Lumber for building has two size references: nominal and dimensional. Nominal dimensions refer to lumber before it is cut and dressed to size at the mill. Dimensional lumber is the product that leaves a lumber mill after sizing and dressing.
Lumber of nominal size is exactly the size noted. For example, a 2 by 4 inch piece of lumber is actually 2 inches thick and 4 inches wide. Green lumber, before cutting to length and planing to finished size, is referred to as nominal.
Rough cut lumber of nominal dimensions is planed to finished dimensions and cut to length. A 2 by 4 inch piece of dimensional lumber is actually 1 1/2 inches by 2 1/2 inches after is passes through planing machines that smooth the product on four sides. Lumber finished in this manner is referred to as being surfaced on four sides.
Nominal lumber is green and wet, and retains a high percentage of moisture. Dimensional lumber is air dried to 19 percent or less with an average moisture content of 15 percent, according to the American Softwood Lumber Standard.
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