Forest

Products

Laboratory

General

Technical 

Report

FPL–GTR–228

In cooperation

with the

United States

Department of

Transportation

Federal

Highway

Administration

United States Department of Agriculture

Forest Service

Species Identification and 

Design Value Estimation 

of Wooden Members in 

Covered Bridges

Alex C. Wiedenhoeft

David E. Kretschmann
Abstract

Covered timber bridges are historic structures with unique 

aesthetic value. To preserve this value and maintain bridges 

in service, robust evaluation of their performance and safety 

is necessary. The strength of the timber found in covered 

bridges can vary considerably, not only because of age 

and condition, but also because of species and grade.  For 

the practicing engineer or inspector evaluating a covered 

bridge, design values for existing wood members must be 

determined with as much accuracy as possible by accounting for bridge-specific conditions. The current practice is to 

assign wood values for strength and moduli from existing 

specifications without specific reference to timber species, 

condition, or grade, which can result in an inaccurate assessment of the structure. This manual provides a walk-through 

for basic wood identification, inspection, condition assessment, field grading, and design value estimation for wood in 

covered bridges.

Keywords: field grading, species identification, covered 

bridges, inspection, wood anatomy, condition assessment, 

design value estimation

June 2014

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.

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

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

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

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

United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin.

The use of trade or firm names in this publication is for reader information and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

The USDA prohibits discrimination in all its programs and activities on the 

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

sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or a part 

of an individual’s income is derived from any public assistance program. 

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Avenue, S.W., Washington, D.C. 20250–9410, or call (800) 795–3272 

(voice) or (202) 720–6382 (TDD). USDA is an equal opportunity provider 

and employer.

This study is part of the Research, Technology and 

Education portion of the National Historic Covered 

Bridge Preservation (NHCBP) Program administered by the Federal Highway Administration. The 

NHCBP program includes preservation, rehabilitation and restoration of covered bridges that are listed 

or are eligible for listing on the National Register of 

Historic Places; research for better means of restoring, and protecting these bridges; development of 

educational aids; and technology transfer to disseminate information on covered bridges in order to 

preserve the Nation’s cultural heritage.

This study is conducted under a joint agreement 

between the Federal Highway Administration–Turner 

Fairbank Highway Research Center, and the Forest 

Service–Forest Products Laboratory. 

Federal Highway Administration Program Manager—

Sheila Rimal Duwadi, P.E. 

Forest Products Laboratory Program Manager—

Michael A. Ritter, P.E.
U.S. Department of Agriculture ■ Forest Service ■ Forest Products Laboratory

Species Identification and 

Design Value Estimation 

of Wooden Members in 

Covered Bridges

Alex C. Wiedenhoeft

David E. Kretschmann
Contents

Introduction...................................................................................................................... 1

Background....................................................................................................................... 1

Objective and scope of the manual.................................................................................. 1

1 | Wood identification and pattern recognition............................................................. 3

2 | Basic wood biology—anatomy for identification....................................................... 5

3 | Correct use of a hand lens.......................................................................................... 11

4 | Wood specimen preparation for hand lens observation.......................................... 13

5 | Basic characters used in the identification of wood with a hand lens..................... 17

6 | Identification key ....................................................................................................... 25

7 | Species description pages.......................................................................................... 29

8 | Additional information about wood identification.................................................. 55

9 | Estimation of size and grade of members................................................................. 57

10 | Condition assessment of bridge members.............................................................. 75

11 | Design value estimation........................................................................................... 79

References....................................................................................................................... 93
Species Identification and Design Value Estimation of 

Wooden Members in Covered Bridges

Alex C. Wiedenhoeft, Research Botanist

David E. Kretschmann, Research General Engineer

Forest Products Laboratory, Madison, Wisconsin

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. An inspection 

should determine the species, evaluate the condition of the 

member, and determine an estimate of the grade before estimating the capacity of the members and thus allow for the 

estimation of the safe loading conditions for the bridge. This 

evaluation process is complicated, and even with the best 

available information, can require considerable engineering 

judgment.

Objective and scope of the manual

The objective of this publication is to provide a field inspection manual for covered bridge inspection engineers that 

addresses the topics of wood identification, grading, condition assessment, and a possible method of design value estimation for wood members used in covered timber bridges.

The continued use of covered bridges is dependent on 

an affirmation that they meet appropriate strength thresholds given their 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. For this 

reason, this manual addresses 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. It is intended for use by professionals who, though 

lacking the formal background in wood science, are committed to mastering the skills and information necessary to 

arrive at a reliable result. Each section presents the level of 

detail needed to make a valid estimate. The content will also 

empower the user to make a judgment about the advisability of securing the services of a consulting engineer—if the 

reader is comfortable with using it, this manual provides the 

user with what is needed. If a consulting engineer is needed, 

the reader will be able to more easily communicate with 

them about the problems and concerns found in the bridge.

Introduction

To extend the service life of historic covered bridges in 

the United States, methods for accurately estimating the 

strength and safety of the wooden structural members 

are necessary. In the absence of dynamic loading or other 

experimental load assessment data, performance estimates 

must be based on the specific details of a bridge’s construction. There are three critical steps needed to make a useful 

estimate: (1) determine the woods used, (2) estimate the 

grade of wood used, and (3) assess the condition of the 

timber being graded. With accurate information, an engineer or other professional can make determinations about 

maintenance, repair, restoration, or decommissioning the 

structure. If accurate information is not available, historic 

bridges may be prematurely replaced. Engaging a skilled and 

experienced inspector familiar with wood identification, 

timber grading, and condition assessment is best, but that 

is not always possible. This field guide describes the most 

critical considerations when examining a historic covered 

bridge and can empower an inspector to make an informed 

judgment about the need for further, detailed analysis of the 

bridge in question.

Background

When a new wooden bridge is constructed, a structural 

engineer relies on the current design values for a known species and the local building codes to determine the appropriate size and grade of timber required for particular service 

loads. The strength of timber found in historic covered 

bridges 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. 

Most historic wooden bridges 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 

3

1 | Wood identification and pattern recognition

Wood identification is a combination of art and science. 

Whereas the bulk of the wood identification section of this 

manual focuses on the scientific characteristics used to make 

accurate field identifications of wood, the contribution of 

the artistic component to the identification process should 

be neither overlooked nor understated. There is nothing 

specific you must do to train yourself in the artistic aspects 

of wood identification; your mind will do this automatically. 

The scientific aspects, on the other hand, are not as easy as 

the artistic portion of the process and will come only with 

diligent study, practice, and focus.

Wood identification is a process of pattern recognition. 

The patterns you will learn to recognize are patterns of 

wood structure; that is, specific cells in specific arrangements, many of which have specific terminology associated 

with them. At first the terminology may be daunting, but 

with practice the terms will take on meaning. 

The process of identifying a wood specimen is a microcosm of the process of learning to identify any object. It is 

very similar to learning to identify anything—wood, birds, 

chairs, people. All these identifications are governed by 

the same processes of cognition resulting in recognition of 

a pattern (Figs. 1.1, 1.2). Identification begins with careful 

observation, proceeds to mental comparison with similar woods, and then makes reference to a formal means 

of confirming an identification (e.g., reference to images 

in this manual or a small collection of correctly identified 

specimens). 

To be able to gather information about the characters in 

a piece of wood, you must understand the basic biology of 

wood; use the correct tools; be able to prepare the wood for 

observation; and make careful observations of wood characters. These characters are employed in an identification key, 

and then from the key the user is directed to species pages 

that provide more detailed information for confirming an 

identification.

Before the topics of using a hand lens, preparing wood for 

observation, and understanding the characters used in wood 

identification can be tackled, a general introduction to the 

biology of wood is needed. The discussion of wood biology 

is restricted to trees here, though much of what is presented 

is also true of shrubs and lianas.

The treatments of wood structure and wood identification 

in this manual are fairly rigorous and will provide the foundation necessary to identify not just the common woods 

found in North American covered bridges but also tropical 

woods, though the latter are not dealt with in the identification key presented here. Additional references for wood 

identification are presented in Chapter 8, so if you wish to 

extend your knowledge of wood identification, the fundamentals in this manual will set you on a firm path.

Figure 1.1—We use pattern recognition in all aspects of our daily 

lives. All three of these objects are chairs, despite the differences in 

materials and design.

Figure 1.2—The process of learning to 

distinguish identical twins is similar to the 

process of learning to identify wood.

5

It is not a simple matter to organize the gross structure and 

biology of wood into an easy-to-understand text. It may be 

necessary to move back and forth between some concepts 

in this chapter in an effort to present a unified concept of 

wood structure for wood identification. Some sections of 

this chapter will be difficult to understand fully the first 

time through; read it a second time after thinking about the 

material, and the once-unfamiliar concepts will be clearer. 

Keep in mind that in addition to permitting us to identify wood, the cells and their characteristics give wood its 

physical and mechanical properties such as stiffness. Other 

features of timber that influence strength and performance, 

such as knots, are also biologically derived. The condition 

of a timber is often affected by external biological agents 

such as decay fungi or insects, and the ability of a timber to 

resist such attack is likewise founded on the biology of the 

tree from which the wood came. The distance between the 

details of the anatomical structure of wood and the ultimate 

determination of the load capacity of a timber bridge is not 

as great as it might seem at first.

2.1 Wood as a cellular material

Wood, like all living or once-living materials, is made from 

millions and millions of interconnected cells that form a 

larger structure, in much the same way your heart or liver 

is made of many different cells. Cells are the basic building 

blocks of larger organisms, and most of the identification 

sections of this manual focus on understanding different 

types, shapes, orientations, and groups of cells in wood. 

2 | Basic wood biology—anatomy for identification

2.2 Two systems of cells in wood

Wood is made of two interlocking systems of cells, the axial 

system and the radial system (Fig. 2.1). The axial system is 

all the cells running up and down the trunk of the tree or 

along the grain of a board. The radial system is made up of 

rays, groups of cells running from the center of the tree out 

toward the bark. Rays and the radial system in general are 

less familiar to the casual observer of wood but are critical 

to understanding and identifying wood. Because tree trunks 

are generally round, the cells in the axial system are lined up 

in overlapping, curved layers.

2.3 Heartwood, sapwood, and pith

When looking at a cut stump or a disc from a tree (Fig. 2.2), 

there are two basic domains in most woods: the central typically darkly colored (and sometimes odiferous) area called 

heartwood, and the typically lighter colored outer strip of 

wood under the bark, the sapwood. There is no appreciable 

structural difference between heartwood and sapwood; the 

difference is one of accumulated biochemicals in the heartwood. The center of the tree is called the pith; this is the 

tissue formed before the first wood is made by the tree.

2.4 Growth rings

Wood is formed of discrete layers called growth rings. They 

represent the basic building blocks of wood structure that 

span the microscopic, cellular nature of wood and that 

which can be seen with the naked eye (Fig. 2.3). A growth 

ring is all the wood formed during one period of growth, 

Axial system Axial system

Rays

Radial system

Figure 2.1—Axial and radial systems in wood. The diagram on the left shows a simplification of the axial and radial 

systems. The photograph on the right shows a decayed stump in which much of the axial system has been removed by 

fungi. The card-like tabs protruding are the rays of the radial system.

Rays

5
6

Heartwood Sapwood

Pith

Figure 2.2—The end grain of a log. Bark surrounds 

the log. Inside this is the wood. The light colored 

wood is sapwood, the dark colored wood is heartwood. The small dot representing the center of the 

tree is the pith.

for our purposes one year, that can 

be distinguished by some anatomical characteristic from the wood 

formed during a different growth 

period. When you look at the end 

grain of a board or the cut end of a 

log or a stump, the concentric circles 

you can see are most likely growth 

rings. The portion of a growth ring 

formed at the beginning of the growing season is known as earlywood, 

and it is found closer to the pith. The 

portion of the growth ring formed 

later in the season is called latewood, 

and it is found toward the outside of 

the tree (this is discussed in greater 

detail in Chapter 5). The rays of the 

radial system appear as lines perpendicular to the growth rings when the 

end grain of a board or a stump is 

examined.

2.5 The planes of section

The relationship between the cells of the axial system and 

the rays of the radial system, the round shape of a tree trunk, 

and the concentric nature of growth rings around the trunk 

combine to give us three primary ways to view a piece of 

wood. These three views are known as planes of section, 

and understanding them is one of the most difficult aspects 

of, but also absolutely necessary for, wood identification.

There are two fundamental ways to cut wood: across the 

grain and along the grain. When we cut wood, whether to 

view it with a hand lens or to produce a board, the easiest 

cut to understand is across the grain. This is the transverse 

cut that loggers make when they fell a tree. This cut is perpendicular to the trunk and thus across the grain of the 

wood—across the axial system (Fig. 2.4). The surface you 

see once this cut is made is the transverse plane of section 

Figure 2.3—Growth rings in a softwood and a hardwood. Although the structure of these two woods is 

quite distinct, both have growth rings and the growth rings can be divided into earlywood and latewood. 

