It's official: effective June 1, Southern Pine #2 2x4 lumber has been downgraded. Testing of randomly sampled 2x4s from sawmills across the Southern Pine-producing region has shown that the strength of the lumber has declined significantly since the official design values were published several decades ago. As a result of the new testing, official "design values" for Southern Pine #2 2x4 wood have dropped by about 30%. Other dimensions and grades are currently undergoing sampling and testing; it appears likely that when this testing is complete, the design values for larger dimensions and higher grades of Southern Pine will likewise be decreased — by how much, is yet unknown.
Coastal Connection has been following this story closely, keeping in touch with the agencies involved, as well as with industry organizations and academic researchers who are studying the issue. It will take years for the full implications of the Southern Pine strength downgrade to play out across the construction industry. For now, here is a quick Q&A on the important questions raised by this event.
• What are design values?
— Design values are published by official lumber grading agencies based on randomized statistical sampling and testing of lumber produced for market. Design values describe the key characteristics of wood as an engineering material: strength in bending, density, and tensile strength. The published design values for all grades and species of lumber are the numbers that engineers use to design wood structures. Design values also determine the allowable spans for floor joists and rafters, developed by the American Wood Council (AWC), which are published in span tables and referenced by the prescriptive code to guide the construction of buildings where case-by-case design engineering is not required.
Because wood is a highly variable material, design values are based on average strength measured in testing of many individual samples. The design values are extremely conservative: they are based on the tested value of the weakest 5th percentile of the wood. In other words, 95% of the wood in the marketplace in any grade can be expected to perform better than the published design value for that grade.
• Why is Southern Pine wood getting weaker?
— Not all Southern Pine lumber has gotten weaker. But the average strength of Southern Pine lumber has gone down. In particular, the weakest 5th percentile of Southern Pine lumber in the marketplace has become weaker. This appears to be the result of changes in the way Southern Pine is grown, harvested, and sawn. Southern Pine is mostly produced in plantations, rather than sawn from trees in naturally occurring forests. Plantation growing practices have resulted in a high proportion of young trees which were grown rapidly for frequent harvest, with more weak "juvenile wood" and fewer annual growth rings per inch. In addition, sawmill technology has changed in recent decades to allow production of lumber from logs as small as 6 inches in diameter. As a result, a greater proportion of 2x4 lumber on the market now shows lower values for bending strength and tensile strength than in decades past. However, some Southern Pine, such as wood harvested from naturally reproducing stands in the National Forest, is still as strong as ever.
Note: Forestry experts saw this coming a long time ago. For a 1991 report from USDA Forest Products Laboratory's David Kretschmann about weakness of Southern Pine sawn from 16-inch-diameter, 28-year-old trees, see: "Ultimate Tensile Strength and Modulus of Elasticity of Fast-Grown Plantation Loblolly Pine Lumber," by David E. Kretschmann and B. Alan Bendtsen.
• Are other species of wood also getting weaker?
— Apparently not, but the jury is still out. After testing of Southern Pine #2 2x4 lumber showed a surprising decline in strength, the American Lumber Standards Council (ALSC) Board of Review instructed grading agencies for all other species of lumber in the U.S. market to conduct comprehensive testing of all sizes, grades, and species of lumber. Results of that testing for S-P-F #2 2x4 (Spruce-Pine-Fir), a lumber produced in Northern and Western regions, have been reported — and the testing indicates no decline in strength for S-P-F wood. In the coming year or two, updated values for all species, sizes, and grades will be reported. If the grading agencies see a change in strength for any type of lumber, design values for that lumber will eventually be changed.
• Will the change in Southern Pine design values affect the building codes?
— Eventually, but not right away. The Structural Building Components Association (SBCA), an organization of wood truss manufacturers, wrote a formal letter to the International Code Council asking if the design value change would result in an immediate change in building code requirements in the field. On May 10, Tom Frost, Senior Vice President for Technical Services at the ICC, responded: "The ICC has made no determination regarding the lumber design value reductions, the Board having denied the American Wood Council's (AWC) request for an emergency amendment at this time." But Frost repeated the comment of the ALSC Board of Review from January 5, 2012: "All design professionals are advised in the strongest terms by the Board to evaluate this information in formulating their designs in the interim period."
Even if the ICC were to adopt an emergency amendment to the International Residential Code (IRC) and the International Building Code (IBC), that change would not take effect in any states or cities unless the local authorities endorsed the change. Wrote Frost: "Please note that section 2.4.3 of CP #28 states, in part, that ‘Emergency amendments to any Code shall not be considered as a retro-active requirement to the Code. Incorporation of the emergency amendment into the adopted Code shall be subjected to the process established by the adopting authority.'" (For more, see: "ICC Indicates Hands-Off Approach Regarding SP Design Value Changes," SBC Magazine.)
• Does this mean the buildings I've already framed with Southern Pine are unsafe?
— No. Nobody is predicting any kind of structural failures as a result of this issue. Conventional wall, floor, and roof framing is very conservative, with multiple members sharing the loads and with many factors of safety built into the span and load tables. There have been no failures reported of wood in service related to this issue, as far as we are aware.
• What about wood trusses made with Southern Pine?
— Trusses are an example of value-engineered design, where the strength of a single truss chord or web member could affect the performance of the entire truss or the entire roof. However, even truss systems benefit from the redundancy of using multiple trusses to share a roof or floor load. In any case, manufacturers have had at least six months to adjust their designs to reflect the new values for Southern Pine. Truss makers now should be using the new design values in all their engineering calculations.
• Should I adjust my framing practices to reflect this change?
— That's a judgment call. If your building includes any design engineering, your engineers should always use the latest published design values. If you‘re already in construction on a project framed with Southern Pine, with an engineered design that used the old design values, you may want to have your engineer re-evaluate those designs. If the building is designed using the prescriptive code and the current span tables based on the old Southern Pine design values, your building still complies with code until such time as your local jurisdiction changes its code.
Wall framing is extremely conservative, and will likely not be affected greatly by this change. Roof and floor framing may be affected more significantly. The biggest concern would be with rare cases where a single member is relied on to handle some sort of critical load — for example, in the case of a knee brace on a deck frame. In those situations, using larger dimension lumber, doubling up members, and carefully selecting lumber with close grain spacing, straight grain, and no knots, would all be good practice.
If you want to make allowances now for the changing design values, there are updated, more conservative span tables available, but they have not yet been incorporated into the building code. The American Wood Council has published addenda to its 2012 Wood Frame Construction Manual, as well as its recommended amendments to the building code span tables, at this link: "Codes and Standards Addenda and Amendments Related to Design Value Changes."
And remember: if you're framing a structure or building a component where it's important to have full assurance of the lumber's actual strength, you can always use machine stress rated (MSR) lumber. MSR lumber is mechanically tested piece by piece at the mill, rather than visually graded. MSR lumber's performance is much more predictable and more uniform, making it ideal for engineered applications or critical uses.