Wood and Timber 

Condition Assessment 

Manual

Second Edition

General Technical Report

FPL–GTR–234

United States Department of Agriculture

Forest 

Service

Forest Products

Laboratory

November

2014

In cooperation with the United States Department of Transportation Federal Highway Administration
November 2014

White, Robert H.; Ross, Robert J. (Eds.). 2014. Wood and timber condition assessment manual: second edition. General Technical Report FPLGTR-234. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory. 

93 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 

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Abstract

This report summarizes information on condition assessment of in-service wood, including visual inspection of 

wood and timbers, use of ultrasound and probing/boring 

techniques for inspection, and assessment of wood and 

timbers that have been exposed to fire. The report also 

includes information on assigning allowable design values 

for in-service wood. 

Keywords: Wood, timbers, condition assessment, visual, 

coring, probing, ultrasound, post-fire assessment
Wood and Timber 

Condition Assessment 

Manual

Second Edition

Edited by

Robert H. White

Robert J. Ross
Contents

Foreword..........................................................................................................iii

Preface ............................................................................................................. v

Chapter 1—Visual Inspection...........................................................................1

Chapter 2—Sounding, Probing, Moisture Content, and Resistance 

Drilling Techniques...........................................................................................5

Chapter 3—Inspection of Timber Structures Using Stress Wave Timing 

Nondestructive Evaluation Tools.................................................................... 13

Chapter 4—New Techniques and Technologies............................................. 23

Chapter 5—Post-Fire Assessment of Structural Wood Members................... 33

Chapter 6—Estimation of Design Values for In-Service Wood...................... 45

Chapter 7—Condition Assessment Report ..................................................... 77

Chapter 8—Summaries from Actual Inspections ........................................... 83
iii

Foreword

Since it was first published in 2004, the Wood and Timber Condition Assessment Manual

has become an extremely valuable resource for wood design professionals. Dr. Robert Ross 

and his team continue to be leading experts in this field and are the “go-to source” for information regarding condition assessment. To my knowledge, there is no other source for the 

type of information found here.

This update from the first edition contains significant new information, including the latest 

assessment techniques and technology and a new chapter on estimation of allowable properties for in-service wood. Other chapters were updated, and example assessment and summary reports remain as practical resources. 

Many thanks to Dr. Ross and his team for persisting with this topic and providing valuable 

tools and guidance. 

John “Buddy” Showalter, P.E.

Vice President, Technology Transfer

American Wood Council

v

and provide estimates of the properties of in-service wood; and 

(3) to make the manual available in digital format. 

I worked with several well-respected technical authorities in 

preparing this edition. The original chapters on visual inspection, drilling/coring/probing techniques, and stress wave timing 

techniques (Chapters 1–3) were reviewed and revised slightly. 

The chapter on post-fire assessment of structural members 

(Chapter 5) was modified significantly to add new material on 

the use of nondestructive testing techniques. Because of repeated requests from field professionals, a new chapter (Chapter 6) 

was developed that addresses estimation of allowable properties for in-service wood. 

I thank the technical contributors to this edition of the manual: 

Brian K. Brashaw, PhD, Program Director, Wood Materials & 

Manufacturing, University of Minnesota Duluth;

Zhiyong Cai, PhD, P.E., Supervisory Research Materials Engineer, USDA Forest Products Laboratory; 

David E. Kretschmann, P.E., Research General Engineer, 

USDA Forest Products Laboratory;

Xiping Wang, PhD, Research Forest Products Technologist, 

USDA Forest Products Laboratory; 

Brian Kukay, PhD, P.E., Associate Professor, General Engineering, Montana Tech.

I also acknowledge Professor (retired) Roy F. Pellerin, Washington State University, for his valuable contributions to the 

first edition of the manual. Special thanks to Greg Fehr, P.E., 

Adam Senalik, P.E., and Tom Williamson, P.E., for their technical reviews.

I acknowledge the following for the excellent artwork on the 

cover—Beijing Research Institute of Architectural Heritage; 

Judy Hesterberg, Hancock, Michigan; and Tivoli Gough, 

USDA Forest Products Laboratory.

Financial support for this second edition of Wood and Timber 

Condition Assessment Manual from the USDA Forest Service 

is gratefully acknowledged. 

This edition of the Wood and Timber Condition Assessment 

Manual is dedicated to my good friend and colleague, 

Dr. Robert Hawthorne White, Research Wood Scientist, USDA 

Forest Products Laboratory. Robert was an internationally 

recognized leader in the wood science profession, where he 

conducted research on fire performance issues with wood 

products. He was very active in ASTM International—and was 

recognized by ASTM at its highest levels. Robert was a wellrespected scientist, worldwide, and he served the public in a 

professional manner for nearly 40 years. His reputation is impeccable, and his voice still carries much weight in the building 

codes and international standards arenas. 

The Wood and Timber Condition Assessment Manual—

Second Edition is available in digital format from the USDA 

Forest Service Forest Products Laboratory website.

Robert J. Ross

Preface

Deterioration of an in-service wood member may result from a 

variety of causes during the life of a structure. Periodic inspection of wood used in structures is important for determining 

the extent of deterioration so that degraded members may be 

replaced or repaired to avoid structural failure.

Inspection professionals use a wide variety of techniques to assess the condition of wood in service. Visual, mechanical probing, and stress wave or ultrasound-based techniques are all used 

either individually or in combination by inspectors. Although 

these techniques are based on solid technical information and 

supporting research, prior to publication of the Wood and Timber Condition Assessment Manual in 2004, no practical, comprehensive manual provided information on inspection of wood 

in service.

The Wood and Condition Assessment Manual was prepared to 

address this need. The manual was prepared from numerous research studies, inspections, and lectures dealing with assessing 

the condition of in-service wood and timber. It was intended 

for inspection professionals. A concerted effort was made to 

provide clear and concise explanations of various aspects of 

inspecting in-service wood and timber. To this end, a number 

of photographs and drawings obtained from actual inspections 

were included.

The 2004 Wood and Timber Condition Assessment Manual

proved to be a widely used reference document and a primary 

technical source for inspection professionals worldwide. User 

feedback included many positive comments about description 

of the various inspection tools available and post-fire assessment of structural wood members. Users did express a strong 

desire to have information on the estimation of allowable design values for in-service wood in future editions.

We relied heavily on this feedback in preparation of the Wood 

and Timber Condition Assessment Manual—Second Edition. It 

is organized into eight chapters. Chapters 1 to 3 present background information on techniques currently used by inspectors, including information on visual inspection techniques, 

mechanical coring or probing techniques, and stress wave or 

ultrasound-based techniques. Included in each chapter is a detailed description of the technique, a list of currently available 

tools and where they can be obtained, and guidelines for their 

use. Each chapter concludes with a list of references that serve 

as the technical base for the technique. Chapter 4 is devoted to 

a review of techniques currently under development for inspection of wood structures. Chapter 5 covers the topic of inspection of fire-damaged wood. A procedure for estimating allowable design values for in-service wood is included in Chapter 6. 

Chapter 7 is a sample condition assessment report. Summaries 

of several inspections are included in Chapter 8.

In preparing this second edition of the Wood and Timber Condition Assessment Manual, I had three objectives: (1) to update 

the existing chapters to reflect advancements in inspection 

methods; (2) to develop new material that focuses on a wide 

range of new techniques and technologies that have been investigated for use in assessing the condition of wood structures 

1

Chapter 1

Visual Inspection

Robert J. Ross

The simplest method for locating deterioration is visual 

inspection—observing the structure for signs of actual or 

potential deterioration, noting areas that require further investigation. When assessing the condition of a structure, visual inspection may be the sole method used. For example, 

substantial modifications of structurals are easily identified 

visually. Visual inspection requires strong light and is useful 

for detecting intermediate or advanced surface decay, water 

damage, mechanical damage, or failed members. Visual 

inspection cannot detect early stage decay, when remedial 

treatment is most effective. During an inspection, the following signs of deterioration should be investigated.

Fruiting Bodies

Although they do not indicate the amount or extent of decay, 

fruiting bodies provide a positive indication of fungal attack. 

Some fungi produce fruiting bodies after small amounts of 

decay have occurred; others develop only after decay is extensive. When fruiting bodies are present, they indicate the 

possibility of a serious decay problem.

Figures 1.1 and 1.2 show fruiting bodies that were observed 

during inspection of Washington State University’s football 

stadium. Figure 1.2 shows the extent of deterioration inside 

the beam. Note that in Figure 1.1, the fruiting body is located where two timber beams (placed end-to-end) rest on a 

column. Exposed end grain allows for moisture accumulation and subsequent absorption by the timbers. Figure 1.3 illustrates the potential extent of deterioration in both beams.

Sunken Faces or Localized 

Collapse

Sunken faces or localized surface depressions can indicate 

underlying decay. Decay voids or pockets may develop 

close to the surface of the member, leaving a thin, depressed layer of intact or partially intact wood at the surface. 

Crushed wood can also be an indicator of decay.

Figure 1.4 shows a timber beam that failed in bearing (compression perpendicular to grain). Such collapse indicates 

that the inner portion of the timber is severely deteriorated. 

Figure 1.5 illustrates that when the inner portion of the 

timber is severely deteriorated, the outer shell can collapse 

when the timber is stressed in bearing.

Figure 1.1—Fruiting body between two beams.

Figure 1.2—Fruiting bodies can indicate extensive damage 

inside the beam.
General Technical Report FPL–GTR–234

2

Staining or Discoloration

Staining or discoloration of wood indicates that it has been 

subjected to water and potentially has a high moisture content suitable to support decay. Rust stains from connection 

hardware also are a good indication of wetting.

Inspection of the Red Cliff Recreation Hall near Bayfield, 

Wisconsin, revealed severe staining and discoloration of the 

fiberboard product used in the ceiling (Figs. 1.6 and 1.7). 

Discoloration and mold indicated that the structure had been 

exposed to moisture and the wood could possibly be deteriorated. Removal of several ceiling panels revealed that the 

fiberglass insulation above them was saturated with water 

(Fig. 1.8). This moisture provided an ideal environment for 

the growth of mold and decay fungi. As a consequence of 

this moisture exposure, wood members in the roof and walls 

also contained areas of deterioration (Fig. 1.9).

Insect Activity

Insect activity is visually characterized by holes, frass, and 

powder posting. The presence of insects may also indicate 

the presence of decay.

Figure 1.10 is a piling from a bridge. Initially deteriorated 

by decay, carpenter ants were attracted to the softened wood 

and continued to destroy the piling. Once the flashing was 

removed, the severe deterioration could be readily observed.

Figure 1.3—Potential deterioration in both beams.

Figure 1.4—The timber beam’s outer shell 

buckled as a result of interior deterioration.

Figure 1.5—Because of interior deterioration, 

the outer shell can collapse when the timber is 

stressed in bearing.

Figure 1.6—The ceiling panels were stained and discolored.
Wood and Timber Condition Assessment Manual • Chapter 1

3

Plant or Moss Growth

Plant or moss growth in splits and cracks, or soil accumulation on the structure, indicates that adjacent wood has been 

at a relatively high moisture content for a sustained period 

and may sustain growth of decay fungi.

Missing Members

Frequently, it is observed that entire structural members 

have been removed. Figures 1.11 and 1.12 show an inservice wood truss that had its webs removed by the homeowner with the intent of creating living space in the attic. 

Figure 1.7—Close-up of the severely stained and discolored 

ceiling panels.

Figure 1.8—Removing the ceiling panels revealed that the 

fiberglass insulation was saturated with water.

Figure 1.9—As a consequence of the moisture exposure, 

wood members in the roof and walls also contained areas 

of deterioration.

Figure 1.10—Damage to a bridge piling.

Figure 1.11—Metal plate connectors were cut and bent 

so that the webs could be removed. This should never be 

done.
General Technical Report FPL–GTR–234

4

This should never be done and must be noted in any inspection report. Any change to web or chord members must be 

reported.

Check and Splits

Checks and splits in members can indicate a weakened 

member. Checks vary in depth; splits extend through the 

entire cross section. The large timber shown in Figure 1.13 

was treated with preservatives. Note that only the outer 

portion of the timber contains preservatives. Depth of preservative treatment depends on species of wood and characteristics of treatment. If a check or split develops in the 

timber that is of sufficient depth, the inner untreated wood is 

susceptible to moisture and decay fungi.

Alterations

Alterations can weaken members. Figure 1.14 shows a floor 

joist found during an inspection of the Red Cliff Recreation 

Hall. A section of the bottom, or tension, side of the joist 

was removed to facilitate plumbing. This type of modification can severely weaken the joist. This should never be 

done and must be noted in any inspection report.

Figure 1.14—A floor joist was cut to make room for 

plumbing. This should never be done.

Figure 1.12—The homeowner removed the webs from the truss leaving only the chords to 

support the roof. This should never be done.

Figure 1.13—Only 

the outer portion of 

this timber contains 

preservatives.
5

Chapter 2

Sounding, Probing, Moisture Content, 

and Resistance Drilling Techniques

Brian Brashaw

Simple mechanical tests are frequently used for in-service 

inspection of wood elements in timber structures. For example, hammer sounding and probing are used in combination with visual inspection to conduct an initial assessment 

of the condition of a member. The underlying premise for 

such tests is that degraded wood is relatively soft and might 

sound hollow, with low resistance to penetration.

Sounding and Probing 

One of the most commonly used techniques for detecting 

deterioration is to hit the surface of a member with a hammer or other object. Based on the sound quality or surface 

condition, an inspector can identify areas of concern for 

further investigation using advanced tools like a stress 

wave timer or resistance microdrill. Deteriorated areas typically have a hollow or dull sound that may indicate internal 

decay. A pick hammer commonly used by geologists is 

recommended for use in timber structures because it allows 

inspectors to combine the use of sound and the pick end to 

probe the element (Fig. 2.1). 

Probing with a moderately pointed tool, such as an awl or 

knife, locates decay near the wood surface as indicated by 

excessive softness or a lack of resistance to probe penetration and the breakage pattern of the splinters. A brash, or 

brittle break indicates decayed wood, whereas a splintered 

break indicates sound wood. Although probing is a simple 

inspection method, experience is required to interpret results. Care must be taken to differentiate between decay and 

water-softened wood, which may be sound but somewhat 

softer than dry wood. Assessing damage in soft-textured 

woods, such as western redcedar, is sometimes difficult. 

Figure 2.2 shows an awl probe inserted into a split to assess 

decay that is visible on the railing end.

Probes can also be used to assess the depth of splits and 

checks in timber members. Flat bladed probes, such as 

pocket knives or feeler gauges, are recommended for use 

in this process. This is also important to understanding the 

effect of checks and cracks in other advanced techniques 

such as stress wave inspection. Figure 2.3 shows the use of 

probes to assess the depth of checks and cracks in timber 

bridge elements.

Figure 2.1—A hammer pick is an effective tool for initial assessments of timber bridge elements.

Figure 2.2—An awl is used to assess depth and presence of 

decay in a horizontal split.
General Technical Report FPL–GTR–234

6

Screw withdrawal testing has been used as an indicator of 

biological deterioration in timber members. Additional research has correlated screw withdrawal resistance to physical and mechanical properties such as density, modulus of 

elasticity, modulus of rupture, and shear modulus. These 

tests are typically conducted by extracting a screw inserted 

at an angle perpendicular to the surface of the timber specimen. Although these tests are dependent on local material 

condition, information from both deteriorated and good 

material locations can be helpful during wood structure 

evaluations. 

A quantitative test based on the premise that underlies 

mechanical probing tests (relative softness of degraded 

wood and consequent low resistance to probe penetration) 

was developed by Talbot (1982). His test differed from the 

probing-type test in that instead of evaluating penetration 

resistance of a probe, it examined withdrawal resistance 

of a threaded probe, similar to a wood screw, inserted into 

a member. Talbot believed that a correlative relationship 

between withdrawal resistance and residual strength should 

exist and would be relatively easy to determine. He conducted an experiment using several small Douglas-fir beams 

in various stages of degradation as a result of exposure to 

decay fungi. Prior to testing the wood to failure in bending, 

probe withdrawal resistance was measured at the neutral 

axis of the beams. Bending strength and corresponding 

probe resistance values were then compared. The results 

revealed a relationship between withdrawal resistance and 

residual strength (Fig. 2.4). Talbot used this test in conjunction with stress wave techniques (described in detail in 

Chapter 3) to assess the extent of damage to solid-sawn timbers in the football stadium at Washington State University.

A modification to Talbot’s technique was developed for use 

in evaluating in-service strength of fire-retardant-treated 

(FRT) plywood in the early 1990s. A portable screw withdrawal system was utilized to inspect FRT plywood and 

identify substantial degrade. An extensive research effort 

was devoted to the use of Talbot’s test for assessing residual 

strength of in-service panel products treated with fire retardant. Winandy et al. (1998) designed and conducted tests 

on a large sample of plywood specimens that were treated 

with fire-retardant chemicals and subsequently exposed to 

elevated temperatures. Screw withdrawal loads were determined for each specimen prior to static bending tests. Correlative relationships were then established between screw 

withdrawal strength and static bending properties (Fig. 2.5).

Recent evaluations have been completed by other researchers and commercial companies (Cai et al. 2002, Fakopp 

2014). Figure 2.6 shows one commercial screw withdrawal 

resistance unit.

Moisture Content Inspection

Moisture meters can be used effectively in inspecting timber 

elements. The presence of moisture is required for decay 

to occur in timber. Typically, moisture conditions in timber 

of less than 20% will not allow decay to occur; as moisture 

increases above 20%, the potential for decay to occur 

increases.

Serious decay occurs only when the moisture content of the 

wood is above 30%. This occurs when dry wood is exposed 

to direct wetting through rain, moisture infiltration, or contact with ground water or bodies of water. Wood decay fungi 

will not affect wood that is fully saturated with water and 

without oxygen. Timber members (such as bridge piling) 

should be carefully inspected near the water line because 

rivers and streams have varying water levels throughout 

the year and from year to year. Figure 2.7 shows the use of 

moisture meters with long insulated pins (up to 3 in. 

Figure 2.3—Probes are used to assess depth of cracks, 

checks, and through splits in timber bridge elements.

Figure 2.4—Relationship between withdrawal resistance 

and residual strength. MOR, modulus of rupture. 
Wood and Timber Condition Assessment Manual • Chapter 2

7

(75 mm) long) to assess moisture content of timber abutment caps. Pin-style moisture meters measure electrical 

resistance between two metal pins that are driven into the 

member and use this measurement to determine moisture 

content. 

Drilling

Drilling and coring are the most common methods used 

to detect internal deterioration in wood members. Both 

techniques are used to detect the presence of voids and to 

determine the thickness of the residual shell when voids 

are present. Drilling utilizes an electric power drill or handcrank drill equipped with a 9.5- to 19-mm- (3/8- to 3/4-in.-) 

diameter bit. Power drilling is faster, but hand drilling allows the inspector to monitor drilling resistance and may 

be more beneficial in detecting pockets of deterioration. In 

general, the inspector drills into the member in question, 

noting zones where drilling becomes easier and observing 

drill shavings for evidence of decay. The presence of common wood defects, such as knots, resin pockets, and abnormal grain, should be anticipated while drilling and should 

not be confused with decay. If decay is detected, remedial 

treatments, such as copper naphthenate, can be added to 

the wood through the inspection hole to help retard further 

decay. The inspection hole is probed with a bent wire or a 

thickness gauge to measure shell thickness. Because these 

holes are typically 3/8 to 3/4 in. diameter, they should be 

filled with a wood dowel section that has been soaked in a 

preservative.

Coring

Coring with an increment borer (often used for determining 

the age of a tree) also provides information on the presence 

of decay pockets and other voids. The resultant solid wood 

core can be carefully examined for evidence of decay. In 

addition, the core can be used to obtain an accurate measure of the depth of preservative penetration and retention. 

Figure 2.8 shows an increment core tool and the extracted 

core. The core can also be used to determine the wood speFigure 2.5—Relationship of screw-withdrawal force to 

bending strength for 12-mm- (1/2-in.-) thick plywood. 

MAP, mono-ammonium phosphate; PA, phosphoric 

acid; KD, dry-kiln drying temperature; and MOR, modulus of rupture. 

Figure 2.6—Screw withdrawal resistance meter that can 

be used to assess local wood quality.

Figure 2.7—A pin-style moisture meter is used to 

determine moisture content of timber elements.
General Technical Report FPL–GTR–234

8

cies. To prevent moisture and insect entry, a bored-out core 

hole should be filled with a wood plug treated with copper 

napthenate.

Resistance Drilling

Another commercially developed drilling technique is the 

resistance drill system. Developed in the late 1980s, this 

system was originally developed for use by arborists and 

tree care professionals to assess tree rings, evaluate the condition of urban trees, and locate voids and decay. This technology is now being utilized to identify and quantify decay, 

voids, and termite galleries in wood beams, columns, poles, 

and piles. This technique is now the preferred drilling and 

coring technique for timber elements. Figure 2.9 shows a 

resistance microdrill being used to assess the level of decay 

in a timber bridge pile. 

Several machine types are available from different manufacturers. They operate under the same general principle of 

measuring electrical power consumption of a needle rotation 

motor. This value is proportional to mechanical torque at the 

needle and primarily depends on wood density (Rinn 2013). 

The purpose of the equipment is to identify areas in timber 

elements that have low density, indicating decay or 

deterioration. 

Resistance drill equipment measures the resistance of 

wood members to a 0.6-in.- (1.5-mm-) diameter drill bit 

with a 0.18-in. (3.0-mm) head. This flat tipped drill bit 

travels through the member at a defined movement rate and 

generates information that allows an inspector to determine 

the exact location and extent of any damaged area. Figure 

2.10 shows several drill bit ends that are used in resistance 

drills. Although the unit is usually drilled into a member in 

a direction perpendicular to the surface, it is also possible to 

drill into members at an angle (Fig. 2.11).

Figure 2.8—An increment core can be used to conduct 

inspections of timber bridge elements. This image shows 

an extracted core from an in-service timber pile ready for 

examination.

Figure 2.9—A resistance microdrill used for inspection 

of a historic Civilian Conservation Corps log cabin.

Figure 2.10—Flat-tipped resistance drill bits used to inspect timber materials.

Figure 2.11—Drilling can take place at an angle to assess 

the area below ground line.
Wood and Timber Condition Assessment Manual • Chapter 2

9

Resistance drills collect data electronically and can also 

product a chart or printout showing relative resistance over 

the drilling path. Modern tools are also promoting the ability 

to view the data on a tablet computer or hand-held mobile 

phone. Areas of sound wood have various levels of resistance, depending on the density of the species; voids show 

no resistance. The inspector can determine areas of low, 

mild, and high levels of decay with this tool, and quantify 

the level of decay in the cross section. Figure 2.12 shows a 

timber abutment cap being assessed with a resistance microdrill and the resulting chart image, which shows minimal 

drilling resistance and indicates that the majority of the cap 

is decayed. Figure 2.13 shows a commercial model with an 

electronic display that allows allow data to be reviewed in 

the field and then further processed using a computer in the 

office. All holes should be filled after drilling, especially if 

no decay is present. For the microdrill, this can be accomplished by injecting a small amount of silicone sealant or 

marine adhesive into the small opening (Fig. 2.14).

Interpreting Drilling Data Charts

Charts or printouts should be reviewed in the field and notes 

taken to ensure understanding of the testing location. Notes 

should be taken on a graphical data chart. Care should be 

exercised to ensure that low density profiles from intact 

but soft wood (such as conifers) not be misinterpreted as 

decay. The very center of softwood species near the pith 

will have low resistance and lack the defined growth rings 

visible in the outer sections. It is important to understand the 

type of wood that is being drilled. Sound wood from many 

hardwood species may have high levels of resistance (over 

50%), whereas sound wood from softwood conifers may 

have low levels of resistance (in the range of 15% to 50+%, 

depending on its inherent density). It is important to evaluate the levels of decay across the full dimension, as some 

species have low resistance values but are not decayed. Further, each piece of commercial equipment provides different 

scales and may indicate different resistance levels. Table 2.1 

shows a general assessment rating index that can provide 

support for the bridge inspector in evaluating resistance data 

collected during testing. Example electronic drilling charts 

for a southern yellow pine pile and a Douglas-fir pile cap 

are shown in Figures 2.15 and 2.16, respectively. Note that 

while the fundamental concept utilized by these pieces of 

equipment is essentially the same, data analysis and subsequent display of the data can vary significantly, depending 

on equipment manufacturer.

Figure 2.12—Resistance microdrilling showing significant 

decay in the bridge pile cap. The inlay shows the paper 

chart readout from a commercial drilling unit.

Figure 2.13—Electronic display on a resistance drill.

Figure 2.14—Silicone is used to fill the small drilling hole.

Table 2.1—General assessment of resistance drilling 

data for Douglas-fir and southern yellow pine bridge 

members

Drilling 

resistance 

(%) Decay level Comments

0 Severe Decay resulting in an internal void

5–15 Moderate Often adjacent to the internal void 

areas

20+ Low to none Sound material often has resistance 

that is consistent across the full 

width

Note: These data must be carefully interpreted because of differences 

between commercial equipment.
General Technical Report FPL–GTR–234

10

Figure 2.15—Electronic view of a southern yellow pine timber piling showing a decay pocket between 8 

and 10 in. of the drilling profile.

Figure 2.16—Electronic resistance chart of a Douglas-fir pile cap showing a large crack between 180 

and 200 mm along the drilling path.
Wood and Timber Condition Assessment Manual • Chapter 2

11

Commercial Equipment

Several companies produce equipment that is suitable for 

inspecting timberstructures based on these concepts. Additional details for these companies and their equipment 

follow.

Increment Borers

Forestry Suppliers Inc.

Jackson, MS 39284-8397 USA 

Telephone: (800) 647-5368

Website: www.forestry-suppliers.com

Ben Meadows Company 

Janesville, WI USA 53547-5277

Telephone: (608) 743-8001

Fax: (608) 743-8007 

Website: www.benmeadows.com

Resistance Microdrills

IML-RESI PD- and F-Series

IML North America, LLC

Moultonborough, NH 03254 USA

Telephone: 603-253-4600

Website: www.iml-na.com

Resistograph 4- and 5-Series

RINNTECH, Inc.

St. Charles, IL 60174, USA

Telephone: (630) 377-2477

Website: www.rinntech.de

Digital microProbe

Sibtec Scientific

Sibert Technology Limited

2a Merrow Business Centre, Guildford

Surrey GU4 7WA England

Telephone: +44 1483 440 724

Fax: +44 1483 440 727 

Website: www.sibtec.com

Screw Withdrawal Resistance Meter

FAKOPP Enterprise

Agfalva, Hungary

Telephone: +36 99 33 00 99

Website: www.fakopp.com 

References

Cai, Z.; Hunt, M.; Ross, R.J.; Soltis, L.A. 2002. Screw 

withdrawal—a means to evaluate densities of in-situ wood 

members. In: Proceedings, 13th international symposium on 

nondestructive testing of wood. Madison, WI: Forest Products Society: 277–281.

Fakopp. 2014. User’s guide: screw withdrawal resistance 

meter. Agfalva, Hungary: Fakopp Enterprise. http://www.

fakopp.com/site/downloads/Screw_Withdrawal.pdf. (Accessed April 24, 2014)

Rinn, F. 2013. From damage maps to condition inventories: 

a proven concept for documentation of results of inspection 

of timber bridges and other timber structures. International 

Conference on Timber Bridges. Las Vegas, NV. http://www.

woodcenter.org/docs/ICTB2013/technical/papers/ID_139_

Rinn.pdf

Talbot, J. 1982. Unpublished research. Pullman, WA: Wood 

Materials and Engineering Laboratory, Washington State 

University.

Winandy, J.E.; Lebow, P.K.; Nelson, W. 1998. Predicting 

bending strength of fire-retardant-treated plywood from 

screw withdrawal tests. Res. Pap. FPL-RP-568. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

13

Chapter 3

Inspection of Timber Structures Using 

Stress Wave Timing Nondestructive 

Evaluation Tools

Robert J. Ross

Roy F. Pellerin

The purpose of this chapter is to provide guidelines on the 

application and use of the stress wave timing inspection 

method in locating and defining areas of decay in timber 

structures. Practical procedures for field testing, workable 

forms for gathering evaluation data, and guidelines for interpreting data are provided. This information was derived 

from research that quantified the ability of stress wave 

timers to detect decay in wood, from laboratory and field 

studies of deteriorated timber bridges, and most importantly 

from the experience of timber bridge inspectors familiar 

with the use of commercially available devices. A table that 

lists current manufacturers of these devices is included. 

Properties of wood and important aspects of wood deterioration are also reviewed to provide those who are unfamiliar 

with wood the basic information necessary to detect decay.