Softwood Hardwood

Growth ring

Growth ring

Latewood Latewood

Earlywood

Earlywood

or transverse surface, also known as 

the end-grain of the wood. On the 

transverse surface we can see the 

curving nature of the growth rings 

and the way in which the rays of 

the radial system run perpendicular 

to that curvature; the rays appear 

as straight lines running from the 

center of the tree out toward the 

bark. To understand the other two 

planes of section, we must understand this relationship between rays, 

growth rings, and the round form of 

a tree trunk.

The second fundamental way to 

cut wood is along the grain; doing 

this produces a longitudinal cut 

and exposes a longitudinal surface. 

Although it is possible to cut along 

the grain in any orientation with 

respect to the rays and growth rings, 

the structure of wood shows us that there are two basic 

ways to cut it, either parallel to the rays, or perpendicular to 

them. Because the rays and growth rings are perpendicular 

to each other, we can cut perpendicular to the growth rings, 

or parallel to them. Not all woods, however, have obvious 

growth rings, so we will orient ourselves by speaking of the 

cut in reference to the orientation of the rays. 

When we cut along the grain and parallel to the rays, 

we produce a radial surface or radial plane of section. This 

exposes the rays like lines running across the surface and is 

little used in the hand lens identification of wood. If a round 

piece of firewood is split exactly in half, from one side to 

the other through the center of the tree, the flat surface is a 

radial surface.

The longitudinal cut perpendicular to the rays is the tangential plane of section, or tangential surface. This exposes 

the cut ends of the rays and allows us to understand the size, 
7

spacing, and arrangement of the rays. This is the view of 

wood we would have if we could walk up to a standing tree 

and tear away the bark to view the wood.

We can use two analogies to clarify these ideas: geometry 

and apple pie (Fig. 2.4). In the geometry example, we draw 

a circle. Any line from the center of the circle to the edge of 

the circle is a radius of that circle. The radius in geometry is 

the ray in wood (and indeed in the source of the name term 

“ray”). A line that touches a circle at one point is called a 

tangent line, and any radius of a circle that intersects with a 

tangent line is perpendicular to it at that point, and parallel 

to the edge of the circle at that point. The edge of the circle 

represents a growth ring, the radius a ray, and thus if we are 

perpendicular to the ray, we are parallel to the growth ring. 

By extending this two-dimensional circle geometry to a 

three-dimensional cylinder and extending either the radius 

or the tangent line up and down the cylinder, we produce a 

radial or tangential plane of section, respectively. 

The second analogy is more accessible. If we start with a 

freshly baked, delicious apple pie, we have a round object. 

When we cut the pie to share it with our family, we make 

cuts from the edge of the pie into and across the center until 

we have a number of wedge-shaped pie pieces. Each pie 

piece shows all three planes of section. The top and bottom 

crusts of the pie are the transverse surfaces. The exposed 

apple filling along the two cut edges are radial surfaces, and 

the outer, curved portion of crust that was against the pie 

pan is a curving tangential surface. 

We identify people we know mostly by facial characteristics. Hand-lens wood identification uses primarily features 

on the transverse surface, thus it can be said to be the “face” 

of wood. For this reason, any reference to the appearance of 

a character will be on the transverse surface unless otherwise stated. We will do little or nothing with the radial or 

tangential surfaces. One way to perfect your understanding is 

to make careful three-dimensional drawings of the relations 

between the planes of section without looking at the figures. 

When you can explain the planes of section to someone else, 

your understanding of planes of section is complete.

2.6 Softwoods and hardwoods

The topics we have covered thus far in this chapter are true 

of all woods in the manual, regardless of species or origin. 

The remainder of the chapter is devoted to drawing some 

fundamental distinctions between different kinds of woods 

and understanding the anatomical and cellular bases for their 

differences.

There are two fundamental types of wood: softwoods and 

hardwoods (Fig. 2.5). Softwoods come from needle-leaved 

and cone-bearing trees like pines, spruces, firs, and cedars. 

Hardwoods come from broad-leaved and flower-bearing 

trees like walnut, oak, ash, and elm. The distinction between 

hardwoods and softwoods is thus a botanical one, and in 

the forest can most easily be seen by differences in external 

characteristics like leaf shape, the nature of the reproductive 

structures (cones vs. flowers), and the architecture of the 

tree itself. In addition to these external features, the cells that 

comprise the wood of softwoods and hardwoods are quite 

different structurally. Thus, hardwoods and softwoods are 

easily distinguishable from each other in the process of wood 

Figure 2.4—The planes of section illustrated three different ways. On the left is a photograph of a small log that has been cut across the grain to 

expose the transverse surface and cut along to grain to expose the radial and tangential surfaces. On the upper right is an illustration showing the 

relationship between radial and tangential planes in a simple cylinder. On the bottom right, a piece of apple pie shows the three planes of section. 

The top and bottom of the piece of pie represent transverse surfaces and the cut sides represent radial surfaces. The curved outer crust of the pie 

represents the tangential surface.

Transverse surface

Tangential surface Radial surface

Tangential surface

Radial surface

Transverse surface

Radial plane

Transverse plane Radius

Tangential plane

Tangent line
8

identification. Though softwoods and hardwoods are structurally distinct (as will be explained in detail below) both 

are still wood. They have axial and radial systems, heartwood and sapwood, growth rings, and can be cut to produce 

transverse, radial, and tangential planes of section. Cells in 

wood are either dead at functional maturity (one type of cell 

in softwoods, two types of cells in hardwoods) or are alive 

at functional maturity. In both softwoods and hardwoods, 

these living cells are called parenchyma cells. Parenchyma 

cells can be a part of the axial system of both softwoods and 

hardwoods, and make up virtually all the cells in the radial 

system (that is, rays are composed of parenchyma cells).

2.7 The specific cellular anatomy of softwoods

The cellular anatomy of softwoods is simple compared to 

that of hardwoods (Fig. 2.6). The axial system is formed 

predominantly of one kind of cell, the tracheid. With a hand 

lens, tracheids appear as small boxes or rectangles. In species with obvious growth rings, the earlywood tracheids are 

typically thin-walled and the open spaces in the cells, the 

lumina, can be seen as dark centers. In such species, the cell 

walls of the latewood are much thicker, and often the lumina 

of the cells cannot be seen. The second and less prevalent 

cell type in the axial system of softwoods is axial parenchyma, either as scattered solitary cells, or forming special 

structures called resin canals. Resin canals are discussed 

later as an important character for the identification of 

Softwoods Hardwoods

Figure 2.5—Two examples of the transverse surface of softwoods and two examples of hardwoods. On the 

left, top and bottom, are two softwoods, as well as a silhouette of a generic softwood tree. On the right, top 

and bottom, are two hardwoods, as well as a generic hardwood silhouette. 

softwoods. The radial system, as in all woods, is composed 

of ray parenchyma cells. It is not possible to see individual 

softwood ray parenchyma cells with a hand lens, but the 

rays they compose are faintly visible as straight lines running perpendicular to the growth rings. Rays in softwoods 

are generally only one cell wide, and this is why they appear 

only faintly.

2.8 The specific cellular anatomy of hardwoods

The cellular wood anatomy of hardwoods is more complex 

than that of softwoods (Fig. 2.6) because there are three 

main cell types in the axial system: vessels, axial parenchyma, and fibers. Vessels are the characteristic cell type of 

hardwoods; all hardwoods in the manual have vessels, and 

no softwoods in the manual have them. Thus, the presence 

of vessels is definitive proof that a wood is a hardwood. Vessels are dead cells specialized for water conduction and are 

generally much larger and rounder than tracheids in softwoods. They appear as round pipes or holes in the wood, 

and understanding the variation in their structure, size, distribution, and relative proportions is a major component of 

hardwood identification.

Axial parenchyma cells in hardwoods are, as with all 

parenchyma cells, living cells when functioning in the tree. 

Axial parenchyma in hardwoods is much more common, 

and its distribution much more complex, than in softwoods. 

Axial parenchyma is generally only visible with a hand lens 
9

Softwoods

Tracheids

Resin canals

Rays

Rays

Fibers

Hardwoods

Vessels

Figure 2.6—Anatomical differences between softwoods (top) and hardwoods (bottom). Softwoods are composed mostly 

of tracheids, which can be thin-walled in the earlywood and thick-walled in the latewood, and rays running vertically 

in the image on the right. The illustration on the left shows more detail and is drawn at a higher magnification than the 

hand-lens view of the transverse surface of a softwood on the right. Shown in both images are resin canals. Hardwoods 

are structurally more complex than softwoods, with vessels, fibers, axial parenchyma, and rays. The vessels are the dark 

holes scattered throughout the wood. The rays are the lines running vertically in the image. The axial parenchyma in this 

wood appears as light-colored, round halos around the vessels. The individual fibers are too small to be seen, but together 

form the brown tissue in which the other cells are embedded. As with the softwood above, the illustration on the left 

is presented at a higher magnification than the hand-lens view of the transverse surface on the right. Note that in this 

hardwood, there is no clear distinction between earlywood and latewood. This is discussed in more detail in Chapter 5.
10

when it occurs in groups of many parenchyma cells. It is 

generally seen as a lighter colored tissue compared to the 

other cells in the wood. Hardwoods present a great range 

of axial parenchyma patterns and abundance, ranging from 

barely visible with a microscope to easily seen with the 

naked eye. Along with variations in vessel characteristics, 

the amount, distribution, and type of axial parenchyma 

present in a wood is one of the major sets of characters used 

in hardwood identification.

Fibers are dead, thick-walled cells and are not usually 

individually visible with a hand lens, but in any given hardwood, groups of fibers usually form a large percentage of 

the visible cells. Though it is an awkward way to describe 

their appearance, fibers are most easily seen and understood 

by subtraction; if you are looking at cells of the axial system 

that are not vessels or axial parenchyma, they are fibers. 

The radial system of hardwoods is composed of ray parenchyma, and like softwoods, it is usually not possible to 

discern individual ray parenchyma cells. Unlike softwoods, 

hardwoods have a tremendous variety of ray widths, ranging 

from rays one cell wide and barely visible (as in softwoods) 

to rays more than 60 cells wide, and easily seen at a distance 

with the naked eye.

2.9 Understanding scientific names

From this point forward, this manual refers to woods using 

the trade names listed in the National Design Specification 

(NDS) Supplement Design Values for Wood Construction

(AWC 2012). The use of a standardized list for American woods simplifies the use of this manual in conjunction 

with the NDS Supplement. Ultimately, however, the NDS 

Supplement trade names refer back to woods whose official 

names are scientific names.

Scientific names are made of two words, a genus name 

and a specific epithet. Both the genus and the specific epithet are italicized or underlined when printed. The genus 

name is the first word of the two and is always capitalized. 

The specific epithet is the second word of the two and is not 

capitalized. In scientific names, the genus name (or genus) is 

like a surname; it designates to the group to which the species most closely belongs. The specific epithet is like a given 

name (first name), distinguishing that species from all others 

in the genus. 

For ponderosa pine, the scientific name is Pinus ponderosa. The word Pinus is the genus name, and the word 

ponderosa is the specific epithet. There are many other species in the genus Pinus, including P. taeda (loblolly pine), 

and P. strobus (eastern white pine). Note that the genus 

was abbreviated by placing a period after the first letter and 

omitting the rest of the word. This is done only after the 

genus has been spelled out already, and is a convention that 

saves space. Note also that it did not say “There are many 

other species in the genus Pinus including, taeda and strobus.” Although it would have been clear that the subject was 

a species of Pinus, in other cases such a shortcut might lead 

to confusion in much the same way using only a given name, 

John, is not specific enough to distinguish him from all the 

other people with that given name. If his surname were a 

common name such as Smith, there would likely be hundreds or thousands of other people named John Smith in 

the world. Such confusion is prevented in scientific names 

by a simple rule; the same combination of genus name 

and specific epithet may not be used to refer to more than 

one species. Thus, there is only one Pinus ponderosa in the 

world, and its scientific name can never be confused with 

that of any other species. 

Learning to use scientific names correctly empowers you to communicate precisely. Sometimes the group 

to which a wood belongs is between the level of species 

and genus. This is common in genera (the plural of genus) 

with many species, or in genera where the wood of different species is dramatically different. For example, the 

genus Pinus has many species scatted across the northern 

hemisphere and different groups of species have different anatomical characteristics. For example, Pinus taeda is 

a species that belongs to the yellow pine group, and so its 

identity would be reported as Pinus sp., yellow pine group, 

because though it can be determined that it is not a member 

of the red pine group, the exact species within the yellow 

pine group cannot be determined. There is no special or 

convenient scientific word to use, so the scientific name 

is combined with additional information written in plain 

language to indicate the proper level of specificity in the 

identification.
11

3 | Correct use of a hand lens

A hand lens is a powerful 

tool for the identification 

of wood, but like all tools 

it must be used correctly 

to take full advantage of its 

powers. The hand lens has 

two main parts (Fig. 3.1), 

a lens that magnifies the 

object of interest (generally we use 10× or 14× 

lenses in wood identification; a 14× lens is recommended 

for use with this manual) and a housing to hold and protect 

the lens. Unlike a microscope that is able to focus at different distances depending on the specimen and the configuration of the various lenses, thus requiring much adjusting, a 

hand lens has a fixed focal length. There is only one distance between the lens and the object that will produce a 

sharp image. This distance is easily found by examining a 

coin or other familiar object with the lens, but is generally 

only 1–4 cm between the lens and the specimen. Most hand 

lenses have no “front” 

or “back” to them; 

you can look through 

either side of the lens 

at a specimen.

