR to the corresponding parameter 

measured from the post flashover gases during the roomcorner fire test. Thermal degradation of white smoke from 

wood into simpler gases within the underventilated fire test 

room during post flashover is not presently well understood 

and can have dramatic effects on thermal radiation within 

the room, which in turn affects wood pyrolysis rates.

Flame Spread

The spread of flames over solids is a very important phenomenon in the growth of compartment fires. Indeed, in 

fires where large fuel surfaces are involved, increase in HRR 

with time is primarily due to increase in burning area. Much 

data have been acquired with the flame spread tests used in 

building codes. Table 18–1 lists the FSI and smoke index 

of ASTM E 84 for solid wood. Some consistencies in the 

FSI behavior of the hardwood species can be related to their 

density (White 2000). Considerable variations are found for 

wood-based composites; for example, the FSI of four structural flakeboards ranged from 71 to 189.

As a prescriptive regulation, the ASTM E 84 tunnel test is 

a success in the reduction of fire hazards but is impractical 

in providing scientific data for fire modeling or in useful 

bench-scale tests for product development. Other full-scale 

tests (such as the room-corner test) can produce quite different results because of the size of the ignition burner or test 

geometry. This is the case with foam plastic panels that melt 

and drip during a fire test. In the tunnel test, with the test 

material on top, a material that melts can have low flammability because the specimen does not stay in place. With an 

adequate burner in the room-corner test, the same material 

will exhibit very high flammability.

A flame spreads over a solid material when part of the 

fuel, ahead of the pyrolysis front, is heated to the critical 

18–12

General Technical Report FPL–GTR–190

0

50

100

150

200

250

300

350

100 200 300 400 500 600 700 800 900 1,000

Time (s)

Heat release rate (kW m-2)

65 kW m-2

50 kW m-2

35 kW m-2

20 kW m-2

Figure 18–2. Heat release rate curves for 12-mm-thick 

oriented strandboard (OSB) exposed to constant heat 

flux of 20, 35, 50 and 65 kW m–2.
condition of ignition. The rate of flame spread is controlled 

by how rapidly the fuel reaches the ignition temperature in 

response to heating by the flame front and external sources. 

The material’s thermal conductivity, heat capacitance, 

thickness, and blackbody surface reflectivity influence the 

material’s thermal response, and an increase in the values of 

these properties corresponds to a decrease in flame spread 

rate. On the other hand, an increase in values of the flame 

features, such as the imposed surface fluxes and spatial 

lengths, corresponds to an increase in the flame spread rate.

Flame spread occurs in different configurations, which are 

organized by orientation of the fuel and direction of the 

main flow of gases relative to that of flame spread. Downward and lateral creeping flame spread involves a fuel orientation with buoyantly heated air flowing opposite of the 

flame spread direction. Related bench-scale test methods are 

ASTM E 162 for downward flame spread, ASTM E 648 for 

horizontal flame spread to the critical flux level, and ASTM 

E 1321 (LIFT apparatus) for lateral flame spread on vertical specimens to the critical flux level. Heat transfer from 

the flame to the virgin fuel is primarily conductive within a 

spatial extent of a few millimeters and is affected by ambient conditions such as oxygen, pressure, buoyancy, and external irradiance. For most wood materials, this heat transfer 

from the flame is less than or equal to surface radiant heat 

loss in normal ambient conditions, so that excess heat is not 

available to further raise the virgin fuel temperature; flame 

spread is prevented as a result. Therefore, to achieve creeping flame spread, an external heat source is required in the 

vicinity of the pyrolysis front (Dietenberger 1994).

Upward or ceiling flame spread involves a fuel orientation 

with the main air flowing in the same direction as the flame 

spread (assisting flow). Testing of flame spread in assisting 

flow exists in both the tunnel tests and the room-corner burn 

tests. The heat transfer from the flame is both conductive 

and radiative, has a large spatial feature, and is relatively unaffected by ambient conditions. Rapid acceleration in flame 

spread can develop because of a large, increasing magnitude 

of flame heat transfer as a result of increasing total HRR in 

assisting flows (Dietenberger and White 2001). These complexities and the importance of the flame spread processes 

explain the many and often incompatible flame spread tests 

and models in existence worldwide.

Charring and Fire Resistance

As noted earlier in this chapter, wood exposed to high temperatures will decompose to provide an insulating layer of 

char that retards further degradation of the wood (Figure 

18–3). The load-carrying capacity of a structural wood 

member depends upon its cross-sectional dimensions. Thus, 

the amount of charring of the cross section is the major 

factor in the fire resistance of structural wood members.

When wood is first exposed to fire, the wood chars and 

eventually flames. Ignition occurs in about 2 min under the 

standard ASTM E 119 fire-test exposures. Charring into the 

depth of the wood then proceeds at a rate of approximately 

0.8 mm min–1 for the next 8 min (or 1.25 min mm–1). Thereafter, the char layer has an insulating effect, and the rate 

decreases to 0.6 mm min–1 (1.6 min mm–1). Considering the 

initial ignition delay, the fast initial charring, and then the 

slowing down to a constant rate, the average constant charring rate is about 0.6 mm min–1 (or 1.5 in. h–1) (Douglas-fir, 

7% moisture content). In the standard fire resistance test, 

this linear charring rate is generally assumed for solid wood 

directly exposed to fire. There are differences among species 

associated with their density, anatomy, chemical composition, and permeability. In a study of the fire resistance of 

structural composite lumber products, the charring rates 

of the products tested were similar to that of solid-sawn 

lumber. Moisture content is a major factor affecting charring rate. Density relates to the mass needed to be degraded 

and the thermal properties, which are affected by anatomical features. Charring in the longitudinal grain direction 

is reportedly double that in the transverse direction, and 

chemical composition affects the relative thickness of the 

char layer. Permeability affects movement of moisture being driven from the wood or that being driven into the wood 

beneath the char layer. Normally, a simple linear model for 

charring where t is time (min), C is char rate (min mm–1), 

and xc is char depth (mm) is 

(18–1)

The temperature at the base of the char layer is generally 

taken to be 300 °C or 550 °F (288 °C). With this temperature criterion, empirical equations for charring rate have 

18–13

Chapter 18 Fire Safety of Wood Construction

Figure 18–3. Illustration of charring of wood slab.
been developed. Equations relating charring rate under 

ASTM E 119 fire exposure to density and moisture content 

are available for Douglas-fir, Southern Pine, and white oak. 

These equations for rates transverse to the grain are

C = (0.002269 + 0.00457m)r + 0.331 for Douglas-fir 

(18–2a)

C = (0.000461 + 0.00095m)r + 1.016 for Southern Pine

(18–2b)

C = (0.001583 + 0.00318m)r + 0.594 for white oak

(18–2c)

where m is moisture content (fraction of ovendry mass) and 

r is density, dry mass volume at moisture content m (kg 

m–3).

A nonlinear char rate model has been found useful. This alternative model is

(18–3)

where m is char rate coefficient (min mm–1.23).

A form of Equation (18–3) is used in the NDS Method for 

calculating the fire resistance rating of an exposed wood 

member. Based on data from eight species (Table 18–3), the 

following equation was developed for the char rate 

coefficient:

 m = -0.147 + 0.000564r + 1.21m + 0.532fc (18–4)

where r is density, ovendry mass and volume, and fc is char 

contraction factor (dimensionless).

The char contraction factor is the thickness of the residual 

char layer divided by the original thickness of the wood 

layer that was charred (char depth). Average values for the 

eight species tested in the development of the equation are 

listed in Table 18–3. These equations and data are valid 

when the member is thick enough to be a semi-infinite slab. 

For smaller dimensions, the charring rate increases once the 

temperature has risen above the initial temperature at the 

center of the member or at the unexposed surface of the panel. As a beam or column chars, the corners become rounded.

Charring rate is also affected by the severity of the fire exposure. Data on charring rates for fire exposures other than 

ASTM E 119 have been limited. Data for exposure to constant temperatures of 538, 815, and 927 °C are available in 

Schaffer (1967). Data for a constant heat flux are given in 

Table 18–3.

The temperature at the innermost zone of the char layer is 

assumed to be 300 °C. Because of the low thermal conductivity of wood, the temperature 6 mm inward from the base 

of the char layer is about 180 °C. This steep temperature 

18–14

General Technical Report FPL–GTR–190

Table 18–3. Charring rate data for selected wood species

Wood exposed to ASTM E 119 exposurea

Wood exposed to a constant heat fluxb

Linear charring ratee

(min mm–1)

Thermal penetration 

depth dg

( mm) 

Average mass 

loss rate 

(g m–2 s–1)

Species

Densityc

(kg m–3)

Char

contraction

factord

Linear

charring

ratee

(min 

mm–1)

Nonlinear

charring

ratef

(min 

mm–1.23)

Thermal 

penetration

depthg

(mm) 

18- 

kW m–2

heat

flux 

55- 

kW m–2

heat

flux 

18- 

kW m–2

heat

flux 

55- 

kW m–2

heat

flux 

18- 

kW m–2

heat

flux 

55- 

kW m–2

heat

flux 

Softwoods 

Southern 

Pine

509 0.60 1.24 0.56 33 2.27 1.17 38 26.5 3.8 8.6 

Western 

 redcedar 

310 0.83 1.22 0.56 33 — — — — — — 

Redwood 343 0.86 1.28 0.58 35 1.68 0.98 36.5 24.9 2.9 6.0 

Engelmann 

 spruce 

425 0.82 1.56 0.70 34 — — — — — — 

Hardwoods 

Basswood 399 0.52 1.06 0.48 32 1.32 0.76 38.2 22.1 4.5 9.3 

Maple, hard 691 0.59 1.46 0.66 31 — — — — — — 

Oak, red 664 0.70 1.59 0.72 32 2.56 1.38 27.7 27.0 4.1 9.6 

Yellow- 

 poplar 

504 0.67 1.36 0.61 32 — — — — — — 

a

Moisture contents of 8% to 9%. 

b

Charring rate and average mass loss rate obtained using ASTM E 906 heat release apparatus. Test durations were 50 to 98 min for 18-kW m–2 heat 

flux and 30 to 53 min for 55-kW m–2 heat flux. Charring rate based on temperature criterion of 300 °C and linear model. Mass loss rate based on 

initial and final weight of sample, which includes moisture driven from the wood. Initial average moisture content of 8% to 9%. c

Based on weight and volume of ovendried wood. d

Thickness of char layer at end of fire exposure divided by original thickness of charred wood layer (char depth). e

Based on temperature criterion of 288 °C and linear model. 

f

Based on temperature criterion of 288 °C and nonlinear model of Equation (18–3). 

g

As defined in Equation (18–6). Not sensitive to moisture content. 
gradient means the remaining uncharred cross-sectional area 

of a large wood member remains at a low temperature and 

can continue to carry a load. Once a quasi-steady-state charring rate has been obtained, the temperature profile beneath 

the char layer can be expressed as an exponential term or a 

power term. An equation based on a power term is

( )

2

i i 300 1 



 



 = + - - d

x T T T (18–5)

where T is temperature (°C), Ti initial temperature (°C), x

distance from the char front (mm), and d thermal penetration depth (mm).

In Table 18–3, values for the thermal penetration depth parameter are listed for both the standard fire exposure and the 

constant heat flux exposure. As with the charring rate, these 

temperature profiles assume a semi-infinite slab. The equation does not provide for the plateau in temperatures that 

often occurs at 100 °C in moist wood. In addition to these 

empirical data, there are mechanistic models for estimating 

the charring rate and temperature profiles. The temperature 

profile within the remaining wood cross section can be used 

with other data to estimate the remaining load-carrying capacity of the uncharred wood during a fire and the residual 

capacity after a fire.

Fire-Retardant-Treated Wood

Wood products can be treated with fire retardants to improve 

their fire performance. Fire-retardant treatments results 

in delayed ignition, reduced heat release rate, and slower 

spread of flames. HRRs are markedly reduced by fire-retardant treatment (Fig. 18–4). In terms of fire performance, 

fire-retardant treatments are marketed to improve the flame 

spread characteristics of the wood products as determined 

by ASTM E 84, ASTM E 108, or other flammability tests. 

Fire-retardant treatment also generally reduces the smokedeveloped index as determined by ASTM E 84. A fireretardant treatment is not intended to affect fire resistance of 

wood products as determined by an ASTM E 119 test in any 

consistent manner. Fire-retardant treatment does not make 

a wood product noncombustible as determined by ASTM E 

136 nor does it change its potential heat as determined by 

NFPA 259.

Because fire-retardant treatment does reduce the flammability of the wood product, FRT wood products are often 

used for interior finish and trim in rooms, auditoriums, and 

corridors where codes require materials with low surface 

flammability. Although FRT wood is not a noncombustible 

material, many codes have specific exceptions that allow 

the use of FRT wood and plywood in fire-resistive and noncombustible construction for framing of non-load-bearing 

partitions, nonbearing exterior walls, and roof assemblies. 

Fire-retardant-treated wood is also used for such special 

purposes as wood scaffolding and for the frame, rails, and 

stiles of wood fire doors. 

To meet specifications in building codes and various standards, FRT lumber and plywood is wood that has been 

pressure treated with chemicals to reduce its flame spread 

characteristics. In the case of other composite wood products, chemicals can be added during the manufacture of the 

wood product. Fire-retardant treatment of wood generally 

improves the fire performance by reducing the amount of 

flammable volatiles released during fire exposure or by 

reducing the effective heat of combustion, or both. Both 

results have the effect of reducing HRR, particularly during 

the initial stages of fire, and thus consequently reducing the 

rate of flame spread over the surface. The wood may then 

self-extinguish when the primary heat source is removed. 

FRT products can be found in the Underwriters Laboratories, Inc., “Building Materials Directory,” evaluation reports 

of ICC Evaluation Service, Inc. (ICC–ES), and other such 

listings.

Pressure Treatments

In impregnation treatments, wood is pressure impregnated 

with chemical solutions using pressure processes similar to 

those used for chemical preservative treatments. However, 

considerably heavier absorptions of chemicals are necessary 

for fire-retardant protection. Penetration of chemicals into 

the wood depends on species, wood structure, and moisture 

content. Because some species are difficult to treat, the 

degree of impregnation needed to meet the performance requirements for FRT wood may not be possible.

Inorganic salts are the most commonly used fire retardants 

for interior wood products, and their characteristics have 

been known for more than 50 years. These salts include 

monoammonium and diammonium phosphate, ammonium 

sulfate, zinc chloride, sodium tetraborate, and boric acid. 

Guanylurea phosphate is also used. Chemicals are combined 

in formulations to develop optimum fire performance yet 

still retain acceptable hygroscopicity, strength, corrosivity, 

machinability, surface appearance, glueability, and 

18–15

Chapter 18 Fire Safety of Wood Construction

0

50

100

150

200

250

100 200 300 400 500 600 700 800

Time (s)

Rate of heat release (kW m-2)

Untreated Douglas fir 

plywood, 12.5 mm thick

FRT treated

Figure 18–4. Heat release curves for untreated and fireretardant-treated (FRT) Douglas-fir plywood, 12.5 mm 

thick.
paintability. Cost is also a factor in these formulations. Actual formulations of commercial fire-retardant treatments 

are generally proprietary. For the two interior fire-retardant 

treatments listed in American Wood Protection Association 

(AWPA) (formerly American Wood-Preservers’ Association) 

standards, the chemicals listed are guanylurea phosphate 

and boric acid for FR-1 and phosphate, boric acid, and ammonia for FR-2. Species-specific information on the depth 

of chemical penetration for these two formulations can be 

found in Section 8.8 of AWPA Standard T1. Traditional fireretardant salts are water soluble and are leached out in exterior applications or with repeated washings. Water-insoluble 

organic fire retardants have been developed to meet the 

need for leach-resistant systems. Such treatments are also 

an alternative when a low-hygroscopic treatment is needed. 

These water-insoluble systems include (a) resins polymerized after impregnation into wood and (b) graft polymer 

fire retardants attached directly to cellulose. An amino resin 

system based on urea, melamine, dicyandiamide, and related 

compounds is of the first type.

There are AWPA standards that describe methods for testing 

wood for the presence of phosphate or boron. Such tests can 

be used to determine the presence of fire-retardant treatments that contain these chemicals. AWPA Standard A9 is a 

method for analysis of treated wood and treating solutions 

by x-ray spectroscopy. The method detects the presence 

of elements of atomic number 5 or higher including B(5) 

and P(15). AWPA Standard A26 has a method for analysis 

of fire retardant FR1 solutions or wood by titration for the 

percentages of boric acid and guanylurea phosphate. AWPA 

Standard A3 describes methods for determining penetration of fire retardants. Included are two methods for boroncontaining preservatives and fire retardants and one method 

for phosphorus-containing fire retardants. The compositions 

of commercial fire-retardant treatments are proprietary. In 

the case of boron, tests for its presence cannot distinguish 

between treatments for preservation and those for fire retardancy. Such chemical tests are not an indicator of the 

adequacy of the treatment in terms of fire retardancy. Smallscale fire tests such as the cone calorimeter (ASTM E 1354), 

oxygen index (ASTM D 2863), fire tube (ASTM E 69), and 

various thermal analysis methodologies can also be used to 

determine the presence of fire retardant treatment.

Performance Requirements

The IBC has prescriptive language specifying performance 

requirements for FRT wood. The fire performance requirement for FRT wood is that its FSI is 25 or less when tested 

according to the ASTM E 84 flame spread test and that it 

shows no evidence of significant progressive combustion 

when this 10-min test is continued for an additional 

20 min. In addition, it is required that the flame front in the 

test shall not progress more than 3.2 m beyond the centerline of the burner at any given time during the test. In the 

IBC, FRT wood must be a wood product impregnated with 

chemicals by a pressure process or other means during 

manufacture. In applications where the requirement being 

addressed is not for “fire-retardant-treated wood” but only 

for Class A or B flame spread, the treatment only needs to 

reduce the FSI to the required level in the ASTM E 84 flame 

spread test (25 for Class A, 75 for Class B).

In addition to requirements for flame spread performance, 

FRT wood for use in certain applications is required to meet 

other performance requirements. Wood treated with inorganic fire-retardant salts is usually more hygroscopic than is 

untreated wood, particularly at high relative humidities. Increases in equilibrium moisture content of this treated wood 

will depend upon the type of chemical, level of chemical retention, and size and species of wood involved. Applications 

that involve high humidity will likely require wood with low 

hygroscopicity. Requirements for low hygroscopicity in the 

IBC stipulate that interior FRT wood shall have a moisture 

content of not more than 28% when tested in accordance 

with ASTM D 3201 procedures at 92% relative humidity.

Exterior fire-retardant treatments should be specified whenever the wood is exposed to weather, damp, or wet conditions. Exterior type treatment is one that has shown no increase in the listed flame spread index after being subjected 

to the rain test of ASTM D 2898. Although the method of 

D 2898 is often not specified, the intended rain test is usually Method A of ASTM D 2898. Method B of D 2898 includes exposures to UV bulbs in addition to water sprays, is 

described in FPL publications, and is an acceptable method 

in AWPA Standard U1 for evaluating exterior treatments. 

The ASTM D 2898 standard practice was recently revised to 

include Methods C and D. Method C is the “amended rain 

test” described in the acceptance criteria for classified wood 

roof systems (AC107) of the ICC Evaluation Service, Inc. 

Method D is the alternative rain test described in ASTM E 

108 for roof coverings.

Fire-retardant treatment generally results in reductions in the 

mechanical properties of wood. Fire-retardant-treated wood 

is often more brash than untreated wood. For structural applications, information on mechanical properties of the FRT 

wood product needs to be obtained from the treater or chemical supplier. This includes the design modification factors 

for initial strength properties of the FRT wood and values 

for the fasteners. Adjustments to the design values must take 

into account expected temperature and relative humidity 

conditions. In field applications with elevated temperatures, 

such as roof sheathings, there is the potential for further 

losses in strength with time. Fire-retardant-treated wood 

that will be used in high-temperature applications, such as 

roof framing and roof sheathing, is also strength tested in 

accordance with ASTM D 5664 (lumber) or ASTM D 5516 

(plywood) for purpose of obtaining adjustment factors as 

described in ASTM D 6841 (lumber) and ASTM D 6305 

(plywood). The temperatures used to obtain the adjustment 

18–16

General Technical Report FPL–GTR–190
factors also become the maximum temperature that can be 

used in kiln drying of lumber or plywood after treatment.

Corrosion of fasteners can be accelerated under conditions 

of high humidity and in the presence of fire-retardant salts. 

For fire-retardant treatments containing inorganic salts, 

the types of metal and chemical in contact with each other 

greatly affect the rate of corrosion. Thus, information on 

proper fasteners also needs to be obtained from the treater or 

chemical supplier. Other issues that may require contacting 

the treater or chemical supplier include machinability, gluing characteristics, and paintability.

Fire-retardant treatment of wood does not prevent the wood 

from decomposing and charring under fire exposure (the rate 

of fire penetration through treated wood approximates the 

rate through untreated wood). Fire-retardant-treated wood 

used in doors and walls can slightly improve fire resistance 

of these doors and walls. Most of this improvement is associated with reduction in surface flammability rather than any 

changes in charring rates.

There are specifications for FRT wood issued by AWPA 

and NFPA. In terms of performance requirements, these 

specifications are consistent with the language in the codes. 

The AWPA standards C20 and C27 for FRT lumber and 

plywood have recently been deleted by AWPA. They have 

been replaced by AWPA “Use Category System Standards” 

for specifying treated wood. The specific provisions are 

Commodity H of Standard U1 and Section 8.8 of Standard 

T1. The fire protection categories are UCFA for interior 

applications where the wood is protected from exterior 

weather and UCFB for exterior applications where any 

water is allowed to quickly drain from the surface. Neither 

category is suitable for applications involving contact with 

the ground or with foundations. Commodity Specification 

H is fire-retardant treatment by pressure processes of solid 

sawn and plywood. The performance requirements for Commodity Specification H treatments are provided in Standard 

U1. Section 8.8 of Standard T1 provides information on the 

treatment and processing (that is, drying) of the products.

There is also NFPA standard 703 for FRT wood and fireretardant coatings. In addition to the performance and 

testing requirements for FRT wood products impregnated 

with chemicals by a pressure process or other means during 

manufacture, this NFPA standard provides separate specifications for fire-retardant coatings.

For parties interested in developing new fire-retardant treatments, there are documents that provide guidelines on the 

data required for technical acceptance. In the AWPA Book 

of Standards, there is “Appendix B: Guidelines for evaluating new fire retardants for consideration by the AWPA.” The 

ICC–ES has issued an “Acceptance criteria for fire-retardant-treated wood” (AC66), which provides guidelines for 

what is required to be submitted for their evaluation reports. 

There is also “Acceptance criteria for classified wood roof 

systems” (AC107). Because of the relative small size of the 

specimen, FPL uses the cone calorimeter in its research and 

development of new FRT products.

Fire-Retardant Coatings

For some applications, applying the fire-retardant chemical 

as a coating to the wood surface may be acceptable to the 

authorities having jurisdiction. Commercial coating products are available to reduce the surface flammability characteristics of wood. The two types of coatings are intumescent 

and nonintumescent. The widely used intumescent coatings 

“intumesce” to form an expanded low-density film upon 

exposure to fire. This multicellular carbonaceous film insulates the wood surface below from high temperatures. Intumescent formulations include a dehydrating agent, a char 

former, and a blowing agent. Potential dehydrating agents 

include polyammonium phosphate. Ingredients for the char 

former include starch, glucose, and dipentaerythritol. Potential blowing agents for the intumescent coatings include 

urea, melamine, and chlorinate parafins. Nonintumescent 

coating products include formulations of the water-soluble 

salts such as diammonium phosphate, ammonium sulfate, 

and borax.

NFPA standard 703 includes specifications for fire-retardant 

coatings. Because coatings are not pressure impregnated or 

incorporated during manufacture, fire-retardant coated wood 

is not FRT wood as defined in most codes or standards including NFPA 703. In NFPA 703, a fire-retardant coating is 

defined as a coating that reduces the flame spread of Douglas-fir and all other tested combustible surfaces to which it 

is applied by at least 50% or to a flame spread classification 

value of 75 or less, whichever is the lesser value, and has a 

smoke developed rating not exceeding 200 when tested in 

accordance with ASTM E 84, NFPA 255, or UL 723. There 

is no requirement that the standard test be extended for an 

additional 20 min as required for FRT wood. NFPA 703 differentiates between a Class A coating as one that reduces 

flame spread index to 25 or less and a Class B coating as 

one that reduces flame spread index to 75 or less.

Fire-retardant coatings for wood are tested and marketed to 

reduce flame spread. Clear intumescent coatings are available. Such coatings allow the exposed appearance of old 

structural wood members to be maintained while providing 

improved fire performance. This is often desirable in the 

renovation of existing structures, particularly museums and 

historic buildings. Studies have indicated that coatings subjected to outdoor weathering are of limited durability and 

would need to be reapplied on a regular basis.

Although their use to improve the resistance ratings of wood 

products has been investigated, there is no general acceptance for using coatings to improve the fire resistance rating 

of a wood member. There is a lack of full-scale ASTM E 

119 test data to demonstrate their performance and validate 

a suitable calculation methodology for obtaining the rating.

18–17

Chapter 18 Fire Safety of Wood Construction
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18–18

General Technical Report FPL–GTR–190
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NFPA 101. Life safety code.

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Fire Test Standards

ASTM. [Current edition]. West Conshohocken, PA: ASTM 

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ASTM D 2863. Measuring the minimum oxygen 

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ASTM D 2898. Accelerated weathering of fire-retardant 

treated wood for fire testing.

ASTM D 6513. Calculating the superimposed load on 

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ASTM E 69. Combustible properties of treated wood 

by fire-tube apparatus.

ASTM E 84. Surface burning characteristics of building 

materials.

ASTM E 108. Fire tests of roof coverings.

ASTM E 119. Fire tests of building construction and 

materials.

ASTM E 136. Behavior of materials in a vertical tube 

furnace at 750 °C.

ASTM E 162. Surface flammability of materials using a 

radiant heat energy source.

ASTM E 648. Critical radiant flux of floor-covering 

systems using a radiant heat energy source.

ASTM E 662. Specific optical density of smoke 

generated by solid materials.

ASTM E 814. Fire tests of through-penetration fire 

stops.

ASTM E 906. Heat and visible smoke release rates 

from materials and products.

ASTM E 970. Critical radiant flux of exposed attic floor 

insulation using a radiant heat energy source.

ASTM E 1321. Determining material ignition and flame 

spread properties.

ASTM E 1354. Heat and visible smoke release rates for 

materials and products using an oxygen consumption 

calorimeter.

ASTM E 1623. Determination of fire and thermal 

parameters of materials, products, and systems using an 

intermediate scale calorimeter (ICAL).

ASTM E 1678. Measuring smoke toxicity for use in fire 

hazard analysis.

ASTM E 2257. Room fire test of wall and ceiling 

materials and assemblies.

ASTM E 2579. Specimen preparation and mounting of 

wood products to assess surface burning characteristics.

NFPA. [Current edition]. Quincy, MA: National Fire 

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NFPA 253. Critical radiant flux of floor covering 

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NFPA 259. Potential heat of building materials.

NFPA 286. Evaluating contribution of wall and ceiling 

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exposed wood members. Tech. Rep. 10. Washington, DC: 

American Forest & Paper Association—American Wood 

Council. www.awc.org.

AWC. [Current edition]. Chapter M16: Fire design. In: 

ASD/LRFD manual for engineered wood construction. 

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American Wood Council. www.awc.org.

AWC. [Current edition]. Component additive method 

(CAM) for calculating and demonstrating assembly fire 

endurance. Design for code acceptance No. 4. Washington, 

DC: American Forest & Paper Association—American 

Wood Council. www.awc.org.

AWC. [Current edition]. Design of fire-resistive exposed 

wood members. Design for code acceptance No. 2. 

Washington, DC: American Forest & Paper Association—

American Wood Council. www.awc.org.

AWC. [Current edition]. Fire rated wood floor and wall 

assemblies. Design for code acceptance No. 3. Washington, 

DC: American Forest & Paper Association—American 

Wood Council. www.awc.org.

Gypsum Association. [Current edition]. Fire resistance 

design manual. Washington, DC: Gypsum Association. 

www.gypsum.org.

HUD. 1980. Guideline on fire ratings of archaic materials 

and assemblies. Rehabilitation guidelines, part 8. 

Washington, DC: U.S. Department of Housing and Urban 

Development, Superintendent of Documents. http://www.

huduser.org/Publications/PDF/fire.pdf.

Intertek. [Current edition]. Directory of listed product. 

www.intertek-etlsemko.com.

Janssens, M. 1994. Thermo-physical properties for 

wood pyrolysis models. In: Proceedings, Pacific timber 

engineering conference; 1994 July 11–15; Gold Coast, 

Australia. Fortitude Valley MAC, Queensland, Australia: 

Timber Research and Development Advisory Council: 

607–618.

Janssens, M.L.; White, R.H. 1994. Short communication: 

temperature profiles in wood members exposed to fire. Fire 

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Ross, R.J.; Brashaw, B.K.; Wang, X.; White, R.H.; Pellerin, 

R.F. 2004. Wood and timber condition assessment manual. 

Madison, WI: Forest Products Society.

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charring rate of wood. Res. Note FPL–145. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

Schaffer, E.L. 1977. State of structural timber fire 

endurance. Wood and Fiber. 9(2): 145–170.

Schaffer, E.L. 1984. Structural fire design: Wood. Res. Pap. 

FPL 450. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory.

Tran, H.C.; White, R.H. 1992. Burning rate of solid wood 

measured in a heat release calorimeter. Fire and Materials. 

16: 197–206.

UL. [Current edition]. Fire resistance directory. Northbrook, 

IL: Underwriters Laboratories, Inc. www.ul.com.

White, R.H. 2003. Fire resistance of engineered wood rim 

board products. FPL Res. Pap. 610. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 22 p.

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members. In: Proceedings, 5th international wood & fire 

safety conference. Zvolen, Slovak Republic: Technical 

University of Zvolen, Faculty of Wood Science and 

Technology. 337–44.

White, R.H. 2006. Fire resistance of structural composite 

lumber products, Res. Pap. FPL–RP–633. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 28 p.

White, R.H. 2008. Analytical methods for determining fire 

resistance of timber members. In: SFPE handbook of fire 

protection engineering. 4th ed. Quincy, MA: National Fire 

Protection Association.

18–21

Chapter 18 Fire Safety of Wood Construction
White, R.H.; Nordheim, E.V. 1992. Charring rate of wood 

for ASTM E 119 exposure. Fire Technology. 28: 5–30.

White, R.H.; Tran, H.C. 1996. Charring rate of wood 

exposed to a constant heat flux. In: Proceedings, wood & 

fire safety, 3rd international conference. The High Tatras; 

1996 May 6–9; Zvolen, Slovakia. Zvolen, Slovakia: Faculty 

of Wood Technology, Technical University: 175–183.

Fire-Retardant-Treated Wood

ASTM. [Current edition]. West Conshohocken, PA: ASTM 

International. www.astm.org.

ASTM D 3201. Hygroscopic properties of fire-retardant 

wood and wood-base products.

ASTM D 5516. Evaluating the mechanical properties 

of fire-retardant treated softwood plywood exposed to 

elevated temperatures.

ASTM D 5664. Evaluating the effects of fire-retardant 

treatments and elevated temperatures on strength 

properties of fire-retardant treated lumber.

ASTM D 6305. Calculating bending strength design 

adjustment factors for fire-retardant-treated plywood 

roof sheathing.

ASTM D 6841. Calculating design value treatment 

adjustment factors for fire-retardant-treated lumber.

AWPA. [Current edition]. Gransbury, TX: American 

Wood Protection Association (formerly American WoodPreservers’ Association). www.awpa.com.

Standard A2. Analysis of waterborne preservatives and 

fire-retardant formulations.

Standard A3. Determining penetration of preservative 

and fire retardants.

Standard A9. Analysis of treated wood and treating 

solutions by X-ray spectroscopy.

Standard A26. Analysis of fire retardant solutions and 

wood by titration.

Standard T1. Use category system: processing and 

treatment standard.

Standard U1. Use category system: user specification 

for treated wood.

Holmes, C.A. 1977. Effect of fire-retardant treatments on 

performance properties of wood. In: Goldstein, I.S., ed. 

Wood technology: chemical aspects. Proceedings, ACS 

symposium series 43. Washington, DC: American Chemical 

Society.

ICC-ES. [Current edition]. Whittier, CA: ICC Evaluation 

Service, Inc. www.icc-es.org.

AC66. Acceptance criteria for fire-retardant-treated 

wood.

18–22

General Technical Report FPL–GTR–190

AC107. Acceptance criteria for classified wood roof 

systems.

Lebow, P.K., Winandy, J.E. 2003. Using kinetic models 

to predict thermal degradation of fire-retardant-treated 

plywood roof sheathing. Paper 048. In: Proceedings, 31st 

annual conference of North American Thermal Analysis 

Society; 2003 September 22–24; Albuquerque, NM.

LeVan, S.L. 1984. Chemistry of fire retardancy. In: Rowell, 

R.M., ed. The chemistry of solid wood. Advances in 

Chemistry series 207. Washington, DC: American Chemical 

Society.

LeVan, S.L.; Tran, H.C. 1990. The role of boron in flameretardant treatments. In: Hamel, M., ed. Proceedings 47355, 

1st international conference on wood protection with 

diffusible preservatives; 1990 November 28–30; Nashville, 

TN. Madison, WI: Forest Products Research Society: 39–41.

LeVan, S.L.; Winandy, J.E. 1990. Effects of fire-retardant 

treatments on wood strength: a review. Wood and Fiber 

Science. 22(1): 113–131.

LeVan, S.L.; Ross, R.J.; Winandy, J.E. 1990. Effects of fire 

retardant chemicals on the bending properties of wood at 

elevated temperatures. Res. Pap. FPL–RP–498. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

NAHB. 1990. Home builders guide to fire retardant treated 

plywood. Evaluation, testing, and replacement. Upper 

Marlboro, MD: National Association of Home Builders, 

National Research Center.

NFPA. [Current edition]. Quincy, MA: National Fire 

Protection Association. www.nfpa.org.

NFPA 703. Fire retardant-treated wood and fireretardant coatings for building materials.

Winandy, J.E. 1995. Effects of fire retardant treatments 

after 18 months of exposure at 150 °F (66 °C). Res. 

Note FPL–RN–0264. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

Winandy, J.E.; LeVan, S.L.; Ross, R.J.; Hoffman, S.P.; 

McIntyre, C.R. 1991. Thermal degradation of fire-retardanttreated plywood—development and evaluation of a test 

protocol. Res. Pap. FPL–RP–501. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.
CHAPTER 19

Specialty Treatments

Rebecca E. Ibach, Research Chemist

19–1

Contents 

Plasticizing Wood 19–1

 Principles of Plasticizing and Bending 19–1

 Bent Wood Members 19–2

 Laminated Members 19–2

 Veneered Curved Members 19–3

 Bending of Solid Members 19–3

 Selection of Stock 19–3

 Moisture Content of Bending Stock 19–3

 Bending Operation and Apparatus 19–4

 Fixing the Bend 19–4

 Characteristics of Bent Wood 19–4

Modified Woods 19–4

 Resin-Treated Wood—Not Compressed 

 (Impreg) 19–4

 Resin-Treated Wood—Compressed 

 (Compreg) 19–5

 Heat Treatments 19–9

 Heating Wet Wood 19–9

 Heating Wet Wood Followed by 

 Compression 19–10

 Heating Dry Wood 19–10

 Heating Dry Wood Followed by 

 Compression 19–10

 Wood Treated with Polyethylene Glycol 

 (PEG) 19–10

 Wood–Polymer Composites 19–11

 Chemical Modification 19–12

Paper-Based Plastic Laminates 19–12

 Industrial Laminates 19–12

 Decorative Laminates 19–13

 Lignin-Filled Laminates 19–14

 Paper-Face Overlays 19–14

References 19–14

Many specialty treatments can be applied to wood to either improve its performance or change its properties. 

Treatments addressed in this chapter are those that make 

permanent changes in the shape of a wood product, improvements in dimensional stability, or improvements in 

performance through combinations with nonwood resources.

Plasticizing Wood

Principles of Plasticizing and Bending

In simple terms, the wood cell wall is a composite made of 

a rigid cellulose polymer in a matrix of lignin and the hemicelluloses. The lignin polymer in the middle lamella and 

S2 layer is thermoplastic; that is, it softens upon heating. 

The glass transition temperature Tg of the lignin in the matrix is approximately 170 °C (338 °F). Above the matrix Tg, 

it is possible to cause the lignin to undergo thermoplastic 

flow and, upon cooling, reset in the same or modified configuration. This is the principle behind bending of wood.

The matrix can be thermoplasticized by heat alone, but the 

Tg of the unmodified matrix is so high that some fiber decomposition can occur if high temperatures are maintained 

for a lengthy period. The Tg of the matrix can be decreased 

with the addition of moisture or through the use of plasticizers or softeners.

Heat and moisture make certain species of wood sufficiently 

plastic for bending operations. Steaming at atmospheric or 

a low gauge pressure, soaking in boiling or nearly boiling 

water, or microwave heating moist wood are satisfactory 

methods of plasticizing wood. Wood at 20% to 25% moisture content needs to be heated without losing moisture; at 

lower moisture content, heat and moisture must be added. 

As a consequence, the recommended plasticizing processes 

are steaming or boiling for about 15 min cm–1 (38 min in–1) 

of thickness for wood at 20% to 25% moisture content and 

steaming or boiling for about 30 min cm–1 (75 min in–1) of 

thickness for wood at lower moisture content levels. Steaming at high pressures causes wood to become plastic, but 

wood treated with high pressure steam generally does not 

bend as successfully as does wood treated at atmospheric 

or low pressure. Microwave heating requires much shorter 

times.

Wood can be plasticized by a variety of chemicals in addition to water. Common chemicals that plasticize wood 

include urea, dimethylol urea, low-molecular-weight 

phenol-formaldehyde resin, dimethyl sulfoxide, and liquid 

ammonia. Urea and dimethylol urea have received limited 
19–2

General Technical Report FPL–GTR–190

commercial attention, and a bending process using liquid 

ammonia has been patented. Wood members can be readily molded or shaped after immersion in liquid ammonia or 

treatment under pressure with ammonia in the gas phase. As 

the ammonia evaporates, the lignin resets, the wood stiffens 

and retains its new shape. Plasticization of the lignin matrix 

alone can be done using chemical modification technologies, which are covered later in this chapter.

It is also possible to bend wood without softening or plasticizing treatments. However, the stability of the final product 

may not be as permanent as from treatments in which softening and plasticizing methods are used.

Bent Wood Members

Bending can provide a variety of functional and esthetically 

pleasing wood members, ranging from large curved arches 

to small furniture components. The curvature of the bend, 

size of the member, and intended use of the product determine the production method.

Laminated Members

At one time in the United States, curved pieces of wood 

were laminated chiefly to produce small items such as parts 

for furniture and pianos. However, the principle was extended to the manufacture of arches for roof supports in farm, 

industrial, and public buildings and other types of structural 

members (see Chap. 11). The laminations are bent without 

end pressure against a form and adhesively bonded together. 

Both softwoods and hardwoods are suitable for laminated 

bent structural members, and thin material of any species 

can be bent satisfactorily for such purposes. The choice of 

species and adhesive depends primarily on the cost, required 

strength, and demands of the application.

Laminated curved members are produced from dry stock 

in a single bending and adhesive bond formation operation. 

This process has the following advantages compared with 

bending single-piece members:

•	 Bending thin laminates to the required radius involves 

only moderate stress and deformation of the wood fibers, eliminating the need for treatment with steam or 

hot water and associated drying and conditioning of 

the finished product. In addition, the moderate stresses 

involved in curving laminated members result in stronger members when compared with curved single-piece 

members.

•	 The tendency of laminated members to change shape 

with changes in moisture content is less than that of 

single-piece bent members.

•	 Ratios of thickness of member to radius of curvature 

that are impossible to obtain by bending single pieces 

can be attained readily by laminating.

•	 Curved members of any desired length can be 

produced.

Straight-laminated members can be steamed and bent after 

they are bonded together. However, this type of procedure 

requires an adhesive that will not be affected by the steaming or boiling treatment and complicates conditioning of the 

finished product.

Curved Plywood

Curved plywood is produced either by bending and adhesive 

bonding the plies in one operation or by bending previously 

bonded flat plywood. Plywood curved by bending and bonding simultaneously is more stable in curvature than plywood 

curved by bending previously bonded material.

Plywood Bent and Adhesively Bonded Simultaneously

In bending and bonding plywood in a single operation, 

adhesive-coated pieces of veneer are assembled and pressed 

over or between curved forms. Pressure and sometimes 

heat are applied through steam or electrically heated forms 

until the adhesive sets and holds the assembly to the desired 

curvature. Some laminations are at an angle, usually 90°, 

to other laminations, as in the manufacture of flat plywood. 

The grain direction of the thicker laminations is normally 

parallel to the axis of the bend to facilitate bending.

A high degree of compound curvature can be obtained in an 

assembly made up of a considerable number of thin veneers. 

First, for both the face and back of the assembly, the two 

outer plies are bonded at 90° to each other in a flat press. 

The remaining veneers are then adhesive-coated and assembled at any desired angle to each other. The entire assembly 

is hot-pressed to the desired curvature.

Bonding the two outer plies before molding allows a higher 

degree of compound curvature without cracking the face 

plies than could otherwise be obtained. Where a high degree 

of compound curvature is required, the veneer should be 

relatively thin (less than 3 mm (1/8 in.)) with a moisture 

content of about 12%.

The molding of plywood with fluid pressure applied by flexible bags of some impermeable material produces plywood 

parts of various degrees of compound curvature. In “bag 

molding,” fluid pressure is applied through a rubber bag by 

air, steam, or water. The veneer is wrapped around a form, 

and the whole assembly is enclosed in a bag and subjected 

to pressure in an autoclave, the pressure in the bag being 

“bled.” Or the veneer may be inserted inside a metal form 

and, after the ends have been attached and sealed, pressure 

is applied by inflating a rubber bag. The form may be heated 

electrically or by steam.

The advantages of bending and bonding plywood simultaneously to form a curved shape are similar to those for 

curved-laminated members. In addition, the cross plies give 

the curved members properties that are characteristic of 

cross-banded plywood. Curved plywood shells for furniture 

manufacture are examples of these bent veneer and adhesive-bonded products.
19–3

Chapter 19 Specialty Treatments

Plywood Bent after Bonding

After the plies are bonded together, flat plywood is often 

bent by methods that are somewhat similar to those used in 

bending solid wood. To bend plywood properly to shape, 

it must be plasticized by some means, usually moisture or 

heat, or a combination of both. The amount of curvature 

that can be introduced into a flat piece of plywood depends 

on numerous variables, such as moisture content, direction 

of grain, thickness and number of plies, species and quality 

of veneer, and the technique applied in producing the bend. 

Plywood is normally bent over a form or a bending mandrel.

Flat plywood bonded with a waterproof adhesive can be 

bent to compound curvatures after bonding. However, no 

simple criterion is available for predetermining whether a 

specific compound curvature can be imparted to flat plywood. Soaking the plywood prior to bending and using 

heat during forming are aids in manipulation. Usually, the 

plywood to be postformed is first thoroughly soaked in hot 

water, and then dried between heated forming dies attached 

to a hydraulic press. If the use of postforming for bending 

flat plywood to compound curvatures is contemplated, exploratory trials to determine the practicability and the best 

procedure are recommended. Remember that in postforming plywood to compound curvatures, all the deformation 

must be by compression or shear because plywood cannot 

be stretched. Hardwood species, such as birch, poplar, and 

gum, are usually used in plywood that is to be postformed.

Veneered Curved Members

Veneered curved members are usually produced by bonding 

veneer to one or both faces of a curved solid-wood base. 

