The American Lumber Congress first established nationwide standards for lumber dimensions in 1924 in the midst of the construction boom that followed World War I. The standards govern the sizes of dressed, or surfaced, and undressed, or rough-sawn, lumber. Standardization of lumber sizes and grades goes hand in hand with the Uniform Building Code to maintain safety of structures and regularity of building practices across the United States.
Early American builders had ready sources of lumber that they could cut to any size they needed. As local sources dwindled, however, and lumber increasingly had to be shipped, the need for standardization of sizes became evident. When it established those standards in 1924, Congress allowed leeway for surfacing, or dressing, of dried lumber, so that the actual width and thickness of a board were less than the nominal values. The current standards for dimensioned lumber sold in the United States were set by the American Lumber Standards Committee in 1963.
Nominal and Actual Thickness
Prior to the 1963 modifications, the actual thickness of a surfaced 1-inch board was 25/32 inches, and before 1929, it was 13/16 inches. As of 2012, it is 3/4 inches. Surfaced boards with nominal thicknesses of 2 inches or more are actually 1/2 inch thinner than their nominal values. Similarly, there is a 1/2-inch difference between the nominal and actual values of the widths of surfaced boards, no matter what the thickness. Consequently, the actual dimensions of a 2-by-4-inch framing stud are 1 1/2 by 3 1/2 inches, and those of a 1-by-4-inch trim board are 3/4 by 3 1/2 inches.
Surfaced construction lumber, whether fir, pine, redwood or cedar, is sold by the foot and available in 2-foot increments from 6 to 16 feet at most lumber yards. Interior trim lumber may be available in shorter or longer lengths. An exception is 2-by-4 framing studs, which have a length of exactly 92 5/8 inches. This length allows them to fit between 2-by-4 top and bottom plates to form an 8-foot wall with a small allowance at the bottom for easier drywall installation. 1-inch lumber is also available at many outlets in 4-foot lengths for use as fencing.
The nominal dimensions of unsurfaced, or rough-sawn, lumber are the same as the actual dimensions, so a 4-by-6 rough-sawn beam is actually 4 inches thick by 6 inches wide. Builders usually avoid mixing surfaced and unsurfaced lumber in a project to avoid mismatched joints. Moreover, retailers usually conform to a different standard when sizing hardwood lumber. Nominal thicknesses, expressed in increments of 1/4 inch, are the same as actual thicknesses. Thus a 4/4 length of hardwood is actually 1 inch thick, and a 5/4 length is 1 1/4 inches thick. Nominal and actual widths are also equal, and the wood is sold by the board-foot.
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