There are three main size categories for lumber:
1. Boards: Lumber that nominally is less than 2 in. (50.8 mm) thick and 2 in. (50.8 mm) or more wide. Boards’ thicknesses refers to the smallest cross-section dimension of lumber and the term width refers to the largest dimension. Boards less than 6 in. (152.4 mm) wide are classified as strips. Boards are used for sheathing, roofing, siding, and paneling.
2. Dimension lumber: Lumber with a nominal thickness of 2-5 in. (50.8-127 mm) and a nominal width of 2 in. (50.8 mm) or more. Dimension lumber ranges in size from 2 X 2 in. (50.8 X 50.8 mm) to 4 – 16 in. (101.6 X 406.4 mm). Dimension lumber is used for general construction where appearance is not a factor, such as studding, blocking, and bracing.
3. Timber: Lumber that is nominally 5 or more inches (127 mm) in the smallest dimension. Timber is used for columns, posts, beams, stringers, struts, caps, sills, and girders.
Lumber is also grouped according to size and intended use into several categories.
1. Light framing: 2-4 in. (50.8-101.6 mm) thick, 2-4 in. (50.8-101.6 mm) wide. Typical grades are Construction Standard, Utility, and Economy and are widely used for general framing purposes. Lumber under this category is of fine appearance but is graded primarily for strength and serviceability. Utility lumber is used where a combination of high strength and economical construction is
desired. An example would be for general framing, blocking, bracing, and rafters.
2. Studs: 2-4 in. (50.8-101.6 mm) thick, 2-6 in (50.8-152.4 mm) wide, 10 ft (3048.0 mm) long and shorter. Primary use is for walls, whether they are load-bearing or nonloadbearing walls.
3. Structural light framing: 2-4 in. (50.8-101.6 mm) thick and 2-4 in. (50.8-101.6 mm) wide. Typical grades are Select Structural No. 1, No. 2, and No. 3. This is intended primarily for use where high strength, stiffness, and fine appearance are desired. An example of a use would be in trusses.
4. Appearance framing: 2-4 in (50.8-101.6 mm) thick, 2 in. (50.8 mm) and wider. For use in general housing and light construction where knots are permitted but high strength and fine appearance are desired.
5. Structural joists and planks: 2-4 in. (50.8-101.6 mm) thick and 5 in. (127.0 mm) or more wide. Typical grades are Select Structural No. 1, No. 2, and No. 3. Intended primarily for use where high strength, stiffness, and fine appearance are required.