What Is Dimensional Lumber?

What Is Dimensional Lumber?

Dimensional lumber is a term that refers to wood that is cut and finished in standardized sizes. This type of lumber is commonly used for residential framing and wall paneling. There are two main types of dimensional lumber, nominal and actual. Both types have a set of measurements that are based on the depth and width of the lumber.

Nominal And Actual Sizes Of Lumber

Oftentimes, lumber yards or home improvement stores will advertise and sell framing and softwood boards in dimensions that are larger than the actual size of the board itself. That can be very confusing to someone who isn’t familiar with woodworking and math. The answer is that boards are referred to by their “nominal” sizes before they’ve been surfaced or milled (surfaced on all four sides). When lumber is actually surfaced, it becomes the final, or actual size of the board. This is different from plywood, which has a nominal thickness but the actual size will be thinner because of the process of sanding the panel during manufacturing.

​Historically, softwood logs were cut green into 2×4 boards that were close to full width and thickness but would naturally shrink as the boards dried in the kiln and after planing and milling. Today, lumber mills account for this by shaving off a small amount of each board so that they can produce more from each tree they harvest.

Thickness And Width Of Lumber

The actual size of a piece of lumber depends on how much it has been surfaced or planed. Lumber is sold either rough-sawn (S1S) or surfaced on one, two, three or all four sides (S2S, S3S, S4S). When a board is surfaced, it loses about 1/4 of an inch from each of its four sides. This reduction in thickness and width is called a “rough dimension.” When a board is surfaced on all four sides, it is considered to be the final or actual size. This difference is what makes dimensional lumber sizes misleading.

Length And Width Of Lumber

When shopping for dimensional lumber, it’s important to understand the difference between length and width of the lumber. For example, a 2×4 is actually 1 1/2 inches thick and 3 1/2 inches wide, not two inches by four inches and eight feet long as you might expect. This discrepancy is due to the way lumber is manufactured. Lumber mills measure a fresh board and mark it with a nominal size before it’s dried and planed to match a consistent profile and dimension. As the board dries, its dimensions shrink. Depending on the type of wood and its moisture content, the shrinkage can range from 2% to 15%.

Length And Thickness Of Lumber

The actual dimensions of a lumber board can vary slightly, depending on the wood species and moisture content of the logs. Lumber mills typically account for this shrinkage during kiln drying and planing, so the final dimensions are consistent. Some dimensional lumber is also treated with chemicals that protect the wood from insects and fungi. Treated lumber is a popular choice for outdoor projects that are exposed to changing Midwestern weather and heavy use. Dimensional lumber is typically available at home centers and building supply stores. Most building supplies stock only a few grades of lumber, but you can order non-stocked grades from your local dealer or lumber trader!