Dimension lumber is softwood lumber that is nominally 2 inches thick and of various lengths and widths. It is the structural softwood lumber used in most wood-based housing construction (2x4 platform-frame construction) in North America. Some engineered wood products, such as laminated veneer lumber, can be used in place of larger dimension lumber pieces.
Dimension lumber is produced in various widths (in nominal increments of 2 inches) and various lengths (in increments of 2 feet). The actual cross-sectional dimensions are less than the nominal dimensions, because the wood shrinks as it is dried and it is planed in its final production step. A “two-by-four” is therefore actually 1.5 inches thick and 3.5 inches wide in its finished form.
In Canada, most dimension lumber is produced from spruce-pine-fir (SPF). In the U.S., most dimension lumber is produced from southern yellow pine. Dimension lumber is usually graded on the basis of the quantity of natural characteristics present in each piece of wood (e.g., size and location of knots, slope of grain, checks, wane and warp) and any manufacturing defects. Dimension lumber is usually dried to a moisture content not exceeding 19%. Some dimension lumber is machine stress rated (MSR).