Bob Falk, FPL wood engineer

>Hi, I am a wood engineer at the USDA Forest Products Lab and I believe I have the answer...here is a sidebar that I wrote for a recent Fine Homebuilding book I wrote...they didn't include it, however I think you might find it informative.

Why a 2x4 isn�t 2 inches x 4 inches?

Answering this question requires a bit of history about the evolution of lumber standards. As our country developed during the 18th and 19th centuries, sawmills sprang up to satisfy local demand for construction lumber. This lumber was not kiln dried (called green) and was supplied roughly sawn from the mill.

Lumber sizes varied by region because mills produced lumber based on local building customs and demand. Before 1895, no standards existed for the grading and sizing of lumber, though two inches had evolved over time as the most common rough thickness for joists, rafters, and studs and one inch as the most common thickness for boards.

During the second half of the 19th century, the wood products industry realized that they could broaden their markets geographically using the rapidly developing railroad system, which forced them to produce sizes matching the distant markets they supplied. Also, they realized that shipping green lumber (which was much heavier) was much costlier than shipping dry lumber. So, kiln drying of lumber became more common.

Like a cotton T-shirt, lumber shrinks with drying, and the once rough, green, full-sized 2 x 4 becomes smaller. Though dry lumber has the added advantage of staying straighter at the jobsite, the different drying rates of the various species of lumber being shipped around meant that there was still considerable variability in lumber sizes. The advent of surface milling, or planning, helped the move towards a more uniform (though smaller) size and easier-to-handle lumber (a smooth surface meant less splinters for carpenters). However, it wasn�t until 1969 that the U.S. Department of Commerce once and for all unified lumber sizes across the country. So, the once full-sized 2x4 was reduced to today�s 1-1/2� x 3-1/2� through a combination of drying, machining, as well as unifying the wide variety of species used to make the ubiquitous 2x4. Bob Falk