Lumber grading and stamps ensure that the lumber type chosen for each project has the characteristics to provide the load-bearing strength, quality, and appearance appropriate to that particular application. SBCA Magazine published the following article on the importance of understanding lumber grading and stamps as a critical component of a project’s success.
Seeing the Lumber Amongst the Trees. (Published with permission)
Included in the cutting-edge software used to design roof and floor trusses are engineering principles outlined in the National Design Specification, Design Values for Wood Construction (NDS), as well as the published design values for the sizes and grades of softwood lumber used in North America. For component manufacturers utilizing truss design software, it’s important to know how those published design values are derived and how they impact truss designs, as well as understand the impact subtle differences between species groups have on expected truss performance.
How Lumber Design Values are Established
The NDS Supplement provides a succinct description of the published lumber design values:
“Reference design values for most species and grades of visually graded dimension lumber are based on provisions of ASTM Standard D 1990-16 (Establishing Allowable Properties for Visually Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens)…Reference design values for machine stress rated (MSR) lumber and machine evaluated lumber (MEL) are based on nondestructive testing of individual pieces…The stress rating systems used for MSR and MEL lumber is regularly checked by the responsible grading agency for conformance to established certification and quality control procedures.”
In other words, softwood lumber species groups are regularly subjected to established testing protocols to ensure the published design values are present in the lumber that is produced. In North America, there are six grading agencies that have jurisdiction over establishing softwood lumber design values and are responsible for conducting ongoing testing to verify them. They are: National Lumber Grades Authority (NLGA), Northeastern Lumber Manufacturers Association (NeLMA), Redwood Inspection Services (RIS), Southern Pine Inspection Bureau (SPIB), Pacific Lumber Inspection Bureau (PLIB/WCLIB), and Western Wood Products Association (WWPA).
Each of these grading agencies follow grading rules approved by the American Lumber Standard Committee’s (ALSC) Board of Review (BOR) and are certified for conformance with the U.S. Department of Commerce Voluntary Product Standard (VPS) process. For softwood lumber used in the U.S., the VPS is maintained by the ALSC and is referred to as American Softwood Lumber Standard PS 20 (PS 20). The current edition is PS 20-20, which can be found at: https://bit.ly/3VY6vHi.
Once each grading agency completes its regular testing protocols, it’s required to report to the ALSC BOR the resulting data, along with any recommendations it has regarding changes to current published design values. Fortunately, changes are rare. The most significant in recent years involved SPIB and their recommended changes to the design values of Southern Pine (SP) in 2012.
Again, these published design values are integrated into component design software. The parameters of each truss assume that the lumber species, grade, and size used in the manufacturing process match the design input. This assumption emphasizes the importance of the lumber grade stamp placed on each stick of dimensional lumber that helps users identify key structural properties.