Lumber Sizes

Difference between Nominal and Actual Lumber Sizes

There are two types of Lumber: Softwood Lumber and Hardwood Lumber

The softwood species are Douglas fir, spruce, or Hem-Fir which are used for manufacturing of trusses and wall panels.

The hardwood species are oak, maple, birch, and mahogany which are used in fine cabinetry and furniture.

As truss designers our focus is on Softwood Lumber.

The term Dimensional Lumber is used with softwood lumber. When softwood lumber are cut to pre-defined standard sizes they are called Dimensional lumber. Dimensional lumber sizes (such as 2x4) refer to the depth and width of the material, not to its length.

Originally, Softwood was cut to exact dimension. But, after the wood was dried the Lumber boards shrank. This created the nominal and actual dimensions.

Nominal dimension is the name of the lumber: 2x4

Actual dimension is the real size of nominal lumber: 1 1/2 inches by 3 1/2 inches

In the United States, softwood lumber is governed by the National Institute of Standards American Softwood Lumber Standard (PS 20)

Note: When referring to a specific piece of lumber, the nominal size is used.