Because there is 

only one distance 

between the lens and 

the object that will 

produce a good image, 

the remaining variable is the distance 

between the lens and 

your eye. For the best 

results, the hand lens 

should be placed as 

close to your eye as 

possible (Fig. 3.2); it is 

common for your eyelashes to brush against 

the lens when it is used correctly. Many people find this 

unnatural at first, but it is essential that this close distance 

be used. Before trying to look at a specimen of wood, use 

the lens to examine common objects: the cloth of a shirt, 

the print in this manual, a fingernail, or other things of 

interest. Force yourself to keep the lens close to your eye, 

and practice adjusting the distance between the lens and the 

object. At first, it will probably be helpful to close your other 

eye, but after a few minutes of examining common objects, 

practice using the lens with both eyes open. Although this 

can be difficult for many people, it is a good skill to practice 

because it will reduce eye strain. 

It can be difficult to ensure that enough light reaches the 

specimen when using a hand lens; your hands and head will 

block light and shade the specimen (Fig. 3.3), making it hard 

to observe details. It is tempting to move the lens away from 

your eye at this point so that more light falls on the object. 

Do not do this. Instead, you must reorient your body so 

that light falls on the specimen. Sometimes this process of 

finding sufficient lighting requires you to turn to a different 

angle, bend at the waist, or take other action to find ample 

light. If you are identifying wood in the field, it may be necessary to remove a small specimen and move out from the 

shade of the structure into the direct sunlight. Although this 

may seem like a minor point, taking pains to secure good 

lighting has several benefits. First, it allows the hand lens to 

perform to its full capacity as a tool. Second, it ensures that 

the object, ultimately an unknown wood specimen, can be 

observed in sufficient detail to permit identification. Last, 

it reduces eye strain, allowing you to gain more experience 

and practice wood identification more comfortably.

Lens

Housing

Fig. 3.1—A hand lens has two main 

parts, the lens and the housing.

Figure 3.2—Using a hand lens to observe a wood specimen. On the left, the distance between the lens and the 

observer’s eye is much too great, but the distance between the lens and wood specimen is correct. The top photo 

in the center shows the correct distance between the lens and the wood specimen. The bottom photo in the center 

shows the correct distance between the lens and the eye. The photograph on the right shows the correct distances 

between the lens, the specimen, and the eye.

Figure 3.3—On the left, the observer’s head is blocking the light, 

and the specimen cannot be observed properly. On the right, 

the observer is allowing light to fall on the specimen to ensure 

accurate observation.
12
Figure 4.1—Slicing a tomato illustrates the principles of cutting wood: (a) Pushing a sharp knife 

downward through a tomato crushes it. (b) Sawing back and forth through the tomato makes 

uneven slices (c). (d) Passing the knife through the tomato with one even stroke maximizes the 

cutting ability of the blade and produces clean slices (e). (f ) Pushing the blade into the thumb 

does not cut the skin.

a

b

c

d

e

f

13

4 | Wood specimen preparation for hand lens observation

The single most difficult physical skill involved in wood 

identification is producing a smoothly prepared surface for 

observing anatomical features. This skill must be practiced 

patiently; it takes time to become proficient at this task. 

Producing a cleanly cut surface is also the only appreciably dangerous aspect of wood identification with a hand 

lens; the tools used to cut the wood are necessarily sharp, 

and injuries from minor cuts to major lacerations can result 

from carelessness, fatigue, or poor technique. Safety must 

always be the first priority, followed closely by the secondary importance of making a serviceable cut of the wood. 

Because this skill is both necessary and inherently dangerous, patience and prudence must be exercised while learning. Before trying to cut a specimen of wood with these 

techniques, please read and understand this entire chapter.

4.1 The principles of cutting

A few basic principles apply to cutting any material, and you 

are likely already familiar with them from everyday experience. The first and most important principle is the idea 

of drawing the edge of your cutting tool across the surface 

you intend to cut. One of the easiest ways to think about 

this is in terms of slicing a tomato (Fig. 4.1): if you try to 

push the edge of a knife straight through the tomato, even 

the sharpest knife will crush the tomato rather than make a 

nice slice (Fig. 4.1a). If you saw back and forth many times 

with the knife edge as you slice the tomato, you produce 

ragged and uneven slices (Fig. 4.1b,c). The best technique 

is to draw the edge of the knife a single time through the 

tomato as the downward motion is applied (Fig. 4.1d). 

This gives you the maximum cutting ability of the edge of 

the knife and produces clean, even slices (Fig. 4.1e). If I 

place the edge of the same knife on my thumb with slight 

pressure, it does not cut me (Fig. 4.1f ). If I slide the edge 

of the knife along my thumb under the same pressure, I 

will be cut. When we cut wood to view it with a hand lens, 

we must draw the edge of the cutting tool across the area 

we are preparing; no other technique will suffice.

A second principle of cutting has to do with the relationships between the blade and the material to be cut. 

Harder materials generally require a steeper cutting angle 

than softer materials (Fig. 4.2). Harder materials generally 

require sturdier blades to cut them as well. For example, 

you can shave hair with a thin razor, but to cut copper 

wire, you will use much thicker wire cutters. To prepare 

wood for observation with a hand lens, we will use the 

utility knife (Fig. 4.3). The harder the wood, the steeper 

the cutting angle, and the softer the wood, the shallower 

the angle. For woods in covered bridges, the utility knife is 

the necessary tool. 
14

4.2 Holding the knife

To cut wood safely and effectively, it is necessary to hold 

the knife correctly. The safest and most powerful method 

of cutting wood might contradict what you have been told 

about using a knife. Most people are taught to cut away 

from themselves, to minimize the chances of a laceration. 

For surfacing wood specimens, you will cut toward yourself, but you will do it in a controlled and failsafe way that, 

if done correctly, makes it impossible to cut yourself. This 

is accomplished by using the correct knife grip (Fig. 4.4). 

Begin with an open hand, thumb extended, fingers held 

loosely together. Lay the knife in the palm of your hand 

such that the base of the blade (where it joins the handle) 

is close to but not in contact with the web of skin between 

your thumb and index finger, with the edge of the blade 

facing your thumb (Fig. 4.4a,b). If the edge is extended too 

far from your hand, you will lose the force needed to make a 

good cut (Fig. 4.4c); if the edge of the blade is touching your 

skin, you will cut yourself badly (Fig. 4.4d) Now, taking care 

that the knife does not shift either closer to or farther from 

the web of skin between the index finger and the thumb, 

Figure 4.3—The preferred basic tool for cutting 

wood, utility knives will make clean cuts of all 

but the softest woods. 

Figure 4.2—Illustrations of cutting angles for hard, dense 

woods and softer woods. On the left, the angle of the knife to 

the wood is high because the wood is dense, and the thicker 

blade is needed. On the right, the angle of the knife is low 

because the wood is light and soft, and the thinner, sharper 

blade is needed.

Figure 4.4—Holding the utility knife safely. (a) Begin with an open hand, and orient the utility knife as shown. (b) Close the fingers and wrap the 

thumb over the fingers; this is the basic grip. Note that the base of the blade is not touching the web of skin between the index finger and the thumb 

(arrow). (c) The distance between the blade and the hand is much too large and it will be difficult to have the strength to make a clean cut of a wood 

specimen. (d) The blade of the knife is in contact with the skin between the index finger and the thumb (arrow), and will cut the skin. 

a b c d

close your fingers firmly over the handle of the knife. Wrap 

your thumb loosely over your fingers. This is the basic grip. 

When it is time to cut a specimen, you will use your thumb 

as a fulcrum to pull the knife through the specimen by closing your fist tightly. 

▶ DO NOT try to cut a wood specimen yet.

4.3 Holding the specimen

With the knife occupying your dominant hand, your other 

hand must hold the specimen. There are two simple rules 

for holding the specimen. First, you must grip the specimen quite firmly, as it takes considerable force to produce 

a cleanly cut surface on a wood specimen. Too loose a grip, 

and the specimen will sail from your grasp and you might 

cut yourself. Second, keep all parts of your hand away from 

the path of the blade; do not in any way hold the specimen 

near the area you intend to cut (Fig. 4.5). All body parts 

must be well away from the cutting area or shielded on the 

leeward side of the specimen to have a safe and effective 

grasp on the specimen.

▶ DO NOT try to cut a wood specimen yet.

4.4 Choosing a specimen

When working in the field, you will likely not have much 

control over the size and shape of wood specimen you must 

identify, but when you are practicing these techniques, wise 

choice of specimens is critical. Select a specimen that is 

approximately the length and width of your hand. The thickness of the specimen should be between 1 and 3 cm; any 

thinner and it may flex or break, any thicker and it will be 

difficult to hold firmly.

▶ DO NOT try to cut a wood specimen yet.

4.5 Orienting the specimen

In addition to questions of knife angle outlined in the section on the principles of cutting, you must also orient the 

edge of the knife relative to the specimen to produce a precise plane of section. If you are cutting a transverse surface, 

the edge of the knife must cut through the specimen perpendicular to the axial system, not merely perpendicular to 
15

knife through the wood, starting your cut with the base of 

the blade at the farthest point from your thumb, and pulling 

the knife toward your hand by firmly making a fist (Fig. 4.7). 

The exposed portion of a utility knife blade is approximately 2 cm long, so your total cutting motion as you draw 

the knife through the wood will only be maybe 3–4 cm in 

length, because of the angle of the blade and the way you 

pull it though the specimen. This is not a large motion, and 

it should be done smoothly and confidently. You will make 

a comparatively small surface, especially at first. This is 

acceptable because the surface is meant to be viewed with 

your hand lens, and this will make the area seem much 

larger.

The first few times you try this you will almost certainly 

fail. As long as your failure does not involve blood, it is actually a success, as failure is a part of the learning process. If 

your knife angle is too steep, the blade will bury in the specimen. If it is too shallow, the edge of the knife will not bite 

into the wood. Practice thoughtfully, rereading the chapter 

as needed to understand what went wrong. If the edge of the 

the surface of the specimen (Fig. 4.6a,b); if you are out of 

plane by more than one or two degrees, the anatomy of the 

wood will take a stretched appearance and not be interpretable (Fig. 4.6c,d). Likewise, if you are making a tangential 

surface, the edge of the knife must be perpendicular to the 

rays as seen on the transverse surface, and the edge must cut 

evenly along (and not across at an angle) the grain. In either 

case, careful orientation of the specimen and the knife is 

critical. 

Consider a section of plastic pipe: if you make a true 

transverse cut through the pipe, the end of the pipe will 

appear as a perfect circle, but if you cut at some angle 

through the pipe, the opening in the end of the pipe will be 

elliptical (Fig. 4.6e). Because the cells in wood are essentially tiny pipes, the orientation of the knife edge relative to the 

structure of the wood is critical. 

It is important to note that your job is to orient the knife 

edge relative to the direction of the cells in the wood, not 

relative to the cut surfaces of your block. It is common for 

wood specimens to be cut imperfectly; what appears with 

your naked eye as a transverse surface may in fact be 5 or 

10 degrees out of plane. 

▶ DO NOT try to cut a wood specimen yet.

4.6 Making the cut

If you have followed the critical directions for holding the 

knife and the specimen, you now need only be mindful of 

the position of your thumb on the knife hand. It should be 

pressed to the near side of the wood specimen, well below 

the top of the specimen so that it is physically impossible 

for you to slip and cut yourself. Review the instructions for 

holding the knife and the comments about the primacy of 

safety before continuing.

You now have a knife, a wood specimen, an initial sense 

of what you are trying to accomplish, and know where to 

put your fingers so that you end the day with as many as you 

began it. The next question is where on the block should 

you cut? Because you are using your thumb as a fulcrum 

for the cutting motion and protecting it away from the path 

of the knife, the best way to maximize the force in your cut 

is to cut the corner nearest your thumb. This is true for all 

three planes of section. By keeping the blade closer to your 

thumb, your hand doesn’t need to stretch far and you can 

have maximum leverage. To cut, you draw the edge of the 

Figure 4.5—The correct grip for holding a specimen for 

cutting (left). Note that the fingers and thumb are below 

the top of the specimen and will not be cut. On the right, 

the specimen is held with the fingers and thumbs in a way 

that will result in injury when the specimen is cut.

a

b d

c

Axial Axial

Figure 4.6—When cutting a transverse surface for observation 

with a hand lens, the cut must be oriented relative to the cells 

of the axial system, not relative to the surface of specimen (a). 

A good cut is perpendicular to the axial system and produces 

a surface in which the anatomical features appear normal (b). 

Cutting at an angle to the axial system produces a surface 

in which the anatomical features appear stretched (d). A 

perpendicular (left) and angled (right) cut through a pipe (e). 

