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The Straight Dope

Up a Tree

Why's a 2x4 not two by four?

By Cecil Adams
Posted // October 15,2012 -

Why is a two-by-four not actually 2 inches by 4 inches? I went to our local hardware store (no Home Depots in our little burg) and asked for a piece of lumber that was, as I had measured it, three-quarters of an inch by 3 1/2 inches. The girl looked at me funny and said, “You mean a one-by-four?” I said, a little embarrassed, “Um, yeah, I guess that’s what I need.” Then I got to looking around, and it dawned on me that lumber sizes have nothing to do with their actual dimensions. With all their professing to “measure twice and cut once,” why don’t carpenters seem to care about the actual size of the stuff with which they work? —Hunter, Craig, Colo.

Oh, they care. In fact, they’ve developed a special vocabulary to deal with the situation. Those in the building trades know one-by-four, two-by-four and so on are “nominal” dimensions—that is, in name only. If you want an honest-to-Jesus 1-inch-by-4-inch board rather than the usual anorexic stick, the magic words are “true size.” Better yet, tell the clerk you want “five-quarter” stock. Five-quarter boards, commonly used for exterior trim, are actually four-quarters of an inch thick—that is, 1 inch true size.

That’s absurd, you say. Few carpenters would argue. However, they have reality to contend with. Also greed. You’ll appreciate these two factors show considerable overlap.

First, reality. Years ago, cutting logs into lumber involved a lot of guesswork. The chief variable was the moisture content of the wood—green lumber shrinks as it dries. How much depends on how wet it was to start with. Typically, a 2-inch green board loses an eighth-inch of thickness once seasoned, but the actual difference may be more or less.

When sawmill operators adjust the “set-off” on their equipment—that is, the amount the log is advanced after each pass through the blade—they must allow for the kerf (or width) of the blade plus shrinkage. In the 19th century, they lacked an accurate way to gauge moisture content. So, they made the set-off a little wider than the nominal size, knowing the true size of the seasoned lumber would probably be a little less. The difference between nominal and true size was known as “scant” allowance.

The process was far from exact. I know this from examining the boards in the museum of antique lumber known as my house, which was built in the early 1890s. The nominal thickness of rafters and such clearly was 2 inches, and I’d say on average, true size was slightly less than that. But there’s quite a bit of variation, from 1 3/4 inches to 2 1/8 inches.

Carpenters in the 1890s dealt with this as best they could, judging from my house. They used thicker lumber as headers—that is, the horizontal boards above windows or on top of a line of studs, which carried a lot of weight.

As time went on, builders began demanding lumber of uniform dimension, so sawmill operators began planing boards after cutting them. Assuming you waited till the boards dried out and adjusted your planer accordingly, you’d wind up with a product of reliable size. However, it was also thinner. Now, the true size of lumber wasn’t slightly less than nominal, but a lot less.

How much less? That’s where the greed comes in.

Whether you’re milling lumber or making Hershey bars, smaller is cheaper. The driving concern at the turn of the century wasn’t so much the wood itself but the cost of freight. The virgin forests close to civilization had been cut down, and lumber had to be shipped from increasingly distant locations. In the early 1900s, you might pay $10 per thousand board feet at the mill and $20 to ship it. It occurred to lumber tycoons that if they did all their finishing out in the woods, thereby reducing the product’s bulk, they’d save a ton of money on freight. So that’s what they did.

But a problem soon emerged: Once nominal and true sizes parted ways, everything was up in the air. Southerners argued that southern yellow pine was stronger than northern white pine, and therefore could be cut thinner. Thus while two-by stock was 1 3/4 inches thick in most of the country, southern yellow pine manufacturers made theirs 1 5/8 inches. As lumber became a national commodity, builders complained about getting wood of different sizes. After World War I, a push for standardization began.

Years of wrangling ensued, as lumbering regions jockeyed for competitive advantage and debated arcane issues, at one point arguing over a thirty-second of an inch. It wasn’t until 1963 that modern sizes were agreed on, but the standard has endured ever since. Today, nominal one-by-fours are three-quarters by 3 1/2 inches, while nominal two-by-fours are 1 1/2 inches by 3 1/2 inches—confusing for novice carpenters and a bother for those rehabbing century-old houses, but otherwise a triumph of rationality over nature and the buck.

Send questions to Cecil via StraightDope.com or write him c/o Chicago Reader, 11 E. Illinois, Chicago 60611. Subscribe to the Straight Dope podcast at the iTunes Store.

 
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