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Visual Grading, Lumber Strength, and Liability 
Question
Forum Responses
From Professor Gene Wengert, forum technical advisor: For a given grade, there are design values associated with that grade. As a general rule, the strength that is used for design is about 1/6 of the clear wood strength. Studies (Dr. Thomas McLain at Oregon State Univ. is a key person) have shown that there is a poor relationship between grade and strength. Therefore, a huge safety factor is included for the visual grades. When building a house or other frame structure, if we do get a weak piece, it is very likely that the adjacent pieces will be stronger and help carry the load (called load sharing) along with the OSB or plywood. So, using visually graded lumber, a structure is tremendously over-designed, strength-wise. Further, the design of a floor using visually graded material is actually based on the deflection limits, which are the governing factor before the strength is. So that gives even more safety, strength-wise. Of course, some argue that over-design is wasting wood. So, a trend is to test each individual piece of wood and come up with a good estimate of the actual strength and stiffness and then use these values to develop design values for a structure. This lumber is called MSR lumber (machine stress rated). In some countries, they also test the wood and then if they find a weak spot, they cut out the weak wood (like a knot) and glue the remaining pieces together (finger joint) and end up with a strong piece (which is tested again to be sure). A lawyer will probably tell you that you can be sued for anything, but the key for having to pay is that it must be shown that you were negligent. However, as trials of this sort are very expensive ($300,000 minimum for each side), it is likely that there will be a settlement prior to trial. If you have insurance, your company will handle this aspect, as it is their money.
From contributor R: One thing people have left out is that no grading agencies require 100% compliance with their grade rules. Usually 95-97% on-grade... This allows for a small number of errors to be made by graders, and I would think could possibly raise issues with the grading agency, but not the individual mill. From Professor Gene Wengert, forum technical advisor: The errors permitted are a total of all errors, including moisture, warp, knots, stain, etc... not all errors reduce strength. From contributor D: There was a board we had a tough time calling the grade on. Some called it a #3, others a #2. It technically made it as a 2. It broke at #1 design values. Typically when a board misses grade, it still doesn't drop below allowable strength values. At the sawbench you can often do more to boost grade. The ALSC, who oversees the various grading agencies, can probably answer the original question best. From Professor Gene Wengert, forum technical advisor: It is well to remember that 95% of the pieces of lumber in a grade will be stronger than the given value. This is because we use the 5% weakest piece to establish the strength. Hence, if you have a piece such as contributor D has, it would be common to see it break at a level for much higher grade... the average strength for a grade is higher than the 5% limit. Would you like to add information to this article? Interested in writing or submitting an article? Have a question about this article? Have you reviewed the related Knowledge Base areas below? 
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