The perpendicular cut produces an accurate representation 

of the circular shape of the pipe, whereas the angled cut 

produces an oval representation.

e

Transverse cut Cut at angle
16

utility knife begins to feel dull, replace it. As your technique 

improves, blades will last longer, but in the beginning you 

will use several blades in a few hours of cutting and observing wood.

At first you may lack the physical strength needed to make 

clean cuts of wood, but this will come with practice as you 

exercise those muscles. Physical strength is far less important than good technique, but your strength will increase 

with practice. If either of your hands are feeling weak or 

beginning to fatigue, stop practicing and allow your strength 

to return before you continue; shaking your hands to loosen 

them can be helpful. Controlling the progress of the blade 

through the wood depends on technique, but fatigue can 

cause sloppy technique and result in injury, so you must not 

cut if your hands are tired.

▶ You are almost ready to try to cut a specimen; read the 

section on inspecting the cut first.

4.7 Inspecting the cut

Now you have made a cut. Specifically, you have exposed 

one of the planes of section of the wood, the transverse surface. Inspect it with your hand lens (Fig. 4.8). Pay special 

attention to the plane you produced and its orientation with 

respect to the cells; the cells of the axial system should be 

nearly perpendicular to your cut. If they are not, you must 

Figure 4.7—Safely making the cut. (a) The grip of the specimen and knife and the placement of fingers and thumbs. (b–d) The progress of the 

knife through the specimen. The cut begins at the base of the knife (a, b) and the knife is pulled through the specimen, drawing the edge along the 

wood as the cut is made. Note that at the end of the cut, the tip of the blade is all that is in contact with the wood (d). In all images, the thumb of 

the knife hand is below the top surface of the specimen (shown clearly in a). 

a b c d

Figure 4.8—Cuts of different quality. The left image shows the specimen cut with a sharp table saw. The middle image shows the 

specimen cut cleanly with a utility knife. The right image shows the specimen prepared in a laboratory, resulting in a researchquality image.

make a new cut, taking special care to orient the edge of the 

blade at the angle that will produce a true transverse plane. 

For a tangential plane of section, your cut surface must be 

perpendicular to the rays as seen on the transverse surface. 

It must follow the cells of the axial system and it must cut 

along rather than across the grain.

▶ Now you may cut a transverse surface on your practice 

specimen. 

4.8 Cutting with a razor blade

For soft or low-density woods, a utility knife is the wrong 

tool for the job, and you will need to use a razor blade. The 

basic principles of cutting and the safety rules are the same 

for this tool as for a utility knife. You will hold the specimen the same way as well. The only differences are in how 

to hold the razor blade, and the direction of the cutting 

motion. Razor blades are not strong implements, and when 

using them you do not need the force used with a utility 

knife. Simply pinch the razor blade firmly between your 

thumb and forefinger, and push (not pull) the blade from 

the corner of the specimen across the wood to produce a 

clean surface. Focus on safety, and allow the blade (rather 

than brute force) to cut the specimen.
17

5 | Basic characters used in the identification of wood with a hand lens

The characters presented in this chapter represent the minimal set of terms and concepts necessary to build skill and 

proficiency in wood identification with a hand lens. This list 

is not exhaustive, nor is it in complete agreement with the 

characters used by other authors, in other references, or in 

traditional microscopic wood anatomy. The ways in which 

these characters and definitions deviate from other authors 

are not important for our purposes, but it is important to 

note that if you also use other wood identification references, you must compare those authors’ definitions of characters to those you learn here. The approach of this manual 

was chosen based on experience teaching hand lens wood 

identification at various workshops in Central America, 

Asia, and the United States. The intent here is to streamline 

the learning process and have you working with real wood 

specimens as quickly as possible. As you use this manual in 

the field, you will doubtless come across characters from 

species not covered here. Make note of what you see and 

compare it to the characters outlined below. If, after careful study and observation, you are certain you have found 

something not described here, that is excellent; it means you 

have learned not only the meaning of the characters here, 

but also have gained wisdom sufficient to understand that 

you are seeing something new.

5.1 Anatomical characters in

wood identification

The identification portions of this manual focus on the use 

of anatomical characteristics to identify unknown wood 

specimens. With the basic biology of wood familiar to you 

from the section on wood biology, we will now revisit the 

cells and structures and define them as characters for identification. In each case, we will be thinking of the structures 

as either being present or absent in a specimen. This process requires constant observation and questioning: is the 

character present or absent, or is it impossible to tell? The 

difference between understanding wood anatomy as a biological field and interpreting that anatomy as characters for 

identification is a subtle but important one. 

The images throughout the manual are not perfect; they 

show marks left by the knives that cut them or the sander 

that smoothed them for photography. Rather than try to 

explain away the difference between perfect images and 

surfaces you can produce in the real world, more realistic photos of wood are shown in this manual, especially in 

Chapter 7. In many of the images presented, the notation 

“M” describes the artifacts of preparation in addition to the 

anatomical features. As you become proficient in the preparation and observation of wood specimens, you will train 

your mind to ignore artifacts like knife marks, but in the initial phases of learning these skills, knife marks can appear to 

the untrained eye as actual anatomical structures. 

5.1.1 Softwood or hardwood?

The first question to ask yourself when confronting an 

unknown wood specimen is whether it is a softwood or 

a hardwood. Softwoods are defined anatomically by the 

absence of vessels (pores), and hardwoods by their presence 

(Fig. 5.1). 

5.1.2 Softwoods

For the softwoods treated in this manual, only two anatomical features can be observed with a hand lens—the 

presence or absence of resin canals and the characteristics 

of the transition within the growth rings. Both of these 

features are seen on the transverse surface.

Resin canals

Resin canals are the structures that produce pitch or resin 

and are found in Pinus, Picea, Larix, and Pseudotsuga. When 

you are first learning wood identification, resin canals can 

look much like vessels in hardwoods, but there are two easy 

Figure 5.1—Distinguishing softwoods from hardwoods based on the absence or presence of vessels. In the softwood on the left, there are no 

vessels, but several resin canals are circled. In the hardwood on the right, several vessels are circled. Note that the general appearance, number, 

and distribution of vessels and resin canals are quite different. With practice, you will automatically recognize the characteristics that distinguish 

vessels from resin canals.
18

ways to differentiate them. First, resin canals, if present at 

all, are usually found in the latewood or in the area between 

the earlywood and the latewood. Vessels in hardwoods will 

be found in all portions of the growth ring. Second, even in 

softwoods with many resin canals, there are still far fewer 

resin canals in a hand lens view than there are vessels in 

most hardwoods. With some practice, especially comparing 

a hardwood and a softwood with resin canals side by side, 

these specific differences will not be important; you will 

recognize the pattern of a softwood with or without resin 

canals and you will not confuse resin canals with vessels.

Growth ring transitions

In the growth rings of softwoods, the transition from earlywood to latewood can be abrupt, gradual, or absent 

(Fig. 5.2a–c). In woods with abrupt transition, there is a 

clearly visible zone where the tracheid cell wall thickness 

changes from thin-walled earlywood cells to thick-walled 

latewood cells (Fig. 5.2a). A gradual transition from earlywood to latewood is one in which the tracheid cell wall 

thickness becomes slightly greater toward the latewood but 

with no clear line where the earlywood ends and the latewood begins (Fig. 5.2b). No transition between earlywood 

and latewood is comparatively rare in softwoods and is generally only seen in tropical species that lack obvious growth 

rings (Fig. 5.2c).

5.1.3 Hardwoods

As mentioned earlier, the anatomical variability of hardwoods is many times greater than that of softwoods, and 

much of this variability can be seen with a hand lens.

Growth rings

There are three classifications of growth rings in hardwoods, depending on transitions in the relative size, pattern, 

distribution, or abundance of vessels. The three classifications are ring-porous, semi-ring-porous, and diffuse porous 

(Fig. 5.2d–f ).

In ring-porous woods, the earlywood vessels are distinctly larger (usually from 3 to 10 times the diameter) than the 

latewood vessels, and also commonly more closely packed 

together (Fig. 5.2d). The transition between large earlywood 

vessels and much smaller latewood vessels is distinct.

Semi-ring-porous woods are like ring-porous woods in 

that the earlywood vessels are large and the latewood vessels are half the diameter or smaller, but unlike ring-porous 

woods, there is a gradual rather than distinct decrease in 

vessel diameter from the earlywood through the latewood. 

This results in a clear distinction between the latewood 

of one growth ring and the earlywood of the next, but in 

no clear distinction between the earlywood and latewood 

within the same growth ring (Fig. 5.2e). Semi-ring-porous 

woods are used comparatively rarely in timber bridges.

Diffuse-porous woods, the most common type of hardwood, do not show a large difference in diameter between 

earlywood and latewood vessels. From the beginning of a 

growth ring to the end, the vessels are more or less evenly 

distributed and are of similar size, though in many diffuse-porous woods there will be a slight decrease in vessel 

diameter in the latewood (Fig. 5.2f ). This small difference in 

diameter should not be confused with the semi-ring-porous 

pattern, in which the difference is substantial.

Vessel arrangement 

Vessel arrangement is a term used to describe the pattern, 

if any, of vessels within a growth ring. This is different from 

the concept of growth ring classification. Most woods have a 

random or even distribution of vessels in the wood; this pattern is not generally considered worthy of a special name as 

a character, because it is the assumed default condition. The 

concept of vessel arrangement involves two distinct aspects: 

the number of vessels in contact with each other and the distribution of vessels throughout the growth ring.

When vessels occur one at a time and are not in contact with other vessels, they are called solitary vessels 

(Fig. 5.3a). Most woods have at least a few solitary vessels, but comparatively few woods have exclusively solitary 

vessels; when this character is present, it is a strong character. When two or more vessels are touching each other 

and extend in the radial direction (parallel to the rays), 

they form a vessel multiple. Vessel multiples are commonly 

formed from two to four vessels (Fig. 5.3b); multiples with 

more than four vessels are less common. When several vessels are in contact radially and tangentially, they form vessel 

clusters. Vessel clusters are much less common than vessel 

multiples, and thus the presence of this character is a useful 

one for identification.

Vessels, whether solitary, in multiples, or in clusters, can 

also have patterns of distribution within the growth ring 

that receive special names. One pattern is radially aligned 

vessels and occurs when vessels are organized into radial 

lines or files, often lined up between rays. This pattern can 

occur in woods with solitary, multiple, or clustered vessels, 

though it is most common in the former two than the latter. 

A related pattern is vessels arranged in echelon (Fig. 5.3c); 

this is like radial arrangement, but instead of running parallel to the rays, the vessels form a pattern at a diagonal to the 

rays. The last pattern is called dendritic vessel arrangement 

(Fig. 5.3d), and is characterized by v-shaped, flame-shaped, 

or tree-like groups of vessels that are narrower on the pith 

side of the growth ring (toward the earlywood) and widen 

out toward the bark side (toward and into the latewood). 

It is important to note that several different vessel arrangements can be present in one wood at the same time.

Rays

As mentioned earlier, rays in hardwoods can be narrow like 

those of softwoods, or quite wide (Fig. 5.4). They can also be 

numerous, or a species can have few rays. These characters 

are best observed from the transverse surface, can be critical 

in hardwood identification, and are a major part of the wood 

anatomical pattern. Because the concepts of narrow, wide, 
19

Softwoods

Ring-porous

Small

Small

Large

 Large

Semi-ring-porous

Diffuse-porous

Gradual

Figure 5.2—Growth ring classifications in softwoods (a–c) and hardwoods (d–f ). (a) Abrupt transition within a 

growth ring from the earlywood to latewood is the most common transition type in the softwoods in this manual. 

Note how the earlywood appears lighter in color than the latewood, and how the change from earlywood to 

latewood happens at a distinct place in the growth ring. (b) Gradual transition from earlywood to latewood results 

in a subtler pattern. (c) No transition within a growth ring is not seen in any of the softwoods in this manual. (d) 

Ring-porous hardwoods show an abrupt change in vessel diameter at the boundary between the earlywood and 

the latewood. (e) Semi-ring-porous hardwoods show a gradual reduction of vessel diameter from earlywood to 

latewood. (f ) Diffuse-porous hardwoods, the most common type of growth ring transition in the woods in this 

manual, show no clear change in vessel size or distribution from the earlywood to the latewood, and sometimes 

lack obvious growth rings altogether.

Hardwoods

Abrupt

Gradual

Absent

a

b

d

e

c f

Growth ring transitions

No change in size
Figure 5.3—Patterns in vessel arrangement. (a) Solitary vessels are those that are not in contact with other vessels. They 

usually appear distinctly rounded. (b) Vessel multiples are groups of two or more vessels with cell walls in contact. The 

individual vessels in a vessel multiple often appear slightly flattened where the two vessels touch. (c) Vessels in echelon 

arrangement can be either solitary vessels, vessel multiples, or a combination of both. The pattern of echelon vessel 

arrangement must be observed over a wider area, because it is a pattern formed by many vessels and how they are arranged 

relative to the direction of the rays. (d) Dendritic vessel arrangement is rare or absent in the woods in this manual, and 

it usually found in woods with extremely small-diameter vessels. Most of the vessels in this image are too small to see 

individually with a hand lens.

a

b d

c

Figure 5.4—Differences in ray width and ray number. The image on the left shows comparatively few, wide rays; however, careful observation will 

show that there are narrow rays between the wide rays. Observing only the wide rays, this wood has few rays. The middle image shows a wood with 

an average number and size of rays. The image on the right shows a wood with numerous, narrow rays.

