What are lumber dimensions classified as in the building code?

Lumber dimensions are classified as nominal dimensions in building codes. This classification refers to the standard sizes used to identify lumber before it is processed and planed. For example, a piece of lumber that is marketed as a 2x4 actually measures approximately 1.5 inches by 3.5 inches when finished. This discrepancy arises from the reduction in size that occurs during milling and planing.

Nominal dimensions provide a consistent terminology for builders and inspectors, facilitating clearer communication and understanding within the construction industry. By using nominal dimensions in regulations and standards, building codes ensure that everyone involved in construction projects knows precisely what size lumber to expect, which aids in planning, ordering materials, and meeting design specifications.

In contrast, actual dimensions, structural dimensions, and engineered dimensions do not accurately capture the conventional method of classifying lumber sizes as nominal. Actual dimensions refer to the true size of a piece of lumber after processing, whereas structural and engineered dimensions pertain to specific performance characteristics or design parameters rather than standard sizing. Thus, nominal dimensions are the appropriate classification in the context of building codes.