Principles of Stress Wave 

Nondestructive Testing for 

Condition Assessment

As an introduction, a schematic of the stress wave concept 

for detecting decay within a rectangular wood member is 

shown in Figure 3.1. First, a stress wave is induced by striking the specimen with an impact device (such as the hammer shown in the illustration) that is instrumented with an 

accelerometer that emits a start signal to a timer. A second 

accelerometer, which is held in contact with the other side 

of the specimen, senses the leading edge of the propagating 

stress wave and sends a stop signal to the timer. The elapsed 

time for the stress wave to propagate between the accelerometers is displayed on the timer.

The velocity at which a stress wave travels in a member is 

dependent upon the properties of the member only. The term 

ultrasonic and sonic refer only to the frequency of excitation used to impart a wave into the member. (Ultrasound 

frequencies begin at 20 kHz; sonic frequencies are between 

20 Hz and 20 kHz.) All commercially available timing units, 

if calibrated and operated according to the manufacturer’s 

recommendations, yield comparable results.

The use of stress wave velocity to detect wood decay in timber bridges and other structures is limited only by access to 

the structural members under consideration. It is especially 

useful on thick timbers or glued-laminated (glulam) timbers 

≥89 mm (≥3.5 in.) where hammer sounding is not effective. 

Access to both sides of the member is required.

Because wood is an organic substance, material properties 

and strength vary in accordance with the direction wood 

is hammered compared with the cell structure orientation. 

Hammering the end grain of a beam or post will cause a 

primarily longitudinal shock wave along the length of the 

cell structure in the timber. Hammering the side or top of 

the beam will cause a wave across or transverse to the wood 

cells. Cells are arranged in rings around the center of the 

tree.

The velocity at which a stress wave propagates in wood is a 

function of the angle at which the fibers of wood are aligned 

(which is also a determinant of other physical and mechanical properties). For most structural members, fibers of the 

Figure 3.1—Stress wave timer.
General Technical Report FPL–GTR–234

14

wood align more or less with the longitudinal axis of the 

member (Fig. 3.2).

Stress wave transmission times on a per length basis for various wood species are summarized in Table 3.1. Stress wave 

transmission times are shortest along the grain (with the 

fiber) and longest across the grain (perpendicular to fiber). 

For Douglas-fir and Southern Pine, stress wave transmission 

times parallel-to-the-fiber are approximately 200 μs/m 

(60 μs/ft). Stress wave transmission times perpendicular to 

the fiber range from 850 to 1,000 μs/m (259 to 305 μs/ft).

Effect of Ring Orientation

Researchers have determined that the longest transverse-tograin transmission times are found at a 45° orientation to 

the annual rings. The shortest is about 30% faster in a path 

that is radial (Fig. 3.3). Table 3.2 and Figure 3.4 show stress 

wave transmission time for wood of good quality at 12% 

moisture content. These values can vary ±10% for species 

variation. These times are based on an assumed stress wave 

transmission time of 668 μs/m radially, 800 μs/m tangentially, and 995 μs/m at 45° to grain.

Effect of Decay

The presence of decay greatly affects stress wave transmission time in wood. Table 3.3 summarizes stress wave transmission values obtained from field investigations of various 

wood members subjected to degradation from decay. Stress 

wave transmission times perpendicular to the grain are drastically increased when the member is degraded. Transmission times for nondegraded Douglas-fir are approximately 

800 μs/m (244 μs/ft), whereas severely degraded members 

exhibit values as high as 3,200 μs/m (975 μs/ft) or greater.

A 30% increase in stress wave transmission times implies 

a 50% loss in strength. A 50% increase indicates severely 

decayed wood (Fig. 3.5). Transverse travel paths are best 

for finding decay. Parallel-to-grain travel paths can bypass 

regions of decay.

Figure 3.2—Three principal axes of wood with respect to 

grain direction and growth rings.

Table 3.1—Summary of research on stress wave transmission times for various species 

of nondegraded wood 

Reference Species 

Moisture 

content 

(% OD)a

Stress wave transmission time (μs/m (μs/ft)) 

Parallel to grain Perpendicular to grain 

Smulski 1991 Sugar maple 12 256 to 194 (78 to 59) — 

Yellow birch 11 230 to 180 (70 to 55) — 

White ash 12 252 to 197 (77 to 60) — 

Red oak 11 262 to 200 (80 to 61) — 

Armstrong et al. 

1991 

Birch 4 to 6 213 to 174 (65 to 53) 715 to 676 (218 to 206) 

Yellow-poplar 4 to 6 194 to 174 (59 to 53) 715 to 676 (218 to 206) 

Black cherry 4 to 6 207 to 184 (63 to 56) 689 to 620 (210 to 189) 

Red oak 4 to 6 226 to 177 (69 to 54) 646 to 571 (197 to 174) 

Elvery and 

Nwokoye 1970 

Several 11 203 to 167 (62 to 51) — 

Jung 1979 Red oak 12 302 to 226 (92 to 69) — 

Ihlseng 1878, 1879 Several — 272 to 190 (83 to 58) — 

Gerhards 1978 Sitka spruce 10 170 (52) — 

Southern pine 9 197 (60) — 

Gerhards 1980 Douglas-fir 10 203 (62) — 

Gerhards 1982 Southern pine 10 197 to 194 (60 to 59) — 

Rutherford 1987 Douglas-fir 12 — 1,092 to 623 (333 to 190) 

Ross 1982 Douglas-fir 11 — 850 to 597 (259 to 182) 

Hoyle and Pellerin 

1978 

Douglas-fir — — 1,073 (327) 

Pellerin et al. 1985 Southern pine 9 200 to 170 (61 to 52) — 

Soltis et al. 1992 Live oak 12 — 613 to 1,594 (187 to 486) 

Ross et al. 1994 Northern red and 

white oak 

green — 795 (242) 

a

 OD, ovendry. 
15

Weight loss is not a good indicator of decay because considerable strength loss can occur without significant weight 

loss. As Figure 3.5 illustrates, significant loss of strength 

occurs before noticeable weight loss.

Effect of Moisture Content

Considerable research has shown the effect of moisture in 

wood on stress wave transmission time. Several studies 

have revealed that stress wave transmission times 

perpendicular to the grain of wood follow the relationship 

shown in Figure 3.6. At moisture contents less than approximately 30%, transmission time decreases with decreasing 

moisture content. Corrections for various moisture content 

values are summarized in Table 3.4.

Figure 3.3—Transverse stress wave paths and transmission 

times: (a) timber, (b) glulam beam.

Table 3.2—Typical stress wave transmission 

times for nondecayed Douglas-fir at 12% 

moisture content 

Path length 

(mm (in.)) 

Stress wave transmission time (μs) 

Radial Tangential 45° to grain 

64 (2.5) 43 51 64 

89 (3.5) 60 71 88 

140 (5.5) 94 112 139 

184 (7.25) 123 147 183 

235 (9.25) 157 188 234 

292 (11.5) 195 234 290 

342 (13.5) 229 274 340 

394 (15.5) 264 315 392 

444 (17.5) 297 355 442 

495 (19.5) 331 396 492 

Figure 3.4—Transverse stress wave transmission time compared with annual ring orientation.

Figure 3.5—Relationship between stress wave transmission 

time and fungal degradation (Pellerin et al. 1985) (1 lb/in2 = 

6.9 kPa).

Wood and Timber Condition Assessment Manual • Chapter 3
General Technical Report FPL–GTR–234

16

At moisture content values greater than approximately 30%, 

little or no change in transmission time occurs. Consequently, there is no need to adjust the measured values for wood 

that is tested in a wet condition.

Effect of Preservative Treatment

Treatment with waterborne salts has almost no effect on 

stress wave transmission time. Treatment with oilborne 

preservatives increases transmission time to about 40% 

greater than that of untreated wood. Round poles are usually 

penetrated to about 37 to 61 mm (1.5 to 2.5 in.). Table 3.5 

was calculated to show expected travel time for round poles 

treated with oilborne preservatives. Although these data illustrate the effect oilborne treatments have on transmission 

time, these values should not be used to estimate level of 

penetration.

Interpretation of Stress Wave 

Velocity Readings

The guidelines in this chapter are useful in interpreting 

readings that are less than those for sound wood. Voids and 

checks will not transmit stress waves. Knots will act as parallel-to-grain wood but are usually oriented perpendicular to 

the long axis of timber.

Based on the direction and length of the stress wave path 

in the wood, moisture content of the wood, and whether or 

not preservative treatment is present, the velocity and travel 

time for sound wood can be determined. For the transverse 

direction, the annual ring orientation and the existence of 

seasoning checks should be recorded.

Table 3.3—Summary of research on use of stress waves for detecting decay in timber structures 

Reference Structure Wood product Test Analysis 

Volny 1992 Bridge Douglas-fir 

glulam, creosote 

pressure treated 

Stress wave transmission time 

perpendicular to grain, across 

laminations at 0.3-m (0.98-ft) 

intervals 

Sound wood: 1,279 μs/m (390 μs/ft) 

Moderate decay: 1,827 μs/m (557 μs/ft) 

Severe decay: 2,430 μs/m (741 μs/ft) 

Ross 1982 Football 

stadium 

Solid-sawn 

Douglas-fir, 

creosote pressure 

treated 

Stress wave transmission time 

perpendicular to grain, near 

connections 

Sound wood: 853 μs/m (260 μs/ft) 

Incipient decay: 

 –Center of members: 

 1,276 μs/m (389 μs/ft) 

 –38-mm-thick solid wood shell: 

 2,129 μs/m (649 μs/ft) 

Severe decay: >3,280 μs/m (1,000 μs/ft) 

Hoyle and 

Pellerin 

1978 

School 

gymnasium 

Douglas-fir 

glulam arches 

Velocity of stress wave 

transmission time 

perpendicular to grain, near 

end supports 

Sound wood: 1,073 μs/m (327 μs/ft) 

Decayed wood: 1,574 μs/m (480 μs/ft) 

Figure 3.6—Transverse stress wave transmission times in 

Southern Pine and red oak piling.

Table 3.4—Stress wave transmission time adjustment factors for 

temperature at various moisture contents for Douglas-fir 

Moisture

content (%) 

Adjustment factors 

–18 °C (0 °F) 3 °C (38 °F) 27 °C (80 °F) 49 °C (120 °F) 

1.8 0.94 0.95 0.97 0.98 

3.9 0.95 0.96 0.98 0.99 

7.2 0.93 0.98 1.00 1.01 

12.8 0.97 0.99 1.00 1.01 

16.5 0.99 1.01 1.03 1.05 

23.7 1.05 1.07 1.09 1.14 

27.2 1.07 1.10 1.12 1.17 
17

Measurement of Stress Wave 

Transmission Time

General Measurement

Several techniques can be used to measure stress wave 

transmission time in wood. The most common technique 

uses simple time-of-flight-type measurement systems. Two 

commercially available systems that use this technique are 

illustrated in Figures 3.7, 3.8, and 3.9. Note that for ultrasonic measurement systems, a couplant is used between the 

transducers and the specimen.

With these systems, a mechanical or ultrasonic impact is 

used to impart a wave into the member. Piezoelectric sensors are placed at two points on the member and used to 

detect passing of the wave. The time required for the wave 

to travel between sensors is then measured.

Commercially Available Equipment

Several equipment development firms are producing portable equipment based on this concept. Table 3.6 presents contact information for obtaining information on these devices. 

Field Considerations and Use of 

Stress Wave Methods

Stress Wave Transmission Time

Figure 3.10 outlines the general procedures used with stress 

wave nondestructive evaluation methods for field work. 

Before venturing into the field, it is useful to estimate stress 

wave transmission time for the size of the members to be 

inspected. Preceding sections provided information on various factors that affect transmission time in wood. This 

information can be summarized, as a starting point, by 

simply using a baseline transmission time of 1,300 μs/m 

(400 μs/ft). Transmission time, on a per length basis, less 

than this would indicate sound material. Conversely, transmission time greater than this value would indicate potentially degraded material. Using this value, you can estimate 

the transmission time for a member by knowing its thickness (path length) and the following formula:

Tbaseline(μs) = 1,300 × WTD μ

where Tbaseline is baseline transmission time (μs) and WTD is 

wave transmission distance (path length) (m).

[Inch–pound formula: Tbaseline (μs) = 400 × WTD, where 

WTD is wave transmission distance (path length) (ft)]

By knowing this number for various thicknesses, field work 

can proceed rapidly.

Field Data Form

An example of a typical field data acquisition form is shown 

in Figure 3.11. Key items to include on the form are structure name, location, number, inspector, and date of 

inspection.

Field Measurements

Field use should be conducted in accordance with instructions provided by equipment manufacturers. In the field, 

extra batteries, cables, and sensors are helpful. Testing 

should be conducted in areas of the member that are highly 

susceptible to degrading, especially in the vicinity of connections and bearing points.

The baseline values provided serve as a starting point in 

the inspection. It is important to conduct the test at several 

points at various distances away from the suspect area. In a 

Table 3.5—Stress wave transmission times for 

round poles treated with oil-borne preservatives 

Pole diameter 

(mm) 

Stress wave transmission time (μs) 

for various levels of penetration 

37 mm 61 mm Full penetration 

294 222 240 300 

343 254 271 350 

392 286 305 400 

441 321 338 450 

490 350 370 500 

539 386 403 550 

588 422 436 600 

Figure 3.7—Technique used to measure impact-induced 

stress wave transmission times in various wood products.

Figure 3.8—Ultrasonic measurement system used to 

measure stress wave transmission times in various 

wood products.

Wood and Timber Condition Assessment Manual • Chapter 3
General Technical Report FPL–GTR–234

18Table 3.6—Commercially available stress wave timing equipment

Manufacturer Product Website Telephone Fax Address Additional Information Agricef USLab www.agricef.com.brCBS-CBT Sylvatest-Duo www.cbs-cbt.com 33 1 56 70 43 80 33 1 48 92 05 85 126 Avenue d'Alfortville, PA Les 

Gondoles, batiment E 

Fakopp Enterprise Various models www.fakopp.com 36 99 33 00 99 36 99 33 00 99 Fakopp Bt., Fenyo U 26, H-9423 

Agfalva, Hungary 

Distributors in Japan, China, Hong Kong, Macau, Italy, USA 

IML GmbH IML Micro Hammer www.imlusa.com 603 253 4600 IML North America, 15 Glidden 

Rd., Moultonborough, NH 03254 

Distributors and sales partners worldwide 

Metriguard, Inc. Models 239A Stress 

Wave Timer 

www.metriguard.com 509 332 7526 509 332 0485 2465 NE Hopkins Court, Pullman, 

WA 99163 

James Instruments, Inc.

V-Meter MK IV www.ndtjames.com 773 463 6565 773 463 0009 3727 N. Kedzie Avenue, Chicago, 

IL 60618-4545 USA 

Distributor in Europe 

Olson Instruments, Inc.

Various models www.olsoninstruments.com 303 423 1212 303 423 6071 12401 W. 49th Avenue, Wheat 

Ridge, CO 80033-1927 USA 
19

Figure 3.10—General procedures used to prepare and use 

stress wave timing methods for field work.

Wood and Timber Condition Assessment Manual • Chapter 3

Figure 3.9— Ultrasonic measurement system. Note couplant 

between transducer and specimen being tested. 

Figure 3.11—Typical field data acquisition form.
General Technical Report FPL–GTR–234

20

sound member, little deviation is observed in transmission 

times. If a significant difference in values is observed, the 

member should be considered suspect.

Data Analysis and Summary Form

When data have been gathered, it is useful to present them 

in an easy-to-read manner. Figure 3.12 illustrates several 

data summaries. From these, the presence and extent of 

degradation can readily be seen. The top and bottom illustrations show a timber cross section with visual defects and 

stress wave times noted. The center drawing illustrates the 

side view of a timber, with stress wave travel times noted. 

References

Armstrong, J.P.; Patterson, D.W.; Sneckenberger, J.E. 1991. 

Comparison of three equations for predicting stress wave 

velocity as a function of grain angle. Wood and Fiber Science. 23(1): 32–43.

Elvery, R.H.; Nwokoye, D.N. 1970. Strength assessment of 

timber for glued laminated beams. In: Proceedings, Paper II, 

Nondestructive testing of concrete and timber, organized by 

the Institution of Civil Engineering and the British Commission for Nondestructive Testing; 1969 June 11–12. London, 

BC: Institute of Civil Engineering: 105–110.

Gerhards, C.C. 1978. Effect of earlywood and latewood on 

stress-wave measurements parallel to the grain. Wood Science. 11(2): 69–72.

Gerhards, C.C. 1980. Effect of cross grain on stress waves 

in lumber. Res. Pap. FPL–RP–368. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Gerhards , C.C. 1982. Effect of knots on stress waves in 

lumber. Res. Pap. FPL–RP–384. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Hoyle, R.J.; Pellerin, R.F. 1978. Stress wave inspection of a 

wood structure. In: Proceedings, 4th symposium on nondestructive testing of wood; 1978 August 28–30; Vancouver, 

WA. Pullman, WA: Washington State University: 33–45.

Ihlseng, M.C. 1878. The modulus of elasticity in some 

American woods, as determined by vibration. Van Nostrand’s Engineering Magazine. Vol. XIX: 8–9.

Ihlseng, M.C. 1879. On a mode of measuring the velocity 

of sound in wood. American Journal of Science. 3d Series. 

17I(98): 125–133.

Jung, J. 1979. Stress wave grading techniques on veneer 

sheets. Gen. Tech. Rep. FPL–GTR–27. Madison, WI: U.S. 

Department of Agriculture, Forest Service, ForestProducts 

Laboratory.

Pellerin, R.F.; De Groot, R.C.; Esenther, G.R. 1985. Nondestructive stress wave measurements of decay and termite 

attack in experimental wood units. In: Proceedings, 5th 

symposium on nondestructive testing of wood; 1985 

September 9–11; Pullman, WA. Pullman, WA: Washington 

State University: 319–353.

Ross, R.J. 1982. Quality assessment of the wooden beams 

and columns of Bay C of the east end of Washington State 

University’s football stadium. Unpublished research. Pullman, WA: Washington State University.

Ross, R.J.; Ward, J.C.; TenWolde, A. 1994. Stress wave nondestructive evaluation of wetwood. Forest Products Journal. 

44(7/8): 79–83.

Rutherford, P.S. 1987. Nondestructive stress wave measurement of incipient decay in Douglas Fir. Pullman, WA: Washington State University. M.S. thesis.

Smulski, S.J. 1991. Relationship of stress wave and static 

bending determined properties of four northeastern hardwoods. Wood and Fiber Science. 23(1): 44–57.

Soltis, L; Baker. A.; Ferge, L.; [et al.]. 1992. USS Constitution—inspection and evaluation procedures for live and 

white oak members. Report submitted to U.S.Navy, Maintenance and Repair, USS Constitution, Boston Naval Shipyard, Charlestown, MA.

Volny, N.J. 1992. Timber bridge inspection case studies in 

use of stress wave velocity equipment. In: Proceedings, 8th 

symposium on nondestructive testing of wood; 1991 September 23–25; Vancouver, WA. Pullman, WA: Washington 

State University.
21

Figure 3.12—Examples of summary form (top) and data summary form.

Wood and Timber Condition Assessment Manual • Chapter 3

23

Chapter 4

New Techniques and Technologies

Xiping Wang

Zhiyong Cai

The wood products research community has invested considerable resources investigating the suitability of several 

techniques for assessing the condition of in-service wood. 

Acoustic tomography, transverse vibration, and threedimensional laser scanning are three techniques that have 

shown promise for in-place assessment of wood structures. 

Advances in computing technologies have resulted in a 

strong interest in using acoustic tomography to assess inservice timbers. Originally developed for assessing the condition of urban trees, this technique builds upon the successful use of acoustic methods to detect deterioration in wood. 

It provides a more detailed assessment than commonly used 

techniques and processes baseline results to provide a twodimensional (2D) image of the cross section of a timber. 

The methods and technologies described in the first three 

chapters are widely used to assess the condition of individual members or small areas within a wood member. Recently, 

a series of experiments were conducted to examine the 

feasibility of in-place testing of entire structural wood systems. Transverse vibration nondestructive testing techniques 

were used to estimate in-place stiffness of short-span timber 

bridges and building floor systems. 

Use of three-dimensional (3D) laser scanning technologies 

to obtain as-built records for historic covered bridges has 

been investigated. These technologies show promise as 

part of a comprehensive inspection approach to historic 

structures. 

Overviews of these techniques and results obtained from 

their use are presented in this chapter. A list of relevant 

technical publications is included. 

Acoustic Tomography

Stress wave timing, although widely used in building 

inspection, has its limitations. A single-pass stress wave 

measurement can only detect internal decay that is greater 

than 20% of total cross-sectional area (Wang et al. 2004). To 

increase the reliability of the inspection and better define extent and location of any internal decay in a structural timber 

would require multiple acoustic measurements in different 

orientations at one cross section, especially in suspect areas. 

Two-dimensional tomographic inversions of stress wave 

data from multiple path measurements could allow inspectors to obtain a color map, or tomogram, of the distribution 

of stress wave velocity in the cross section, enabling a more 

direct imagery-based assessment. 

Acoustic tomography is a nondestructive evaluation technology for decay detection in urban trees and structural 

timbers. Commonly used tomographic image reconstruction is based on time of flight (TOF) data obtained through 

multipath measurements. It allows users to visualize the 

velocity distribution of acoustic waves as the waves propagate through the cross section of a wood member. Because 

acoustic velocity is directly related to density and modulus 

of elasticity of wood, acoustic velocity mapping of a cross 

section can be used as a diagnostic image to detect internal 

wood decay (Bucur 2003). Application of this technology 

was initially demonstrated by Tomikawa et al. (1990) for 

inspecting the internal condition of wooden poles. Later, 

researchers investigated the applicability of the technology 

to detect internal decay in live trees (Divos and Szalai 2002; 

Gilbert and Smiley 2004; Wang et al. 2004, 2009; Wang 

and Allison 2007; Allison et al. 2008; Deflorio et al. 2008). 

Although field use of acoustic tomography is largely with 

urban tree inspections, the potential of this technology for 

assessing condition of large timber piles or columns in wood 

structures has been demonstrated and its application in the 

field of structural inspection will likely increase (Divos 

2006, Wang and Wacker 2006, Yu et al. 2009, Dackermann 

et al. 2014). 

Basics of Acoustic Tomography

Acoustic tomography is a diffraction-type tomography that 

utilizes acoustic impulses as an energy source. In contrast 

to ionizing radiation, acoustic waves do not travel along 

straight rays and the projections are not line integrals. The 

accuracy of tomography with acoustic energy is limited 

by the diffraction of the energy field and also affected by 

frequency of the acoustic waves, number of sensors used, 

and the applied evaluation algorithm. The main benefits of 

this technique are that (1) it is a noninvasive method, (2) the 

equipment is safe to operate, and (3) the equipment is relatively low cost. Three main types of algorithms can be used 

to form tomographic images from acoustic data: transform 

techniques, iterative methods, and direct inversion techniques (Bucur 2003). The most commonly used parameter 

in acoustic imaging is time of flight (TOF), which can be 

easily obtained through a pair of sensors. 
General Technical Report FPL–GTR–234

24

For TOF data acquisition, multiple sensors must be attached 

around the circumference of a timber. Figure 4.1 shows an 

example of acoustic tomography measurement with a chain 

of sensors attached to the base of a timber post through sensor pins (Rinn 2006, Dackermann et al. 2014). Typically, 8 

to 12 acoustic sensors are used in most applications, including trees and structural timbers (posts, columns, and beams), 

to obtain a reasonably good resolution of tomographic images. In cases where higher resolution is desired for critical 

structural timbers, the number of sensors can be increased to 

24 or 36, depending on availability of sensors. 

Coupling of the acoustic sensors to the timber under inspection is typically achieved through pins or nails when impulses are generated by a hammer impact. When ultrasonic 

sensors are used to test solid-sawn or glued-laminated timbers, gel-type coupling can provide good acoustic impedance. Field tomography measurement can also be expedited 

by using a specially designed flexible mounting frame that 

facilitates the sensor installation process. 

Figure 4.2 illustrates the sensor arrangement and paths of 

acoustic waves for a tomography test on a round timber. 

Acoustic TOF data is obtained by sequentially tapping each 

pin using a steel hammer. A complete TOF data matrix can 

be obtained through this measurement process at each test 

location. 

Equipment and Operating Procedures

Three commercial tools are currently available for performing TOF acoustic tomography testing on trees and structural 

timbers: PiCus Sonic Tomograph (Argus Electronic Gmbh, 

Rostock, Germany), Arbotom 3D Impulse Tomograph 

(RinnTech, Heidelberg, Germany), and ArborSonic 3D 

Acoustic Tomograph (Fakoop Enterprise, Sopron, Hungary). 

An acoustic tomography unit generally comes with a series 

of low-noise piezoelectric sensors, sensor attachment pins, 

amplifier boxes, connection cables, a rechargeable battery 

pack, a tape or caliper for measuring sensor positions, a steel 

hammer for tapping the sensors, and a software package for 

mapping geometry of the cross section, collecting TOF data, 

and generating tomograms. Total measurement time for one 

cross section of a timber is from 20 to 30 min., depending 

on the number of sensors used and ease of accessing the test 

location. Measurements at several heights can be assembled 

into a 3D model when needed. 

Acoustic tomography measurement is generally conducted 

based on following operating procedures:

1. Select the cross section of the timber to be tested; measure the circumference of the cross section; determine 

the number of sensors to be used. 

2. Mark the starting point (sensor No. 1) at the cross section, place a tape measure around the cross section, and 

then mark the locations of rest of the sensors. 

3. Drive attachment pins into the timber using a hammer 

and attach the piezoelectric sensors on the pins (spiked 

sensors can be inserted into timber directly).

4. Measure the distances between the sensors with a caliper, and enter the measured data into a computer.

5. Determine the geometry of the cross section using the 

software package.

6. Conduct acoustic measurements: starting with sensor 

No. 1 and proceeding to each sensor in turn, tap each 

sensor with a steel hammer to generate sound waves. 

The tool measures the time of flight with microsecond 

precision to each transducer and transmits the data to 

the computer.

Figure 4.1—Acoustic tomography test on a timber post. 

(Rinn 2006)

Figure 4.2—Sensor arrangement and paths of acoustic 

measurement on a round timber.
25

7. The software calculates the internal acoustic velocity 

distribution in the cross section and displays as a colored tomogram.

Applications 

Glued-Laminated Timber Specimen: Divos (2006) demonstrated the use of acoustic tomography on a section of 

glued-laminated timber (glulam) with a simulated void 

(Fig. 4.3). Sixteen sensors were strategically placed on four 

faces of the specimen (Fig. 4.3a). The center void can be 

visualized as a red color zone in the tomogram (Fig. 4.3b). 

Wood Structural Members: Yu et al. (2009) inspected the 

wood structural members of the Imperial Palace, Beijing, 

China, using an impulse stress wave tomography tool. An 

Wood and Timber Condition Assessment Manual • Chapter 4

Figure 4.3—Acoustic tomography test on a glued-laminated specimen with a simulated 

void (Divos 2006).

Figure 4.4—Acoustic tomogram revealed internal crack of an old timber at the Imperial 

Palace in Beijing (Yu et al. 2009).

internal crack in the timber was properly identified in the 

tomogram (Fig. 4.4). The results suggested that at least 

12 sensors should be placed around a 54 by 30 cm timber 

in order to obtain a reasonably accurate tomographic image. 

In-Service Masts of Historic Wooden Ship: Wang and Wacker (2006) inspected the masts of the U.S. Brig Niagara, a 

reconstruction of an early 19th century wooden warship 

of the U.S. Navy, using acoustic tomography technique 

(Fig. 4.5). The fore mast and main mast are both fabricated 

from southern yellow pine glulam. Primary concerns were 

locations at the main deck level where water often comes 

in and creates moist conditions. Tomography testing on 

both masts indicated sound cross sections at the deck level, 

which were further confirmed by resistance drilling tests. 
General Technical Report FPL–GTR–234

26

Tomography Complemented with Micro Resistance Drilling: Riggio and Piazza (2011) used a multi-channel device 

(TDAS 16 – Boviar SRL) with an ultrasonic probe of 

55 kHz to map decayed areas in timbers. Acoustic velocity 

maps were represented in a diagram by 256 levels of grey, 

with the white level corresponding to maximum velocity and the black level corresponding to minimum velocity. Complementary to TOF tomography, micro resistance 

drilling tests were carried out in portions of the sections to 

provide data for confirmation of the tomograms. Figure 4.6 

shows the tomogram acquired from a decayed cross section 

superimposed with a relative resistance profile. The x axis of 

the plot is positioned along the corresponding drilling path. 