The bases are ordinarily sawn to the desired shape or bent 

from a piece grooved with saw kerfs on the concave side at 

right angles to the direction of bend. Pieces bent by making 

saw kerfs on the concave side are commonly reinforced and 

kept to the required curvature by bonding splines, veneer, or 

other pieces to the curved base. Veneering over curved solid 

wood is used mainly in furniture. The grain of the veneer is 

commonly laid in the same general direction as the grain of 

the curved wood base. The use of crossband veneers, that is, 

veneers lay with the grain at right angles to the grain of the 

back and face veneer, decreases the tendency of the member 

to split.

Bending of Solid Members

Wood of certain species that is steamed, microwaved, or 

soaked in boiling water can be compressed as much as 

25% to 30% parallel to the grain. The same wood can be 

stretched only 1% to 2%. Because of the relation between 

attainable tensile and compressive deformations, if bending 

involves severe deformation, then most of the deformation 

must be compression. The inner or concave side must assume the maximum amount of compression, and the outer 

or convex side must experience zero strain or a slight tension. To accomplish this, a metal strap equipped with end 

fittings is customarily used. The strap makes contact with 

the outer or convex side and, acting through the end fittings, 

places the whole piece of wood in compression. The tensile 

stress that would normally develop in the outer side of the 

piece of wood during bending is borne by the metal strap. A 

bending form is shown in Figure 19–1.

Selection of Stock

In general, hardwoods possess better bending quality than 

softwoods, and certain hardwoods surpass others in this 

quality. This is interesting from a theoretical point of view 

because hardwoods contain less lignin than softwoods. 

Hardwoods also contain much more hemicelluloses in the 

matrix than do softwoods. The species commonly used to 

produce bent members are white oak, red oak, elm, hickory, 

ash, beech, birch, maple, walnut, sweetgum, and mahogany. 

As stated, most softwoods have a poor bending quality and 

are not often used in bending operations. However, Pacific 

yew and yellow-cedar are exceptions to this rule. In addition, Douglas-fir, southern yellow pine, northern and Atlantic white-cedar, and redwood are used for ship and boat 

planking for which purpose they are often bent to moderate 

curvature after being steamed or soaked.

Bending stock should be free from serious cross grain and 

distorted grain, such as may occur near knots. The slope of 

cross grain should not be steeper than about 1 to 15. Decay, 

knots, shake, pith, surface checks, and exceptionally light or 

brittle wood should be avoided.

Moisture Content of Bending Stock

Although green wood can be bent to produce many curved 

members, difficulties are encountered in drying and fixing 

the bend. Another disadvantage with green stock is that 

hydrostatic pressure may be developed during bending. 

Hydrostatic pressure can cause compression failures on the 

concave side if the wood is compressed by an amount 

greater than the air space in the cells of the green wood. 

Figure 19–1. Chair back being bent through an arc of 

180° in a bending machine.
19–4

General Technical Report FPL–GTR–190

Bending stock that has been dried to low moisture content 

level requires a lengthy steaming or soaking process to increase its moisture content to the point where it can be made 

sufficiently plastic for successful bending. For most chair 

and furniture parts, the moisture content of the bending 

stock should be 12% to 20% before it is steamed or microwave heated. The preferred moisture content level varies 

with the severity of the curvature to which the wood is bent 

and the method used in drying and fixing the bent member. 

For example, chair-back slats, which have a slight curvature and are subjected to severe drying conditions between 

steam-heated platens, can be produced successfully from 

stock at 12% moisture content. For furniture parts that need 

a more severe bend where the part must be bent over a form, 

15% to 20% moisture content is recommended.

Bending Operation and Apparatus

After being plasticized, the stock should be quickly placed 

in the bending apparatus and bent to shape. The bending 

apparatus consists essentially of a form (or forms) and a 

means of forcing the piece of steamed wood against the 

form. If the curvature to be obtained demands a difference 

of much more than 3% between lengths of the outer and inner surfaces of the pieces, then the apparatus should include 

a device for applying end pressure. This generally takes the 

form of a metal strap or pan provided with end blocks, end 

bars, or clamps.

Fixing the Bend

After being bent, the piece should be cooled and dried while 

held in its curved shape. One method is to dry the piece in 

the bending machine between the plates of a hot-plate press. 

Another method is to secure the bent piece to the form and 

place both the piece and the form in a drying room. Still another is to keep the bent piece in a minor strap with tie rods 

or stays so that it can be removed from the form and placed 

in a drying room. When the bent member has cooled and 

dried to moisture content suitable for its intended use, the 

restraining devices can be removed and the piece will hold 

its curved shape.

Characteristics of Bent Wood

After a bent piece of wood is cooled and dried, the curvature 

will be maintained. An increase in moisture content may 

cause the piece to lose some of its curvature. A decrease in 

moisture content may cause the curve to become sharper, 

although repeated changes in moisture content bring about 

a gradual straightening. These changes are caused primarily 

by lengthwise swelling or shrinking of the inner (concave) 

face, the fibers of which were wrinkled or folded during the 

bending operation.

A bent piece of wood has less strength than a similar unbent 

piece. However, the reduction in strength brought about by 

bending is seldom serious enough to affect the utility value 

of the member.

Modified Woods 

Wood can be chemically modified to improve water repellency, dimensional stability, resistance to acids or bases, 

ultraviolet radiation, biodeterioration, and thermal degradation. Wood can also be chemically treated, then compressed 

to improve dimensional stability and increase hardness. 

Sheets of paper treated with resins or polymers can be laminated and hot pressed into thick panels that have the appearance of plastic rather than paper. These sheets are used 

in special applications because of their structural properties 

and in items requiring hard, impervious, and decorative surfaces.

Modified woods, modified wood-based materials, and paperbased laminates are usually more expensive than wood because of the cost of the chemicals and the special processing 

required producing them. Thus, modified wood use is generally limited to special applications where the increased cost 

is justified by the special properties needed.

Wood is treated with chemicals to increase hardness and 

other mechanical properties, as well as its resistance to decay, fire, weathering, and moisture. The rate and extent of 

swelling and shrinking of the wood when in contact with 

water is decreased by application of water-resistant chemicals to the surface of wood, impregnation of the wood with 

such chemicals dissolved in water or volatile solvents, or 

bonding chemicals to the cell wall polymer. Such treatments 

may also decrease the rate at which wood changes dimension as a result of humidity, even though these treatments do 

not affect the final dimensional changes caused by lengthy 

duration exposures. Paints, varnishes, lacquers, wood-penetrating water repellents, and plastic and metallic films retard 

the rate of moisture absorption but have little effect on total 

dimensional change if exposure to moisture is extensive and 

prolonged.

Resin-Treated Wood—Not Compressed 

(Impreg)

Permanent stabilization of the dimensions of wood is 

needed for certain specialty uses. This can be accomplished 

by depositing a bulking agent within the swollen structure 

of the wood fibers. The most successful bulking agents that 

have been commercially applied are highly water-soluble, 

thermosetting, phenol-formaldehyde resin-forming systems, 

with initially low molecular weights. No thermoplastic resins have been found that effectively stabilize the dimensions 

of wood.

Wood treated with a thermosetting, fiber-penetrating resin 

and cured without compression is known as impreg. The 

wood (preferably green veneer to facilitate resin pickup) is 

soaked in the aqueous resin-forming solution or, if air dry, is 

impregnated with the solution under pressure until the resin 

content equals 25% to 35% of the weight of dry wood. The 

treated wood is allowed to stand under nondrying conditions 
19–5

Chapter 19 Specialty Treatments

for 1 to 2 days to permit uniform distribution of the solution 

throughout the wood. The resin-containing wood is dried at 

moderate temperatures to remove the water, and then heated 

to higher temperatures to cure the resin.

Uniform distribution of the resin has been effectively accomplished with thick wood specimens only in sapwood of 

readily penetrated species. Although thicker material can 

be treated, the process is usually applied to veneers up to 

about 8 mm (0.3 in.) thick, because treating time increases 

rapidly with increases in thickness. Drying thick, resintreated wood may result in checking and honeycombing. For 

these reasons, treatments should be confined to veneer and 

the treated-cured veneer used to build the desired products. 

Any species can be used for the veneer except the resinous 

pines. The stronger the original wood, the stronger the end 

product.

Impreg has a number of properties differing from those of 

normal wood and ordinary plywood. These properties are 

given in Table 19–1, with similar generalized findings for 

other modified woods. Data for the strength properties of 

yellow birch impreg are given in Table 19–2. Information on 

thermal expansion properties of ovendry impreg is given in 

Table 19–3.

The good dimensional stability of impreg is the basis of 

one use where its cost is not a deterrent. Wood dies of automobile body parts serve as the master from which the 

metal-forming dies are made for actual manufacture of 

parts. Small changes in moisture content, even with the most 

dimensionally stable wood, produce changes in dimension 

and curvature of an unmodified wood die. Such changes create major problems in making the metal-forming dies where 

close final tolerances are required. The use of impreg, with 

its high antishrink efficiency (ASE) (Table 19–4), almost 

entirely eliminated the problem of dimensional change during the entire period that the wood master dies were needed. 

Despite the tendency of the resins to dull cutting tools, pattern makers accepted the impreg readily because it machines 

with less splitting than unmodified wood.

Patterns made from impreg are also superior to unmodified 

wood in resisting heat when used with shell-molding techniques where temperatures as high as 205 °C (400 °F) are 

required to cure the resin in the molding sand.

Resin-Treated Wood—Compressed (Compreg)

Compreg is similar to impreg except that it is compressed 

before the resin is cured within the wood. The resin-forming 

chemicals (usually phenol-formaldehyde) act as plasticizers for the wood so that it can be compressed under modest 

pressure (6.9 MPa, 1,000 lb in–2) to a specific gravity of 

1.35. Some properties of compreg are similar to those of impreg, and others vary considerably (Tables 19–1 and 19–2). 

Compared with impreg, the advantages of compreg are its 

natural lustrous finish that can be developed on any cut 

surface by sanding with fine-grit paper and buffing, its 

greater strength properties, and its ability to mold (Tables 

19–1 and 19–2). However, thermal expansion coefficients of 

ovendry compreg are also increased (Table 19–3).

Compreg can be molded by (a) gluing blocks of resintreated (but still uncured) wood with a phenolic glue so that 

the gluelines and resin within the plies are only partially set; 

(b) cutting to the desired length and width but two to three 

times the desired thickness; and (c) compressing in a split 

mold at about 150 °C (300 °F). Only a small flash squeeze 

out at the parting line between the two halves of the mold 

needs to be machined off. This technique was used for motor-test propellers and airplane antenna masts during World 

War II.

A more satisfactory molding technique, known as expansion molding, has been developed. The method consists 

of rapidly precompressing dry but uncured single sheets 

of resin-treated veneer in a cold press after preheating the 

sheets at 90 to 120 °C (195 to 250 °F). The heat-plasticized 

wood responds to compression before cooling. The heat is 

insufficient to cure the resin, but the subsequent cooling sets 

the resin temporarily. These compressed sheets are cut to the 

desired size, and the assembly of plies is placed in a split 

mold of the final desired dimensions. Because the wood was 

precompressed, the filled mold can be closed and locked. 

When the mold is heated, the wood is again plasticized and 

tends to recover its uncompressed dimensions. This exerts 

an internal pressure in all directions against the mold equal 

to about half the original compressing pressure. On continued heating, the resin is set. After cooling, the object may 

be removed from the mold in finished form. Metal inserts 

or metal surfaces can be molded to compreg or its handles 

are molded onto tools by this means. Compreg bands have 

been molded to the outside of turned wood cylinders without compressing the core. Compreg tubes and small airplane 

propellers have been molded in this way.

Past uses of compreg were related largely to aircraft; however, it is a suitable material where bolt-bearing strength is 

required, as in connector plates, because of its good specific 

strength (strength per unit of weight). Layers of veneer making up the compreg for such uses are often cross laminated 

(alternate plies at right angles to each other, as in plywood) 

to give nearly equal properties in all directions.

As a result of its excellent strength properties, dimensional 

stability, low thermal conductivity, and ease of fabrication, 

compreg is extremely useful for aluminum drawing and 

forming dies, drilling jigs, and jigs for holding parts in place 

while welding.

Compreg has also been used in silent gears, pulleys, waterlubricated bearings, fan blades, shuttles, bobbins, and picker 

sticks for looms, nuts and bolts, instrument bases and cases, 

musical instruments, electrical insulators, tool handles, and 

various novelties. At present, compreg finds considerable 
19–6

General Technical Report FPL–GTR–190

Table 19–1. Properties of modified woods

Property Impreg Compreg Staypak 

Specific gravity 15% to 20% greater than 

normal wood 

Usually 1.0 to 1.4 1.25 to 1.40 

Equilibrium swelling 

and shrinking 

1/4 to 1/3 that of normal wood 1/4 to 1/3 that of normal wood at right angle 

to direction of compression, greater in 

direction of compression but very slow to 

attain

Same as normal wood at right angle to 

direction of compression, greater in 

direction of compression but very slow to 

attain

Springback None Very small when properly made Moderate when properly made 

Face checking Practically eliminated Practically eliminated for specific gravities 

less than 1.3 

About the same as in normal wood 

Grain raising Greatly reduced Greatly reduced for uniform-texture woods, 

considerable for contrasting grain woods 

About the same as in normal wood 

Surface finish Similar to normal wood Varnished-like appearance for specific 

gravities greater than about 1.0; cut surfaces 

can be given this surface by sanding and 

buffing 

Varnished-like appearance; cut surfaces 

can be given this surface by sanding and 

buffing 

Permeability to water 

vapor 

About 1/10 that of normal 

wood 

No data, but presumably much less than 

impreg 

No data, but presumably less than impreg 

Decay and termite 

resistance 

Considerably better than 

normal wood 

Considerably better than normal wood Normal, but decay occurs somewhat more 

slowly

Acid resistance Considerably better than 

normal wood 

Better than impreg because of 

impermeability 

Better than normal wood because of 

impermeability, but not as good as 

compreg 

Alkali resistance Same as normal wood Somewhat better than normal wood because 

of impermeability 

Somewhat better than normal wood 

because of impermeability 

Fire resistance Same as normal wood Same as normal wood for long exposures, 

somewhat better for short exposures 

Same as normal wood for long exposures, 

somewhat better for short exposures 

Heat resistance Greatly increased Greatly increased No data 

Electrical 

conductivity 

1/10 that of normal wood 

at 30% RH; 1/1,000 that of normal 

wood at 90% RH 

Slightly more than impreg at low relative 

humidity values due to entrapped water 

No data 

Heat conductivity Slightly increased Increased about in proportion to specific 

gravity increase 

No data, but should increase about in 

proportion to specific gravity increase 

Compressive strength Increased more than 

proportional to specific gravity 

increase 

Increased considerably more than 

proportional to specific gravity increase 

Increased about in proportion to specific 

gravity increase parallel to grain, increased 

more perpendicular to grain 

Tensile strength Decreased significantly Increased less than proportional to 

specific gravity increase 

Increased about in proportion to specific 

gravity increase 

Flexural strength Increased less than 

proportional to specific gravity 

increase 

Increased less than proportional to specific 

gravity increase parallel to grain, increased 

more perpendicular to grain 

Increased proportional to specific gravity 

increase parallel to grain, increased more 

perpendicular to grain 

Hardness Increased considerably more 

than proportional to specific 

gravity increase 

10 to 20 times that of normal wood 10 to 18 times that of normal wood 

Impact strength 

Toughness About 1/2 of value for normal 

wood, but very susceptible 

to the variables of manufacture 

1/2 to 3/4 of value for normal wood, but 

very susceptible to the variables of 

manufacture 

Same to somewhat greater than normal 

wood 

Izod About 1/5 of value for normal 

wood 

1/3 to 3/4 of value for normal wood Same to somewhat greater than normal 

wood 

Abrasion resistance 

(tangential)

About 1/2 of value for normal 

wood 

Increased about in proportion to specific 

gravity increase 

Increased about in proportion to specific 

gravity increase 

Machinability Cuts cleaner than normal wood, 

but dulls tools more 

Requires metalworking tools and 

metalworking tool speeds 

Requires metalworking tools and 

metalworking tool speeds 

Moldability Cannot be molded but can be 

formed to single curvatures at 

time of assembly 

Can be molded by compression and 

expansion molding methods 

Cannot be molded 

Gluability Same as normal wood Same as normal wood after light sanding or 

in the case of thick stock, machining 

surfaces plane 

Same as normal wood after light sanding, 

or in the case of thick stock, machining 

surfaces plane 
19–7

Chapter 19 Specialty Treatments

Table 19–2. Strength properties of normal and modified laminatesa of yellow birch and a laminated paper plastic

Property 

Normal 

laminated 

woodb

Impreg 

(impregnated, 

uncompressed)c

Compreg 

(impregnated,

highly 

compressed)c

Staypak 

(unimpregnated, 

highly 

compressed)b

Paper laminate 

(impregnated, 

highly 

compressed)d

Thickness of laminate (mm (in.)) 23.9 (0.94) 26.2 (1.03) 16.0 (0.63) 12.2 (0.48) 3.2 (0.126) 

13.0 (0.512) 

Moisture content at time of test (%) 9.2 5.0 5.0 4.0 — 

Specific gravity (based on weight and volume at test) 0.7 0.8 1.3 1.4 1.4 

Parallel laminates

Flexure—grain parallel to span (flatwise)e

Proportional limit stress (MPa (lb in–2)) 79.3 (11,500) 109.6 (15,900) 184.1 (26,700) 138.6 (20,100) 109.6 (15,900) 

Modulus of rupture (MPa (lb in–2)) 140.6 (20,400) 129.6 (18,800) 250.3 (36,300) 271.6 (39,400) 252.3 (36,600) 

Modulus of elasticity (GPa (1,000 lb in–2)) 16.0 (2,320) 16.4 (2,380) 25.4 (3,690) 30.7 (4,450) 20.8 (3,010) 

Flexure—grain perpendicular to span (flatwise)e

Proportional limit stress (MPa (lb in–2)) 6.9 (1,000) 9.0 (1,300) 29.0 (4,200) 22.1 (3,200) 72.4 (10,500) 

Modulus of rupture (MPa (lb in–2)) 13.1 (1,900) 11.7 (1,700) 31.7 (4,600) 34.5 (5,000) 167.5 (24,300) 

Modulus of elasticity (GPa (1,000 lb in–2)) 1.0 (153) 1.5 (220) 4.3 (626) 4.2 (602) 10.2 (1,480) 

Compression parallel to grain (edgewise)e

Proportional limit stress (MPa (lb in–2)) 44.1 (6,400) 70.3 (10,200) 113.1 (16,400) 66.9 (9,700) 49.6 (7,200) 

Ultimate strength (MPa (lb in–2)) 65.5 (9,500) 106.2 (15,400) 180.0 (26,100) 131.7 (19,100) 144.1 (20,900) 

Modulus of elasticity (GPa (1,000 lb in–2)) 15.8 (2,300) 17.0 (2,470) 26.1 (3,790) 32.2 (4,670) 21.5 (3,120) 

Compression perpendicular to grain (edgewise)f

Proportional limit stress (MPa (lb in–2)) 4.6 (670) 6.9 (1,000) 33.1 (4,800) 17.9 (2,600) 29.0 (4,200) 

Ultimate strength (MPa (lb in–2)) 14.5 (2,100) 24.8 (3,600) 96.5 (14,000) 64.8 (9,400) 125.5 (18,200) 

Modulus of elasticity (GPa (1,000 lb in–2)) 1.1 (162) 1.7 (243) 3.9 (571) 4.0 (583) 11.0 (1,600) 

Compression perpendicular to grain (flatwise)e

Maximum crushing strength (MPa (lb in–2)) — 29.5 (4,280) 115.1 (16,700) 91.0 (13,200) 291.0 (42,200) 

Tension parallel to grain (lengthwise) 

Ultimate strength (MPa (lb in–2)) 153.1 (22,200) 108.9 (15,800) 255.1 (37,000) 310.3 (45,000) 245.4 (35,600) 

Modulus of elasticity (GPa (1,000 lb in–2)) 15.8 (2,300) 17.3 (2,510) 27.2 (3,950) 31.8 (4,610) 25.1 (3,640) 

Tension perpendicular to grain (edgewise) 

Ultimate strength (MPa (lb in–2)) 9.6 (1,400) 9.6 (1,400) 22.1 (3,200) 22.8 (3,300) 137.9 (20,000) 

Modulus of elasticity (GPa (1,000 lb in–2)) 1.1 (166) 1.6 (227) 4.3 (622) 4.0 (575) 11.8 (1,710) 

Shear strength parallel to grain (edgewise)f

Johnson double shear across laminations (MPa (lb in–2)) 20.5 (2,980) 23.8 (3,460) 50.8 (7,370) 43.9 (6,370) 122.7 (17,800) 

Cylindrical double shear parallel to laminations 

(MPa (lb in–2)) 

20.8 (3,020) 24.5 (3,560) 39.2 (5,690) 21.2 (3,080) 20.7 (3,000) 

Shear modulus 

Tension method (GPa (1,000 lb in–2)) 1.2 (182) 1.8 (255) 3.1 (454) — — 

Plate shear method (FPL test) (GPa (1,000 lb in–2)) — — — 2.6 (385) 6.3 (909) 

Toughness (FPL test edgewise)f

 (J (in-lb)) 26.6 (235) 14.1 (125) 16.4 (145) 28.2 (250) — 

Toughness (FPL test edgewise)f

 (J mm–1 of width 

(in-lb in–1 of width)) 

1.1 (250) 0.53 (120) 1.0 (230) 2.3 (515) — 

Impact strength (Izod)—grain lengthwise 

Flatwise (notch in face) (J mm–1 of notch (ft-lb in–1 of 

notch)) 

0.75 (14.0) 0.12 (2.3) 0.23 (4.3) 0.68 (12.7) 0.25 (4.7) 

Edgewise (notch in face) (J mm–1 of notch (ft-lb in–1 of 

notch)) 

0.60 (11.3) 0.10 (1.9) 0.17 (3.2)g — 0.036 (0.67) 

Hardness 

Rockwell flatwisee

 (M–numbers) — 22 84 — 110 

Load to embed 11.3-mm (0.444-in.) steel ball to 1/2 

its diameter (kN (lb)) 

7.1 (1,600) 10.7 (2,400) — — — 

Hardness modulus (HM)

h

 (MPa (lb in–2)) 37.2 (5,400) 63.4 (9,200) 284.8 (41,300) 302.0 (43,800) 245.4 (35,600) 

Abrasion—Navy wear-test machine (flatwise)e

 wear per 

1,000 revolutions (mm (in.)) 

0.76 (0.030) 1.45 (0.057) 0.46 (0.018) 0.38 (0.015) 0.46 (0.018) 
19–8

General Technical Report FPL–GTR–190

 Table 19–2. Strength properties of normal and modified laminatesa of yellow birch and a laminated paper plastic—con.

 Property 

Normal 

laminated 

woodb

Impreg 

(impregnated, 

uncompressed)c

Compreg 

(impregnated,

highly 

compressed)c

Staypak 

(unimpregnated, 

highly 

compressed)b

Paper laminate 

(impregnated, 

highly 

compressed)d

Water absorption (24-h immersion) increase in weight (%) 43.6 13.7 2.7 4.3 2.2 

Dimensional stability in thickness direction 

Equilibrium swelling (%) 9.9 2.8 8.0 29 — 

Recovery from compression (%) — 0 0 4 — 

Crossband laminates

Flexure—face grain parallel to span (flatwise)e

Proportional limit stress (MPa (lb in–2)) 47.6 (6,900) 55.8 (8,100) 99.3 (14,400) 78.6 (11,400) 86.9 (12,600) 

Modulus of rupture (MPa (lb in–2)) 90.3 (13,100) 78.6 (11,400) 157.2 (22,800) 173.0 (25,100) 215.8 (31,300) 

Modulus of elasticity (GPa (1,000 lb in–2)) 9.0 (1,310) 11.5 (1,670) 17.1 (2,480) 20.0 (2,900) 15.4 (2,240) 

Compression parallel to face grain (edgewise)f

Proportional limit stress (MPa (lb in–2)) 22.8 (3,300) 35.8 (5,200) 60.0 (8,700) 35.8 (5,200) 34.5 (5,000) 

Ultimate strength (MPa (lb in–2)) 40.0 (5,800) 78.6 (11,400) 164.8 (23,900) 96.5 (14,000) 130.3 (18,900) 

Modulus of elasticity (GPa (1,000 lb in–2)) 9.4 (1,360) 10.3 (1,500) 15.8 (2,300) 18.6 (2,700) 16.3 (2,370) 

Tension parallel to face grain (lengthwise) 

Ultimate strength (MPa (lb in–2)) 84.8 (12,300) 54.5 (7,900) 113.8 (16,500) 168.9 (24,500) 187.5 (27,200) 

Modulus of elasticity (GPa (1,000 lb in–2)) 8.9 (1,290) 10.1 (1,460) 15.1 (2,190) 17.7 (2,570) 18.6 (2,700) 

Toughness (FPL test edgewise)f

 (J mm–1 of width 

(in-lb in–1 of width)) 

0.47 (105) 0.18 (40) 0.51 (115) 1.4 (320) — 

a

Laminates made from 17 plies of 1.6-mm (1/16-in.) rotary-cut yellow birch veneer. b

Veneer conditioned at 27 °C (80 °F) and 65% relative humidity before assembly with phenol resin film adhesive. c

Impregnation, 25% to 30% of water-soluble phenol-formaldehyde resin based on the dry weight of untreated veneer. d

High-strength paper (0.076-mm (0.003-in.) thickness) made from commercial unbleached black spruce pulp (Mitscherlich subtile), phenol resin content 

36.3% based on weight of treated paper, Izod impact abrasion, flatwise compression, and shear specimens, all on 12.7-mm- (1/2-in.-) thick laminate. e

Load applied to the surface of the original material (parallel to laminating pressure direction). f

Forest Products Laboratory (FPL) test procedure: load applied to edge of laminations (perpendicular to laminating pressure direction). g

Values as high as 0.53 J mm–1 (10.0 ft-lb in–1) of notch have been reported for compreg made with alcohol-soluble resins and 0.37 J mm–1 (7.0 ft-lb in–1)

with water-soluble resins. 

h

Values based on the average slope of load–penetration plots where HM is an expression for load per unit of spherical area of penetration of the 11.3-mm 

(0.444-in.) steel ball expressed in MPa (lb in–2). 

Table 19–3. Coefficients of linear thermal expansion per degree Celsius of wood, hydrolyzed wood, 

and paper productsa

 

Linear expansion per °C 

(values multiplied by 106

)

Materialb

Specific

gravity of

product

Resin

contentc

(%)

Fiber or

machine

direction 

Perpendicular 

to fiber 

or machine 

direction 

in plane of 

laminations 

Pressing 

direction 

Cubical

expansion

per °C 

(values 

multiplied

by 106)

Yellow birch laminate 0.72 3.1 3.254 40.29 36.64 80.18 

Yellow birch staypak laminate 1.30 4.7 3.406 37.88 65.34 106.63 

Yellow birch impreg laminate 0.86 33.2 4.648 35.11 37.05 76.81 

Yellow birch compreg laminate 1.30 24.8 4.251 39.47 59.14 102.86 

1.31 34.3 4.931 39.32 54.83 99.08 

Sitka spruce laminate 0.53 6.0d

 3.887 37.14 27.67 68.65 

Parallel-laminated paper laminate 1.40 36.5 5.73 15.14 65.10 85.97 

Crossbanded paper laminate 1.40 36.5 10.89 11.0e

 62.2 84.09 

Molded hydrolyzed-wood plastic 1.33 25 42.69 42.69 42.69 128.07 

Hydrolyzed-wood sheet laminate 1.39 18 13.49 224.68 77.41 115.58 

a

These coefficients refer to bone-dry material. Generally, air-dry material has a negative thermal coefficient, because the shrinkage

resulting from the loss in moisture is greater than the normal thermal expansion. b

All wood laminates made from rotary-cut veneer, annual rings in plane of sheet. c

On basis of dry weight of product. 

d

Approximate. 

e

Calculated value. 
use in handles for knives and other cutlery. The expansionmolding techniques of forming and curing of the compreg 

around the metal parts of the handle as well as attaching 

previously made compreg with rivets are two methods used. 

Compreg is currently manufactured worldwide, including 

the United States, United Kingdom, Pakistan, and India.

Veneer of any nonresinous species can be used for making 

compreg. Most properties depend upon the specific gravity to which the wood is compressed rather than the species 

used.

Heat Treatments

Heating wood changes the properties of wood. It can decrease the hygroscopicity and improve the dimensional 

stability and decay resistance. Yet, at the same time, the 

increase in stability and durability also increases the brittleness and loss in some strength properties, including impact 

toughness, modulus of rupture, and work to failure. The 

treatments usually cause a darkening of the wood and the 

wood has a tendency to crack and split.

Wood can be heated various ways: heating in the presence 

of moisture, heating in the presence of moisture followed by 

compression, heating dry wood, and heating dry wood followed by compression. The effect of the heating process on 

wood properties depends on the process itself. As the wood 

is heated, the first weight loss is due to the loss of water, followed by a variety of chemistries that produce degradation 

products and volatile gasses. As the temperature increases, 

wood cell wall polymers start to degrade. Pyrolysis of the 

hemicelluloses takes place about 270 °C followed closely by 

cellulose. Lignin is much more stable to high temperature.

Many of the commercial heat treating processes take place 

in the absence of air at temperatures ranging from 180 to 

260 °C for times ranging from a few minutes to several 

hours. Temperatures lower than 140 °C result in less change 

in physical properties, and heating above 300 °C results in 

severe wood degradation. Wood has been heated in steam, 

in an inert gas, below molten metal, and in hot oil baths. 

Improved dimensional stability and durability are thought 

to be due to a loss of hydroscopic hemicellulose sugars and 

their conversion to furan-based polymers that are much less 

hydroscopic, and the lost sugars decrease the ability of fungi 

to attack the heated wood. The weight loss is proportional to 

the square of the reduction in swelling.

A variety of thermal modification processes have been 

developed. The results of the process depend on several 

variables, including time and temperature, treatment atmosphere, wood species, moisture content, wood dimensions, 

and the use of a catalyst. Temperature and time of treatment 

are the most critical elements. Treatments done in air result 

in oxidation reactions not leading to the desired properties 

of the treated wood. Generally, weight loss occurs to 

a greater extent in hardwoods than in softwoods.

Several names have been given to the various heat-treated 

products and treatments for wood, including Staypak and 

Staybwood in the United States, Lignostone and Lignofol in 

Germany, Jicwood and Jablo in the United Kingdom, ThermoWood in Finland, Plato in the Netherlands, and Perdure 

and Retification in France.

Heating wood under a variety of conditions is an environmentally benign process requiring no added chemicals and 

gives rise to a variety of products with decreased moisture 

contents and some durability against biological degradation. 

However, it is not recommended to be used in ground contact. Most physical properties are decreased, especially abrasion resistance and toughness, and it is therefore not suitable 

for load-bearing applications.

Heating Wet Wood

Wood with moisture content close to its equilibrium moisture content (EMC) that is heated to 180 to 200 °C results in 

a wood with greatly decreased moisture content. The high 

temperature degrades the hemicellulose sugars to furanbased intermediates and volatile gasses. The furan intermediates have a lower EMC than the sugars and increase bonding of the wood structure. At a weight loss of approximately 

25%, the EMC is lowered by almost the same percentage. 

Dimensional stability is also increased but not as much as 

heating followed by compression (discussed in the following section).

Two current processes are based on heating wet wood for 

stability and increased biological resistance. ThermoWood 

was developed by VTT in Finland and is a three-stage process done in the presence of steam, which helps protect the 

wood from oxidative reactions. In the first stage, the wood 

is heated to 100 °C for almost 20 h. In the second stage, the 

wood is heated to 185 to 230 °C for 10 h, followed by the 

lowering of the temperature in the presence of a water spray. 

Plato (Proving Lasting Advanced Timber Option) wood was 

developed by Royal Dutch Shell in The Netherlands and 

involves a four-stage process. The first stage involves heating the wood to 150 to 180 °C under high-pressure steam 

for 4 to 5 h. The wood is then dried to a moisture content of 

8% to 10% and then heated again at 150 to 190 °C for 12 to 

19–9

Chapter 19 Specialty Treatments

Table 19–4. Comparison of wood treatments 

and the degree of dimensional stability achieved

Treatment 

Antishrink efficiency 

(%)

Simple wax dip 2 to 5 

Wood–plastic combination 10 to 15 

Staypak/Staybwood 30 to 40 

Impreg 65 to 70 

Chemical modification 65 to 75 

Polyethylene glycol 80 to 85 

Formaldehyde 82 to 87 

Compreg 90 to 95 
16 h, resulting in a drop in moisture content to less than 1%. 

The wood is then conditioned to 4% to 6% moisture over 

a 3-day period. The wood is dark brown in color but will 

weather to the normal gray color in time. It has a 5% to 20% 

decrease in modulus of rupture but a slightly higher modulus of elasticity.

The Le Bois Perdure process was developed by the French 

company BCI in the mid-1990s and has been commercialized by PCI Industries, Inc., based in Quebec. The process 

involves drying and heating the wood at 200 to 230 °C in 

steam.

All heat-treated wood is gluable and paintable and can be 

used for furniture, flooring, decking, door and window components, and exterior joinery.

Heating Wet Wood Followed by Compression

When wet wood is heated to 180 to 220 °C and compressed, 

the wood structure is compressed and remains in this compressed state when dried. The compressed wood is much 

harder and has a much higher modulus of rupture and elongation. Re-wetting the compressed wood reverses the process and it swells back to its original thickness.

Heating Dry Wood

Heating wood under drying conditions at higher temperatures (95 to 320 °C (200 to 600 °F)) than those normally 

used in kiln drying produces a product known as Staybwood 

that decreases the hygroscopicity and subsequent swelling 

and shrinking of the wood appreciably. However, the stabilization is always accompanied by loss of mechanical properties. Toughness and resistance to abrasion are most seriously 

affected.

Under conditions that cause a reduction of 40% in shrinking 

and swelling, the toughness is decreased to less than half 

that of the original wood. Extensive research to minimize 

this loss was not successful. Because of the reduction in 

strength properties from heating at such high temperatures, 

wood that is dimensionally stabilized in this manner was 

never commercialized.

One commercial process produces dry-heated wood products. Retification is a process developed in France by École 

des Mines de St. Etienne and involves heating wood in a 

nitrogen atmosphere to 180 to 250 °C for several hours.

Heating Dry Wood Followed by Compression

To meet the demand for a tougher compressed product than 

compreg, a compressed wood containing no resin (staypak) 

was developed. A temperature range of 150 to 170 °C is 

used, and the wood is compressed while heated. It does not 

lose its compression under swelling conditions as does untreated compressed wood. In making staypak, the compressing conditions are modified so that the lignin-cementing 

material between the cellulose fibers flows sufficiently to 

eliminate internal stresses.

Staypak is not as water resistant as compreg, but it is about 

twice as tough and has higher tensile and flexural strength 

properties (Tables 19–1 and 19–2). The natural finish of 

staypak is almost equal to that of compreg. Under weathering conditions, however, it is definitely inferior to compreg. 

For outdoor use, a good synthetic resin varnish or paint finish should be applied to staypak.

Staypak can be used in the same way as compreg where 

extremely high water resistance is not needed. It shows 

promise in tool handles, forming dies, connector plates, 

propellers, and picker sticks and shuttles for weaving, where 

high impact strength is needed. Staypak is not impregnated; 

therefore, it can be made from solid wood as well as from 

veneer. The cost of staypak is less than that of compreg.

A material similar to staypak was produced in Germany 

prior to World War II. It was a compressed solid wood 

with much less dimensional stability than staypak and was 

known as lignostone. Another similar German product was a 

laminated compressed wood known as lignofol.

Wood Treated with Polyethylene Glycol (PEG)

The dimensional stabilization of wood with polyethylene 

glycol-1000 (PEG), also known as Carbowax, is accomplished by bulking the fiber to keep the wood in a partially 

swollen condition. PEG acts in the same manner as does 

the previously described phenolic resin. It cannot be further 

cured. The only reason for heating the wood after treatment is to drive off water. PEG remains water soluble in the 

wood. Above 60% relative humidity, it is a strong humectant 

and, unless used with care and properly protected, PEGtreated wood can become sticky at high levels of relative 

humidity. Because of this, PEG-treated wood is usually finished with a polyurethane varnish.

Treatment with PEG is facilitated by using green wood. 

Here, pressure is not applied because the treatment is based 

on diffusion. Treating times are such that uniform uptakes of 

25% to 30% of chemical are achieved (based on dry weight 

of wood). The time necessary for this uptake depends on 

the thickness of the wood and may require weeks. The PEG 

treatment is being effectively used for cross-sectional wood 

plaques and other decorative items. Table tops of high quality furniture stay remarkably flat and dimensionally stable 

when made from PEG-treated wood.

Another application of this chemical is to decrease the 

checking of green wood during drying. For this application, 

a high degree of PEG penetration is not required. This method of treatment has been used to decrease checking during 

drying of small wood blanks or turnings.

Cracking and distortion that old, waterlogged wood undergoes when it is dried can be substantially decreased by 

treating the wood with PEG. The process was used to dry 

200-year-old waterlogged wooden boats raised from Lake 

George, New York. The “Vasa,” a Swedish ship that sank 

19–10

General Technical Report FPL–GTR–190
on its initial trial voyage in 1628, was also treated after it 

was raised. There have been many applications of PEG 

treatment for the restoration of waterlogged wood from 

archeological sites.

Wood–Polymer Composites

In the modified wood products previously discussed, most 

of the chemical resides in cell walls; the lumens are essentially empty. If wood is vacuum impregnated with certain 

liquid vinyl monomers that do not swell wood and are 

later polymerized in situ by gamma radiation or chemical 

catalyst-heat systems, the resulting polymer resides almost 

exclusively in the lumens. Methyl methacrylate is a common monomer used for wood–polymer composites. It is 

converted to polymethyl methacrylate. The hygroscopic 

characteristics of the wood substance are not altered because 

little, if any, polymer penetrates the cell walls. However, 

because of the high polymer content (70% to 100% based 

on the dry weight of wood), the normally high void volume 

of wood is greatly decreased. With the elimination of this 

very important pathway for vapor or liquid water diffusion, 

the response of the wood substance to changes in relative 

humidity or water is very slow, and moisture resistance or 

water-repellent effectiveness (WRE) is greatly improved. 

Water-repellent effectiveness is measured as follows:

WRE 100

2

1 = × S

S (19–1)

where S1 is the swelling or moisture uptake of the control 

specimen during exposure to water for t minutes, and S2 is 

the swelling or moisture uptake of the treated specimen 

during exposure to water also for t minutes.

Wood–polymer composite materials offer desirable aesthetic appearance, high compression strength and abrasion 

resistance, and increase in hardness and are much stronger 

than untreated wood (Table 19–5). Commercial application 

of these products is largely based on increased strength and 

hardness properties. Improvements in physical properties 

of wood–polymer composites are related to polymer loading. This, in turn, depends not only on the permeability of 

the wood species but also on the particular piece of wood 

being treated. Sapwood is filled to a much greater extent 

than heartwood for most species. The most commonly used 

monomers include styrene, methyl methacrylate, vinyl acetate, and acrylonitrile. Industrial applications include certain 

sporting equipment, musical instruments, decorative objects, 

and high-performance flooring.

At present, the main commercial use of wood–polymer 

composites is hardwood flooring. Comparative tests with 

conventional wood flooring indicate that wood–polymer 

materials resisted indentation from rolling, concentrated, 

and impact loads better than did white oak. This is largely 

attributed to improved hardness. Abrasion resistance is 

also increased. A finish is usually used on these products to 

increase hardness and wear resistance even more. Wood–

polymer composites are also being used for sporting goods, 

musical instruments, and novelty items.

In addition to the use of vinyl monomers for wood–polymer 

composites, polysaccharides from renewable resources are 

also used. Examples include the use of furfuryl alcohol from 

primarily corn cobs and the use of modified polysaccharides 

primarily from soy and corn starch. The process (Indurite) 

involves the impregnation of wood with a water-soluble 

polysaccharide solution made from soy and corn starch, followed by a curing step at 70 °C. The treatment improves the 

dimensional stability and hardness of wood and is used in 

production of flooring materials.

Modification of wood with furfuryl alcohol is called furfurylation. Stamm started research on furfurylation at the 

Forest Products Laboratory in the 1950s. The process was 

industrialized in the mid-1960s in the United States, and 

furfurylated wood products included knife handles, bench 

tops, and rotor blades, but production ceased by the 1970s. 

Interest renewed in the late 1980s, and now products are 

marketed in the United States and Europe. Furfurylation 

19–11

Chapter 19 Specialty Treatments

Table 19–5. Strength properties of wood–polymer compositesa

Strength property Unit Untreatedb

 Treatedb

Static bending 

Modulus of elasticity MPa (×103 lb in–2) 9.3 (1.356) 11.6 (1.691) 

Fiber stress at proportional limit MPa (lb in–2) 44.0 (6,387) 79.8 (11,582) 

Modulus of rupture MPa (lb in–2) 73.4 (10,649) 130.6 (18,944) 

Work to proportional limit J mm–3 (in-lb in–3) 11.4 (1.66) 29.1 (4.22) 

Work to maximum load J mm–3 (in-lb in–3) 69.4 (10.06) 122.8 (17.81) 

Compression parallel to grain 

Modulus of elasticity GPa (×106

 lb in–2) 7.7 (1.113) 11.4 (1.650) 

Fiber stress at proportional limit MPa (lb in–2) 29.6 (4,295) 52.0 (7,543) 

Maximum crushing strength MPa (lb in–2) 44.8 (6,505) 68.0 (9,864) 

Work to proportional limit J mm–3 (in-lb in–3) 77.8 (11.28) 147.6 (21.41) 

Toughness J mm–3 (in-lb in–3) 288.2 (41.8) 431.6 (62.6) 

a

Methyl methacrylate impregnated basswood. 

b

Moisture content 7.2%. 

\
involves a full cell impregnation step of the treatment solution, an intermediate drying step, a reaction curing step, and 

a final kiln-drying step. Products are available for decking, 

marine application, cladding, window joinery, poles, roofs, 

garden furniture, building materials, and flooring. Impact 

strength is strongly decreased (from –25% at 15% WPG to 

–65% at 125% WPG). Stiffness increases from 30% to 80%. 

The ASE ranges from 30% to 80%. Fungal durability and 

insect resistance are high at high weight gains.

Chemical Modification

Through chemical reactions, it is possible to add an organic 

chemical to the hydroxyl groups on wood cell wall components. This type of treatment bulks the cell wall with a permanently bonded chemical. Many reactive chemicals have 

been used experimentally to chemically modify wood. For 

best results, chemicals used should be capable of reacting 

with wood hydroxyls under neutral or mildly alkaline conditions at temperatures less than 120 °C. The chemical system 

should be simple and must be capable of swelling the wood 

structure to facilitate penetration. The complete molecule 

should react quickly with wood components to yield stable 

chemical bonds while the treated wood retains the desirable 

properties of untreated wood. Reaction of wood with chemicals such as anhydrides, epoxides, isocyanates, acid chlorides, carboxylic acids, lactones, alkyl chlorides, and nitriles 

result in antishrink efficiency (ASE) values (Table 19–4) of 

65% to 75% at chemical weight gains of 20% to 30%. Antishrink efficiency is determined as follows:

100

1

2 1 × − = V

V V S (19–2)

where S is volumetric swelling coefficient, V2 is wood volume after humidity conditioning or wetting with water, and 

V1 is wood volume of ovendried sample before conditioning 

or wetting.Then,

ASE 100

1

2 1 × − = S

S S (19–3)

where ASE is reduction in swelling or antishrink efficiency 

resulting from a treatment, S2 is treated volumetric swelling 

coefficient, and S1 is untreated volumetric swelling coefficient.

Reaction of these chemicals with wood yields a modified 

wood with increased dimensional stability and improved 

resistance to termites, decay, and marine organisms.

Mechanical properties of chemically modified wood are essentially unchanged compared with untreated wood.

Modification of wood with acetic anhydride has been researched extensively. The acetylation process involves 

impregnation of acetic anhydride followed by heat to start 

the reaction. The last step is to remove the acetic acid byproduct and any remaining acetic anhydride. The hydroxyl 

groups of the cell wall polymers are converted to acetyl 

groups, making the wood hydrophobic. As a result, biological durability and dimensional stability increase significantly compared with unmodified wood. Acetylated wood is 

now commercially available.

