Q. I am trying to use 2 x 4 and 2 x 6s from the local lumber yard, but I see that the actual size varies a little bit. Why is this, as I thought all pieces were supposed to be the same?
A: There is a lumber manufacturing standard that every manufacturer abides by. With respect to sizes, a dry 2 x 4, after planing, is 1.5 inches x 3.5 inches. A 2 x 6 is 1.5 inches x 5.5 inches.
Here is the issue: This size is the size at the time of grading (which usually is done right after planing to size). It is common to have a moisture change occur after grading when used in an interior location which is usually quite dry.
As part of the grading stamp that is put on the lumber, the moisture condition is noted, with phrases such as S-GRN (surfaced green, which means somewhere over 20 percent MC), DRY (indicating an average of around 19 percent MC), KD15 or MC15 (indicating that the wood has been kiln dried to around 15 percent MC and perhaps a few pieces as high as 18 percent MC, but not many) and others.
Also, the grading ink stamp will indicate the agency under which the lumber was graded, so you can go on line with that agency to obtain precise terms and meanings.
So, now for furniture and other interior wood products, the final, in-use MC in much of North America will be close to 8 percent MC, a percent or two drier in the winter and a bit higher in the summer.
As a very rough rule of thumb, pine shrinks 1 percent when the moisture drops 4 percent MC. So, with varying MC at the time of grading and this natural shrinkage as wood dries, plus variations in wood of the same species, there will be some variation (minor variation) in final, in-use size, unless you resize it yourself.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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