The presence of void and poor quality of the timber can be 

identified by low values of both acoustic velocity and relative drilling resistance. The void area in the left portion of 

the timber is confirmed by the almost flat resistance profile, 

and the sound wood in the central part is confirmed by the 

high resistance profiles. Results indicated that integral of 

relative resistance profile divided by penetration depth and 

average acoustic velocity values in tomograms can be used 

to give general interpretation of the tests. 

Limitations

Although acoustic tomography has been successfully used 

to detect moderate to severe internal decay in structural 

timbers, the detection of early stage of decay using such 

technology still constitutes a challenge because (1) acoustic 

tomography techniques currently used are largely based on 

TOF measurements, which limit the accuracy and resolution 

of the tomography images obtained; (2) the construction of 

tomogram from TOF data is affected by pronounced anisotropy of timber in terms of microstructures and wood properties; and (3) interpretation of tomograms is not quantitative 

in nature and often needs further confirmation or verification 

by other means. For field application, it is recommended 

that complementary nondestructive test methods be used to 

obtain local quantitative information, such as a resistance 

drilling test, to verify the internal condition of the area. 

Transverse Vibration of Structural 

Systems

Inspection and evaluation of existing timber structures 

has historically been limited to evaluating each structural 

member individually, which requires a time-consuming inspection. Sometimes individual members are not accessible 

and therefore difficult to inspect. Low-frequency transverse 

vibration has been studied for over 50 years as a potential 

method for evaluating wood-based products. However, the 

nondestructive evaluation of a timber floor system using 

transverse vibration was researched about 15 years ago. 

Transverse vibration was used to evaluate properties of 

new and salvaged individual floor joists (Cai et al. 2000), 

and then these joists were used to fabricate floor systems 

of five joists and attached flooring deck in the laboratory. 

The responses of these floor systems were then evaluated in the laboratory using transverse vibration (Cai et al. 

2002). These pilot studies used an impact load to initially 

displace the structure; the free vibration characteristics of 

individual joists or the whole floor system were measured. 

The salvaged joists tested in these studies had seasoning 

checks, splits, and some edge decay, which resulted in lower 

stiffness values than those of the new joists. Later, forced 

vibration was imposed by a motor with an eccentric rotating 

mass attached to the floor decking (Soltis et al. 2002). Motor speed could be manually changed, and the rotating mass 

forced the floor to vibrate at its frequency. The resonant vibration was observed by changing the speed of the rotating 

mass until maximum deflection was reached. The observed 

resonant frequency of the wood floor system should be close 

to its natural frequency, which is highly related to its stiffness and its supporting boundary condition. The transverse 

vibration technique was used later to assess single-span 

timber bridges (Wang et al. 2005). 

Fundamentals

Jayne (1959) applied a forced transverse vibration technique 

for evaluating a single wood member. Pellerin (1965) 

Figure 4.6—Tomogram of a timber section superimposed 

with the relative resistance profile (Riggio and Pizza 2011).

Figure 4.5—Inspection of the masts of the U.S. Brig Niagara 

using acoustic tomography.
27

significantly improved the technique by using free transverse vibration with dimension lumber. A vibrating beam 

can be modeled as the vibration of a mass M that is attached 

to a weightless spring and internal damping. The generalized equation of motion of a mass under damped forced 

vibration is derived from the classical spring-dash-pot analogy. When a forcing function equaling P0 sin ωt (P is force 

and ω is frequency, or zero) is applied for forced (or free) 

vibration, the equation of motion of M can be expressed as

 

(1)

where K is the elastic constant of the spring and D is the 

damping coefficient of the dashpot. 

A solution for Equation (1) is obtained as

(2)

where

 

(3)

When the beam is at resonance (dx/dω = 0), the solution for 

K (with correction for mass and substitutions of appropriate constants) leads to the following equation for dynamic 

modulus of elasticity (MOE) of the beam:

(4)

where fr is resonant frequency (Hz), W is beam weight (lb or 

kg), L is beam span (in. or m), I is beam moment of inertia 

(in4 or m4); g is gravitational acceleration (386 in/s2 or 

9.8 m/s2 ), n is a constant dependent on mode of support 

(equal to 12.65 for freely supported at two nodal points and 

2.46 for supported at the ends).

The logarithmic decrement, δ, is used as a measure of energy dissipation and can be expressed as (for free vibrations)

 

(5)

where A1 and An are the amplitudes of two oscillations n – 1 

cycles apart. For forced vibrations,

where ∆ f is the difference in frequency of two points of 

amplitude A on each side of a resonance curve, fr is the frequency at resonance and Ar

 is the amplitude at resonance. 

Note that Equation (4) represents the relationship for a simply supported idealized beam. This formula, however, is 

frequently used to estimate the relationship for simply supported systems as well.

Exciting and Measuring System

Floor systems were subjected to both free and forced vibration. Free vibration was initiated by impact from a large 

hammer. Forced vibration was imposed by a motor with an 

eccentric rotating mass attached to the floor decking. 

Motor speed could be manually changed from rest up to a 

maximum of 2,500 rpm. The weight of the rotating mass 

was based on the total weight of the floor system (up to 

1 kg) and the eccentricity from the rotating axis. The response to vibration was measured at the bottom of the joists 

using a linear variable differential transducer (LVDT). The 

time-deflection signal was recorded by an oscilloscope. 

For free vibration, the damped natural frequency was determined as the inverse of the period measured from the 

time-deflection signal; the damping ratio was determined 

from the same signal using the classic log-decrement technique. For forced vibration, the damped resonant frequency 

was determined by increasing motor speed until maximum 

deflection resonance was observed and then measuring frequency from the time deflection signal.

Results and Implementation

A total of 30 in-place floors and four configurations of two 

salvaged floor sections were tested both dynamically and 

statically (Forsman and Erickson 2001, Soltis et al. 2002, 

Wang et al. 2005, Hunt et al. 2007). The forced natural frequencies and configuration were measured. According to 

Equation (4), the square of forced damped natural frequency 

for each floor is correlated to the factor EI/WL3, where EI

was determined by the static load test, W is estimated weight 

of the floor/section, and L is measured floor span. The solid 

curve in Figure 4.7 is the hypothesized response of the natural frequencies with different EI/WL3 configurations. Data 

points in Figure 4.7 are measured natural frequencies of 

the prior tested laboratory-built floor sections. A least 

squares analysis yields a coefficient of determination 

r2 = 0.85 for comparing empirical data versus theory. Based 

on these favorable results, the hypothesis that in-place wood 

floors, such as those tested in this project, can be modeled as 

a simply supported, one-way beam system under transverse 

vibration with viscous damping was not rejected.

The goal of implementing this technique was to develop 

a methodology of using the measured damped natural frequency of a floor to safely predict the floor’s EI. Although 

the theory fits experimental data extremely well, 95% confident limits could be used to account for variability. Similar 

to establishing allowable bending stress for lumber, the fifth 

Wood and Timber Condition Assessment Manual • Chapter 4

 M + D + Kx = P0 sin ωt d2x

dt ( 2 ) dx ( dt )

x(t) = sin(ωt – φ)

P0

(–ω2M + K)2 + (ωD)2

 φ = tan–1 ωD

–ω2M + K

MOE = fr

2WL3

n2Ig

δ = 1

n – 1

ln A1

An

δ = πΔf

fr

1

(Ar /A)2 – 1 (6)
General Technical Report FPL–GTR–234

28

percentile exclusion limit is used as the control curve to estimate allowable EI/WL3. In practice, a wood structural system (i.e., an in-place floor) would be transversely resonated 

to determine its damped natural frequency; the frequency 

value would be used to obtain the associated value of 

EI/WL3 with 95% confidence level for the system. The EI

of the system can be isolated from this value by measuring 

the floor span and estimating the weight of the system. The 

EI will provide a good estimation of overall structural condition of the wood structural system. 

Laser Scanning of Structures

Three-dimensional (3D) laser scanners are instruments that 

record precise and accurate surface data of objects in a nondestructive manner. These instruments use a beam of infrared light to calculate and record the distance to an object, 

typically as data points with spatial coordinates. These data 

are then analyzed using various types of computer software 

to generate a detailed image of coordinates and dimensions. 

Three-dimensional laser scanners have been used successfully to digitize objects of various sizes ranging from small 

diagnostic artifacts to large, complex sites of monumental 

architecture (FARO Technologies 2011a,b).

A number of companies manufacture various types of 3D 

laser scanners. Generally, these units use light detection and 

ranging technology (LiDAR), where laser pulses determine 

the distance to an object or surface. The distance to an object is determined by using time-of-flight between transmission of a pulse and detection of the reflected signal. A point 

cloud of data is then collected and can be converted into the 

true shape of the object.

Laser scanning technologies have been investigated for use 

in the assessment of bridges. The Pennsylvania Department 

of Transportation (PennDOT), for example, completed an 

initial study in 1999 with the goal of evaluating the technology for creating as-built drawings. A comparison of traditional and 3D scanning estimated an overall time savings of 

100+ person-hours through the use of 3D scanning (Foltz 

2000). Based on these results, PennDOT purchased two laser scanners in 2000. A second assessment was completed in 

2003 showing that laser scanning could be used cost effectively for preliminary surveys to develop triangular network 

(TIN) meshes of roadway surfaces and to measure bridge 

beam camber more safely and quickly than with conventional approaches. Other applications noted in this publication 

showed potential applications for laser scanning to include 

developing as-built drawings of historical structures, such 

as the bridges of Madison County, Iowa.

Covered bridges are part of the fabric of American history. 

Although much effort is expended to preserve these structures, many are lost forever. The National Park Service’s 

Historic American Engineering Record (HAER) has efforts 

under way to document historic structures. Their Level I 

documentation is defined in the Secretary of the Interior’s 

Standards and Guidelines for Architectural and Engineering 

Documentation and consists of measured and interpretive 

drawings, large-format photographs, and written historical 

reports. To assist in this effort, newer technologies needed to 

be explored that can provide as-built records at a faster rate 

and with more accuracy.

The University of Minnesota Duluth’s Natural Resources 

Research Institute (UMD NRRI), in cooperation with the 

USDA Forest Products Laboratory and Federal Highway 

Administration, recently completed a study to examine the 

use of laser scanning technologies for providing as-built records for historic covered timber bridges. Additional support 

for the study was provided by the City of Zumbrota, Minnesota, via funds from the Operational Research Program 

(OPERA) of the Minnesota Local Road Research Board. 

Following is a brief description of that study; detailed 

documentation is presented in Ross and others (2012).

The bridges for scanning and assessment used in this study 

are located in Wisconsin, Iowa, and Minnesota. Key background information (Table 4.1), detailed descriptions, and 

photographs are summarized in the following paragraphs. 

Contact was made with the appropriate government or 

administrative staff for each bridge, and permission was 

also secured to obtain scans. Note that four of the bridges 

scanned included the historic Madison County, Iowa, 

bridges.

Laser scanning was completed by Sightline, LLC (Milwaukee, Wisconsin), a private firm that specializes in the use 

of laser scanning technologies with structures. A FARO LS 

880 model and a Photon 120 model (FARO Technologies, 

Lake Mary, Florida) used similar technology to acquire the 

scan points. A laser beam is emitted from a rotating mirror toward the area being scanned. The laser beam is then 

Frequency (Hz)

35

30

25

20

15

10

5

0

0 0.1 0.2 0.3 0.4

Theory

FPL

RR

MTU

AG

FPRD

AGs

IWU

x

x

x

x

x x x

x

x x

x

x

x

x

xxx

x

El/WL3

(1/cm)

Figure 4.7—Scatter plot of in-place floor test data and previously tested lab-built floor sections.
29

reflected back to the scanner by objects in its path. The 

distance to the objects defining an area is calculated, as 

well as their relative vertical and horizontal angles (FARO 

Technologies 2011a,b). Approximately 20 to 30 scans were 

completed for each bridge from a variety of angles using a 

FARO LS 880 laser scanner. Figure 4.8 shows a FARO laser 

scanner being used to inspect a historic covered bridge. The 

scanning process consisted of scans that were completed by 

Sightline, and data processing was completed by the University of Minnesota Duluth Natural Resources Research Institute (UMD NRRI). The steps in the process were as follows 

(with an estimate of time required for each step):

1. Paper “targets” were placed in numerous locations on 

the bridge for use in linking up to 30 individual laser 

scans together. Time duration: 2 person-hours.

2. A FARO LS 880 3D laser scanner was used to conduct 

the scan. The scanner was placed at several vantage 

points inside and outside the bridge, so that all visible 

surfaces of the bridge could be documented. Individual 

scans were completed in approximately 15 to 20 min. 

Each completed scan was saved to a computer as an .ls 

file. (A computer with substantial computing power and 

RAM is highly recommended to process the millions of 

data points created during the scanning process.) Time 

duration: 10 person-hours.

3. After all visible portions of the bridge were scanned, 

the software files were linked using FARO Scene software. This software allows an individual to identify 

the targets placed prior to the scanning process and use 

them to link or attach one scan to another. It is also possible to filter, using various techniques or software, extraneous images (for instance, a vehicle traveling across 

the bridge or background vegetation). The process of 

linking two individual scans was repeated several times 

until all scans were compiled into one large scan depicting the entire bridge. Time duration: 9 person-hours.

4. Once a bridge was completely assembled using all the 

individual scans, it was exported as a point cloud, depicting all visible aspects and actual dimensions of the 

bridge. This cloud of data was then exported into 

AutoCAD software (Autodesk, Inc., San Rafael, California) using an add-in provided by Kubit USA (Houston, Texas). This add-in allows a user to import point 

cloud files in addition to the ones inherently recognized 

by AutoCAD 2011 and has additional modeling tools 

for working directly with point cloud data in AutoCAD. 

Once a point cloud has been exported into AutoCAD, it 

can be divided into multiple cross sections. This is done 

so that specific components of the bridge can be seen 

more clearly. From this point, two dimensional (2D) 

and 3D models of the bridge were generated. Time 

duration: 40+ person-hours.

Several types of images can be presented from processing 

point cloud data. These images include a point cloud image 

resulting from only one scan, a point cloud image created 

from multiple scans, a parametric picture created from a 

point cloud scan, a point cloud image imbedded in AutoCAD, and 2D/3D AutoCAD images. These images can be 

created using FARO Scene software or AutoCAD 2011 with 

a Kubit USA add-in. As to project activities, the majority of 

the scan processing for the Cedarburg, Wisconsin, bridge 

was completed by Sightline, LLC. Processing of scan data 

for the Madison County, Iowa, bridges and the Zumbrota, 

Minnesota, bridge was completed by the UMD NRRI. The 

Wood and Timber Condition Assessment Manual • Chapter 4

Table 4.1—Background information on the historic covered timber bridges used in 

this study 

State City Bridge 

Year

built

Span

(ft)

Placement 

on National 

Register of 

Historic Places Design 

Wisconsin Cedarburg Red 1876 120 March 14, 1973 Lattice through truss 

Iowa Winterset Roseman 1883 106 September 1, 1976 Lattice through truss 

Iowa St. Charles Imes 1870 81 February 9, 1979 Lattice through truss 

Iowa Winterset Hogback 1884 106 August 28, 1976 Lattice through truss 

Iowa Winterset Cutler–Donahoe 1871 79 October 8, 1976 Lattice through truss 

Minnesota Zumbrota Zumbrota 1869 120 February 20, 1975 Lattice through truss 

Figure 4.8—A FARO laser scanner used to scan a historic 

covered bridge. 
General Technical Report FPL–GTR–234

30

project team decided that based on processing time estimates, detailed in-point cloud and 3D AutoCAD data would 

be provided for the Cedarburg, Imes, and Zumbrota Bridges, 

with only point cloud data for the Hogback, Cutler–Donahoe, and Roseman Bridges. All digital points could be further processed to develop detailed dimensional information 

because point cloud images are considered accurate data. 

Point cloud images could also be further processed using 

Kubit USA add-ins for AutoCAD 2011. Each bridge scan 

and AutoCAD image is shown in the following sections. 

Cedarburg, Wisconsin, Covered Bridge

Figures 4.9 to 4.11 show various images created from 3D laser scanning conducted during the project for the Cedarburg 

Bridge. This includes point cloud images and AutoCAD 

drawings created from the point cloud images. 

Imes Bridge, Madison County, Iowa

The Imes Bridge is the only Madison County Bridge for 

which the point cloud data were used to create AutoCAD 

drawings. For the Imes Bridge, the data show accurate 

dimensions and shape of the bridge at the time of scanning. No corrections were made to straighten any bridge 

members, such as would be done to create new construction 

drawings.

References

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in the USFS Nesbit Lake Camp, Sidnaw, Michigan. In: 

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43–48.

Bucur, V. 2003. Nondestructive characterization and imaging of wood. Berlin, Heidelberg, Germany: Springer-Verlag: 

354 p.

Cai, Z.; Hunt, M.O.; Ross, R.J.; Soltis, L.A. 2000. Static and 

vibration moduli of elasticity of salvaged and new joists. 

Forest Products Journal. 50(2): 35–40.

Cai, Z.; Ross, R.J.; Hunt, M.O.; Soltis, L.A. 2002. Pilot 

study to examine use of transverse vibration nondestructive 

evaluation for assessing floor systems. Forest Products Journal. 52(1): 89–93.

Dackermann, U.; Crews, K.; Kasal, B.; Li, J.; Riggio, M.; 

Rinn, F.; Tannert T. 2014. In situ assessment of structural 

timber using stress-wave measurements. Materials and 

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Deflorio, G.; Fink, S.; Schwarze, F.W.M.R. 2008. Detection 

of incipient decay in tree stems with sonic tomography after 

wounding and fungal inoculation. Wood Science and Technology. 42:117–132

Figure 4.9—Point cloud image of the Cedarburg Bridge 

(SightLine, LLC). 

Figure 4.10—3D AutoCAD image of the Cedarburg Bridge 

embedded in point cloud data (SightLine, LLC). 

Figure 4.11—Dimensioned point cloud image of bridge 

entry on Cedarburg Bridge (SightLine, LLC). 
31

Divos, F. 2006. Stress wave based acoustic tomography. 

IUFRO Workshop on Nondestructive Testing of Wood, 

Harbin, China. August 7–9, 2006 (unpublished presentation, 

personal communication).

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tomography. In: Proceedings of the 13th international symposium on nondestructive testing of wood. Madison, WI: 

Forest Products Society: 339–349.

FARO Technologies. 2011a. FARO laser scanner training 

[Print presentation]. Lake Mary, FL: FARO Technologies.

FARO Technologies. 2011b. Measuring arms, laser tracker, 

laser scanner. In: FARO laser scanner focus 3D—applications. Lake Mary, FL: FARO Technologies. http://www.faro.

com/focus/applications.

Foltz, L. 2000. 3D laser scanner provides benefits for 

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Forsman, J. W., and Erickson, J. R. _2001_. “Final report on 

a study of methods to determine the effect of decayed lumber in floor systems.” School of Forestry and Wood Products, Michigan Technological Univ., Houghton, Mich.

Gilbert, E.A.; Smiley, E.T. 2004. PiCUS sonic tomography for the quantification of decay in white oak (Quercus 

alba) and hickory (Carya spp.). Journal of Arboriculture. 

30(5):277–280.

Hunt, M.O., R.J. Ross, X. Wang, L.A. Soltis, R.R. Leavitt, 

and Z. Cai. 2007. Assessment of in-place wood floor systems. J. of Architectural Engineering. 13( 1): 30-35.

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Ross, R.J; Brashaw, B.K.; Anderson, S.J. 2012. Use of laser 

scanning technology to obtain as-built records of historic 

covered bridges. Res. Paper. FPL-RP-669. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory: 18 p.

Soltis, L.A.; Wang, X.; Ross, R.J.; Hunt, M.O. 2002. Vibration testing of timber floor systems. Forest Products Journal. 

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inspection of wooden poles using ultrasonic computed tomography. IEEE Transactions UFFC. 33(4): 354–358.

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trees using a combination of visual inspection, acoustic testing, and resistance microdrilling. Journal of Arboric Urban 

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structural members of the U.S. Brig Niagara. Inspection 

report, NRRI/TR-2006/25. Duluth, MN: University of Minnesota: 61 p.

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Pellerin, R.F. 2004. Assessment of decay in standing timber 

using stress wave timing non-destructive evaluation tools—

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single-span timber bridges using a vibration-based method. 

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Wood and Timber Condition Assessment Manual • Chapter 4

33

Chapter 5

Post-Fire Assessment of Structural 

Wood Members

Robert H. White

Brian M. Kukay

Because the interior of a charred wood member normally 

retains its structural integrity, large structural wood members often do not need to be replaced after a fire. Engineering judgment is required to determine which members can 

remain and which members need to be replaced or repaired. 

The lack of established methods to directly determine 

residual capacity of damaged wood members dictates a 

systematic approach, starting with assessment of likely fire 

exposure. Assessment includes visual inspection of damaged members, visual inspection of connections, and visual 

inspection of any protective membranes (such as gypsum 

board). Potential methods for nondestructive evaluation of 

structural properties of a fire-damaged wood member are 

discussed after a brief review of the degradation of wood 

when exposed to fire.

Thermal Degradation of Wood

Wood degrades when exposed to elevated temperatures. Fire 

exposure causes thermal degradation or pyrolysis of wood 

whereby the wood is converted to volatile gases and a char 

residue. The extent of any thermal degradation depends on 

both temperature and duration of exposure. A temperature of 

≈300 °C (≈550 °F) is commonly associated with the base 

of the char layer for wood subjected to direct fire exposure 

in the standard fire-resistance test (ASTM E 119). Vigorous 

production of flammable volatiles occurs in the temperature 

range of 300 to 450 °C (550 to 842 °F). Temperatures below 

a threshold of 300 °C can still have a demonstrated detrimental effect on wood. The National Design Specification 

for Wood Construction (NDS) indicates that properties of 

wood heated to 150 °F (66 °C) for brief periods will essentially return to their original levels when the wood is cooled 

(AWC 2012). However, prolonged exposure to temperatures 

above 66 °C may result in a permanent loss in properties 

(Kretschmann 2010). The change in residual strength properties of similarly sized individual wood members can differ 

at elevated temperatures, depending, in part, on how they 

are loaded. For example, permanent loss in compressive 

strength has been shown to occur at higher temperatures 

than a comparative loss in tensile strength (Schaffer 1977).

Kinetic parameters are used to model rate of thermal degradation. Detailed discussions of the processes involved can 

be found in the literature (Browne 1958, White and Dietenberger 2001). Degradation resulting in weight loss is associated with temperatures exceeding 100 °C. For temperatures 

less than 200 °C (≈392 °F), charring of wood requires 

prolonged exposure. Significant degradation occurs in the 

temperature range of 200 to 300 °C (392 to 572 °F). Products such as plywood and particleboard have ignition properties very similar to those of solid wood, so the solid wood 

results will generally be applicable to them. Piloted ignition 

at heat fluxes sufficient to cause a direct-flaming ignition 

normally occurs at surface temperatures of 300 to 

365 °C (Babrauskas 2001). Sudden surface heating of a 

wood member in a fire results in surface charring and a 

steep temperature gradient. Thus, the stages of thermal 

wood degradation previously discussed become zones of 

degradation in a structural wood member exposed to fire. In 

a broad sense, there is an outer char layer, a pyrolysis zone, 

a zone of elevated temperatures, and the cool interior 

(Fig. 5.1). These zones of degradation reflect the temperature profile through the cross section.

Figure 5.1—Degradation zones in a charred piece of wood.
General Technical Report FPL–GTR–234

34

Fire-Damaged Wood

For wood members that have charred, the char layer can 

be scraped off. Obviously, any charred portion of a fireexposed wood member has no residual load capacity. The 

wood beneath the char layer has residual load capacity, but 

this residual capacity will be less than the load capacity prior to the fire because of the reduced cross section. Members 

that have only visual smoke damage or slight browning of 

the surface also have significant residual load capacity.

ASTM E 119 (ASTM 2009) standard test method is the 

test for determining the fire-resistance rating of a structural 

member or assemblies for building code purposes. This 

severe and direct fire exposure results in rapid surface charring, the development of a char layer with a base temperature of ~300 °C (~550 °F), and a steep temperature gradient 

of 177 °C (350 °F) at 6 mm (0.2 in.) and 104 °C (220 °F) 

at 13 mm (0.5 in.) beneath the char layer (Fig. 5.2). The 

standard fire exposure is a specified time–temperature curve 

of 538 °C (1,000 °F) at 5 min, 843 °C (1,550 °F) at 30 min, 

and 927 °C (1,700 °F) at 1 h. For a large wood member 

directly exposed to the standard fire exposure, the char rate 

is approximately 0.6 mm/min (38 mm/h, 1.5 in/h, 1/40 in/

min). Char rate in the standard test depends on species, density, moisture content, and duration of exposure (White and 

Nordheim 1992). Most research on fire endurance of wood 

members has been directed toward predicting or understanding their performance in this test (White 2008, Buchanan 

2001). Fire endurance research on wood for other fire exposures or post-fire situations is limited.

Without extinguishment, a fire has three phases:

1. Growth of the fire from ignition to flashover

2. Fully developed post-flashover fire

3. Decay period of declining temperatures as fuel 

is consumed

Although fire exposure transitions through three phases, the 

fire exposure of the standard fire-resistance test approximates only the second phase, or post-flashover portion, of 

the fire. Information gathered in a NFPA 921 investigation 

will help establish likely maximum temperatures in various 

locations. The fire exposure of a standard fire-resistance test, 

such as ASTM E 119 (ASTM 2009), is used to approximate 

the second phase, or post-flashover portion, of the fire of 

structural members and assemblies that are in the immediate vicinity. Standard fire exposure represents exposure of 

a structural member or assembly in the immediate vicinity 

of a fully developed post-flashover fire. Flashover is the full 

involvement of combustible contents of the compartment 

and is associated with flames coming out the door in the 

standard room-corner test. The following situations are all 

examples of fire exposures inconsistent with an assumption 

of the standard fire exposure: 

•	 Exposure of wood components a distance away from the 

fully developed post-flashover fire (such as roof rafters 

exposed to hot gases from a fire in a room below)

•	 Smoldering cellulosic insulation fire near wood rafters

•	 High-intensity fire that is quickly extinguished

•	 Prolonged heating of wood after extinguishment

•	 Wood behind gypsum wallboard or other protective 

membranes 

It is advisable to first obtain an informed understanding of 

the fire itself and the fire exposure to the structural members 

being evaluated. The general rules for reducing the cross 

section for a fire equivalent to the standard exposure are 

based on assumptions of temperature gradients within uncharred wood during the fire. 

Fire Investigation

As noted by Buchanan (2001), it is valuable to visit the 

fire scene immediately after the fire to make notes of all of 

the damage that occurred. The post-fire situation after the 

mid-1990s fire in a building at the USDA Forest Service, 

Forest Products Laboratory, is illustrated in Figure 5.3. For 

most fire investigations conducted by fire departments and 

other investigators, the intent is to establish the cause for 

initial ignition and fire growth. The standard guide for such 

investigations is NFPA 921 Guide for Fire and Explosions 

Investigations (NFPA 2008). This guide advocates a methodology based on a systematic approach and attention to all 

relevant details. For the purpose of a post-fire assessment of 

structural wood members, the intent of an immediate investigation is to better estimate the intensity and duration of fire 

exposure to wood members during and after the fire. Such 

insight will be helpful in making engineering judgments on 

likely temperatures within the charred and uncharred wood 

members. NFPA 921 provides information on various observations for estimating temperatures developed during a fire. 

Figure 5.2—Charring wood member exposed to the 

standard fire exposure of 815 to 1,038 °C.
35

For post-fire assessment, exposure of structural wood members to elevated temperatures during the decay period of fire 

development should be considered. Although temperatures 

are lower during the decay period, duration of exposure 

can be prolonged compared with the duration of the fully 

developed post-flashover fire phase. The steep temperature 

gradient near fire-exposed surfaces assumed in normal assessment of residual load capacity is based on transient 

heating coupled with progressive charring of the wood cross 

section. During prolonged cooling, surface temperatures 

will decline while temperatures on the cool inside portion of 

the cross section will increase. Tests have indicated that this 

temperature increase in the interior of a wood member due 

to redistribution of heat after fire exposure is particularly 

the case for wood protected with gypsum board. Because 

the decay or post-extinguishment period is one of reduced 

temperatures, many damage observations made at the fire 

scene will be less helpful in determining intensity and duration of the exposure. More careful and detailed inspections 

of structural members and connections will likely need to be 

done in a subsequent inspection when the general debris has 

been removed.