20

few, and numerous are easily understood by examining photos 

or transverse surfaces of a variety of hardwoods, little explanation is needed.

Axial parenchyma patterns

Axial parenchyma patterns in hardwoods are critical characters 

used in wood identification. Axial parenchyma patterns can be 

divided into three broad groups, depending on the position and 

appearance of the parenchyma (Fig. 5.5).

The first type of axial parenchyma is called marginal 

parenchyma. This name is used because marginal parenchyma occurs at the beginning or end of a growth ring, 

and when present assists in defining the growth rings of 

the wood. Marginal parenchyma appears as a solid line of 

cells running evenly around the growth ring (Fig. 5.5 top); 

it generally does not have a wavy or undulating appearance. With some practice, correctly determining the 

presence of marginal parenchyma is an easy task.
21

Whereas marginal parenchyma is defined as delimiting the boundary of a growth ring, the other two types of 

parenchyma are defined on the basis of their association or 

lack of association with vessels and occur within the body 

of the growth ring. If the parenchyma is not associated with 

the vessels, it is called apotracheal parenchyma (Fig. 5.5 

middle). If it is associated with vessels, it is called paratracheal (Fig. 5.5 bottom). The idea of association with vessels 

is at first a somewhat complex one, as will be seen below. 

Learning the names and patterns of apotracheal and paratracheal parenchyma, and having the words mean the correct 

things in your mind, is one of the hardest aspects of learning 

the characters for wood identification.

Apotracheal parenchyma is parenchyma not associated 

with the vessels, and occurs in three basic patterns: diffuse, diffuse-in-aggregate, and banded. These three patterns 

can be considered part of a continuum (Fig. 5.6b–d); at 

one end is diffuse apotracheal parenchyma, in the middle 

is diffuse-in-aggregate apotracheal parenchyma, and at 

the other end is banded apotracheal parenchyma. Diffuse 

apotracheal parenchyma appears as small dots of generally lighter colored cells scattered out in the fibers and 

not touching the vessels (Fig. 5.6b). Diffuse apotracheal 

parenchyma is often not visible with a hand lens and so will 

only be used as a character in species that show it clearly. 

Diffuse-in-aggregate apotracheal parenchyma has a similar distribution, but instead of single cells, it is formed of 

small tangential lines of two to four cells (Fig. 5.6c). This 

gives it the appearance of short, broken lines of cells running perpendicular to the rays, but still not associated with 

vessels. The aggregates of cells do not usually cross the rays, 

contributing to its broken appearance. In contrast to diffusein-aggregate apotracheal parenchyma, banded apotracheal 

parenchyma is formed of long, wavy lines of cells typically 

crossing several to many rays (Fig. 5.6d). Depending on 

the length of the bands, they may appear to be touching 

Marginal parenchyma

Delimits growth rings

Apotracheal parenchyma

Not associated with vessels

Paratracheal parenchyma

Associated with vessels

Marginal 

parenchyma

Figure 5.5—There are three types of axial parenchyma in hardwoods: marginal, apotracheal, and paratracheal. Marginal parenchyma occurs at 

the beginning or end of a growth ring, and is shown in the top images and illustrations. Both apotracheal and paratracheal parenchyma occur 

within the growth ring, and differ on the basis of their association with vessels. Apotracheal parenchyma is not associated with vessels, and is 

shown in the middle images and illustrations. Paratracheal parenchyma is closely associated with vessels, and is shown in the bottom images 

and illustrations.
Vasicentric Winged

aliform

Confluent Lozenge

aliform

Figure 5.7—Specific axial parenchyma patterns. (a) Vasicentric paratracheal parenchyma appears as an even sheath or halo 

of parenchyma around the vessels or vessel multiples. (b) Lozenge aliform paratracheal parenchyma appears similar to 

vasicentric parenchyma, but the parenchyma surrounding the vessels extends tangentially with short, thick projections. 

(c) Winged aliform paratracheal parenchyma is most similar to lozenge aliform parenchyma, but with narrow, long wings 

extending tangentially. (d) Confluent paratracheal parenchyma appears like a series of vessels connected by lozenge 

aliform or winged aliform parenchyma. The connecting, tangential bands of parenchyma can be narrow or wide, and can 

connect from two to several hundred vessels. Compare this to banded apotracheal parenchyma in Figure 5.6d and note the 

differences in association of parenchyma with the vessels.

a

b

c

d

Marginal

Diffuse

Diffuse-inaggregate

 Banded

Figure 5.6—Specific axial parenchyma patterns. (a) Marginal parenchyma. Note that the bands of marginal parenchyma are 

thick. (b) Diffuse apotracheal parenchyma; this, when observable with a hand lens, appears as small dots among the fibers. (c) 

Diffuse-in-aggregate apotracheal parenchyma appears as short tangential lines among the fibers, and the short lines generally 

do not cross the rays. (d) Banded apotracheal parenchyma forms long tangential lines among the fibers, the bands typically 

cross several rays, and are often wavy. Compare this pattern to marginal parenchyma in (a).

a

b d

c

22
23

vessels. You can think of this pattern, when it contacts vessels, as being interrupted by them, rather than associated 

with them. Individual bands can be from many millimeters 

in length tangentially to less than half a millimeter in length. 

Bands can also be narrow (a single cell) radially, or wide 

(many cells).

Paratracheal parenchyma is always clearly associated with 

the vessels, has a variety of patterns, and much like apotracheal parenchyma, these patterns can be seen as a part of a 

continuum (Fig. 5.7). At one end of the spectrum is vasicentric paratracheal parenchyma, in the middle is aliform 

paratracheal parenchyma, and at the other end is confluent 

paratracheal parenchyma. Vasicentric paratracheal parenchyma appears as a round halo or sheath of parenchyma 

around the vessel, and it can be narrow or wide, referring to 

distance out from the vessel it extends (Fig. 5.7a). Aliform 

paratracheal parenchyma is like vasicentric paratracheal 

parenchyma with tangential extensions. That is, there are 

extensions from the sheath of parenchyma perpendicular to 

the rays. These extensions can be comparatively long and 

narrow, giving a winged aliform paratracheal parenchyma 

pattern (Fig. 5.7c), or thicker and short, giving a lozenge 

aliform paratracheal parenchyma pattern (Fig. 5.7b). If the 

wings of aliform paratracheal parenchyma stretch outward 

and fuse with the wings of adjacent aliform paratracheal 

parenchyma from another vessel, the pattern formed is 

called confluent paratracheal parenchyma (Fig. 5.7d). Confluent paratracheal parenchyma can connect as few as two 

vessels or hundreds of vessels. The radial thickness of the 

parenchyma between vessels can be narrow or wide.

5.1.4 Vessel contents

Although wood identification has many additional anatomical terms and characters, only three more are necessary to 

complete our working list of characters used in this manual, 

and all pertain to materials that can be found in vessels: 

tyloses, gums, and powders. For all three, they are typically 

found only in heartwood; if you are examining sapwood of a 

species for any of the three, you will not find them. Tyloses 

are outgrowths of parenchyma cells into vessels and appear 

in the vessels as bubbles or shiny, angled inclusions. Gums 

are resinous, shiny, sticky-looking, generally dark materials 

that appear to plug the vessels. Powders are like gums, but 

appear as dry, dusty, or powdery materials, and are generally lighter in color.

5.2 Nonanatomical characters in 

wood identification

The characters used in wood identification are primarily 

anatomical ones, though there are some nonanatomical features that will be referenced in the identification key (Chapter 6). These nonanatomical characters include color, odor, 

density and hardness, regularity of grain, and fluorescence.

5.2.1 Color

Color in wood usually refers to the heartwood color 

(Fig. 5.8) and is a difficult topic to discuss for several reasons. The heartwood color in a freshly felled log may be 

quite different from the color of the heartwood once the 

wood has been dried and processed. Many woods change 

color over time and with exposure to light (e.g., American black cherry, Prunus serotina) even after they are dried 

and processed. There can also be significant natural biological variability (a wide range of colors) in the color of 

a given species or timber. Some species have characteristic streaks of color throughout the heartwood that are not 

always present in small specimens. A further complication 

is that people see colors differently, and words to describe 

color often do not mean exactly the same thing to different 

people. For these reasons, the information presented about 

the color of woods should be interpreted loosely. Additionally, the color of a timber in a bridge, at least partly exposed 

to dirt, dust, light, and the elements, may bear no semblance 

at all to the “natural” color of the same wood.

5.2.2 Odor

Odor in wood is another nonanatomical character that can 

be both valuable and difficult to describe. As with color, 

different people will perceive the odor of the same wood 

differently, and there can be significant variability in the 

strength of the odor of wood. Generally speaking, odor is 

not a strong character, and should only be used at the end of 

an identification, when all anatomical characters have been 

observed. As with color, the exposure of timbers in a bridge 

Figure 5.8—Heartwood color is derived from 

chemicals deposited by the tree as it matures. The 

top image shows the heartwood–sapwood transition; 

the sapwood is yellower in color, and the heartwood 

reddish. The bottom image shows a heartwood–

sapwood transition in another wood; the sapwood is 

whitish, and heartwood a rich brown.

Heartwood Sapwood

Sapwood

Heartwood
24

may result in a characteristic odor being absent or masked 

by other scents, such as mold, decay, or chemical treatment 

(e.g., the odor of creosote will overpower any natural scent 

in wood).

5.2.3 Density and hardness

Density and hardness are related physical properties of 

wood and have strong influences on the mechanical properties, and thus on the ultimate strength attributed to the 

members in a bridge. Density describes the weight or 

mass of a specimen compared to its volume, and hardness 

gives information about how easy or difficult it is to cut or 

dent the wood. The apparent density of a piece of wood is 

affected by the amount of moisture in the specimen; wood 

left out in the rain will soak up water and seem denser than 

wood of the same species that has been properly dried. For 

this reason, density is best used as a character only when it is 

clearly quite high or quite low. Most woods are of a medium 

or average density, and in such species density is not a useful 

character. The same thing is true of hardness; if the wood 

is easy to dent with your fingernail, it is soft. If you cannot 

dent it with your fingernail, it is hard. Remember that hardness or softness are physical properties of the wood, and 

have nothing to do with whether the wood is a softwood or 

a hardwood, terms that refer to the botanical origin of the 

wood and the kinds of cells you will see in them. For both 

density and hardness, evaluation of these properties is only 

meaningful for sound wood. Wood that is decayed or otherwise damaged may seem lighter or softer than the species 

would normally be.

5.2.4 Regularity of grain

Regularity of grain refers to whether the cells of the axial 

system of the wood are wavy, interlocked, or straight. Wavy 

grain occurs when the cells of the axial system run up and 

down the trunk of the tree with gradual back-and-forth 

undulations of maybe 1–2 cm. Interlocked grain is formed 

when the cells of the axial system do not run perfectly 

straight up and down the tree but rather grow curving up 

the tree to the right for several years, then curve up the tree 

to the left for several years, then back to the right. This alternating pattern of curvature up the trunk produces wood 

that can be quite attractive (especially in species with luster, 

like Ulmus (elm), where interlocked grain is common), but 

is also difficult to process, and sometimes problematic to 

prepare for hand lens observation because the angle of the 

grain in one part of specimen is often quite different from 

the angle in a different part. Straight-grained wood is neither wavy nor interlocked; the cells of the axial system are 

straight and run up and down the trunk nearly vertically. 

The regularity of grain is generally not a strong character in 

wood identification, but can affect strength properties of 

timbers.

5.2.5 Fluorescence

A last nonanatomical character is fluorescence. If you shine 

a UV lamp (either a small portable light or a larger laboratory model) on freshly sanded or cut surfaces, some woods 

fluoresce. That is, the slightly purple or invisible UV light 

is taken up by the specimen and a different color (usually 

a yellow-green color) is emitted by the wood. This is most 

easily seen in dark conditions. Most woods lack fluorescence, so it can be a powerful character for confirming an 

identification once an initial determination is made based 

on wood anatomical features, but it can be difficult to assess 

in the field. Exposure to the elements can render a normally 

fluorescent wood no longer fluorescent, and some decay 

fungi produce yellow-fluorescent compounds in nonfluorescent woods.

With practice and experience, all the nonanatomical 

features discussed above will play a part in your identification of a wood specimen. Because nonanatomical features 

are generally much weaker than anatomical ones, you will 

tend to use them either as secondary confirmations or as 

an automatic part of pattern recognition, where you do 

not specifically ask yourself “Does this specimen have a 

pinkish cast to its heartwood?” but rather notice automatically whether or not it has this characteristic. That is to say, 

nonanatomical characters are often some of the first characters that will form a part of the artistic aspect of wood 

identification.
25

6.1 How to use the key

The key is written to guide you through the identification process in the most efficient and accurate way possible. 

