Nominal Size vs. Actual Size
Nominal size refers to the size of the board before the drying and planing process. So if you are purchasing lumber that is 2 x 4 inches, you are really getting the shrunken version of that lumber, which is roughly 1-1/2 by 3-1/2 inches. See the chart below for more approximations.
Lumber & Boards Usage
The chart below specifies the uses for lumber and boards in relation to their sizes. The uses for boards applies to all 1 x 2 to 1 x 12 sizes, while the framing lumber sizes each have specific applications. Sizes are represented in inches and in nominal form (see above for actual sizes).
Boards 1 x 2 to 1 x 12 (in.)
Framing Lumber 2 x 4 to 2 x12 (in.)
2 x 4: Studs, Purlins, Plate
2 x 6: Studs, Plate, Sills, Fascia, Ceiling Joints
2 x 8: Floor/Ceiling Joints, Sills, Fascia, Headers
2 x 10 and 2 x 12: Floor/Ceiling Joints, Rafters, Fascia, Headers
Dry Lumber vs. Green Lumber
Dry lumber is also known as “seasoned.” It can be dried in kiln-dried or air dried. The purpose of drying is to prevent shrinkage and warping after construction. Green lumber is freshly cut with no drying process. It is high in moisture and prone to warp as it naturally dries.
Lumber that has been dried through either
evaporation (air dry) or forced via kiln-dried and
heat treatment to 19% (or less) moisture content.
Dry lumber is more resistant to rot and insects,
and is easier to paint or stain.
Lumber that has been recently milled, and not
seasoned or dried, whether from evaporation or
forced via kiln – thus containing more moisture.
The main reasons for using green lumber is for its
cost, flexibility, and ease of use.
Dimensional Lumber Classes
Dimensional lumber is finished/planed and cut to standard widths and depths. It is categorized based on size classification and used in constructing the wall, floor, and roof framing of a structure.
Material that is between 2 and 4 inches thick and varies in length
Material that is between 3 and 4 inches square thickness and varies in length
Material that is between 5 and 20 inches thickness and varies in length
Material that are usually less than 2 inches thick and varies in length