Visual Inspection of Charred 

Members

Wood exposed to temperatures in excess of ~300 °C 

(~550 °F) will form a residual char layer on the surface 

(Fig. 5.4). With prolonged exposure, charring of wood can 

occur at lower temperatures. Although it retains the anatomical structure of uncharred wood, the char layer can be 

scraped or sand-blasted off.

In an inspection of charred members, it is important to understand that the char layer exhibits significant shrinkage, 

resulting in fissures in the char layer. Glowing combustion 

of char also can occur. As a result, thickness of the residual 

char layer is less than depth of charring (Fig. 5.5). The 

profile of the original section will need to be determined 

from construction records or similar uncharred members.

Load Capacity of Damaged 

Members

Thermal degradation of wood results in loss of structural 

properties. Thermal degradation of wood is a kinetic process. Thermal properties of wood result in development of 

a distinct temperature gradient in a wood member when 

it is exposed to fire. Thus, loss of structural properties of 

fire-damaged wood members depends on both temperature 

within the wood member and duration of the elevated 

temperatures.

For an exposed wood member large enough that the temperature of its center or back surface has not increased, the 

temperature gradient within a wood beam or column for 

standard fire exposure has been documented. For such a 

fire exposure, the base of the char layer has a clear 

Figure 5.3—Area of fire origin in Building 2 fire at the Forest 

Products Laboratory.

Figure 5.4—Charred and uncharred wood members in the 

Building 2 fire at the Forest Products Laboratory.

Figure 5.5—Residual cross section of a charred wood 

member.

Wood and Timber Condition Assessment Manual • Chapter 5
General Technical Report FPL–GTR–234

36

demarcation. For standard fire exposure of a semi-infinite 

slab, the temperature profile beneath the base of the char 

layer can be approximated by

(5.1)

where Ti is initial temperature of the wood (°C), Tp is temperature of the base of the char layer (300 °C), x is distance 

beneath the char layer (mm), and a is thickness (mm) of the 

layer of elevated temperatures (Fig. 5.6).

For the data of White and Nordheim (1992), the average 

value of a was 33 mm (1.3 in.) for the eight species tested 

(Janssens and White 1994). An alternative exponential model was developed by Schaffer (1965, 1982b). This temperature profile is valid after a standard fire exposure of about 

20 min. The thickness of the zone of elevated temperatures 

decreases for increased fire exposure severity. For a char 

depth of 12 mm (0.5 in.), the observed depth of elevated 

temperatures decreased from 36 to 30 mm (1.4 to 1.2 in.) 

when the level of a constant heat flux exposure was increased from 15 to 50 kW/m2 (White and Tran 1996). For 

a char depth of 6 mm (0.2 in.), the depths of elevated temperature were 34 and 25 mm (1.3 and 1.0 in.) for heat flux 

levels of 15 and 50 kW/m2, respectively.

The irreversible effects of elevated temperatures on mechanical properties depend on moisture content, heating medium, 

temperature, exposure period, and to some extent species 

and size of the piece involved (Kretschmann 2010). Over a 

period of months, temperatures of 66 °C (150 °F) can significantly reduce modulus of rupture (MOR). Graphs of the 

permanent effect of oven heating for periods up to 200 days 

on MOR and modulus of elasticity (MOE) can be found in 

the Wood Handbook (Kretschmann 2010). After 50 days of 

oven heating at 115 °C (240 °F), MOR at room temperature was approximately 90% that of unheated controls. For 

samples heated at 135 °C (275 °F), MOR at room temperature was approximately 62% that of unheated controls after 

50 days. Permanent losses in strength occurred more rapidly 

with heating temperatures of 155 °C (310 °F) and 175 °C 

(350 °F). Elevated temperatures below charring temperature 

appear to have little effect on MOE.

Using the data of Knudson and Schniewind (1975) and 

Schaffer (1973), Schaffer (1977, 1982a,b) developed graphs 

of temperature effects on tensile (Fig. 5.7) and compressive 

(Fig. 5.8) strength. The data illustrate the reduced impact 

that temperature has on residual strength properties once 

the wood has cooled to room temperature and has been 

reconditioned back to 12% moisture content. At a depth of 

only 8 mm (0.3 in.) beneath the char layer, temperature has 

dropped to 200 °C (Fig. 5.6). At 200 °C, residual strength 

properties still exceed 80% of initial room temperature values (Figs. 5.7 and 5.8). Additional information on the effects 

of temperature and moisture content on strength properties 

of wood are provided by Schaffer (1982a), Gerhards (1982), 

Kretschmann (2010), and Buchanan (2001). During an actual fire, the residual capacity of the wood member is affected 

by steam generated within the member (Buchanan 2001) 

and zones of elevated moisture content (White and Schaffer 

1980). Schaffer (1982a) concluded his discussion of properties of timbers exposed to fire by noting that because of the 

short time that wood just beyond the char line has been at its 

maximum temperature, overall strength loss in heavy sections will be small and residual load-carrying capacity will 

be closely approximated by using the initial strength properties of the uncharred residual cross section as a base.

Thus, the steep temperature gradient allows us to easily 

estimate residual load capacity of the member by reducing 

residual cross section of the uncharred section by an 

additional amount to improve the safety margins of our 

T = Ti + (Tp – Ti)(1 – x/a)2

Figure 5.6—Temperature profile beneath the base of the 

char layer of a semi-infinite wood slab directly exposed to 

the ASTM E 119 standard fire exposure.

Figure 5.7—Fractional tensile strength as function of 

temperature (Schaffer 1977, 1982b, 1984).
37

calculations. In general, fire endurance design of wood 

members is referred to as the reduced or effective crosssection method (Fig. 5.5). A notional char depth defines the 

effective cross section for calculation purposes. Although 

visual char depths may provide some insight, depth of char 

and overall damage can make predicting a time of exposure 

to fire from the char characteristics difficult (Schroeder 

1999). In their model of a large glued-laminated member 

in a fire, Schaffer et al. (1986) calculated a reduction of 

8 mm (0.3 in.) for tensile strength loss. In the U.S. procedure for fire endurance design of wood members, reduction 

for load capacity calculations is an additional 20% of the 

actual depth of charring (AF&PA 2003). For a 1-h fireresistance test, this calculates to 8 mm (0.3 in.) (char depth 

of 38 mm (1.5 in.)). Complicating factors, including grain 

orientation, heat flux exposure, and moisture content, can 

vary significantly, even within a given species. Accordingly, 

a uniform char rate cannot always be supported (Schroeder 

1999). The AF&PA American Wood Council procedure also 

uses a nonlinear char rate (White and Nordheim 1992). In 

calculating the ability of a member to maintain a specified 

load in a fire test, reduced cross-sectional area is multiplied 

by ultimate strength properties. This procedure for calculations of fire-resistance ratings is discussed in the National 

Design Specification for Wood Construction (AWC 2012). 

Increasing the reduction due to charring by 20% is used in 

the calculations to account for rounding at the corners and 

reduction of strength and stiffness of the heated zone (AWC 

2012). 

In calculating residual load capacity of a member after a 

fire, the reduced cross section would however be multiplied 

by the allowable stresses as in normal allowable stress 

design (AWC 2012). Unless the fire is severe, replacing all 

the large members after a fire is usually not necessary. Fox 

(1974) conducted a study using three 50-ft 24F DF stressgrade beams that were salvaged from a warehouse after a 

severe fire had reduced the cross section of each member 

by nearly 10%. Two-point loading tests were conducted on 

each beam after they were cut into six 10-ft sections and 

three 30-ft sections. Results showed no evidence that the fire 

had reduced the strength of the beams beyond the reduced 

cross section and affected glue line (Fox 1974). According 

to Schaffer (1982a), because of the short time that wood just 

beyond the char line is at its maximum temperature, overall 

strength loss in heavy sections will be small and residual 

load-carrying capacity will be closely approximated by using the initial strength properties of the uncharred residual 

cross section as a base. Time of exposure and temperature 

gradient within fire damaged members may, in some instances, warrant a more in-depth analysis by a qualified professional. Using the information of Kukay et al. (2013), four 

glued-laminated beams were exposed to a time-temperature 

curve intended to produce a significant zone of damaged 

wood due to elevated temperatures within the residual wood 

section. In this instance, loss in strength of the residual 

section was arrived at through a longer duration and lower 

temperature fire exposure, with a more gradual temperature 

gradient within the fire damaged member. Results show that 

relative changes in apparent MOE and MOR changed by 

20% and 46%, respectively, on average after the beams had 

been fire damaged in comparison to a control group that was 

not fire damaged. This marked decrease in flexural properties goes beyond simply reducing cross section once the 

char layer has been removed for members that can otherwise 

be expected to remain in service (Kukay et al. 2013).

Any assessment of a glued-laminated beam must take 

into account the grade of the individual laminates. This is 

particularly the case for beams with a high-grade tension 

laminate that likely was subjected to the most charring. In 

his discussion of assessment and repair of fire-damaged 

buildings, Buchanan (2001) notes that residual wood under the charred layer of heavy timber structural members 

can be assumed to have full strength. He continues with 

the comment that the size of the residual cross section can 

be determined by scraping away the charred layer and any 

wood which is significantly discolored. Williamson (1982a) 

recommends that the amount of char/wood that should be 

removed by sandblasting or other means should be equal 

to the char layer plus approximately 6 mm (0.25 in.) or less 

of the wood below the char–wood interface. The exposed 

surface should then have the appearance of normal wood. 

Williamson (1982a) makes a distinction between design 

capacities controlled by compression strength or stiffness 

and those controlled by tensile strength. In the compression 

case, the removal of the additional 6 mm (0.25 in.) of wood 

is sufficient to use the residual cross section in the design 

calculations without any additional adjustment. For the 

case of tensile design calculations, Williamson (1982a) 

Figure 5.8—Fractional compressive strength as function of 

temperature (Schaffer 1982a,b).

Wood and Timber Condition Assessment Manual • Chapter 5
General Technical Report FPL–GTR–234

38

recommends an additional adjustment beyond the removal 

of 6 mm (0.25 in.) of wood. In calculations of residual tensile strength of the member, basic allowable design stress 

values should be reduced by 10%. An alternative is to take 

a reduction of 16 mm (0.625 in.) of wood beyond the char–

wood interface and use 100% of the basic allowable design 

stress values.

In 1970, the glulam industry adopted the use of high-quality 

tension laminations based on research conducted at FPL. If 

a glulam bending member manufactured after that date is 

damaged during a fire and the outermost tension lamination 

is destroyed, the allowable bending stress for the remaining 

cross section must be reduced by 25%. This is in addition to 

the reduction in section size.

Removal or degradation of any wood from a structural 

member will likely require regrading of the member to determine the proper allowable properties to be used in calculations of residual load capacity. Wood members should be 

re-graded after the char is completely removed. The grade of 

the structural member may have changed due to loss of the 

outer layer of wood. Charring of the wood member is similar to ripping the wood member with a saw in terms of its 

impact on mechanical properties and grade of the member. 

Grading procedures take into account the impact of residual 

dimensions on applicable grading rules for the reduced dimension as well as the altered relative locations of strengthreducing characteristics (such as knots) in the cross section. 

Current accredited grading agencies are listed in a document 

of the American Lumber Standard Committee (ALSC 2011). 

Inherently, wood is a natural product and no two samples 

are exactly alike. Variability is introduced naturally as well 

as mechanically, and through manufacturing and construction practices (Garab et al. 2010). Calculation of residual 

load capacity must take into account structural grade variation of individual components within a composite structural 

wood member. This is very important for charred gluedlaminated (glulam) structural members, because glulam 

members are normally manufactured with a graded lay-up 

that has higher grade materials at the outer laminates and 

lower grade materials in the core. In particular, the charred 

bottom laminate may have been a high-grade tension laminate that significantly impacts the bending strength of the 

member. Examples of calculations for fire-damaged glulam 

members are provided by Williamson (1982a). Williamson 

(1982b) also discusses the rehabilitation of fire-damaged 

heavy timbers at the Filene Center for Performing Arts at 

Wolftrap Farm, Virginia. The structure was damaged due to 

a fire while the facility was under construction in 1971.

White and Woeste (2013) point out that often in older buildings, timbers are not grade marked and there are no records 

of any grading of timbers. The recommend steps for analysis 

begin with a paper study that will either warrant if further 

study is required or simply necessitate that a member be 

replaced outright. When additional study is required, 

calculations are further refined and incorporate a cohort of 

field measurements up to and including identification and 

documentation of the species, grade, and size of each 

timber. Additional considerations are presented in Kukay 

et al. (2012). 

Light-Frame Members

Most information on fire-damaged wood focuses on evaluation of large timber members. Evaluation of residual load 

capacity of structural elements in light-frame construction 

does not allow some of the assumptions of the previous 

analysis, such as direct fire exposure and semi-infinite slab.

Wood structural members in light-frame construction are 

generally covered by a membrane of gypsum board. Gypsum board provides very effective fire protection. Gypsum 

is primarily hydrated calcium sulfate. Bound water within 

the gypsum board delays the rise of the temperature at the 

wood–gypsum board interface above 100 °C (212 °F) for 

a significant period of time. The chemically bound water is 

released as steam during this calcination process. Gypsum 

board loses its integrity or cohesion after exposure to fire 

(Cramer et al. 2003). The integrity of the gypsum board can 

be examined by using a sharp blade or by removing samples 

for more careful examination. Spiszman (1994) suggests 

grinding a sample of the gypsum (minus the paper) and 

moistening it with water to a paste-like material. If, after 

2 h, the sample is hard, similar to plaster of Paris, it should 

be considered heat damaged. As with fire-damaged wood, 

similar materials in areas not involved in the fire provide a 

performance level for comparison. A rule of thumb is that 

gypsum board may be assumed to retain its integrity as 

long as the paper envelope has not charred (King 2002). 

The cross section of the gypsum board can be examined for 

visual evidence of the progression of calcination through 

the gypsum board. Where there is evidence of fire damage, 

the gypsum board may need to be removed so that structural 

wood members can be examined. Charring of wood is more 

likely to occur at the joints between sheets of gypsum board. 

Because of protection provided to the sides of the wood 

members, damage to structural members in assemblies with 

cavity insulation may be limited.

In light-frame construction, significantly charred members 

are generally removed (Steven Winters Associates, Inc. 

1999). Application of the guidelines for heavy timbers to 

light-frame construction results in inadequate load capacity 

with even a small amount of charring. However, many lightframe members in an actual fire suffer only smoke damage 

or very superficial charring. Given the high temperature 

of a fire and the low temperature for wood char (300 °C 

(550 °F)), superficial charring reflects very brief exposure 

to a fire. As previously discussed, the depth of elevated 

temperatures is less for initially smaller depths of charring. Charring is much more rapid during initial charring. 

As the thickness of the insulative char layer increases, the 

progression of charring is slowed. It has also been shown 

that as density of a wood species increases, charring rate 

decreases (Yang et al. 2009). Even so, once the temperature 
39

at the center of the light-frame member starts to increase, 

the temperature profile shown in Figure 5.6 is not valid and 

temperatures will increase more rapidly. King (2002) states 

that structural repair of fire-damaged framing is often not 

required if char depth is less than 6 mm (0.2 in.). He also 

notes that treatment of any significant loss of surface should 

have approval of a local building inspector or a qualified 

structural engineer. Other rules of thumb that have been 

recommended for lumber in trusses include (1) no charring, 

(2) charring of up to 10% of the cross section, and (3) charring depth up to 1.6 mm (1/16 in.) (Smith 2000). Engineering judgment on whether a member needs to be replaced 

includes considering the importance of the member to the 

structural integrity of the building and the need for a 

conservative approach.

Light-frame construction contains numerous building cavities. When fire damage is not extensive, heat, smoke, and 

water damage can occur within the building cavities. In a 

cavity, fire-generated heat damage would involve components with higher sensitivities than the surrounding materials (King 2002). In addition to the cavities of structural 

components, many cavities are also associated with the 

routing of utilities in the building.

Testing

Unlike wood damaged by decay, little work has been done 

on suitable methods for field testing fire-damaged wood for 

residual load capacity. Some potential options are those suggested for field testing fire-retardant-treated (FRT) plywood 

for possible thermal degradation (NAHB 1990). Prolonged 

elevated temperatures, associated with roof applications, 

have resulted in degradation of plywood treated with some 

formulations of fire-retardant treatments. Thermal degradation of plywood was similar to degradation of wood in a fire. 

The National Association of Home Builders (NAHB) identified several options for possible degraded FRT plywood:

1. Applying a concentrated proof load

2. Removing small samples for laboratory mechanical 

testing

3. Screw withdrawal test

4. Chemical analysis for chemical composition of the 

wood, such as hemicelluloses

5. Spectral analysis for end products of degradation

Options 1 and 2 are destructive if, for example, the proof 

load exceeds the linear elastic range for the component being tested, thereby reducing its cross-sectional integrity as a 

result (option 1), or if the removal of small specimens from 

the full-size member renders it unrepairable or otherwise 

requires that it be replaced (option 2). The need to classify 

wood by evaluating the change in physical and mechanical 

properties of small clear specimens can be very valuable 

where similar members are exposed to various degrees of 

degradation and are otherwise expected to remain in service. 

It is also noted that the moisture content is an underlying 

factor of wood strength that can readily be verified by the 

use of a moisture meter during a post-fire investigation. 

However, immediately following a fire, moisture content of 

charred members is likely to fall below 6.5%, a value associated with the lower limit of most moisture meters. Generally speaking, a 1% change in moisture content can affect 

wood strength properties by as much as 2% to 6% (Regents 

of the University of Minnesota and the Forest Products 

Management Development Institute 1998).Typically, wood 

gains strength as moisture content decreases. Wood will 

gain back moisture and re-equilibrate after heat damage. As 

such, lasting effects of this nature would occur only if the 

equilibrium moisture content associated with a given relative humidity changed. This type of test has been standardized and is facilitated with the use of equations presented in 

ASTM D 4933-99 (ASTM 2004). For these reasons, when 

comparing strength properties from separate members to 

one another, the pretesting moisture content of each beam 

must be less than 16% and adjusted to a standard moisture 

content of 12% according to ASTM D 2915-03 (ASTM 

2003). Additional information on temperature and moisture 

content effects on strength properties of wood in general are 

provided by Kretschmann (2010).

Research results have supported the use of Options 2 and 3 

when establishing the change in residual flexural properties 

for fire-damaged members that are otherwise expected to remain in service (Kukay et al. 2012). Evidence of changes in 

flexural properties can be obtained from static bending tests 

that are performed on small samples or from screw withdrawal tests (along with other significant factors). Either 

methodology provides a more quantitative approach than 

simply replacing individual members, presuming that depthwise reductions are less than 0.25 in. These methodologies 

are thought to be valid when similar members of like size 

are heat damaged in similar fashion. 

Kukay and Todd (2009) developed equations including 

variables for moisture content (MC), specific gravity (SG), 

moment of inertia (I), maximum screw withdrawal load 

(SW), cross-sectional orientation with respect to the pith 

(O), treatment group (T, charred or uncharred), and various 

interactions between these variables. The following reduced 

model was found to be adequate based on a model comparison F-test procedure:

where the βi’s are regression parameters and ε represents 

experimental error. Essentially this inclusive model represents the parameters that were (through this particular study) 

associated with either residual stiffness or residual strength. 

So depending on which one Y represents, the factors used to 

predict that value change. 

Models like this are believed to be applicable under similar 

field conditions and are methodology-, material-, and gradespecific. Rather than predicting specific values of Ef

 and 

Wood and Timber Condition Assessment Manual • Chapter 5

Y = β0 + β1MC + β2SG + β3I + β4SW + β5O + β6T + β7O • T + ε 
General Technical Report FPL–GTR–234

40

MOR, results obtained through screw-withdrawal tests are 

best represented when the results are compared with those 

of similar members that have obvious degrees of residual 

load capacity. Variability of results stems from changes in 

predrilled pilot-hole size, screw insertion depth, screw tip 

to screw shank diameter, and rate of extraction. For these 

reasons, care was needed when interpreting and extrapolating the results from screw-withdrawal tests. General correlations are likely to lack adequate precision to establish 

actual property values. Results of individual research studies 

that incorporate screw-withdrawal tests are generally not 

extrapolated. Additional work is needed to expand the models to account for the effects of a wider range of species and 

grades of materials. Additional information can be found in 

Kukay and Todd (2009). 

In the case of options 3 and 4, further research is needed 

to identify and document any appropriate correlations and 

methodologies for fire-damaged wood. Because general correlations are likely to lack adequate precision to establish 

actual property values, these options are more likely to be 

fruitful when they are used to compare similar members in 

the fire-damaged building that have obvious degrees of degradation or residual load capacity. Thus, they may be more 

useful in evaluating fire damage in light-frame construction. 

Application of a screw-withdrawal test to the FRT plywood 

situation was investigated by Winandy et al. (1998). In their 

study, the variability and reproducibility of 8-mm (5/16-

in.) screw insertion was compared with that for 16-mm 

(5/8-in.) screw insertion in 16-mm (5/8-in.) plywood. The 

shorter depth measurements were observed to have higher 

coefficients of variation. In many instances, load cell resolution of the apparatus exceeded 25% of the measured value 

compared with less than 10% for the 16-mm (5/8-in.) screw. 

Although there may have been some gradient in the FRTplywood degradation through the thickness of the plywood, 

the application of the method to fire-damaged wood would 

need to be able to identify degradation primarily near the 

surface of the uncharred wood.

Research has been done on modeling strength loss in wood 

by chemical composition (Winandy and Lebow 2001). Potentially, an increment borer could be used to extract wood 

samples for chemical or spectral analysis. Spectral analysis tests use infrared radiation to identify end products of 

chemical processes. A trained organic chemist is required 

to interpret the data gained from a spectrophotometer. More 

information pertaining to spectral analysis is discussed by 

APA (1989).

Connections

All connections will require detailed inspection to assess 

their load-bearing capacity. In his discussion of large firedamaged timbers, Williamson (1982a) notes that the effect 

of fire on the strength of any connection is very difficult to 

determine without a thorough investigation of the affected 

connection, because the amount of damage is dependent on 

the quantity of metal and the surface contact of metal with 

fire along with other factors. There may also be possible 

chemical damage from the corrosive effects of fire residues. 

Metal roof supports, ceilings, and other structural members 

are vulnerable to long-term acid attack from fire residues 

(King 2002). Exposed metal connections provide a means 

for heat conduction into the wood (Fuller et al. 1992).

Degradation of the wood beneath a metal plate connection 

is what results in its failure (Fig. 5.9a). In a situation when 

heating is strictly via radiation, the metal plate may actually 

initially protect the wood beneath the plate from charring as 

much as the adjacent wood (Fig. 5.9b). The test specimens 

shown in Figure 5.9 are from a project to develop a fire 

endurance model for metal-plate-connected wood trusses 

(White et al. 1993, Shrestha et al. 1995). If there is damage 

to the plate area, the plate is discolored, or there is charring 

under the plate, it is recommended that the connection be 

considered ineffective (Smith 2000).

Smoke Damage

The subjects of smoke damage and control of odor are not 

within the scope of this manual. The impact of fire residues 

on wood framing is confined to appearance and odor (King 

2002). Except for possible corrosive effects on metal fasteners, smoke and other fire residues do not affect the load 

capacity of the wood member. The National Institute of 

Disaster Restoration (NIDR) provides guidelines based on 

current practice in restoration technology (King 2002). The 

institute is associated with the Association of Specialists in 

Cleansing and Restoration (www.ascr.org). Actions for 

addressing smoke damage are also discussed in an article 

by the Chicora Foundation (2003).

Figures 5.9—(a) Test specimens of metal plate connections illustrating charred wood failure beneath the plate, 

and (b) metal plate failure with uncharred wood beneath 

the plate.
41

Fire odors should be identified and removed before any 

application of sealers, paints, or other finishes because 

the masking effects of such products are temporary (King 

2002). The presence of fire acids, visible fire residues, and 

odor need to be addressed. The NIDR Guidelines for Fire 

and Smoke Repair (King 2002) provides information on 

methods for removal of fire residues, neutralizing acid residues, removing fire odors, and the use of sealing and encapsulation. Structural members restored after fire damage 

should retain no char or untreated fire residues even when 

they are covered with new framing or other interior finishes 

(King 2002).

Repairs

Once the load capacities of fire-damaged members are 

determined, potential repairs can be identified. When blasting is required, various media can be used, including sand, 

ground corn cob, and baking soda. Once char and other fire 

residues have been removed, wood surfaces can be treated 

for residual odors and sealers can be applied.

Information on rehabilitation of damaged structures is available in the nine-volume series of the PATH program (www.

pathnet.org) of the U.S. Department of Housing and Urban 

Development known as The Rehab Guide. Information on 

moisture damage will help address water damage due to fire 

suppression efforts. With the high level of concern about 

mold damage, any moisture damage associated with fire 

suppression also needs to be addressed. Restoration of wood 

floors is discussed by King (2002).

In the case of partially fire-damaged wood, repairs often 

consist of reinforcing the original damaged member by attaching a supplemental piece of wood to it. This action is referred to as “sistering.” The effect of fire on epoxy-repaired 

timber is discussed by Avent and Issa (1984). They found 

the two epoxies they tested to be sensitive to heat at relatively low temperature (66 to 93 °C (150 to 200 °F)). Buchanan and Barber (1994) found the two epoxies they tested 

lost strength rapidly at 50 °C (122 °F). Epoxy joints should 

be protected by a thick outer wood layer or other protective material such as gypsum board. Available information 

indicates that adhesives (phenol, resorcinol, and melamine) 

normally used in the manufacture of structural wood composites have a fire performance equivalent of solid wood. 

Schaffer (1968) found that separation did not occur at either 

phenol-resorcinol or melamine glue lines in either charred 

or noncharred laminates during fire exposure.

Any repairs should also include the consideration of design 

changes or additional protection to reduce the likelihood of 

future fire damage. Schaffer (1982c) discusses designing 

to avoid problems with fire. Because of concerns about the 

fire-resistance performance of some adhesives being used to 

make finger-jointed lumber, the wood industry established 

performance qualifications, and the “HRA” markings for 

end-jointed lumber are interchangeable with solid-sawn 

lumber in 1-h fire-rated assemblies (AF&PA 2007). Additional information can be found in the Wood Handbook

(White and Dietenberger 2010). Repairs must comply with 

appropriate building code requirements. Conventional repair 

methods may increase dead loads, installation costs, and 

transportation expenses because of heavier reinforcement 

materials. Modern reinforcement methods using fiberreinforced polymers (FRP) have lower strength-to-weight 

and stiffness ratios. This allows FRPs to be used without a 

significant increase to the dead load of a structure.

FRP composites are usually bonded to the higher stress 

zone, more commonly known as the tension side of timber 

beams, which increases their load-carrying capabilities 

while decreasing deflection. FRP composites bonded on 

the tension face are either adhered to the surface as a sheet 

or are inserted into a cut narrow groove and secured into 

place with a bonding agent. Near-surface mounting of FRP 

bars placed in cut grooves is a new technology designed to 

increase the load-carrying capacity and energy absorption 

capacity comparatively (Kim and Harries 2010). Installing 

the bars in a cut groove on the tension side of the members 

means that repairs will typically be hidden from view. This 

is very important to historic covered bridges that need to 

maintain their historic integrity and charm.

Glass-fiber reinforced plastic (GFRP) bars are generally less 

expensive than other FRP bars, carbon fiber, and aramid fiber, but still significantly improve the load-carrying capacity 

and stiffness of a member (Raftery et al. 2009).

It should be mentioned that for all cases of FRP bar installed 

into timber, the bond shear-slip between the two materials 

has a minor effect on ultimate loading capacity of the members. During analysis, a continuous bond between the two 

materials was observed in most of the tested cases (Valipour 

and Crews, 2011).

Concluding Remarks

Often, the end product of the reaction of wood to fire is an 

outer char layer and a cooler inner core of solid wood. In 

many fires, there is a clear demarcation between the char 

layer and the relatively undamaged residual wood. With appropriate analysis, treatment, and repairs, the fire-damaged 

wood members often can be restored instead of being replaced (Fig. 5.10).

References

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Wood and Timber Condition Assessment Manual • Chapter 5
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42

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in-situ testing. Tacoma, WA: American Plywood 

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2858–2875.

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Babrauskas, V. 2001. Ignition of wood: a review of the state 

of the art. London: Interflam, Interscience Communications 

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its control. FPL Report 2136. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Buchanan, Andrew H. 2001. Structural design for fire safety. 

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Buchanan, A.; Barber, D. 1994. Fire resistance of epoxied 

steel rods in glulam timber. In: Proceedings, volume 1, 

Pacific timber engineering conference. Queensland, Australia: Timber Research and Development Advisory Council: 

590–598.