It presents you with a numbered series of questions. The answers you provide will be based on your interpretations 

of the anatomical characters in your unknown specimen and will lead you to a new set of questions. Each time you 

answer a question and proceed to the next you are one step closer to making an identification. Eventually, the key 

will direct you to a species depicted in Chapter 7. If your specimen is a good match for the indicated wood, you 

have successfully identified it. If, however, your unknown specimen is a species not included in the key, the images 

and information in Chapter 7 will not match, and you must conclude that the specimen is not recognized. This will 

happen from time to time, because no key includes all woods. In such an event, seek the assistance of trained wood 

identification expert.

6 | Identification key

Number Key features to evaluate Go to Common name Scientific name

1 Vessels absent (softwood) 2

1 Vessels present (hardwood) 11

2 Resin canals present 3

2 Resin canals absent 9

3 Growth ring transition abrupt 4

3 Growth ring transition gradual 8

4 Resin canals large/conspicuous 5

4 Resin canals small/inconspicuous 7

5 Earlywood often more than half the width of the growth 

ring

6

5 Earlywood typically less than half the growth ring, 

heartwood yellowish to brown, southern origin

Southern Yellow 

Pine Group

Pinus sp., Southern yellow 

pine group (p. 30)

6 Heartwood with a pinkish cast, northern origin Red Pine Pinus resinosa (p. 31)

6 Heartwood brownish or yellowish, western or northern 

origin 

Yellow Pine Group 

(not Southern)

Pinus sp., yellow pine group 

(p. 32)

7 Resin canals occurring singly, heartwood generally 

yellowish in color

Larch/Tamarack Larix sp. (p. 33)

7 Resin canals occurring singly and in pairs, heartwood 

generally with a faint pinkish cast

Douglas-fir Pseudotsuga cf. mensiezii

(p. 34)

8 Resin canals large/conspicuous Soft Pine Group; 

if Eastern origin, 

Eastern White Pine

Pinus sp., soft pine group; 

Pinus strobus if Eastern 

origin (p. 35)

8 Resin canals small/inconspicuous Spruce Picea sp. (p. 36)

9 Growth ring transition abrupt 10

9 Growth ring transition gradual If an Eastern wood, use NDS “Eastern Softwoods.”

If a Western wood, use NDS “Western Woods.”

6.2 Key to common woods in covered timber bridges
26

Number Key features to evaluate Go to Common name Scientific name

10 Southeastern wood, heartwood yellowish to brown, 

often oily

Baldcypress Taxodium distichum (p. 37)

10 Western wood, heartwood distinctly reddish in color, 

without cedar odor 

Redwood Sequoiadendron/

Sequoia sp. (p. 38)

11 Wood ring-porous or semi-ring-porous 12

11 Wood diffuse-porous 19

12 Rays narrow and with frequent broad rays (1 mm wide 

or wider) interspersed, the broad rays easily seen with 

the naked eye

13

12 Rays narrow to medium width 14

13 Latewood vessels comparatively large, solitary, rounded 

in outline 

Red Oak Group Quercus sp., red oak group 

(p. 39)

13 Latewood vessels small, in clusters or flame-like 

arrangement (dendritic), not clearly rounded in outline

White Oak Group Quercus sp., white oak 

group (p. 40)

14 Latewood mostly composed of prominent, long, wavy, 

tangential bands of tiny vessels

Elm or Hackberry Ulmus sp. or Celtis sp. 

(p. 41)

14 Latewood not mostly composed of vessels in wavy 

tangential bands

15

15 Latewood parenchyma pattern predominantly 

apotracheal banded

16

15 Latewood parenchyma pattern not predominantly 

apotracheal banded

17

16 Wood ring-porous, hard and heavy Hickory/Pecan Carya sp. (p. 42)

16 Wood semi-ring-porous, medium density Walnut/Butternut Juglans sp. (p. 43)

17 Rays narrow and numerous, not usually visible without 

a lens

18

17 Rays of medium width, often distinct without a lens— 

everything else northern woods

19 

18 Latewood vessels with faint sheath of aliform 

paratracheal parenchyma, latest latewood vessels 

very small, embedded in confluent parenchyma, 

polychromatic tyloses in earylwood vessels common, 

wood hard and moderately heavy

Ash Fraxinus sp. (p. 44)

18 Latewood vessels with faint sheath of aliform 

paratracheal parenchyma, latest latewood vessels 

very small, embedded in confluent parenchyma, 

polychromatic tyloses in earylwood vessels common, 

wood is soft and light, with a greyish cast

Catalpa Catalpa sp. (p. 45)

18 Latewood vessels in flame-shaped (dendritic) groups, 

earlywood vessel commonly with tyloses

Chestnut Castanea sp. (p. 46)

6.2 Key to common woods in covered timber bridges—con.
27

6.2 Key to common woods in covered timber bridges—con.

Number Key features to evaluate Go to Common name Scientific name

19 Rays all or frequently wide, some woods with occasional 

broad rays of the oak type between narrower rays

20

19 Rays narrow or up to medium width, numerous or few 21

20 All rays medium to wide, wood soft to medium-hard Sycamore Platanus occidentalis (p. 47)

20 Most rays narrow to medium width, occasional broad 

rays of the oak type, such broad rays commonly flared or 

fluted at growth ring boundaries

Beech Fagus grandifolia (p. 48)

20 Rare presence of a wide, disorganized ray, otherwise 

rays barely visible

Alder Alnus sp. (p. 49)

21 Vessels extremely numerous, quite small and often 

difficult to distinguish with a hand lens

Unrecognized 

hardwood

No scientific name (p. 50)

21 Vessels abundant but easy to distinguish with hand lens 22

22 Wood moderately hard and heavy, with marginal 

parenchyma

23

22 Wood comparatively soft and lower density 24

23 Wood whitish to light brown Maple or Birch Acer sp. or Betula sp. (p. 51)

23 Wood light to dark green to black, often with luster Yellow Poplar or 

Magnolia 

Liriodendron tulipifera or 

Magnolia sp. (p. 52)

24 Wood whitish to light brown 25

24 Wood light to dark green to black, often with luster Yellow Poplar or 

Magnolia 

Liriodendron tulipifera or 

Magnolia sp. (p. 52)

25 Wood with marginal and banded apotracheal or diffusein-aggregate parenchymyaBasswood Tilia sp. (p. 53)

25 Wood with only marginal parenchyma Aspen/Poplar/

Cottonwood

Populus sp. (p. 54)
28

Softwoods (vessels absent) Present or 

absent?

Resin canals 

Growth ring transition Abrupt 

Gradual 

Hardwoods (vessels present) Present or 

absent?

Growth rings

Ring-porous 

Semi-ring-porous 

Diffuse-porous 

Vessel arrangement

Solitary 

Multiples 

Clusters 

Radial 

Echelon 

Wavy tangential bands (ulmiform latewood)

Vessel contents

Powders (white, yellow, etc.) 

Tyloses 

Gums or resins 

Vessel size

Small 

Medium 

Large 

Rays

Numerous 

Few 

Wide 

Narrow 

Axial parenchyma

Marginal 

Apotracheal

Diffuse 

Diffuse-in-aggregate 

Banded

Paratracheal

Vasicentric

Aliform

Confluent

Growth rings per cm Many 

Few 

6.3 Checklist of features

A checklist like the one below can be helpful when identifying an unknown specimen, 

because it encourages you to go through the process of observing specific features useful 

in wood identification. It can prevent you from overlooking an important character or failing to check for the presence of all possible features. With practice and experience, such a 

checklist will be less useful to you for wood patterns you recognize automatically, but it can 

help with difficult specimens even when you have years of practice identifying wood.
29

7 | Species description pages
30

Notes

National Design Specification guidance

Use the Southern Pine NDS designation.

Resin canals present, large

Southern Yellow Pine Group

Pinus sp., Southern yellow pine group

Transition abrupt
31

Notes

National Design Specification guidance

Use the NDS Red Pine designation for timber-size members, but the Spruce–Pine–Fir designation is 

recommended for dimension lumber.

Resin canals present, large

Red Pine

Pinus resinosa

Transition abrupt

Wood with a pinkish cast
32

Notes

Resin canals present, large

National Design Specification guidance

Use the Spruce–Pine–Fir designation.

Yellow Pine Group (not Southern)

Pinus sp., Yellow pine group

Latewood less prominent
33

Notes

Resin canals present but small

National Design Specification guidance

Use the Douglas Fir–Larch designation if known to be Western Larch, otherwise use the Eastern Hemlock–

Tamarack designation.

Larch/ Tamarack

Larix sp.

Transition abrupt with

prominent latewood
34

Notes

Resin canals present, 

small, sometimes in pairs

National Design Specification guidance

Use the Douglas Fir–Larch designation. The geographic origin of the material, if known, will determine 

whether to use the North or South category. If unknown, use the lower of the two values.

Douglas-fir

Pseudotsuga cf. mensiezii

Wood with pinkish cast
35

Notes

Resin canals present, large

National Design Specification guidance

If wood is of Eastern U.S. origin, use the Eastern White Pine values. If wood is of Western U.S. origin, 

use the Spruce–Pine–Fir designation.

Soft Pine Group

Pinus sp., soft pine group

Transition gradual
36

Notes

Resin canals present, small

National Design Specification guidance

Use the Spruce–Pine–Fir designation.

Spruce

Picea sp.

Transition gradual
37

Notes

Transition abrupt, latewood narrow,

earlywood tracheids open and rectangular

National Design Specification guidance

Use the Baldcypress designation.

Baldcypress

Taxodium distichum
38

Notes

National Design Specification guidance

Use the Redwood designation.

Redwood

Sequoia/Sequoiadendron sp.

Wood with a reddish cast
39

Notes

Latewood vessels thick-walled

 and round

National Design Specification guidance

Use the geographic origin of the timber, if known, to determine whether to use the Northern or Southern values 

for Red Oak. If origin is unknown, use the lower values.

Red Oak Group

Quercus sp., red oak group

Large rays present, with 

narrow rays between

Ring-porous
40

Notes

National Design Specification guidance

Use the White Oak Group designation.

White Oak Group

Quercus sp., white oak group

Latewood vessels thin-walled

 and polygonal Large rays present, with 

narrow rays between

Ring-porous
41

Notes

National Design Specification guidance

These species are currently not in the NDS specifications, so you will need to make a clear wood estimate.

Elm or Hackberry

Ulmus sp. or Celtis sp. 

Latewood vessels in wavy

tangential bands (ulmiform)

Ring-porous
42

Notes

National Design Specification guidance

Use the Beech–Birch–Hickory designation.

Hickory or Pecan

Carya sp.

Prominent banded

apotracheal parenchyma

Ring-porous
43

Notes

National Design Specification guidance

This species is currently not in the NDS specifications, so you will need to make a clear wood estimate.

Walnut or Butternut

Juglans sp.

Banded

apotracheal 

parenchyma

Semi-ring-porous
44

Notes

National Design Specification guidance

This species is currently not in the NDS specifications, so you will need to make a clear wood estimate.

Ash

Fraxinus sp.

Latewood vessels thick-walled

Narrow rays

Ring-porous
45

Notes

National Design Specification guidance

This species is currently not in the NDS specifications, so you will need to make a clear wood estimate.

Catalpa

Catalpa sp.

Latewood vessels larger,

thinner-walled

Ring-porous
46

Notes

National Design Specification guidance

This species is currently not in the NDS specifications, so you will need to make a clear wood estimate.

Chestnut

Castanea sp.

Latewood vessels 

thin-walled

and polygonal No large rays present, 

only narrow rays

Ring-porous
47

Notes

National Design Specification guidance

This species is currently not in the NDS specifications, so you will need to make a clear wood estimate.

Sycamore

Platanus sp.

Rays mostly wide, 

wide rays numerous

Diffuse-porous
48

Notes

National Design Specification guidance

Use the Beech–Birch–Hickory designation.

Beech

Fagus grandifolia

Occasional broad rays of the 

oak type present

Broad rays commonly 

flared or fluted at 

growth ring boundaries

Diffuse-porous
49

Notes

National Design Specification guidance

This species is currently not in the NDS specifications, so you will need to make a clear wood estimate.

Alder

Alnus sp.

Diffuse-porous Loosely organized wide rays

infrequent but present
50

Notes

National Design Specification guidance

No useful guidance can be given for estimating strength properties of the wood without further identification 

of the wood itself. Removing a small specimen (1/4 in. square across the grain and 1–2 in. along the grain) and 

submitting it to an extension forester or forest products specialist should provide the necessary information to 

allow further estimation of properties.

Unrecognized Hardwood
51

Notes

National Design Specification guidance

Use the Beech–Birch–Hickory designation.

Maple or Birch

Acer sp. or Betula sp.

Diffuse-porous

Marginal parenchyma present
52

Notes

National Design Specification guidance

Use the Yellow Poplar designation.

Yellow Poplar or Magnolia

Liriodendron tulipifera or Magnolia sp.

