Don’t be surprised if your two-by-four turns out to be 1.5 by 3.5; and no, your building supplier isn’t shortchanging you.
Lumber dimensions appear in two forms: nominal and actual. In this case, the two-by-four is the nominal dimension, measuring two inches deep and four inches wide. Meanwhile, the 1.5 by 3.5 is the actual dimension, which builders must learn to compensate. Post-mill use is particularly to blame for the shrinkage.
Don’t take this the wrong way; two-by-four is totally legit but as rough wood. When lumber mills process timber, they cut it into the nominal measurements as specified. However, smoothing and drying wood, among others, cause it to shrink. The amount of shrinkage may vary, but it’s usually up to 25 percent less of the nominal dimensions.
You can opt to use untreated lumber for your projects, ensuring nominal dimensions, but their applications are limited, mostly to gardening. Treated lumber can be applied to practically the entire building, from trusses to walls because of their all-weather and pest resilience. A small reduction in size is a small price to pay for long-term protection.
Nominal dimensions are also used for pricing lumber. Suppliers may either price lumber by its board feet (volume of the board divided by 12) or running foot (length).