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fire. Bethesda, MD: Society of Fire Protection Engineers: 

173–182.

Fox, S.P. 1974. Residual strength of fire-damaged laminated 

beams. Forest Products Journal. 24(1): 48–50.

Fuller, J.J.; Leichti, R.J.; White, R.H. 1992. Temperature 

distribution in a nailed gypsum-stud joint exposed to fire. 

Fire and Materials. 16: 95–99.

Garab, J.; Toth, A.; Szalai, J.; Bejo, L.; Divos, F. 2010. Evaluating glued laminated beams using a nondestructive testing 

technique. Transactions of FAMENA. 34(4): 33–46.

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.

Janssens, M.L.; White, R.H. 1994. Temperatures profiles in 

wood members exposed to fire. Fire and Materials. 18: 263-

265.

Kim, Y.J.;Harries, K.A. 2010. Modeling of timber beams 

strengthened with various CFRP composites. Engineering Structures. 32(10): 3225–3234. doi:10.1016/j.engstruct.2010.06.011

Figure 5.10—Post-fire inspection (top) of residence; post-fire (middle) and post-repair 

(bottom) of Building 2 at the Forest Products 

Laboratory. Top photo courtesy Montana Standard; www.mtstandard.com.
43

King, M.L. 2002. NIDR guidelines for fire and smoke damage repair. Millerville, MD: The National Institute of Disaster Restoration. 153 p.

Knudsen, R.M.; Schniewind, A.P. 1975. Performance of 

structural wood members exposed to fire. Forest Products 

Journal. 25(2):23–32.

Kretschmann, D.E. 2010. Mechanical properties of wood. 

In: Wood handbook, wood as an engineering material. Gen. 

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Kukay, B.M.; Todd, C. 2009. Determining wood’s residual 

flexural properties using nondestructive testing. Wood Design Focus. 19(2): 8–12.

Kukay, B.M.; White, R.H.; Woeste, F. 2011. Fire damage 

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buildings and bridges. Washington, DC: International Code 

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plywood: Evaluation, testing and replacement. Upper Marlboro, MD: National Association of Home Builders National 

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Raftery, G.M.; Harte, A.M.; Rodd, P.D. 2009. Bond quality 

at the FRP wood interface using wood-laminating adhesives. 

International Journal of Adhesion and Adhesives. 29(2): 

101–110. doi:10.1016/j.ijadhadh.2008.01.006

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2:117–132.

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Schaffer, E.L. 1973. Effect of pyrolytic temperatures on the 

longitudinal strength of dry Douglas-fir. ASTM Journal of 

Testing and Evaluation. 1(4):319–329.

Schaffer, E. 1977. State of structural timber fire endurance. 

Wood and Fiber. 9(2):145–170.

Schaffer, E.L. 1982a. 2.3 Temperature of Chapter 2 Factors 

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guide and commentary. New York: American Society of 

Civil Engineers: 69–92.

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maintenance and upgrading of wood structures—A guide 

and commentary. New York: American Society of Civil 

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of wood structures–A guide and commentary. New York: 

American Society of Civil Engineers: 319–338.

Schaffer, E.L.; Marx, C.M.; Bender, D.A.; Woeste, F.E. 

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General Technical Report FPL–GTR–234

44

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45

Chapter 6

Estimation of Design Values for In-Service 

Wood

David Kretschmann

When a new structure is constructed a structural engineer 

relies on current design values for a known species and local building codes to determine the appropriate size and 

grade of timber required for particular service loads. The 

strength of timber found in historic structures can vary 

considerably because of age and condition. Uncertainty can 

also be introduced when estimating strength of members if 

the species and grade are unknown. Many historic wooden 

structures were constructed before standardized grading 

practices for timbers were agreed upon. For evaluation purposes, design value estimates for existing wood members 

must be assigned with as much accuracy as possible. This 

is impossible when the type, grade, and condition of the 

existing wood are uncertain. The current practice for wood 

in service in historical structures is to assign values for 

strength and moduli from existing specifications based on 

assumed species and grade, but often these assumptions are 

inaccurate for the specific conditions and can result in an 

unreliable assessment of the structure. The condition assessment of a structure can be a complicated process and, even 

with the best available information, estimation of allowable 

properties of members can require considerable engineering 

judgment. With accurate information, an engineer or other 

professional can make determinations about maintenance, 

repair, restoration, or decommissioning of the structure. An 

inspecting engineer must take three critical steps to estimate 

design values for wood members: (1) identify the species 

of wood, (2) estimate a size and grade for the member 

(Fig. 6.1), and (3) determine the condition of the wood in 

the member (described in detail in Chapters 1–5). If accurate information is not available, historic structures may be 

prematurely replaced.

The continued use of a structure is dependent on an affirmation that it meets appropriate strength thresholds given 

its intended use. Because this directly affects public safety, 

every step in the process of making a design value estimate must be performed with care, precision, and a robust 

understanding of the factors that affect it. This chapter discusses each step of the process that leads to a design value 

estimate—wood species identification, member grading, 

on-site condition assessment, and calculating an appropriate 

strength—with a high degree of detail. The content will 

also empower the reader to make a judgment about the 

advisability of securing the services of a consulting 

engineer. It is best to engage a skilled and experienced inspector familiar with wood identification, timber grading, 

and condition assessment. The reader will be able to more 

easily communicate with them about the problems and concerns found in the structure.

The scope of this chapter is limited to visually graded structural lumber. It does not address the assignment of design 

values for structural composite lumber products, gluedlaminated timbers, or mechanically graded structural 

lumber.

Significance of Species

The first and probably most critical step to accurately assign 

design values to in-service members is identifying the species of wood in the member. Wood properties for clear wood 

vary considerably from species to species, as illustrated by 

selected examples in Table 6.1 (FPL 2010). Values reported 

in Table 6.1 are average measured properties of small clear 

specimens in a laboratory setting and are not design values. 

Proper identification of wood species is a skill that requires 

a considerable amount of training and experience to become 

proficient at sorting through wood features and characteristics. In most cases it is best if the services of a professional 

are utilized to identify the species of wood. Chapters 1–8 of 

Wiedenhoeft and Kretschmann (2014) detail the features, 

skills, and characters used to make a field identification of 

wooden members likely to be found in wooden bridges in 

the United States; this detailed description is also applicable 

to wooden members in historic structures. Regardless of 

who conducts the identification, following are some useful tips for sampling and prioritization of woods species 

samples. 

Sampling of Members

The best advice that can be given for sampling members is 

“have a plan.” With photos or hand drawings, establish a 

system for identifying, cataloging, and recording property 

estimates for the members. Different species can be used for 

different sections of the structure; therefore, the ability to 

trace the location to positions on the structure is very important. An inspector must also consider which members should 

be prioritized, which can be accomplished by preliminary 

structural analysis to save time and effort in the field.
General Technical Report FPL–GTR–234

46

Yes, Beams and Stringer

Consult the chart for 

maximum knot sizes 

for given species and 

size member in 

Appendix C

Consult the chart for 

maximum knot sizes 

for given species and 

size member in 

Appendix C

Consult the chart for 

maximum knot sizes 

for given species and 

size member in 

Appendix C

Consult the chart for 

maximum knot sizes 

for given species and 

size member in 

Appendix C

Record estimated 

size of maximum 

center knot on wide 

face of member

Record estimated 

size of maximum 

center knot on narrow 

face of member

Record estimated 

size of maximum 

edge knot on wide 

face of member

Record the 

estimated maximum 

slope of grain in 

member

Look for other 

obvious aws 

(checks, shake, 

splits)

Analyze to

determine if

necessary to

replace member

Estimate the 

size of the knots 

in member

Determine 

size

Determine 

species

Use as 

estimate of 

grade

Record estimated 

size of the largest 

unsound knot 

in member

Dimension Timber

lumber

No, Post and Timber

5 in. and 

thicker, width 

more than 2 in. > 

thickness?

Severe 

checks, shake, 

or splits 

present?

Is member 

dimension lumber 

or timber?

Is grade 

stamp

present?

Knot size 

determination

Slope of grain determination

Other obvious aws

Timber (thickness > 4 in.)

Dimension lumber

(thickness ≤ 4 in.)

Consult the chart for 

maximum knot sizes 

for given species and 

size member in 

Appendix B

Consult the chart for 

maximum knot sizes 

for given species and 

size member in 

Appendix B

Take the lowest overall 

grade estimate for 

each characteristic as 

an estimate of the 

grade of the member

Take the lowest overall 

grade estimate for 

each characteristic as 

an estimate of the 

grade of the member

Yes, Joist and Plank

No, Structural Light Framing

2–5-in. thick,

5 in. and wider?

No

No

No

No

Repair or 

replace member

Is damage 

or decay

present?

Yes

Yes

Yes

Yes

Look for other 

obvious aws 

(checks, shake, 

splits)

Severe 

checks, shake, 

or splits 

present?

Figure 6.1—Steps for member grading.
47

Wood and Timber Condition Assessment Manual • Chapter 6

Table 6.1—Strength properties of some commercially important woods grown in the United States (metric)a 

(These values are average measured properties of small clear specimens in a laboratory setting and are not 

design values)

Common 

species 

names

Moisture 

content

Specific

gravityb

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/m3)

Hardwoods

Ash, 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, 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

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, 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

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

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

Elm, 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 — —

Hickory, 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

Maple, Red Green 0.49 53,000 9,600 79 810 2,260 2,800 7,900 — 3,100

12% 0.54 92,000 11,300 86 810 45,100 6,900 12,800 — 4,200

Maple, Sugar Green 0.56 65,000 10,700 92 1020 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, 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

Oak, 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

Oak, 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

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

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
General Technical Report FPL–GTR–234

48

Table 6.1—Strength properties of some commercially important woods grown in the United States (metric)a—con.

(These values are average measured properties of small clear specimens in a laboratory setting and are not 

design values)

Common 

species 

names

Moisture 

content

Specific

gravityb

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/m3)

Softwoods

Baldcypress Green 0.42 46,000 8,100 46 640 24,700 2,800 6,500 2,100 1,700

12% 0.46 73,000 9,900 57 610 43,900 5,000 6,900 1,900 2,300

Cedar, Western 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

Douglas-fir, 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

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

Fir, 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

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

Hemlock, 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

Pine, 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

Pine, 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

Pine, 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

Pine, 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

Pine, 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

Pine, 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 4,800 3,200 2,500

Spruce, 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

Spruce, Sitka Green 0.37 39,000 5,800 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

aResults of tests on clear specimens in the green and air-dried conditions. These values are average measured properties of small clear specimens 

in a laboratory setting and are not design values. 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 stregth; and side hardness is hardness measured when load is perpendicular to grain.

bSpecific gravity is based on weight when ovendry and volume when green or at 12% moisture content.

cModulus 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%.
49

What to Prioritize in Sampling for Wood 

Identification

As is often the case with inspections, limited time and resources are available. The inspector should prioritize the 

members that are most critical to structural performance. 

The species of wood in these members should be the first to 

be identified.

How to Collect Samples for Wood 

Identification

Samples can be obtained by coring a member, drilling with 

a hole saw, or collecting broken splinters. Each sample 

should be gathered in individual containers and labeled so 

that samples can be traced back to the members they came 

from. A large enough sample must be taken so that sample 

preparation techniques can be safely applied, but it should 

not be taken in a way that affects structural integrity of the 

member. 

How Many Samples to Take

It may be likely that the same species would be used for 

most members. However, a sufficient number of samples 

should be taken to ensure that species involved in the most 

critical structural members are thoroughly represented. A 

minimum of 20% of those members should be sampled for 

identification. Approximately 10% of the other less 

critical structural members should also be sampled for 

identification. 

Size of Member

Before an estimated grade can be assigned to a particular 

member, the size of that member must be accurately estimated. If you are inspecting a structure that was built or refurbished since 1970, you will be able to directly use much 

of the information tabulated in the current NIST Product 

Standard PS 20 related to size (Table 6.2, DOC [current edition], Smith and Wood 1964). Wood members are divided 

into three categories: boards, dimension lumber, and timbers. Structural members are classified as either dimension 

lumber or timbers. Most historic structures, however, were 

built before current methods assigning design values to 

structural wood members were standardized. Sizes of members likely will not be the standard sizes you are accustomed 

to today.

A piece of “dimension lumber” is any structural lumber that 

has a nominal thickness of 2 to 4 in. (actual thickness of 38 

to 114 mm (1.5 to 4.5 in.)). A piece of “timber” is a structural member that has a nominal thickness greater than 5 in. 

(actual thickness greater than 114 mm (4.5 in.)). Timbers 

are further subdivided, based on use, into subcategories of 

“Beam and Stringer” (timbers used as bending members) 

and “Post and Timber” (timbers used more as compression 

and tension members). The size of a member directly influences the capacity of the member and will help determine 

which design values for “dimension lumber or timbers” 

should be associated with it. 

All three dimensions of a lumber member need to be determined (Fig. 6.2). The thickness of a member is the measured 

dimension of the narrower face of a rectangular piece of 

lumber. The width of the member is the measured dimension of the larger face of a rectangular piece of lumber. The 

length of the member is the measured dimension of the 

largest dimension of the member. Measurement is best accomplished using digital calipers and measuring tapes on 

exposed cross sections of members. Some more creative 

means are often required to determine sizes of less accessible members. 

Grade of Member 

An estimate of grade will be required for an inspector to 

initially estimate design values for a member. Unfortunately, 

estimating grade for an inspector is an imprecise business 

that requires a great deal of judgment. The information in 

this section is meant to provide a very basic overview of 

grading and allow crude judgments on the grade of members. Becoming proficient in visual grading requires many 

hours of experience. Some basic understanding of the grading process will allow an inspector to judge whether a more 

thorough grading of members is advised. Experienced certified graders should be hired to provide the truest estimate of 

grade for members. 

Purpose of Grading

To more efficiently and economically use wood from logs, 

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. With new 

material, a grade stamp on a piece of lumber tells architects, 

engineers, builders, and building officials the quality of a 

piece of lumber. A typical grade stamp for dimension lumber is shown in Figure 6.3. A grade stamp provides information on the supervising grading agency (WWPA 2005), the 

wood species or species combination (Douglas-fir–Larch), 

the mill number or brand of the firm that produced the board 

(12), the grade requirements the piece meets (No. 1 and better), and the target moisture content to which the wood was 

dried or the moisture content at which it was surfaced (surfaced-green). If a grade stamp is present, the inspector’s job 

is immediately made easier. A certified grader has already 

judged the grade of the member based on the most severe 

defects present. If the inspector determines that the member 

is still undamaged and undecayed, this grade can be used to 

determine the design values. If not, additional judgments on 

the quality of the member must be made.

Brief History of Visual Grading

For many years, lumber has demonstrated the versatility of 

wood by serving as a primary raw material for construction 

and manufacture in the United States. In this role, lumber 

has been produced in a wide variety of products from many 

different species. The first industry-sponsored grading rules 

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

50

(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 across the United 

States were developed and lumber was transported to distant points, each producing region continued to establish its 

own grading rules. 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 markets, confusion and dissatisfaction 

were inevitable. 

Research conducted on wood properties in the early 1900s 

had two distinct camps: full-size testing and small clear 

wood testing. The full-size testing group felt that testing 

Table 6.2—Specifications for dimension of structural members (DOC PS 20)

Thicknesses Widths

Nominal

in.

Minimum dressed

Nominal 

in.

Minimum dressed

Dry Green Dry Green

mm in. mm in. mm in. mm in.

Board

2 38 1-1/2 40 1-9/16

3 64 2-1/2 65 2-9/16

4 89 3-1/2 90 3-9/16

3/8 8 5/16 9 11/32 5 114 4-1/2 117 4-5/8

1/2 11 7/16 12 15/32 6 140 5-1/2 143 5-5/8

5/8 14 9/16 15 19/32 7 165 6-1/2 168 6-5/8

3/4 16 5/8 17 11/16 8 184 7-1/4 190 7-1/2

1 19 3/4 20 25/32 9 210 8-1/4 216 8-1/2

1-1/4 25 1 26 1-1/32 10 235 9-1/4 241 9-1/2

1-1/2 32 1-1/4 33 1-9/32 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

2-1/2 51 2 52 2-1/16

3 64 2-1/2 65 2-9/16

2 38 1-1/2 40 1-9/16 3-1/2 76 3 78 3-1/16

2-1/2 51 2 52 2-1/16 4 89 3-1/2 90 3-9/16

3 64 2-1/2 65 2-9/16 4-1/2 102 4 103 4-1/16

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 13 off 1/2 off 13 off 1/2 off 5 & 6 wide 13 off 1/2 off 13 off 1/2 off

7–15 thick 19 off 3/4 off 13 off 1/2 off 7–15 wide 19 off 3/4 off 13 off 1/2 off

≥ 16 thick 25 off 1 off 13 off 1/2 off ≥ 16 wide 25 off 1 off 13 off 1/2 off
51

programs and any subsequent grading standards should be 

done on full-size members available to consumers. This 

group argued that this approach would reduce the waste 

involved in overbuilt structures and ensure minimum standards for integrity and safety. The small clear wood group, 

primarily foresters, felt that timber tests should focus on the 

qualities of trees rather than the potential design uses for 

lumber. Their group argued that tests of multiple small samples of clear wood should be conducted to provide strength 

averages without incurring the expenses and waste that fullsize testing would generate (Green and Evans 2001). This 

debate continues to this day. 

As grading rules began to develop, a number of conferences 

were sponsored by the U.S. Department of Commerce from 

1919 to 1925 to minimize unnecessary differences in grading rules and to improve and simplify these rules. These 

conferences were attended by representatives of lumber 

manufacturers, distributors, wholesalers, retailers, engineers, 

architects, and contractors. The result of these conferences 

was a relative standardization of sizes, definitions, and 

procedures for deriving allowable design properties and a 

voluntary American Lumber Standard. Two Circulars, Circular 295, “Basic Grading Rules and Working Stresses for 

Structural Timbers,” and Circular 296, “Standard Grading 

Specifications for Yard Lumber,” involving allowable design 

values published by USDA Forest Products Laboratory in 

1923, served as the basis for the grading rules we see in the 

United States today (Newlin and Johnson 1923, Ivory et al. 

1923). In the years that followed initial acceptance of the 

first grading rules, these rules have been modified as more 

information was gathered on the influence of knots, slope of 

grain, growth characteristics, and sawing practices. 

An orderly, voluntary, but circuitous system of responsibilities has evolved in the United States for the development, 

manufacture, and merchandizing of most stress-graded lumber (Fig. 6.4). 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), and 

then applied to product classes. 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. Virtually all commercial 

softwood and hardwood lumber used for structural purposes 

that is manufactured in the United States is stress graded 

under American Lumber Standard practice and is called 

American Lumber Standard program lumber. The American 

Lumber Standard has been modified several times, including the addition of hardwood species to the standard beginning in 1970. The current edition is the American Softwood 

Lumber Standard PS 20–10. Distinctive grade marks for 

each species or species grouping are provided by accredited 

agencies. The principles of stress grading are also applied to 

Wood and Timber Condition Assessment Manual • Chapter 6

Width

Thickness Length

Figure 6.2—Definition of length, width, and thickness in a 

lumber member.

Figure 6.3—Grade stamp from 

WWPA No. 1 & Better dimension 

lumber. Moisture content is surfaced 

green (unseasoned) condition, over 

19% (WWPA 2005).

Formulation of stress grading principles

University staff members

Government researchers

Industry R&D staff

Consumer representatives

Product Standard

American Lumber Standard Committee and its

Board of Review

National Grading Rule Committee

National Institute of Standards and Technology

Formulation and publication of stress-grading rules

Rules-writing agencies

Grading agency accreditation

American Lumber Standard Committee

Grade mark supervision and reinspection

Rules-writing agencies

Independent inspection agencies

Manufacture and marketing of American 

standard stress-graded lumber

Sawmills

Review and adoption

Figure 6.4—Voluntary system of responsibilities for 

stress grading under the American Softwood Lumber 

Standard.
General Technical Report FPL–GTR–234

52

several hardwood species under provisions of the American 

Softwood Lumber Standard, ASTM International, and then 

applied to product classes. The allowable design properties are tabulated in the Supplement to the National Design 

Specification (AWC 2012). 

Responsibilities and Standards for Stress 

Grading

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 (ALSC) 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 re-inspection services to individual lumber manufacturers. Accredited rules-writing and independent agencies are 

listed below. These agencies are in the business of grading 

and have a wealth of experience of determining grades of 

lumber. The continued accreditation of these organizations 

is under the scrutiny of the ALSC Board of Review. For the 

most accurate assessment of grade of a member an accredited grader from one of these organizations should be used. 

(For updated information, contact American Lumber Standard Committee, P.O. Box 210, Germantown, MD 20875; 

alsc@alsc.org; www.alsc.org.)

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.

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

Tables 6.3 to 6.5 provide example grading rules for various grades at the time of this publication. The most current 

rules for visually grading lumber and timber members can 

be obtained from various rules-writing agencies that are 

responsible for supervising lumber mill grading operations. 

The most up-to-date set of rules can be obtained from the 

rules-writing agencies. These rules can provide an inspector with a clear guide as to what is acceptable under current 

visual grading rules.

Current Visual Grading System

The grading rules in use today, like those discussed in 1923, 

are based on the premise that mechanical properties of lumber and timbers differ from mechanical properties of clear 

wood because many growth characteristics affect properties 

and these characteristics can be seen and judged visually. 

For dimension lumber (lumber less than 90 mm (nominal 

4 in.)), a single set of grade names and descriptions is used 

throughout the United States, although the design values 

vary with species. The current National Grading Rule restrictions for dimension lumber are given in Table 6.3. 

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. Structural timbers of Southern Pine are graded without regard to anticipated use (Table 6.4). For most 

species, however, timber grades are classified according to 

intended use (Table 6.5).

Influence of Visual Grading Characteristics on 

Grade

Visual grading characteristics influence wood properties 

and performance, and as such are used as sorting criteria. 

Such characteristics include knots, slope of grain, checks 

and splits, shake, density, decay, heartwood and sapwood, 

pitch pockets, and wane. To make the most exact estimate 

of a structural member’s grade, all these characteristics 

should be considered, but the grade of the member will be 

determined by the most severe of these characteristics. Not 

all these member characteristics will be easy or possible 

to measure in place, but their influence on properties is 

discussed for completeness. The most critical of these characteristics for inspections are highlighted in a later section 

(“What to prioritize in field estimation grading”). With some 

experience, you will be able identify which of the features 

explained below represent the most significant or severe defect in the member and only measure that feature. 

Knots

Knots are branches or portions of branches embedded in a 

piece of wood and cause localized cross grain with steep 

slopes within the timber. 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. The location of a 

knot influences its effect on strength. Centerline knots on the 
53

Wood and Timber Condition Assessment Manual • Chapter 6

Characteristics Select Structural No. 1 No. 2 No. 3

Compression wood Not allowed in damaging form for the grade considered

Slope of grain 1 in 12 1 in 10 1 in 8 1 in 4

Decay Not permitted Not permitted Heart center, 

1/3 thickness × 1/3 width

Heart center, 1/3 cross section. Must 

not destroy nailing edge. 

See para. 710(e).

Holes Same as unsound knots Same as unsound knots See chart below See chart below

Knots Edge 

(in.)

Centerline (in.)

Unsound 

knots (in.)

Edge (in.) Centerline (in.)Unsound 

knots (in.)

Edge

(in.)

Centerline (in.) Holes (in.) Edge

(in.)

Centerline (in.) Holes (in.)

2×4 3/4 7/8 3/4 1 1-1/2 1 1-1/4 2 1-1/4 1-3/4 2-1/2 1-3/4

2×5 1 1-1/2 7/8 1-1/4 1-7/8 1-1/8 1-5/8 2-3/8 1-3/8 2-1/4 3 1-7/8

2×6 1-1/8 1-7/8 1 1-1/2 2-1/4 1-1/4 1-7/8 2-7/8 1-1/2 2-3/4 3-3/4 2

2×8 1-1/2 2-1/4 1-1/4 2 2-3/4 1-1/2 2-1/2 3-1/2 2 3-1/2 4-1/2 2-1/2

2×10 1-7/8 2-5/8 1-1/4 2-1/2 3-1/4 1-1/2 3-1/4 4-1/4 2-1/2 4-1/2 5-1/2 3

2×12 2-1/4 3 1-1/4 3 3-3/4 1-1/2 3-3/4 4-3/4 3 5-1/2 6-1/2 3-1/2

Sound, firm, encased, pith, tight 

and well spaced. 

One hole or equivalent 

smaller holes per 4 lin. ft.

Sound, firm, encased, pith, 

tight and well spaced. 

One hole or equivalent 

smaller holes per 3 lin. ft.

Well spaced knots of any 

quality. One hole or equivalent 

smaller holes per 2 lin. ft.

Well spaced knots of any quality. 

One hole or equivalent 

smaller holes per 1 lin. ft.

Shakes

Ends: Same as splits.

Elsewhere: 2 ft surface; 

none through.

Ends: Same as splits.

Elsewhere: Surface 3 ft or 

1/4 length; 2 ft through.

1/6 length if through at edges 

or ends; elsewhere through 

shakes 1/3 length.

Checks Surface seasoning checks not limited.

Through checks at ends limited as splits.

Skips

Hit and miss in 

10% of the pieces.

See para. 720(f).

Hit and miss. 5% of pieces 

may be hit or miss or 

heavy skip for 2 ft. 

See para. 720(e,f,g).

Hit or miss. 10% of pieces 

may have heavy skip. 

See para. 720(e,g).

Splits Equal to the width Equal to 1-1/2 times 

the width Equal to 1/6 length

Wane

1/4 thickness × 1/4 width × full length or equivalent; 

must not exceed 1/2 thickness × 1/3 width 

for up to 1/4 length. 

Also see para. 750.

1/3 thickness × 1/3 width × full 

length or equivalent; 

must not exceed 2/3 thickness × 

1/2 width for up to 1/4 length. 

Also see para. 750.

1/2 thickness × 1/2 width × full 

length or equivalent; 

must not exceed 7/8 thickness or 3/4 

width for up to 1/4 length. 

Also see para. 750.

Bow 10 ft/1-3/8 in.; 12 ft/1-1/2 in.; 

14 ft/2 in.; 16 ft/2-1/2 in.

10 ft/1-1/2 in.; 12 ft/2 in.; 

14 ft/2-1/2 in.; 16 ft/3-1/4 in.

10 ft/2-3/4 in.; 12 ft/3 in.; 

14 ft/4 in.; 16 ft/5 in.

Crook 10 ft 12 ft 14 ft 16 ft 10 ft 12 ft 14 ft 16 ft 10 ft 12 ft 14 ft 16 ft

2×4 3/8 in. 1/2 in. 5/8 in. 3/4 in. 1/2 in. 11/16 in. 7/8 in. 1 in. 3/4 in. 1 in. 1-1/4 in. 1-1/2 in.

2×6 5/16 in. 7/16 in. 9/16 in. 11/16 in. 7/16 in. 5/8 in. 3/4 in. 7/8 in. 5/8 in. 7/8 in. 1-1/8 in. 1-3/8 in.

2×8 1/4 in. 13/32 in. 1/2 in. 9/16 in. 3/8 in. 1/2 in. 5/8 in. 3/4 in. 1/2 in. 13/16 in. 1 in. 1-1/8 in.

2×10 7/32 in. 3/8 in. 7/16 in. 1/2 in. 1/4 in. 7/16 in. 1/2 in. 5/8 in. 7/16 in. 3/4 in. 7/8 in. 1 in.

2×12 3/16 in. 9/32 in. 3/8 in. 7/16 in. 3/16 in. 3/8 in. 3/8 in. 1/2 in. 3/8 in. 9/16 in. 3/4 in. 7/8 in.

Dense grain: Requires 6 rings/in. and 1/3 summerwood or 4 rings/in. and 1/2 summerwood.

Exceptionally light weight pieces: Should not be placed in No. 2 and higher grades (exceptionally light weight pieces have less than 15% summerwood).

Table 6.3—National Grading Rule Specifications for Dimension Lumber (Standard Grading Rules for Southern Pine 

Lumber 2002; used courtesy of Southern Pine Inspection Bureau)
General Technical Report FPL–GTR–234

54

Table 6.4—Grading Rule Specifications for Southern Pine Timber (Standard Grading Rules for Southern Pine Lumber 

2002; used courtesy of Southern Pine Inspection Bureau)

Grading rules for timbers

Characteristics Select Structural No. 1 No. 2

Compression wood Not allowed in damaging form for the grade considered

Slope of grain 1 in 14 1 in 11 1 in 6

Decay In knots only In knots only Heart-center decay or unsound 

red heart and equiv. streaks 

limited to 10% cross section 

if wholly enclosed within four 

surfaces of each piece and 5% 

otherwise

Holes Medium – well scattered Medium – well scattered Limited to 1-1/2 in. in diameter

Knots

(nominal 

width 

of face)

(in.)