Marginal parenchyma present

Rays numerous, neither wide 

nor narrow, but distinct

Wood with greenish or

purplish cast and luster

Diffuse-porous
53

Notes

National Design Specification guidance

This species is currently not in the NDS specifications, so you will need to make a clear wood estimate.

Basswood

Tilia sp.

Marginal parenchyma present

Diffuse-porous Diffuse-in-aggregate

parenchyma present

but often faint
54

Notes

National Design Specification guidance

Use the Aspen or Cottonwood designations if the origin of the material is known. Western or far Northern 

woods are likely to be Aspen. Eastern or more Southern woods are likely to be Cottonwood.

Aspen/Poplar/Cottonwood

Populus sp.

Diffuse-porous Vessels abundant, rays extremely

narrow and abundant
55

8.1 The limits of wood identification

In using the key and the species description pages, we are 

sure you noticed that identification of an exact species is 

rarely possible. This is due in part to the nature of the tools 

we are using in this manual. A hand lens is a powerful tool 

for wood identification, but it is not the most powerful 

method for identification based on anatomical characteristics. The greatest power in wood identification comes with 

the use of light microscopy in a laboratory with a scientifically valid reference collection (more about reference 

collections below). Even in such a setting and with a microscope, species-level identification is still not possible by 

wood anatomy in most cases. 

This limit, and it is a profound one, is not a result of a 

lack of effort on the part of wood anatomy researchers, but 

rather reflects a basic biological truth; species are defined 

by flowers, fruits, leaves, and other external characteristics. 

Internal characteristics, like wood anatomy, often do not 

vary from one species to another in the same genus, and so 

species cannot be separated using them. At the time of this 

writing, there are a number of efforts around the world to 

use other scientific techniques to identify exact species of 

wood, and in some cases, to match a specific log, board, or 

wood product to a stump left behind in the field. Most of 

these methods depend on either DNA technology or sophisticated techniques in chemistry or spectroscopy. To date 

none of these techniques have proved universally reliable 

and are still costly and time-consuming, and are considered 

experimental. If and when the day arrives that a technological solution is available to ensure species identification of 

wood, it is not likely to be equally available in all countries 

and regions and will probably involve laboratory rather than 

field analysis. 

8.2 Additional wood identification information

Many wood identification references are available in addition to this manual. Many of them are written for professional wood anatomists with specific emphasis on the 

microscopic identification of wood. These resources, though 

parts will exceed your current knowledge, may nonetheless 

prove useful in your work. Other references will be more 

appropriate for your use as you develop your skills in wood 

identification. It is important to keep in mind that not all references are of the same quality; some references, even wellknown ones, have significant flaws or errors. Below we list a 

small selection of resources, as well as some candid comments about the reliability of the information in each.

InsideWood

InsideWood. 2004 onwards. Published on the Internet. 

North Carolina State University, Raleigh, NC. 

(http://insidewood.lib.ncsu.edu/search?2) 

This is a tremendously useful resource for wood identification in the laboratory, with a distinct emphasis on light 

microscopic identification. It incorporates a (usually) 

complete anatomical characterization of each species in 

the database, allows rapid searching, and many of the species it covers are illustrated with a variety of photographs. 

Despite the emphasis on microscopy, many of the species in 

the database have one or more images similar in magnification to those in this manual. InsideWood is being updated, 

improved, and maintained regularly as of this writing, so it 

continues to improve as a resource.

Hoadley’s books

Hoadley, R.B. 2000. Understanding Wood: A Craftsman’s 

Guide to Wood Technology. 2nd Ed. Newtown, CT: Taunton 

Press. 

Hoadley, R.B. 1990. Identifying Wood: Accurate Results with 

Simple Tools. Newtown, CT: Taunton Press.

These books provide basic information on the structure, 

properties, and identification of woods, with a greater 

emphasis on North American species. The information is 

clearly presented with high-quality illustrations, but emphasizes microscopic rather than hand lens observation of wood 

for identification.

Textbook of Wood Technology

Panshin, A.J. and C. deZeeuw. 1980. Textbook of Wood 

Technology. 4th Ed. New York: McGraw-Hill. 

This is a famous but out-of-print college textbook for 

understanding many aspects of wood technology, including 

detailed information on wood identification of North American species. The identification keys in the book are of questionable consistency and reliability. Despite the comparative 

weakness of the keys themselves, the earlier chapters 

reviewing the scientific literature in wood technology are 

useful and relatively up-to-date as of the year of publication.

8 | Additional information about wood identification

57

9 | Estimation of size and grade of members

Figure 9.1—Steps for member grading.

An inspecting engineer must take three critical steps in order to assign the most accurate estimate of design values to 

wood members in a bridge: (1) identify the species of wood, (2) estimate a size and grade for the member (Fig. 9.1), 

and (3) determine the condition of the wood in the member. 

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?
58

9.1 Significance of species

The first and probably most critical step is for the species 

of the member to be identified. Wood properties for clear 

wood vary considerably from species to species, as illustrated in selected examples in Table 9.1 (FPL 2010). Values 

reported in Table 9.1 are average measured properties of 

small clear specimens in a laboratory setting and are not 

design values. Chapters 5, 6, and 7 detail the features, skills, 

and characters used to make a field identification of wooden 

members likely to be found in a timber bridge in the United 

States.

9.1.1 Sampling of bridge members

The best piece of advice that can be given for sampling 

bridge 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 bridge; 

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.

9.1.2 What to prioritize in sampling for wood ID

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 of 

the bridge. The species of wood in these members should be 

the first to be identified. 

9.1.3 How to collect samples for wood ID

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 the techniques 

explained in Chapter 4 can be safely applied, but it should 

not be taken in a way that affects structural integrity of the 

member. 

9.1.4 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 the 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.

9.2 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 bridge 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 9.2, DOC PS 20, 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 bridges, however, were built before the 

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 “dimension lumber or timbers” should 

be associated with it. 

All three dimensions of a lumber member need to be 

determined (Fig. 9.2). 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. 

9.3 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 inspectors with 

a very basic overview of grading and allow them to make 

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. 

Figure 9.2—Definition of length, width, and thickness in a 

lumber member.

Width

Thickness Length
59

9.3.1 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 9.3. A grade stamp 

provides information on the supervising grading agency 

(WWPA), 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 (Chapter 11). If not, additional judgments on the quality of the 

member must be made.

9.3.2 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 

(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 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 full-size 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. 9.4). 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 several hardwood species under 

provisions of the American Softwood Lumber Standard, 

Figure 9.3—Grade stamp from WWPA 

No. 1 & Better dimension lumber. 

Moisture content is surfaced green 

(unseasoned) condition, over 19%.
60

ASTM International, and then applied to product classes. 

The allowable design properties are tabulated in the Supplement to the National Design Specification (ANSI/AWC 

NDS; Current Edition).

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.

Tables 9.3, 9.4, and 9.5 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.

9.3.3 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 

by eye. The typical visual sorting criteria used in grading 

are size of member, knots, slope of grain, checks and splits, 

shake, density, decay, heartwood and sapwood, pitch pockets, and wane. 

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 9.3. 

Timbers (lumber standard 114 mm (nominal 5 in.) or 

more in least dimension) are also structurally graded under 

Rules-writing agencies Independent 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)

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

Sawn lumber grading agencies*

*For updated information, contact American Lumber Standard Committee, P.O. Box 210, Germantown, MD 20875; 

alsc@alsc.org; www.alsc.org.

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 9.4—Voluntary system of responsibilities for stress grading 

under the American Softwood Lumber Standard.
61

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 9.4). For most species, however, timber grades are 

classified according to intended use (Table 9.5).

9.3.4 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 

bridge member’s grade, all these characteristics should be 

considered, but the grade of the member will be determined 

by the most severe of the defects. Not all these member 

characteristics in a bridge 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 bridge inspections are highlighted in Section 9.4. 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 the knot’s effect on strength. Centerline knots on the 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 (knots along the centerline have little or no effect). 

Intergrown (or live) knots resist (or transmit) some kinds 

of stress, but encased knots (unless very tight) or knotholes 

resist (or transmit) little or no stress. On the other hand, distortion of grain is greater around an intergrown knot than 

around an encased (or dead) knot of equivalent size. As a 

result, overall strength effects are roughly equalized, and 

often no distinction is made in stress grading between intergrown knots, dead knots, and knotholes. 

The 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.

Measuring knots

It is important to know the size and location of knots in 

wood members of a timber bridge. 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. 9.5), 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 historic bridges. 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 

Figure 9.5—Adaptation of diagram from a pocket guide showing 

measurement of knots on 2-inch to 4-inch lumber which included 

structural joists and planks (Southern Pine Grading Rule 2002, used 

by permission, Southern Pine Inspection Bureau 2002).

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
62

knots in dimension lumber. When properly aligned, the 

grid allows you to make a good estimate of knot size to the 

closest 1/4 inch (Fig. 9.6). 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.

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. 9.7). 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. 

Measuring slope of grain

Drying checks in timber generally follow the slope of grain 

and can be used to determine the slope of grain on painted 

timbers. Figure 9.8 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. 9.9). 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. But if evidence of extensive decay is present, replacing the member is the best option.

Measuring decay

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 

Figure 9.6—Example of using grid to establish the size of knot in 

bridge member. The measured knot in the example is 1-1/2 in.

Figure 9.7—Definition of slope of grain is the ratio of rise to run (1:x).

Figure 9.8—Example of using checking in a member to measure the 

slope of grain in bridge. The end of the scale points to the start of one 

check.

Figure 9.9—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. 

Average line of the direction of bers

1 x
63

probe can be used to determine if wood structure is weakened (Chapter 10). 

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.

Checks and splits

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 

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.

Measuring checks, splits, and shake

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. 9.10). 

Checks are measured as an average of the penetration perpendicular to the wide face (Fig. 9.11). 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. 9.12). 

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. 9.13). If the member is below 

16% moisture content, the size of the shake may be 1.5 

times the size permitted in the grade.

9.3.5 Density, rate of growth, and percentage 

of latewood

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 

Measure average 

penetration

Figure 9.11—Checks in post and timber.

Whichever 

is the least 

dimension

Figure 9.13—Shake in post and timber. 3W

Measure checks

and shakes

Measure shakes, 

checks, and splits

Measure shake

W/2

W

3W

Figure 9.10—Beams and stringer checks, shake, and split measurement.

Figure 9.12—Split measurement.

Measure 

penetration 

from the end
64

of the wood. Strength is related to mass per unit volume 

(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 lowstrength pieces can be eliminated from a grade by excluding 

those that are exceptionally low in density.

9.3.6 Heartwood and sapwood

Heartwood or sapwood is often not taken into account in 

structural grading because heartwood and sapwood are 

assumed 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.

9.3.7 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. 

9.3.8 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.

9.4 What to prioritize in field estimation 

grading

When inspecting a bridge, not all properties discussed 

above will be easily measured. Fortunately, the most significant strength-reducing factors in structural members of 

wooden bridges (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.

9.5 Examples of grade estimation

Nine example members from different bridges are tabulated 

in Table 9.6 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. 

9.5.1 Dimension lumber grading examples 

Six dimension lumber member grade estimation examples 

are presented in Table 9.6. 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 9.3, 

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. 

The 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 slope of grain for a 2 by 6 shown in Table 9.3. 

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.

The 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 9.3, 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. 

The 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 
65

the criteria found in Table 9.3. One large edge knot on the 

wide face forced a reduction in the estimated grade for 

member 5 to No. 2. 

The 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 9.3 for 

dimension lumber, the recorded knot sizes and slope of 

grain suggest an approximate grade of No. 2 for member 6. 

9.5.2 Timber grading example

Three timber grading examples are presented in Table 9.6. 

The 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 9.4. 

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. 

The 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 9.5 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. 

The 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 9.5. 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.

9.6 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.
66

Table 9.1—Strength properties of some commercially important woods grown in the United States (metric)a

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
67

Table 9.1—Strength properties of some commercially important woods grown in the United States (metric)a—con.

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 a

Results 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.

b

Specific gravity is based on weight when ovendry and volume when green or at 12% moisture content.

c

Modulus of elasticity measured from a simply supported, center-loaded beam, on a span depth ratio of 14/1. To correct for shear deflection, the 

modulus can be increased by 10%.
68

Table 9.2—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
69

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 9.3—National Grading Rule Specifications for Dimension Lumber (Standard Grading Rules for Southern Pine Lumber 

2002; used courtesy of Southern Pine Inspection Bureau)
70

Table 9.4—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
71

Table 9.5—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
72

Table 9.5—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
73

Table 9.6—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

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 Cottonwood

2 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).

75

10 | Condition assessment of bridge members

The principal reason for most inspections of a wooden 

bridge is to determine the condition of the wood components so that a judgment can be made on the capacity of the 

bridge. Helpful tips and methods for conducting condition assessments of wood structures have been described 

in detail by many authors (Wood 1954; Ross et al. 2004; 

Brashaw et al. 2005a,b; Anthony 2007; Anthony et al. 2009; 

Anthony & Associates 2009; Kukay et al. 2013). These 

publications emphasize where problems are most likely 

to occur in historic 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.

