Pop quiz! What are the dimensions of a 2×4?
Wrong answer: 2 inches by 4 inches. WAIT, WHAT?!
Correct answer: 1½ inches by 3½ inches. If you walked into a lumber yard in the last 50 years and asked for a finished 2×4, 1½ by 3½ are the ACTUAL dimensions of the cross section you’d get. “2×4” are the NOMINAL dimensions – they’re a shorthand for the true measurements with one-third of the syllables. Similarly, if you ask your lumber yard for a 2×6, they’ll give you a board measuring 1½ by 5½. And if you ask for a 2×8, they’ll give you a board measuring 1½ by 7½, right?
Wrong again. A 2×8 is actually 1½ by 7¼. HUH?!?
Fortunately, all this stuff is tabulated, and there aren’t all that many commonly used cross sections to begin with. Every practicing carpenter you’ll meet in the US has pretty much memorized the table. I guarantee it.
The lost fractions of an inch aren’t a marketing ploy to give you less than you think you paid for. Back in the old days, the sawmill would take their newly felled “green” lumber and cut it right at 2 inches by 4 inches. Lumber shrinks as it dries, so carpenters don’t usually build with green lumber. Used to be, the lumber yard would sell you whatever finished size the (originally 2 by 4) lumber shrunk to. In the 1960s the finish sizes got standardized, and now the onus is on the sawmill to cut their green lumber so it shrinks to exactly the standard actual dimensions.
When you order your lumber, you’ll also specify a standard length – anything from 6 feet to 22 feet is readily available in two-foot intervals. But the length you get doesn’t necessarily match your order, either. A 10-footer may well measure 9 feet 11 inches, or 10 feet 1 inch. Now, in the end you’ll chop most of your lumber into shorter pieces, so the precise length doesn’t matter much. But if you DO want exactly the length you ask for, ask for a cut length, like 85½ inches. Terry says cut lengths are actually less expensive than standard lengths because the sawmill can cut boards too short for a standard size down to the length you need. They’re often painted on the ends for easy identification.
Lumber that is cut directly from felled trees is known as dimensional lumber, and it’s almost always sold by the sizing system I just described. But many other types of lumber exist. There’s glued laminated lumber, or glulam, made from several narrower pieces of lumber glued together. There’s laminated veneer lumber, or LVLs, made from lots of thin layers of wood. And there are non-rectangular shapes available, like I-beams and trusses, as we’re using for floor joists on the house. In general, these varieties are sold by true dimensions… so if you order a glulam 2×4, what you get will be precisely 2 inches by 4 inches. A bit simpler, yes, but if you want bang for your buck you’ll do most of your building with dimensional lumber. Get used to it.