The reaction of formaldehyde with wood hydroxyl groups is 

an interesting variation of chemical modification. At weight 

gains as low as 2%, formaldehyde-treated wood is not attacked by wood-destroying fungi. An antishrink efficiency 

(Table 19–4) of 47% is achieved at a weight gain of 3.1%, 

55% at 4.1%, 60% at 5.5%, and 90% at 7%. The mechanical 

properties of formaldehyde-treated wood are all decreased 

from those of untreated wood. A definite embrittlement is 

observed, toughness and abrasion resistance are greatly decreased, crushing strength and bending strength are 

decreased about 20%, and impact bending strength is 

decreased up to 50%.

Paper-Based Plastic Laminates

Commercially, paper-based plastic laminates are of two 

types: industrial and decorative. Total annual production is 

equally divided between the two types. They are made by 

superimposing layers of paper that have been impregnated 

with a resinous binder and curing the assembly under heat 

and pressure.

Industrial Laminates

Industrial laminates are produced to perform specific functions requiring materials with predetermined balances of 

mechanical, electrical, and chemical properties. The most 

common use of such laminates is electrical insulation. The 

paper reinforcements used in the laminates are kraft pulp, 

alpha pulp, cotton linters, or blends of these. Kraft paper 

emphasizes mechanical strength and dielectric strength perpendicular to laminations. Alpha paper is used for its electric 

and electronic properties, machineability, and dimensional 

stability. Cotton linter paper combines greater strength than 

alpha paper with excellent moisture resistance.

Phenolic resins are the most suitable resins for impregnating the paper from the standpoint of high water resistance, 

low swelling and shrinking, and high strength properties 

(except for impact). Phenolics also cost less than do other 

resins that give comparable properties. Water-soluble resins 

of the type used for impreg impart the highest water resistance and compressive strength properties to the product, 

but they make the product brittle (low impact strength). 

Alcohol-soluble phenolic resins produce a considerably 

tougher product, but the resins fail to penetrate the fibers as 

well as water-soluble resins, thus imparting less water resistance and dimensional stability to the product. In practice, 

alcohol-soluble phenolic resins are generally used.

Paper-based plastic laminates inherit their final properties 

from the paper from which they are made. High-strength 

papers yield higher strength plastic laminates than do lowstrength papers. Papers with definite directional properties 

19–12

General Technical Report FPL–GTR–190
result in plastic laminates with definite directional properties 

unless they are cross laminated (alternate sheets oriented 

with the machine direction at 90° to each other).

The use of higher strength paper has helped in the development of paper-based laminates suitable for structural use. 

Pulping under milder conditions and operating the paper 

machines to give optimum orientation of the fibers in one 

direction, together with the desired absorbency, contribute 

markedly to improvements in strength.

Strength and other properties of a paper–plastic laminate are 

shown in Table 19–2. The National Electrical Manufacturers 

Association L1–1 specification has additional information 

on industrial laminates. Paper is considerably less expensive than glass fabric or other woven fabric mats and can 

be molded at considerably lower pressures; therefore, the 

paper-based laminates generally have an appreciable price 

advantage over fabric laminates. However, some fabric laminates give superior electrical properties and higher impact 

properties. Glass fabric laminates can be molded to greater 

double curvatures than can paper laminates.

During World War II, a high-strength paper plastic known as 

papreg was used for molding nonstructural and semistructural airplane parts such as gunner’s seats and turrets, ammunition boxes, wing tabs, and the surfaces of cargo aircraft 

flooring and catwalks. Papreg was used to a limited extent 

for the skin surface of airplane structural parts, such as wing 

tips. One major objection to its use for such parts is that it 

is more brittle than aluminum and requires special fittings. 

Papreg has been used to some extent for heavy-duty truck 

floors and industrial processing trays for nonedible materials. Because it can be molded at low pressures and is made 

from thin paper, papreg is advantageous for use where accurate control of panel thickness is required.

Decorative Laminates

Although made by the same process as industrial laminates, 

decorative laminates are used for different purposes and 

bear little outward resemblance to industrial laminate. They 

are used as facings for doors and walls and tops of counters, 

flooring, tables, desks, and other furniture.

These decorative laminates are usually composed of a combination of phenolic- and melamine-impregnated sheets of 

paper. Phenolic-impregnated sheets are brown because of 

the impregnating resins and make up most of the built-up 

thickness of the laminate. Phenolic sheets are overlaid with 

paper impregnated with melamine resin. One sheet of the 

overlay is usually a relatively thick one of high opacity and 

has the color or design printed on it. Then, one or more 

tissue-thin sheets, which become transparent after the resin 

is cured, are overlaid on the printed sheet to protect it in 

service. The thin sheets generally contain more melamine 

resin than do the printed sheets, providing stain and abrasion 

resistance as well as resistance to cigarette burns, boiling 

water, and common household solvents.

The resin-impregnated sheets of paper are hot pressed, 

cured, and then bonded to a wood-based core, usually 

plywood, hardboard, or particleboard. The thin transparent (when cured) papers impregnated with melamine resin 

can be used alone as a covering for decorative veneers in 

furniture to provide a permanent finish. In this use, the impregnated sheet is bonded to the wood surface in hot presses 

at the same time the resin is cured. The heat and stain resistance and the strength of this kind of film make it a superior 

finish.

The overall thickness of a laminate may obviously be varied 

by the number of sheets of kraft-phenolic used in the core 

assembly. Some years ago, a 2-mm (0.08-in.) thickness was 

used with little exception because of its high impact strength 

and resistance to substrate show through. Recently, a 1-mm 

(0.04-in.) thickness has become popular on vertical surfaces 

such as walls, cabinet doors, and vertical furniture faces. 

This results in better economy, and the greater strength of 

the heavier laminate is not necessary. As applications have 

proliferated, a series of thicknesses have been offered, from 

20 to 60 mm (0.8 to 2.4 in.), even up to 150 mm (6 in.) 

when self-supportive types are needed. These laminates may 

have decorative faces on both sides if desired, especially in 

the heavier thicknesses. Replacement bowling lanes made 

from high-density fiberboard core and phenolic-melamine, 

high-pressure laminated paper on the face and back are 

commercially used.

The phenolic sheets may also contain special postformingtype phenolic resins or extensible papers that make it 

possible to postform the laminate. By heating to 160 °C 

(320 °F) for a short time, the structure can readily undergo 

simple bending to a radius of 10 mm (0.4 in.), and 5 to 

6 mm (0.20 to 0.24 in.) with careful control. Rolled furniture 

edges, decorative moldings, curved counter tops, shower 

enclosures, and many other applications are served by this 

technique. Finally, the core composition may be modified to 

yield a fire-retardant, low-smoking laminate to comply with 

fire codes. These high-pressure decorative laminates are 

covered by the National Electrical Manufacturers Association Specification LD–3.

Paper will absorb or give off moisture, depending upon 

conditions of exposure. This moisture change causes paper 

to shrink and swell, usually more across the machine direction than along it. In the same manner, the laminated paper 

plastics shrink and swell, although at a much slower rate. 

Cross laminating minimizes the amount of this shrinking 

and swelling. In many furniture uses where laminates are 

bonded to cores, the changes in dimension as a result of 

moisture fluctuating with the seasons are different than those 

of the core material. To balance the construction, a paper 

plastic with similar properties may be glued to the opposite 

face of the core to prevent bowing or cupping caused by 

moisture variation.

19–13

Chapter 19 Specialty Treatments
Lignin-Filled Laminates

The cost of phenolic resins at one time resulted in considerable effort to find impregnating and bonding agents that 

were less expensive and yet readily available. Lignin-filled 

laminates made with lignin recovered from the spent liquor 

of the soda pulping process were developed as a result of 

this search. Lignin is precipitated from solution within the 

pulp or added in a pre-precipitated form before the paper 

is made. The lignin-filled sheets of paper can be laminated 

without the addition of other resins, but their water resistance is considerably enhanced when some phenolic resin 

is applied to the paper in a second operation. The water 

resistance can also be improved by impregnating only the 

surface sheet with phenolic resin. It is also possible to introduce lignin, together with phenolic resin, into untreated 

paper sheets. The lignin-filled laminates are always dark 

brown or black. They have better toughness than phenolic 

laminates; in most other strength properties, they are 

comparable or lower.

Reduction in cost of phenolic resins has virtually eliminated 

the lignin-filled laminates from U.S. commerce. These laminates have several potential applications, however, where a 

cheaper laminate with less critical properties than phenolic 

laminates can be used.

Paper-Face Overlays

Paper has found considerable use as an overlay material for 

veneer or plywood. Overlays can be classified into three 

different types according to their use—masking, structural, 

and decorative. Masking overlays are used to cover minor 

defects in plywood, such as face checks and patches, minimize grain raising, and provide a more uniform paintable 

surface, thus making possible the use of lower grade veneer. 

Paper for this purpose need not be of high strength, because 

the overlays do not need to add strength to the product. For 

adequate masking, a single surface sheet with a thickness of 

0.5 to 1 mm (0.02 to 0.04 in.) is desirable. Paper impregnated with phenolic resins at 17% to 25% of the weight of the 

paper gives the best all-around product. Higher resin content 

makes the product too costly and tends to make the overlay 

more transparent. Appreciably lower resin content gives a 

product with low scratch and abrasion resistance, especially 

when the panels are wet or exposed to high relative 

humidities.

The paper faces can be applied at the same time that the 

veneer is assembled into plywood in a hot press. Thermal 

stresses that might result in checking are not set up if the 

machine direction of the paper overlays is at right angles to 

the grain direction of the face plies of the plywood.

The masking-paper-based overlays or vulcanized fiber 

sheets have been used for such applications as wood house 

siding that is to be painted. These overlays mask defects in 

the wood, prevent bleed-through of resins and extractives 

in the wood, and provide a better substrate for paint. The 

19–14

General Technical Report FPL–GTR–190

paper-based overlays improve the across-the-board stability 

from changes in dimension as a result of changes in moisture content.

The structural overlay, also known as the high-density overlay, contains no less than 45% thermosetting resins, generally phenolic. It consists of one or more plies of paper similar 

to that used in the industrial laminates described previously. 

The resin-impregnated papers can be bonded directly to the 

surface of a wood substrate during cure of the sheet, thus 

requiring only a single pressing operation.

The decorative-type overlay is described in the Decorative 

Laminates section.

References

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289–306.

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General Technical Report FPL–GTR–190
CHAPTER 20

Heat Sterilization of Wood

Xiping Wang, Research Forest Products Technologist

20–1

Contents 

Heat Treatment Standards 20–1

Factors Affecting Heating Times 20–2

 Energy Source 20–2

 Heating Medium 20–2

 Air Circulation 20–2

 Size and Configuration of Wood 20–2

 Species 20–2

 Stacking Methods 20–2

 Heating Times for Wood in Various Forms 20–4

Methods for Estimating Heating Times 20–5

 MacLean Equations 20–5

 Multiple Regression Models 20–7

American Lumber Standard Committee (ALSC) 

Enforcement Regulations 20–9

Quality Mark 20–11

Other Considerations 20–11

Literature Cited 20–13

Insects and other pests can travel between countries in pallets and other wood packaging materials through international trade. Because these pests can cause significant ecological damage, their invasion into non-native countries is 

undesirable. Heat sterilization is currently the most practical 

and environmentally friendly treatment to kill pests in solid 

wood materials and prevent their transfer between continents and regions. Consequently, regulations requiring heat 

sterilization are becoming more and more common.

Two important questions should be considered in heat sterilizing solid wood materials: First, what temperature–time 

regime is required to kill a particular pest? Second, how 

much time is required to heat the center of any wood configuration to the kill temperature? The entomology research 

on the first question has facilitated the development of international standards for heat sterilization of various solid 

wood materials. This chapter primarily addresses the second 

question. It focuses on various factors that should be considered when planning and implementing a heat treatment 

process, discusses experimentally derived heating times for 

commonly used wood products, and presents analytical and 

empirical methods for estimating heating times that can be 

used as starting points in the development of heat treatment 

schedules. Current wood packaging material enforcement 

regulations and several additional practical considerations 

for heat treatment operations are also presented.

The preferred units of measure for this chapter are in the 

in–lb system because of the current high demand for this 

information in the United States. Metric units or conversion 

factors are also provided.

Heat Treatment Standards

The current international standard for heat sterilization of 

solid wood packaging materials is the International Standard 

for Phytosanitary Measures (ISPM) Pub. No.15, “Guidelines 

for Regulating Wood Packaging Material in International 

Trade,” which requires heating wood to a minimum core 

temperature of 133 °F (56 °C) for a minimum of 30 min 

(IPPC 2002, APHIS 2004). These guidelines are for all 

forms of wood packaging material that may serve as a pathway for plant pests posing a threat mainly to living trees. 

This temperature–time regime is chosen in consideration of 

the wide range of pests for which this combination is documented to be lethal and a commercially feasible treatment. 

Table 20–1 lists the pest groups associated with wood packaging material that can be practically eliminated by heat 

treatment under ISPM 15 standard. Although some pests 

are known to have a higher thermal tolerance, quarantine 
20–2

General Technical Report FPL–GTR–190

pests in this category are managed by the National Plant 

Protection Organizations (NPPOs) on a case-by-case basis 

(IPPC 2002). Future development may identify other temperature–time regimes required to kill specific insects or 

fungi.

Factors Affecting Heating Times

From a practical standpoint, the time required for the center 

of solid wood material to reach the kill temperature depends 

on many factors, including the type of energy source used 

to generate the heat, the medium used to transfer the heat 

(for example, wet or dry heat), the effectiveness of the air 

circulation in the heating facility, the species and physical 

properties (configurations, specific gravity, moisture content, 

initial wood temperature) of the wood and wood products 

being sterilized, and the stacking methods used in the heat 

treatment process.

Energy Source

Energy is the amount of heat supplied during the heat treatment process. Heat-treating chambers typically employ 

systems that utilize steam, hot air (direct fire), electricity, 

and hot water or hot oil as mechanisms to generate the heat 

necessary to sterilize the wood. The choice of heat energy 

primarily depends on the heat treatment method, energy 

resources available, and the cost of the energy.

Heating Medium

The temperature and humidity of the heating medium significantly affect heating times. Higher heating temperatures 

obviously yield shorter heating times, and heating wood 

in saturated steam (wet heat) results in the shortest heating 

times. When the heating medium is air that is not saturated with steam, the relative humidity is less than 100% 

(wet-bulb depression > 0 °F), and drying occurs as water 

evaporates from the wood surface. As the heating medium 

changes from wet to dry heat, the time needed to reach 

the required temperature increases. This is illustrated in 

Figure 20–1, which shows experimentally derived heating 

times as a function of wet-bulb depression for a series of 

lumber and timber products.

When the wet-bulb temperature in the heating medium approaches or falls below the target center temperature, heating time becomes much longer than with wet heat (Simpson 

2002, Simpson and others 2003) because evaporation of 

water from the wood surface with dry heat cools the surface 

and lowers its temperature, reducing the surface-to-center 

temperature gradient that is the driving force for transferring 

heat. With wet heat there is little or no evaporation of moisture and thus little surface cooling to slow heat transfer.

Air Circulation

Maintaining adequate air circulation is also important in 

heat sterilization. The circulating air performs two functions, as it does in kiln drying: it carries heat to the wood 

to effect evaporation, and it removes the evaporated water 

vapor. Good air circulation ensures uniform heat distribution in the chamber and keeps the wood surface temperature 

high so that the surface-to-center temperature gradient is as 

high as possible. This is usually accomplished with fans and 

baffles in a treatment chamber.

Size and Configuration of Wood

The heat treatment process is affected by wood configuration and size, as would be expected. Heating time increases 

with size and at a rate that is more than proportional to the 

configuration. For example, heating time can range from 

only a few minutes for thin boards to many hours for large 

timbers. The effect of wood configuration on heating time 

can be seen in Figure 20–1 for a series of web-bulb 

depressions.

Species

Studies of five hardwood species (red maple, sugar maple, 

red oak, basswood, and aspen) at the USDA Forest Service 

Forest Products Laboratory (FPL) have indicated that the 

actual effect of species was not large (Simpson and others 

2005). In fact, the differences in heating times of different 

species are of a similar magnitude to the expected natural 

variability between individual boards and square timbers. In 

heat treatment operation, there is no practical reason to heattreat different hardwood species separately. Figure 20–2 

illustrates the effects of species on heating times of boards 

and square timbers for five hardwood species.

No data are currently available to directly assess the effect 

of species in heat-treating softwood products. However, 

there are practical reasons to separate species in drying 

softwood lumber, and heat treatment for softwood products 

is often accomplished as part of the wood drying process. 

Detailed information on heating times for softwood products 

is presented in the sections of stacking methods, heating 

times for wood in various forms, and methods for estimating 

heating times.

Stacking Methods

Proper stacking of lumber or timbers is an essential aspect 

of the heat treatment process because it directly affects heat 

Table 20–1. Pest groups that are 

practically eliminated by heat treatment 

under ISPM 15 standard

Insects 

Anobiidae

Bostrichidae 

Buprestidae

Cerambycidae 

Curculionidae 

Isoptera

Lyctidae (with some exceptions for HT) 

Oedemeridae

Scolytidae 

Siricidae 

Nematodes 

Bursaphelenchus	xylophilus	
20–3

Chapter 20 Heat Sterilization of Wood

Figure 20–1. Dependence of heating time on wet-bulb depression for (a) 1- to 2-in.-thick 

ponderosa pine boards; (b) 4- to 12-in. ponderosa pine timbers; (c) 3/4- to 1-1/2-in.-thick 

Douglas-fir boards; and (d) 3-1/2- by 3-1/2-in. Douglas-fir timbers (initial temperature: 60 °F). 

(°C = (°F – 32)/1.8; 1 in. = 25.4 mm)

Figure 20–2. Effect of species on heating times of boards and squares. RM, red maple; SM, sugar 

maple; RO, red oak; BA, basswood; AS, aspen. The solid rectangle represents 2 °F (1.1 °C) wetbulb depression. The entire rectangle represents 10 °F (5.6 °C) wet-bulb depression.
20–4

General Technical Report FPL–GTR–190

transfer and, consequently, heating times. If a heat treatment 

facility receives solid-piled bundles of lumber or timbers, it 

may be desirable to heat-treat in the solid-piled configuration. However, a solid bundle of lumber or timbers requires 

much longer heating times than a comparable quantity of 

stickered lumber or timbers. Figure 20–3, for example, 

shows the ratio of heating times for equal quantities of 

lumber or timbers, one being heat treated as a solid bundle 

(4 by 3.2 ft) and the other treated after stickering. Note that 

the ratio ranges from about 2 for 12- by 12-in. timbers to 

more than 14 for 1- by 6-in. boards, which indicates that 

heat-treating stickered materials can result in substantial 

decreases in heating times. In addition, a higher degree of 

variation in heating times for solid-piled materials than for 

stickered materials results from how closely the individual 

pieces fit together in a stacking bundle (Simpson and others 

2003). Gaps between individual pieces allow hot air to penetrate and thus warm the surface more than where adjacent 

pieces fit tightly together. In commercial practice, this high 

variability would cause complications in estimating heating 

times.

Heating Times for Wood in Various Forms

A series of heating experiments were conducted at the FPL 

(Simpson 2001, 2002; Simpson and others 2003, 2005). 

Tables 20–2 and 20–3 summarize experimental heating 

times for ponderosa pine and Douglas-fir boards and square 

timbers to a center temperature of 133 °F (56 °C) in a heating environment of 160 °F (71 °C) dry-bulb temperature and 

various wet-bulb depressions. Table 20–4 summarizes average heating times required to reach 133 °F (56 °C) for six 

sizes of five hardwood species (red maple, sugar maple, red 

oak, basswood, and aspen) at two wet-bulb depressions 

(0 and 10 °F (0 and 5.6 °C)). Note that heating times in 

these tables are for wood in green condition and that these 

data were obtained through laboratory experiments in a 

small-scale dry kiln (approximately 1,500 board foot 

(3.5 m3) capacity) under well-controlled heating conditions. 

Although the experimental results have not been calibrated 

to commercial operation, they have served as the bases for 

developing heat treatment schedules for industrial applications (ALSC 2009).

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

Ratio

of solid to stickered heating time

(a)

(b)

(c)

(d)

12 by 12

6 by 6

4 by 4

2 by 6

1 by 6

12 by 12

6 by 6

4 by 4

2 by 6

1 by 6

12 by 12

6 by 6

4 by 4

2 by 6

1 by 6

12 by 12

6 by 6

4 by 4 2 by 6

1 by 6

Figure 20–3. Ratio of heating times of solid-piled boards and timbers (4 by 3.2 ft) to stickered boards 

and timbers for (a) Douglas-fir, 1.5 °F/2.2 °F (0.8 °C/1.2 °C) wet-bulb depression; (b) Douglas-fir, 12.5 

°F/13.8 °F (7.0 °C/7.7 °C) wet-bulb depression; (c) ponderosa pine, 2.5 °F/2.8 °F (1.4 °C/1.6 °C) wetbulb depression; and (d) ponderosa pine, 12.0 °F/13.4 °F (6.7 °C/7.5 °C) wet-bulb depression.
20–5

Chapter 20 Heat Sterilization of Wood

Methods for Estimating Heating 

Times

Many combinations of wood configurations, heating temperatures, wet-bulb depressions, and initial wood temperatures 

are possible. No one experiment of practical scope would 

cover them all. Therefore, analytical methods are needed 

to estimate the heating times for combinations not directly 

measured experimentally.

MacLean Equations

MacLean (1930, 1932, 1941) developed equations for estimating heating times in steam and showed experimentally 

that they worked well. The equations are for two-dimensional heat flow (heating is from all four cross-sectional faces) 

and apply only to heating in a saturated steam environment. 

Heat conduction is considered to be about 2.5 times faster 

in the longitudinal grain direction than across the grain. 

However, because the length of many typical timbers and 

rounds is much greater than the cross-sectional dimension, 

longitudinal conduction is ignored and the equations thus 

simplified.

Round Cross Section

The heat conduction equations for round cross sections are 

taken from MacLean (1930), further refined by Ingersoll and 

Zobel (1948). The temperature T at any point on radius r is 

given by

(20–1)

Table 20–2. Summary of experimental heating times to heat 

ponderosa pine boards and square timbers to a center 

temperature of 133 °F (56 °C) in a heating environment of nominal 

160 °F (71 °C) dry-bulb temperature and various wet-bulb 

depressions

Wet-bulb 

depression

(°F (°C)) 

Experimental heating times (min)a

1 by 6b 2 by 6 4 by 4 6 by 6 12 by 12 

Stickered 

2.5 (1.4) 17 (8.1) 43 (13.1) 153 (8.9) 299 (17.7) 1,006 (15.5) 

6.2 (3.4) 16 (5.9) 53 (2.4) 180 (6.0) 271 (6.2) 980 (12.1) 

12.0 (6.6) 23 (3.1) 67 (15.0) 207 (17.3) 420 (28.3) 1,428 (8.2) 

26.8 (14.9) 188 (45.2) 137 (12.5) 256 (19.0) 568 (7.2) 1,680 (13.9) 

47.5 (26.4) 427 (18.1) 361 (30.7) 817 (53.9) 953 (38.1) 2,551 (22.2) 

Solid-piledc

2.8 (1.6) 166 (70.3) 361 (64.9) 831 (14.0) 1,201 (30.1) 1,736 (26.4) 

13.4 (7.4) 201 (22.7) 391 (23.4) 710 (48.1) 1,617 (26.7) 2,889 (22.4) 

a

Values in parentheses are coefficients of variation (%). b

Actual sizes are the same as nominal sizes. 

c

Solid pile 4 ft wide and 3.2 ft high. 

Table 20–3. Summary of experimental heating times to heat 

Douglas-fir boards and square timbers to a center temperature of 

133 °F (56 °C) in a heating environment of nominal 160 °F (71 °C) 

dry-bulb temperature and various wet-bulb depressions

Wet-bulb 

depression

(°F (°C)) 

Experimental heating times (min)a

1 by 6b 2 by 6 4 by 4 6 by 6 12 by 12 

Stickered 

2.2 (1.2) 7 (22.2c) 21 (21.3) 78 (12.5) 209 (8.9) 840 (8.8) 

6.3 (3.5) 8 (10.3) 25 (21.9) 91 (10.5) 202 (11.6) 914 (13.9) 

12.5 (6.9) 10 (6.7) 34 (22.3) 138 (17.8) 262 (7.7) 1,153 (7.0) 

27.1 (15.0) 216 (39.9) 157 (23.1) 255 (25.1) 715 (22.8) 1,679 (3.1) 

44.2 (24.6) 233 (62.8) 223 (20.3) 362 (28.0) 849 (6.1) 2,005 (23.3) 

Solid-piledc

1.5 (0.8) 103 (45.2) 137 (46.9) 432 (27.2) 977 (9.3) 1,931 (13.5) 

13.8 (7.7) 143 (69.1) 195 (77.4) 521 (54.7) 1,847 (25.7) 1,847 (25.7) 

a

Values in parentheses are coefficients of variation (%). b

Nominal sizes. 

c

Solid pile 4 ft wide and 3.2 ft high.
where

Ts is surface temperature (which must be attained 

 immediately),

T0 initial temperature,

J0 zero-order Bessel function,

J1 first-order Bessel function,

zn 	 nth root of J0(zn) = 0,

r any point on radius of cross section,

R radius of cross section,

α thermal diffusivity (dimension2/time), and

t heating time.

To calculate the temperature at the center of the cross section, r = 0, Equation (20–1) becomes

(20–2)

Equations (20–1) and (20–2) converge quickly, so only the 

first few terms are necessary. The first few terms of Equation (20–2) are

From Watson (1958), the first five roots of J0(zn) = 0 are

z1 = 2.405

z2 = 5.520

z3 = 8.654

z4 = 11.792

z5 = 14.931

and the first five values of J1(zn) are

J1(2.405) = 0.5191

J1(5.520) = -0.3403

J1(8.654) = 0.2714

J1(11.792) = -0.2325

J1(14.931) = 0.2065

Rectangular Cross Section

The equation for rectangular cross sections is taken from 

MacLean (1932) and is the solution to the differential equation of heat conduction in the two dimensions of a rectangular cross section. The temperature T at any point x and y is 

given by

T = Ts + (T0 – Ts)(16/π2) ×

 {sin(πx/a) sin(πy/b) exp[-π2t(αx/a2+αy/b2)]

+(1/3) sin(3πx/a) sin(πy/b) exp[-π2t(9αx/a2+αy/b2]

+(1/3) sin(πx/a) sin(3πy/b) exp[-π2t(αx/a2+9αy/b2]

+(1/5) sin(5πx/a) sin(πy/b) exp[-π2t(25αx/a2+αy/b2]

+(1/5) sin(πx/a) sin(5πy/b) exp[-π2t(αx/a2+25αy/b2]

+(1/7) sin(7πx/a) sin(πy/b) exp[-π2t(49αx/a2+αy/b2]

+(1/7) sin(πx/a) sin(7πy/b) exp[-π2t(αx/a2+49αy/b2] 

 +...}

20–6

General Technical Report FPL–GTR–190

Table 20–4. Summary of experimental heating times to 133 °F (56 °C) for six 

sizes of five hardwood species heated at a nominal dry-bulb temperature of 

160 °F (71 °C) and two wet-bulb depressionsa

Wet-bulb 

depression

(°F (°C)) 

Piece size 

(in.)c

Heating time (min)b

Red

maple 

Sugar

maple 

Red

oak Basswood Aspen 

0 (0) 1 by 6 14 (15) 13 (14) 14 (15) 12 (14) 13 (14) 

1-1/2 by 6 29 (31) 28 (30) 26 (28) 26 (28) 29 (32) 

2 by 6 50 (52) 48 (49) 49 (53) 46 (48) 50 (54) 

3 by 3 59 (64) 58 (61) 57 (60) 51 (58) 61 (64) 

4 by 4 115 (119) 107 (113) 109 (112) 100 (108) 113 (117) 

6 by 6 265 (283) 255 (277) 252 (259) 226 (243) 262 (278) 

10 (5.6) 1 by 6 17 (18) 14 (15) 15 (16) 15 (17) 15 (16) 

1-1/2 by 6 36 (38) 31 (34) 32 (33) 29 (31) 32 (33) 

2 by 6 59 (62) 53 (56) 56 (59) 54 (58) 57 (62) 

3 by 3 85 (96) 63 (67) 66 (69) 63 (69) 69 (74) 

4 by 4 137 (143) 121 (127) 124 (129) 114 (120) 129 (133) 

6 by 6 294 (304) 284 (299) 284 (298) 262 (284) 285 (195) 

a

Heating times were adjusted to a common initial temperature of 60 °F (16 °C) and the overall actual 

average heating temperature of 157 °F (69 °C). 

b

Values in parentheses are 99% upper confidence bounds of heating times. c

Actual sizes. 

(20–4)

(20–3)
where

Ts is surface temperature (which must be attained 

 immediately),

T0 initial temperature,

a one cross-sectional dimension,

b other cross-sectional dimension,

αx thermal diffusivity in the x direction 

 (dimension2/time),

αy thermal diffusivity in the y direction, and

t heating time.

Equation (20–4) converges quickly, so only the first few 

terms are necessary. Because thermal conductivity and thermal diffusivity do not differ much in the radial and tangential directions of wood, in Equation (20–4) we can set 

αx = αy (MacLean 1941). Equation (20–4) can easily be converted to calculate the temperature at the center of the cross 

section by setting x = a/2 and y = b/2.

Gu and Garrahan (1984) experimentally confirmed that 

MacLean’s equations were valid for estimating heating 

times. Figure 20–4 shows close agreement of experimental 

heating times of Gu and Garrahan (1984) with times calculated using MacLean’s heat conduction equation. Simpson 

(2001) further confirmed the validity of MacLean’s equations and used them to develop a series of tables of heating times (to the center) of round and rectangular sections. 

Variables in the tables were wood specific gravity, moisture 

content, initial temperature, heating temperature, and target 

center temperature.

Specific gravity and moisture content values were chosen 

to represent several species that might be subjected to heat 

sterilization. Target center temperatures other than 133 °F 

(56 °C) were included because future heat sterilization requirements are not known and might include higher 

temperatures. As an example, Table 20–5 tabulates the estimated heating times to heat lumber of selected sizes to 

133 °F (56 °C) for wood specific gravity of 0.35 (Cheung 

2008). Tables for other combinations of variables are presented in Simpson (2001).

Heat experiments at the Forest Products Laboratory indicated that MacLean’s equations are able to estimate heating 

times in steam to a degree of accuracy that is within about 

5% to 15% of measured heating times. The equations offer a 

powerful way to include the effects of all the variables that 

affect heating time—specific gravity, moisture content, initial temperature, heating temperature, target center temperature, and cross-sectional dimensions.

MacLean’s approach requires full access of all four faces 

to the heating medium. This might not be achieved in the 

close edge-to-edge contact of the stickered configuration or 

the solid-piled configuration. In practice, his approach will 

probably require some small level of gapping between adjacent boards or timbers.

Multiple Regression Models

MacLean’s equations apply only to heating in a saturated 

steam environment. When the heating medium is air that is 

not saturated with steam, there is a wet-bulb depression (the 

relative humidity is less than 100%), and drying occurs as 

water evaporates from the wood surface. The consequence 

is that heating time increases and MacLean’s equations no 

longer apply. An alternative method to estimate the heating 

time when simultaneous drying occurs is to use a strictly 

empirical approach.

The following multiple regression model proved to have a 

good ability to predict heating time from size, wet-bulb 

depression, and initial wood temperature as long as the 

20–7

Chapter 20 Heat Sterilization of Wood

Figure 20–4. Comparison of experimental heating 

times of Gu and Garrahan (1984) with times calculated using MacLean equations for white birch and 

red pine.
20–8

General Technical Report FPL–GTR–190

Table 20–5. Estimated heating times to heat lumber to 133 °F (56 °C) for wood with a specific gravity of 0.35

Thickness 

(t) and width 

(w) (in.) 

Heat 

temp. 

(°F)

Estimated heating time (min) from four initial wood temperatures and four MC levels 

30 °F 50 °F 70 °F 90 °F 

25% 70% 100% 130% 25% 70% 100% 130% 25% 70% 100% 130% 25% 70% 100% 130% 

t = 1.0 

w = 4.0 

140 21 21 20 19 19 19 18 17 17 17 16 15 15 14 13 12 

150 15 15 14 13 14 13 13 12 12 11 11 10 10 9 9 8 

160 13 12 12 11 11 11 10 9 10 9 9 8 8 7 7 6 

170 11 10 10 9 10 9 8 8 8 7 7 7 6 6 6 5 

180 9 9 9 8 8 8 7 7 7 6 6 6 6 5 5 4 

190 9 8 8 7 7 7 7 6 6 6 5 5 5 4 4 4 

200 8 7 7 6 7 6 6 5 6 5 5 4 5 4 4 3 

210 7 7 6 6 6 6 5 5 5 5 4 4 4 4 3 3 

t = 1.0 

w = 6.0 

140 21 21 20 19 19 19 18 17 17 17 16 15 15 14 13 12 

150 15 15 14 13 14 13 13 12 12 11 11 10 10 9 9 8 

160 13 12 12 11 11 11 10 9 10 9 9 8 8 7 7 6 

170 11 10 10 9 10 9 8 8 8 7 7 7 6 6 6 5 

180 9 9 9 8 8 8 7 7 7 6 6 6 6 5 5 4 

190 9 8 8 7 7 7 7 6 6 6 5 5 5 4 4 4 

200 8 7 7 6 7 6 6 5 6 5 5 4 5 4 4 3 

210 7 7 6 6 6 6 5 5 5 5 4 4 4 4 3 3 

t = 2.0 

w = 4.0 

140 75 74 70 66 69 67 64 59 62 59 56 53 54 50 48 45 

150 56 55 52 49 51 49 46 43 45 42 40 38 38 35 33 31 

160 46 45 43 40 42 40 38 35 37 34 33 30 30 28 26 25 

170 41 39 37 35 36 34 33 30 32 29 28 26 26 24 22 21 

180 36 35 33 31 32 30 29 27 28 26 24 23 23 21 20 18 

190 33 31 30 28 29 27 26 24 25 23 22 20 21 18 17 16 

200 30 28 27 25 27 25 24 22 23 21 20 19 19 17 16 15 

210 28 26 25 23 25 23 22 20 22 19 18 17 18 15 15 14 

t = 2.0 

w = 8.0 

140 86 85 81 76 79 77 73 68 71 67 64 60 61 57 54 50 

150 63 62 59 55 57 55 52 49 50 47 45 42 41 38 36 34 

160 52 50 48 45 46 44 42 39 40 37 35 33 32 30 28 26 

170 44 43 41 38 39 37 35 33 34 31 30 28 27 25 24 22 

180 39 37 36 33 35 32 31 29 30 27 26 24 24 21 20 19 

190 35 33 32 30 31 29 27 26 27 24 23 21 21 19 18 17 

200 32 30 29 27 29 26 25 23 24 22 21 19 19 17 16 15 

210 30 28 26 24 26 24 23 21 22 20 19 18 18 16 15 14 

t = 4.0 

w = 4.0 

140 188 186 177 166 173 168 160 150 157 149 142 132 136 127 120 112 

150 141 138 131 123 128 123 117 110 114 107 102 95 96 89 85 79 

160 118 114 109 102 107 102 97 90 94 88 83 78 79 72 69 64 

170 103 99 94 88 93 88 83 78 82 76 72 67 68 62 59 55 

180 93 88 84 78 84 78 74 69 73 67 64 59 61 55 52 49 

190 85 80 76 71 76 71 67 63 67 61 58 54 56 50 47 44 

200 79 74 70 65 71 65 62 57 62 56 53 49 52 46 43 40 

210 74 68 65 60 66 60 57 53 58 52 49 46 48 43 40 37 

t = 4.0 

w = 12.0 

140 335 332 316 296 309 300 286 267 278 265 252 235 239 224 213 198 

150 248 243 232 217 225 216 206 192 198 187 178 166 165 153 145 135 

160 205 199 190 177 184 175 167 156 160 150 142 133 131 120 114 106 

170 177 171 162 152 158 149 142 133 136 126 120 112 111 101 95 89 

180 158 150 143 133 140 131 124 116 120 110 105 98 97 87 83 77 

190 143 135 128 119 126 117 111 104 108 98 93 87 87 78 74 69 

200 131 122 116 108 115 106 101 94 98 89 84 78 79 70 67 62 

210 121 112 106 99 107 97 92 86 91 81 77 72 73 64 61 57 
20–9

Chapter 20 Heat Sterilization of Wood

wet-bulb temperature in the heating chamber is greater than 

the target center temperature:

ln T133 = ln a +b (ln t)n + c ln (WBD) + d ln (Ti) (20–5)

where

T133 is time for the center to reach 133 °F (56 °C) 

 (min),

t thickness of boards or cross-sectional 

dimension of timbers (in.),

WBD wet-bulb depression (°F),

Ti initial wood temperature (°F),

a,b,c,d regression coefficients,

n either 1 or 2.

Simpson and others (2003) developed a series of regression models to estimate heating times for ponderosa pine 

and Douglas-fir boards and timbers. The regression coefficients (a, b, c, and d) and coefficients of determination (R2) 

are shown in Table 20–6. The models worked well when 

the wet-bulb depression was less than or equal to about 12 

°F (6.7 °C) and the boards or timbers were stickered. The 

heating time estimates for a series of sizes, wet-bulb depressions, and initial temperature generated using these equations are presented in Tables 20–7 to 20–10. The estimates 

for ponderosa pine cover initial temperatures from 40 to

 80 °F (4.4 to 26.7 °C) (in 10 °F (5.6 °C) increments). The 

estimates for Douglas-fir cover only initial temperature of 

60 to 80 °F (15.6 to 26.7 °C) because of the seasonal timing 

of the experiments.

The estimated heating times in Tables 20–7 to 20–10 are 

average times and give a reasonable general estimate of the 

time required to heat the center of wood to 133 °F (56 °C). 

In any group of lumber and timbers, the average time does 

not ensure that all pieces will achieve the target temperature because some will require more than the average time. 

Therefore, the upper statistical confidence levels for the 

heating times need to be considered. Equations for calculating the upper confidence levels of heating times for ponderosa pine and Douglas-fir boards and timbers are provided 

in Simpson and others (2003). In Tables 20–7 to 20–10, the 

heating time values of 99% upper confidence bounds are 

presented in parentheses.

American Lumber Standard 

Committee (ALSC) Enforcement 

Regulations

Heat treatment of wood is typically accomplished in a heat 

chamber. Heat chamber is defined as any enclosed equipment used to heat-treat lumber or wood packaging material 

and includes kiln, heat boxes, or any other appropriate apparatus. Depending on the treating schedules used, products 

from heat treatment processes are of two types:

1. Heat treated (HT)—lumber or used, previously assembled or repaired wood packaging that has been placed 

in a closed chamber with artificial heat added until the 

lumber or packaging achieves a minimum core temperature of 133 °F (56 °C) for a minimum of 30 min.

2. Kiln-dried heat-treated (KD HT)—lumber or used, previously assembled or repaired wood packaging that has 

been placed in a closed chamber with artificial heat added until the lumber or packaging achieves a minimum 

core temperature of 133 °F (56 °C) for a minimum of 

30 min and that is dried to a maximum moisture content 

of 19% or less.

ALSC enforcement regulations require that a heat treatment 

facility should be inspected and verified by an accredited 

third-party agency for initial qualification. Agencies will 

verify the accuracy of temperature-measuring and recording 

devices in the heating chamber and require that thermocouples be located to accurately measure the temperature 

achieved in the heat chamber and that an appropriate number of thermocouples are utilized given the chamber configuration. A thermocouple verification study is needed for 

any kiln schedule operating in a heat chamber using (1) both 

dry and wet heat (steam) with wet-bulb temperature of less 

than 140 °F (60 °C) or (2) only dry heat of less than 160 °F 

(71 °C). In such a verification study, an appropriate number 

Table 20–6. Coefficients for multiple regression models (Eq. (20–5)) for estimating time 

required to heat stickered ponderosa pine and Douglas-fir boards and timbers to a 133 

°F (56 °C) center temperature in a 160 °F (71 °C) heating mediuma

Application 

Coefficients 

ln a b	 c	 d	 R2

Ponderosa pine, 1- and 2-in. boards, WBD < 12 F 5.04 1.55 0.257 0.627 0.978 

Ponderosa pine, 4-, 6-, and 12-in. timbers, WBD < 12 F 4.59 1.61 0.205 –0.521 0.967 

Douglas-fir, 1- and 2-in. boards, WBD < 12 F 8.04 1.63 0.265 –1.35 0.925 

Douglas-fir, 4-, 6-, and 12-in. timbers, WBD < 12 F 15.03 0.455 0.336 –2.70 0.984 

a

T°C = (T°F – 32)/1.8; °C = °F/1.8; 1 in. = 25.4 mm. 
20–10

General Technical Report FPL–GTR–190

of thermocouples are used to accurately measure the temperature conditions of the chamber and the wood to ensure 

that time and temperature requirements for heat treating are 

met. Any equipment variance of more than ±5 °F (±2.8 °C) 

requires recalibration or replacement.

Heat treatment facilities are also required to monitor temperatures throughout the heat treatment cycle by any of the 

following options:

1. Wet- and dry-bulb temperature

2. Dry-bulb only—unless the specific schedule has been 

verified, required heating times shall be equal to or 

greater than the time specified for the applicable schedule assuming the maximum wet-bulb depression as provided in either of the following: 

a. FPL–RP–607, Heat sterilization time of ponderosa

pine and Douglas-fir boards and square timbers

(Simpson and others 2003); or

b. FPL–RP–604, Effect of wet-bulb depression on

heat sterilization time of slash pine lumber (Simpson 2002); or

c. CFIA PI–07, The technical heat treatment guidelines and operating conditions manual, Option C 

(CFIA 2006).