10.1 Sampling for condition assessment from 

covered bridges

For bridges, inspections should focus on areas where problems are known to be common, such as places where wood 

is in ground contact, moisture stains are present, or visible 

signs of decay are indicated. Areas where different types of 

material come in contact (such as wood and masonry) are 

other priority locations for condition assessment. These are 

areas such as beam pockets, floor joists and girders, roof 

framing, sill beams and plates (particularly when in contact 

with masonry), and top plates. Of less structural importance 

are the exterior wood work components, including cladding, shingles, and soffits. Flaws in the exterior wood cladding of a bridge, though, can be the point where moisture 

enters and damages structural elements. Finally, areas of the 

bridge that may have been modified should be closely examined to see if the modifications weakened the bridge.

10.2 Moisture-related degradation

An inspector should also be alert to areas where periodic leaks in roofs or walls or seasonal flooding may have 

caused serious problems with decay or warping. This is best 

accomplished by looking for evidence of moisture stains. 

It is important to differentiate moisture stains that resulted 

from a single exposure to moisture (Fig. 10.1) and those 

that represent a repeated exposure to moisture. Different 

hues of stain colors are an indication of repeated moisture 

exposure. One-time moisture exposures likely have little 

impact on the structural integrity of wood as long as there 

is sufficient air circulation to dry the member out. Repeated 

exposure to moisture, however, leads to decay and insect 

attack (Fig. 10.2). Determining member moisture content 

with moisture meters, described in Section 10.4.2, is a good 

way to determine if the moisture exposure has persisted to 

the time of inspection.

10.3 Physical degradation

An inspector should look for signs of damaged or missing 

members (Fig. 10.3). Physical degradation can either be a 

result of prolonged changes in climate (causing weathering 

or drying checks from shrinkage of large members as they 

Figure 10.2—Example of insect damage.

Figure 10.1—Indication of a water exposure event. Note the single 

color of stained material. If this were from multiple wetting events, 

multiple colors or bands of stains would be evident.

Figure 10.3—Failed load bearing member in need of replacement.
76

dry) or be a result of use (such as overloading or mechanical 

damage by careless drivers). Members that have clear evidence of physical damage should be replaced. 

10.4 Nondestructive evaluation techniques

Nondestructive evaluation tests on various bridge members can help an inspector identify degradation of members 

caused by decay or physical damage. Many available methods are available for nondestructive evaluation (Fig. 10.4). 

The most widely applied techniques are described in greater 

detail in publications by Ross et al. (2004) and Brashaw et 

al. (2005a,b). A summary of the more common techniques 

follows.

10.4.1 Visual inspection

A quick walk around the bridge gives an inspector invaluable information and will suggest a starting point for more 

in-depth assessment. Look for any indication of decay or 

damage of structural members. Identify failed abutments 

(missing pieces, cracks in surface of abutment, sloping 

bridge deck, or other evidence of settling) or signs of overall deterioration of the bridge (failed members, excessive 

deflections, large gaps, missing members). Examine the surfaces of existing members looking for fungal fruiting bodies, 

sunken or collapsed faces or localized surface depressions, 

staining or discoloration, plant or moss growth in splits, 

cracks, insect damage, or fire damage. If present, these features are areas that will require the closest inspection.

10.4.2 Moisture meter

As previously discussed, elevated moisture content is an 

indicator of a board likely to have decay. To determine if 

excess moisture is present, a moisture content measurement can be performed with an electrical-resistance-type 

moisture meter and 76-mm- (3-in.-) long insulated probe 

pins (Fig. 10.5) or surface moisture meters in accordance 

with ASTM D 4444 (ASTM 2013) requirements. Several 

types of moisture content measuring devices are available. 

Most require some correction for temperature and species 

(Pfaff and Garrahan 1984). Conditions are right for decay if 

moisture content is above 20%. Moisture content of sound 

wood should be between 6% and 12% for interior applications and between 10% and 20% for exterior applications. 

Decaying wood will often have moisture contents that register between 20% and 30% or higher. Previously decayed 

wood can be found in wet or dry members.

10.4.3 Probing

Another useful tool when looking for decay is an awl or 

knife to probe areas of suspected decay or insect damage. 

The probe should be used to see how solid the woody material is and how it fails when broken. A splintered break 

when you pry up a wood flake reveals sound wood while a 

brittle, brash (even, smooth transverse surface) break indicates decayed wood (Fig. 10.6). Care should be taken so that 

water-softened wood is not mistaken for decayed wood.

10.4.4 Increment boring

A forestry increment borer is not truly a nondestructive 

tool for inspecting (using an increment borer will leave a 

noticeable mark), but it is truly a useful tool for an inspector 

(Fig. 10.7). An increment borer can assist in identification of 

voids, indicate the presence or absence of decay, reveal the 

presence and penetration of treatment, and provide a potential sample for species identification. The hole created by 

boring should be plugged with a preservative-treated dowel 

to prevent a pathway for unwanted moisture exposure.

Figure 10.4—Various tools and techniques available to conduct condition assessments (used with permission of 

Natural Resources Research Institute, University of Minnesota Duluth, Brashaw et al. 2005b).

Figure 10.5—Example of a moisture meter in use.
77

less resistance to the drill in unsound wood. The drill measurement also provides a good density profile of the wood 

member. This method is further described by Ross et al. 

(2004) and Rinn (2013) (Fig. 10.9).

10.5 Importance of condition assessment

A member that is in poor condition, regardless of grade, 

can compromise the safety and integrity of a bridge. The 

inspector’s confidence in the condition of members directly 

affects the design value estimate that can be assigned to that 

member. The more tools brought to bear on judging the 

condition of a member, the more confident an inspector can 

be in judging the condition of a member. This chapter gives 

only a few possible ways for an inspector to judge the condition of a member. Whole books are devoted to the topic. 

Any of the referenced articles, reports, or books are good 

resources for inspectors to improve their condition assessment skills.

10.6 Site-specific bridge condition assessment

In addition to the condition of individual members, some 

aspects of overall bridge condition should be evaluated 

(such as loss of bridge camber, changes in loading with 

changes in use) in a site-specific way that cannot be generalized for a field manual.

10.4.5 Stress wave 

The calculated stress wave velocities (meters or feet per 

microsecond) of a bridge member can help identify potentially decayed locations in the member. Measurements of 

time-of-flight of induced stress waves within a member can 

be performed with stress wave timing equipment using 

well-established procedures (Fig. 10.8) (Ross et al. 1999). At 

locations of interest, the width of the member is measured 

along with the wave transmission time. A pulse is introduced into the member, and an accelerometer monitors the 

wave’s arrival and stops the timing clock. Stress waves travel 

faster through sound wood than through unsound wood. 

Differences in transit time are used to indicate the location 

of decay. This method requires access to both sides of the 

member.

10.4.6 Resistance drilling

Another method of detecting decayed locations in bridge 

members is micro-resistance drilling. A micro-resistance 

drill measures drill bit resistance, and monitoring this resistance is a means of detecting unsound wood. There is far 

Figure 10.7—Increment borer.

Figure 10.8—Transverse stress wave velocity measurement method.

Figure 10.6—Example of a knife being used to probe wood: 

(top) Checking the soundness of wood and (bottom) an 

example of abrupt failure surfaces indicating decay.

Figure 10.9—Examples of nondestructive instruments in use.

79

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 

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.

11.1 Must-have documents

There are three must-have documents for someone trying to 

estimate a design value: 

•The National Design Specification (NDS) Supplement 

Design Values for Wood Construction, which lists all currently approved design values for visually graded lumber 

and timber (AWC 2012) 

•ASTM standard D 2555 Standard Practice for Establishing 

Wood Strength Values (ASTM 2012b) 

•ASTM standard D 245 Standard Practice for Establishing 

Structural Grades and Related Allowable Properties for 

Visually Graded Lumber (ASTM 2012a)

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).

11.2 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.

11.3 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. 11.1 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 11.1.

Species listed in the NDS Supplement include the most 

common species that are present in wooden bridges. Procedures for estimating design values for species that are found 

in the NDS Supplement are given in Section 11.4. Note that 

some species listed in Table 11.1 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 11.1, then you should proceed directly 

to Section 11.5. 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 Section 11.4 for an initial estimate of 

design values.

Member grading examples in Table 9.6 are also used to 

illustrate design value assignment. Members 1, 2, 3, 4, 6, 

and 7 in Table 9.6 are all species listed in the NDS supplement. No design value need be calculated for Member 1 in 

Table 9.6 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 Section 11.4. Members 5 and 8 in Table 9.6 are not in 

the NDS Supplement and are assigned estimated design 

values in Section 11.5. All estimated design values reported 

below apply the rounding rules specified in ASTM D 245 

(Table 11.2). 

11 | Design value estimation
80

Figure 11.1—Flow chart for estimated design value assignment.

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?
81

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. 

11.4.1 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 11.3. Members 2, 3, and 4 in 

Table 9.6 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 11.4, 11.5, and 11.6, respectively.

Table 11.4 shows steps for calculating estimated design 

values for member 2. Member 2 in Table 9.6 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 11.3 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 11.4. The estimated Fb value, for 

example, is 900 lb/in2.

Table 11.5 shows estimated design values for member 3 in 

Table 9.6, 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 11.3 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 11.5.

Table 11.5 shows estimated design values for member 4 

in Table 9.6, 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. 

11.4 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 9.6 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 exception of Southern Pine timber, are further subdivided into 

use categories with different values for timbers that can 

Abbreviations used in this chapter

Cf: Size factors

E: Mean modulus of elasticity 

Emin: Lower bound of modulus of elasticity

Fb: Design value for stress in bending

Ft: Design value for stress in parallel to grain in tension

Fv: Design value for stress in horizontal shear

Fcperp: Design value for stress perpendicular to grain in 

compression

Fc//: Design value for stress parallel to the grain in 

tension

Un: Uncertainty factor that reflects the inspector’s 

confidence in the observed grade or condition

Str. LF: Structural Light Framing (2–4 in. thick, 

2–4 in. wide)

Str. J&P: Structural Joist and Plank (2–4 in. thick, 

5 in. and wider)

B-S: Beam or Stringer (5 in. and thicker, width more 

than 2 in. greater than thickness)

P-T: Post or Timber (5 by 5 in. and larger, width not 

more than 2 in. greater than thickness)
82

The appropriate size factors Cf shown in Table 11.3 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 11.6. 

Southern Pine dimension lumber in the 

NDS Supplement

Table 11.7 shows steps for determining an estimated design 

value for member 5. Member 5 in Table 9.6 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.

11.4.2 Timber values found in NDS Supplement

The 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 11.8 shows estimated design values for member 7. 

Member 7 in Table 9.6 was identified as a 5.5- by 7.5-in. 

Southern Pine member whose observable knots and slope 

of 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 11.8.

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 11.9 shows estimated design values for member 8. 

Member 8 in Table 9.6 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 11.9.

11.5 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 9.1 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 11.10) 

times the average clear wood strength. The species’ mean 

clear wood modulus of elasticity (MOE) and compression 

Fn = Fo

12

d

1/9 ⎛

⎜

⎝

⎞

⎟

⎠

∙Grade∙MC∙Size∙Un 1

adjfac Fx = (Property)∙
83

perpendicular to grain (Cperp) stress values can be used to 

determine an estimate of MOE and Cperp design values. 

The adjustment factors (adjfac) suggested in D 245 for 

various clear wood properties are given in Table 11.11. 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 11.12. 

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 

covered bridges can be found in Table 9.1. Two examples 

from Table 9.6, 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).

11.5.1 Member is dimension lumber size

Table 11.13 shows steps for estimating design values for 

member 6. Member 6 of Table 9.6 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 9.1, 

which 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 11.10 and the estimated 

grade strength ratio from Table 11.11 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.

11.5.2 Member is timber size

Table 11.14 shows steps for estimating design values for 

member 9. Member 9 of Table 9.6 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 9.1, 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 11.10 and the estimated grade 

strength ratio from Table 11.11 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. 

11.6 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 bridge 

members. It gives the inspector a basic method for estimating design value for members. The most accurate estimate of 

the in-place 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 

wooden bridge, 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 field guide is based on publicly available 

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 for confirming what the current rules are.

Size = 2

d

1/9 ⎛

⎜

⎝

⎞

⎟

⎠
84

Table 11.1—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
85

Table 11.1—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
86

Table 11.1—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.
87

Table 11.3—Size factors, Cf

Grades

Width 

(in.)

Fb

Ft Fc//

Thickness

2 and 3 in. 4 in.

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

Table 11.4—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 

value

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

Table 11.2—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)

a

Un, uncertainty.
88

Table 11.6—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 

value

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

Table 11.5—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 

value

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

a

Un, uncertainty.

a

Un, uncertainty.
89

Table 11.7—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 

value

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

Table 11.8—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 

value

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

a

Un, uncertainty.

a

Un, uncertainty.
90

Table 11.9—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 

value

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

Table 11.11—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 11.10—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 11.12—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

a

Un, uncertainty.
91

Table 11.13—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

Table 11.14—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

a

adjfac, adjustment factor.

b

Un, uncertainty.

a

adjfac, adjustment factor.

b

Un, uncertainty.

93

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