Narrow 

face and 

at edge of 

wide face 

(in.) 

(2)

Centerline 

wide 

face

(in.)

Unsound 

knots 

(in.)

(1)

Narrow 

face and at 

edge of 

wide face 

(in.) 

(2)

Centerline 

wide 

face

(in.)

Unsound 

knots (in.) 

(1)

Narrow face 

and at edge 

of wide face; 

centerline 

wide face (in.)

Unsound 

knots

(in.)

(1)

5 1-3/8 1 1-3/4 1-3/8 2-1/2 2

6 1-5/8 1-5/8 1-1/4 2-1/8 2-1/8 1-5/8 3 2-1/4

8 1-7/8 2-1/4 1-1/2 2-1/2 2-3/4 2 4-1/2 2-3/4

10 2-1/8 2-3/4 2 2-3/4 3-1/2 2-1/2 5-1/2 3

12 2-3/8 3-1/4 2-1/8 3-1/8 4-1/4 2-7/8 6-1/2 3-1/2

14 2-1/2 3-5/8 2-1/4 3-3/8 4-3/4 3-1/8 7-1/2 3-3/4

16 2-3/4 3-7/8 2-1/2 3-1/2 5 3-3/8 8 4

18 2-7/8 4-1/8 2-1/2 3-1/2 5-1/4 3-1/2 8-1/2 4

20 3 4-3/8 3 3-1/2 5-1/2 3-1/2 9 4

Sound, firm, encased, and pith knots Sound, firm, encased, and pith knots Sound, firm, encased, and pith 

knots

(1) In unsound knots as allowed, decay must be confined to the knot itself 

and not be in surrounding wood and not penetrate deeper than 1-1/2 in.

(2) In timbers of equal face, knots are permitted throughout as specified for 

narrow faces regardless of location.

(1) In unsound knots as allowed, 

decay must not penetrate deeper 

than 2 in.

Shakes, checks, 

splits

Splits not longer than thickness of 

piece; shakes and surface checks not 

deeper than 1/3 thickness if not dry 

and 3/8 thickness if dry

Splits not longer than thickness of 

piece; shakes and surface checks not 

deeper than 1/3 thickness if not dry 

and 3/8 thickness if dry

Splits not longer than 1-1/4 times 

thickness of piece; shakes and 

surface checks not deeper than 

1/2 thickness

Skips Hit and Miss in 10% of pieces Hit or Miss dressing Hit or Miss dressing except 

occassional scant width and 

thickness from full length skip 

limited to 1/8 in., must be No. 1 

otherwise throughout any 

portion scant over 1/16 in.

Stain Medium if dry; not limited if 

ordered green

Medium if dry; not limited if ordered 

green

Medium if dry; not limited if 

ordered green

Wane 1/8 the width of face and 1/4 length 1/6 the width of face and 1/3 length 1/4 face on one edge and 

1/3 face on both edges

Warp Very light Very light Light
55

Wood and Timber Condition Assessment Manual • Chapter 6

Table 6.5—Grading Rules Specifications for NeLMA (WWPA) Timbers 

Beams and stringers

Characteristics Select Structural No. 1 No. 2

Slope of grain 1 in 14 1 in 10 1 in 6

Decay None None Small spots of unsound 

wood well scattered, 

1/6 the face width

Knots 

(nominal 

width 

of face) 

(in.)

Edge 

wide 

face 

(in.)

Centerline 

wide 

face 

(in.)

Unsound 

knots

Edge 

wide 

face 

(in.)

Centerline 

wide 

face 

(in.)

Unsound 

knots

Edge of wide 

face and 

centerline 

of wide face 

(in.)

Unsound 

knots

8 1-7/8 2 2-5/8 3 4-1/2

10 2 2-5/8 2-7/8 3-3/4 5-5/8

12 2-1/8 3-1/8 3-1/4 4-1/2 6-7/8

14 2-3/8 3-3/8 3-1/2 5 7-1/2

16 2-1/2 3-5/8 3-3/4 5-1/4 8-1/8

18 2-3/4 3-5/8 3-7/8 5-5/8 8-5/8

20 2-7/8 3-7/8 4-1/8 5-7/8 9-1/8

22 3 4 4-3/8 6-1/4 9-1/2

24 3-1/8 4-1/4 4-1/2 6-1/2 10

Sound, tight and well-spaced Sound, tight and well-spaced Sound, not firmly fixed or 

holes, well-spaced

Shakes 1/6 the thickness on end 1/6 the thickness on end 1/2 length, 1/2 thickness. 

If through at ends, limited 

as splits.

Splits Splits equal in length to 1/2 the width of 

the piece or equivalent of end checks

Splits equal in length to width of the 

piece or equivalent of end checks

Medium or equivalent 

end checks

Checks Seasoning checks, single or opposite 

each other with a sum total equal to 

1/4 the thickness of the piece

Seasoning checks, single or opposite 

each other with a sum total equal to 

1/2 the thickness of the piece

Seasoning checks

Skips Occasional skips 1/16 in. deep, 

2 ft in length

Occasional skips 1/8 in. deep, 

2 ft in length

1/8 in. deep, 2 ft in length, or 

1/16 in. skip full length

Stain Stained sapwood. Firm heart stain, 

10% of width or equivalent.

Stained sapwood. Firm stained 

heartwood.

Stained wood

Wane 1/8 of any face, or equivalent slightly 

more for a short distance

1/4 of any face or equivalent slightly 

more for a short distance

1/3 of any face, or equivalent 

slightly more 

for a short distance
General Technical Report FPL–GTR–234

56

Table 6.5—Grading Rules Specifications for NeLMA (WWPA) Timbers—con. 

Posts and timbers

Characteristics Select Structural No. 1 No. 2

Slope of grain 1 in 12 1 in 10 1 in 6

Decay None None Small spots of unsound 

wood well scattered, 

1/6 the face width

Knots 

(nominal width 

of face) 

(in.)

Anywhere 

on 

wide face 

(in.)

Unsound 

knots 

(in.)

Anywhere 

on 

wide face 

(in.)

Unsound 

knots 

(in.)

Anywhere 

on 

wide face 

(in.)

Unsound 

knots 

(in.)

5 1 1-1/2 2-1/2 1-1/4

6 1-1/4 1-7/8 3 1-1/2

8 1-5/8 2-1/2 3-3/4 1-7/8

10 2 3-1/8 5 2-1/2

12 2-3/8 3-3/4 6 3

14 2-1/2 4 6-1/2 3-1/4

16 2-3/4 4-1/4 7 3-1/2

18 3 4-1/2 7-1/2 3-3/4

Sound, tight and well-spaced Sound, tight and well-spaced Sound, not firmly fixed or holes, 

well-spaced

Shakes 1/3 the thickness on end 1/3 the thickness on end 1/2 length, 1/2 thickness. 

If through at ends, limited as splits.

Splits Splits equal in length to 3/4 

the thickness of the piece or 

equivalent of end checks

Splits equal in length to width 

of the piece or equivalent of 

end checks

Medium or equivalent end checks

Checks Seasoning checks, single or 

opposite each other with a 

sum total equal to 1/2 the 

thickness of the piece

Seasoning checks, single or 

opposite each other with a 

sum total equal to 1/2 the 

thickness of the piece

Seasoning checks

Skips Occasional skips 1/16 in. 

deep, 2 ft in length

Occasional skips 1/8 in. deep, 

2 ft in length

1/8 in. deep, 2 ft in length, 

or 1/16 in. skip full length

Stain Stained sapwood. 

Firm heart stain, 10% of 

width or equivalent.

Stained sapwood. 

Firm stained heartwood.

Stained wood

Wane 1/8 of any face, or equivalent 

slightly more for a short 

distance

1/4 of any face, or equivalent 

slightly more for a short 

distance

1/3 of any face, or equivalent 

slightly more for a short distance
Face side Reverse

Measure two sides 

and average

1-1/2-in. narrow 

face knot 

extending 

1/2 of width

1-in. narrow 

face knot 

extending 

3/4 of width

A B

Knot size on Knot may extend

Grade narrow face (in.) into piece

No. 1 1-1/2 1/2 of width (A)

 1 3/4 of width (B)

No. 2 1-1/2 Slightly less than

 3/4 of width

No. 3 1-1/2 Slightly less

 than width

Figure 6.5—Diagram from a pocket guide showing measurement of knots on 2- to 4-in. lumber 

that included structural joists and planks (SPIB 

2002). Used courtesy of Southern Pine Inspection Bureau.

Figure 6.6—Example of using grid to establish the size of 

knot in bridge member. The measured knot in the example 

is 1-1/2 in.

57

wide face have the least effect on strength grade; edge knots 

on the wide or narrow face have the most effect on strength 

and grade. 

In general, knots have 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. Small 

knots along the centerline have little or no effect, but a very 

large knot along the centerline can have a significant influence on strength. 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 presence of a knot has a greater effect on most strength 

properties than on stiffness. 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.

It is important to know the size and location of knots 

in wood members of a structure. The effect of knots 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 member. 

Grading criteria thus place limits on knot sizes in relation 

to the width of the face and location on the face in which 

the knot appears (Fig. 6.5), and the influence of the worst or 

most severe knot determines whether the knot is the limiting factor for determining the grade estimate. 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 mid-height. These facts are recognized in 

some grades by different limitations on the sizes of knots in 

different locations.

Knot sizes are likely to be difficult to assess on many members in structures. A scale is often used for quick estimates 

of knot size. Anthony et al. (2009) discuss another useful 

technique (a grid on a clear film) for estimating knots in dimension lumber. When properly aligned, the grid allows you 

to make a good estimate of knot size to the closest 1/4 in. 

(Fig. 6.6). Accurately capturing maximum knot size for the 

centerline and edge of the member is highly important; these 

are usually the controlling features for establishing grade.

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

58

Slope of Grain 

Slope of grain is a ratio expressing the amount the grain 

slopes up or down within a set distance along the long axis 

of the member (Fig. 6.7). A nonzero slope of grain (cross 

grain) reduces mechanical properties of lumber. 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, straight-grained 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. 

Drying checks in timber generally follow the slope of grain 

and can be used to determine the slope of grain on any visual surface, including painted members. Figure 6.8 shows 

an example of how checks indicate the slope of grain of a 

member. Other times the grain will be obvious on the unfinished surfaces of a member.

To measure slope of grain, the clear sheet with a printed 1/2-

in. grid can once again be put to use (Fig. 6.9). In this example, the area that appears to have significant slope of grain 

should be measured from an axis parallel to the long axis 

of the member. The grid can be used to establish the 

rise-over-run ratio. To do so, the total number of inches for 

the rise can be determined along the vertical axis by counting from the lowest point of the rise to the highest point 

of the rise (or wherever the grain crosses the edge of the 

acetate sheet). The total number of inches in the run can 

be determined along the horizontal axis by counting across 

from the lowest point of the rise to the highest point of the 

rise (or wherever the grain crosses the edge of the acetate 

sheet). These measurements can then be used to represent 

the actual slope of grain over a given length to determine the 

appropriate grade. As with knots, the most severe slope of 

grain in a member should be identified and, depending on its 

severity, may be the limiting factor for grade.

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 (Gonzalez and Morrell 2012). 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. If evidence of extensive decay is present, 

for example, if most of the section is deteriorated, replacing 

the member is the best option.

In a field inspection, decay can be detected by a close examination of texture and color of the wood. Decay can be 

indicated by color and/or texture changes such as brown, 

cubical, friable wood, or by soft, white, brittle wood. Decay 

would also clearly be suggested by the presence of mushrooms or other fruiting bodies. Also, the absence of wood 

can be suggestive of decay. Decay is often more severe 

where moisture collects or is trapped, such as where two 

pieces of wood overlap, or where fasteners penetrate the 

wood. A probe can be used to determine if wood structure is 

weakened (Chapter 2). 

Checks, Splits, and Shake

Checks and splits have little influence on Post and Timber 

members but can affect the behavior of Beam and Stringer 

members. Shake in a Beam and Stringer is measured between lines enclosing the shake and parallel to the wide 

face.

Average line of the direction of bers

1 x

Figure 6.7—Definition of slope of grain is the ratio 

of rise to run (1:x).

Figure 6.8—Example of using checking in a member to 

measuring the slope of grain in bridge. The end of the scale 

points to the start of one check.

Figure 6.9—Use of grid to easily estimate the slope of grain. 

The 2:18 grids above implies 1 in 9 slope of grain. The black 

line shows the direction of slope of grain. 
59

Checks are separations on the surface of the wood that normally occur as a result of drying wood. Splits are a separation of the wood through the piece to the opposite surface or 

to an adjoining face. 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 

(below).

Shake (or “ring-shake”) is the separation of annual rings 

caused by weak or absent bonds between them and is presumed to extend lengthwise within a member 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 (Beam 

and Stringer) where shear stresses are highest. In members 

with limited cross-grain, which are subjected only to tension or compression (Post and Timber), shake does not affect strength greatly. Shake may also be limited in a grade 

because of appearance and because it permits entrance of 

moisture, which can result in decay.

How checks, splits, and shake are measured depends on the 

intended use of the member. For all grades of members intended to be used as Beam and Stringer, checks, splits, and 

shake are measured in the middle half of the width. Restrictions on checks apply for a distance from the ends equal to 

three times the width of the wide face. Shake is measured 

from the end grain of the member between lines enclosing 

the shake and parallel to the wide face (Fig. 6.10). Checks 

are measured as an average of the penetration perpendicular 

to the wide face (Fig. 6.11). Where two or more checks appear on the same face, only the deepest one is measured. 

Where two checks are directly opposite each other, the sum 

of their deepest penetration is considered. 

Splits are measured as the penetration of a split from the end 

of the piece and parallel to the edges of the piece (Fig. 6.12).

In Post and Timber, shake is measured at the ends of pieces, 

between lines parallel with the two faces that give the least 

dimension (Fig. 6.13). If the member is below 16% moisture 

content, the size of the shake may be 1.5 times the size permitted in the grade.

Density, Rate of Growth, and Percentage of 

Late-Wood

The strength and stiffness of clear wood are related to its 

density, with denser wood generally being stronger and 

stiffer than less dense wood. In some species, based on the 

structure of the wood, rate of growth (rings per inch) and 

percentage of latewood can give an indication of the density 

of the wood. Strength is related to mass per unit volume 

Wood and Timber Condition Assessment Manual • Chapter 6

3W

Measure checks

and shakes

Measure shakes, 

checks, and splits

Measure shake

W/2

W

3W

Figure 6.10—Beams and stringer checks, shake, and split 

measurement.

Measure average 

penetration

Figure 6.11—Checks in post and 

timber.

Measure 

penetration 

from the end

Figure 6.12—Split measurement.

Whichever 

is the least 

dimension

Figure 6.13—Shake in post 

and timber.
General Technical Report FPL–GTR–234

60

(density) of clear wood. Properties assigned to lumber are 

sometimes modified by using rate of growth and percentage 

of latewood as measures of density. Typically, selection for 

density requires that rings per unit length and percentage 

of latewood be within a specified range. Some very low 

strength pieces can be eliminated from a grade by excluding 

those that are exceptionally low in density.

Heartwood and Sapwood

Heartwood or sapwood is often not taken into account in 

structural grading because heartwood and sapwood are assumed erroneously to have essentially equal mechanical 

properties. However, heartwood is sometimes specified in 

a visual grade because heartwood of some species is more 

resistant to decay than is sapwood. If heartwood of a particular species is decay resistant, heartwood may be specified 

for replacement members if the wood is to be untreated and 

will be exposed to a decay hazard. On the other hand, sapwood takes preservative treatment more readily than does 

heartwood and 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, and extremely 

large pitch pockets may function as areas of ring shake and 

thus can be treated as such for grading purposes. 

Wane

Wane refers to bark or lack of wood on the edge or corner of 

a piece of lumber. 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 those bases rather than for its 

influence on timber strength.

What to Prioritize in Field 

Estimation Grading

When inspecting a structure, not all properties discussed 

above will be easily measured. Fortunately, the most significant strength-reducing factors in structural members of 

wooden structures (size and location of the largest knots, 

slope of grain, and presence of decay) are some of the most 

easily field-assessable characteristics. The most time and 

greatest care should be taken to measure these properties 

on the critical members. Next in importance would be determining rings per inch, if possible, to give an indication 

of relative density of the member. If these properties are 

available, an approximation of grade can be established and 

thereby a reasonable first estimate of design values for the 

member should be attainable.

Condition Assessment

The principal reason for most inspections of a wooden structure is to determine the condition of the wood components 

so that a judgment can be made on the capacity of the structure. Helpful tips and methods for conducting condition assessments of wood structures have been described in detail 

in Chapters 1 to 5. These chapters emphasize where problems are most likely to occur in wooden structures. Missing 

or failed components, wood in ground contact, wood with 

moisture stains, wood with the presence of fungal fruiting 

bodies, decayed wood at material interfaces, wood with insect bore holes and mud tubes, sill beams and plates in contact with masonry, burned or charred wood, and corroded 

or damaged fasteners or connections are all areas that are 

referred to as needing close inspection. If extensive decay or 

evidence of large-scale insect attack are present, the member 

is most likely compromised and should be replaced. Decay 

and insect attack can result in severe reduction in strength of 

the members (Gonzalez and Morrell 2012). Also, structurally damaged members (members that have already failed) 

need to be replaced. Choices of members to inspect depend 

on the goal of the inspection. Regardless the condition of the 

member needs to be considered when establishing an estimate of grade.

Examples of Grade Estimation

Nine example members from different structures are tabulated in Table 6.6 to illustrate estimation of grade for a 

member. Entries 1 to 6 look at dimension lumber members; 

entries 7 to 9 look at the grade of timber size members. 

Dimension Lumber Grading Examples 

Six dimension lumber member grade estimation examples 

are presented in Table 6.6. Member 1 was identified as a 

piece of red oak and had a measured dimension of a full 

3 by 8 in., making it a structural joist and plank. No grade 

stamp was present. There was, however, considerable decay present in this member. Even though the knot sizes and 

slope of grain for a 3 by 8 member, based on the criteria in 

Table 6.3, would have allowed for an estimated grade of 

No. 1, the presence of severe decay made this member a 

candidate for replacement. 

Member 2 was identified as Douglas-fir, with measured dimensions of 1.5 by 5.5 in. It was in good shape with a clearly marked No. 1 grade stamp. Because there was no indication of decay or damage, the stamped grade will be used. 

Initial inspection of member 3 indicated that the member 

was northern red oak and in good shape with no damage or 

decay present. Member 3 was a full 2 by 6 in. and can be 

considered a structural joist and plank. No grade stamp was 

present. There was good access to all sides of the member 

for over 70% of its length. Measured slope of grain and knot 

dimensions were compared with allowable knot sizes and 
61

Table 6.6—Examples for estimation of member grade (members are not all from same bridge)

Member 

and 

species

Member 

size 

(in.) Use

Percent 

access 

to the 

member 

and 

sides

Damage 

or decay 

present

Grade 

Stamp 

Largest 

unsound 

knot 

(in.)

Largest 

edge 

knot on 

wide 

face 

(in.)

Largest 

centerline 

knot on 

wide 

face 

(in.)

Largest 

narrow 

face 

knot 

(in.)

Slope of 

grain 

(1:x)

Final 

estimated 

grade/

recommendations

1 Red Oak 3 by 8 Str. 

J&P

60% 

3 of 4

Yes No N/A N/A N/A N/A N/A

Grade estimate Replace

2 Douglas-fir 1.5 by 

5.5

Str. 

J&P

70%

all

No Yes N/A N/A N/A N/A N/A

Grade estimate No. 1 No. 1

3 Northern 

Red Oak

2 by 6 Str. 

J&P

70% 

all

No No 1.5 1.75 2.75 1.5 1:12

Grade estimate No. 2 No. 2 No. 2 N/A Sel. Str. No. 2

4 Cottonwood2 by 7.5 Str. 

J&P

50% 

3 of 4

No No 1.25 2.0 2.75 2.0 1:9

Grade estimate Sel. Str. No. 1 No. 1 N/A No. 2 No. 2

5 Southern 

Pine

2 by 4 Str. 

LF

100% 

all

No No 0.75 1.25 1.25 0.5 1:18

Grade estimate Sel. Str. No. 2 No. 1 N/A Sel. Str. No. 2

6 American 

Chestnut

2 by 6 Str. 

J&P

80% 

all

No No 1.5 2.0 2.5 2.75 1:8

Grade estimate No. 2 No. 1 No. 2 N/A No. 2 No. 2

7 Southern 

Pine

5.5 by 

7.5

P-T 50% 

all 

3 of 4 

rest

No No 1.5 2.25 2.75 2.0 1:12

Grade estimate Sel. Str. No. 1 No. 1 No. 1 No. 1 No. 1

8 Douglas-fir 7.5 by 

9.5

P-T 100% 

all

No No 1 1 1.25 1 1:18

Grade estimate Sel. Str. Sel. Str. Sel. Str. Sel. Str. Sel. Str. Sel. Str.

9 White Ash 5 by 8 B-S 70% 

3 of 4

No No 1.5 3.0 3.5 2.75 1:6

Grade estimate No. 1 No. 1 No. 1 No. 1 No. 2 No. 2

Str. J&P: Structural Joist and Plank (2–4 in. thick, 5 in. and wider). 

Str. LF: Structural Light Framing (2–4 in. thick, 2–4 in. wide). 

P-T: Post or Timber (5 by 5 in. and larger, width not more than 2 in. greater than thickness).

B-S: Beam or Stringer (5 in. and thicker, width more than 2 in. greater than thickness).

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

62

slope of grain for a 2 by 6 shown in Table 6.3. Member 3 

grades out as a Select Structural grade member according 

to its slope of grain, but the knot characteristics are that of a 

No. 2 member. The grade level that a member qualifies for 

always takes the lowest estimated grade characteristic for 

the member. Therefore, the grade for member 3 is estimated 

to be No. 2.

Initial inspection of member 4 identified the species as cottonwood and again indicated that the member was in good 

shape with no damage or decay present. Member 4 had 

measured dimensions that were closest to the dimensions 

of a piece of nominal 2 by 8 dimension lumber and could 

be considered a structural joist and plank. There was very 

good access to three of the four sides for 50% of this member, with limited access to the wide face for the remaining 

portions of the member. Lack of access to the faces of the 

member makes the estimate of grade of this member more 

uncertain. Using the criteria detailed in Table 6.3, the tabulated knot information available suggests the grade is 

No. 1, whereas the slope of grain value only qualifies for 

No. 2. Therefore estimated grade for member 4 is No. 2. 

Initial inspection identified member 5 as Southern Pine with 

no decay present. Member 5 had a measured dimension of a 

full 2 by 4 in. and can be classified as a structural light framing member. No grade stamp was present. There was very 

good access to all sides of the member. The knot dimensions 

and slope of grain were compared against the criteria found 

in Table 6.3. One large edge knot on the wide face forced a 

reduction in the estimated grade for member 5 to No. 2. 

Initial inspection identified member 6 as American chestnut. 

Member 6 had a measured cross-sectional dimension of 2 by 

6 in. There was good access to all sides of the member for 

80% of its length. A probe test revealed no severe decay, and 

no other damage was observed. Applying the criteria in the 

national grading rule in Table 6.3 for dimension lumber, the 

recorded knot sizes and slope of grain suggest an approximate grade of No. 2 for member 6. 

Timber Grading Example

Three timber grading examples are presented in Table 6.6. 

Initial inspection identified member 7 as Southern Pine. 

Member 7 had measured dimensions of 5.5 by 7.5 in. The 

dimension of this member allows it to be treated as a nominal 6 by 8 Post and Timber. Because the member is Southern Pine and Southern Pine grading rules do not distinguish 

between Beam and Stringer or Post and Timber, only one 

criterion for knots and slope of grain is given in Table 6.4. 

Initial inspection of the member indicated that the member 

was in good shape with no damage present or obvious signs 

of decay. There was good access to all sides of the member 

for over 50% of its length and access to three sides for the 

remaining length of the member. The estimated slope of 

grain information for member 7 is No. 1. 

Initial inspection identified member 8 as Douglas-fir and 

indicated that the member was in good shape with no damage present or obvious signs of decay. Member 8 had a 

measured dimension of 7.5 by 9.5 in. This member was being used as a support column and had all sides visible. The 

dimension and use of this timber make it a Post or Timber. 

The Post and Timber rules presented in Table 6.5 can be applied to Douglas-fir timbers. There were very few and quite 

small knots present in this member. Member 8 easily met 

the criteria for Select Structural. 

Initial inspection identified member 9 as ash and indicated 

that the member was in good shape with no damage present 

or obvious signs of decay. Member 9 had a measured dimension of 5 by 8 in. The dimension and use of this member 

makes it a Beam and Stringer. The grading rules for timber 

Beam and Stringer are given in Table 6.5. There was good 

access to only three sides of the member for over 70% of 

its length. The member knot sizes indicated the beam could 

be graded out as a No. 1 but a severe slope of grain downgraded member 9 to No. 2.

Words of Advice for Size and Grade 

Estimation 

This manual is meant to provide the reader with very basic 

information about how grading is conducted and a very basic method for estimating grade for members. The most accurate estimate of the in-place grade of a member would be 

obtained by certified graders that have extensive experience 

and knowledge of various structural lumber grades. The 

field inspector, however, should be able to get a good idea of 

the species of the wood and the size and quality of the members that are in the structure. 

Keep good records indicating the amount of each member 

that was accessible and your confidence of the estimated 

grade. Many times it will be impossible to see a large portion of a member, and you truly will need to make an educated guess that, based on what you can see, the same knots 

and slope of grain conditions may extend to the rest of the 

member. To arrive at an estimate of the design value for the 

member, the condition and your confidence in the estimate 

of grade is important. 

Estimating Design Values 

Wanting to assign structural lumber values to lumber found 

in historic structures is certainly nothing new. A 1954 article 

by Lyman Wood in Southern Lumberman explored issues 

associated with assigning structural design values to old 

lumber (Wood 1954). Loferski and others used nondestructive evaluation and testing techniques to determine mechanical properties (Loferski et al. 1996). Falk and others looked 

at assigning design values to material from wood salvaged 

from building deconstruction projects (Falk et al. 2008, 

Lantz and Falk 1997). Even for newly produced lumber, a 

certain amount of uncertainty will always be associated with 
63

design value estimation. Assigning design values to in situ 

lumber and timber is more difficult because it requires judgment about the condition of members that you cannot fully 

access. It also requires you to judge how the prior load and 

environmental history has affected the member. An inspector must always be aware that judgments can only be made 

on what you can see and something detrimental could be 

lurking in the unexposed portions of the member.

Words of Caution about Estimating Design 

Values

Many times an inspector will not be able to see all sides of 

a member when trying to estimate its grade and condition. 

Even for very experienced certified graders, all that can truly 

be said about members that are observed in place is that no 

defects were found for the sections observed that exclude a 

member from a grade. There is no guarantee that this grade 

estimate applies to the unreachable portions of the member. 

The size of defects on exposed surfaces will provide some 

clue as to unexposed surfaces. Some confidence in a grade 

assignment may be gained from the grade assessment of 

other more accessible members. Great care, however, should 

be taken when determining the design values for members 

in place. Reducing the design value of the member by an additional uncertainty factor (Un)—perhaps as great as 2—is 

a reasonable thing to do.

Steps in Making a First Estimate of Design 

Values for Members

Once you have identified the species of wood involved, 

determined the dimension, established the condition, and 

estimated a grade, follow the steps (Fig. 6.14) to establish an 

estimate of design values for the members you have chosen. 

The path to assigning design values may seem confusing at 

first glance. Examples of each are provided to help guide the 

reader through the design value estimation process. Only 

Select Structural, No. 1, No. 2, and No. 3 grades are considered for assigning design values. 

After the species, size, condition, and grade of a member 

is established, the next step in estimating a design value is 

to determine whether the species can be found in the list 

of species that have design values in the NDS supplement 

(AWC 2012). If a wood member has remained dry and does 

not have excessive decay, damage, splitting, or checking, 

one can assume that the member has equivalent properties 

of material produced today. A list of current species in the 

NDS supplement is given in Table 6.7. 

Species listed in the NDS Supplement include the most 

common species that are present in wooden structures. 

