Why Lumber Variability Is a Problem for CMs


Why Lumber Variability Is a Problem for CMs

ALSC’s PS 20-15 has fundamentally
changed what this grade stamp represents.

Few issues have demanded the attention of the SBCA Board of Directors more over the past five years than the variability of lumber design values. SBCA Legal Counsel Kent Pagel provided guidance to component manufacturers (CMs) on ways to mitigate the potential risk and liability that have arisen in the market as a result of how the lumber industry has chosen to deal with the issue of variable design properties.

Visually Graded Lumber

While it is impossible to get around the fact that lumber is an organic substance, and therefore has inherent variability, it has always been used as the primary raw material of the residential construction industry. In fact, the growing reliance on the structural design properties of lumber by the US housing industry led to the development of lumber grading processes and grade stamps in the 1920s. The process today assigns global strength properties to an individual stick of lumber by a visual inspection of qualities like knots and grain. 

This process works well as long as the global strength properties assigned to a particular species, size and grade of lumber exist for each particular stick of lumber. As Pagel made clear, that’s not always the case. The Southern Pine Inspection Bureau (SPIB), the entity responsible for establishing the global strength properties of Southern Pine (SP), determined the published design values that existed in 2010 were dramatically overstated. In turn, they published both Supplement Numbers 9 and 13 to the Standard Grading Rules for Southern Pine Lumber, recommending design value reductions of 25-30 percent for almost all sizes and grades of SP.

It has been well documented in the pages of SBC Magazine the lengths to which SBCA went to protect the best interests of CMs throughout SPIB’s process to change the design values of SP lumber (see September/October 2014). Indeed, the SP market transitioned to the new lumber design values rather seamlessly, but the process did create a problem for CMs that has quietly festered.

Strength Properties Represent a Range of Values

While they did not publish this fact, Pagel pointed out that SPIB realized it had created a problem for SP lumber producers. If a structure failed due to an individual stick of lumber not having the strength properties published by SPIB, the lumber producer could be held responsible. SPIB added the following language to Appendix A of the Standard Grading Rules for Southern Pine Lumber in an effort to disclaim or avoid any warranty with respect to published design values:

Wood is a natural product subject to variations […]. Each piece or lot of visually graded lumber is not mechanically tested to verify strength properties. Since the stress ratings are representative of the entire producing region, lots from a specific location may have physical properties at the extremes of the property range or statistical distribution representing that range of strength values.

This change was immediately adopted by the American Lumber Standards Committee (ALSC), the body created by the U.S. Department of Commerce to oversee the lumber writing agencies such as SPIB. The new SPIB language essentially states visually graded lumber design values are not reliable for any particular stick of lumber.

This language creates a problem for CMs, Pagel said, as there is a growing reliance on the structural properties of visually graded lumber, and lumber producers rarely provide any type of express warranty with respect to the strength properties defined by the grade stamp on their lumber. The concern that arises is whether this renders CMs responsible for the performance of the engineered products they manufacture, even when the lumber design values on which that engineering depends don’t accurately represent the actual strength of the lumber CMs use. As the lumber industry refuses to accept responsibility for design properties in visually graded lumber, responsibility appears to shift to any vendor or entity that assumes the accuracy of the design values, including CMs, their suppliers and their customers.

This tactic of the lumber industry was further solidified last April when ALSC adopted the latest version of its American Softwood Lumber Standard, also known as PS 20-15. PS 20-15 now defines design values as “published design data that are representative of the strength and stiffness of specific grades and species/species groups of lumber.” In practice, this definition means engineers who use lumber design values in NDS equations are working with approximate numbers. The design vales that should be ensuring structural components provide resistance to code-defined loading conditions are now merely representative of the strength and stiffness of actual materials.

SBCA advocated for many changes to PS 20-15 that would have created greater transparency in how lumber design values were determined and the inherent limitations of that approach. These recommendations were summarily rejected by ALSC, but are now under appeal by SBCA.

Define Your Scope of Work

In response to the lumber industry’s attempt to shift liability, Pagel recommended CMs clearly define their scope of work and utilize language for their customers, project design professionals and building inspectors in their contracts, submittal cover letters, truss design drawings and truss placement diagrams. SBCA has developed template language for all these documents clarifying how CMs rely on lumber grade stamps and use the corresponding design values in their design software. Pagel stressed that incorporating this language into their documentation is one of the most effective things CMs can do to limit their legal responsibility in the future.

This article was based on the 2015 BCMC Educational Session How Lumber Design Value Variability Affects Legal Responsibilities. Many thanks to Kent Pagel (Pagel, Davis & Hill, P.C.).