Q. We have been buying some quartersawn oak lumber and it seems to be thinner than the lumber we have been buying. The supplier says that is normal. True or false?
A. There are specifications for the minimum thickness for hardwood lumber (NHLA) and softwood lumber (various grading agencies). These specifications apply to the thickness at the time of grading. If quartersawn lumber that is graded before final drying loses moisture through intentional drying, air drying, predrying, kiln drying, or through drying in its environment, it will shrink nearly twice as much in thickness as flatsawn lumber will. (On the other hand flatsawn will shrink nearly twice as much in width as quartersawn will.)
So for your oak lumber, if you have quartersawn, undried lumber that has the thinnest pieces around 1-1/16-inch thick, the lumber will shrink about 8 percent (almost 3/32-inch) or a bit more (depends on the oak specs) as it dries to 6 percent MC.
Note that because quartersawn dries perhaps 15 percent more slowly than flatsawn, it is not uncommon to find that quartersawn in a load with mostly flatsawn will be several percent higher in MC than flatsawn when it leaves the kiln, unless the kiln was equalized specifically to avoid this. Being higher in MC, it will dry as it reaches equilibrium with its environment and shrink very little in width, but more in thickness. If this final drying occurs in the customer’s home or office, some thickness defects related to flatness or smoothness could occur.
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