by Nick Gromicko, CMI®, Kenton Shepard and Kate Tarasenko
A grade stamp is printed on lumber produced in North America, but what does it mean?
Since lumber comes from a natural source, much of it has naturally occurring defects, such as large knots or splits, and these can reduce its strength. Because of these and less obvious defects, lumber that leaves a sawmill must be appraised by trained inspectors and assigned a grading stamp.
Why are grade stamps required?
- 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.
What information is included in a grade stamp?
- 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.
CCA Pressure-Treated Wood
Wood-constructed homes, roof shakes and shingles, exterior decks, and children's play equipment that were built before 2004 and not made of cedar or redwood were likely constructed using wood that was pressure-treated with CCA, or chromated copper arsenate. CCA is comprised of arsenic, chromium and copper. In wood, CCA is used as a chemical preservative. CCA is also a pesticide.
The U.S. Consumer Product Safety Commission (CPSC) and the U.S. EPA (Environmental Protection Agency) received several petitions in 2001 asking that the use of CCA in wood playground equipment and other outdoor structures be banned because of concerns that physical contact with the wood and surrounding soil could increase the risk of certain kinds of cancer caused by the arsenic contained in CCA. Although an official ban was never imposed, several manufacturers voluntarily discontinued using CCA in their wood products.
However, wooden structures and residential components that use CCA pressure-treated wood may still exist in or on a property. Homeowners who are concerned about this can contact their local health department for information on how to have those structures and components tested for arsenic, along with information about their risk of exposure.
Home inspectors can check suspect lumber for an end-stamp that has the following information:
(Image courtesy of the CPSC and the EPA)
Some Final 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.
In summary, information about the wood's quality is contained in grading stamps, which are placed on every piece of manufactured lumber.
Some information in this article was provided by the U.S. CPSC and the EPA.