I was relaxing a few days ago reading an article in the October issue of Fine Woodworking entitled "Build Perfect Drawers" which had a section called "Wood matters, a lot" that contained this statement (p. 45):
So if you need a 1/16"-gap at the top of a drawer made with the pine, [for flatsawn white oak] you'd need to leave a gap that's five times as big: a whopping 5/16 in.I omit their development of this conclusion but I don't think I am taking it out of context or misrepresenting it. I reacted with incredulity, thinking that, even though my white oak is rift sawn, a gap anywhere close to this would make the drawers look absolutely awful. How could this be? White oak would shrink a full quarter of an inch more than pine on a 3" drawer? I've owned white oak furniture and I never saw anything like this. After doing some research, I think the article is incorrect and I want to provide an explanation. The silver lining for me is that I think I understand this subject fairly well now but, if you think I am wrong, please, please comment below.
The best treatment of this subject I could find is by the National Wood Flooring Association (NOFMA), which you can find here. It is worth reading, but I'll give you the short version.
Wood reaches an equilibrium moisture content (EMC) based on its environment, not instantaneously but with a lag. I am discussing indoor furniture so it is the indoor environment that is relevant here. The indoor environment is influenced by the outdoor environment but it is obviously not the same. A sufficiently sophisticated HVAC system could maintain constant indoor environmental conditions year round regardless of the local climate and, if it did, the wood would not expand and contract. In reality indoor conditions do vary with the seasons so the question is how much the EMC of wood indoors changes in reality. The USDA Forest Products laboratory (FPL) has done empirical studies and produced a map showing ranges for different regions of the country. As it happens, both my son and I live in an area with extreme variation, the west coast along the Pacific Ocean, where the average range of EMC is 8-13%. There are the usual problems with averages, but the map is pretty detailed. So, we would expect the equilibrium moisture content to vary by 5 percentage points during the year in this region.
How much shrinkage and expansion will result from this 5 percentage point variation in EMC? Once again the FPL has developed coefficients based on wood species. For white oak, they are .00365 for plainsawn lumber and .00180 for quartersawn lumber. To use them you multiply the coefficient by the percentage change in EMC and multiply the result by the width of the piece of wood. I have rift sawn lumber so it is reasonable to choose the midpoint of plain sawn and quarter sawn coefficients, which is .00273, multiply it by 5 (percentage points) and multiply the result by 3" (the height of my drawers) for an expected shrinkage/expansion of .04", a bit more than 1/32"!
If you don't care to do the calculations by hand, Woodweb offers an online shrinkage calculator here. It provides results consistent with those above. For example, predicted shrinkage for a 3" flatsawn white oak drawer is .0563" vs. .0548" using the coefficient above.
Maybe my seat of the pants method isn't so bad after all. There is a rub though. I really need to know my starting point. What was the moisture content of my lumber when I built the drawer? After all, it could be at the high point, the low point, in between or even, conceivably, outside of the range. Given the time of the year, I guessed that the EMC was at the bottom end of the range so I made the drawer a little loose. In order to do any better, I will need to buy a moisture meter. Maybe the gap will be a little too big. Doesn't really matter. My son just graduated from law school so he should be used to big gaps by now! :-)