3. Direct measurement of wood core temperature of the 

thickest piece(s) by use of thermocouple(s) properly 

sealed with non-conductive material

Heat treatment facilities are currently required to annually 

calibrate the temperature-monitoring and recording equipment for each facility heat-treating chamber and requalify 

a heat-treating chamber any time there is a major change 

in equipment or remodeling of the chamber. Except in the 

case of wood core temperature of the thickest piece(s) being directly measured by using thermocouples, when wood 

moisture content is not determined at the beginning of the 

heat treatment cycle, facilities are required to select and use 

Table 20–7. Summary of heating times (at 160 °F (71 °C)) to 

133 °F (56 °C) for ponderosa pine boards estimated by multiple 

regression modelsa

Wet-bulb 

depression

(°F)

Initial 

temperature

(°F)

Heating time (min)b

1.00 in.

thick 

1.25 in.

thick 

1.50 in.

thick 

1.75 in.

thick 

2.00 in. 

thick 

2 40 18 (39) 26 (53) 34 (67) 43 (82) 53 (98) 

4 40 22 (45) 31 (60) 41 (76) 52 (93) 64 (112) 

6 40 24 (48) 34 (65) 45 (83) 58 (101) 71 (121) 

8 40 26 (51) 37 (69) 49 (87) 62 (107) 76 (128) 

10 40 28 (54) 39 (72) 52 (92) 66 (112) 81 (134) 

12 40 29 (56) 41 (75) 54 (95) 69 (117) 85 (139) 

2 50 16 (28) 22 (37) 30 (47) 38 (58) 46 (70) 

4 50 19 (31) 27 (42) 36 (54) 45 (66) 55 (80) 

6 50 21 (34) 30 (46) 39 (59) 50 (72) 62 (87) 

8 50 23 (36) 32 (49) 42 (62) 54 (77) 66 (92) 

10 50 24 (38) 34 (51) 45 (65) 57 (80) 70 (97) 

12 50 25 (39) 36 (53) 47 (68) 60 (84) 74 (101) 

2 60 14 (21) 20 (28) 27 (36) 34 (45) 41 (55) 

4 60 17 (24) 24 (33) 32 (42) 40 (52) 49 (63) 

6 60 19 (26) 27 (35) 35 (46) 45 (57) 55 (70) 

8 60 20 (28) 29 (38) 38 (49) 48 (61) 59 (75) 

10 60 21 (29) 30 (40) 40 (52) 51 (65) 63 (79) 

12 60 22 (30) 32 (42) 42 (54) 53 (68) 66 (83) 

2 70 13 (17) 18 (24) 24 (31) 31 (39) 38 (48) 

4 70 15 (20) 22 (27) 29 (36) 37 (46) 45 (57) 

6 70 17 (22) 24 (30) 32 (40) 41 (51) 50 (64) 

8 70 18 (23) 26 (33) 34 (43) 44 (56) 54 (70) 

10 70 19 (25) 27 (35) 36 (46) 46 (59) 57 (74) 

12 70 20 (26) 29 (36) 38 (45) 48 (63) 60 (78) 

2 80 12 (15) 17 (21) 22 (29) 28 (37) 35 (46) 

4 80 14 (18) 20 (26) 26 (35) 34 (45) 41 (56) 

6 80 16 (20) 22 (29) 29 (39) 37 (51) 46 (64) 

8 80 17 (22) 24 (31) 32 (42) 40 (55) 49 (70) 

10 80 18 (23) 25 (33) 33 (45) 43 (59) 52 (75) 

12 80 19 (24) 26 (35) 35 (48) 45 (63) 55 (79) 

a

T°C = (T°F – 32)/1.8; °C = °F/1.8; 1 in. = 25.4 mm. b

Values in parentheses are 99% upper confidence bounds of heating times. 
20–11

Chapter 20 Heat Sterilization of Wood

appropriate time–temperature schedules assuming the lowest initial wood moisture content from one of the following 

publications:

a) FPL–GTR–130, Heating times for round and rectangular cross sections of wood in steam (Simpson 2001);

b) FPL–RP–607, Heat sterilization time of ponderosa pine

and Douglas-fir boards and square timbers (Simpson 

and others 2003); 

c) FPL–RP–604, Effect of wet-bulb depression on heat

sterilization time of slash pine lumber (Simpson 2002); 

or

d) CFIA PI–07, The technical heat treatment guidelines

and operating conditions manual, Option C (CFIA 

2006).

Quality Mark

ISPM 15 requires that treated packaging must be marked 

with an official stamp that includes an International Plant 

Protection Convention (IPPC) symbol, an International 

Standards Organization (ISO) two-letter country code, and 

abbreviation of the type of treatment used (heat treatment is 

indicated by the mark HT), and a unique number assigned 

by the country’s national plant protection organization to the 

producer of the wood packaging material, who is responsible for ensuring that appropriate wood is used and properly 

marked (Figure 20–5). If wood packaging materials arrive in 

a member country without this quality mark, officials at the 

port of arrival have the right to refuse entry or require treatment (such as fumigation) at the port—a costly situation. 

Recycled, remanufactured, or repaired wood packing material should be recertified and remarked. All components of 

such material are required to be properly treated.

Other Considerations

Heating capacity—It is critical in heat sterilization that the 

heating and humidification system be designed to meet the 

production schedule. Typically, the heating capacity of a 

Table 20–8. Summary of heating times (at 160 °F (71 °C)) to 133 °F (56 °C)) for 

ponderosa pine square timbers estimated by multiple regression modelsa

Wet-bulb 

depression

(°F)

Initial 

temperature 

(°F)

Heating time (min)b

4 by 4 6 by 6 8 by 8 10 by 10 12 by 12 

2 40 155 (225) 297 (429) 473 (682) 677 (980) 90 (1,321)

4 40 178 (259) 343 (492) 545 (782) 780 (1,123) 1,04 (1,512)

6 40 194 (282) 372 (535) 592 (850) 848 (1,220) 1,13 (1,643)

8 40 206 (299) 395 (569) 628 (903) 899 (1,296) 1,20 (1,745)

10 40 215 (314) 413 (597) 657 (947) 941 (1,359) 1,26 (1,830)

12 40 223 (327) 429 (621) 682 (986) 977 (1,414) 1,31 (1,904)

2 50 138 (200) 265 (382) 421 (609) 603 (878) 80 (1,185)

4 50 159 (229) 305 (437) 485 (697) 695 (1,003) 93 (1,354)

6 50 173 (249) 332 (475) 527 (756) 755 (1,088) 1,01 (1,468)

8 50 183 (264) 352 (504) 559 (802) 801 (1,155) 1,07 (1,558)

10 50 192 (277) 368 (529) 585 (841) 838 (1,210) 1,12 (1,633)

12 50 199 (288) 382 (550) 607 (875) 870 (1,258) 1,16 (1,697)

2 60 125 (182) 241 (350) 383 (559) 548 (807) 73 (1,091)

4 60 144 (208) 278 (400) 441 (638) 632 (921) 84 (1,245)

6 60 157 (226) 302 (433) 479 (692) 687 (998) 92 (1,349)

8 60 166 (240) 320 (460) 508 (734) 728 (1,058) 97 (1,430)

10 60 174 (251) 335 (482) 532 (769) 762 (1,108) 1,02 (1,497)

12 60 181 (261) 348 (501) 552 (799) 791 (1,151) 1,06 (1,555)

2 70 116 (169) 222 (326) 353 (523) 506 (755) 67 (1,022)

4 70 133 (193) 256 (372) 407 (596) 583 (860) 78 (1,164)

6 70 145 (210) 278 (403) 442 (645) 634 (932) 85 (1,260)

8 70 154 (222) 295 (427) 469 (684) 672 (987) 90 (1,335)

10 70 161 (233) 309 (448) 491 (716) 703 (1,033) 94 (1,398)

12 70 167 (242) 321 (465) 510 (743) 730 (1,073) 97 (1,451)

2 80 108 (160) 207 (308) 330 (494) 472 (715) 63 (968)

4 80 124 (182) 239 (351) 380 (563) 544 (814) 73 (1,102)

6 80 135 (197) 260 (380) 413 (609) 591 (880) 79 (1,192)

8 80 143 (209) 275 (403) 438 (645) 627 (932) 84 (1,262)

10 80 150 (219) 288 (421) 458 (675) 656 (975) 88 (1,321)

12 80 156 (227) 299 (438) 476 (701) 681 (1,013) 91 (1,371)

a

T°C = (T°F – 32)/1.8; °C = °F/1.8; 1 in. = 25.4 mm. b

Values in parentheses are 99% upper confidence bounds of heating times. 
20–12

General Technical Report FPL–GTR–190

Table 20–9. Summary of heating times (at 160 °F (71 °C)) to 

133 °F (56 °C) for Douglas-fir boards estimated by multiple 

regression modelsa

Wet-bulb 

depression

(°F)

Initial 

temperature

(°F)

Heating time (min)b

0.75 in.

thick 

1.00 in.

thick 

1.25 in.

thick 

1.50 in. 

thick 

2 60 9 (25) 14 (37) 21 (53) 28 (70) 

4 60 11 (29) 17 (44) 25 (62) 34 (82)

6 60 12 (32) 19 (49) 28 (68) 38 (91) 

8 60 13 (34) 21 (52) 30 (74) 41 (98) 

10 60 14 (36) 22 (55) 32 (78) 43 (104) 

12 60 15 (38) 23 (58) 34 (82) 45 (109) 

2 70 7 (15) 12 (22) 17 (32) 23 (42) 

4 70 9 (17) 14 (26) 20 (37) 27 (49)

6 70 10 (19) 16 (29) 23 (41) 31 (55) 

8 70 11 (20) 17 (31) 24 (44) 33 (59) 

10 70 11 (22) 18 (33) 26 (47) 35 (63) 

12 70 12 (23) 19 (35) 27 (49) 37 (66)

2 80 6 (10) 10 (16) 14 (23) 19 (31)

4 80 7 (12) 12 (19) 17 (27) 23 (37) 

6 80 8 (13) 13 (21) 19 (30) 25 (41) 

8 80 9 (15) 14 (23) 20 (32) 28 (44) 

10 80 9 (15) 15 (24) 22 (35) 29 (47)

12 80 10 (16) 16 (25) 23 (36) 31 (49) 

a

T°C = (T°F – 32)/1.8; °C = °F/1.8; 1 in. = 25.4 mm. b

Values in parentheses are 99% upper confidence bounds of heating times. 

Table 20–10. Summary of heating times (at 160 °F (71 °C)) to 133 °F (56 °C) for Douglas-fir 

square timbers estimated by multiple regression modelsa

Wet-bulb 

depression

(°F)

Initial 

temperature 

(°F)

Heating time (min)b

4 by 4 6 by 6 8 by 8 10 by 10 12 by 12 

2 60 159 (229) 285 (406) 473 (667) 738 (1,034) 1,098 (1,534)

4 60 200 (298) 360 (526) 597 (862) 932 (1,334) 1,386 (1,974)

6 60 229 (349) 412 (615) 684 (1,007) 1,068 (1,556) 1,588 (2,299)

8 60 253 (391) 454 (689) 754 (1,126) 1,176 (1,739) 1,749 (2,567)

10 60 272 (427) 489 (752) 812 (1,229) 1,267 (1,897) 1,885 (2,799)

12 60 289 (459) 520 (809) 863 (1,321) 1,347 (2,038) 2,004 (3,006)

2 70 105 (143) 188 (256) 312 (426) 487 (669) 724 (1,003)

4 70 132 (181) 237 (323) 394 (535) 614 (836) 914 (1,251)

6 70 151 (209) 272 (372) 451 (615) 704 (959) 1,047 (1,432)

8 70 167 (232) 299 (412) 497 (680) 775 (1,061) 1,153 (1,580)

10 70 179 (252) 323 (447) 535 (737) 835 (1,148) 1,243 (1,709)

12 70 191 (270) 343 (478) 569 (788) 888 (1,226) 1,321 (1,824)

2 80 73 (103) 131 (188) 217 (315) 339 (499) 505 (753)

4 80 92 (127) 165 (230) 274 (386) 428 (609) 637 (918)

6 80 105 (144) 189 (261) 314 (436) 491 (688) 730 (1,036)

8 80 116 (159) 209 (286) 346 (477) 540 (752) 804 (1,130)

10 80 125 (171) 225 (307) 373 (513) 582 (807) 866 (1,212)

12 80 133 (182) 239 (326) 397 (544) 619 (855) 921 (1,283)

a

T°C = (T°F – 32)/1.8; °C = °F/1.8; 1 in. = 25.4 mm. b

Values in parentheses are 99% upper confidence bounds of heating times. 
20–13

Chapter 20 Heat Sterilization of Wood

hardwood kiln ranges from 0.5 to 1.5 boiler horsepower per 

thousand board feet of lumber (7,100 to 21,300 Btu/h per 

cubic meter of lumber). To get the rapid heating needed, 

the boiler horsepower needs to be sized from 6.0 to 12.5 

boiler horsepower per thousand board feet (85,100 to 

177,300 Btu/h per cubic meter), depending on the lumber 

used and starting temperature (Denig and Bond 2003).

Structure damage—The environment used for heat sterilization of wood can be extremely corrosive and damaging to 

some structures. In addition to using the proper materials, a 

floor drain system should be used, especially when using the 

high-humidity schedules.

Mold prevention—Heat sterilization kills only mold, fungus, and insects that are present when the material is sterilized. In certain cases, mold and fungus have rapidly infested 

heat-sterilized lumber that was not dry (Denig and Bond 

2003). It is critical for the pallet operator and user to keep 

their production facility free of waste wood, minimize pallet 

inventory of heat-treated pallets, and ensure some air movement around green pallets that have been heat-treated.

Literature Cited

ALSC. 2009. Wood packaging material enforcement regulations. Germantown, MD: American Lumber Standards 

Committee, Inc.

APHIS. 2004. Rules and regulations—importation of wood 

packaging material. In: Convention, animal and plant 

protection convention. 7 CFR, Part 319. Federal Register. 

69(179): 55,719–55,733.

CFIA. 2006. The technical heat treatment guidelines and 

operating conditions manual. CFIA PI–09. Ottawa, Canada: 

Canadian Food Inspection Agency, Plant Health Division. 

27 p.

Cheung, K.C.K. 2008. Rules and regulations regarding 

the heat treatment of wood—an American perspective. In: 

Conference, quality drying for the 21st Century: energy and 

market realities; 2006 November 15–17; Bellingham, WA. 

Madison, WI: Forest Products Society: 83–85.

Denig, J.; Bond, B. 2003. Heat sterilization of hardwood 

pallets and pallet material. Pallet Phytosanitary Project Tech. 

Rep. No. TP–1. North Carolina: A cooperative effort of the 

limestone bluffs resources conservation and development 

area and the wood education and resources center. September 2003. 8 p.

Gu, L.B.; Garrahan, P. 1984. The temperature and moisture 

content in lumber during preheating and drying. Wood Science and Technology. 18: 121–135.

Ingersoll, L.R.; Zobel, A.C. 1948. Heat conduction. New 

York, NY: McGraw-Hill Book Co., Inc. 278 p.

IPPC. 2002. Guidelines for regulating wood packaging material in international trade. ISPM Pub. No. 15. FAO, Rome: 

International Plant Protection Convention.

MacLean, J.D. 1930. Studies of heat conduction in wood. 

Pt. I. Results of steaming green round Southern Pine timbers. In: Proceedings, American Wood Preservers’ Association. 26: 197–217.

MacLean, J.D. 1932. Studies of heat conduction in wood. 

Pt. II. Results of steaming green sawed Southern Pine timbers. In: Proceedings, American Wood Preservers’ Association. 28: 303–329.

MacLean, J.D. 1941. Thermal conductivity of wood: heating, piping and air conditioning. In: Proceedings, American 

Wood Preservers’ Association. 13: 380–391.

Simpson, W.T. 2001. Heating times for round and rectangular cross sections of wood in steam. General Technical 

Report FPL–GTR–130. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

103 p.

Simpson, W.T. 2002. Effect of wet bulb depression on 

heat sterilization time of slash pine lumber. Research Paper 

FPL–RP–604. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 6 p.

Simpson, W.T.; Wang, X.; Verrill, S. 2003. Heat sterilization 

time of ponderosa pine and Douglas-fir boards and square 

timbers. Research Paper FPL–RP–607. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 24 p.

Simpson, W.T.; Wang, X.; Forsman, J.W.; Erickson, J.R. 

2005. Heat sterilization times of five hardwood species. Research Paper FPL–RP–626. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 

10 p.

Watson, G.N. 1958. A treatise on the theory of Bessel functions. 2nd ed. Cambridge, UK: Cambridge University Press. 

804 p.

Figure 20–5. ISPM 15 requires the use of a quality 

mark on wood packaging materials to certify that 

proper treatment has occurred.

GLOSSARY 

G–1

Adherend. A body that is held to another body by an 

adhesive.

Adhesion. The state in which two surfaces are held together 

by interfacial forces, which may consist of valence forces or 

interlocking action or both. 

Adhesive. A substance capable of holding materials together 

by surface attachment. It is a general term and includes ce‐

ments, mucilage, and paste, as well as glue.

Assembly Adhesive—An adhesive that can be used for 

bonding parts together, such as in the manufacture of a 

boat, airplane, furniture, and the like.

Cold‐Setting Adhesive—An adhesive that sets at tem‐

peratures below 20 °C (68 °F).

Construction Adhesive—Any adhesive used to assemble 

primary building materials into components during build‐

ing construction—most commonly applied to elasto‐

mer‐based mastic‐type adhesives.

Contact Adhesive—An adhesive that is apparently dry 

to the touch and that will adhere to itself instantaneously 

upon contact; also called contact bond adhesive or dry 

bond adhesive.

Gap-Filling Adhesive—An adhesive capable of form‐

ing and maintaining a bond between surfaces that are not 

close fitting.

Hot‐Melt Adhesive—An adhesive that is applied in a 

molten state and forms a bond on cooling to a solid state.

Hot‐Setting Adhesive—An adhesive that requires a tem‐

perature at or above 100 °C (212 °F) to set it.

Room‐Temperature-Curing Adhesive—An adhesive 

that sets in the temperature range of 20 to 30 °C (68 to 

86 °F), in accordance with the limits for Standard Room 

Temperature specified in the Standard Methods of Con‐

ditioning Plastics and Electrical Insulating Materials for 

Testing (ASTM D 618).

Solvent Adhesive—An adhesive having a volatile or‐

ganic liquid as a vehicle. (This term excludes water‐based 

adhesives.)

Structural Adhesive—A bonding agent used for transfer‐

ring required loads between adherends exposed to service 

environments typical for the structure involved.

Air‐Dried. (See Seasoning.)

Allowable Property. The value of a property normally 

published for design use. Allowable properties are identified 

with grade descriptions and standards, reflect the orthotropic 

structure of wood, and anticipate certain end uses.

Allowable Stress. (See Allowable Property.)

American Lumber Standard. The American Softwood 

Lumber Standard, Voluntary Product Standard PS–20 (Na‐

tional Institute of Standards and Technology), establishes 

standard sizes and requirements for the development and 

coordination of lumber grades of various species, the assign‐

ment of design values when called for, and the preparation 

of grading rules applicable to each species. It provides for 

implementation of the standard through an accreditation and 

certification program to assure uniform industry-wide mark‐

ing and inspection. A purchaser must, however, make use of 

grading association rules because the basic standards are not 

in themselves commercial rules.

Anisotropic. Exhibiting different properties when measured 

along different axes. In general, fibrous materials such as 

wood are anisotropic.

Assembly Joint. (See Joint.)

Assembly Time. (See Time, Assembly.)

Balanced Construction. A construction such that the forces 

induced by uniformly distributed changes in moisture con‐

tent will not cause warping. Symmetrical construction of 

plywood in which the grain direction of each ply is perpen‐

dicular to that of adjacent plies is balanced construction.

Bark Pocket. An opening between annual growth rings that 

contains bark. Bark pockets appear as dark streaks on radial 

surfaces and as rounded areas on tangential surfaces.

Bastard Sawn. Lumber (primarily hardwoods) in which the 

annual rings make angles of 30° to 60° with the surface of 

the piece.

Beam. A structural member supporting a load applied trans‐

versely to it.

Bending, Steam. The process of forming curved wood 

members by steaming or boiling the wood and bending it to 

a form.

Bent Wood. (See Bending, Steam.)

Bird Peck. A small hole or patch of distorted grain result‐

ing from birds pecking through the growing cells in the tree. 

The shape of bird peck usually resembles a carpet tack with 

the point towards the bark; bird peck is usually accompanied 

by discoloration extending for considerable distance along 

the grain and to a much lesser extent across the grain.

Birdseye. Small localized areas in wood with the fibers in‐

dented and otherwise contorted to form few to many small 

circular or elliptical figures remotely resembling birds’ eyes 

on the tangential surface. Sometimes found in sugar maple 

and used for decorative purposes; rare in other hardwood 

species.

Blister. An elevation of the surface of an adherend, some‐

what resembling in shape a blister on human skin; its bound‐
G–2

General Technical Report FPL–GTR–190

aries may be indefinitely outlined, and it may have burst and 

become flattened. (A blister may be caused by insufficient 

adhesive; inadequate curing time, temperature, or pressure; 

or trapped air, water, or solvent vapor.)

Bloom. Crystals formed on the surface of treated wood by 

exudation and evaporation of the solvent in preservative 

solutions.

Blow. In plywood and particleboard especially, the devel‐

opment of steam pockets during hot pressing of the panel, 

resulting in an internal separation or rupture when pressure 

is released, sometimes with an audible report.

Blue Stain. (See Stain.)

Board. (See Lumber.)

Board Foot. A unit of measurement of lumber represented 

by a board 12 in. long, 12 in. wide, and 1 in. thick or its 

cubic equivalent. In practice, the board foot calculation for 

lumber 1 in. or more in thickness is based on its nominal 

thickness and width and the actual length. Lumber with a 

nominal thickness of less than 1 in. is calculated as 1 in.

Bole. The main stem of a tree of substantial diameter—

roughly, capable of yielding sawtimber, veneer logs, or large 

poles. Seedlings, saplings, and small‐diameter trees have 

stems, not boles.

Bolt. (1) A short section of a tree trunk. (2) In veneer pro‐

duction, a short log of a length suitable for peeling in a 

lathe.

Bond. (1) The union of materials by adhesives. (2) To unite 

materials by means of an adhesive.

Bondability. Term indicating ease or difficulty in bonding a 

material with adhesive.

Bond Failure. Rupture of adhesive bond.

Bondline. The layer of adhesive that attaches two 

adherends.

Bondline Slip. Movement within and parallel to the bond‐

line during shear.

Bond Strength. The unit load applied in tension, compres‐

sion, flexure, peel impact, cleavage, or shear required to 

break an adhesive assembly, with failure occurring in or 

near the plane of the bond.

Bow. The distortion of lumber in which there is a deviation, 

in a direction perpendicular to the flat face, from a straight 

line from end‐to‐end of the piece.

Box Beam. A built‐up beam with solid wood flanges and 

plywood or wood-based panel product webs.

Boxed Heart. The term used when the pith falls entirely 

within the four faces of a piece of wood anywhere in its 

length. Also called boxed pith.

Brashness. A condition that causes some pieces of wood 

to be relatively low in shock resistance for the species and, 

when broken in bending, to fail abruptly without splintering 

at comparatively small deflections.

Breaking Radius. The limiting radius of curvature to which 

wood or plywood can be bent without breaking.

Bright. Free from discoloration.

Broad-Leaved Trees. (See Hardwoods.)

Brown Rot. (See Decay.)

Brown Stain. (See Stain.)

Built-Up Timbers. An assembly made by joining layers of 

lumber together with mechanical fastenings so that the grain 

of all laminations is essentially parallel.

Burl. (1) A hard, woody outgrowth on a tree, more or less 

rounded in form, usually resulting from the entwined growth 

of a cluster of adventitious buds. Such burls are the source 

of the highly figured burl veneers used for purely ornamen‐

tal purposes. (2) In lumber or veneer, a localized severe 

distortion of the grain generally rounded in outline, usually 

resulting from overgrowth of dead branch stubs, varying 

from one to several centimeters (one-half to several inches) 

in diameter; frequently includes one or more clusters of sev‐

eral small contiguous conical protuberances, each usually 

having a core or pith but no appreciable amount of end grain 

(in tangential view) surrounding it.

Butt Joint. (See Joint.)

Buttress. A ridge of wood developed in the angle between a 

lateral root and the butt of a tree, which may extend up the 

stem to a considerable height.

Cambium. A thin layer of tissue between the bark and 

wood that repeatedly subdivides to form new wood and bark 

cells.

Cant. A log that has been slabbed on one or more sides. 

Ordinarily, cants are intended for resawing at right angles 

to their widest sawn face. The term is loosely used. (See 

Flitch.)

Casehardening. A condition of stress and set in dry lumber 

characterized by compressive stress in the outer layers and 

tensile stress in the center or core.

Catalyst. A substance that initiates or changes the rate of 

chemical reaction but is not consumed or changed by the 

reaction.

Cell. A general term for the anatomical units of plant tissue, 

including wood fibers, vessel members, and other elements 

of diverse structure and function.

Cellulose. The carbohydrate that is the principal constituent 

of wood and forms the framework of the wood cells.

Cellulosic Fiberboard. (See Wood-Based Composite 

Panel.)
G–3

Glossary

Check. A lengthwise separation of the wood that usually 

extends across the rings of annual growth and commonly 

results from stresses set up in wood during seasoning.

Chemical Brown Stain. (See Stain.)

Chipboard. A paperboard used for many purposes that may 

or may not have specifications for strength, color, or other 

characteristics. It is normally made from paper stock with a 

relatively low density in the thickness of 0.1524 mm (0.006 

in.) and up.

Cleavage. In an adhesively bonded joint, a separation in the 

joint caused by a wedge or other crack-opening-type action.

Close Grained. (See Grain.)

Coarse Grained. (See Grain.)

Cohesion. The state in which the constituents of a mass of 

material are held together by chemical and physical forces.

Cold Pressing. A bonding operation in which an assembly 

is subjected to pressure without the application of heat.

Collapse. The flattening of single cells or rows of cells in 

heartwood during the drying or pressure treatment of wood. 

Often characterized by a caved‐in or corrugated appearance 

of the wood surface.

Compartment Kiln. (See Kiln.)

Composite Assembly. A combination of two or more mate‐

rials bonded together that perform as a single unit.

Composite Panel. (See Wood-Based Composite Panel.)

Compound Curvature. Wood bent to a compound curva‐

ture, no element of which is a straight line.

Compreg. Wood in which the cell walls have been impreg‐

nated with synthetic resin and compressed to give it reduced 

swelling and shrinking characteristics and increased density 

and strength properties.

Compression Failure. Deformation of the wood fibers re‐

sulting from excessive compression along the grain either 

in direct end compression or in bending. It may develop in 

standing trees due to bending by wind or snow or to internal 

longitudinal stresses developed in growth, or it may result 

from stresses imposed after the tree is cut. In surfaced lum‐

ber, compression failures may appear as fine wrinkles across 

the face of the piece.

Compression Wood. Abnormal wood formed on the lower 

side of branches and inclined trunks of softwood trees. 

Compression wood is identified by its relatively wide an‐

nual rings (usually eccentric when viewed on cross section 

of branch or trunk), relatively large amount of latewood 

(sometimes more than 50% of the width of the annual rings 

in which it occurs), and its lack of demarcation between 

earlywood and latewood in the same annual rings. Compres‐

sion wood shrinks excessively longitudinally, compared 

with normal wood.

Conditioning (pre and post). The exposure of a material to 

the influence of a prescribed atmosphere for a stipulated pe‐

riod of time or until a stipulated relation is reached between 

material and atmosphere.

Conifer. (See Softwoods.)

Connector, Timber. Metal rings, plates, or grids that are 

embedded in the wood of adjacent members, as at the bolted 

points of a truss, to increase the strength of the joint.

Consistency. That property of a liquid adhesive by virtue of 

which it tends to resist deformation. (Consistency is not a 

fundamental property but is composed of rheological prop‐

erties such as viscosity, plasticity, and other phenomena.)

Construction Adhesive. (See Adhesive.)

Contact Angle. The angle between a substrate plane and the 

free surface of a liquid droplet at the line of contact with the 

substrate.

Cooperage. Containers consisting of two round heads and a 

body composed of staves held together with hoops, such as 

barrels and kegs.

Slack Cooperage—Cooperage used as containers for 

dry, semidry, or solid products. The staves are usually 

not closely fitted and are held together with beaded steel, 

wire, or wood hoops.

Tight Cooperage—Cooperage used as containers for 

liquids, semisolids, or heavy solids. Staves are well fitted 

and held tightly with cooperage-grade steel hoops.

Copolymer. Substance obtained when two or more types of 

monomers polymerize.

Corbel. A projection from the face of a wall or column sup‐

porting a weight.

Core Stock. A solid or discontinuous center ply used in pan‐

el‐type glued structures (such as furniture panels and solid 

or hollowcore doors).

Coupling Agent. A molecule with different or like function‐

al groups that is capable of reacting with surface molecules 

of two different substances, thereby chemically bridging the 

substances.

Covalent Bond. A chemical bond that results when elec‐

trons are shared by two atomic nuclei.

Creep. (1) Time-dependent deformation of a wood member 

under sustained wood. (2) In an adhesive, the time-depen‐

dent increase in strain resulting from a sustained stress.

Crook. The distortion of lumber in which there is a devia‐

tion, in a direction perpendicular to the edge, from a straight 

line from end-to‐end of the piece.

Crossband. To place the grain of layers of wood at right 

angles in order to minimize shrinking and swelling; also, 

in plywood of three or more plies, a layer of veneer whose 

grain direction is at right angles to that of the face plies.
G–4

General Technical Report FPL–GTR–190

Cross Break. A separation of the wood cells across the 

grain. Such breaks may be due to internal stress resulting 

from unequal longitudinal shrinkage or to external forces.

Cross Grained. (See Grain.)

Cross‐Link. An atom or group connecting adjacent mol‐

ecules in a complex molecular structure.

Cup. A distortion of a board in which there is a deviation 

flatwise from a straight line across the width of the board.

Cure. To change the properties of an adhesive by chemi‐

cal reaction (which may be condensation, polymerization, 

or vulcanization) and thereby develop maximum strength. 

Generally accomplished by the action of heat or a catalyst, 

with or without pressure.

Curing Agent. (See Hardener.)

Curing Temperature. (See Temperature, Curing.)

Curing Time. (See Time, Curing.)

Curly Grained. (See Grain.)

Curtain Coating. Applying liquid adhesive to an adherend 

by passing the adherend under a thin curtain of liquid falling 

by gravity or pressure.

Cut Stock. (See Lumber for Dimension.)

Cuttings. In hardwoods, portions of a board or plank having 

the quality required by a specific grade or for a particular 

use. Obtained from a board by crosscutting or ripping.

Decay. The decomposition of wood substance by fungi.

Advanced (Typical) Decay—The older stage of decay 

in which the destruction is readily recognized because 

the wood has become punky, soft and spongy, stringy, 

ringshaked, pitted, or crumbly. Decided discoloration or 

bleaching of the rotted wood is often apparent.

Brown Rot—In wood, any decay in which the attack con‐

centrates on the cellulose and associated carbohydrates 

rather than on the lignin, producing a light to dark brown 

friable residue—hence loosely termed “dry rot.” An ad‐

vanced stage where the wood splits along rectangular 

planes, in shrinking, is termed “cubical rot.”

Dry Rot—A term loosely applied to any dry, crumbly rot 

but especially to that which, when in an advanced stage, 

permits the wood to be crushed easily to a dry powder. 

The term is actually a misnomer for any decay, since all 

fungi require considerable moisture for growth.

Incipient Decay—The early stage of decay that has not 

proceeded far enough to soften or otherwise perceptibly 

impair the hardness of the wood. It is usually accompa‐

nied by a slight discoloration or bleaching of the wood.

Heart Rot—Any rot characteristically confined to the 

heartwood. It generally originates in the living tree.

Pocket Rot—Advanced decay that appears in the form of 

a hole or pocket, usually surrounded by apparently sound 

wood.

Soft Rot—A special type of decay developing under very 

wet conditions (as in cooling towers and boat timbers) 

in the outer wood layers, caused by cellulose‐destroying 

microfungi that attack the secondary cell walls and not the 

intercellular layer.

White‐Rot—In wood, any decay or rot attacking both 

the cellulose and the lignin, producing a generally whit‐

ish residue that may be spongy or stringy rot, or occur as 

pocket rot.

Delamination. The separation of layers in laminated wood 

or plywood because of failure of the adhesive, either within 

the adhesive itself or at the interface between the adhesive 

and the adherend.

Delignification. Removal of part or all of the lignin from 

wood by chemical treatment.

Density. As usually applied to wood of normal cellular 

form, density is the mass per unit volume of wood substance 

enclosed within the boundary surfaces of a wood-plus-voids 

complex. It is variously expressed as pounds per cubic foot, 

kilograms per cubic meter, or grams per cubic centimeter at 

a specified moisture content.

Density Rules. A procedure for segregating wood according 

to density, based on percentage of latewood and number of 

growth rings per inch of radius.

Dew Point. The temperature at which a vapor begins 

to deposit as a liquid. Applies especially to water in the 

atmosphere.

Diagonal Grained. (See Grain.)

Diffuse-Porous Wood. Certain hardwoods in which the 

pores tend to be uniform in size and distribution throughout 

each annual ring or to decrease in size slightly and gradually 

toward the outer border of the ring.

Dimension. (See Lumber for Dimension.)

Dipole–Dipole Forces. Intermolecular attraction forces 

between polar molecules that result when positive and nega‐

tive poles of molecules are attracted to one another.

Dote. “Dote,” “doze,” and “rot” are synonymous with “de‐

cay” and are any form of decay that may be evident as either 

a discoloration or a softening of the wood.

Double Spread. (See Spread.)

Dry‐Bulb Temperature. The temperature of air as indicated 

by a standard thermometer. (See Psychrometer.)

Dry Kiln. (See Kiln.)

Dry Rot. (See Decay.)
G–5

Glossary

Dry Strength. The strength of an adhesive joint deter‐

mined immediately after drying under specified conditions 

or after a period of conditioning in a standard laboratory 

atmosphere.

Drywall. Panel product used as an interior wall and ceiling 

covering made of gypsum plaster with paper facings. The 

gypsum plaster may be reinforced with recycled fiber.

Durability. A general term for permanence or resistance to 

deterioration. Frequently used to refer to the degree of re‐

sistance of a species of wood to attack by wood‐destroying 

fungi under conditions that favor such attack. In this con‐

nection, the term “decay resistance” is more specific. As 

applied to bondlines, the life expectancy of the structural 

qualities of the adhesive under the anticipated service condi‐

tions of the structure.

Earlywood. The portion of the growth ring that is formed 

during the early part of the growing season. It is usually less 

dense and weaker mechanically than latewood.

Edge Grained. (See Grain.)

Edge Joint. (See Joint.)

Elastomer. A macromolecular material that, at room tem‐

perature, is deformed by application of a relatively low force 

and is capable of recovering substantially in size and shape 

after removal of the force.

Embrittlement. A loss in strength or energy absorption 

without a corresponding loss in stiffness. Clear, straightgrained wood is generally considered a ductile material; 

chemical treatments and elevated temperatures can alter the 

original chemical composition of wood, thereby embrittling 

the wood. 

Encased Knot. (See Knot.)

End Grained. (See Grain.)

End Joint. (See Joint.)

Equilibrium Moisture Content. The moisture content at 

which wood neither gains nor loses moisture when sur‐

rounded by air at a given relative humidity and temperature.

Excelsior. (See Wood Wool.)

Extender. A substance, generally having some adhesive ac‐

tion, added to an adhesive to reduce the amount of the pri‐

mary binder required per unit area.

Exterior Plywood. (See Wood-Based Composite Panel.)

Extractive. Substances in wood, not an integral part of the 

cellular structure, that can be removed by solution in hot or 

cold water, ether, benzene, or other solvents that do not react 

chemically with wood components.

Extrusion Spreading. A method of adhesive application 

in which adhesive is forced through small openings in the 

spreader head.

Factory and Shop Lumber. (See Lumber.)

Failure, Adherend. Rupture of an adhesive joint, such that 

the separation appears to be within the adherend.

Failure, Adhesive. Rupture of an adhesive joint, such that 

the plane of separation appears to be at the adhesive–adher‐

end interface.

Failure, Cohesive. Rupture of an adhesive joint, such that 

the separation appears to be within the adhesive.

Feed Rate. The distance that the stock being processed 

moves during a given interval of time or operational cycle.

Fiber, Wood. A wood cell comparatively long (≤40 to 300 

mm, ≤1.5 to 12 in.), narrow, tapering, and closed at both 

ends.

Fiberboard. (See Wood-Based Composite Panel.)

Fiber Saturation Point. The stage in the drying or wet‐

ting of wood at which the cell walls are saturated and the 

cell cavities free from water. It applies to an individual cell 

or group of cells, not to whole boards. It is usually taken 

as approximately 30% moisture content, based on ovendry 

weight.

Fibril. A threadlike component of cell walls, invisible under 

a light microscope.

Figure. The pattern produced in a wood surface by annual 

growth rings, rays, knots, deviations from regular grain such 

as interlocked and wavy grain, and irregular coloration.

Filler. In woodworking, any substance used to fill the holes 

and irregularities in planed or sanded surfaces to decrease 

the porosity of the surface before applying finish coatings. 

As applied to adhesives, a relatively nonadhesive substance 

added to an adhesive to improve its working properties, 

strength, or other qualities.

Fine Grained. (See Grain.)

Fingerjoint. (See Joint.)

Finish (Finishing). (1) Wood products such as doors, stairs, 

and other fine work required to complete a building, espe‐

cially the interior. (2) Coatings of paint, varnish, lacquer, 

wax, or other similar materials applied to wood surfaces to 

protect and enhance their durability or appearance.

Fire Endurance. A measure of the time during which a ma‐

terial or assembly continues to exhibit fire resistance under 

specified conditions of test and performance.

Fire Resistance. The property of a material or assembly 

to withstand fire or give protection from it. As applied to 

elements of buildings, it is characterized by the ability to 

confine a fire or to continue to perform a given structural 

function, or both.

Fire Retardant. (See Flame Retardant.)
Fire-Retardant-Treated Wood. As specified in building 

codes, a wood product that has been treated with chemicals 

by a pressure process or treated during the manufacturing 

process for the purpose of reducing its flame spread perfor‐

mance in an ASTM E 84 test conducted for 30 min to per‐

formance levels specified in the codes. 

Flake. A small flat wood particle of predetermined dimen‐

sions, uniform thickness, with fiber direction essentially 

in the plane of the flake; in overall character resembling a 

small piece of veneer. Produced by special equipment for 

use in the manufacture of flakeboard.

Flakeboard. (See Wood-Based Composite Panel.)

Flame Retardant. A treatment, coating, or chemicals that 

when applied to wood products delays ignition and reduces 

the flame spread of the product. 

Flame Spread. The propagation of a flame away from the 

source of ignition across the surface of a liquid or a solid, or 

through the volume of a gaseous mixture.

Flat Grained. (See Grain.)

Flat Sawn. (See Grain.)

Flecks. (See Rays, Wood.)

Flitch. A portion of a log sawn on two or more faces—com‐

monly on opposite faces leaving two waney edges. When 

intended for resawing into lumber, it is resawn parallel to its 

original wide faces. Or, it may be sliced or sawn into veneer, 

in which case the resulting sheets of veneer laid together 

in the sequence of cutting are called a flitch. The term is 

loosely used. (See Cant.)

Framing. Lumber used for the structural member of a 

building, such as studs and joists.

Full‐Cell Process. Any process for impregnating wood with 

preservatives or chemicals in which a vacuum is drawn to 

remove air from the wood before admitting the preservative. 

This favors heavy adsorption and retention of preservative 

in the treated portions.

Furnish. Wood material that has been reduced for incorpo‐

ration into conventional wood-based composites; including 

flakes, particles, and fiber.

Gelatinous Fibers. Modified fibers that are associated with 

tension wood in hardwoods.

Girder. A large or principal beam used to support concen‐

trated loads at isolated points along its length.

Gluability. (See Bondability.)

Glue. Originally, a hard gelatin obtained from hides, 

tendons, cartilage, bones, etc., of animals. Also, an adhe‐

sive prepared from this substance by heating with water. 

Through general use, the term is now synonymous with the 

term “adhesive.”

Glue Laminating. Production of structural or nonstructural 

wood members by bonding two or more layers of wood to‐

gether with adhesive.

Glued Laminated Timber (Glulam). A manufactured 

structural timber product composed of layers of dimensional 

lumber glued together. 

Glueline. (See Bondline.)

Grade. The designation of the quality of a manufactured 

piece of wood or of logs.

Grain. The direction, size, arrangement, appearance, or 

quality of the fibers in wood or lumber. To have a specific 

meaning the term must be qualified.

Close‐Grained (Fine-Grained) Wood—Wood with nar‐

row, inconspicuous annual rings. The term is sometimes 

used to designate wood having small and closely spaced 

pores, but in this sense the term “fine textured” is more 

often used.

Coarse‐Grained Wood—Wood with wide conspicuous 

annual rings in which there is considerable difference 

between earlywood and latewood. The term is sometimes 

used to designate wood with large pores, such as oak, 

keruing, meranti, and walnut, but in this sense, the term 

“open-grained” is more often used.

Cross‐Grained Wood—Wood in which the fibers deviate 

from a line parallel to the sides of the piece. Cross grain 

may be either diagonal or spiral grain or a combination of 

the two.

Curly‐Grained Wood—Wood in which the fibers are dis‐

torted so that they have a curled appearance, as in “birds‐

eye” wood. The areas showing curly grain may vary up to 

several inches in diameter.

Diagonal‐Grained Wood—Wood in which the annual 

rings are at an angle with the axis of a piece as a result of 

sawing at an angle with the bark of the tree or log. A form 

of cross‐grain.

Edge‐Grained Lumber—Lumber that has been sawed 

so that the wide surfaces extend approximately at right 

angles to the annual growth rings. Lumber is considered 

edge grained when the rings form an angle of 45° to 90° 

with the wide surface of the piece.

End‐Grained Wood—The grain as seen on a cut made 

at a right angle to the direction of the fibers (such as on a 

cross section of a tree).

Fiddleback-Grained Wood—Figure produced by a 

type of fine wavy grain found, for example, in species of 

maple; such wood being traditionally used for the backs 

of violins.

Flat‐Grained (Flat-Sawn) Lumber—Lumber that has 

been sawn parallel to the pith and approximately tangent 

G–6

General Technical Report FPL–GTR–190
to the growth rings. Lumber is considered flat grained 

when the annual growth rings make an angle of less than 

45° with the surface of the piece.

Interlocked‐Grained Wood—Grain in which the fibers 

put on for several years may slope in a right‐handed di‐

rection, and then for a number of years the slope reverses 

to a left‐handed direction, and later changes back to a 

right‐handed pitch, and so on. Such wood is exceedingly 

difficult to split radially, though tangentially it may split 

fairly easily.

Open‐Grained Wood—Common classification for 

woods with large pores such as oak, keruing, meranti, and 

walnut. Also known as “coarse textured.”

Plainsawn Lumber—Another term for flat‐grained 

lumber.

Quartersawn Lumber—Another term for edge‐grained 

lumber.

Side‐Grained Wood—Another term for flat‐grained 

lumber.

Slash‐Grained Wood—Another term for flat‐grained 

lumber.

Spiral‐Grained Wood—Wood in which the fibers 

take a spiral course about the trunk of a tree instead of 

the normal vertical course. The spiral may extend in a 

right‐handed or left‐handed direction around the tree 

trunk. Spiral grain is a form of cross grain.

Straight‐Grained Wood—Wood in which the fibers run 

parallel to the axis of a piece.

Vertical‐Grained Lumber—Another term for 

edge‐grained lumber.

Wavy‐Grained Wood—Wood in which the fibers collec‐

tively take the form of waves or undulations.

Green. Freshly sawed or undried wood. Wood that has be‐

come completely wet after immersion in water would not 

be considered green but may be said to be in the “green 

condition.”

Growth Ring. The layer of wood growth put on a tree dur‐

ing a single growing season. In the temperate zone, the an‐

nual growth rings of many species (for example, oaks and 

pines) are readily distinguished because of differences in the 

cells formed during the early and late parts of the season. 

In some temperate zone species (black gum and sweetgum) 

and many tropical species, annual growth rings are not eas‐

ily recognized.

Gum. A comprehensive term for nonvolatile viscous plant 

exudates, which either dissolve or swell up in contact with 

water. Many substances referred to as gums such as pine and 

spruce gum are actually oleoresins.

Hardboard. (See Wood-Based Composite Panel.)

Hardener. A substance or mixture of substances that is part 

of an adhesive and is used to promote curing by taking part 

in the reaction.

Hardness. A property of wood that enables it to resist 

indentation.

Hardwoods. Generally one of the botanical groups of trees 

that have vessels or pores and broad leaves, in contrast to 

the conifers or softwoods. The term has no reference to the 

actual hardness of the wood.

Heart Rot. (See Decay.)

Heartwood. The wood extending from the pith to the sap‐

wood, the cells of which no longer participate in the life 

processes of the tree. Heartwood may contain phenolic com‐

pounds, gums, resins, and other materials that usually make 

it darker and more decay resistant than sapwood.

Hemicellulose. A celluloselike material (in wood) that is 

easily decomposable as by dilute acid, yielding several dif‐

ferent simple sugars.

Hertz. A unit of frequency equal to one cycle per second.

High Frequency Curing. (See Radiofrequency Curing.)

Hollow‐Core Construction. A panel construction with 

faces of plywood, hardboard, or similar material bonded to a 

framed‐core assembly of wood lattice, paperboard rings, or 

the like, which support the facing at spaced intervals.

Honeycomb Core. A sandwich core material constructed of 

thin sheet materials or ribbons formed to honeycomb-like 

configurations.

Honeycombing. Checks, often not visible at the surface, 

that occur in the interior of a piece of wood, usually along 

the wood rays.

Hot‐Setting Adhesive. (See Adhesive.)

