The American Lumber Congress first launched nationwide criteria for timber dimensions in 1924 at the middle of the building boom that followed World War I. The criteria govern the dimensions of dressed, or surfaced, and undressed, or rough-sawn, timber. Standardization of timber sizes and levels goes together with the Uniform Building Code to maintain safety of structures and regularity of building practices across america.
Early American contractors had ready sources of timber which they may cut into any size they needed. As local resources dwindled, however, and timber increasingly had to be shipped, the demand for standardization of dimensions became evident. When it established those criteria in 1924, Congress allowed leeway for surfacing, or dressing, of dried timber, so the true width and thickness of a plank were less than the nominal worth. The present criteria for dimensioned lumber sold in the USA were set from the American Lumber Standards Committee in 1963.
Nominal and Actual Thickness
Prior to the 1963 alterations, the true thickness of a surfaced 1-inch plank was 25/32 inches, and before 1929, it had been 13/16 inches. As of 2012, it is 3/4 inches. Surfaced boards with nominal thicknesses of 2 inches or longer are actually 1/2 inch thinner than their minimal values. In the same way, there’s a 1/2-inch gap between the nominal and real values of the widths of surfaced boards, no matter what the thickness. Consequently, the real 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 timber, whether walnut, pine, redwood or cedar, is sold by the foot and also available in 2-foot increments from 6 to 16 ft at most lumber yards. Interior trim timber might be available in shorter or longer lengths. An exception is 2-by-4 framing studs, which have a span of just 92 5/8 inches. This length allows them to fit between 2-by-4 upper and bottom plates to create an 8-foot wall having a little allowance at the bottom for easier drywall installation. 1-inch timber can be available at several outlets in 4-foot lengths for use as fencing.
The nominal dimensions of unsurfaced, or rough-sawn, timber are just like the real dimensions, so a 4-by-6 rough-sawn beam is actually 4 inches thick by 6 inches wide. Builders usually avoid blending surfaced and unsurfaced timber in a project to prevent 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 real thicknesses. Thus a 4/4 period of hardwood is actually 1 inch thick, and also a 5/4 span is 1 1/4 inches thick. Nominal and real widths will also be equal, and the timber is sold by the board-foot.