Rough cut lumber is wood that has been cut by a sawmill from a large log. When a log is processed in a sawmill, it is cut into pieces that will be sold as boards, dimensional lumber, veneers and other components. Very rough in texture, this wood nevertheless conforms to the nominal sizing standards for dimensional lumber and boards.
Dimensional lumber, used extensively for framing houses and other structures, is originally cut to certain dimensions of width and thickness. Measured in inches, the most commonly known dimensional lumber is the “two-by-four,” or 2x4 (approximately 5.1 x 10.2 cm). Dimensional lumber is commonly available as various combinations of 1, 2, 3, and 4 inches (approximately 2.5, 5.1, 7.6, and 10.2 cm), although larger sizes are available as well. Lengths are specified separately, as in “an 8-foot 2x4.”
Most dimensional lumber is softwood, that is, from coniferous trees like pine. Hardwood, from deciduous trees like maple and oak, is more often used to make veneers and boards and plywood, which in turn are used for paneling, furniture construction, and other such items. The measurement of hardwood boards is different from the “two-by” standards in that only the thickness is generally measured, and boards of varying, non-standard lengths and widths are available in a lumberyard. These boards are delivered to the lumberyard as rough cut lumber, or with one or both sides surfaced.
Logs are cut shortly after harvesting, but the rough cut lumber still has a very high moisture content and must be dried. The product of a living organism, lumber reacts to environmental influences such as heat and humidity. Much rough-cut lumber will twist and warp during the drying process, and to recover it for productive use, it will be surfaced &emdash; cut, jointed, and planed &emdash; to produce a straight, flat, smooth piece of lumber. This process removes significant quantities of stock, so the final piece of lumber offered for sale will be markedly smaller than the original &emdash; “nominal” &emdash; size of the lumber. A finished 2x4, for example, will measure exactly 1.5 x 3.5 inches (38mmx89mm). Likewise, a hardwood board that's been rough cut to 1 inch (2.54 cm) &emdash; usually referred to as “4/4” &emdash; will be 7/8-inch (2.22 cm) thick if surfaced on one side only, and 13/16-inch (2.06 cm) if surfaced on both sides.
Rough cut lumber is often used by woodworkers because it's much less expensive than finished lumber, which must be surfaced smooth and flat before being sold. In addition to its relatively low cost, rough cut wood is thicker than the surfaced lumber available in a lumberyard, giving more room for error in their projects. In addition, when woodworkers do their own surfacing, they can plane or joint the wood exactly to their specifications, and many report that they can often produce a smoother, flatter surface on the wood than they'd be able to buy from the lumberyard. A major drawback of buying rough cut lumber, though, is that it's still green, and must be dried. A rule of thumb for drying rough cut lumber is to give it a year of drying time for every inch of thickness.