Hydrogen Bond. An intermolecular attraction force that 

results when the hydrogen of one molecule and a pair of 

unshared electrons on an electronegative atom of another 

molecule are attracted to one another.

Hydrophilic. Having a strong tendency to bind or absorb 

water.

Hydrophobic. Having a strong tendency to repel water.

Impreg. Wood in which the cell walls have been impregnat‐

ed with synthetic resin so as to reduce materially its swell‐

ing and shrinking. Impreg is not compressed.

Incising. A pretreatment process in which incisions, slits, or 

perforations are made in the wood surface to increase pen‐

etration of preservative treatments. Incising is often required 

to enhance durability of some difficult-to-treat species, but 

incising reduces strength. 

Increment Borer. An augerlike instrument with a hollow 

bit and an extractor, used to extract thin radial cylinders of 

G–7

Glossary
wood from trees to determine age and growth rate. Also 

used in wood preservation to determine the depth of pen‐

etration of a preservative.

Inorganic-Bonded Composites. Manufactured wood-based 

composites where an inorganic binder, typically gypsum, 

Portland-cement, or magnesia-cement, acts as a continuous 

matrix and fully encapsulates the wood elements.

Intergrown Knot. (See Knot.)

Interior Plywood. (See Wood-Based Composite Panel.)

Interlocked Grained. (See Grain.)

Interlocking Action. (See Mechanical Adhesion.)

Internal Stresses. Stresses that exist within an adhesive 

joint even in the absence of applied external forces.

Interphase. In wood bonding, a region of finite thickness 

as a gradient between the bulk adherend and bulk adhesive 

in which the adhesive penetrates and alters the adherend’s 

properties and in which the presence of the adherend in‐

fluences the chemical and/or physical properties of the 

adhesive.

Intumesce. To expand with heat to provide a low‐density 

film; used in reference to certain fire‐retardant coatings.

Isotropic. Exhibiting the same properties in all directions.

Joint. The junction of two pieces of wood or veneer.

Adhesive Joint—The location at which two adherends 

are held together with a layer of adhesive.

Assembly Joint—Joints between variously shaped parts 

or subassemblies such as in wood furniture (as opposed to 

joints in plywood and laminates that are all quite similar).

Butt Joint—An end joint formed by abutting the squared 

ends of two pieces.

Edge Joint—A joint made by bonding two pieces of 

wood together edge to edge, commonly by gluing. The 

joints may be made by gluing two squared edges as in a 

plain edge joint or by using machined joints of various 

kinds, such as tongued‐and‐grooved joints.

End Joint—A joint made by bonding two pieces of wood 

together end to end, commonly by finger or scarf joint.

Fingerjoint—An end joint made up of several meshing 

wedges or fingers of wood bonded together with an adhe‐

sive. Fingers are sloped and may be cut parallel to either 

the wide or narrow face of the piece.

Lap Joint—A joint made by placing one member partly 

over another and bonding the overlapped portions.

Scarf Joint—An end joint formed by joining with ad‐

hesive the ends of two pieces that have been tapered or 

beveled to form sloping plane surfaces, usually to a feath‐

eredge, and with the same slope of the plane with respect 

to the length in both pieces. In some cases, a step or hook 

may be machined into the scarf to facilitate alignment of 

the two ends, in which case the plane is discontinuous and 

the joint is known as a stepped or hooked scarf joint.

Starved Joint—A glue joint that is poorly bonded be‐

cause an insufficient quantity of adhesive remained in the 

joint.

Sunken Joint—Depression in wood surface at a joint 

(usually an edge joint) caused by surfacing material too 

soon after bonding. (Inadequate time was allowed for 

moisture added with the adhesive to diffuse away from 

the joint.)

Joint Efficiency or Factor. The strength of a joint 

expressed as a percentage of the strength of clear 

straight‐grained material.

Joist. One of a series of parallel beams used to support floor 

and ceiling loads and supported in turn by larger beams, 

girders, or bearing walls.

Kiln. A chamber having controlled air‐flow, temperature, 

and relative humidity for drying lumber. The temperature is 

increased as drying progresses, and the relative humidity is 

decreased.

Kiln Dried. (See Seasoning.)

Knot. That portion of a branch or limb that has been sur‐

rounded by subsequent growth of the stem. The shape of the 

knot as it appears on a cut surface depends on the angle of 

the cut relative to the long axis of the knot.

Encased Knot—A knot whose rings of annual growth are 

not intergrown with those of the surrounding wood.

Intergrown Knot—A knot whose rings of annual growth 

are completely intergrown with those of the surrounding 

wood.

Loose Knot—A knot that is not held firmly in place by 

growth or position and that cannot be relied upon to re‐

main in place.

Pin Knot—A knot that is not more than 12 mm (1/2 in.) 

in diameter.

Sound Knot—A knot that is solid across its face, at least 

as hard as the surrounding wood, and shows no indication 

of decay.

Spike Knot—A knot cut approximately parallel to 

its long axis so that the exposed section is definitely 

elongated.

Laminate. A product made by bonding together two or 

more layers (laminations) of material or materials.

Laminate, Paper‐Based. A multilayered panel made by 

compressing sheets of resin‐impregnated paper together into 

a coherent solid mass.

G–8

General Technical Report FPL–GTR–190
Laminated Strand Lumber (LSL). (See Structural Com‐

posite Lumber.)

Laminated Veneer Lumber (LVL). (See Structural Com‐

posite Lumber.)

Lap Joint. (See Joint.)

Latewood. The portion of the growth ring that is formed af‐

ter the earlywood formation has ceased. It is usually denser 

and stronger mechanically than earlywood.

Latex Paint. A paint containing pigments and a stable water 

suspension of synthetic resins (produced by emulsion po‐

lymerization) that forms an opaque film through coalescence 

of the resin during water evaporation and subsequent curing.

Lathe Checks. In rotary-cut and sliced veneer, the fractures 

or checks that develop along the grain of the veneer as the 

knife peels veneer from the log. The knife side of the veneer 

where checks occur is called the loose side. The opposite 

and log side of the veneer where checking usually does not 

occur is called the tight side.

Layup. The process of loosely assembling the adhe‐

sive‐coated components of a unit, particularly a panel, to be 

pressed or clamped.

Lbs/MSGL. Abbreviation for rate of adhesive application 

in pounds of adhesive per 1,000 ft2 of single glueline (bon‐

dline). (See Spread.) When both faces of an adherend are 

spread as in some plywood manufacturing processes, the 

total weight of adhesive applied may be expressed as Lbs/

MDGL (pounds per 1,000 ft2 double glueline). 

Lignin. The second most abundant constituent of wood, 

located principally in the secondary wall and the middle 

lamella, which is the thin cementing layer between wood 

cells. Chemically, it is an irregular polymer of substituted 

propylphenol groups, and thus, no simple chemical formula 

can be written for it.

London Dispersion Forces. Intermolecular attraction forces 

between nonpolar molecules that result when instantaneous 

(nonpermanent) dipoles induce matching dipoles in neigh‐

boring molecules. London forces also exist between polar 

molecules.

Longitudinal. Generally, parallel to the direction of the 

wood fibers.

Loose Knot. (See Knot.)

Lumber. The product of the saw and planing mill for which 

manufacturing is limited to sawing, resawing, passing 

lengthwise through a standard planing machine, crosscutting 

to length, and matching. Lumber may be made from either 

softwood or hardwood (See also Lumber for Dimension.)

Board—Lumber that is less than 38 mm standard (2 in. 

nominal) thickness and greater than 38 mm standard (2 in 

nominal) width. Boards less than 140 mm standard (6 in. 

nominal) width are sometimes called strips. 

Dimension—Lumber with a thickness from 38 mm 

standard (2 in. nominal) up to but not including 114 mm 

standard (5 in. nominal) and a width of greater than 38 

mm standard (2 in. nominal).

Dressed Size—The dimensions of lumber after being 

surfaced with a planing machine. The dressed size is 

usually 1/2 to 3/4 in. less than the nominal or rough size. 

A 2‐ by 4‐in. stud, for example, actually measures about 

1‐1/2 by 3‐1/2 in. (standard 38 by 89 mm).

Factory and Shop Lumber—Lumber intended to be cut 

up for use in further manufacture. It is graded on the per‐

centage of the area that will produce a limited number of 

cuttings of a specified minimum size and quality.

Matched Lumber—Lumber that is edge dressed and 

shaped to make a close tongued‐and‐grooved joint at the 

edges or ends when laid edge to edge or end to end.

Nominal Size—As applied to timber or lumber, the size 

by which it is known and sold in the market (often dif‐

fers from the actual size).

Patterned Lumber—Lumber that is shaped to a pat‐

tern or to a molded form in addition to being dressed, 

matched, or shiplapped, or any combination of these 

workings.

Rough Lumber—Lumber that has not been dressed 

(surfaced) but has been sawed, edged, and trimmed.

Shiplapped Lumber—Lumber that is edge dressed to 

make a lapped joint.

Shipping‐Dry Lumber—Lumber that is partially dried 

to prevent stain and mold in transit.

Shop Lumber—(See Factory and Shop Lumber.)

Side Lumber—A board from the outer portion of the 

log—ordinarily one produced when squaring off a log 

for a tie or timber.

Structural Lumber—Lumber that is intended for use 

where allowable properties are required. The grading of 

structural lumber is based on the strength or stiffness of 

the piece as related to anticipated uses.

Surfaced Lumber—Lumber that is dressed by running 

it through a planer. 

Timbers—Lumber that is standard 114 mm (nominal 5 

in.) or more in least dimension. Timbers may be used as 

beams, stringers, posts, caps, sills, girders, or purlins.

Yard Lumber—A little‐used term for lumber of all sizes 

and patterns that is intended for general building pur‐

poses having no design property requirements.

Lumber for Dimension. The National Dimension Manu‐

facturers Association defines both hardwood and softwood 

dimension components as being cut to a specific size from 

G–9

Glossary
kiln-dried rough lumber, bolts, cants, or logs. Dimension 

components include Flat Stock (solid and laminated) for 

furniture, cabinet, and specialty manufactures. This term has 

largely superceded the terms “hardwood dimension” and 

“dimension parts.” (See also Lumber).

Lumen. In wood anatomy, the cell cavity. 

Manufacturing Defects. Includes all defects or blemishes 

that are produced in manufacturing, such as chipped grain, 

loosened grain, raised grain, torn grain, skips in dressing, hit 

and miss (series of surfaced areas with skips between them), 

variation in sawing, miscut lumber, machine burn, machine 

gouge, mismatching, and insufficient tongue or groove.

Mastic. A material with adhesive properties, usually used in 

relatively thick sections, that can be readily applied by ex‐

trusion, trowel, or spatula. (See Adhesive.)

Matched Lumber. (See Lumber.)

Mechanical Adhesion. Adhesion between surfaces in which 

the adhesive holds the parts together by interlocking action.

Medium‐Density Fiberboard. (See Wood-Based Compos‐

ite Panel.)

Millwork. Planed and patterned lumber for finish work in 

buildings, including items such as sash, doors, cornices, 

panelwork, and other items of interior or exterior trim. Does 

not include flooring, ceiling, or siding.

Mineral Streak. An olive to greenish‐black or brown dis‐

coloration of undetermined cause in hardwoods.

Modified Wood. Wood processed by chemical treatment, 

compression, or other means (with or without heat) to im‐

part properties quite different from those of the original 

wood.

Moisture Content. The amount of water contained in the 

wood, usually expressed as a percentage of the weight of the 

ovendry wood.

Molecular Weight. The sum of the atomic weights of the 

atoms in a molecule.

Moulding. A wood strip having a curved or projecting sur‐

face, used for decorative purposes.

Monomer. A relatively simple molecular compound that can 

react at more than one site to form a polymer.

Mortise. A slot cut into a board, plank, or timber, usually 

edgewise, to receive the tenon of another board, plank, or 

timber to form a joint.

Nanoindentation Hardness. A hardness measurement 

conducted at the nanometer scale. Nanoindentation hard‐

ness uses an extremely small indenter of a hard material and 

specified shape to press into the surface of a specimen with 

sufficient force to cause deformation.

Naval Stores. A term applied to the oils, resins, tars, and 

pitches derived from oleoresin contained in, exuded by, or 

extracted from trees, chiefly species of pines (genus Pinus). 

Historically, these were important items in the stores of 

wood sailing vessels.

Nominal‐Size Lumber. (See Lumber for Dimension.)

Nonpolar. (See Polar.)

Nonpressure Process. Any process of treating wood with a 

preservative or fire retardant where pressure is not applied. 

Some examples are surface applications by brushing or brief 

dipping, soaking in preservative oils, or steeping in solu‐

tions of waterborne preservatives; diffusion processes with 

waterborne preservatives; and vacuum treatments.

Oil Paint. A paint containing a suspension of pigments in 

an organic solvent and a drying oil, modified drying oil, 

or synthetic polymer that forms an opaque film through a 

combination of solvent evaporation and curing of the oil or 

polymer.

Old Growth. Timber in or from a mature, naturally estab‐

lished forest. When the trees have grown during most if not 

all of their individual lives in active competition with their 

companions for sunlight and moisture, this timber is usually 

straight and relatively free of knots.

Oleoresin. A solution of resin in an essential oil that oc‐

curs in or exudes from many plants, especially softwoods. 

The oleoresin from pine is a solution of pine resin (rosin) in 

turpentine.

Open Assembly Time. (See Time, Assembly.)

Open Grain. (See Grain.)

Oriented Strandboard. (See Wood-Based Composite 

Panel.)

Oriented Strand Lumber (OSL). (See Structural Com‐

posite Lumber.)

Orthotropic. Having unique and independent properties in 

three mutually orthogonal (perpendicular) planes of symme‐

try. A special case of anisotropy.

Ovendry Wood. Wood dried to a relatively constant weight 

in a ventilated oven at 102 to 105 °C (215 to 220 °F).

Overlay. A thin layer of paper, plastic, film, metal foil, or 

other material bonded to one or both faces of panel products 

or to lumber to provide a protective or decorative face or a 

base for painting.

Paint. Any pigmented liquid, liquifiable, or mastic composi‐

tion designed for application to a substrate in a thin layer 

that converts to an opaque solid film after application.

Pallet. A low wood or metal platform on which material can 

be stacked to facilitate mechanical handling, moving, and 

storage.

Paperboard. The distinction between paper and paperboard 

is not sharp, but broadly speaking, the thicker (greater than 

G–10

General Technical Report FPL–GTR–190
0.3 mm (0.012 in.)), heavier, and more rigid grades of paper 

are called paperboard.

Papreg. Any of various paper products made by impregnat‐

ing sheets of specially manufactured high‐strength paper 

with synthetic resin and laminating the sheets to form a 

dense, moisture‐resistant product.

Parallel Strand Lumber (PSL). (See Structural Compos‐

ite Lumber.)

Parenchyma. Short cells having simple pits and functioning 

primarily in the metabolism and storage of plant food mate‐

rials. They remain alive longer than the tracheids, fibers, and 

vessel elements, sometimes for many years. Two kinds of 

parenchyma cells are recognized—those in vertical strands, 

known more specifically as axial parenchyma, and those in 

horizontal series in the rays, known as ray parenchyma.

Particleboard. (See Wood-Based Composite Panel.)

Particles. The aggregate component of particleboard manu‐

factured by mechanical means from wood. These include 

all small subdivisions of wood such as chips, curls, flakes, 

sawdust, shavings, slivers, strands, wafers, wood flour, and 

wood wool.

Peck. Pockets or areas of disintegrated wood caused by ad‐

vanced stages of localized decay in the living tree. It is usu‐

ally associated with cypress and incense‐cedar. There is no 

further development of peck once the lumber is seasoned.

Peel. To convert a log into veneer by rotary cutting. In an 

adhesively bonded joint, the progressive separation of a 

flexible member from either a rigid member or another flex‐

ible member.

Phloem. The tissues of the inner bark, characterized by 

the presence of sieve tubes and serving for the transport of 

elaborate foodstuffs.

Pile. A long, heavy timber, round or square, that is driven 

deep into the ground to provide a secure foundation for 

structures built on soft, wet, or submerged sites (for exam‐

ple, landing stages, bridge abutments).

Pin Knot. (See Knot.)

Pitch Pocket. An opening extending parallel to the annual 

growth rings and containing, or that has contained, pitch, 

either solid or liquid.

Pitch Streaks. A well‐defined accumulation of pitch in a 

more or less regular streak in the wood of certain conifers.

Pith. The small, soft core occurring near the center of a tree 

trunk, branch, twig, or log.

Pith Fleck. A narrow streak, resembling pith on the surface 

of a piece; usually brownish, up to several centimeters long; 

results from burrowing of larvae in the growing tissues of 

the tree.

Plainsawn. (See Grain.)

Planing Mill Products. Products worked to pattern, such as 

flooring, ceiling, and siding.

Plank. A broad, thick board laid with its wide dimension 

horizontal and used as a bearing surface.

Plasticizing Wood. Softening wood by hot water, steam, or 

chemical treatment to increase its moldability.

Plywood. (See Wood-Based Composite Panel.)

Pocket Rot. (See Decay.)

Polar. Characteristic of a molecule in which the positive and 

negative electrical charges are permanently separated, as op‐

posed to nonpolar molecules in which the charges coincide. 

Water, alcohol, and wood are polar in nature; most hydro‐

carbon liquids are not.

Polymer. A compound formed by the reaction of simple 

molecules having functional groups that permit their combi‐

nation to proceed to high molecular weights under suitable 

conditions. Polymers may be formed by polymerization 

(addition polymer) or polycondensation (condensation poly‐

mer). When two or more different monomers are involved, 

the product is called a copolymer.

Polymerization. A chemical reaction in which the mol‐

ecules of a monomer are linked together to form large mol‐

ecules whose molecular weight is a multiple of that of the 

original substance. When two or more different monomers 

are involved, the process is called copolymerization.

Pore. (See Vessel Elements.)

Postformed Plywood. (See Wood-Based Composite 

Panel.)

Post Cure. (1) A treatment (normally involving heat) ap‐

plied to an adhesive assembly following the initial cure, 

to complete cure, or to modify specific properties. (2) To 

expose an adhesive assembly to an additional cure, follow‐

ing the initial cure; to complete cure; or to modify specific 

properties.

Pot Life. (See Working Life.)

Precure. Condition of too much cure, set, or solvent loss of 

the adhesive before pressure is applied, resulting in inad‐

equate flow, transfer, and bonding.

Preservative. Any substance that, for a reasonable length of 

time, is effective in preventing the development and action 

of wood‐rotting fungi, borers of various kinds, and harmful 

insects that deteriorate wood.

Pressure Process. Any process of treating wood in a closed 

container whereby the preservative or fire retardant is forced 

into the wood under pressures greater than one atmosphere. 

Pressure is generally preceded or followed by vacuum, as in 

the vacuum‐pressure and empty‐cell processes respectively; 

G–11

Glossary
or they may alternate, as in the full‐cell and alternating‐pres‐

sure processes.

Progressive Kiln. (See Kiln.)

Psychrometer. An instrument for measuring the amount of 

water vapor in the atmosphere. It has both a dry‐bulb and 

wet‐bulb thermometer. The bulb of the wet‐bulb thermom‐

eter is kept moistened and is, therefore, cooled by evapora‐

tion to a temperature lower than that shown by the dry‐bulb 

thermometer. Because evaporation is greater in dry air, the 

difference between the two thermometer readings will be 

greater when the air is dry than when it is moist.

Quartersawn. (See Grain.)

Radial. Coincident with a radius from the axis of the tree 

or log to the circumference. A radial section is a lengthwise 

section in a plane that passes through the centerline of the 

tree trunk.

Radiofrequency (RF) Curing. Curing of bondlines by the 

application of radiofrequency energy. (Sometimes called 

high‐frequency curing.)

Rafter. One of a series of structural members of a roof de‐

signed to support roof loads. The rafters of a flat roof are 

sometimes called roof joists.

Raised Grain. A roughened condition of the surface of 

dressed lumber in which the hard latewood is raised above 

the softer earlywood but not torn loose from it.

Rays, Wood. Strips of cells extending radially within a tree 

and varying in height from a few cells in some species to 4 

or more inches in oak. The rays serve primarily to store food 

and transport it horizontally in the tree. On quartersawn oak, 

the rays form a conspicuous figure, sometimes referred to as 

flecks.

Reaction Wood. Wood with more or less distinctive ana‐

tomical characters, formed typically in parts of leaning or 

crooked stems and in branches. In hardwoods, this consists 

of tension wood, and in softwoods, compression wood.

Relative Humidity. Ratio of the amount of water vapor 

present in the air to that which the air would hold at satura‐

tion at the same temperature. It is usually considered on the 

basis of the weight of the vapor but, for accuracy, should be 

considered on the basis of vapor pressures.

Resilience. The property whereby a strained body gives up 

its stored energy on the removal of the deforming force.

Resin. (1) Solid, semisolid, or pseudosolid resin—An or‐

ganic material that has an indefinite and often high molecu‐

lar weight, exhibits a tendency to flow when subjected to 

stress, usually has a softening or melting range, and usually 

fractures conchoidally. (2) Liquid resin—an organic poly‐

meric liquid that, when converted to its final state for use, 

becomes a resin.

Resin Ducts. Intercellular passages that contain and trans‐

mit resinous materials. On a cut surface, they are usually 

inconspicuous. They may extend vertically parallel to the 

axis of the tree or at right angles to the axis and parallel to 

the rays.

Retention by Assay. The determination of preservative 

retention in a specific zone of treated wood by extraction or 

analysis of specified samples.

Rheology. The study of the deformation and flow of matter.

Ring Failure. A separation of the wood during seasoning, 

occurring along the grain and parallel to the growth rings. 

(See Shake.)

Ring‐Porous Woods. A group of hardwoods in which the 

pores are comparatively large at the beginning of each an‐

nual ring and decrease in size more or less abruptly toward 

the outer portion of the ring, thus forming a distinct inner 

zone of pores, known as the earlywood, and an outer zone 

with smaller pores, known as the latewood.

Ring Shake. (See Shake.)

Rip. To cut lengthwise, parallel to the grain.

Roll Spreading. Application of a film of a liquid material to 

a surface by means of rollers.

Room‐Temperature‐Setting Adhesive. (See Adhesive.)

Rot. (See Decay.)

Rotary‐Cut Veneer. (See Veneer.)

Rough Lumber. (See Lumber.)

Sap Stain. (See Stain.)

Sapwood. The wood of pale color near the outside of the 

log. Under most conditions, the sapwood is more suscep‐

tible to decay than heartwood.

Sash. A frame structure, normally glazed (such as a win‐

dow), that is hung or fixed in a frame set in an opening.

Sawn Veneer. (See Veneer.)

Saw Kerf. (1) Grooves or notches made in cutting with a 

saw. (2) That portion of a log, timber, or other piece of wood 

removed by the saw in parting the material into two pieces.

Scarf Joint. (See Joint.)

Schedule, Kiln Drying. A prescribed series of dry‐ and 

wet‐bulb temperatures and air velocities used in drying a 

kiln charge of lumber or other wood products.

Seasoning. Removing moisture from green wood to im‐

prove its serviceability.

Air Dried—Dried by exposure to air in a yard or shed, 

without artificial heat.

Kiln Dried—Dried in a kiln with the use of artificial heat.

G–12

General Technical Report FPL–GTR–190
Second Growth. Timber that has grown after the removal, 

whether by cutting, fire, wind, or other agency, of all or a 

large part of the previous stand.

Semitransparent Stain. A suspension of pigments in either 

a drying oil–organic solvent mixture or a water–polymer 

emulsion, designed to color and protect wood surfaces by 

penetration without forming a surface film and without hid‐

ing wood grain.

Set. A permanent or semipermanent deformation. In refer‐

ence to adhesives, to convert an adhesive into a fixed or 

hardened state by chemical or physical action, such as con‐

densation, polymerization, oxidation, vulcanization, gela‐

tion, hydration, or evaporation of volatile constituents.

Shake. A separation along the grain, the greater part of 

which occurs between the rings of annual growth. Usually 

considered to have occurred in the standing tree or during 

felling.

Shakes. In construction, shakes are a type of shingle 

usually hand cleft from a bolt and used for roofing or 

weatherboarding.

Shaving. A small wood particle of indefinite dimensions 

developed incidental to certain woodworking operations 

involving rotary cutterheads usually turning in the direc‐

tion of the grain. This cutting action produces a thin chip of 

varying thickness, usually feathered along at least one edge 

and thick at another and generally curled.

Shear. In an adhesively bonded joint, stress, strain, or fail‐

ure resulting from applied forces that tends to cause adjacent 

planes of a body to slide parallel in opposite directions.

Sheathing. The structural covering, usually of boards, 

building fiberboards, plywood, or oriented strandboard, 

placed over exterior studding or rafters of a structure.

Shelf Life. (See Storage Life.)

Shiplapped Lumber. (See Lumber.)

Shipping‐Dry Lumber. (See Lumber.)

Shop Lumber. (See Lumber.)

Side Grained. (See Grain.)

Side Lumber. (See Lumber.)

Siding. The finish covering of the outside wall of a frame 

building, whether made of horizontal weatherboards, verti‐

cal boards with battens, shingles, or other material.

Slash Grained. (See Grain.)

Sliced Veneer. (See Veneer.)

Soft Rot. (See Decay.)

Softwoods. Generally, one of the botanical groups of trees 

that have no vessels and in most cases have needlelike or 

scalelike leaves, the conifers, also the wood produced by 

such trees. The term has no reference to the actual hardness 

of the wood.

Solid Color Stains (Opaque Stains). A suspension of pig‐

ments in either a drying oil–organic solvent mixture or a wa‐

ter–polymer emulsion designed to color and protect a wood 

surface by forming a film. Solid color stains are similar to 

paints in application techniques and in performance.

Solids Content. The percentage of weight of the nonvolatile 

matter in an adhesive.

Solvent Adhesive. (See Adhesive.)

Sound Knot. (See Knot.)

Specific Adhesion. Adhesion between surfaces that are held 

together by valence forces of the same type as those that 

give rise to cohesion.

Specific Gravity. As applied to wood, the ratio of the oven‐

dry weight of a sample to the weight of a volume of water 

equal to the volume of the sample at a specified moisture 

content (green, air dry, or ovendry).

Spike Knot. (See Knot.)

Spiral Grained. (See Grain.)

Spread. The quantity of adhesive per unit joint area applied 

to an adherend. (See Lbs/MSGL.)

Single spread—Refers to application of adhesive to only 

one adherend of a joint.

Double spread—Refers to application of adhesive to 

both adherends of a joint.

Squeezeout. Bead of adhesive squeezed out of a joint when 

pressure is applied.

Stain. A discoloration in wood that may be caused by such 

diverse agencies as micro‐organisms, metal, or chemicals. 

The term also applies to materials used to impart color to 

wood.

Blue Stain—A bluish or grayish discoloration of the 

sapwood caused by the growth of certain dark‐colored 

fungi on the surface and in the interior of the wood; made 

possible by the same conditions that favor the growth of 

other fungi.

Brown Stain—A rich brown to deep chocolate‐brown 

discoloration of the sapwood of some pines caused by a 

fungus that acts much like the blue‐stain fungi.

Chemical Brown Stain—A chemical discoloration of 

wood, which sometimes occurs during the air drying or 

kiln drying of several species, apparently caused by the 

concentration and modification of extractives. 

Sap Stain—A discoloration of the sapwood caused by the 

growth of certain fungi on the surface and in the interior 

of the wood; made possible by the same conditions that 

favor the growth of other fungi. 

G–13

Glossary
Sticker Stain—A brown or blue stain that develops in 

seasoning lumber where it has been in contact with the 

stickers.

Starved Joint. (See Joint.)

Static Bending. Bending under a constant or slowly applied 

load; flexure.

Staypak. Wood that is compressed in its natural state (that 

is, without resin or other chemical treatment) under con‐

trolled conditions of moisture, temperature, and pressure 

that practically eliminate springback or recovery from com‐

pression. The product has increased density and strength 

characteristics.

Stickers. Strips or boards used to separate the layers of lum‐

ber in a pile and thus improve air circulation.

Sticker Stain. (See Stain.)

Storage Life. The period of time during which a packaged 

adhesive can be stored under specific temperature conditions 

and remain suitable for use. Sometimes called shelf life.

Straight Grained. (See Grain.)

Strand. (1) A type of wood flake with a high aspect ratio 

which allows for orientation. It is used in oriented strand‐

board, oriented strand lumber, and laminated strand lumber. 

(2) A wood element with a high aspect ratio manufactured 

from veneer. It is used in parallel strand lumber.

Strength. (1) The ability of a member to sustain stress 

without failure. (2) In a specific mode of test, the maximum 

stress sustained by a member loaded to failure.

Strength Ratio. The hypothetical ratio of the strength of 

a structural member to that which it would have if it con‐

tained no strength‐reducing characteristics (such as knots, 

slope‐of‐grain, shake).

Stress‐Wave Timing. A method of measuring the apparent 

stiffness of a material by measuring the speed of an induced 

compression stress as it propagates through the material.

Stressed‐Skin Construction. A construction in which pan‐

els are separated from one another by a central partition of 

spaced strips with the whole assembly bonded so that it acts 

as a unit when loaded.

Stringer. A timber or other support for cross members in 

floors or ceilings. In stairs, the support on which the stair 

treads rest.

Structural Composite Lumber (SCL). (Wood elements 

glued together to form products that are similar in size to 

solid-sawn lumber)

Laminated Strand Lumber (LSL)—Similar to ori‐

ented strand lumber with somewhat longer strands.

Laminated Veneer Lumber (LVL)—Structural com‐

posite lumber manufactured from veneers laminated 

into a panel with the grain of all veneer running parallel 

to each other. The resulting panel is ripped to common 

lumber dimensions.

Oriented Strand Lumber (OSL)—Structural com‐

posite lumber made from wood strand elements similar 

to those used in oriented strand board. The strands are 

oriented primarily along the length of the member. 

Parallel Strand Lumber (PSL)—Structural composite 

lumber made from high aspect ratio wood strand ele‐

ments manufactured from veneer oriented primarily 

along the length of the member. It is manufactured in 

billets and cut to lumber dimensions.

Structural Lumber. (See Lumber.)

Structural Timbers. Pieces of wood of relatively large size, 

the strength or stiffness of which is the controlling element 

in their selection and use. Examples of structural timbers are 

trestle timbers (stringers, caps, posts, sills, bracing, bridge 

ties, guardrails); car timbers (car framing, including upper 

framing, car sills); framing for building (posts, sills, gird‐

ers); ship timber (ship timbers, ship decking); and crossarms 

for poles.

Stud. One of a series of slender wood structural members 

used as supporting elements in walls and partitions.

Substrate. A material upon the surface of which an adhe‐

sive‐containing substance is spread for any purpose, such 

as bonding or coating. A broader term than adherend. (See 

Adherend.)

Surface Inactivation. In adhesive bonding to wood, physi‐

cal and chemical modifications of the wood surface that re‐

sult in reduced ability of an adhesive to properly wet, flow, 

penetrate, and cure.

Surface Tension. The force per unit length acting in the 

surface of a liquid that opposes the increase in area of the 

liquid (spreading).

Surfaced Lumber. (See Lumber.)

Symmetrical Construction. Panels in which the plies on 

one side of a center ply or core are essentially equal in thick‐

ness, grain direction, properties, and arrangement to those 

on the other side of the core.

Tack. The property of an adhesive that enables it to form a 

bond of measurable strength immediately after adhesive and 

adherend are brought into contact under low pressure.

Tangential. Strictly, coincident with a tangent at the cir‐

cumference of a tree or log, or parallel to such a tangent. In 

practice, however, it often means roughly coincident with 

a growth ring. A tangential section is a longitudinal section 

through a tree or limb perpendicular to a radius. Flat-grained 

lumber is sawed tangentially.

Temperature, Curing. The temperature to which an adhe‐

sive or an assembly is subjected to cure the adhesive. The 

G–14

General Technical Report FPL–GTR–190
temperature attained by the adhesive in the process of curing 

(adhesive curing temperature) may differ from the tempera‐

ture of the atmosphere surrounding the assembly (assembly 

curing temperature).

Temperature, Setting. (See Temperature, Curing.)

Tenon. A projecting member left by cutting away the wood 

around it for insertion into a mortise to make a joint.

Tension. In an adhesively bonded joint, a uniaxial force 

tending to cause extension of the assembly, or the counter‐

acting force within the assembly that resists extension.

Tension Wood. Abnormal wood found in leaning trees of 

some hardwood species and characterized by the presence 

of gelatinous fibers and excessive longitudinal shrinkage. 

Tension wood fibers hold together tenaciously, so that sawed 

surfaces usually have projecting fibers and planed surfaces 

often are torn or have raised grain. Tension wood may cause 

warping.

Texture. A term often used interchangeably with grain. 

Sometimes used to combine the concepts of density and 

degree of contrast between earlywood and latewood. In 

this handbook, texture refers to the finer structure of the 

woodrather than the annual rings. (See also Grain.)

Thermoplastic. (1) Capable of being repeatedly softened 

by heat and hardened by cooling. (2) A material that will 

repeatedly soften when heated and harden when cooled.

Thermoset. A cross-linked polymeric material.

Thermosetting. Having the property of undergoing a chem‐

ical reaction by the action of heat, catalyst, ultraviolet light, 

and hardener, leading to a relatively infusible state.

Timbers, Round. Timbers used in the original round form, 

such as poles, piling, posts, and mine timbers.

Timber, Standing. Timber still on the stump.

Timbers. (See Lumber.)

Time, Assembly. The time interval between the spread‐

ing of the adhesive on the adherend and the application of 

pressure or heat, or both, to the assembly. (For assemblies 

involving multiple layers or parts, the assembly time begins 

with the spreading of the adhesive on the first adherend.)

Open Assembly Time—The time interval between the 

spreading of the adhesive on the adherend and the com‐

pletion of assembly of the parts for bonding.

Closed Assembly Time—The time interval between 

completion of assembly of the parts for bonding and the 

application of pressure or heat, or both, to the assembly.

Time, Curing. The period during which an assembly is sub‐

jected to heat or pressure, or both, to cure the adhesive.

Time, Setting. (See Time, Curing.)

Toughness. A quality of wood that permits the material to 

absorb a relatively large amount of energy, to withstand 

repeated shocks, and to undergo considerable deformation 

before breaking.

Tracheid. The elongated cells that constitute the greater part 

of the structure of the softwoods (frequently referred to as 

fibers). Also present in some hardwoods.

Transfer. In wood bonding, the sharing of adhesive between 

a spread and an unspread surface when the two adherends 

are brought into contact.

Transverse. Directions in wood at right angles to the wood 

fibers. Includes radial and tangential directions. A transverse 

section is a section through a tree or timber at right angles to 

the pith.

Treenail. A wooden pin, peg, or spike used chiefly for fas‐

tening planking and ceiling to a framework.

Trim. The finish materials in a building, such as moldings, 

applied around openings (window trim, door trim) or at the 

floor and ceiling of rooms (baseboard, cornice, and other 

moldings).

Truss. An assembly of members, such as beams, bars, rods, 

and the like, so combined as to form a rigid framework. All 

members are interconnected to form triangles.

Twist. A distortion caused by the turning or winding of the 

edges of a board so that the four comers of any face are no 

longer in the same plane.

Tyloses. Masses of parenchyma cells appearing somewhat 

like froth in the pores of some hardwoods, notably the white 

oaks and black locust. Tyloses are formed by the extension 

of the cell wall of the living cells surrounding vessels of 

hardwood.

Ultrasonics. (See Stress‐Wave Timing.)

van der Waal Forces. Physical forces of attraction between 

molecules, which include permanent dipole, induced dipole, 

hydrogen bond, and London dispersion forces.

Vapor Retarder. A material with a high resistance to vapor 

movement, such as foil, plastic film, or specially coated 

paper, that is used in combination with insulation to control 

condensation.

Veneer. A thin layer or sheet of wood.

Rotary‐Cut Veneer—Veneer cut in a lathe that rotates a 

log or bolt, chucked in the center, against a knife.

Sawn Veneer—Veneer produced by sawing.

Sliced Veneer—Veneer that is sliced off a log, bolt, or 

flitch with a knife.

Vertical Grained. (See Grain.)

G–15

Glossary
Vessel Elements. Wood cells in hardwoods of compara‐

tively large diameter that have open ends and are set one 

above the other to form continuous tubes called vessels. The 

openings of the vessels on the surface of a piece of wood are 

usually referred to as pores.

Virgin Growth. The growth of mature trees in the original 

forests.

Viscoelasticity. The ability of a material to simultaneously 

exhibit viscous and elastic responses to deformation.

Viscosity. The ratio of the shear stress existing between 

laminae of moving fluid and the rate of shear between these 

laminae.

Wane. Bark or lack of wood from any cause on edge or cor‐

ner of a piece except for eased edges.

Warp. Any variation from a true or plane surface. Warp 

includes bow, crook, cup, and twist, or any combination 

thereof.

Water Repellent. A liquid that penetrates wood that mate‐

rially retards changes in moisture content and dimensions 

of the dried wood without adversely altering its desirable 

properties.

Water‐Repellent Preservative. A water repellent that con‐

tains a preservative that, after application to wood and dry‐

ing, accomplishes the dual purpose of imparting resistance 

to attack by fungi or insects and also retards changes in 

moisture content.

Weathering. The mechanical or chemical disintegration and 

discoloration of the surface of wood caused by exposure 

to light, the action of dust and sand carried by winds, and 

the alternate shrinking and swelling of the surface fibers 

with the continual variation in moisture content brought by 

changes in the weather. Weathering does not include decay.

Wet Strength. The strength of an adhesive joint determined 

immediately after removal from water in which it has been 

immersed under specified conditions of time, temperature, 

and pressure.

Wet‐Bulb Temperature. The temperature indicated by the 

wet‐bulb thermometer of a psychrometer.

Wettability. A condition of a surface that determines how 

fast a liquid will wet and spread on the surface or if it will 

be repelled and not spread on the surface.

Wetting. The process in which a liquid spontaneously ad‐

heres to and spreads on a solid surface.

White‐Rot. (See Decay.)

Wood-Based Composite Panel. A generic term for a ma‐

terial manufactured from wood veneer, strands, flakes, 

particles, or fibers or other lignocellulosic material and a 

synthetic resin or other binder.

Cellulosic Fiberboard—A generic term for a low-density 

panel made from lignocellulosic fibers characterized by 

an integral bond produced by interfelting of the fibers, to 

which other materials may have been added during manu‐

facture to improve certain properties, but which has not 

been consolidated under heat and pressure as a separate 

stage in manufacture; has a density of less than 496 kg 

m–3 (31 lb ft–3) (specific gravity 0.50 ) but more than 160 

kg m–3 (10 lb ft–3) (specific gravity 0.16).

Exterior Plywood—A general term for plywood bonded 

with a type of adhesive that by systematic tests and ser‐

vice records has proved highly resistant to weather; mi‐

croorganisms; cold, hot, and boiling water; steam; and dry 

heat. 

Fiberboard—A generic term inclusive of panel products 

of various densities manufactured of refined or partially 

refined wood (or other lignocellulosic) fibers. Bonding 

agents may be added.

Flakeboard—A generic term indicating a manufactured 

panel product composed of flakes bonded with a synthetic 

resin.

Hardboard—A generic term for a panel manufactured 

primarily from interfelted lignocellulosic fibers (usually 

wood), consolidated under heat and pressure in a hot press 

to a density of 496 kg m–33 (31 lb ft–3) or greater. May be 

manufactured using either a dry-process or wet-process.

Interior Plywood—A general term for plywood manu‐

factured for indoor use or in construction subjected to 

only temporary moisture. The adhesive used may be inte‐

rior, intermediate, or exterior.

Medium‐Density Fiberboard—A dry-process fiberboard 

manufactured from lignocellulosic fibers combined with 

a synthetic resin or other suitable binder. The panels are 

manufactured to a density of 496 kg m–3 (31 lb ft–3) (0.50 

specific gravity) to 880 kg m–3 (55 lb ft–3) (0.88 specific 

gravity) by the application of heat and pressure by a pro‐

cess in which the interfiber bond is substantially created 

by the added binder.

Oriented Strandboard—A type of flakeboard prod‐

uct composed of strand‐type flakes that are purpose‐

fully aligned in directions that make a panel stronger, 

stiffer, and with improved dimensional properties in the 

alignment directions than a panel with random flake 

orientation.

Particleboard—A panel product manufactured from 

wood particles usually in three layers. For good surface 

characteristics, the outer layers have smaller particles and 

the interior uses coarser particles. The particles in the core 

may or may not be aligned.

Plywood—A glued wood panel made up of relatively thin 

layers of veneer with the grain of adjacent layers at right 

G–16

General Technical Report FPL–GTR–190
angles or of veneer in combination with a core of lumber 

or of reconstituted wood. The usual constructions have an 

odd number of layers.

Wood Failure. The rupturing of wood fibers in strength 

tests of bonded joints usually expressed as the percentage of 

the total area involved that shows such failure. (See Failure, 

Adherend.)

Wood Flour. Wood reduced to finely divided particles, ap‐

proximately the same as those of cereal flours in size, ap‐

pearance, and texture, and passing a 40 to 100 mesh screen.

Wood Substance. The solid material of which wood is 

composed. It usually refers to the extractive‐free solid 

substance of which the cell walls are composed, but this is 

not always true. There is not a wide variation in chemical 

composition or specific gravity between the wood substance 

of various species. (The characteristic differences of species 

are largely due to differences in extractives and variations in 

relative amounts of cell walls and cell cavities.)

Wood-Thermoplastic Composite. Manufactured com‐

posite materials consisting primarily of wood elements and 

thermoplastic. The wood element may either serve as a rein‐

forcement or filler in a continuous thermoplastic matrix, or 

the thermoplastic may act as a binder to the wood element.

Wood Wool. Long, curly, slender strands of wood used 

as an aggregate component for some particleboards and 

cement-bonded composites. Sometimes referred to as 

excelsior.

Workability. The degree of ease and smoothness of cut ob‐

tainable with hand or machine tools.

Working Life. The period of time during which an adhe‐

sive, after mixing with catalyst, solvent, or other compound‐

ing ingredients, remains suitable for use. Also called pot life.

Working Properties. The properties of an adhesive that 

affect or dictate the manner of application to the adherends 

to be bonded and the assembly of the joint before pressure 

application (such as viscosity, pot life, assembly time, set‐

ting time).

Xylem. The portion of the tree trunk, branches, and roots 

that lies between the pith and the cambium (that is the 

wood).

Yard Lumber. (See Lumber.)