Procedures for estimating design values for species that are 

found in the NDS Supplement are given in the following 

section. Note that some species listed in Table 6.7 are found 

as groups or individually. For example, a mixed oak group 

and northern red oak are both listed. If you are confident 

of your wood identification, use values for the individual 

species; otherwise it is best to use the more conservative 

estimate for the species group. If the species you have is 

not in the list of species given in Table 6.7, then you should 

proceed directly to the section on “Assigning estimated design values for species not found in the NDS Supplement.” 

Your estimate of design values will depend on whether 

small clear wood property data for your species exist. Alternatively, you might use the low “sweeper group” design 

values (such as Northern Species or Western woods) and the 

methods of the following section for an initial estimate of 

design values. 

Member grading examples in Table 6.6 are also used to illustrate design value assignment. Members 1, 2, 3, 4, 6, and 

7 in Table 6.6 are all species listed in the NDS supplement. 

No design value need be calculated for member 1 in Table 

6.6 because it was identified through close inspection to 

have severe decay and needs to be replaced. The other five 

members are assigned estimated design values in the following section. Members 5 and 8 in Table 6.6 are not in the 

NDS Supplement and are assigned estimated design values 

in the section on “Assigning estimated design values for 

species not found in the NDS Supplement.” All estimated 

design values reported below apply the rounding rules 

specified in ASTM D 245 (Table 6.8). 

Assigning Estimated Design Values for 

Species Found in the NDS Supplement

If the species of the member you are interested in is in 

the NDS Supplement, your assignment of design values 

is rather straightforward. Once a grade estimate has been 

established, determining the design values for a member is 

a matter of reading the design values for that particular species from a table in the NDS supplement and applying the 

appropriate moisture, size, and uncertainty reduction factors. 

Depending on size and grade of the member, design values 

for dimension lumber and timber can be found in one of 

three tables (4A, 4B, or 4D) in the NDS supplement. The 

members in Table 6.6 provide examples that use each type 

of table. 

Visually graded structural wood products are divided into 

two broad use categories—dimension lumber and timbers. 

If the smallest dimension of the member is less than 4.5 in. 

thick, the material can be considered dimension lumber. 

If the member is 5 in. or larger in smallest dimension, it is 

considered a timber. 

Dimension lumber properties are tabulated in two different 

tables depending on species. If the member is not Southern 

Pine, you would look up the design values in table 4A; otherwise, if the member is Southern Pine you would look up 

the properties in table 4B of the NDS Supplement.

For timbers, design values for all species can be found 

in table 4D of the NDS Supplement. Timbers, with the 

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

64

Timber 

(thickness > 4 inches)

Dimension lumber

(thickness ≤ 4 inches)

For a given grade, 

look up current 

design value in 

Table 4A of 

NDS Supplement

Apply Table 4A 

adjustment factors

Additional factor 

for uncertainty

For a given size and 

grade, look up 

current design value 

in Table 4B of 

NDS Supplement

Apply Table 4B 

adjustment factors

Additional factor 

for uncertainty

For a given species 

and grade, look up 

current design value 

in Table 4D of 

NDS Supplement

Apply Table 4D 

adjustment factors

Additional factor 

for uncertainty

Assign estimated

design value

For a given grade, 

look up current 

Southern Pine 

design value in 

Table 4D of 

NDS Supplement

Apply Table 4D 

adjustment factors

Additional factor 

for uncertainty

For a given species 

and grade, look up 

current design value 

in Table 4D of 

NDS Supplement

Apply Table 4D 

adjustment factors

Additional factor 

for uncertainty

Use the mean clear 

wood value and 

standard deviation 

to estimate the 

5th percentile

Reduce for duration 

of load

Additional factor 

for uncertainty

Adjust for 

moisture content

Reduce for

grade

Minimum

Determine 

mean clear 

wood value 

for member

Reduce for duration 

of load

Additional factor 

for uncertainty

Adjust for 

moisture content

Reduce for

grade

Mean

Posts and Timbers

Beams and Stringers

No

Yes

Yes

Yes

No

No

Yes

Is species in 

NDS Supplement?

Use ASTM D 245 

method to estimate 

design value

No design value 

estimate possible 

for member

Determine

species

Determine

size

Determine

condition

Determine

grade

Is the 

property a 

mean or 

near minimum

property?

Beam and 

Stringer or Post and 

Timber?

Is there 

clear wood 

property 

information 

available?

Timber

No

Dimension

Is member 

Southern Pine?

Is member 

Southern Pine?

Dimension 

lumber or timber?

Figure 6.14—Flow chart for design value assignment.
65

Table 6.7—Species with design values in the NDS Supplement (AWC 2012)

NDS species or species combination Species included in combination Table with design value

Alaska Cedar 4A

Alaska Hemlock 4A

Alaska Spruce Alaska Sitka Spruce

Alaska White Spruce

4A

Alaska Yellow Cedar 4A

Aspen Big Tooth Aspen

Quaking Aspen

4A

Baldcypress 4A, 4D

Balsam Fir 4D

Beech-Birch-Hickory American Beech

Bitternut Hickory

Mockernut Hickory

Nutmeg Hickory

Pecan Hickory

Pignut Hickory

Shagbark Hickory

Shellbark Hickory

Sweet Birch

Water Hickory

Yellow Birch

4A, 4D

Coast Sitka Spruce 4A, 4D, 4E

Douglas Fir-Larch Douglas-fir

Western Larch

4A, 4C, 4D, 4E

Douglas Fir-Larch (North) Douglas-fir

Western Larch

4A, 4C, 4D, 4E

Douglas Fir-South 4A, 4C, 4D, 4E

Eastern Hemlock 4D

Eastern Hemlock-Balsam Fir Balsam Fir

Eastern Hemlock

Tamarack

4A

Eastern Hemlock-Tamarack Eastern Hemlock

Tamarack

4A, 4D, 4E

Eastern Hemlock-Tamarack (North) Eastern Hemlock

Tamarack

4D, 4E

Eastern Softwoods Balsam Fir

Black Spruce 

Eastern Hemlock

Eastern White Pine

Jack Pine

Norway (Red) Pine

Pitch Pine

Red Spruce

Tamarack

White Spruce

4A

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

66

Table 6.7—Species with design values in the NDS Supplement (AWC 2012)—con.

NDS species or species combination Species included in combination Table with design value

Eastern Spruce Black Spruce

Red Spruce

White Spruce

4D, 4E

Eastern White Pine 4A, 4D, 4E

Eastern White Pine (North) 4E

Hem-Fir California Red Fir

Grand Fir

Noble Fir

Pacific Silver Fir

Western Hemlock

White Fir

4A, 4C, 4D, 4E

Hem-Fir (North) Amabilis Fir

Western Hemlock

4A, 4C, 4D, 4E

Mixed Maple Black Maple

Red Maple

Silver Maple

Sugar Maple

4A, 4D

Mixed Oak All Oak species under NeLMA rules 4A, 4D

Mixed Southern Pine Any species in the Southern Pine species 

combination, plus either or both of the 

following: Pond Pine, Virginia Pine

4B, 4C, 4D

Mountain Hemlock 4D

Northern Pine Jack Pine

Norway (Red) Pine

Pitch Pine

4D, 4E

Northern Red Oak Black Oak

Northern Red Oak

Pin Oak

Scarlet Oak

4A, 4D

Northern Species Any species graded under NLGA rules 

except Red Alder, White Birch, and 

Norway Spruce

4A, 4D, 4E

Northern White Cedar 4A, 4D, 4E

Ponderosa Pine 4D, 4E

Red Maple 4A, 4D

Red Oak Black Oak

Cherrybark Oak

Laurel Oak

Northern Red Oak

Pin Oak

Scarlet Oak

Southern Red Oak

Water Oak

Willow Oak

4A, 4D

Red Pine 4D, 4E

Redwood 4A, 4D, 4E

Sitka Spruce 4D, 4E
67

Table 6.7—Species with design values in the NDS Supplement (AWC 2012)—con.

NDS species or species combination Species included in combination Table with design value

Southern Pine Loblolly Pine

Longleaf Pine

Shortleaf Pine

Slash Pine

4A, 4C, 4D, 4E

Spruce-Pine-Fir Alpine Fir

Balsam Fir

Black Spruce

Engelmann Spruce

Jack Pine

Lodgepole Pine

Red Spruce

White Spruce

4A, 4C, 4D, 4E

Spruce-Pine-Fir (South) Balsam Fir

Black Spruce

Engelmann Spruce

Jack Pine

Lodgepole Pine

Norway (Red) Pine

Red Spruce

Sitka Spruce

White Spruce

4A, 4C, 4D, 4E

Western Cedars Alaska Cedar

Incense Cedar

Port Orford Cedar

Western Red Cedar

4A, 4C, 4D, 4E

Western Cedars (North) Pacific Coast Yellow Cedar

Western Red Cedar

4A, 4C, 4D, 4E

Western Hemlock 4A, 4C, 4D, 4E

Western Hemlock (North) 4A, 4C, 4D, 4E

Western White Pine 4A, 4C, 4D, 4E

Western Woods Any species in the Douglas Fir-Larch, Douglas 

Fir-South, Hem-Fir, and Spruce-Pine-Fir 

(South) species combinations, plus any or all 

of the following: Alpine Fir, Idaho White Pine, 

Mountain Hemlock, Ponderosa Pine, Sugar Pine

4A, 4C, 4D, 4E

White Oak Bur Oak 

Chestnut Oak

Live Oak

Overcup Oak

Post Oak

Swamp Chestnut Oak

Swamp White Oak

White Oak

Yellow Cedar 4A

Yellow Poplar 4A

Table 4A: Reference Design Values for Visually Graded Dimension Lumber (2–4 in. thick).

Table 4B: Reference Design Values for Visually Graded Southern Pine Dimension Lumber (2–4 in. thick).

Table 4C: Reference Design Values for Mechanically Graded Dimension Lumber.

Table 4D: Reference Design Values for Visually Graded timbers (5 by 5 in. and larger).

Table 4E: Reference Design Values for Visually Graded Decking.

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

68

exception of Southern Pine timber, are further subdivided 

into use categories with different values for timbers that can 

be described as a Beam or Stringer or as Post and Timber. 

Southern Pine members have no distinction in design values 

based on use category. 

Dimension Lumber Species Found in NDS 

Supplement

Two subcategories of dimension lumber are (1) species that 

are not Southern Pine and (2) species that are Southern Pine. 

Design value estimates for each category are determined differently.

Non-Southern Pine Dimension Lumber Species—For 

members that are dimension lumber size and are not 

Southern Pine, design values found in table 4A in the NDS 

Supplement apply (AWC 2012). The values in table 4A are 

primarily for nominal 2- by 12-in. dimension lumber. These 

tabulated values must be corrected to the desired size using 

the size factors shown in Table 6.9. Members 2, 3, and 4 in 

Table 6.6 fall into this category of dimension lumber. The 

step-by-step estimation of design values for members 2, 3, 

and 4 are given in Tables 6.10, 6.11, and 6.12 respectively.

Table 6.10 shows steps for calculating estimated design values for member 2. Member 2 in Table 6.6 was identified as 

a piece of 2 by 6 Douglas-fir dimension lumber with a grade 

stamp that identified the grade of the member as No. 1. It is 

an example of a member that fits into the dimension lumber 

size category (1.5 to 4 in. thick) and is not Southern Pine. 

The member was in good condition, and current design values for 2 by 12 No. 1 in table 4A of the NDS supplement apply. The appropriate size adjustments given in Table 6.9 are 

needed to convert these values to 2 by 6. Finally, because 

only 70% of the member was easily accessible, an additional 

uncertainty factor is applied as a precaution, giving the estimated design values listed in the far right column of Table 

6.10. The estimated Fb value, for example, is 900 lb/in2. 

Table 6.11 shows estimated design values for member 3 in 

Table 6.6, which was identified as a northern red oak 2- by 

6-in. member with an approximate grade of No. 2. This is 

another example of a member that fits into the dimension 

lumber size category (1.5- to 4-in. thick) and is not Southern 

Pine. There was good access to 70% of the member. The 

baseline design values for northern red oak can be determined from table 4A of the NDS Supplement. Once again 

the appropriate size factors Cf shown in Table 6.9 must be 

applied to convert the member to a 2 by 6 size. Finally, a 

similar uncertainty factor of 0.7 is applied because not all 

the member was visible when graded. The resulting estimated design values for member is given in the far right column 

of Table 6.11.

Table 6.8—Rounding Rules (ASTM D 245-11)

Bending, tension parallel, and 

compression parallel to grain

For design value stresses 1,000 lb/in2 (6.9 MPa) or greater,

round to nearest 50 lb/in2 (340 kPa)

For design value stresses less than 1,000 lb/in2 (6.9 MPa),

round to nearest 25 lb/in2 (170 kPa)

Horizontal shear and compression 

perpendicular to grain

Round to nearest 5 lb/in2 (34 kPa)

Modulus of elasticity Round to nearest 100,000 lb/in2 (69 GPa)

Table 6.9—Size factors, Cf (AWC 2012)

Grades Width (in.)

Fb

Thickness

2 and 3 in. 4 in. Ft Fc//

Select Structural, 

No. 1, No. 2, and No. 3

2 and 3 1.5 1.5 1.5 1.15

5 1.4 1.4 1.4 1.1

6 1.3 1.3 1.3 1.1

8 1.2 1.3 1.2 1.05

10 1.1 1.2 1.1 1.0

12 1.0 1.1 1.0 1.0

14 0.9 1.0 0.9 0.9
69

Table 6.10—Example of design value estimation for No. 1 Douglas-fir 2 by 6 

Species

Estimated 

member 

size

Estimated 

member 

grade Purpose Property

Table 4A 

baseline 

design 

valueb

Cf

Size 

adjustment 

factor Una

Estimated 

design

value

Douglas-fir 1.5 by 5.5 in. No. 1 Str. J&P Fb

(lb/in2)

1,000 1.3 0.7 900

Ft

(lb/in2)

675 1.3 0.7 600

Fv

(lb/in2)

180 1.0 0.7 125

Fcperp

(lb/in2)

625 1.0 0.7 440

Fc//

(lb/in2)

1,500 1.1 0.7 1,150

E

(106 lb/in2)

1,700,000 1.0 0.7 1,200,000

Emin

(106 lb/in2)

620,000 1.0 0.7 400,000

aUn, uncertainty.

bAWC (2012).

Table 6.11—Example of design value estimation for No. 2 Northern Red Oak 2 by 6 

Species

Estimated 

member

size

Estimated 

member 

grade Purpose Property

Table 4A 

baseline 

design 

valueb

Cf

Size 

adjustment 

factor Una

Estimated 

design

value

Northern Red 

Oak

2 by 6 No. 2 Str. J&P Fb

(lb/in2)

975 1.3 0.7 890

Ft

(lb/in2)

575 1.3 0.7 525

Fv

(lb/in2)

220 1.0 0.7 155

Fcperp

(lb/in2)

885 1.0 0.7 620

Fc//

(lb/in2)

725 1.1 0.7 560

E

(106 lb/in2)

1,300,000 1.0 0.7 910,000

Emin

(106 lb/in2)

470,000 1.0 0.7 329,000

aUn, uncertainty.

bAWC (2012).

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

70

Table 6.12 shows estimated design values for member 4 in 

Table 6.6, which was identified as a 2- by 7.5-in. piece of 

cottonwood, which is closest to the current dimension of a 

nominal 2 by 8. The approximate grade for this member was 

determined to be No. 2. The baseline design values for cottonwood can be found in table 4A of the NDS supplement. 

The appropriate size factors Cf shown in Table 6.9 must be 

applied to convert the member to a 2 by 8 size. During inspection, notes indicated that there was not good access to 

this member; only 50% of the member was visible on three 

of the four sides. This lack of access introduces a great deal 

of uncertainty into the estimate of grade, so a considerable 

reduction (Un = 0.5) in design value estimates is certainly 

advised. The resulting estimated design values are given in 

the far right column of Table 6.12. 

Southern Pine Dimension Lumber in the NDS Supplement—Table 6.13 shows steps for determining an estimated 

design value for member 5. Member 5 in Table 6.6 was 

identified as a 2- by 4-in. Southern Pine member whose approximate grade was No. 2. The size-specific design values 

for Southern Pine dimension lumber are tabulated in table 

4B of the NDS supplement. To estimate design values, the 

inspector needs to find the entry that corresponds to the 

grade and closest size of the member of interest. In this 

case, for example, the 2 by 4 No. 2 Fb given in table 4B 

is 1,100 lb/in2. Because the values are size-specific, a size 

adjustment factor is not necessary. The inspection notes also 

indicated easy access to all sides, so an uncertainty reduction is not needed. Estimated design values listed in the far 

right column for member 5 is, in this case, the value given 

for Southern Pine 2 by 4 No. 2 of table 4B of the NDS 

Supplement.

Timber Values Found in NDS Supplement

Timber design values are found in table 4D of the NDS Supplement. There are again differences between how you treat 

Southern Pine and other species, but this time the difference 

is not as exaggerated. A different set of size adjustments 

apply to timbers than did to dimension lumber. For timbers 

that are subjected to loads applied to the narrow face, tabulated design values for bending are adjusted by a size adjustment found in ASTM D 245. Other properties do not receive 

a size adjustment. If the member depth is greater than 12 in., 

a new Fn can be calculated using

where d is member depth, Fo is original bending strength, 

and Fn is adjusted bending strength.

Southern Pine Timbers—Southern Pine timbers have no 

distinction between the use classes of Beams or Stringers 

and Post and Timbers. Therefore, you only need to find the 

Southern Pine timber values for the grade you are interested 

in and apply them to the member.

Table 6.14 shows estimated design values for member 7. 

Member 7 in Table 6.6 was identified as a 5.5- by 7.5-in. 

Southern Pine member whose observable knots and slope of 

Table 6.12—Example of design value estimation for No. 2 Cottonwood 2 by 8

Species

Estimated 

member 

size

Estimated 

member 

grade Purpose Property

Table 4A 

baseline 

design 

valueb

Cf

Size 

adjustment 

factor Una

Estimated 

design 

value

Cottonwood 2 by 8 No. 2 Str. J&P Fb

(lb/in2)

625 1.2 0.5 375

Ft

(lb/in2)

350 1.2 0.5 210

Fv

(lb/in2)

125 1 0.5 65

Fcperp

(lb/in2)

320 1 0.5 160

Fc//

(lb/in2)

475 1.05 0.5 250

E

(106 lb/in2)

1,100,000 1 0.5 550,000

Emin

(106 lb/in2)

400,000 1 0.5 200,000

aUn, uncertainty.

bAWC (2012).

Fn = Fo

12

d

1/9 ( (
71

Table 6.13—Example of design value estimation for No. 2 Southern Pine 2 by 4 

Species

Estimated 

member

size

Estimated 

member

grade Purpose Property

Table 4B 

baseline 

design 

valueb

Cf

Size 

adjustment 

factor Una

Estimated 

design

value

Southern Pine 2 by 4 No. 2 Str. LF Fb

(lb/in2)

1,100 1 1.0 1,100

Ft

(lb/in2)

675 1 1.0 675

Fv

(lb/in2)

175 1 1.0 175

Fcperp

(lb/in2)

565 1 1.0 565

Fc//

(lb/in2)

1,450 1 1.0 1,450

E

(106 lb/in2)

1,400,000 1 1.0 1,400,000

Emin

(106 lb/in2)

510,000 1 1.0 510,000

aUn, uncertainty.

bAWC (2012).

Table 6.14—Example of design value estimation for No. 1 Southern Pine 6 by 8

Species

Estimated 

member

size

Estimated 

member 

grade Purpose Property

Table 4D 

baseline 

design 

valueb

Cf

Size 

adjustment 

factor Una

Estimated 

design

value

Southern Pine 6 by 8 No. 1 P-T Fb

(lb/in2)

1,350 1 0.5 675

Ft

(lb/in2)

900 1 0.5 450

Fv

(lb/in2)

165 1 0.5 85

Fcperp

(lb/in2)

375 1 0.5 190

Fc//

(lb/in2)

825 1 0.5 415

E

(106 lb/in2)

1,500,000 1 0.5 750,000

Emin

(106 lb/in2)

550,000 1 0.5 275,000

aUn, uncertainty.

bAWC (2012).

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

72

grain classified the member as a No. 1 timber. Design values 

for Southern Pine for this size and grade can be obtained 

directly from table 4D of the NDS Supplement. Inspection 

notes indicate that there was not good access to this member; only 50% of the member was visible on three of four 

sides. This suggests the need for a considerable uncertainty 

factor of 0.5. Estimated design values for member 7 are 

given in the far right column of Table 6.14.

Non-Southern Pine Timbers—For non-Southern Pine 

members, the value in the NDS Supplement that is appropriate for the member is linked to its species, dimensions, 

estimated grade, and use. If the member is 5 in. thick and its 

width is more than 2 in. greater than thickness, the member 

is a Beam or Stringer. If the member is 5 by 5 in. and larger 

with a width not more than 2 in. greater than thickness, the 

member is a Post and Timber.

Table 6.15 shows estimated design values for member 8. 

Member 8 in Table 6.6 was identified as a 7.5- by 9.5-in. 

Douglas-fir member that had observable knots and slope of 

grain that would approximate a Select Structural piece of 

timber. The member size means that it can be classified as 

a Post and Timber. The design values for Select Structural 

Douglas-fir Post and Timber can be obtained in table 4D of 

the NDS Supplement. Inspection notes indicate that there 

was easy access to all sides, so no uncertainty factor is applied. Estimated design values for member 8 are given in the 

far right column of Table 6.15.

Assigning Estimated Design Values for Species Not Found in the NDS Supplement

If the species of the member is not in the NDS Supplement, 

design value estimates are even more uncertain. Strength 

and stiffness information on that species must be found. If 

small clear wood test values are available for the species 

you have identified, using the methods listed in ASTM D 

245 as a guide, the equation

will allow for a design value estimate of Fx, which is the 

particular member property times an adjustment for duration 

of load and factor of safety (adjfac), grade (Grade), moisture 

content (MC), size (Size), and uncertainty (Un).

For bending, tension, shear, and compression parallel to 

grain design values, the estimated 5th percentile for each 

property is needed to estimate the design value. The 5th 

percentile for the species can be estimated by the (Avg – 

1.645σ), where Avg is the average green small clear strength 

value for the small clear test result in Table 6.1 and σ is the 

standard deviation for the species. The standard deviation 

can be approximated by using the average coefficient of 

variation for most wood properties (given in Table 6.16) 

times the average clear wood strength. The species’ mean 

clear wood modulus of elasticity (MOE) and compression 

perpendicular to grain (Cperp) stress values can be used to 

determine an estimate of MOE and Cperp design values. 

Table 6.15—Example of design value estimation for Select Structural Douglas-fir 8 by 10

Species

Estimated 

member

size

Estimated 

member 

grade Purpose Property

Table 4D 

baseline 

design 

valueb

Cf

Size 

adjustment 

factor Una

Estimated 

design

value

Douglas-fir 8 by 10 Sel. Str. P-T Fb

(lb/in2)

1,500 1 1.0 1,500

Ft

(lb/in2)

1,000 1 1.0 1,000

Fv

(lb/in2)

170 1 1.0 170

Fcperp

(lb/in2)

625 1 1.0 625

Fc//

(lb/in2)

1,150 1 1.0 1,150

E

(106 lb/in2)

1,600,000 1 1.0 1,600,000

Emin

(106 lb/in2)

580,000 1 1.0 580,000

aUn, uncertainty.

bAWC (2012).

Fx = (Property)· ·Grade·MC·Size·Un 1

adjfac
73

The adjustment factors (adjfac) suggested in D 245 for 

various clear wood properties are given in Table 6.17. This 

factor is meant to account for duration of load and a safety 

factor. 

The Grade adjustment is based on the assumed strength 

ratio for a particular visual grade. Estimated strength ratios 

for cross grain and density have been obtained empirically; 

strength ratios for other growth characteristics have been 

derived theoretically. Particular grades have had approximate strength ratios associated with them. The strength ratio 

estimates for Grade are given in Table 6.18. 

The Size adjustment is based on a size adjustment factor 

applied to the bending clear wood value. This factor adjusts 

the small clear wood specimen test result obtained at a 2-in. 

dimension to the dimension of the member. The adjustment 

for size can be determined by using the member depth of the 

member d and 

Finally, as in previous examples, an uncertainty factor (Un) 

adjustment is recommended to reflect the uncertainty associated with determining the grade and condition of the 

member. 

Most species not in the NDS supplement but used in wooden structures can be found in Table 6.1. Two examples from 

Table 6.6, dimension lumber member 6 and timber member 

9, are given below. Other example calculations of design 

values using clear wood information can be found in Bendtsen and Galligan (1978).

Member is Dimension Lumber Size

Table 6.19 shows steps for estimating design values for 

member 6. Member 6 of Table 6.6 was identified as a 2- by 

6-in. American chestnut Structural Joist and Plank that had 

observable defects and a slope of grain that classified it as a 

No. 2 piece of dimension lumber. The average clear wood 

mean values for American chestnut from Table 6.1, which 

Table 6.16—Average coefficient of variation for clear wood properties (FPL 2010)

Bending

Modulus 

of

elasticity

Tensile 

strength 

parallel to 

grain

Compressive 

strength

parallel

to grain

Horizontal 

shear 

strength

Proportional limit 

stress at deformation 

in compression 

perpendicular to grain

Coefficient of variation 16 22 25 18 14 28

Table 6.17—Adjustment factors to be applied to the clear wood properties (ASTM D 245-11)

Bending

Modulus 

of 

elasticity

Tensile 

strength 

parallel to 

grain

Compressive 

strength 

parallel 

to grain

Horizontal 

shear 

strength

Proportional limit 

stress at deformation 

in compression 

perpendicular to grain

Softwoods 2.1 0.94 2.1 1.9 2.1 1.67

Hardwoods 2.3 0.94 2.3 2.1 2.3 1.67

Table 6.18—Assumed strength ratio values for rough estimate of design values 

(Bendtsen and Galligan 1978)

Grades

Assumed strength ratio

Bending Tension Shear Cperp Compression MOE

Clear wood 100 100 100 100 100 100

Select Structural 65 65*55 50 100 69 100

No. 1 55 55*55 50 100 62 100

No. 2 45 45*55 50 100 52 90

No. 3 26 26*55 50 100 33 80

Size = 2

d

1/9 ( (

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

74

have been converted to pounds per square inch, are in column 1. Where applicable, these values are adjusted to a 5th 

percentile estimate in column 2. The hardwood adjustment 

factors from Table 6.17 and the estimated grade strength 

ratio from Table 6.18 are in columns 3 and 4, respectively. 

Moisture adjustment factors for American chestnut from 

green to dry are available in table X1.1 of ASTM D 2555 

and are listed in column 5. Only the bending stress value 

is adjusted for size in column 6. Inspection notes indicate 

that there was quite good access, resulting in an uncertainty 

factor of 0.8. The estimated design values for member 6 are 

given in far right column. 

Member is Timber Size—Table 6.20 shows steps for estimating design values for member 9. Member 9 of Table 6.6 

was identified as a 5- by 8-in. ash beam and stringer that had 

observable defects approximating a No. 2 grade timber. The 

average clear wood mean values for white ash from Table 

6.1, which have been converted to pounds per square inch, 

are tabulated in column 1. Where applicable, these values 

are adjusted to a 5th percentile estimate in column 2. The 

hardwood adjustment factors from Table 6.17 and the estimated grade strength ratio from Table 6.18 are in columns 

3 and 4, respectively. Moisture adjustment factors for white 

ash from green to dry are available in table X1.1 of ASTM 

D 2555 and are listed in column 5. Only the bending stress 

value is adjusted for size in column 6. Inspection notes indicate that only 70% of the member was accessible, so an uncertainty factor of 0.7 is applied to the estimate. Estimated 

design values for member 9 are given in far right column. 

Final Words of Advice for Design Value 

Estimation 

This manual is meant to provide the reader with basic information about design value assignment to wooden members. 

It gives the inspector a basic method for estimating design 

value for members. The most accurate estimate of the inplace design value for a member would be obtained by 

employing a certified grader with extensive experience and 

knowledge of the various structural lumber grades. The field 

inspector, however, should be able to get a good sense of the 

quality of the members that are in the structure and determine if current members are in the “ball park” of capacity 

needed. 

An inspector will often not be able to see a large portion of 

a member, and judgment will be required based on the quality of wood in members that are visible in accessible areas. 

Confidence in the condition of members also plays a big 

role. Keep good records indicating the amount of the member that was accessible and your confidence of the estimated 

grade. Design values for the member may require an adjustment based on your confidence in your determined grade. 

The uncertainty factor that has been applied in the example 

estimations is one such adjustment. 

