By Bud Coburn
- Builders, inspectors and other professionals use these grades to ensure that quality lumber is used where it is needed.
- Structural engineers take these grades into consideration when designing structures.
- Building codes widely used in the U.S. and Canada typically require graded, stamped lumber to be used in framing.
- In the U.S., there are six associations that develop and publish grade rules and issue grade stamps. They can be identified on a stamp by the following abbreviations:
- Redwood Inspection Service (R1S);
- Northeastern Lumber Manufacturers Association (NELMA);
- Northern Hardwood and Pine Manufacturers Association (NHPMA);
- Southern Pine Inspection Bureau (SPIB);
- West Coast Lumber Inspection Bureau (WCLB); and
- Western Wood Products Association (WWPA).
Several other agencies are licensed to use these grade rules and apply stamps of their own. Canada’s agencies, such as the National Lumber Grading Authority (NLGA), operate similarly and their rules are essentially the same as those in the United States. But beware that not all grade stamps are legitimate! One InterNACHI inspector reported seeing a set of stamps that was used for falsifying lumber grades.
- The species of lumber is stamped, and is also abbreviated. Some common examples include:
- “S-P-F” represents spruce-pine-fir, a common grouping for some of the Eastern softwoods.
- “DF-L” refers to Douglas fir and Western larch.
- “Hem-fir” stands for Western hemlock and true firs.
- The mill identification name or number is also included. For advertising purposes, mills pay grading agencies for the right to place a grade stamp on their lumber. When the mill subscribes, they are assigned an identification number by that grading agency. Some mills stamp their name or trademark on the lumber as well.
- The grade itself is indicated. A lumber grade is the quality-control standard for lumber that has been in place since such standards were instituted in 1960, following a revision to Canadian and U.S. building codes.
Lumber is graded using the American Lumber Standards, which are based on the structural integrity of a board. These grades take into account the size and location of defects, as well as the slope of grain, in order to predict the load-bearing capacity of the board. These factors are used to determine the percentage of clear wood in the board that, in turn, determines the grade. The most common grades and their clear-wood requirements are as follows:
- “Select” = at least 80% clear wood
- “#1 Structural” = at least 75% clear wood;
- “#2 Structural” = at least 66% clear wood;
- “#3 Structural” (“stud” grade) = at least 50% clear wood;
- “Construction Grade” = at least 57% clear wood;
- “Standard Grade” = at least 43% clear wood; and
- “Utility Grade” = at least 29% clear wood.
Inspectors are most likely to encounter #2 structural grade wood in houses.
- You’ll also find the moisture content of the wood, which is determined at the mill when the stamp is applied. Under the National Grading Rule, there are three moisture-content conditions:
- “S-GRN” (surfaced green) means that the moisture content is above 19%.
- Most lumber is dried to the “S-DRY” (surfaced dry) condition, meaning that that the moisture content is less than 19%.
- “MC15” means that the moisture content is less than 15%.
Lumber is dried for the following purposes:
- to reduce the risk of insect and fungal damage;
- to reduce shipping weight and costs;
- to control the amount of shrinkage that takes place; and
- to make gluing and finishing more feasible.
Damper regions of the world often require kiln-dried wood for construction, which must have a moisture content of 19% or less. The additional expense of kiln-dried wood is the reason it is used in only a small portion of construction.Keep in mind that lumber, which may leave the mill very wet, is wrapped in plastic and stays wrapped until it’s uncovered at the job site. It can retain a lot of moisture and develop large mold colonies, which are then incorporated into the walls of the home when that lumber is used for framing.
A few notes…
- Inspectors should not use these guidelines to guess the grade of lumber that has no visible grade stamp. There are plenty of #3 grade panels that that appear clear of all knots, and #2 grade panels may have knots, splits and generally look awful.
- Inspectors may also find stamps signifying that wood has been harvested sustainably. “Eco-labels,” as they are sometimes called, are an easy way to identify materials that have been grown, harvested and milled in an ecologically sensitive manner. As of December 2006, more than 200 million acres of forest in 76 countries have received certification by the Forest Stewardship Council (FSC) in an effort to promote sustainable harvesting. Lumber bearing an FSC stamp has been approved by that organization.
- Lumber that has been treated with flame-resistant chemicals may bear a “D-BLAZE” notation on its grade stamp.