G–17

Glossary

INDEX

I–1

Accelerated air-drying, 13-6

Acid copper chromate (ACC):

components, 15-3

effectiveness and leaching, 15-3

retention levels for various wood 

products, 15-4t

Southern pine sapwood stakes retention 

and life span test results, 15-7t to 15-8t

temperature for pressure treating, 15-20

Adherends:

bond strength properties, 10-21

density and porosity, 10-6

ease of bonding by species, 10-7t

extractives on surface, effect on, 10-4

knife- and abrasive-planed surfaces, 

10-2 to 10-3, 10-3fig

mechanical testing of bonded 

 assemblies, 10-20 to 10-21

moisture content:

and dimensional change, 10-6 to 10-7

control, 10-14 to 10-15

U.S. averages, 10-15, 10-15fig

surface preparation, 10-15

surface properties, 10-2 to 10-3

veneer surfaces, 10-3 to 10-4

wettability, 10-4

wood and nonwood composites, 10-5

Adhesives:

affected by:

physical properties, 10-5 to 10-8

surface properties, 10-5

wood density, 10-6

wood moisture content, 10-6

wood porosity, 10-8

analytic chemical and mechanical testing 

of polymers, 10-20 to 10-21

assembly and pressing, 10-16 to 10-17, 

10-17fig

composition, 10-8 to 10-9

consistency, effect of, 10-16 to 10-17, 

10-17fig

definition, 10-1

ease of bonding by species, 10-7t

form and color of types, 10-11t to 10-13t

health and safety, 10-14

mechanical testing of bonded 

 assemblies, 10-20 to 10-21

post-cure conditioning, 10-17

preparation and application, 10-11t to 

10-13t

quality assurance programs, 10-22

short- and long-term performance, 10-21 

to 10-22, 10-22fig

selection, 10-10 to 10-14

spreading, 10-15, 10-16fig

strength and durability, 10-9 to 10-10, 

10-10t, 10-11t to 10-13t

strength properties, 10-21

uses, 10-1, 10-11t to 10-13t

use on veneers, 10-3 to 10-4

use on wood and nonwood composites, 

10-5

use with chemically modified wood, 

10-4

use with fire-retardant-treated woods, 

10-4

use with preservative-treated wood, 10-4

working life, 10-10, 10-14

Adjustment of properties for design use, 

7-10 to 7-13

Advantages of using wood for structures, 

2-2

Afara. See Limba

Afrormosia:

characteristics, 2-19

decay resistance, 2-19

ease of bonding, 10-7t

locality of growth, 2-19

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

uses, 2-19

Air-drying advantages and limitations, 13-6

Albarco:

characteristics, 2-19

locality of growth, 2-19

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

uses, 2-19

workability, 2-19

Alder, red:

characteristics, 2-3

characteristics for painting, 16-5t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

erosion of planed surfaces, 16-12t

locality of growth, 2-3

moisture content, 4-2t

nomenclature, 6-5t

shock resistance, 2-3

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

uses, 2-3

Alkaline copper quat (ACQ):

common types, 15-5

composition of common types, 15-6t

retention levels for various wood 

products, 15-4t

uses, 15-5

Alkyl ammonium compound:

effectiveness, 15-12

in ammoniacal copper quat, 15-5

solubility, 15-9

Almon. See Meranti

Alpha paper, 19-12

Amaranth. See Purpleheart

American Lumber Standard Committee 

(ALSC):

accepting design values for foreign 

species, 7-3, 7-4t

design properties, 7-4

stress grading, 7-2 to 7-3

American Society of Testing and Materials 

(ASTM):

calculating design properties, 7-8 to 7-9

calculating strength ratios, 7-3

calculating wood properties for visual 

stress grades, 7-3

critical radiant flux of floorcovering 

systems using a radiant heat energy 

source (ASTM E 648), 18-5

related test methods, 18-8 to 18-9

cone calorimeter (ASTM E 1354), 18-6, 

18-7

fire tests of roof covering (ASTM E 

108), 18-3

fire-resistance test (ASTM E 119), 18-6 

to 18-7

flame spread (ASTM E 84), 18-3 to 18-

5, 18-4t, 18-6, 18-12

flame spread index for solid sawn 

lumber, 18-4t

NBS smoke chamber (ASTM E 662), 

18-6

American Softwood Lumber Standard, 6-2, 

6-7, 6-8, 7-2

American standard lumber sizes, 6-10, 

6-11t

American Wood Preservers’ Association:

fire-retardant-treated wood, 18-16 to 

18-17

Ammonia for plasticizing wood, 19-2

Ammoniacal copper citrate (CC):

retention levels for various wood 

products, 15-4t

solution percentages, 15-9

temperature for pressure treating, 15-18

Ammoniacal copper zinc arsenate (ACZA):

composition, 15-3

replacement for ACA, 15-3

retention levels for various wood 

products, 15-4t

temperature for pressure treating, 15-18

uses, 15-3

use with Douglas-fir, 15-3

Anani. See Manni

Anaura. See Marishballi

Andiroba:

characteristics, 2-19

decay resistance, 14-5t

dimensional change coefficient, 13-17t

durability, 2-19

ease of bonding, 10-7t

locality of growth, 2-19

mechanical properties, 5-18t, 5-22t
I–2

General Technical Report FPL–GTR–190

nomenclature, 2-19, 2-43t

resistance to decay and insects, 2-19

shrinkage values, 4-8t

uses, 2-19

workability, 2-19

Angelin (See also Sucupira):

ease of bonding, 10-7t

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

Angelique:

characteristics, 2-19 to 2-20

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

machining properties, 2-20

mechanical properties, 5-18t, 5-22t

resistance to decay and insects, 2-20

shrinkage values, 4-8t

uses, 2-20

Animal adhesives:

structural performance, 10-10t

working and strength properties, and 

uses, 10-11t to 10-13t

Ants, carpenter, 14-13

Annual growth rings. See Growth rings

Apa. See Wallaba

Apamate. See Roble

Apitong. (See also Keruing):

dimensional change coefficient, 13-17t

shrinkage values, 4-8t

Apple:

dimensional change coefficient, 13-16t

moisture content, 4-2t

Ash:

decay resistance, 14-5t

for flooring, 6-4, 6-6

Ash, black:

characteristics, 2-3

dimensional change coefficient, 13-16t

locality of growth, 2-3

moisture content, 4-2t

nomenclature, 6-5t

shrinkage values, 4-6t

species, 2-3

specific gravity, 2-3

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

uses, 2-3

Ash, blue:

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

Ash, green:

dimensional change coefficient, 13-16t

moisture content, 4-2t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

Ash, Oregon:

dimensional change coefficient, 13-16t

nomenclature, 6-5t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

Ash, pumpkin:

dimensional change coefficient, 13-16t

shrinkage values, 4-6t

Ash, white:

characteristics, 2-3 to 2-4

characteristics for painting, 16-5t

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

locality of growth, 2-3

moisture content, 4-2t, 5-34t

nomenclature, 6-5t

penetration, 15-16t

Poisson ratio, 5-3t

shrinkage values, 4-6t

shock resistance, 2-3

species, 2-3

thermal conductivity, 4-13t

uses, 2-4

Aspen:

characteristics, 2-4

connector joint strength, 8-21t

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-4

moisture content, 4-2t

nomenclature, 6-5t

shock resistance, 2-4

species, 2-4

uses, 2-4

workability, 2-4

Aspen, bigtooth:

characteristics for painting, 16-5t

mechanical properties, 5-14t, 5-16t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

Aspen, quaking:

dimensional change coefficient, 13-16t

mechanical properties, 5-14t, 5-16t

Poisson ratio, 5-3t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

Avodire:

characteristics, 2-20

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-20

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

shock resistance, 2-20

uses, 2-20

workability, 2-20

Axial load, deformation, 9-1, 9-1eq

Azobe:

characteristics, 2-20

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-20

mechanical properties, 5-18t, 5-22t

resistance to decay and insects, 2-20

marine borers, 14-13 to 14-15

shrinkage values, 4-8t

workability, 2-20

uses, 2-20

Bacteria causing decay, 14-9

Back priming, 16-25

Bagtikan. See Seraya, white

Balata:

characteristics, 2-20

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-20

resistance to decay and insects, 2-20

shrinkage values, 4-8t

uses, 2-20

workability, 2-20

Balau:

characteristics, 2-21

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-21

species, 2-21

uses, 2-21

Baldcypress:

characteristics, 2-11

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

flame spread index, 18-4t

locality of growth, 2-11

moisture content, 4-2t

nomenclature, 6-13t

pecky cypress, 2-11

penetration, 15-16t

Poisson ratio, 5-3t

tensile strength, 5-26t

thermal conductivity, 4-14t

uses, 2-11

Balsa:

characteristics, 2-21, 2-44t

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

elastic ratio, 5-2t

locality of growth, 2-21

mechanical properties, 5-18t, 5-22t

Poisson ratio, 5-3t

shrinkage values, 4-8t

uses, 2-21

Banak:

characteristics, 2-21

decay resistance, 14-5t
I–3

Index

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-21

machining properties, 2-21

mechanical properties, 5-18t, 5-22t

nomenclature, 2-21, 2-44t

resistance to decay and insects, 2-21

shrinkage values, 4-8t

uses, 2-21

Bark:

growth, 3-2

inner and outer, 3-2fig

Basswood:

characteristics, 2-4

characteristics for painting, 16-5t

charring rate data, 18-14t

connector joint strength, 8-21t

decay resistance, 14-5t

ease of bonding, 10-7t

elastic ratio, 5-2t

flammability data, 18-4t

locality of growth, 2-4

machinability, 2-4

moisture content, 4-2t

nomenclature, 6-5t

strength properties, 5-4t, 5-9t

in wood–polymer composites, 19-11t

thermal conductivity, American, 4-13t

uses, 2-4

workability, 2-4

Basswood, American:

dimensional change coefficient, 13-16t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

Bastard sawn lumber, definition, 3-14

Beams:

bending deflection, 9-3 to 9-4, 9-2eq, 

9-2t, 9-4eq

bending stress, 9-4, 9-5eq, 9-9

combined bending and axial load, effect 

of, 9-3, 9-6

compressive stress, 9-4

deformation, 9-1 to 9-4

end loading, effect of, 9-6

lateral buckling, 9-9 to 9-10, 9-9eq, 9-9t

modulus of rupture, 9-4, 9-4eq

notches and holes, effect of, 9-2, 9-6 to 

9-7

shear deflection, 9-2, 9-2eq, 9-2t

shear stress, 9-4, 9-4eq

size, effect on strength, 9-5, 9-5eq, 9-6eq

size, effects on modulus of rupture, 9-5, 

9-6eq

stability, 9-7 to 9-10

tapered beams:

bending stress, 9-5 to 9-6, 9-5eq

deflections, 9-3, 9-3eq, 9-3fig

glulam combinations, 11-17 to 11-18

shear stresses, 9-4 to 9-6, 9-5fig

tensile, stress, 9-4, 9-4eq

time, effect of, 9-2, 9-7

twist, 9-4, 9-4eq

waterponding, effect of, 9-2, 9-3eq, 9-6, 

9-8, 9-8eq

Beech:

characteristics for painting, 16-5t

decay resistance, 14-5t

for flooring, 6-4, 6-6

nomenclature, 6-5t

penetration, 15-16t

Beech, American:

characteristics, 2-4

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-4

machinability, 2-4

moisture content, 4-2t

shock resistance, 2-4

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

tensile strength, 5-26t

thermal conductivity, 4-13t

uses, 2-4

Bees, carpenter, 14-13

Beetles:

ambrosia beetles, 14-9 to 14-10

damage caused by, 14-8t

bark beetles, 14-9

old house borers, 14-10

damage caused by, 14-8t

powder-post beetles, 14-10

damage caused by, 14-9fig, 14-8t

Bending:

creep, 5-39

strength and temperature, 5-37t

Bending of glulam, 11-17

Bending properties, affected by 

temperature, 5-37, 5-37t

Bending stiffness:

bending strength, stress grading, 7-6

Bending stress:

derivations for machine-graded lumber, 

7-8 to 7-10, 7-12fig

Bending of wood:

apparatus, 19-4

chemicals used, 19-2

fixing the bend, 19-4

moisture content of stock, 19-3 to 19-4

principles of plasticizing and bending, 

19-1

selection of stock, 19-3

solid members, 19-3, 19-3fig

steaming, 19-1

Benge:

characteristics, 2-21

decay resistance, 14-5t

ease of bonding, 10-7t

mechanical properties, 5-18t, 5-22t

nomenclature, 2-21

shrinkage values, 4-8t

uses, 2-22

workability, 2-21

Bent wood members:

characteristics, 19-4

solid wood members, species used, 19-3

uses, 19-2

Birch:

connector joint strength, 8-21t

decay resistance, 14-5t

for casing and base, 6-17

for flooring, 6-4, 6-6

heat release data, 18-12fig

nomenclature, 6-5t

Birch, gray, shrinkage values, 4-6t

Birch, paper:

characteristics, 2-4 to 2-5

dimensional change coefficient, 13-16t

locality of growth, 2-5

moisture content, 4-2t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

uses, 2-5

Birch, river:

dimensional change coefficient, 13-16t

penetration, 15-16t

shrinkage values, 4-6t

Birch, sweet:

characteristics, 2-5

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-5

moisture content, 4-2t

penetration, 15-16t

shock resistance, 2-5

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

uses, 2-5

Birch, yellow:

characteristics, 2-5

characteristics for painting, 16-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

flame spread index, 18-4t

laminated:

strength properties, 19-7t to 19-8t

thermal expansion coefficients, 19-8t

locality of growth, 2-5

moisture content, 4-2t, 5-34t

Poisson ratio, 5-3t

penetration, 15-16t

shrinkage values, 4-6t

shock resistance, 2-5

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

toughness values, 5-27t

uses, 2-5
I–4

General Technical Report FPL–GTR–190

Bird peck:

description, 5-33

effect on strength, 5-33

species involved, 5-33

Black locust. See Locust, black

Blood adhesive, working and strength 

properties, and uses, 10-11t to 10-13t

Blue stain, description, 14-2

Boats, wood:

control of decay in, 14-9

control of marine borers, 14-14 to 14-15

use of varnish on, 16-25

Bolts:

bearing stress of wood under:

bolt diameter, effect perpendicular to 

grain, 8-15fig

intermediate angle to grain loading, 

8-14, 8-13fig

L/D ratios, 8-14

parallel to grain loading, 8-14, 8-14fig

perpendicular to grain loading, 8-14, 

8-14fig

bearing stress with steel side plates, 8-15

bolt holes, effect on, 8-16 to 8-17, 

8-16fig

bolt quality, effect on joint strength, 

8-15, 8-14fig, 8-15fig

drift, 8-10

member thickness, effect of, 8-15, 

8-15fig

multiple bolt joints, 8-15 to 8-16

pre-1991 allowable loads:

parallel to grain, 8-17 to 8-18, 8-17t

perpendicular to grain, 8-17, 8-18t, 

8-19eq

post-1991 yield model, 8-17, 8-19eq, 

8-19t

spacing, edge and end distance, 8-16

Bondability:

of metals and plastics to wood, 10-5

of wood species, 10-6, 10-7t

Bonded joints:

basic stress modes, 10-20 to 10-21

construction joints, 10-19 to 10-20, 10-

19fig

edge-grain joints, 10-18, 10-18fig

end-grain joints, 10-18, 10-18fig

end-to-edge grain joints, 10-18, 10-18fig

mechanical testing of, 10-19 to 10-21

Bonding:

adhesive selection, 10-10 to 10-15

assembly and pressing, 10-16 to 10-17, 

10-17fig

effect of:

moisture content and dimensional 

change, 10-14 to 10-15

wood density, 10-5 to 10-6

wood porosity, 10-6

elements of, 10-1 to 10-2

moisture content control, 10-14 to 10-16

quality assurance programs, 10-22

short- and long-term performance of 

adhesives, 10-21 to 10-22, 10-22fig

strength and durability of adhesives, 

10-9 to 10-10, 10-10t, 10-18

strength properties of adhesives, 10-21

surface preparation, 10-15

with veneers, 10-3 to 10-4

wettability of wood, 10-2, 10-4fig

wood and nonwood composites, 10-5

Boxelder, nomenclature, 6-5t

Box nails, 8-2, 8-2t

Bridges, 17-8 to 17-9, 17-9fig

Brown rot, 14-4

Bubinga (See also Benge):

decay resistance, 14-5t

ease of bonding, 10-7t

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

Buckeye:

characteristics, 2-5

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-5

machinability, 2-5

nomenclature, 6-5t

shock resistance, 2-5

species, 2-5

uses, 2-5

yellow, shrinkage values, 4-6t

Buckling:

interaction of buckling modes, 9-9 to 

9-10, 9-9eq, 9-10eq

of beams, 9-9 to 9-10, 9-9eq, 9-9t, 

9-10fig

of columns, 9-7 to 9-9, 9-7eq, 9-8fig

of deck beams, 9-9, 9-10fig

of roof beams, 9-9

Building codes:

classifications of construction, 18-2

fire safety:

containment to compartment of origin, 

18-6 to 18-8

fire growth within compartments, 18-3 

to 18-6

types of construction, 18-2

organizations producing, 18-2

Buildings, control of decay in, 14-4, 14-7

Built-up columns, 9-8

Bulletwood (See also Balata):

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

Butternut:

characteristics, 2-5

characteristics for painting, 16-5t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-5

machinability, 2-5

nomenclature, 6-5t

shock resistance, 2-5

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

uses, 2-5

Buttonwood. See Sycamore

Cambium, 3-2fig, 3-4, 3-6fig

Carapa. See Andiroba

Carbon impact:

construction performance indices, 1-2t

contained, 1-3

deforestation, 1-2

emissions, net carbon, 1-3, 1-4t

photosynthesis, 1-2

process, 1-2

sequestration, 1-2

sustainability, defined, 1-3

Carbon monoxide, 18-6

Carpenter ants:

damage caused by, 14-11fig, 14-8t

discussed, 14-13

Carpenter bees:

damage caused by, 14-8t

discussed, 14-13

Casing and base, availability, 6-17

Casein adhesive:

structural performance, 10-10t

working and strength properties, and 

uses, 10-11t to 10-13t

Catalpa:

decay resistance, 14-5t

dimensional change coefficient, 13-16t

Catalysts and adhesion, 10-10

Cativo:

characteristics, 2-22

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-22

mechanical properties, 5-18t, 5-22t

resistance to decay and insects, 2-22

shrinkage values, 4-8t

uses, 2-22

Cedar:

availability at retail lumber yards, 6-16

for finish boards, 6-16

Cedar, Atlantic-white:

characteristics, 2-18

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-18

nomenclature, 6-13t

shock resistance, 2-18

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-14t

uses, 2-18

Cedar, eastern red:

characteristics, 2-16
I–5

Index

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

heat release data, 18-12fig

locality of growth, 2-16

moisture content, 4-2t

nomenclature, 6-13t

shock resistance, 2-16

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-14t

uses, 2-16

Cedar, incense:

characteristics, 2-12 to 2-13

characteristics for painting, 16-5t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-12

moisture content, 4-2t

pecky, 2-13

nomenclature, 6-13t

shock resistance, 2-13

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

uses, 2-13

Cedar, northern white:

characteristics, 2-18

characteristics for painting, 16-5t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

for shingles, 6-17

locality of growth, 2-18

mechanical properties, 5-14t, 5-15t

nomenclature, 6-13t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-18

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-14t

uses, 2-18

Cedar, Port-Orford:

characteristics, 2-16

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-16

moisture content, 4-2t

nomenclature, 6-13t

shock resistance, 2-16

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

tensile strength, 5-26t

thermal conductivity, 4-14t

uses, 2-16

Cedar, western red:

characteristics, 2-16

characteristics for painting, 16-5t

charring rate data, 18-14t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

for shingles and shakes, 6-17

for siding, 6-16

locality of growth, 2-16

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

nomenclature, 6-13t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-16

shrinkage values, 4-6t

strength properties, 5-12t, 5-7t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

used for poles, 6-19

uses, 2-16

Cedar, yellow:

characteristics, 2-18

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

flame spread index, 18-4t

locality of growth, 2-18

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

nomenclature, 6-13t

shrinkage values, 4-6t

shock resistance, 2-18

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

toughness values, 5-28t

uses, 2-18

Cedro. See Spanish-cedar

Cedro macho. See Andiroba

Cedro-Rana. See Tornillo

Ceiba:

characteristics, 2-22

decay resistance, 14-5t

locality of growth, 2-22

mechanical properties, 5-18t, 5-22t

resistance to decay and insects, 2-22

shrinkage values, 4-8t

uses, 2-22

workability, 2-22

Cells of wood:

description, 3-7, 3-9 to 3-12

fibers, 3-11

functions of various types, 3-9 to 3-12

parenchyma, 3-9, 3-11

rays, 3-9, 3-11

tracheids, 3-9

vessels, 3-10

Cellulose, discussion, 3-7

Cellulose insulation, product safety 

standard, 18-2 to 18-3

Cement-bonded composites, 11-23 to 11-

25, 11-24fig

magnesia-cement-bonded, 11-23

mechanical properties, 12-9, 12-10t

Portland-cement-bonded, 11-24

problems, 11-24

treatment, 11-24

Cement-coated nails, 8-2fig, 8-3

Ceramic-bonded composites, 11-25

Chalking, 16-28

Checks:

development due to weathering, 16-6, 

16-11, 16-11fig

during drying, 13-7, 13-9 to 13-10, 13-

12fig

in decking boards, 16-6

in lumber stress grading, 7-4

in glued-laminated timbers, 11-19

in veneer adhesion, 10-4

Chemical discolorations during drying, 

13-10, 13-13fig

Chemicals, effect on wood:

discussion, 5-41

strength and mechanical properties, 5-41 

to 5-42

Cherry:

characteristics for painting, 16-5t

ease of bonding, 10-7t

nomenclature, 6-5t

Cherry, black:

characteristics, 2-5

decay resistance, 14-5t

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

locality of growth, 2-5

machinability, 2-5

moisture content, 4-2t

Poisson ratio, 5-3t

shock resistance, 2-5

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

uses, 2-1, 2-5

Cherry, pin, penetration, 15-16t

Chestnut:

characteristics for painting, 16-5t

nomenclature, 6-5t

Chestnut, American:

availability, 2-5 to 2-6

characteristics, 2-6

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-5
I–6

General Technical Report FPL–GTR–190

moisture content, 4-2t, 5-34t

penetration, 15-16t

shock resistance, 2-6

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

uses, 2-6

workability, 2-6

Chewstick. See Manni

Chlorothalonil/chlorpyrifos:

component rations, 15-12

effectiveness, 15-12

Chlorpyrifos:

effectiveness, 15-12

in combination, 15-12

Chromated copper arsenate (CCA):

common types, 15-4

component substitutions, 15-4

composition of common types, 15-5t

effectiveness of common types, 15-4

effect on adhesion, 10-4

finishing wood treated with, 16-24

resistance to marine borers, 15-5

retention levels for various wood 

products, 15-5t

Southern pine sapwood stakes retention 

and life span test results, 15-7t to 15-8t

temperature for pressure treating, 15-18

use with Douglas-fir, 15-3

Coal-tar creosote. See Creosote

Cocobolo, shrinkage values, 4-8t

Coefficient of friction, 4-17

Coefficient of thermal expansion, 4-14

Coefficient of variation, 5-26t

Collapse during drying, 13-10, 13-12fig

Columns:

built-up and spaced columns, 9-8

flanged columns, 9-8

long columns, compressive stress, 9-7, 

9-7eq

short columns, compressive stress, 9-7 to 

9-8, 9-8eq

Composite products:

classification, 11-2t

conventional wood-based composite 

panels, 11-2 to 11-17

additives, 11-5

adhesives, 11-3 to 11-5

classification, 11-2t

elements, 11-2, 11-6fig

fiberboard, 11-12 to 11-15

oriented strandboard, 11-7 to 11-10

particleboard, 11-10 to 11-12

performance and standards, 11-1, 

11-3t

plywood, 11-5 to 11-7

specialty composites, 11-15

glued-laminated timber (glulam), 11-17 

to 11-20

mechanical properties, 12-2t

specific gravity, 12-2t

standards, 11-3t

structural composite lumber, 11-20 to 

11-22

description, 11-20

laminated strand lumber, 11-21

laminated veneer lumber, 11-21

oriented strand lumber, 11-21

parallel strand lumber, 11-21

test standards, 12-5 to 12-7

wood elements, 11-4fig

wood–nonwood composites, 11-22 to 

11-27

description, 11-22

inorganic-bonded composites, 11-22 

to 11-25

wood–thermoplastic composites, 11-

25 to 11-27

Compreg:

advantages, 19-5

dimensional stability, 19-9t

molding, 19-5, 19-9

plasticizers, 19-5

properties, 19-5, 19-6t

thermal expansion coefficients, 19-8t

uses, 19-5 to 19-9

Compressed wood, untreated (staypak) 

appearance, 19-10

dimensional stability, 19-9t

purpose, 19-7t to 19-6t

strength properties, 19-7t to 19-8t

uses, 19-10

Compression failures:

causes, 5-33

description, 5-33, 5-33fig

effect on strength, 5-33

Compression wood:

definition, 5-31

density increase, 5-31

in juvenile wood, 5-32

shrinkage, 4-5, 5-32

Compression strength parallel to grain:

affected by temperature, 5-36t

coefficient of variation, 5-26t

defined, 5-3

Compressive stress of columns, 9-7 to 9-9, 

9-7eq

Compression stress perpendicular to grain:

coefficient of variation, 5-26t

defined, 5-3

Condensation in crawl spaces, 14-7

Conductance meters and moisture content, 

13-2 to 13-3

Conductivity, thermal:

definition, 4-10

discussion, 4-10 to 4-15

Cone calorimeter, 18-12

Connector joints:

cross bolts, 8-24

design loads, 8-20, 8-20fig, 8-21fig

end distance and spacing, 8-23

multiple connectors, 8-23 to 8-24, 

8-24fig

net section stress, 8-23

parallel to grain loading, 8-19, 8-20fig

perpendicular to grain loading, 8-19 to 

8-20, 8-20fig

shear plate connector, 8-19fig

split-ring connector, 8-19fig

strength components, 8-18 to 8-19

strength ratios, 8-25t

working loads:

exposure and moisture condition, 

effect of, 8-21

grade and quality of lumber, 8-22

loads at angle with grain, 8-22, 8-7fig, 

8-13eq,

species grouping, 8-21t, 8-22t

steel side plates, effect of, 8-21

thickness of member, effect of, 8-22, 

8-22t, 8-23fig

width of member, effect of, 8-23

Consortium for Research on Renewable 

Industrial Materials (CORRIM):

calculations, 1-2

standards, 1-2

Construction, design factors affecting 

dimensional change, 13-18

Construction logs:

availability, 6-20

form, 6-21, 6-21fig

standards and specifications, 6-18t

strength properties, 7-14

uses, 6-20

Copper azole - Type A (CBA-Type A):

retention levels for various wood 

products, 15-4t

solution percentages, 15-6

temperature for pressure treating, 15-18

Copper bis(dimethyldithiocarbamate) 

(CDDC):

retention levels for various wood 

products, 15-4t, 15-5t

solution percentages, 15-6

temperature for pressure treating, 15-18

uses, 15-6

Copper HDO (CXA), 15-6

Copper naphthenate:

color transfer and changes, 15-6

effectiveness, 15-6

retention levels for various wood 

products, 15-4t

solution values, 15-6

Southern pine sapwood stakes retention 

and life span test results, 15-7t to 15-8t

treatment for cutting pretreated wood, 

15-25

Copper naphthenate (waterborne), 15-6

Cotton linter paper, 19-12
I–7

Index

Cottonwood:

characteristics, 2-6

connector joint strength, 8-21t

decay resistance, 14-5t

ease of bonding, 10-7t

elastic ratio, 5-2t

flame spread index, 18-4t

locality of growth, 2-6

moisture content, 4-2t

nomenclature, 6-5t

penetration, 15-16t

shock resistance, 2-6

species, 2-6

uses, 2-6

workability, 2-6

Cottonwood, balsam poplar:

mechanical properties, 5-14t, 5-15t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

Cottonwood, black:

dimensional change coefficient, 13-16t

mechanical properties, 5-14t, 5-15t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

Cottonwood, eastern:

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

characteristics for painting, 16-5t

mechanical properties, 5-14t, 5-15t

Poisson ratio, 5-3t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

Courbaril:

characteristics, 2-22 to 2-23

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-22

machinability, 2-23

mechanical properties, 5-18t, 5-22t

resistance to decay and insects, 2-23

shrinkage values, 4-8t

uses, 2-23

Covalent chemical bonds, 10-1 to 10-2

Crabwood. See Andiroba

Crack propagation systems, 5-17

Creep:

defined, 5-39

discussed, 5-39

influence of stress on, 5-39fig

Creosote, coal-tar:

advantages, 15-10

appearance, 15-10

composition variability, 15-10

EPA-approved customer

information sheet, 15-2

effect on mechanical properties, 15-25 to 

15-26

for non-pressure treatments, 15-12

handling precautions, 15-2

in pressure treatment process, 15-18

odor and vapors, 15-2

retention levels for various wood 

products, 15-4t, 15-5t

standards, 15-2 to 15-3

temperature for pressure treating, 15-25

treatment for cutting pretreated wood, 

15-25

use site precautions, 15-2

volatility, 15-10

Creosote-coal-tar solutions:

properties, 15-10

retention levels for various wood 

products, 15-4t to 15-5t

standards by volume, 15-10

temperature for pressure treating, 5-42

Creosote-petroleum oil solutions:

retention levels for various wood 

products, 15-4t to 15-5t

Cristobal. See Macawood

Critical radiant flux test, 18-5

Cross grain:

effect on mechanical properties, 5-27 to 

5-30

types, 5-27 to 5-30

Cuangare (See also Banak):

dimensional change coefficient, 13-17t

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

Cucumber, nomenclature, 6-5t

Cypress:

for siding, 6-16

Cypress, Arizona, decay resistance, 14-5t

Cypress, Mexican

characteristics, 2-38 to 2-39

locality of growth, 2-38

machinability, 2-38

mechanical properties, 5-18t, 5-22t

uses, 2-39

Dead trees, strength, 5-34

Decay:

and exposure to weather, 16-10, 16-13

appearance of fungi, 14-4

brown rot, 14-4

conditions favoring, 14-4

control in:

boats, wood, 14-7

buildings, 14-7

logs, poles, piling, or ties, 14-6

lumber, 14-6

plywood, 14-7, 14-9

cycle, 14-3fig

dead trees, 5-34

dry rot, 14-4

dry rot fungi, 14-4

effect on mechanical properties, 5-44

effect on strength, 5-43 to 5-44, 14-4 to 

14-5, 14-6

fungal stain and molds, 14-4

heartwood, susceptibility to, 14-3 to 

14-4

incipient, 14-4

sapwood, susceptibility to, 14-3

soft rot, 14-4

white rot, 14-4

Decay hazard climate index, 14-2fig

Decay resistance:

extractives, 3-3

heartwood, 3-3, 4-2t, 14-3 to 14-4

in visual stress grading, 7-5

Decks:

discussed, 17-4

finishes, suitability and expected service 

life, 16-17t

finishing, 16-24

supports, 9-9, 9-10fig

use of cleaners, 16-34 to 16-35

Deflections of beams:

effect of time, creep, 9-2

straight beams, 9-1 to 9-2, 9-2t, 9-1eq

tapered beams, 9-2, 9-2eq, 9-3fig

Deformation equations:

axial load, 9-1, 9-1eq

bending:

effect of notches and holes, 9-2

effect of time, creep deflection, 9-3

straight beam deflection, 9-1 to 9-2, 

9-1eq, 9-2t

tapered beam deflection, 9-2, 9-2eq, 

9-3fig

water ponding, 9-2, 9-3eq

combined bending and axial load:

concentric load, 9-3 to 9-4, 9-4eq

eccentric load, 9-7, 9-7eq

torsion, 9-7, 9-7eq, 9-7fig

Deformed shank nails, strength of, 8-8

Degame:

characteristics, 2-23

locality of growth, 2-23

machinability, 2-23

mechanical properties, 5-18t, 5-22t

resistance to decay and insects, 2-23

shrinkage values, 4-8t

strength, 2-23

uses, 2-23

Delamination and adhesive failure, 10-21

Density:

as function of specific gravity and 

moisture content, 4-7 to 4-10, 4-11t, 

4-12t

definition, 3-12

effect on adhesive bonding, 10-5 to 10-6

effect on withdrawal resistance of nails, 

8-2 to 8-3

in sorting machine-graded lumber, 7-8

in visual stress grading, 7-5

variation, 4-7

Design factors affecting dimensional 

change in structures:

flooring, 13-18
I–8

General Technical Report FPL–GTR–190

framing lumber in house construction, 

13-17

heavy timber construction, 13-18

interior finish, 13-18

Design properties:

and stress grading, 7-3

procedures for deriving, 7-5 to 7-7

Design values:

for foreign species, approval

process, 7-4t

Determa:

characteristics, 2-23

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-23

machinability, 2-23

mechanical properties, 5-18t, 5-22t

resistance to decay and insects, 2-23

uses, 2-23

Diagonal grain, 5-28 to 5-31

Dielectric constant, definition, 4-15, 4-16

Dielectric meters and moisture content, 

13-2 to 13-3

Dielectric power factor, 4-16, 4-17

Dimensional change and wood paintability, 

16-2, 16-3

Dimensional changes in wood:

affected by design factors, 13-17

calculation based on green dimensions, 

13-17, 13-17eq

care during construction, 13-18 to 13-19

estimation using coefficients, 13-15 to 

13-17, 13-15eq, 13-16t, 13-17t

Discoloration:

by mildew, 16-12, 16-28, 16-28fig

by mold and fungal stains, 14-2

by nonmicrobial or chemical stains, 14-2

from water-soluble extractives, 16-28 to 

16-30

of paint due to extractives, 16-25, 16-29 

to 16-31

use of wood cleaners, 16-34 to 16-36

Dogwood, nomenclature, 6-5t

Doors, fire resistance in frame construction, 

18-3 to 18-6

Douglas-fir:

availability at retail yards, 6-16

characteristics, 2-11

characteristics for painting, 16-5t

charring rate equation, 18-13

connector joint strength, 8-21t

decay resistance, 14-5t

ease of bonding, 10-7t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

for flooring, 6-16

for siding, 6-16

fracture toughness, 5-28t

kiln drying schedule, 13-11t

locality of growth, 2-11

mechanical properties, 5-14t, 5-15t

moisture content, 5-34t

nomenclature, 6-13t

penetration, 15-16t

Poisson ratio, 5-3t

preservative pressure and temperature, 

15-18, 15-20

preservative treatment to prevent marine 

borer attack on piles, 15-5t

preservatives used, 15-10 to 15-12

shrinkage, 4-5, 4-7fig

used for poles, 6-19

uses, 2-11

Douglas-fir, coast:

dimensional change coefficient, 13-16t

moisture content, 4-2t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

toughness values, 5-28t

Douglas-fir, interior north

dimensional change coefficient, 13-16t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

Douglas-fir, interior west

dimensional change coefficient, 13-16t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

toughness values, 5-28t

Douglas-fir, interior south

strength properties, 5-7t, 5-12t

toughness values, 5-28t

Draftstops, 18-6

Dried wood, moisture control during transit 

and storage, 13-14 to 13-15, 13-15t

Drift bolts, 7-10

Dry kilns, 13-6 to 13-8, 13-8fig

Drying of wood:

accelerated air drying and predrying, 

13-6

advantages, 13-5

affect of properties, 13-5

air drying, 13-6

drying mechanism, 13-6 to 13-7, 13-6fig

drying defects:

discoloration, 13-10, 13-13fig, 

13-14fig

fracture or distortion, 13-10, 13-12fig

warp, 13-10, 13-13fig

drying schedules, 13-8 to 13-9, 13-11t

drying stresses, 13-7, 13-7fig

kiln drying, 13-6

hardwood lumber targets, 13-5

softwood lumber targets, 13-5

Duration of load:

adjustment of design properties, 7-10 to 

7-13, 7-12t, 7-13fig

defined, 5-15, 5-39

effect on mechanical properties, 5-39 to 

5-40

relationship to failure, 5-40fig

Dutch elm disease, 2-6

Earlywood:

effect on paintability, 16-2, 16-3fig, 16-

4fig, 16-3 to 16-4

description, 3-4

erosion of planed surfaces, 16-12t

Ebony, shrinkage values, 4-8t

Edge-grained lumber:

preferred for painting, 16-8, 16-6fig

weathering and check development, 

16-11

Ehie (See also Benge):

decay resistance, 14-5t

Ekki. See Azobe

Ekop:

characteristics, 2-23

decay resistance, 14-5t

locality of growth, 2-23

machinability, 2-23

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

uses, 2-23

EL2, 15-8

Elastic properties of clear wood:

discussion, 5-2

values of:

modulus of elasticity ratios, 5-2t

modulus of rigidity ratios, 5-3t

Poisson’s ratio, 5-2

Elastomeric adhesives:

in light-frame construction, 10-19, 

10-19fig

performance over time, 10-21 to 10-22, 

10-22fig

structural performance, 10-10t

working and strength properties, and 

uses, 10-11t to 10-13t

Electrical properties of wood:

conductivity, 4-15 to 4-17

dielectric constant, 4-15, 4-16

power factor, 4-16, 4-17

resistance and moisture content, 4-16fig

Elm:

characteristics, 2-6

connector joint strength, 8-21t

decay resistance, 14-5t

disease, 2-6

locality of growth, 2-6

moisture content, 4-2t

species, 2-6

uses, 2-6
I–9

Index

Elm, American:

characteristics for painting, 16-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

moisture content, 4-2t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

Elm, cedar:

dimensional change coefficient, 13-16t

moisture content, 4-2t

shrinkage values, 4-6t

tensile strength, 5-26t

Elm, rock:

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

moisture content, 4-2t

nomenclature, 6-5t

penetration 15-16t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

Elm, slippery:

dimensional change coefficient, 13-16t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

Elm, soft:

nomenclature, 6-5t

Elm, winged:

dimensional change coefficient, 13-16t

shrinkage values, 4-6t

Encased knots, definition, 5-27

Encino. See Oak

End joints in glued laminated timber, 11-19

Engineered trusses and light-frame 

construction, 17-4

Environmental benefits of wood:

discussion, 1-1

wood industry fuel sources, 1-2t

Epoxy:

performance over time, 10-21 to 10-22

structural performance, 10-10t

use with wood and nonwood composites, 

10-5

working and strength properties, and 

uses, 10-11t to 10-13t

Erosion:

of earlywood and latewood planed 

surfaces, 16-12t

of finishes, 16-13

of wood, discussed, 16-11

rates for hardwood and softwoods, 

16-12t

Equilibrium moisture content:

definition, 4-3, 16-6

relative humidity as related to, 4-4t

values for U.S. cities, 13-3, 13-4t

Extractives:

and mill glaze, 16-26 to 16-27

discoloration, water-soluble, 16-25, 

16-29

discussion, 3-3, 5-34

effect on painting, 16-7, 16-12 to 16-13, 

16-29

effect on strength, 5-34

heartwood, 3-3

species involved, 5-34

Factory-finished wood products, 16-25

Factory lumber (See also Hardwood 

lumber):

grades, 6-2

standard dimensions, 6-3

Fastener head embedment, 8-26, 8-27fig

Fasteners:

and iron stain, 16-30

corrosion and fire-retardant treated 

wood, 18-15

fire resistance in frame construction, 

18-7 to 18-8

multiple-fastener joints, 8-24 to 8-25

Fatigue:

defined, 5-15, 5-40

discussed, 5-40 to 5-41

summary of fatigue studies, 5-40t

Fences, finishing, 16-13, 16-24

Fiberboard:

attrition milling or refining, 11-12

cellulosic fiberboard, 11-14

grade stamp, 11-15fig

manufacture, 11-14

products, 11-14

sizing agents, 11-14

standards, 11-3t

classification, 11-1, 11-2t, 11-5fig

fibers used, 11-12

finishing techniques, 11-14 to 11-15

hardboard:

dry-process, 11-13

grade stamp, 11-15fig

heat treatment, 11-13

humidification, 11-13

manufacture, 11-13

mechanical properties, 12-6t

specific gravity, 12-6t

standards, 11-3t

tempering, 11-13

wet-process, 11-13

medium-density fiberboard (MDF):

density, 12-6t

grade stamp, 11-14fig

mechanical properties, 12-6t

resins used, 11-13

standards, 11-3t

uses, 11-13

with veneer overlay, 11-15, 11-16fig

Fiber orientation, related to slope of grain, 

5-28 to 5-31, 5-30fig

Fibers:

description, 3-11

length, 3-11

shape, 3-11

Fiber saturation point:

average, 4-2

definition, 4-2

Finger joints, in laminated members, 11-19

Finger-jointed lumber:

defined, 16-9

finishing, 16-10, 16-32

Finish board, availability, 6-16

Finish failure:

chalking, 16-28

cross-grain cracking, 16-26

discoloration from water-soluble 

extractives, 16-29, 16-29fig

intercoat peeling, 16-27, 16-27fig

mill glaze, 16-26 to 16-27

moisture blisters, 16-26, 16-26fig

stain:

blue, 16-30, 16-30fig

brown stain over knots, 16-25, 16-31

iron, 16-25, 16-30

rust, 16-30

Finishes:

application and maintenance, 16-14t, 

16-17t, 16-19t

application of:

paint, 16-22

semitransparent penetrating stain, 

16-20

caution in use, 16-20, 16-19t

solid-color stain, 16-22

water-repellant preservative, 16-13

drying oils, 16-21

function, 16-21

factors affecting performance:

extractives, 16-7, 16-29 to 16-30

knots, 16-31

texture of wood, 16-8

weathering:

as a natural finish, 16-16

effect on paint adhesion, 16-12

effect on wood, 16-11

effect on wood finish, 16-13

wood moisture content, 16-6 to 16-7

back-priming, 16-25

wood product characteristics:

finger-jointed lumber, 16-9, 16-32

lumber, 16-8 to 16-9

plywood, 16-9

reconstituted wood products:

fiberboard, 16-8 to 16-10

particleboard, 16-10

treated wood:

fire-retardant treated, 16-11

preservative treated, 16-10
I–10

General Technical Report FPL–GTR–190

wood properties, 16-2 to 16-3, 

16-32

moisture-excluding effectiveness of, 

16-6 to 16-8, 16-13

on ponderosa pine, 16-14t

nondrying oils, 16-34

on:

butcher blocks and cutting boards, 

16-34

floors, 16-33 to 16-34

interior wood, 16-32

items used for food, 16-34

porches, decks, fences, 16-24

treated wood, 16-24

wood exposed to marine 

environments, 16-24 to 16-25

paintability values, 16-7, 16-12

paraffin wax, 16-33 to 16-34

refinishing, 16-22 to 16-23

suitability and expected service life for 

exterior wood surfaces, 16-17t

types:

film-forming:

effect on water and vapor 

absorption, 16-13

fire-retardant coatings, 16-11, 16-25

paint, 16-7, 16-12, 16-22

solid-color stain, 16-22

varnish:

clear, 16-21

pigmented, 16-21

opaque, 16-32

penetrating, 16-17 to 16-18

lightly colored, 16-16 to 16-17

semitransparent stains, 16-19 to 

16-20

transparent clear, 16-17

oils, 16-21

stains, 16-16 to 16-24

surface coats, 16-33

transparent, 16-33

use of fillers, 16-33

use of sealers, 16-33

VOC regulation compliance, 16-15 to 

16-16

See also Paint and Water-repellant 

preservatives

Fire performance characteristics of wood:

charring:

differences in wood species, 18-13 to 

18-15

discussed, 18-13 to 18-15

equations for charring rates, 18-13 to 

18-15

moisture content, 18-10

flame spread:

and heat release rate, 18-11

configurations, 18-9

factors influencing, 18-11

flammability data for wood species, 

18-14t

heat release rate:

discussed, 18-11 to 18-13

measuring, 18-11

ignition:

piloted, 18-9

unpiloted, 18-10

smoke:

approaches for dealing with, 18-11

carbon monoxide, 18-6

defined, 18-6

release rate, 18-8

tests for determining yield, 18-6 to 

18-8

toxicity, 18-6

smouldering, 18-6 to 18-7

stages of degradation, 18-8

Fire resistance:

calculating, 18-13

defined, 18-13

failure criteria, 18-14

in heavy timber construction, 18-7

in light-frame construction, 18-7

in glued laminated members, 18-2

ratings, 18-2

Fire-retardant coatings, 16-11, 16-25, 

18-17

Fire-retardant-treated wood:

and hygroscopicity, 18-15

and mechanical properties, 18-17

discussed, 18-15

effect on adhesion, 10-4

fastener corrosion, 18-17

heat release rates, 18-12 to 18-13

in high temperature applications, 18-11

paintability, 16-22

performance requirements, 18-11

uses, 18-11

Fire-retardant treatments:

application methods, 18-15

chemical penetration, 18-16

inorganic salts, 18-15

Fire safety design:

cellulosic insulation, 18-3

code organizations, 18-2

code requirements, 18-1

components, 18-1

containment to compartment of origin:

firestops and draftstops, 18-6

fire resistance, 18-6

of heavy timber construction, 18-2

in light-frame construction, 18-7

of glued-laminated members, 18-2, 

18-7

fire safety engineering, 18-7

flame spread index, 18-3 to 18-5

flashover, 18-5

flooring, 18-5

roof covering materials, 18-5

types of construction:

heavy timber, 18-2

light frame, 18-7

ordinary, 18-2

Fire tests:

critical radiant flux, 18-5

flame spread index:

discussed, 18-3

values for solid lumber, 18-4t

roof coverings, 18-5

room/corner test, 18-5

tunnel test, 18-3

wood ignition, 18-9

Firestops, 18-6

Fir, balsam:

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

mechanical properties, 5-14t, 5-15t

nomenclature, 6-13t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

Fir, California red:

dimensional change coefficient, 13-16t

nomenclature, 6-13t

penetration, 15-16t

tensile strength, 5-26t

toughness values, 5-28t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

Fir, grand:

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

moisture content, 4-2t

nomenclature, 6-13t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

Fir, noble:

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

moisture content, 4-2t

nomenclature, 6-13t

penetration, 15-16t

shrinkage values, 4-6t

toughness values, 5-28t

strength properties, 5-7t, 5-12t

Fir, Pacific silver:

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

flame spread index, 18-4t

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

nomenclature, 6-13t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

tensile strength, 5-26t

toughness values, 5-28t

Fir, subalpine:

dimensional change coefficient, 13-16t
I–11

Index

elastic ratio, 5-2t

mechanical properties, 5-14t, 5-15t

nomenclature, 6-13t

Poisson’s ratio, 5-3t

shrinkage values, 4-6t

Firs, true (Eastern species):

characteristics, 2-11

decay resistance, 14-5t

locality of growth, 2-11

shock resistance, 2-11

strength properties, 2-11

uses, 2-11

Firs, true (Western species):

decay resistance, 14-5t

locality of growth, 2-12

species, 2-12

uses, 2-12

Fir, white:

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

moisture content, 4-2t

nomenclature, 6-13t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

toughness values, 5-28t

Flame-retardant composites, 11-15

Flame spread index (FSI):

classes for, 18-2

discussed, 18-3

values for solid lumber, 18-4t

wood usage, 18-4

Flat-sawn lumber:

and mill glaze, 16-26 to 16-27

disadvantages for finishing, 16-1, 16-25

Flashover, discussed, 18-5

Flooring:

care during construction, 13-19

design factors affecting dimensional 

change, 13-17

recommended moisture content, 13-5t

retail yard availability, 6-16, 6-17

Forest certification programs:

discussion, 1-3 to 1-4

systems used, 1-4

American Tree Farm System (ATFS):

standards, 1-4

program location, 1-4

Canadian Standards Association (CSA):

standards, 1-5

program location, 1-5

Forest Stewardship Council (FSC):

standards, 1-4

program location, 1-4

Programme for the Endorsement of 

Forest Certification (PEFC) Schemes:

standards, 1-5

program location, 1-5

Sustainable Forestry Initiative (SFI):

standards, 1-4

program location, 1-4

Formaldehyde adhesives:

performance over time, 10-21 to 10-22, 

10-22fig

safety concerns, 10-14

structural performance, 10-11t to 10-13t

use with composite products, 11-3 to 

11-4

Formosan termite, 14-11

Foundations for light-frame buildings, 17-2

Fracture toughness, defined, 5-15

Friction, coefficient of, 4-17

Fungi:

appearance of, 14-4

conditions favorable to growth, 14-1

definition, 14-1

discoloration caused by, 14-1 to 14-2

effect on wood, 14-2

prevention of damage from, 14-6

Fungus damage:

causes, 14-1 to 14-2

stains, 14-2

Glued-laminated timber (Glulam):

advantages, 11-17

architectural effects, 11-7

combinations:

axial members, 11-18

bending members, 11-18 to 11-19

curved members, 11-18

tapered straight members, 11-18

cross section variation, 11-17

finishes, suitability and expected service 

life, 15-15t

fire resistance, 17-5

grade variation, 11-17

manufacture:

end jointing, 11-19

face bonding, 11-19 to 11-20

finishing and fabrication, 11-20

lumber drying and grading, 11-19

preservative treatment, 11-20

mechanical properties, 12-7t

preservative retention levels, 14-5t to 

14-6t

seasoning advantages, 11-17

size capabilities, 11-17

standards 11-18 to 11-19, 11-3t

Glulam. See Glued-laminated timber 

(Glulam)

Glulam beam construction, 16-8

Glulam timber bridge, 16-10, 16-10fig

Gluing properties of different wood, 

classification of species, 9-8t

Gmelina, shrinkage values, 4-8t

Gola. See Ekop

Goncalo alves:

characteristics, 2-24

decay resistance, 14-5t

locality of growth, 2-24

mechanical properties, 5-18t, 5-22t

resistance to fungus attack, 2-24

shrinkage values, 4-8t

species, 2-24, 2-43t

strength, 2-24

uses, 2-24

Grades and lumber, purchasing 

consideration, 6-17 to 6-18

Grading hardwood lumber, 6-2 to 6-4

Grading, machine, 7-7 to 7-11

Grading softwood lumber, 6-7 to 6-10

Grading, visual, 7-3 to 7-7

Grain and texture of wood:

and finish performance, 16-6fig

and paintability, 16-7 to 16-8

discussion, 3-13 to 3-15

Green building:

bioenergy, 1-2

biomass, 1-2

carbon neutral, 1-2

defined, 1-1

embodied energy, 1-1

fossil fuels, 1-2

production uses, 1-1 to 1-2

wood products, 1-2

Greenheart:

characteristics, 2-24

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-24

machinability, 2-24

mechanical properties, 5-18t, 5-22t

shrinkage values, 4-8t

resistance to fungi and insects, 2-24

marine borers, 14-13 to 14-15

uses, 2-24

Green wood:

and clinched nails, 8-5

bending properties, 19-3

creep and relaxation under load, 5-39

definition, 4-1

relationship of mechanical properties to 

specific gravity, 5-29t

treatment with polyethylene glycol 

(PEG), 19-10

Growth rings:

cross section showing, 3-5fig, 3-6fig

discussion, 3-4, 3-6

effect on strength properties, 5-30 to 

5-31

grain, 3-14

in sawn boards, 3-14

principal axes with respect to, 5-2

shrinkage, 4-5 to 4-7, 4-7fig

Guatambu. See Pau marfim

Guayacan. See Ipe

Gum:
I–12

General Technical Report FPL–GTR–190

nomenclature, 6-5t

Gurjun. See Apitong

Gypsum board, use in wood-frame 

construction to provide fire resistance, 18-7 

to 18-8

Gypsum-bonded composites, 11-23

Hackberry:

characteristics, 2-6

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-6

moisture content, 4-2t

nomenclature, 6-5t

penetration, 15-16t

shock resistance, 2-6

shrinkage values, 4-6t

strength properties, 5-4t, 5-9t

thermal conductivity, 4-13t

uses, 2-6

Hardness:

coefficient of variation, 5-26t

definition, 5-3

Hardwood flooring, grading rules:

Maple Flooring Manufacturers 

Association, 6-4, 6-4t

National Oak Flooring Manufacturers 

Association, 6-4, 6-4t

Hardwood lumber:

drying targets, 13-5

finished market products:

flooring:

grading rules, 6-2 to 6-6

standard dimensions, 6-3 to 6-4

types, 6-4, 6-6

kiln drying schedules, 13-8 to 13-9, 

13-11t

grades, 6-2, 6-2fig, 6-3t

grading associations and rules, 6-4, 6-3t, 

6-4t

minimum widths, 6-3

standard dimensions, 6-3

standard thicknesses, 6-6t

uses, 6-2

Hardwoods:

availability, 2-1 to 2-3

bending properties, l9-3

charring rates, 18-14t

definition, 2-2

flame spread index, 18-4t

flammability data, 18-10t

heat release data, 18-12fig

imported, 2-19 to 2-38

locality of growth, 2-2

moisture content, heartwood and

sapwood, 4-2t

preservative penetration, 15-16t

relationship of mechanical properties to 

specific gravity, 5-29t

species by region, 2-3t

uses, 2-2

vessels, 3-10 to 3-11

Heartwood:

color, 4-2t

decay resistance in different species, 3-3, 

14-5t

extractives content, 3-3

for shakes, 6-17

in visual stress grading, 7-5

moisture content and drying, 13-7

Heat capacity:

defined, 4-11 to 4-12

discussed, 4-11

of solid wood, 4-15t

Heat release rate:

and smoke release rate, 18-11

discussed, 18-11 to 18-12

Heat sterilization:

American Lumber Standard Committee 

(ALSC) regulations, 20-9 to 20-11

quality mark, 20-11, 20-13fig

standards:

discussion, 20-1

International Standard for 

Phytosanitary Measures (ISPM), 20-1 to 

20-2

pest groups, 20-1, 20-2t

heating times:

experimental, 20-4t

factors affecting:

air circulation, 20-2

energy source, 20-2

heating medium, 20-2, 20-3fig

size and configuration of wood, 

20-2, 20-3fig

species, 20-2, 20-3fig

stacking methods, 20-2, 20-4fig

methods for estimating:

MacLean equations, 20-5, 20-5eq, 

20-6eq, 20-7fig

degree of accuracy, 20-7

heat conduction, 20-5eq

summary of estimates, 20-8t

multiple regression models, 20-7

coefficients for, 20-9t

summary of estimates, 20-10t, 

20-11t, 20-12t

Heat treatment. See Heat sterilization

Heavy-timber construction:

fire resistance of, 18-2, 18-7 to 18-8

wood used in, 18-2

Hemicellulose, 3-7

Hemlock:

availability at retail yards, 6-17

for siding, 6-16

Hemlock, eastern:

characteristics, 2-12

dimensional change coefficient, 13-17t

connector joint strength, 8-21t

locality of growth, 2-12

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

nomenclature, 6-13t

penetration, 15-16t

shock resistance, 2-12

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

uses, 2-12

Hemlock, mountain:

characteristics, 2-12

locality of growth, 2-12

nomenclature, 6-13t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

toughness values, 5-28t

uses, 2-12

Hemlock, western:

characteristics, 2-12

characteristics for painting, 16-5t

connector joint strength, 8-21t

dimensional change coefficient, 13-17t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

fracture toughness, 5-28t

locality of growth, 2-12

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t, 5-34t

nomenclature, 6-13t

Poisson ratio, 5-3t

shock resistance, 2-12

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

uses, 2-12

Hickory:

decay resistance, 14-5t

nomenclature, 6-5t

Hickory, bitternut:

moisture content, 4-2t

strength properties, 5-5t, 5-10t

Hickory, mockernut:

moisture content, 4-2t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

toughness values, 5-27t

Hickory, nutmeg, strength properties, 5-5t, 

5-10t

Hickory, pecan:

characteristics, 2-6 to 2-7

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

for flooring, 6-4, 6-6

locality of growth, 2-6 to 2-7
I–13

Index

shrinkage values, 4-6t

species, 2-6

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

uses, 2-7

Hickory, pignut:

moisture content, 4-2t

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

toughness values, 5-27t

Hickory, red, moisture content, 4-2t

Hickory, sand:

moisture content, 4-2t

toughness values, 5-27t

Hickory, shagbark:

characteristics for painting, 16-5t

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

Hickory, shellbark:

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

Hickory, water:

moisture content, 4-2t

strength properties, 5-5t, 5-10t

Hickory, true:

characteristics, 2-7

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-7

shrinkage values, 4-6t

species, 2-7

strength properties, 5-5t, 5-10t

uses, 2-7

Holly, nomenclature, 6-5t

Holly, American:

dimensional change coefficient, 13-16t

shrinkage values, 4-6t

Honeylocust:

availability, 2-7

characteristics, 2-7

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-7

shock resistance, 2-7

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

uses, 2-7

Hot melt adhesives:

bonding, 10-9

structural performance, 10-10t

working and strength properties, and 

uses, 10-11t to 10-13t

Hot-press bonding, affect on moisture 

content, 10-15

Hot pressing:

fiberboard, 11-13 to 11-14

oriented strandboard, 11-8, 11-10

particleboard, 11-11

Hura:

characteristics, 2-24

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-24

machinability, 2-24

mechanical properties, 5-18t, 5-22t

resistance to fungi and insects, 2-24

shrinkage values, 4-8t

uses, 2-24

Identification of wood, 3-16

Ignition of wood:

piloted, 18-9

unpiloted, 18-10

Ilomba:

characteristics, 2-24 to 2-25

locality of growth, 2-24

machinability, 2-25

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-25

shrinkage values, 4-8t

uses, 2-25

Imbuia, shrinkage values, 4-8t

Imported woods, commercially important, 

2-18 to 2-40

Impact bending:

coefficient of variation, 5-26t

defined, 5-3

Impreg:

bulking agents, 19-4 to 19-5

dimensional stability, 19-9t

process, 19-4 to 19-5

properties, 19-6t

species, 19-5

strength properties, 19-7t to 19-8t

Incising:

and fire-retardant treatments, 18-15 to 

18-16

effect on strength properties, 5-43

Incense-cedar. See Cedar, Incense

Inorganic boron (borax/boric acid):

acceptable compounds, 15-13 to 15-15

effectiveness, 15-13 to 15-15

solubility, 15-6, 15-8

temperature for pressure treating, 15-20

uses, 15-6, 15-8

Insect damage and control:

beetles, 14-9 to 14-10

carpenter ants, 14-13

bees, 14-13

effect on strength, 5-44

naturally termite-resistant wood, 14-12 

to 14-13

termites, 14-11 to 14-12

types of damage, 14-8t, 14-9fig

Insulation for sound control for wood 

buildings, 17-11, 17-12t

Insulation in wood building, 17-11 to 17-12

Intergrown knots, 3-15, 5-27

Interlocked grain:

definition, 3-15

effect on strength, 5-30

in Sweetgum, 2-9

Interior finishes, care during construction, 

13-19

Internal friction, 5-17, 5-21

Invasive species. See Heat sterilization

Ipe:

decay resistance, 14-5t

locality of growth, 2-25

machinability, 2-25

mechanical properties, 5-19t, 5-23t

resistance to decay and insects, 2-25

shrinkage values, 4-8t

uses, 2-25

Ipil. See Merbau

Iroko:

characteristics, 2-25

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-25

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-25

shrinkage values, 4-8t

species, 2-25

workability, 2-25

uses, 2-25

Iron stain, 14-2, 16-25, 16-30

Ironwood, nomenclature, 6-5t

Isocyanate adhesives:

performance over time, 10-21

structural performance, 10-11t to 10-13t

use with composite products, 11-4

working and strength properties, and 

uses, 10-11t to 10-13t

Jacaranda. See Rosewood, Brazilian

Jarrah:

characteristics, 2-25 to 2-26

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-25

machinability, 2-26

mechanical properties, 5-19t, 5-23t

resistance to decay and insects, 2-25 to 

2-26

marine borers, 14-13 to 14-15

shrinkage values, 4-8t

uses, 2-26

Jatoba. See Courbaril

Jelutong:

characteristics, 2-26

decay resistance, 14-5t

locality of growth, 2-26

mechanical properties, 5-19t, 5-23t

shrinkage values, 4-8t

uses, 2-26

workability, 2-26
I–14

General Technical Report FPL–GTR–190

Jequitiba. See Albarco

Joints, glued, strength, 10-4 to 10-5

Joists and light-frame construction, 17-1 

to 17-2

Joists and rafter systems in light-frame 

construction, 17-3

Juniper, decay resistance, 14-5t

Juvenile wood:

discussion, 3-13

effect on mechanical properties, 5-32 to 

5-33

effect on strength properties, 5-32 to 

5-33

properties, 5-33, 5-33fig

shrinkage of, 5-32

Kakaralli. See Manbarklak

Kaneelhart:

characteristics, 2-26

ease of bonding, 10-7t

locality of growth, 2-26

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-26

shrinkage values, 4-8t

uses, 2-26

Kapur:

characteristics, 2-26

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-26

machinability, 2-26

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-26

shrinkage values, 4-8t

strength properties, 2-26

uses, 2-26

Karri:

characteristics, 2-26 to 2-27

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-26 to 2-27

machinability, 2-27

mechanical properties, 5-19t, 5-23t

shrinkage values, 4-8t

uses, 2-27

Kauta. See Marishballi

KDS, 15-8

Kempas:

characteristics, 2-27

decay resistance, 14-5t

locality of growth, 2-27

machinability, 2-27

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-27

shrinkage values, 4-8t

uses, 2-27

Keruing:

characteristics, 2-27

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

durability, 2-27

locality of growth, 2-27

machinability, 2-27

mechanical properties, 5-19t, 5-23t

shrinkage values, 4-8t

uses, 2-27

Khaya (See Mahogany, African):

dimensional change coefficient, 13-17t

Kiln drying:

advantages,13-6

importance of air circulation, 13-6

kiln schedules, 13-8 to 13-9, 13-11t

types of kilns, 13-6 to 13-8, 13-10fig

Knots:

changes during drying, 13-10, 13-12fig

definition, 5-26

discoloration through paint, 16-25

effect on:

in determining strength ratios, 7-5 to 

7-6, 7-6fig

mechanical properties of wood, 5-26 

to 5-28, 5-34

stiffness, 7-4

strength properties, 7-3 to 7-4

encased, 5-27, 7-4

in lumber stress grades, 7-4

intergrown, 5-27, 7-4

knotholes, 7-4

Kokrodua (See also Afrormosia):

dimensional change coefficient, 13-17t

Korina. See Limba

Krabak. See Mersawa

Kraft paper, 19-12

Kwila. See Merbau

Lag screws. See Screws, lag

Laminated members, curved:

advantages, 19-2 to 19-3

species, choice of, 19-2 to 19-3

uses, 19-2

Laminated members, glued. See Glued 

structural members.

Laminated strand lumber, 11-21

Laminated veneer lumber, 11-21

mechanical properties, 12-8t

Laminated wood, strength properties, 19-7t 

to 19-8t

Laminates. See Paper-based plastic 

laminates

Laminating grades, 6-8

Lapacho (See also Ipe):

ease of bonding, 10-7t

Lap marks, 16-19, 16-20fig

Lapuna. See Ceiba

Larch, western:

characteristics, 2-13

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

elastic ratio, 5-2t

for flooring, 6-16

locality of growth, 2-13

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t, 5-34t

nomenclature, 6-13t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-13

strength properties, 5-7t, 5-12t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

used for poles, 6-19

uses, 2-13

Lateral buckling, of beams, 9-9 to 9-10

Lateral resistance:

and the National Design Specification 

for Wood Construction, 8-1

of lag screws, 8-12 to 8-14

of nails, 8-5

of wood screws, 8-10 to 8-11

Latewood:

description, 3-4

erosion of planed surfaces, 16-12t

paintability, 16-7 to 16-8, 16-12

Lauans:

dimensional change coefficient, 13-17t

shrinkage values, 4-8t

Lemonwood. See Degame

Light-frame construction:

balloon framing, 17-1

ceiling and roof, 17-4

decks, 17-4

exterior walls, 17-3 to 17-4

fire resistance of, 18-2 to 18-3

floors, 17-2, 17-3fig

foundations, 17-2

platform framing, 17-1 to 17-2

use of elastomeric adhesives, 10-19 to 

10-20, 10-19fig

wood use in, 18-2

Lignin:

discussion, 3-7

weathering, 16-11 to 16-12

Lignumvitae:

characteristics, 2-27 to 2-28

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-28

mechanical properties, 5-19t, 5-23t

species, 2-27 to 2-28

uses, 2-28

working and strength properties, and 

uses, 10-11t to 10-13t

Limba:

characteristics, 2-28

decay resistance, 14-5t
I–15

Index

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-28

machinability, 2-28

mechanical properties, 5-19t, 5-23t

resistance to decay and insects, 2-28

shrinkage values, 4-8t

uses, 2-28

Limnoria, 14-14

Locust, black:

characteristics, 2-7

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-7

nomenclature, 6-5t

penetration, 15-16t

shock resistance, 2-7

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

uses, 2-1, 2-7

Lodgepole pine, used for poles, 6-19

Log homes, 6-21, 17-6, 17-7fig 

Logs, control of mold, stain, decay, 14-6

Longitudinal shrinkage of wood, 4-5

Lumber:

commonly used abbreviations, 6-23 to 

6-25

development of grading rules, 6-7

grading organization, 6-9t, 6-12

hardwood:

drying targets, 13-5

finished market products:

flooring:

grading rules, 6-6 to 6-7

standard dimensions, 6-3 to 6-4

types, 6-6

kiln drying schedules, 13-8 to 13-9, 

13-11t

grades, 6-2, 6-3t, 6-2fig

grading associations and rules, 6-2, 

6-3t

minimum widths, 6-3

standard dimensions, 6-3

standard thicknesses, 6-6t

uses, 6-2

purchase:

distribution yards, 6-12

primary manufacturers:

customers, 6-12

retail yard inventory:

availability of hardwood and 

softwoods, 6-12, 6-16

boards and yard lumber, 6-16

casing and base, 6-17

dimension and structural lumber 

stocked, 6-16

finish boards, 6-16

flooring, 6-16, 6-17

shingles and shakes, 6-17

siding, 6-16

purchase consideration, 6-17 to 

6-18

softwood:

American Lumber Standards, 6-7

classification by grades:

factory and shop lumber:

factory (shop) grades, 6-10

industrial clears, 6-10

ladder and pole stock, 6-10

moulding stock, 6-10

pencil stock, 6-10

tank stock, 6-10

structural lumber:

dimension lumber, 6-8

structural laminations, 6-8

yard lumber:

select lumber, 6-7

common lumber, 6-7, 6-8fig

development of grading rules, 6-7

drying targets, 13-5

grading organizations, 6-9t

kiln drying schedules, 13-8 to 13-9, 

13-11t

manufacture:

size, 6-10 to 6-11, 6-11t

surfacing, 6-11

patterns, 6-11, 6-12fig

species, 6-13t to 6-15t, 6-16

transportation, 6-12

Macacauba. See Macawood

Macawood:

characteristics, 2-28

common names, 2-44t

locality of growth, 2-28

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-28

shrinkage values, 4-8t

uses, 2-28

workability, 2-28

Machine-graded structural lumber:

allowable stress for bending, 7-8 to 7-10

common grades, 7-7, 7-7t

components of system, 7-8

design stresses for other properties, 7-10

machine sorting criteria, 7-8

procedures for deriving design 

properties, 7-8

quality control, 7-10

Machinmango. See Manbarklak

Madrone, Pacific:

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

nomenclature, 6-5t

shrinkage values, 4-6t

Magnesia-cement-bonded composites, 

11-23

Magnolia:

characteristics, 2-7 to 2-8

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-7

moisture content, 4-2t

nomenclature, 6-5t

shock resistance, 2-8

shrinkage values, 4-6t

species, 2-7

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

uses, 2-7

Mahogany:

species, 2-28

Mahogany, African:

characteristics, 2-28

dimensional change coefficients, 13-17t

decay resistance, 2-29, 14-5t

ease of bonding, 10-7t

elastic ratio, 5-2t

locality of growth, 2-28

machinability, 2-29

mechanical properties, 5-19t, 5-23t

Poisson ratio, 5-3t

shrinkage values, 4-8t

species, 2-28

uses, 2-29

Mahogany, American

characteristics, 2-29

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-29

machinability, 2-29

mechanical properties, 5-19t, 5-23t

uses, 2-29

Mahogany, Honduras:

elastic ratio, 5-2t

Poisson ratio, 5-3t

Mahogany, Phillippine, availability at retail 

yards, 6-16

Manbarklak:

characteristics, 2-29

locality of growth, 2-29

mechanical properties, 5-19t, 5-23t

nomenclature, 2-29, 2-44t

shrinkage values, 4-8t

resistance to fungi and insects, 2-29

marine borers, 14-13 to 14-15

uses, 2-29

workability, 2-29

Manni:

characteristics, 2-29

decay resistance, 14-5t

locality of growth, 2-29

machinability, 2-29

mechanical properties, 5-19t, 5-23t

resistance to insects, 2-29

shrinkage values, 4-8t

uses, 2-29

Maple, black:

dimensional change coefficients, 13-16t
I–16

General Technical Report FPL–GTR–190

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

Maple, bigleaf:

dimensional change coefficients, 13-16t

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

Maple, hard:

characteristics, 2-8

charring rate data, 18-14t

connector joint strength, 8-21t

decay resistance, 14-5t

ease of bonding, 10-7t

heat release data, 18-12fig

locality of growth, 2-8

nomenclature, 6-5t

shock resistance, 2-8

species, 2-8

uses, 2-8

Maple, Oregon, nomenclature, 6-5t

Maple, red:

dimensional change coefficients, 13-16t

elastic ratio, 5-2t

Poisson ratio, 5-3t

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

Maple, silver:

dimensional change coefficients, 13-16t

moisture content, 4-2t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

Maple, soft:

connector joint strength, 8-21t

ease of bonding, 10-7t

locality of growth, 2-8

nomenclature, 6-5t

species, 2-8

uses, 2-8

Maple, sugar:

characteristics for painting, 16-5t

dimensional change coefficients, 13-16t

elastic ratio, 5-2t

fracture toughness, 5-27t, 5-28t

moisture content, 4-2t

penetration, 15-16t

Poisson ratio, 5-3t

shrinkage values, 4-6t

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

toughness values, 5-27t

Maple flooring:

availability, 6-16 to 6-17

grading, 6-6

Maple Flooring Manufacturers Association, 

grading rules, 6-6

Marishballi:

characteristics, 2-29 to 2-30

locality of growth, 2-29 to 2-30

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-30

shrinkage values, 4-8t

uses, 2-30

Marine borer damage and control, 14-13 

to 14-15

Malayapis. See Lauans

Mata-mata. See Manbarklak

Mayflower. See Roble

Mechanical interlocking and adhesives, 

10-1 to 10-2

Mechanical properties of wood adjusted for 

design use, 7-10 to 7-13

affected by:

adhesion to metals, 10-5

age, 5-41

changes in moisture content, 5-34

chemicals:

exposure to, 5-41

treatment, 5-41 to 5-43

cross grain, 5-30

decay, 5-43 to 5-44

duration of load, 5-39 to 5-40

fire-retardant treatments, 18-15

insect damage, 5-44

knots, 5-26 to 5-27

juvenile wood, 5-32 to 5-33

mold and stain fungi, 5-43

rate of loading, 5-38

slope of grain, 5-30t

temperature, 5-35 to 5-38

waterborne preservatives, 5-42

relation to specific gravity, 5-26, 5-29t

relation to stress grades, 7-2

Medium-density fiberboard:

grade stamp, 11-14fig

properties, 11-13, 12-5, 12-6t

resins used, 11-13

standards, 11-3t

uses, 11-13

with veneer overlay, 11-15, 11-16fig

Medium-density hardboard, finishes, 

suitability and expected service life, 16-17t

Medium density overlays:

finishes, suitability and expected service 

life, 16-17t

to improve paintability of plywood, 16-9

Melamine adhesives:

performance over time, 10-21 to 10-22

use with composite products, 11-4

working and strength properties, and 

uses, 10-11t to 10-13t

Meranti:

characteristics, 2-30

color, 2-30t

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-30

machinability, 2-30

mechanical properties, 5-21t, 5-25t

uses, 2-30

Merbau:

characteristics, 2-30

locality of growth, 2-30

machinability, 2-30

mechanical properties, 5-19t, 5-23t

resistance to insects, 2-30

shrinkage values, 4-8t

uses, 2-30

Mersawa:

characteristics, 2-31

decay resistance, 14-5t

locality of growth, 2-31

machinability, 2-31

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-31

shrinkage values, 4-8t

uses, 2-31

Metal bonding, 10-5

Metal plate connectors, 8-25, 8-26fig

Mildew:

discoloration of wood, 16-28 to 16-29, 

16-28fig

removal, 16-28 to 16-29

Mill glaze, 16-26 to 16-27

Mill work, finishes, suitability and 

expected service life, 16-17t

Modified woods:

formaldehyde-treated wood:

dimensional stability, 19-9t, 13-12

mechanical properties, 19-11 to 19-12

resistance to fungi, 19-12

chemical modification:

antishrink efficiency calculations, 

19-12

conditions for, 19-12

chemicals used, 19-12

dimensional stability, 19-9t, 19-11

mechanical properties, 19-11 to 19-12

cost, 19-5

dimensional stability, 19-9t

purposes, 19-4 to 19-5

resin-treated compressed wood 

(Compreg):

advantages, 19-5

dimensional stability, 19-9t

molding, 19-5, 19-9

plasticizers, 19-5

properties, 19-5, 19-6t

strength properties, 19-7t to 19-8t

thermal expansion coefficients, 19-8t

uses, 19-9

resin-treated wood (Impreg):

bulking agents, 19-4 to 19-5

dimensional stability, 19-9t

process, 19-5

properties, 19-6t

strength properties, 19-7t to 19-8t
I–17

Index

untreated compressed wood (Staypak):

appearance, 19-10

dimensional stability, 19-9t

properties, 19-9 to 19-10, 19-6t

purpose, 19-9

strength properties, 19-7t to 19-8t

uses, 19-9 to 19-10

untreated heated wood (Staybwood):

loss of mechanical and strength 

properties, 19-9

purpose, 19-9

wood–polymer composites:

advantages, 19-11 to 19-12

changing characteristics, 19-11 to 

19-12

monomers, 19-11

species used, 19-11

strength properties, 19-11t

uses, 19-11

wood treated with polyethylene glycol 

(PEG):

dimensional stability, 19-9t

finishing, 19-10

process, 19-10

uses, 19-10

Modulus of elasticity:

and columns with flanges, 9-8

coefficient of variation, 5-26t

discussed, 5-3

effect of temperature, 5-35, 5-36fig, 

5-37t

of machine graded lumber, 7-7 to 7-8

of visually graded lumber, 7-6 to 7-7

values, 5-2t

Modulus of rigidity:

discussion, 5-3

ratios, 5-3t

Modulus of rupture:

and moisture content, 7-11, 7-11t, 

7-12fig

coefficient of variation, 5-26t

defined, 5-3

effect of temperature, 5-36fig, 5-36t, 

5-37fig

of beams, 9-5, 9-6

temperature effect, 5-36

Moisture blisters, 16-26

Moisture content:

adsorption/desorption curve, 4-5fig

adjustment for, 7-12

and decay, 14-4

and electrical resistance, 4-15

and heat capacity, 4-11

changes in finished ponderosa pine 

sapwood, 16-14fig

definition, 4-2, 16-6

determined by:

electrical method, 13-2 to 13-3

ovendrying method, 13-2, 13-2eq

dimensional changes, 10-6 to 10-8

effect on bonded joints, 10-9

effect of strength properties, 5-34, 

5-35fig

equilibrium:

discussion, 4-3, 4-3fig

relative humidity as related to, 4-4t

green wood, 4-1, 4-2t

heartwood, 4-2t

in wood exposed outdoors, 16-7

recommended for:

exterior siding, 13-5t

flooring, 13-5t

furniture, 13-5t

interior woodwork, 13-5t

laminated members, 13-5, 13-5t

lumber, 13-3, 13-4t, 13-5t

plywood, 13-5, 13-5t

sheathing, 13-5t

timbers, 13-3

trim, 13-5t

veneer, 13-3

sapwood, 4-2t

shrinkage as related to, 4-5 to 4-7, 4-10

shrinkage curves, 4-7fig

specific gravity, 4-7, 4-9, 4-10fig

Moisture content of dried lumber:

air-dry, 13-11

kiln dry, 13-12

shipping dry, 13-11

Moisture content during transit and 

storage:

finish and factory lumber, 13-14

general, 13-14, 13-15t

plywood and structural items, 13-14

Moisture-gradient, typical in lumber, 13-6, 

13-7fig

Molding resin-treated compressed wood 

(Compreg), 19-5, 19-9

Molds:

appearance of, 14-2

distinction from stain, 14-2

effect on wood, 14-2

Moment capacity, 9-5, 9-5eq

Mora:

characteristics, 2-31

locality of growth, 2-31

mechanical properties, 5-19t, 5-23t

resistance to fungi and insects, 2-31

shrinkage values, 4-8t

uses, 2-31

workability, 2-31

Multiple-fastener joints, 8-24 to 8-25, 

8-24eq

Nails:

lateral resistance of common wire nails:

pre-1991:

equations, 8-8

lateral load coefficients, 8-6t

load-slip curve, 8-6, 8-6fig

post-1991:

direction of driving, effect of, 8-6

load-slip curves, 8-8, 8-8eq, 8-9fig

moisture content, effect of, 8-8

seasoning, effect of, 8-8

shank form, effect of, 8-8

spacing, 8-7

yield model theory, 8-6, 8-7eq, 8-9fig, 

8-7t

sizes:

box nails, 8-2t

common, 8-2fig

helically and annularly threaded, 8-2, 

 8-2t

penny size, 8-2

wire nails, 8-2, 8-2t

toenailed joints, strength of, 8-5

withdrawal resistance:

affected by, 8-2, 8-3

allowable loads,8-6

clinched nails, 8-5

corrosion, 8-5

density of wood, effect of, 8-3

direction of driving, 8-5

etched nails, 8-4

load displacement curve, 8-3, 8-3fig

moisture content, effect of, 8-3, 8-4

nail heads, 8-4

nail points, effect of, 8-4

plywood, 8-6

prebored lead holes, effect of, 8-5

seasoning, effect of, 8-3

shank form, effect of, 8-4

surface coatings, effect of, 8-3 to 8-4

surface coating, used for 8-4

Nanoindentation hardness, 5-15

Naphthenate, copper:

color transfer and changes, 15-6

effectiveness, 15-6

retention levels for various wood 

products, 15-4t, 15-5t

solution values, 15-3 to 15-9

Southern pine sapwood stakes retention 

and life span test results, 15-7t, 15-8t

treatment for cutting pretreated wood, 

15-25

Naphthenate, zinc:

effectiveness, 15-11

inappropriate uses, 15-11

properties, 15-11

National Grading Rule, 7-2, 7-3, 7-3t

National Fire Protection Association, 18-2, 

18-5

National Hardwood Lumber Association, 

6-2, 6-4t

National Oak Flooring Manufacturers 

Association, grading rules, 6-6 to 6-7

Nomenclature, 6-5t, 6-13t

Nuclear radiation:

discussion, 4-17

effect on wood strength, 5-43
I–18

General Technical Report FPL–GTR–190

Oak:

for casing and base, 6-17

used in piles, 6-20

Oak, black:

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

Oak, bur:

strength properties, 5-6t, 5-11t

thermal conductivity, 4-13t

Oak, California black, moisture content, 

4-2t

Oak, cherrybark, strength properties, 5-5t, 

5-10t

Oak, chestnut, strength properties, 5-6t, 

5-11t

Oak, laurel, strength properties, 5-5t, 5-10t

Oak, live, strength properties, 5-6t, 5-11t

Oak, northern red:

characteristics for painting, 16-5t

fracture toughness, 5-28t

strength properties, 5-5t, 5-10t

Oak, overcup:

strength properties, 5-6t, 5-11t

tensile strength, 5-26t

toughness values, 5-27t

Oak, pin:

strength properties, 5-5t, 5-10t

tensile strength, 5-26t

toughness values, 5-27t

Oak, post, strength properties, 5-6t, 5-11t

Oak, red:

availability for purchase, 6-17

characteristics, 2-8

charring rate data, 18-14t

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

flammability data, 18-10t

heat release data, 18-12fig

locality of growth, 2-8

mechanical properties, 5-19t, 5-23t

nomenclature, 6-5t

Poisson ratio, 5-3t

penetration, 15-16t

shrinkage values, 4-6t

species, 2-8

uses, 2-8

Oak, scarlet:

strength properties, 5-5t, 5-10t

toughness values, 5-27t

Oak, southern red:

moisture content, 4-2t

strength properties, 5-5t, 5-10t

thermal conductivity, 4-13t

Oak, swamp chestnut, strength properties, 

5-6t, 5-11t

Oak, swamp white, strength properties, 

5-6t, 5-11t

Oak, tropical:

characteristics, 2-31

locality of growth, 2-31

resistance to fungi, 2-31

specific gravity, 2-31

uses, 2-31

Oak, water:

moisture content, 4-2t

strength properties, 5-5t, 5-11t

Oak, white:

characteristics, 2-1, 2-8 to 2-9

characteristics for painting, 15-16t

charring rate equation, 18-13

connector joint strength, 8-21t

decay resistance, 2-9, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

flame spread index, 18-4t

locality of growth, 2-8 to 2-9

nomenclature, 6-5t

penetration, 15-16t

Poisson ratio, 5-3t

shrinkage values, 4-6t

species, 2-8 to 2-9

strength properties, 5-6t, 5-11t

uses, 2-9

Oak, willow, moisture content, 4-2t

Oak flooring, 6-6

Obeche:

characteristics, 2-31 to 2-32

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-44

machinability, 2-32

mechanical properties, 5-19t, 5-23t

shrinkage values, 4-8t

uses, 2-32

Ofram. See Limba

Oilborne preservatives. See preservatives, 

oilborne

Oil-type preservatives, strength loss, 5-42

Okoume:

characteristics, 2-32

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-32

machinability, 2-32

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

uses, 2-32

Old house borer, 14-10

Oligomeric alkylphenol polysulfide 

(PXTS), 15-9

Opaque finishes, 16-32

Opepe:

characteristics, 2-32

ease of bonding, 10-7t

locality of growth, 2-32

machinability, 2-32

mechanical properties, 5-20t, 5-24t

resistance to decay and insects, 2-32

shrinkage values, 4-8t

uses, 2-32

Ordinary construction:

code requirements for fire protection, 

18-2

description, 18-2

Oriented strandboard:

adhesive application or blending, 11-10

adhesives used, 11-7

certification, 11-10, 11-8fig

defined, 11-7

drying process, 11-8, 11-10

finishes, suitability and expected service 

life, 16-17t

grade stamps, 11-8fig

hot pressing, 11-10

manufacturing process, 11-8, 11-10, 

11-10fig

mat formation, 11-10

mechanical properties, 12-5t

species used, 11-8

specific gravity, 12-5t

standards, 11-3t

stranding process, 11-8

Oriented strand lumber, 11-21

Orthotropic nature of wood, 5-1

Osage-orange:

decay resistance, 14-5t

ease of bonding, 10-7t

nomenclature, 6-5t

Ossol. See Manni

Otie. See Ilomba

Ovangkol (See also Benge):

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

Ovendry weight, specific gravity, 4-9, 4-9t

Oxine copper:

composition, 15-11

corrosiveness, 15-11

retention levels for various wood 

products, 15-4t, 15-5t

Southern pine sapwood stakes retention 

and life span test results, 15-7t to 15-8t

toxicity, 15-11

Pacific yew, nomenclature, 6-14t

Paint:

adhesion bandage test, 16-28

application and maintenance, 16-19t, 

16-20 to 16-21

characteristics of wood, 16-5t

cracking, 16-26

discussed, 16-26 to 16-27

disposal, 16-36

failure caused by:

dimensional changes in wood, 16-7
I–19

Index

earlywood/latewood band, 16-2

face checks in plywood, 16-9, 16-9fig

grain orientation, 16-8

moisture content of wood, 16-6 to 

16-7

temperature at time of painting, 16-22 

to 16-23

texture of wood, 16-9

water soluble extractives, 16-29

weathering, 16-11

failure in finger-jointed lumber, 16-9 to 

16-10

lead-based:

dust from, 16-36 to 16-37

health effects, 16-36 to 16-37

removal of, 16-36 to 16-37

use of, 16-36

mildew removal, 16-29

moisture blisters, 16-26

on treated wood, 16-24

on preweathered panels, 16-12

peeling:

avoiding with proper painting, 16-24

caused by water, intercoat, 16-27

protection against moisture, 16-13

preweathering before painting, 16-13

repainting, 16-24, 16-32 to 16-33

removal, 16-35 to 16-36

service life, 16-25

strippers:

chemical:

alkali-based, 16-36

solvent-based, 16-36

mechanical, 16-35

“safe”, 16-36

VOC regulation compliance, 16-15 to 

16-16

Pallet and container stock, 6-17

Palosapis. See Mersawa

Panel products:

classification, 11-5fig

fiberboard, 11-12 to 11-15

oriented strandboard, 11-7 to 11-10

particleboard, 11-10 to 11-12

performance standards, 11-3t

plywood, 11-5 to 11-7

product standards, 11-3t

Paper-based plastic laminates:

decorative laminates:

process, 19-12 to 19-13

thicknesses, 19-13

uses, 19-12 to 19-13

industrial laminates:

cost advantage, 19-12

papreg, 19-13

resins used, 19-13

strength, 19-12, 19-7t to 19-8t

uses, 19-12

lignin-filled laminates:

strength properties, 19-14

resins used, 19-14

shrinking and swelling, 19-13

Paper-faced overlays:

manufacture, 19-14

types:

decoratives, 19-13

masking, 19-14

structural, 19-14

uses, 19-13

Papreg, 19-13

Para-Angelim (See also Sucupira):

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

Parallell strand lumber, 11-21

Parana pine:

characteristics, 2-39

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-39

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

uses, 2-39

Particleboard, 11-10 to 11-12

adhesives, 11-11

certification, 11-12

finishing, 11-12

hot pressing, 11-12

mat formation, 11-11

mechanical properties, 12-5t

particle classifications and conveying, 

11-11

particle drying, 11-11

production, 11-11

resins and wax, 11-11

screw withdrawal, 8-10

specific gravity, 12-5t

standards, 11-3t

uses, 11-11

Pau marfim:

characteristics, 2-32 to 2-33

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-32

machinability, 2-32

mechanical properties, 5-20t, 5-24t

resistance to decay, 2-32

shrinkage values, 4-8t

specific gravity, 2-32

strength values, 2-32

uses, 2-33

Pecan (See also Hickory, pecan):

nomenclature, 6-5t

Pecky cypress, 2-11

Pentachlorophenol:

and nonpressure treatments, 15-20 to 

15-21

approved uses, 15-10

effect on mechanical properties of wood, 

15-20

effectiveness, 15-10

EPA-approved consumer information 

sheet, 15-2

handling precautions, 15-2

ineffective against marine borers, 15-10

paintability, 15-10

retentions for various wood products, 

15-4t, 15-5t

solution standards, 15-10

solution performance, 15-10

use site precautions, 15-2

Peroba, white. See Peroba de Campos

Peroba de campos:

characteristics, 2-33

locality of growth, 2-33

machinability, 2-33

mechanical properties, 5-20t, 5-24t

resistance to decay, 2-33

shrinkage values, 4-8t

uses, 2-33

Peroba rosa:

characteristics, 2-33

ease of bonding, 10-7t

locality of growth, 2-33

mechanical properties, 5-20t, 5-24t

resistance to fungi and insects, 2-33

shrinkage values, 4-8t

uses, 2-33

Persimmon:

