Nominal Sizes: When Is a Two-by-Four Not 2 by 4

Nominal Lumber Sizes

A two-by-six is not a 2 x 6 when it’s construction lumber.

The framing materials we use for the walls and ceilings of our modular homes are mostly two-by-sixes, two-by-tens, and two-by-fours.  You might assume, as I did when I first started selling modular homes, that these designations refer to the actual dimensional sizes of the lumber.  But a two-by-six is not 2” x 6”.  It’s actually 1 ½” x 5 ½”.  In fact, the 1 ½” dimension can be as little as 1 3’8” or as much as 1 5/8” and the 5 ½” dimension can be as little as 5 3’8” or as much as 5 5/8”.

Why Is Lumber Labeled with Nominal Sizes

Lumber sizes for residential construction are designated by the "nominal" values assigned to each size.
Lumber sizes for residential construction are designated by the “nominal” values assigned to each size.

In residential construction in the United States the framing materials are designated with a “nominal” value, which approximates its size.  For example, a 2 x 10, which is close to 2” x 10” but actually 1 ½” x 9 ½”, is given the name “two-by-ten”.  This makes sense when you understand a little history.

In the past, the nominal dimensions given to the lumber were the sizes of green lumber before it was dried and planed smooth.  This process shrunk the lumber by about ½” in each dimension.  The lumber sold today for residential construction is already dried and planed.  But it’s still sold in the historical sizes with each size retaining its nominal name.  That’s why your modular home will built with “two-by-sixes”, “two-by-tens”, and “two-by-fours”.

Nominal Sizes of Modular Floor Plans

Nominal values also play a role in designating the width of modular home floor plans.  For example, a “twenty-eight x forty-four” home is actually 27’6” x 44’.  In this case, the width is rounded up by 6”.

For more information about nominal lumber size, see Designing a Modular Home in my book The Modular Home.

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