Decisions made by the inspector on the condition of a 

structure, based on the inspector’s analysis of calculations 

or technical data, are ultimately the responsibility of the 

inspector. Understanding the basis of the information or 

data upon which they rely is the primary responsibility of 

the inspector. This manual is based on publicly available 

Table 6.19—Example of design value estimation for No. 2 American Chestnut 2 by 6 

Species

Estimated 

member 

size

Estimated 

member 

grade Purpose Property

Clear 

wood

mean

(lb/in2)

5th 

Pctl.

Est. adjfaca Grade MC Size Unb

Estimated 

design 

value

(lb/in2)

American 

Chestnut

2 by 6 Sel. Str. Str. J&P Fb

(lb/in2)

5,655 4,170 2.3 0.45 1.53 0.894 0.8 900

Ft

(lb/in2)

5,655 3,330 2.3 0.247 1.53 1 0.8 450

Fv

(lb/in2)

800 615 2.3 0.5 1.36 1 0.8 145

Fcperp

(lb/in2)

305 N/A 1.67 1 2.00 1 0.8 290

Fc//

(lb/in2)

2,465 1,735 2.1 0.50 2.15 1 0.8 650

E

(106 lb/in2)

930,000 N/A 0.94 0.9 1.32 1 0.8 900,000

aadjfac, adjustment factor.

bUn, uncertainty.
75

information. Some of that information comes from grading rules published by various trade associations or grades 

identified within the National Design Specification for Wood 

Construction. The Federal Highway Administration and the 

USDA Forest Service, while providing financial and administrative support, are not responsible for the content or use 

of the information by individuals. Be aware that changes in 

grading rules occasionally take place, and the inspector is 

responsible to confirm what the current rules are.

Must-Have Documents

There are three must-have documents for someone trying to 

estimate a design value. Be sure to obtain the most current 

editions.

•	 The National Design Specification (NDS) Supplement 

Design Values for Wood Construction, which lists all 

currently approved design values for visually graded 

lumber and timber 

•	 ASTM standard D 2555 Standard Practice for Establishing Wood Strength Values

•	 ASTM standard D 245 Standard Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber

The NDS Supplement can be obtained from the American 

Wood Council, 222 Catoctin Circle SE, Suite 201, Leesburg, VA 20175 (www.awc.org). The ASTM standards can 

be obtained from ASTM International, 100 Barr Harbor 

Drive, P.O. Box C700, West Conshohocken, PA 19428–2959 

(www.ASTM.org).

References

Anthony, R.W.; Dugan, K.D.; Anthony, D.J. 2009. A grading 

protocol for structural lumber and timber in historic structures. APT Bulletin. 40(2): 3–9.

ASTM. 2013. Annual book of standards, Vol. 04.10. West 

Conshohocken, PA: American Society for Testing and Materials. 

a. ASTM D 245–11. Standard methods for establishing 

structural grades for visually graded lumber.

b. ASTM D 2555–06. Standard methods for establishing 

clear wood strength values.

AWC. 2012. National design specification (NDS) supplement: design values for wood construction 2012 edition. 

Leesburg, VA: American Wood Council.

Bendtsen, B.A.; Galligan, W.L. 1978. Deriving allowable 

properties of lumber (A practical guide for interpretation of 

ASTM standards). Gen. Tech. Rep. FPL–GTR–20. Madison 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 31 p.

DOC. [Current edition]. American softwood lumber standard. Voluntary product standard PS–20. Washington, D.C.: 

U.S. Department of Commerce.

Green, D.W.; Evans, J.W. 2001. Evolution of standardized 

procedures for adjusting lumber properties for change in 

moisture content. Gen. Tech. Rep. FPL–GTR–127. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory.

Table 6.20—Example of design value estimation for No. 2 White Ash 5 by 8

Species

Estimated 

member 

size

Estimated 

member 

grade Purpose Property

Clear 

wood

mean

(lb/in2)

5th 

Pctl.

Est. adjfaca Grade MC Size Unb

Estimated 

design 

value

(lb/in2)

White Ash 5 by 8 No. 2 B-S Fb

(lb/in2)

9,570 7,055 2.3 0.45 1.57 0.857 0.7 1,500

Ft

(lb/in2)

5,265 5,635 2.3 0.247 1.57 1 0.7 650

Fv

(lb/in2)

1,350 1,040 2.3 0.5 1.41 1 0.7 225

Fcperp

(lb/in2)

665 N/A 1.67 1 1.73 1 0.7 385

Fc//

(lb/in2)

3,990 2,805 2.1 0.50 1.86 1 0.7 975

E

(106 lb/in2)

1,435,000 N/A 0.94 0.9 1.21 1 0.7 1,200,000

aadjfac, adjustment factor.

bUn, uncertainty.

Wood and Timber Condition Assessment Manual • Chapter 6
General Technical Report FPL–GTR–234

76

Falk, R.H.; Maul, D.G.; Cramer, S.M.; Evans, J.; Herian, 

V. 2008. Engineering properties of Douglas-fir lumber reclaimed from deconstructed buildings. Res. Pap. FPL–RP–

650. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 47 p. 

FPL. 2010. Wood handbook—wood as an engineering material. Gen. Tech. Rep. FPL–GTR–190. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 508 p. 

Gonzalez, J.M.; Morrell, J.J. 2012. Effects of environmental factors on decay rates of selected white- and brown-rot 

fungi. Wood and Fiber Science. 44(4): 343–356.

Ivory, E.P.; White, D.G.; Upson, A.T. 1923. Standard grading specifications for yard lumber. Circular 296. Washington, DC: U.S. Department of Agriculture. 75 p.

Lantz, S.F.; Falk, R.H. 1997. Feasibility of recycling timber 

from military industrial buildings. In: The use of recycled 

wood and paper in building applications. Proceedings 7286. 

Madison, WI: Forest Products Society: 41–48.

Loferski, J.R.; Dolan, J.D.; Lang E. 1996. Determining mechanical properties by nondestructive evaluation and testing 

methods in wood buildings. In: Kelley, S.J., ed. Standards 

for preservation and rehabilitation. West Conshohocken, 

PA.: American Society for Testing and Materials: 175–185. 

Newlin, J.A.; Johnson, R.P.S. 1923. Basic grading rules and 

working stresses for structural timbers. Circular 295. Washington, DC: U.S. Department of Agriculture.

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.

SPIB. 2002. Standard grading rules for southern pine lumber. Pensacola, FL: Southern Pine Inspection Bureau.

Wiedenhoeft, Alex C.; Kretschmann, David E. 2014. Species identification and design value estimation of wooden 

members in covered bridges. General Technical Report 

FPL-GTR-228. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 94 p.

Wood, L.W. 1954. Structural values in old lumber. Southern 

Lumberman. No. 189, Dec. 15, part 2. 

WWPA. 2005. Western lumber grading rules. Portland, OR: 

Western Wood Products Association.
77

Chapter 7

Condition Assessment Report

Robert J. Ross

This chapter presents the complete condition assessment 

report for the Quincy Mine Blacksmith Shop in Hancock, 

Michigan. The assessment was conducted by Robert J. Ross, 

Project Leader, Condition Assessment and Rehabilitation of 

Structures, USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin; Xiping Wang, Senior Research 

Associate, Natural Resources Research Institute, University 

of Minnesota Duluth, Duluth, Minnesota; John W. Forsman, 

Assistant Scientist, Michigan Technological University, 

Houghton, Michigan; and John R. Erickson, Director (retired), USDA Forest Service, Forest Products Laboratory, 

Madison, Wisconsin. The report was completed September 

28, 2001.

Condition Assessment of 

Timbers from Quincy Mine 

Blacksmith Shop—Scanning 

Timbers for Deterioration

Summary

Fifteen 12- by 12-in. white pine timbers removed from the 

Quincy Mine Blacksmith Shop were nondestructively evaluated at the USDA Forest Service, Forest Products Laboratory, through intensive stress wave scanning. The timbers 

were part of the original roof truss structure in the Blacksmith Shop. Scan results indicated that some timbers or 

some sections of the timbers had severely deteriorated and 

lost structural integrity. Most of the timbers still contained 

a substantial amount of solid wood, and some timbers were 

only slightly decayed or mechanically damaged. Further 

data analysis indicated that 61% of the total materials could 

be potentially recovered from these timbers and used in the 

reconstruction project.

Background

The Quincy Mining Company’s Drill and Blacksmith Shop, 

located in the Keweenaw National Historical Park, was constructed in 1900 by the Quincy Mining Company. Of great 

interest is the preservation and use of the remaining roof 

trusses and floor joists. The 12- by 12-in. white pine roof 

truss timbers had been exposed to weather for several years. 

These structural members, manufactured from old-growth 

timber, are a tremendous resource that should be appropriately used in the renovation of the Drill and Blacksmith 

Shop.

Condition assessment of the heavy timbers from the Blacksmith Shop was an important first step in the restoration of 

this structure. An early on-site assessment found that a substantial amount of timbers contained solid wood in spite of 

visually observed deterioration of parts of the members. The 

intent of this assessment was to follow up on the promising 

cursory assessment results and to further nondestructively 

evaluate selected white pine timbers in the laboratory.

Objectives

The objectives of this assessment were as follows:

1. To detect the location and severity of deterioration in 

12- by 12-in. white pine timbers removed from the 

Quincy Mine Blacksmith Shop through intensive scanning using a stress wave transmission technique

2. To determine how much of the original structural wood 

from these timbers can be potentially recovered and 

used in the reconstruction project

Materials and Method

Fifteen 12- by 12-in. white pine timbers removed from 

the Quincy Mine Blacksmith Shop were transported to the 

USDA Forest Service, Forest Products Laboratory (FPL) in 

Madison, Wisconsin, on August 2, 2001 (Fig. 7.1). The 

timber specimens ranged from 25.5 to 50 ft in length; 

13 of the specimens were about 50 ft long (originally used 

Figure 7.1—Fifteen 12- by 12-in. white pine timbers removed 

from the Quincy Mine Blacksmith Shop.
General Technical Report FPL–GTR–234

78

as tension members in the roof trusses) and 3 of the specimens were from 26 to 34 ft long (probably used as pitched 

chord or web materials). These timbers were part of the 

original roof structures and had been exposed to weather for 

several years. Surface decay, splits, mechanical damage, and 

notches were the major defects observed (App. A).

Figure 7.2 shows a typical scanning diagram used to conduct stress wave transmission tests. The timbers were supported on two beams. Each timber was stress wave scanned 

along three lines (upper line A, middle line B, and lower line 

C) on the side surfaces through the height of each timber. 

Because the ends of the timber were usually considered to 

be the areas having a high risk of decay, stress wave transmission tests were more intensively performed at ends than 

other parts of the timber. Intervals between the two scanning points in the longitudinal direction changed from 6 in. 

beginning at the end of the timber to 1, 2, and 4 ft, respectively, as the scanning position approached the middle part 

of the timber.

A commercially available stress wave timing unit was used 

to measure wave transmission time, perpendicular to grain, 

at a series of points as described above (described in detail 

in FPL (2000)). Three stress wave readings were taken at 

each scanning point, and the average wave transmission 

time for each point was used as an indicator of the soundness of wood in that area.

Results

Results obtained from stress wave scans are shown in 

Figure 7.3. Stress wave transmission time measured perpendicular to grain was in microseconds. Given the same width 

of 12 in. for all timbers, the stress wave data expressed on 

the y axis can also be deemed as values in a unit of microseconds per foot (μs/ft). Therefore, scan results for all 

15 timbers can be conveniently compared with baseline 

data given in previous publications (Ross and Pellerin 1994; 

Ross et al. 1998, 2001; FPL 2000; Clausen et al. 2001).

Stress wave transmission time may vary from species to 

species, but it is mainly controlled by the soundness of 

wood in terms of deterioration detection. For most species, 

stress wave transmission time perpendicular to grain ranges 

Figure 7.2—Scanning diagram for nondestructive evaluation of 12- by 12-in. white pine timbers.

Figure 7.3—Typical scan results from 12- by 12-in. white pine 

timbers.
79

Wood and Timber Condition Assessment Manual • Chapter 7

from 180 to 400 μs/ft for solid wood (FPL 2000). This baseline range was used as an evaluation standard to assess scan 

results obtained from the fifteen 12- by 12-in. white pine 

timbers.

As shown in wave transmission-time/length-of-timber 

graphs, these white pine timbers are in various conditions, 

from solid, to slightly to severely deteriorated, to almost 

fully deteriorated. Deterioration was mainly caused by decay (surface and internal), splits, and mechanical damage. 

From scans shown in Figure 7.3, it is possible to distinguish 

whether deterioration occurs in surface areas or inside the 

timber as stress wave transmission times obtained from 

three scan lines (A, B, and C) are plotted on the same graph 

for each timber.

The solid wood in each timber was determined by comparing scan results with baseline data of stress wave transmission times for solid wood. The approximate length of solid 

wood for each timber that could have potential structural 

uses was also determined and indicated below each scan 

graph. A summary of potential recovery of the solid wood 

from all tested white pine timbers is detailed in Table 7.1.

From the scans and Table 7.1, it is evident that a substantial 

amount of solid wood could be recovered from these timbers except for one short timber, FPL-12, which was almost 

fully deteriorated. Potential recovery rate could be as low 

as 16% to 17% because of severe deterioration found in 

timbers (such as FPL-4 and FPL-8); it could also be as high 

as over 90% (such as timber FPL-3 and FPL-10). Overall, 

an average of 61% of total materials (not including FPL-12) 

could be potentially recovered from these white pine timbers and used in the reconstruction project.

Conclusions and Recommendations

Fifteen 12- by 12-in. white pine timbers removed from 

the Quincy Mine Blacksmith Shop were nondestructively 

evaluated through intensive stress wave scanning. The scan 

results indicated that some timbers or parts of timbers had 

seriously deteriorated and lost structural integrity. It was 

also found that most of the timbers still contain a substantial 

amount of solid wood, and some timbers were only slightly 

decayed or mechanically damaged on the ends. Further data 

analysis indicated that overall 61% of the total materials 

could be potentially recovered from these timbers and used 

or displaced in the reconstruction project.

Three recommendations were based on preliminary results 

obtained from the 15 white pine timbers:

1. Complete stress wave scanning tests on the remaining white pine timbers, including 12- by 12-in. roof 

truss timbers and 3- by 14-in. floor joists. Use the same 

procedure as employed in this assessment to determine 

location and severity of deterioration that might occur 

in the remaining structural members.

2. Cut off deteriorated sections from the timbers and obtain solid material based on scan results.

3. Mechanically proof-load all solid wood sections to 

supply engineering design values for their reuse in the 

reconstruction project.

References

Clausen, C.A.; Ross, R.J.; Forsman, J.W.; Balachowski, 

J.D. 2001. Condition assessment of Quincy Mine Blacksmith Shop in Keweenaw National Historical Park. Res. 

Table 7.1—Potential recovery of solid wood from 12- by 12-inch (30- by 30-cm) 

white pine timbers

Timber 

ID

Length of 

timber 

(ft)a

Length of 

solid 

material 

(ft)a

Potential 

recovery 

(%) Major defects of timber

FPL-1 50 39 78 Decay on end, notch

FPL-2 48.4 28 58 Surface check

FPL-3 45.8 45 98 —

FPL-4 50 8 16 Severe decay, split, notch

FPL-5 47.8 28 59 Decay on end, surface check, notch

FPL-6 50 42 84 Decay on end, surface check, notch

FPL-7 50 26 52 Decay, notch

FPL-8 46 8 17 Severe surface decay

FPL-9 50 42 84 Surface check, notch

FPL-10 48.3 44.5 92 Surface decay on one end

FPL-11 50 24 48 Severe decay, notch

FPL-12 25.5 0 0 Severe decay, split, notch

FPL-13 33.8 22 65 Split on ends

FPL-14 32 16 50 Decay and split on ends, notch

FPL-15 50 24 48 Severe decay, notch a

1 ft = 0.3 m.
General Technical Report FPL–GTR–234

80

Note FPL–RN–0281. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 4 p.

FPL. 2000. Stress wave timing nondestructive evaluation 

tools for inspecting historic structures. Gen. Tech. Rep. 

FPL–GTR–119. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 15 p.

Ross, R.J.; Pellerin, R.F. 1994. Nondestructive testing for 

assessing wood members in structures: A review. Gen. Tech. 

Rep. FPL–GTR–70 (Rev.). Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 

40 p.

Ross, R.J.; Soltis, L.A.; Otton, P. 1998. Assessing wood 

members in the USS Constitution using non-destructive 

evaluation methods. APT Bulletin (The Journal of Preservation Technology). 29(2):21-25.

Ross, R.J.; Pellerin, R.F.; Forsman, J.W.; Erickson, J.R.; 

Lavinder, J.A. 2001. Relationship between stress wave 

transmission time and compression properties of timbers 

removed from service. Res. Note FPL–RN–280. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 4 p.
81

Wood and Timber Condition Assessment Manual • Chapter 7

Surface decay

Severe decay damage

End decay

End decay

Large, deep check

Appendix A—Major Defects 

Observed in 12- by 12-inch White 

Pine Timbers
General Technical Report FPL–GTR–234

82

Surface decay and multiple cracks

Mechanical damage
83

Chapter 8

Summaries from Actual Inspections

Robert J. Ross

This chapter presents summaries of inspections from a wide 

range of structures, from a sports stadium to a mummy 

coffin. Included are a brief overview of the structure, its 

location, a brief history of the structure, and several photographs. Note that these were selected to emphasize the 

broad range of structures that can be inspected using the 

techniques presented in earlier chapters. Most include a 

photo of the structure, visual observations, and equipment in 

use. No attempt is made to discuss repairs made or other actions taken based on inspection results. Citations for papers 

prepared based on the inspections are included, if available, 

for further details. 

Washington State University 

Football Stadium

Structure: Martin Stadium

Location: Pullman, Washington

Special Consideration(s): Arid environment

Brief history of structure: 

Estimated age: 50 years at time of inspection

Inspection date: 1982

Construction details: Heavy timber, creosote-treated 

Douglas-fir

Washington State University’s football stadium was inspected using visual, coring, and stress wave timing techniques. 

This stadium was originally constructed in the 1930s; the 

north and south grandstands were replaced after a fire in 

the 1960s. In the early 1980s, the structural integrity of the 

horseshoe section that joined the north and south grandstands was inspected. This horseshoe section was part of the 

original stadium and was constructed from large solid-sawn 

timbers. An informal inspection revealed that the structural 

members in the horseshoe section were badly decayed. Further evaluation using stress wave timing equipment showed 

that speed-of-sound transmission was significantly lower in 

decayed members than in sound wood. Subsequent probing 

of those areas indicated that the decay was so extensive that 

only a thin shell of sound wood remained. These results led 

to the dismantling of the horseshoe section of the stadium.
General Technical Report FPL–GTR–234

84
85

Grangeville and Kooskia High 

School Gymnasiums

Structure: School gymnasiums

Location: Grangeville, Idaho; Kooskia, Idaho

Special Consideration(s): None

Brief history of structure:

Estimated age: 20 years at time of inspection

Inspection date: 1975

Construction details: Series of curved, glued-laminated 

timber arches

Two school gymnasiums constructed with laminated barrel 

arches were evaluated. These glued-laminated arches were 

the main support structure for the gymnasiums. Each arch 

end was exposed to the weather and rested in a metal shoe 

(which was closed, so the water could not drain properly) 

fastened to a concrete pier foundation. These conditions and 

the heavy nonbreathing paint that was used on the exposed 

portions of the arches created an environment that would 

support the growth of decay fungi. Cracking and peeling 

paint were the first indications that decay was present in the 

arch ends. When school personnel realized the condition of 

the gymnasium, the problem was one of determining where 

decay was present and where the wood was sound and did 

not require replacement. It was not necessary to pinpoint the 

decayed areas with great precision but to establish how far 

in from the arch ends that decay had progressed. The repair 

procedure was then to replace those arch ends with structurally sound material.

The primary inspection method used was stress wave timing. Inspectors used two papers containing grids attached to 

each side of the arch as a map for taking stress wave timing 

measurements. The recorded times were then used to determine the extent of decay.

Hoyle, R.J.; Pellerin, R.F. 1978. Stress wave inspection of a 

wood structure. In: Proceedings, 4th symposium on nondestructive testing of wood. Pullman, WA: Washington State 

University: 33–45.

Wood and Timber Condition Assessment Manual • Chapter 8
General Technical Report FPL–GTR–234

86

Peshtigo River Timber Bridge

Structure: Timber bridge

Location: Chequamegon–Nicolet National Forest, Laona, 

Wisconsin

Special Consideration(s): None

Brief history of structure: 

Estimated age: 40 years

Inspection date: 2001

Construction details: Five span, heavy timber 

construction, creosote-treated Southern Pine pilings, 

creosote-treated Douglas-fir pile caps, girders, and 

decking

A detailed inspection of this traditional heavy timber bridge 

was completed to evaluate the quality of individual members as part of the bridge inspection system. Visual inspection, sounding, and moisture content techniques were used 

to identify decay and subsequent carpenter ant damage to 

timber piles on the bridge abutment. The Southern Pine pilings showed shell damage caused by ice and damage due to 

flotsam. The use of a snooper vehicle allowed for detailed 

inspection of individual girders, pile caps, and pilings in 

each of the five spans. Two exterior girders and one pile 

cap with substantial decay were located and mapped using 

sounding, stress wave timing, and moisture content techniques. Confirmation of shell thickness was completed using 

increment coring. Members were identified for replacement.
87

Wood and Timber Condition Assessment Manual • Chapter 8

C.A. Thayer

Structure: Three-masted schooner

Location: San Francisco Maritime National Historical Park, 

San Francisco, California

Special Consideration(s): Inspection was conducted as part 

of a preservation effort

Brief history of structure:

Estimated age: 106 years

Inspection date: 2001

Construction details: Douglas-fir, heavy timber 

construction

The C.A. Thayer was launched in 1895 as a lumber carrier. 

During her working career, the C.A. Thayer made 113 voyages: 13 in the salmon trade, 12 as a cod fishing boat, and 

88 as a lumber carrier. All but a handful of the lumber voyages began and ended at Hoquiam, Washington, on Gray’s 

Harbor. She also delivered lumber to Honolulu, Mexico, and 

the Fiji Islands.

During the summer of 2001, an inspection of the keel and 

mizzenmast of C.A. Thayer was conducted. Visual and 

stress wave techniques were used. The keel was found to 

be sound, with a small area of deterioration just below the 

bowspirit support. Deterioration, mechanical and decay, was 

found in the mizzenmast at the deck line.

References

Ross, R.J.; Kirchner, J.; Wang, X.; Forsman, J. 2001. Inspection of keel and mizzenmast on C.A. Thayer. Research 

Report, USDA Forest Service, Forest Products Laboratory, 

Madison, WI.
General Technical Report FPL–GTR–234

88

Esterhazy Castle

Structure: Esterhazy Castle

Location: Sopron, Hungary

Special Consideration(s): Castle complex was damaged during World War II and was minimally maintained after the 

War

Brief history of structure:

Estimated age: 200+ years, the castle was constructed 

from 1720 to 1800

Inspection date: 2000

Construction details: Traditional European heavy 

timber construction

Rehabilitation efforts for this structure have begun. Its roof 

system was visually inspected in 2000. The traditional mortise and tenon timber framing system was found to be in 

excellent condition. Stains from water exposure and subsequent decay were evident in several locations where timbers 

were in direct contact with masonry.
89

Wood and Timber Condition Assessment Manual • Chapter 8

Grey Towers National Historic 

Landmark

Structure: Mansion

Location: Milford, Pennsylvania

Special Consideration(s):None

Brief history of structure:

Estimated age: 108 years at time of inspection

Inspection date: 1994

Construction details: Timber and masonry

Located in the Pocono Mountains of northeastern Pennsylvania, this mansion overlooks the Delaware River Valley 

and the small town of Milford. Designed by Richard Morris 

Hunt, the chateauesque summer mansion was completed in 

1886.

In 1994, as part of an ongoing restoration effort, an inspection was conducted. Visual, stress wave, and coring 

techniques were utilized. Significant water damage was 

observed in several areas of the mansion. Floor joists supporting the structure’s kitchen were severely degraded. Several joists were so deteriorated that little or no contact was 

observed between them and the masonry foundation.
General Technical Report FPL–GTR–234

90

USS Constitution

Structure: Three-masted battleship

Location: Boston, Massachusetts 

Special Consideration(s): None

Brief history of structure:

Estimated age: 200+ years 

Inspection date: 1991, 1992 

Construction details: Heavy timber, live oak

The USS Constitution, “Old Ironsides,” was launched on 

October 21, 1797. She is the oldest commissioned ship 

afloat in the world. She carries this distinction based on 

the fact that a significant portion of the ship’s underwater 

structure is original. Included in this original material are 

the keel, deadwood, floors, lower planking strakes, and 

a number of lower frame futtocks. During the 1990s, the 

U.S. Navy conducted an intensive assessment of the condition of the ship. The sophistication of these tests ran the 

gamut from signs of deterioration observed through visual 

inspection to instrumented probes and advanced chemical 

analyses. Mechanical and decay deterioration was observed. 

Several members were repaired.

References

Ross, R.J.; Soltis, L.A.; Otton, P. 1998. Assessing wood 

members in the USS Constitution using non-destructive 

evaluation methods. APT Bulletin (The Journal of Preservation Technology). XXIX(2):21–25.

Witherell, P.W.; Ross, R.J.; Faris, W.R. 1992. Using today’s 

technology to help preserve USS Constitution. Naval Engineers Journal. 104(3):124–134.
91

Wood and Timber Condition Assessment Manual • Chapter 8

Mummy Coffin

Structure: Mummy coffin

Location: Nelson–Atkins Museum of Art, St. Louis, 

Missouri

Special Consideration(s): Historically important Egyptian 

artifact

Brief history of structure: 

Estimated age: 2,500 years at time of inspection

Inspection date: 2009

Construction details:Natural paints, timber joinery

The country of Egypt provides us with some of the bestpreserved wood artifacts because of several factors. First, 

despite the paucity of indigenous trees capable of producing wood in sufficient sizes to make large timbers, ancient 

Egyptians used wood extensively in a wide variety of applications. Second, the arid climate in Egypt provides an excellent environment for long-term preservation of wood. As a 

consequence, excavations of various archeological sites in 

Egypt have yielded many well-preserved wood artifacts.

Mummy coffins are the best examples of Egyptian wood 

artifacts. As some of the most precious, ancient pieces at 

the largest museums in the world, mummy coffins provide 

scientists very important information about social, religious, 

and cultural beliefs of ancient Egyptians. This wood mummyform coffin, dated to the first millennium B.C., is part 

of a group of funerary objects acquired by the museum in 

2007 and put on display in a newly designed gallery space. 

The assessment was conducted to provide information on 

the condition of the coffin to museum staff responsible for 

designing and constructing appropriate support mechanisms 

for its display. 

The coffin was constructed from sycamore fig trees, Ficus 

sycomorous. This tree species was known as the “tree of 

life” in ancient Egypt and was cultivated commonly. Visual 

inspection and stress wave techniques were used to assess 

the condition of the upper and lower portions of the coffin. 

Much of the wood was solid, with no deterioration present. Estimates of several important properties (modulus of 

elasticity, modulus of rupture, compression parallel to grain) 

were made.

Reference

Ross, R.J.; Dundar, T. 2012. Condition assessment of 2500 

year old wood coffin. Res. Note FPL-RN-0327. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory: 3 pp.
General Technical Report FPL–GTR–234

92

Steam Schooner WAPAMA

Structure: Wooden steam-powered schooner

Location: California State Historical Maritime Park, 

California

Special Consideration(s): Last surviving example of wooden 

steam-powered schooners designed for the 19th- and 20thcentury Pacific Coast lumber trade and coastal service

Brief history of structure: 

Estimated age: 91 years at time of inspection

Inspection date: 2006

Construction details: Built almost entirely of oldgrowth Douglas-fir timber, approximately 217 ft long 

and 50 ft from keel to house top, with a gross tonnage 

(or internal volume) of 945 GT. Its construction is 

unique in its use of sister frames and lack of steel strapping. The hull is single decked and characterized by a 

plum stem, full bows, straight keel, moderate deadrise, 

and an easy turn of bilge. 

A variety of nondestructive test methods were employed to 

locate problem areas and define the severity of deterioration 

on key structural members such as keelsons, keel, ceiling 

planking, hull frames, clamps, and main deck beams. Mechanical and decay-related deterioration was found in several members. They were subsequently replaced.

References

Wang X.; Wacker, J.P.; Ross, R.J.; Brashaw, B.K. 2008. 

Condition assessment of the main structural members of 

historic steam schooner Wapama. Res. Pap. FPL–RP–649. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.