A lumber grader with almost 30 years on the job once watched a buyer reject five good boards for wane (bark or missing wood along an edge), then hand-pick five replacements. All five were below grade. Most of us choose lumber by the price tag and by eye, and the eye picks wrong more often than you’d think. The stamp on lumber tells you what the eye can’t. It says whether a board is rated to carry a floor, what decides how far it can span, and roughly how much shrinking it has left to do after it’s nailed in place.

I framed and renovated houses for years with Wargo Construction, and I didn’t really know what that stamp said until I read the federal standard behind it. It’s a block of black ink a foot or two from one end of the 2x4s, 2x6s and other framing lumber in the aisle, and it names the mill, the grading agency, the grade, the species and how wet the wood was when it was surfaced (run through a planer to its final size).…