A Grading Protocol for Structural Lumber and Timber in Historic Structures (2009-01)

By NCPTT On July 7, 2009 · 1 Comment · In Architecture & Engineering, Product Catalog

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A Brief Summary of Grading of New Lumber and Timbers

Over time, grading rules were modified as more technical data became available on material properties, and the effects of knots, slope of grain, and sawing defects on material properties. Lumber trade associations modified their own grading rules, based on the American Lumber Standards Committee framework.

In the 1970s an in-grade testing program was initiated for dimension lumber. Prior to the in-grade testing program, lumber was assigned design values based on tests of small clear specimens to determine clear wood strength. The clear wood strength values were reduced by a number of factors, such as the size and location of knots and slope of grain as specified in ASTM standards. The ASTM standards and procedures are still in effect today, with numerous revisions over the years. However, tests on full-size lumber, coupled with concerns over changing forest resources, made it apparent that the small clear specimen approach did not accurately reflect the strength of full-size lumber.

The in-grade testing program conducted by the U.S. and Canadian forest products industry led to modifications in the design values assigned to a particular lumber species, or species group, based on the dimensions and defects present. The design values were coordinated through the industry trade associations. For western timber species, the Western Wood Products Association (WWPA) and West Coast Lumber Inspection Bureau (WCLIB) were instrumental in the work on softwood lumber. The Southern Pine Inspection Bureau (SPIB) was involved with the testing and analysis of southern yellow pine data.

These design values are used by architects and engineers to determine the appropriate size of lumber to resist loads on and within a structure. The loads are due to wind, snow, people and furnishings, and the weight of the structure. The design value is known to the architect or engineer by the grade assigned to and stamped on the lumber.

Figure 1. Grade stamp from SPIB for No. 1 Timbers for southern yellow pine. Moisture content is in the green (unseasoned) condition, over 19 percent (SPIB, 2002).

The grade stamp on a piece of lumber is the means by which the end user knows how the piece can be used. It specifies wood species and grade overseen by the industry trade associations in accordance with the American Lumber Standards Committee rules. The grade stamp does not guarantee that a piece of lumber has a particular strength, only that the piece of lumber met the requirements to be assigned a particular design value. The requirements are determined by the grading rules and include species, dimensions, knot size and location, slope of grain and manufacturing defects (such as skip and wane).

The grade stamp is recognized by architects, engineers, builders and building officials as a measure of the quality of a piece of lumber. A typical grade stamp is shown in Figure 1. The grade stamp generally has the following characteristics:

Certification mark – indicates the supervising grading agency
Species – identifies the wood species or species combination
Mill identification – the number or brand of the firm that produced the board
Grade name – indicates the grade for which the piece meets the requirements
Moisture content – indicates the target moisture content to which the wood was dried or the moisture content at which it was surfaced (planed).

Appearance grades are used in structures to satisfy architectural requirements (e.g. free of knots) or for secondary industries, such as furniture manufacturers. Grades for hardwood lumber are established by separate industry trade associations, such as the National Hardwood Lumber Association. Most hardwood species of lumber and timber are typically not used in structural applications today. The difference between structural and non-structural grades is that structural grades are referenced in model building codes and, as such, carry a legal burden of safety. The failure to either be properly specified or to meet the grade requirements may also compromise safety. If a building collapses, the grade of the lumber may be called into question. In contrast, failure to meet an appearance grade is a matter of aesthetics, not safety.

The modern architect or engineer, when first exposed to the interlocking complexity of standards, rules, building codes, governing organizations and trade groups may well feel overwhelmed. The process through which standards are developed and implemented through the various organizing bodies is certainly convoluted. The materials to which the grading rules are applied include round timbers and solid-sawn structural lumber. There are three size classifications for wood products that are commonly found in historic structures:

Boards – less than 2 inches in nominal thickness (before the board is planed to provide a smooth surface). Boards are non-structural and are not included in this grading protocol.
Dimension lumber – 2 to 4 inches in nominal thickness and 2 or more inches wide. Dimension lumber up to 4 inches in width is considered light framing or structural light framing and is not included in the grading protocol. Dimension lumber greater than 4 inches in width is considered Structural Joists and Planks.
Timbers – 5 or more inches in nominal least dimension.

The classifications of structural lumber are, generally, graded using visual criteria, which is the sorting of lumber into specific categories based on characteristics such as knots, slope of grain, wane, and other characteristics. The characteristics most important to historic structures are defined and discussed in more detail in the section entitled “Field Data Required for Use of the Grading Protocol”.

For new lumber and timbers, a trained lumber grader examines a piece and quickly assesses its characteristics, either during or after production. The characteristics examined include:

Definitions of these characteristics are provided in the glossary at the end of this report. Measuring these characteristics, the grader examines a piece and determines the appropriate grade for the piece based on limitations specified for a particular grade and application. As an example, the limitations for these characteristics for No. 1 Timbers (SPIB, 2002) are given in Table 1.