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

nomenclature, 6-5t

shrinkage values, 4-6t

Phenolic adhesives:

performance over time, 10-22, 10-22fig

structural performance, 10-11t to 10-13t

use with composite products, 11-3, 11-5

working and strength properties, and 

uses, 10-11t to 10-13t

Phenolic resins:

for laminates, 19-13

Pholads, 14-14

Photodegradation. See Weathering

Piles:

knots, effect on strength, 5-26 to 5-28

marine, protection against insects, 14-14 

to 14-15

preservative retention levels, 15-4t to 

15-5t

preservative treatments, 6-22

service life, 6-22

standards and specifications, 6-18t, 6-20 

to 6-21

standards for end-bearing piles, 6-21

for friction piles, 6-21

straightness, 6-21

strength properties, 7-14

timber availability, 6-20

weight and volume, 6-21 to 6-22
I–20

General Technical Report FPL–GTR–190

Pilon:

characteristics, 2-33

locality of growth, 2-33

mechanical properties, 5-20t, 5-24t

resistance to insects, 2-33

uses, 2-33

workability, 2-33

Pine, Caribbean:

characteristics, 2-39

ease of bonding, 10-7t

locality of growth, 2-39

machinability, 2-39

mechanical properties, 5-20t, 5-24t

resistance to insects, 2-39

shrinkage values, 4-8t

uses, 2-39

Pine, eastern white:

characteristics, 2-13

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

flame spread index, 18-4t

locality of growth, 2-13

mechanical properties, 5-14t, 5-15t

nomenclature, 6-13t

penetration, 15-16t

shock resistance, 2-13

shrinkage values, 4-6t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

uses, 2-13

workability, 2-13

Pine, jack:

characteristics, 2-13

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-13

mechanical properties, 5-14t, 5-15t

nomenclature, 6-13t

penetration, 15-16t

shock resistance, 2-13

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

toughness values, 5-28t

uses, 2-13

Pine, loblolly:

decay resistance, 14-5t

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

moisture content, 4-2t, 5-34t

penetration, 15-16t

Poisson ratio, 5-3t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

Pine, lodgepole:

characteristics, 2-13 to 2-14

decay resistance, 14-5t

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

flame spread index, 18-4t

locality of growth, 2-13 to 2-14

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

nomenclature, 6-13t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-14

shrinkage values, 2-14, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

toughness values, 5-28t

used for poles, 6-19

workability, 2-14

uses, 2-14

Pine, longleaf:

decay resistance, 14-5t

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

moisture content, 4-2t, 5-34t

nomenclature, 6-13t

Poisson ratio, 5-3t

shrinkage values, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

Pine, ocote:

characteristics, 2-39

locality of growth, 2-39

mechanical properties, 5-20t, 5-24t

resistance to fungi, 2-39

shrinkage values, 4-8t

strength properties, 2-39

uses, 2-39

Pine, pitch:

characteristics, 2-14

decay resistance, 14-5t

locality of growth, 2-14

nomenclature, 6-13t

shock resistance, 2-14

shrinkage values, 2-14, 4-6t

strength properties, 5-7t, 5-12t

thermal conductivity, 4-14t

uses, 2-14

Pine, pond:

characteristics, 2-14

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-14

elastic ratio, 5-2t

Poisson ratio, 5-3t

shock resistance, 2-14

shrinkage values, 2-14, 4-6t

strength properties, 5-8t, 5-13t

uses, 2-14

Pine, ponderosa:

characteristics, 2-14

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

for siding, 6-16

fracture toughness, 5-28t

Jeffrey, 2-14

locality of growth, 2-14

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

changes in finished sapwood samples, 

16-14fig

moisture-excluding effectiveness of 

finishes on, 16-14t

nomenclature, 6-13t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-14

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

toughness values, 5-28t

used for poles, 6-19

uses, 2-14

Pine, radiata:

characteristics, 2-39 to 2-40

ease of bonding, 10-7t

locality of growth, 2-39

machinability, 2-40

mechanical properties, 5-20t, 5-24t

uses, 2-40

Pine, red:

characteristics, 2-14

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

flame spread index, 18-4t

heat release index, 18-10t

locality of growth, 2-14

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t, 5-34t

penetration, 15-16t

preservative treatments to prevent 

marine borer attacks on piles, 15-5t

Poisson ratio, 5-3t

shock resistance, 2-14

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

toughness values, 5-28t

uses, 2-14

Pine, sand, strength properties, 5-8t, 5-13t

Pine, scots, fracture toughness, 5-28t
I–21

Index

Pine, shortleaf:

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

moisture content, 4-2t

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

toughness values, 5-28t

Pine, slash:

dimensional change coefficient, 13-16t

decay resistance, 14-5t

elastic ratio, 5-2t

Poisson ratio, 5-3t

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

toughness values, 5-28t

Pine, southern:

characteristics, 2-14 to 2-15

characteristics for painting, 16-5t

charring rate data, 18-14t

charring rate equation, 18-13

connector joint strength, 8-21t

decay resistance, 14-5t

ease of bonding, 10-6, 10-7t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

flammability data, 18-10t

for flooring, 6-16 to 6-17

fracture toughness, 5-28t

grading, 6-8

kiln drying schedule, 13-8, 13-11t

moisture content and property values, 

5-36t

nomenclature, 6-13t

preservative retention and life span tests, 

15-7t to 15-8t

preservative treatments to prevent 

marine borer attacks on piles, 15-5t

shock resistance, 2-15

shrinkage values, 2-15, 4-6t

species, 2-14 to 2-15

used for piles, 6-20

used for poles, 6-19

uses, 2-15

Pine, spruce:

characteristics, 2-15

decay resistance, 14-5t

locality of growth, 2-15

strength properties, 5-8t, 5-13t

uses, 2-15

Pine, sugar:

characteristics, 2-15

connector joint strength, 8-12t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

locality of growth, 2-15

moisture content, 4-2t

nomenclature, 6-14t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-15

shrinkage values, 2-15, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

uses, 2-15

workability, 2-15

Pine, Virginia:

characteristics, 2-15 to 2-16

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-15

shock resistance, 2-15

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

tensile strength, 5-26t

toughness values, 5-28t

uses, 2-16

Pine, western white:

characteristics, 2-16

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

flame spread index, 18-4t

for siding, 6-17

fracture toughness, 5-28t

heat release data, 18-12fig

locality of growth, 2-16

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

paintability rating, 16-14fig

Poisson ratio, 5-3t

shock resistance, 2-16

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

uses, 2-16

workability, 2-16

Pitch pockets:

description, 5-33

effect on strength, 5-33

in lumber stress grading, 7-5

species involved, 5-33

Pith, 3-2, 3-2fig

Piquia:

characteristics, 2-33 to 2-34

locality of growth, 2-33

machinability, 2-34

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

resistance to fungi and insects, 2-34

uses, 2-34

Plainsawn lumber:

advantages, 3-15t

method of producing, 3-14

shrinkage, 4-5

Plasticizing adhesive polymers, 10-9

Plasticizing wood:

bent wood members, 19-2

bending operation and apparatus, 19-4

characteristics of bent wood, 19-4

chemicals used, 19-2

fixing the bend, 19-4

laminated members, 19-2 to 19-3

moisture content of bending stock, 19-3 

to 19-4

principles of plasticizing and bending, 

19-1

resin-treated compressed wood, 19-5

selection of stock, 19-3

steaming, 19-1

veneered curved members, 19-3

Plastering, care during construction, 13-19

Plastic bonding, 10-5

Plastic-coated nails, 8-4

Port-Orford Cedar, See Cedar, Port-Orford

Plywood:

adhesives, 11-7

advantages over solid wood, 11-6

assembly, 11-5

classification by:

exposure capability, 11-7

grades, 11-7

description, 11-5

erosion of planed surfaced, 16-12t

finishes, suitability and expected service 

life, 16-17t

finishing, 16-16

fire-retardant treated, 18-15

grade stamp, 11-8fig, 11-9fig

in light-frame construction, 17-3

mechanical properties, 11-6, 12-4t

piles, 14-6

plies, 11-6

preservative retention levels, 15-4t to 

15-5t

product standard, 11-7, 11-3t

protecting from decay, 14-6

specialty panels, 11-7

specific gravity, 12-4t

standards, 11-3t

types:

construction and industrial, 11-7

hardwood and decorative, 11-7

weathering of, 16-9

Plywood curved members:

bent after gluing:

common hardwoods for, 19-3

procedure for, 19-2

bent and glued simultaneously:

advantages, 19-2 to 19-3

procedure for, 19-2

Poisson’s ratio:

discussion, 5-2

values, 5-3t
I–22

General Technical Report FPL–GTR–190

Pole buildings, 17-4 to 17-5, 17-5fig, 17-

6fig, 6-20

Poles:

availability, 6-19

characteristics relating to:

Douglas-fir, 6-19

lodgepole pine, 6-19

ponderosa pine, 6-19

southern pine, 6-19

western larch, 6-19

western redcedar, 6-19

control of mold, stain, and decay, 14-6

form and taper, 6-20 to 6-21, 6-20t

preservation and seasoning, 6-22

preservative treatment, retention levels, 

15-4t to 15-5t

specifications, 6-18t

service life, 6-22

species selection, 6-19

standards and specifications, 6-18t

strength properties, 7-13 to 7-14

weight and volume, 6-21 to 6-22

Polymeric methylene diphenyl diisocyanate 

(PMDI) adhesives, 10-14

Polymerization, effect on mechanical 

properties, 5-43

Polymers, organic and synthetic, and 

adhesion, 10-9 to 10-10

Polymers, analytic chemical and 

mechanical testing, 10-19 to 10-20

Polyurethane bonding with wood and 

nonwood composites, 10-5

Polyurethane adhesives:

performance over time, 10-21 to 10-22

structural performance, 10-11t to 10-13t

working and strength properties, and 

uses, 10-11t to 10-13t

Polyvinyl adhesives:

structural performance, 10-11t to 10-13t

working and strength properties, and 

uses, 10-11t to 10-13t

Porches:

cleaning before refinishing, 16-23 to 

16-24

finishing, 16-25

pests, 14-13

Pores. See Vessels

Portland-cement-bonded composites, 

11-24, 12-9

Post-frame buildings, 17-4 to 17-5, 17-5fig, 

17-6fig

Pre-drying, 13-6

Prefinished wood products, 16-25

Preservative penetration:

heartwood, 15-13 to 15-15, 15-16t

incising, effect of, 15-17 to 15-18

sapwood, 15-13 to 15-15, 15-16t

softwoods, 15-16t

Preservative-pressure-treated wood, EPAapproved customer information sheets, 

15-2

Preservative retention tests, pressure 

treated, 15-4t to 15-5t

Preservatives:

copper-containing, control of marine 

borers with, 14-14 to 14-15

effect on mechanical properties, 15-20

effect on paintability, 16-24 to 16-25

effect on strength of oil-type, 5-42

EPA regulations, 15-1 to 15-2

inorganic arsenicals:

EPA-approved information sheet, 15-2

handling precautions, 15-2

use site precautions, 15-2

oilborne, various types:

alkyl ammonium compound:

effectiveness, 15-12

in ammoniacal copper quat, 15-5

solubility, 15-5

chlorothalonil/chlorpyrifos:

component rations, 15-12

effectiveness, 15-12

chlorpyrifos:

effectiveness, 15-12

in combination, 15-12

copper naphthenate:

color transfer and changes, 15-11

effectiveness, 15-11

retention levels for various wood 

products, 15-4t to 15-5t

solution values, 15-11

Southern pine sapwood stakes 

retention and life span test results, 

15-7t to 15-8t

treatment for cutting pretreated 

wood, 15-18

creosote, coal-tar:

advantages, 15-10

appearance, 15-10

composition variability, 15-10

EPA-approved customer 

information sheet, 15-2

effect on mechanical properties, 

15-20 to 15-21

for non-pressure treatments, 15-20

handling precautions, 15-2

in pressure treatment process, 15-18

odor and vapors, 15-2

retention levels for various wood 

products, 15-4t to 15-5t

standards, 15-2

temperature for pressure treating, 

15-18

treatment for cutting pretreated 

wood, 15-18

use site precautions, 15-2

volatility, 15-10

creosote-coal-tar solutions:

properties, 15-10

retention levels for various wood 

products, 15-4t to 15-5t

standards by volume, 15-10

temperature for pressure treating, 

5-41

creosote-petroleum oil solutions:

retention levels for various wood 

products, 15-4t to 15-5t

oxine copper:

composition, 15-11

corrosiveness, 15-11

retention levels for various wood 

products, 15-4t to 15-5t

Southern pine sapwood stakes 

retention and life span test results, 

15-7t to 15-8t

toxicity, 15-11

pentachlorophenal solutions:

and nonpressure treatments, 15-20

solution standards, 15-10 to 15-11

solution performance, 15-10 to 

15-11

pentachlorophenol:

approved uses, 15-10

effect on mechanical properties of 

wood, 15-20

effectiveness, 15-10

EPA-approved consumer 

information sheet, 15-2

handling precautions, 15-2, 15-10

ineffective against marine borers, 

15-10

paintability, 15-10

retention levels for various wood 

products, 15-4t to 15-5t

Southern pine sapwood stakes 

retention and life span test results, 

15-7t to 15-8t

use site precautions, 15-2

propiconazole:

effectiveness, 15-9

solubility, 15-9

uses, 15-9

tebuconazole:

effectiveness, 15-9

solubility, 15-9

zinc naphthenate:

effectiveness, 15-11

inappropriate uses, 15-11

properties, 15-11

3-iodo-2-propynyl butyl carbamate:

effectiveness, 15-11

paintability, 15-11

retail marketing, 15-11

use in combination, 15-11

uses, 15-11

4, 5-dichloro-2-N-octyl-4-

isothiazolin-3-one:

effectiveness, 15-12

inappropriate uses, 15-12

solubility, 15-12

waterborne, general:
I–23

Index

and non-pressure treatment, 15-12

application and maintenance of 

exterior wood finish, 16-19t

effectiveness, 15-12

effect on mechanical properties, 5-41 

to 5-43

effect on strength, 5-41 to 5-43

finishing, 16-24

for marine piles, 15-3

initial kiln-drying temperature, 5-42

paintability, 15-12

post-treatment kiln drying 

temperatures, 5-42 to 5-43

retention levels effect of strength, 

5-15

retention levels for various wood 

products, 15-4t to 15-5t

retentions necessary for marine borer 

protection, 15-5t

temperature considerations, 15-3

waterborne, various types:

acid copper chromate (ACC):

components, 15-3

effectiveness and leaching, 15-3

retention levels for various wood 

products, 15-4t to 15-5t

Southern pine sapwood stakes 

retention and life span test results, 

15-7t to 15-8t

temperature for pressure treating, 

15-21

alkaline copper quat (ACQ): 

common types, 15-5

composition of common types, 

15-5t

retention levels for various wood 

products, 15-4t

ammoniacal copper zinc arsenate 

(ACZA):

composition, 15-3

replacement for ACA, 15-3

retention levels for various wood 

products, 15-4t to 15-5t

temperature for pressure treating, 

15-20

use, 15-3

use with Douglas-fir, 15-3 to 15-4

chromated copper arsenate (CCA):

common types, 15-4 to 15-5

component substitutions, 15-4 to 

15-5

composition of common types, 

15-5t

effectiveness of common types, 

15-4 to 15-5

effect on adhesion, 10-4

finishing wood treated with, 16-24 

to 16-25

resistance to marine borers, 15-5

retention levels for various wood 

products, 15-4t to 15-5t

Southern pine sapwood stakes 

retention and life span test results, 

15-7t to 15-8t

temperature for pressure treating, 

15-20

use with Douglas-fir, 15-3 to 15-4

copper azole - Type A (CBAType A):

retention levels for various wood 

products, 15-4t to 15-5t

solution percentages, 15-6

temperature for pressure treating, 

15-20

copper bis(dimethyldithiocarbamate) 

(CDDC):

retention levels for various wood 

products, 15-4t to 15-5t

solution percentages, 15-6

temperature for pressure treating, 

15-20

uses, 15-6

copper HDO (CXA), 15-6

copper naphthenate (waterborne), 

15-6

EL2, 15-8

ICC-ES evaluation report 

preservatives, 15-9

inorganic boron (borax/boric acid):

acceptable compounds, 15-6, 15-8

effectiveness, 15-6, 15-8

solubility, 15-6, 15-8

temperature for pressure treating, 

15-20

uses, 15-6, 15-8

KDS, 15-8

oligomeric alkylphenol polysulfide 

(PXTS), 15-9

propiconazole, 15-9

tebuconazole, 15-9

propiconazole–tebuconazole–

imidacloprid (PTI), 15-9

Preservatives, recommended retentions, 

15-21, 15-4t to 15-5t

Preservatives, water-repellant, and nonpressure treatments:

effectiveness, 15-12

federal specifications, 15-12

uses, 15-12

Preservative-treated wood:

best management practices, 15-23 to 

15-24

cut surfaces, protection of, 15-18

effect on adhesion, 10-6

finishing, 16-24

handling, 15-23 to 15-24

inspection, 15-25, 15-26fig

quality assurance, 15-25 to 15-26, 

15-26fig

seasoning required, 15-24

service life, 15-25

specifications, 15-25

strength as affected by preservatives, 

15-25

timing of use, 15-25

Preservative treatment for:

composite products, 11-15

light-frame construction, 17-2

Preservative treatment, effect on strength, 

7-13

Preservative treatment, preparing for:

air-drying practices, 15-16

conditioning green lumber, 15-17

Boulton or boiling-undervacuum 

process, 15-17

steaming-and-vacuum process, 15-17

cutting and framing:

common uses, 15-18

potential size changes, 15-18

timing, 15-18

drying, 15-17 to 15-18

incising:

method, 15-17, 15-17fig

purpose, 15-17

primary species, 15-17

peeling, 15-17, 15-17fig

preventing decay while drying, 15-16

Preservative treatments, nonpressure:

Boucherie process for green unpeeled 

poles, 15-21

brushing:

application, 15-22

choice of preservative, 15-22

effectiveness, 15-22

penetration obtained, 15-22

cold-soaking process:

effectiveness, 15-21

method, 15-21

retentions and penetrations, 15-21

compared to pressure treatment, 15-21

diffusion processes:

butt or groundline treatment of poles 

or posts, 15-21

double diffusion, 15-21

dipping:

effectiveness, 15-20

method, 15-20

penetration obtained, 15-20

steeping process:

effectiveness, 15-21

method, 15-21

wood used, 15-21

tire-tube method for green, unpeeled 

fencepost, 15-17

vacuum processes:

contrasted to pressure treating, 15-21

effectiveness, 15-21

methods, 15-21

uses, 15-21

Preservative treatments, pressure:

advantages, 15-20
I–24

General Technical Report FPL–GTR–190

and preservative temperature, 15-20

empty-cell processes:

Lowry process, 15-20

Rueping process, 15-18 to 15-19

full-cell process:

description, 15-18

following other conditioning, 15-18

modified full-cell, 15-18

penetration and retention levels, 15-18

pressures used, 15-18

Primavera:

availability, 2-34

characteristics, 2-34

dimensional change coefficient, 13-17t

locality of growth, 2-34

machinability, 2-34

mechanical properties, 5-20t, 5-24t

resistance to fungi, 2-34

shrinkage values, 4-8t

uses, 2-34

Propiconazole:

effectiveness, 15-9

solubility, 15-9

uses, 15-9

Purlins and glulam beam construction, 17-7

Purpleheart:

characteristics, 2-34

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-34

machinability, 2-34

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

uses, 2-34

Pycnanthus. See Ilomba

Quartersawn lumber:

advantages, 3-15t

method of producing, 3-14

shrinkage, 4-5

Quality control in preservative-treated 

wood, 15-25, 15-26fig

Radius-edged decking, 6-8

Rafters in light-frame construction, 17-3 

to 17-4

Ramin:

characteristics, 2-34

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-34

mechanical properties, 5-20t, 5-24t

resistance to decay, 2-34

shrinkage values, 4-8t

uses, 2-34

workability, 2-34

Rays, definition and discussion, 3-9 to 

3-10, 3-11 to 3-12

Reaction wood:

compression wood, 3-13, 5-31

increase in density, 5-31

shrinkage, 4-5, 5-32

tension wood, 3-13, 5-32

Redcedar, eastern. See Cedar, Eastern red

Redcedar, western. See Cedar, Western red

Redwood:

characteristics, 2-17

characteristics for painting, 16-5t

charring rate data, 18-14t

connector joint strength, 8-21t

decay resistance, 2-17, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

flammability data, 18-10t

for finish board, 6-17

for siding, 6-16

heat release data, 18-12fig

locality of growth, 2-17

moisture content, 4-2t, 5-34t

nomenclature, 6-14t

paintability rating, 16-14fig

penetration, 15-16t

preservative pressure and temperature, 

15-20

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

uses, 2-17

workability, 2-17

Relative humidity:

equilibrium moisture content, 4-3

moisture content of wood, 4-4t, 16-6 to 

16-7

Remedial treatment, 14-7, 14-9

Resorcinol adhesives:

performance over time, 10-21 to 10-22, 

10-22fig

structural performance, 10-11t to 10-13t

working and strength properties, and 

uses, 10-11t to 10-13t

Roble:

characteristics, 2-34

ease of bonding, 10-7t

locality of growth, 2-34 to 2-35

machinability, 2-35

mechanical properties, 5-20t, 5-24t

resistance to decay, 2-35

shrinkage values, 2-35, 4-8t

uses, 2-35

Rolling shear strength, defined, 5-17

Roof beams and water ponding, 9-8

Rosewood, ease of bonding, 10-7t

Rosewood, Brazilian:

characteristics, 2-35

locality of growth, 2-35

machinability, 2-35

mechanical properties, 5-20t, 5-24t

resistance to fungi and insects, 2-35

shrinkage values, 4-8t

uses, 2-35

Rosewood, Indian:

characteristics, 2-35

locality of growth, 2-35

machinability, 2-35

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

uses, 2-35

Round timber and ties:

availability, 6-19 to 6-20

durability, 6-22

form, 6-20 to 6-21

material requirements, 6-18 to 6-19

standards and specifications, 6-18t

strength properties, 7-13

weight and volume, 6-21 to 6-22

Sande:

characteristics, 2-35 to 2-36

decay resistance, 14-5t

locality of growth, 2-35

machinability, 2-36

mechanical properties, 5-20t, 5-24t

resistance to decay and insects, 2-36

shrinkage values, 4-8t

uses, 2-36

fire resistance, 18-7

Santa Maria:

characteristics, 2-36

dimensional change coefficient, 13-17t

locality of growth, 2-36

machinability, 2-36

mechanical properties, 5-20t, 5-24t

resistance to insects, 2-36

uses, 2-36

Sapele:

characteristics, 2-36

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-36

machinability, 2-36

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

uses, 2-36

Sap stain, 13-10, 13-13fig, 14-2

Sapwood:

function, 3-2

in visual stress grading, 7-5

location, 3-2, 3-2fig

moisture content, 4-2t

moisture content and drying, 13-7

preservative retention and stake lifespan, 

15-7t to 15-8t

Sassafras:

characteristics, 2-9
I–25

Index

decay resistance, 2-9, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-9

nomenclature, 6-5t

shock resistance, 2-9

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

uses, 2-9

Screws, lag:

description, 8-12

post-1991 lateral load, yield model 

theory, 8-13 to 8-14, 8-7t, 8-13t

pre-1991 lateral loads, 8-13 to 8-14, 

7-12eq, 8-12t, 8-13fig

lateral resistance, 8-13, 8-14t

lubrication, 8-14

prebored lead hole, size required, 8-11, 

8-13fig

spacing, 8-16

withdrawal resistance, 8-10 to 8-13, 

8-10eq

Screws, tapping:

described, 8-10

use in particleboard, 8-11

withdrawal loads, 8-11, 8-11eq

withdrawal resistance, 8-10

Screws, wood:

gauges, 8-11t

ateral load coefficients, 8-6t

lateral resistance, 8-10 to 8-11, 8-10eq, 

8-11eq

lead hole and load, 8-11

lubrication, 8-10

penetration depth and load, 8-11

sizes, 8-11t

types, 8-10, 8-10fig

withdrawal loads for particleboard, 8-11, 

8-11eq

withdrawal resistance, 8-10 to 8-11, 

8-11eq

yield model theory, 8-8t, 8-11

Seasoning and lumber purchasing 

considerations, 6-17 to 6-18

Selangan Batu. See Balau

Select lumber availability, 6-16 to 6-17

Sepetir:

characteristics, 2-36

decay resistance, 14-5t

locality of growth, 2-36

mechanical properties, 5-20t, 5-24t

shrinkage values, 4-8t

uses, 2-36

workability, 2-36

Seraya, white:

characteristics, 2-36 to 2-37

decay resistance, 14-5t

locality of growth, 2-36

machinability, 2-37

uses, 2-37

Shakes:

in lumber stress grading, 7-4 to 7-5

Shear strength parallel to grain:

coefficient of variation, 5-26t

defined, 5-3

Shear stress of beams, 9-6

Sheathing and light-frame construction, 

17-3, 17-4fig

Shingles and shakes, availability, 6-17

Shingles, finishes, suitability and expected 

service life, 16-17t

Shipworms, 14-13

Shrinkage:

adjustment for design use, 7-10

affect on painting, 16-3, 16-4t

coefficients for changing moisture 

content by species, 13-16t, 13-17t

coefficient of variation, 4-7, 4-9

compression wood, 4-7

discussion, 4-5 to 4-9

fiber saturation point as related to, 4-2

longitudinal, 4-5

moisture content curves, 4-5fig

of domestic woods, 4-6t

of imported woods, 4-8t

radial, 4-6t, 4-8t

species, 4-6t, 4-8t

tangential, 4-6t, 4-8t

volumetric, 4-6t, 4-8t

Siding:

and mill glaze, 16-26 to 16-27

availability, 6-16

back-priming, 16-25

finishes, suitability and expected service 

life, 16-17t

lumber finishing, 16-8 to 16-9

prefinished at the factory, 16-25

removing weathered surface, 16-13

use of plywood, 16-9

Silverballi, brown, See Kaneelhart

Size factor:

for design use, 7-11 to 7-12

procedures for design use:

in-grade test, 7-11 to 7-12

small clear, 7-11

Slash-grained lumber. See Flat-grained

Slope of grain:

determination, 5-28 to 5-30, 5-29eq

in visual sorting, 7-4

relationship to fiber orientation, 5-30fig

types, 5-29 to 5-31

Smoke:

approaches for dealing with, 18-6

carbon monoxide, 18-6

defined, 18-6

release rate, 18-11

tests for determining yield, 18-9 to 18-10

toxicity, 18-10

Soft rot, 14-4

Softwood lumber:

American Lumber Standard, 6-7

classification by grades:

factory and shop lumber:

factory (shop) grades, 6-10

industrial clears, 6-10

ladder and pole stock, 6-10

moulding stock, 6-10

tank stock, 6-10

structural lumber:

dimension lumber, 6-8

structural laminations, 6-8

yard lumber:

common lumber, 6-7, 6-8fig

select lumber, 6-7 to 6-8

development of grading rules, 6-7

drying targets, 13-5

grading organizations, 6-9t

kiln drying schedules, 13-8 to 13-9, 

13-11t

manufacture:

size, 6-10, 6-11t

surfacing, 6-11

patterns, 6-11, 6-12fig

species, 6-13t to 6-15t

Softwoods:

availability, 2-2

bending properties, 19-3

charring rates, 18-14t

definition, 2-2

flame spread index, 18-4t

flammability data, 18-10t

heat release data, 18-12fig

imported, 2-38 to 2-40

moisture content, heartwood and 

sapwood, 4-2t

preservative penetration, 15-7t to 15-8t

relationship of mechanical properties to 

specific gravity, 5-29t

species by region, 2-3t

thermal conductivity, 4-14t

uses, 2-2

Sorption hysteresis, discussed, 4-3

Solvents and adhesion, 10-9

Sound, speed of, 5-17

Southern pine sapwood stakes, preservative 

retention and life span tests, 15-7t to 15-8t

Soybean adhesives:

structural performance, 10-11t to 10-13t

working and strength properties, and 

uses, 10-11t to 10-13t

Spanish-cedar:

characteristics, 2-37

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-37

mechanical properties, 5-21t, 5-25t

shrinkage values, 4-8t

uses, 2-37

workability, 2-37
I–26

General Technical Report FPL–GTR–190

Specific gravity:

and paintability of wood, 16-1, 16-2 to 

16-7

and weathering of wood, 16-11

coefficient of variation, 4-7, 4-9, 4-9t, 

5-26t

definition, 4-7

density as a function of, 4-9t, 4-11t

influence on mechanical properties, 

5-26, 5-29t

moisture content, 4-1, 4-3fig, 4-5fig

of reaction wood, 5-31

Speed of sound, 5-17

Sphaeroma, 14-14

Spikes, 8-9

Spiral grain, 5-30, 5-30fig

Splits:

in lumber stress grading, 7-4

Spruce, black:

dimensional change coefficient, 13-16t

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

Spruce, Eastern:

characteristics, 2-17

flame spread index, 18-4t

locality of growth, 2-17

nomenclature, 6-14t

species, 2-17

uses, 2-17

workability, 2-17

Spruce, Engelmann:

characteristics, 2-17

characteristics for painting, 16-5t

charring rate data, 18-14t

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

locality of growth, 2-17

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t

nomenclature, 6-14t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-17

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

tensile strength, 5-26t

thermal conductivity, 4-14t

toughness values, 5-28t

uses, 2-17

Spruce, red:

connector joint strength, 8-21t

dimensional change coefficient, 13-17t

fracture toughness, 5-28t

mechanical properties, 5-14t, 5-15t

moisture content, 5-34t

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

Spruce, Sitka:

characteristics, 2-17 to 2-18

connector joint strength, 8-21t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

elastic ratio, 5-2t

flame spread index, 18-4t

locality of growth, 2-17

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t, 5-34t

nomenclature, 6-14t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-18

shrinkage values, 4-6t

strength properties, 2-18, 5-8t, 5-13t

tensile strength, 5-26t

thermal conductivity, 4-14t

uses, 2-18

Spruce, white:

connector joint strength, 8-21t

dimensional change coefficient, 13-17t

mechanical properties, 5-14t, 5-15t

penetration, 15-16t

strength properties, 5-8t, 5-13t

thermal conductivity, 4-14t

Stability:

beams:

water ponding, 9-9, 9-9eq

lateral-torsion buckling, 9-9 to 9-10, 

9-9eq, 9-10t

deck support, effect of, 9-9, 9-9eq, 

9-10fig

built-up and spaced columns, 9-8, 9-8eq

column flanges, 9-8, 9-8eq

long columns, 9-7, 9-7eq

interaction of buckling modes, 9-9 to 

9-10, 9-9eq, 9-10eq

short columns, 9-7 to 9-8, 9-8eq, 9-8fig

Stain:

penetrating:

use on exterior plywood, 16-9

semi-transparent:

application and maintenance, 16-19t, 

16-19 to 16-20

discussed, 16-17

refinishing, 16-18 to 16-20

solid color:

and mill glaze, 16-26 to 16-27

application and maintenance, 16-19t, 

16-19 to 16-20

discussed, 16-22

reconstituted wood products, 16-25

refinishing, 16-22

use on floors, 16-33 to 16-34

Stains. See also Discoloration

Stains, chemical, discussed 14-2

Stains, fungal:

blue, 16-30

discussed, 14-1 to 14-2

distinction from mold, 14-2

during drying, 13-10, 13-13fig

effect on wood, 14-2

Stains, iron, 16-30

Stake tests with preservatives, pressuretreated, 15-7t to 15-8t

Standard lengths of lumber, 6-3

Standard lumber abbreviations, 6-23 to 

6-25

Standard thicknesses of lumber:

for flooring, 6-6

table, 6-6t

Standard widths of lumber, 6-3

Staples, 8-9 to 8-10

Starch adhesive, structural performance, 

10-11t to 10-13t

Staybwood, 19-10

Staypak:

appearance, 19-10

dimensional stability, 19-9t

properties, 19-6t, 19-9 to 19-10

purpose, 19-10

strength properties, 19-7t to 19-8t

uses, 19-10

Sterilization of wood. See Heat sterilization

Sticker stain during drying, 13-10, 13-14fig

Stiffeners and glulam beam construction, 

17-7

Stiffness, affected by knots, 7-4

Storage of lumber:

care in yards, 13-14 to 13-15, 14-7

green or partially seasoned, 13-14

storage shed temperature, 13-14t

Storing lumber:

finish and factory items, 13-14

sheathing and structural items, 13-14

Strength and duration of load adjustments, 

7-12

Strength ratio:

definition, 7-3

equations, 7-6, 7-6fig

estimating, 7-5

ranges in visual grading, 7-6

Strength properties:

bird pecks, effect of, 5-33

compression failures, effect of, 5-33

creep, 5-39

dead trees, 5-34

derived for small clear wood, 7-5

duration of load, effect, 5-39 to 5-40

extractives, effect of, 5-34

fire-retardant treatments, 5-41 to 5-42

for commercially important woods, 5-4t 

to 5-13t

insect damage, effect of, 5-44

juvenile wood, 5-32 to 5-33

knots, effect of, 5-26 to 5-28
I–27

Index

moisture content as related to, 5-5

nuclear radiation, 5-43

pitch pockets, effect of, 5-33

rate of loading, 5-38

slope of grain, 5-28 to 5-31

treatment, effect of, 7-13

Strength, related to slope, 5-31t

Stress equations:

axial load:

tensile stress, 9-4, 9-4eq

short-block compressive stress, 9-4, 

9-4eq

bending:

notches, slits and holes, effect 9-6 to 

9-7

size effect, 9-5, 9-5eq

straight beam stresses, 9-4

tapered beam stresses, 9-4 to 9-6, 

9-5fig, 9-5eq, 9-6eq

time effects, 9-6

water ponding, 9-6

combined bending and axial load:

concentric load, 9-6, 9-6eq

eccentric load, 9-7, 9-7eq

torsion, 9-7, 9-7fig, 9-7eq

Stress-graded lumber, American Standard 

lumber sizes, 6-11t

Stress grading:

American Lumber Standard Committee 

(ALSC):

accredited rules-writing and 

independent agencies, 7-2t

American Softwood Standard, 7-2

National Grading Rule, 7-3, 7-3t

American Society of Testing and 

Materials (ASTM):

calculating clear wood properties for 

visual stress grades, 7-3

design properties, 7-1

strength ratio, 7-3

U.S. responsibility structure, 7-2, 7-2fig

Stress, relationship between constant load 

and failure, 5-39 to 5-40, 5-39fig, 5-40fig

Structural composite lumber (SCL) in 

bridges, 17-9

Structural flakeboard and light-frame 

construction, 17-3

Sucupira:

characteristics, 2-37

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-37

mechanical properties, 5-21t, 5-25t

resistance to fungi and insects, 2-37

shrinkage values, 4-8t

uses, 2-37

workability, 2-37

Sugarberry (See also Hackberry)

Suradan. See Pilon

Sweetgum:

characteristics, 2-9

characteristics for painting, 16-5t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

flame spread index, 18-4t

interlocked grain, 2-9

locality of growth, 2-9

moisture content, 4-2t

penetration, 15-16t

shock resistance, 2-9

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-13t

toughness values, 5-27t

uses, 2-9

Swelling, coefficient for changing moisture 

content, by species, 13-16t to 13-17t

Sycamore:

characteristics for painting, 16-5t

decay resistance, 14-5t

ease of bonding, 10-7t

nomenclature, 6-5t

Sycamore, American:

characteristics, 2-9

connector joint strength, 8-21t

dimensional change coefficient, 13-16t

locality of growth, 2-9

moisture content, 4-2t

penetration, 15-16t

shock resistance, 2-9

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-13t

uses, 2-9

Tamarack:

characteristics, 2-18

characteristics for painting, 16-5t

decay resistance, 14-5t

dimensional change coefficient, 13-17t

locality of growth, 2-18

mechanical properties, 5-14t, 5-15t

moisture content, 4-2t, 5-34t

nomenclature, 6-14t

penetration, 15-16t

shrinkage values, 4-6t

strength properties, 5-8t, 5-13t

uses, 2-18

Tangare. See Andiroba

Tanoak:

characteristics, 2-9 to 2-10

decay resistance, 14-5t

dimensional change coefficient, 13-16t

locality of growth, 2-9

machinability, 2-10

nomenclature, 6-5t

shrinkage values, 4-6t

strength properties, 2-9, 5-6t, 5-11t

uses, 2-10

Teak:

characteristics, 2-37 to 2-38

decay resistance, 14-5t

dimensional change coefficient, 13-17t

ease of bonding, 10-7t

locality of growth, 2-37

machinability, 2-38

mechanical properties, 5-21t, 5-25t

shrinkage values, 4-8t

uses, 2-38

Tebuconazole:

effectiveness, 15-9

solubility, 15-9

Temperature, effect on:

fatigue strength, 5-40 to 5-41

mechanical properties:

irreversible effects, 5-37 to 5-38, 

5-38fig

reversible effects, 5-35 to 5-36, 

5-36fig

properties, 7-13, 7-13t

relative humidity and moisture content, 

4-4t

Tensile strength, end-grain bonded joints, 

10-18

Tensile strength parallel to grain:

average values, 5-26t

coefficient of variation, 5-26t

defined, 5-3

Tensile strength perpendicular to grain, 5-3

Tension wood:

definition, 5-32

density increase, 5-31

description, 5-32

effect on strength, 5-32

shrinkage, 5-32

Texture of wood, 3-13 to 3-14

Texture, effect on paintability, 16-6

Termites:

damage caused by, 14-8t, 14-11fig

nonsubterranean, 14-12

subterranean, 14-11 to 14-12

termite-resistant wood, 14-12 to 14-13

Thermal, conductivity of wood:

definition, 4-10

determination, 4-12

factors affecting, 4-10, 4-11

selected species, 4-13t to 4-14t

Thermal diffusivity of wood, 4-12

Thermal expansion, 4-14

Thermal properties of wood, 4-10

Thermoplastics and adhesion, 10-9 to 

10-10

Ties:

availability, 6-20

preservative penetration levels, 15-4t to 

15-5t

service life, 6-23

sizes, 6-20
I–28

General Technical Report FPL–GTR–190

standards and specifications, 6-18t

strength properties, 7-14

weight and volume, 6-21 to 6-22

Timber:

inventory, 2-2

resources, 2-2

Timber bridges:

glulam, 17-7, 17-9, 17-9fig

log stringer, 17-8

sawn lumber, 17-9

structural composite lumber, 17-9

Timber buildings:

arch structure, 17-8

dome, 17-8, 17-8fig

glulam beam, 17-7

mill-type construction:

fire resistance, 17-7

specifications, 17-7

timber frame houses, 17-6 to 17-7, 

17-7fig

Timbers, recommended moisture content, 

13-3

Timber from dead trees, properties of, 5-34

Time, effect on strength:

creep, 5-39

duration of load, 5-39 to 5-40

Tornillo:

characteristics, 2-38

decay resistance, 14-5t

locality of growth, 2-38

mechanical properties, 5-21t, 5-25t

uses, 2-38

workability, 2-38

Torsion, strength, 5-15

Toughness:

average values, 5-27t, 5-28t

coefficient of variation, 5-26t

defined, 5-25

Tracheids, description and function, 3-9

Transverse and volumetric shrinkage of 

wood, 4-5 to 4-7

Treated wood recycling and disposal, 

15-26

Trebol. See Macawood

Trim, exterior:

care during construction, 13-19

recommended moisture content, 13-5t

Trusses:

care during construction, 13-8 to 13-19

in light-frame construction, 17-4

in pole and post-frame construction, 

17-4 to 17-6

Truss plates, 8-25, 8-25fig

Tupelo:

characteristics, 2-10

connector joint strength, 8-21t

locality of growth, nomenclature, 6-5t

shock resistance, 2-10

species, 2-10

uses, 2-10

Tupelo, black:

dimensional change coefficient, 13-16t

moisture content, 4-2t

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-13t

Tupelo, swamp, moisture content, 4-2t

Tupelo, water:

dimensional change coefficient, 13-16t

moisture content, 4-2t

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-13t

Urea and dimethylol urea for plasticizing 

wood, 19-1

Urea adhesives:

performance over time, 10-21 to 10-22, 

10-22fig

structural performance, 10-11t to 10-13t

use with composite products, 11-3

working and strength properties, and 

uses, 10-11t to 10-13t

Van der Waal’s forces, 10-1 to 10-2

Varnish:

application and maintenance, 16-19t

clear, 16-21

use on boats, 16-25

use on floors, 16-33 to 16-34

Veneer:

and adhesion, 10-3 to 10-4

use with plywood, 11-5

Veneered curved members, 19-3

Ventilation and anobiids, 14-10

Vertical-grained lumber. See Edgegrained 

lumber

Vessel:

definition, 2-2

function, 3-10 to 3-11

Vibration properties, 5-17

Virola. See Banak

Visual grades in the National Grading 

Rule, 7-3t

Visual grading:

deriving strength properties for small 

clear wood, 7-5 to 7-6, 7-5fig

deriving modulus of elasticity for small 

clear wood, 7-5, 7-6fig

in-grade procedures, 7-7

sorting criteria, 7-3 to 7-4

Visual sorting criteria in lumber stress 

grading:

checks and splits, 7-4

decay, 7-5

density, 7-5

explanation, 7-3

heartwood and sapwood, 7-5

knots, 7-3 to 7-4

pitch pockets, 7-5

shakes, 7-4

slope of grain, 7-4

wane, 7-5

Waika. See Manni

Walele. See Ilomba

Wallaba:

characteristics, 2-38

decay resistance, 14-5t

ease of bonding, 10-7t

locality of growth, 2-38

machinability, 2-38

mechanical properties, 5-21t, 5-25t

resistance to insects, 2-38

shrinkage values, 4-8t

uses, 2-38

Walnut, black:

availability at retail yards, 6-16

characteristics, 2-10

characteristics for painting, 16-5t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

kiln drying schedule, 13-11t

locality of growth, 2-10

moisture content, 4-2t

nomenclature, 6-5t

Poisson ratio, 5-3t

shock resistance, 2-10

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

uses, 2-10

workability, 2-10

Wane in lumber stress grading, 7-5

Wapa. See Wallaba

Warp:

and finishing, 16-13 to 16-14

and weathering, 16-11

during drying, 13-7

Waterborne preservatives. See 

Preservatives, waterborne

Waterponding, effect on bending stress, 9-8

Water repellants (See also water repellant 

preservative):

application and maintenance, 16-19t, 

16-20

discussed, 16-13

moisture-excluding coatings, different 

from, 16-13

used as a finish, 16-13

Water-repellant preservatives:

application and maintenance, 16-19t

as a first step in finishing, 16-11

back-priming, 16-25

caution in use, 16-25

effect of use on window sash and frame, 

16-18fig

on wood exposed to marine 

environments, 16-24 to 16-25
I–29

Index

paintability, 16-24 to 16-25

refinishing, 16-25

used as a finish, 16-25

use on edges of reconstituted wood 

products, 16-10

use on end grain, 16-6fig

Wax in composite products, 11-5

Weathering of wood:

and mill glaze, 16-26 to 16-27

artist’s rendition of, 16-11fig

as a natural finish, 16-16

effect on extractives, 16-7

effect on lignin, 16-11

effect on paint adhesion, 16-12

erosion rates for hardwoods and 

softwoods, 16-12t

in marine environments, 16-24 to 16-25

of finishes, 16-16

process, 16-24

warp, 16-12 to 16-13

Western redcedar, used for poles, 6-19

White-cedar:

Atlantic. See Cedar, Atlantic white

Northern. See Cedar, Northern white

White rot, 14-4

Willow, black:

characteristics, 2-10

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

locality of growth, 2-10

nomenclature, 6-5t

penetration, 15-16t

shock resistance, 2-10

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

toughness values, 5-27t

uses, 2-10

Withdrawal resistance:

of lag screws, 8-12 to 8-13

of nails, 8-2 to 8-5

of wood screws, 8-10 to 8-11

Wood buildings, consideration:

moisture control:

effect on heat flow, 17-11

mold, mildew, mites and health, 17-10

moisture control strategies, 17-11

paint failure and appearance problems, 

17-11

structural failure, 17-11

sound control, 17-11 to 17-12, 17-12t

structural performance and 

serviceability, 17-9 to 17-10

thermal insulation and air infiltration 

control, 17-9 to 17-10

Wood cells. See cells of wood

Wood cleaners and brighteners, 16-34 to 

16-35

Wood Components Manufacturers 

Association, 6-4, 6-4t

Wood fillers, 16-33

Wood identification, 3-16

Wood-thermoplastic composites:

materials, 11-25

production, 11-26

with high thermoplastic composites:

advantages, 11-26

compounding, 11-26

disadvantages, 11-26

manufacture, 11-26

mechanical properties, 12-9t

specific gravity, 12-9t

with low thermoplastic composites:

manufacture, 11-27

Wood screws. See screws, wood

Work to maximum load in bending, 5-3

Yang. See Keruing

Yellow-cedar. See Cedar, yellow

Yellow-poplar:

characteristics, 2-10 to 2-11

characteristics for painting, 16-5t

charring rate data, 18-14t

connector joint strength, 8-21t

decay resistance, 14-5t

dimensional change coefficient, 13-16t

ease of bonding, 10-7t

elastic ratio, 5-2t

erosion of planed surfaces, 16-12t

flame spread index, 18-4t

fracture toughness, 5-27t, 5-28t

locality of growth, 2-10

moisture content, 4-2t

penetration, 15-16t

Poisson ratio, 5-3t

shock resistance, 2-11

shrinkage values, 4-6t

strength properties, 5-6t, 5-11t

thermal conductivity, 4-13t

toughness values, 5-27t

uses, 2-11

Zinc-coated nails, 8-3

Zinc naphthenate:

effectiveness, 15-11

inappropriate uses, 15-11

properties, 15-11