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Nominal Vs Actual Lumber Sizes

Nominal Vs Actual Lumber Sizes

Softwood Lumber Sizes

To the novice do it yourselfer softwood lumber sizes can be hard to understand. Why isn’t the 2×4 at the lumber yard 2 inches thick and 4 inches wide? The 4×4’s are 3 ½ x 3 ½ and the 1×6’s are ¾ of an inch thick and 5 ½ wide. The first dimensions are its nominal ones, the second measurements are its actual or minimum dressed sizes. The reason for the differences is because lumber sizes are traditionally given in their nominal size. This is a well-established industry practice.

Why are the nominal dimensions of lumber larger than the actual dimensions?

The derivation of nominal measurements was from a board’s size before its drying and surfacing on all 4 sides. The actual measurements are the final sizes after drying and surfacing. Originally, a 2×4 was cut as a rough green piece that was close to a full 2 inches thick and 4 inches wide. However, after drying and surfacing, the boards became smaller, approaching the now standard 2×4 dimensions of 1 ½ x 3 ½ inches. Green southern pine lumber shrinks in width and thickness as it dries.  Surfacing removes up to an additional ¼ inch from both dimensions.

Today, modern sawing, drying, and planing technologies are much more precise than cutting green to a full 2×4 inches. Therefore, you can cut the boards a little smaller than 2×4 inches and still leave it large enough to dress after drying at 1 ½ inches thick and 3 ½ inches wide.

A few years ago consumer groups brought several lawsuits against some of the big box stores. They also brought a few hardware and building supply chains for false advertising by selling lumber that they maintained, was undersized.

The courts eventually found that the US of lumber sizes is in nominal inches in and in millimeters in other countries. Builders, engineers, architects, buyers, and sellers are familiar with the differences between nominal and actual lumber sizes. The courts eventually found that the use of nominal lumber sizes was long-standing.

However, there was one change that the courts mandated. The size labels now have a new requirement. According to the American Wood Council, the National Institute of Standards and Technology permits stores to label lumber with nominal dimensions. This is if the label or other sign clearly states that the size given is a nominal size. The label should also state “the actual or minimum measurements.” 

In most places, you should see both the nominal and actual dimensions on the label or other sign at the point of sales.

Softwood Dimension Lumber Sizes

Dimension lumber is lumber cut and finished S4S, (surfaced four sides) and is available in standardized sizes. Framing of buildings commonly uses the construction lumber and that is where the name framing or Dimensional lumber comes from.

Technically, you can define dimension lumber  as sawed softwood lumber from 2 to 5 inches (5 to 12.7 centimeters) thick and from 4 to 12 inches (10.2 to 30.5 centimeters) wide. The length of dimension lumber can range from 6 to 24 feet. The Nominal and standard sizes of lumber can be found in the Voluntary Product Standard PS 20-20 American Softwood Lumber Standard. This standard lists the nominal and minimum dressed sizes for all categories of structural and nonstructural lumber.

The most commonly available dimension lumber sizes are 2×4 through 2×12. The 2×4’s and 2×6’s are ½” less than nominal in width and the 2×8, 2×10’s and 2×12’s are ¾ of an inch less than nominal. This is common knowledge for the experienced lumber user and is the result of the fact that wider lumber shrinks more in width than the narrow lumber during drying. These differences in width are the same for boards.

Nominal Vs Actual Lumber SizesVoluntary Product Standard PS 20-20 American Softwood Lumber Standard.

 

The Southern Forest Products Association (SFPA) publishes a comprehensive pamphlet, “The Southern Pine Use Guide,”. This pamphlet also lists nominal and standard sizes for all Southern Pine products. It also features the design values, span tables, and grade descriptions.

The table below shows the Standard Sizes of Southern Pine Dimension, Timbers, Boards, Finish, and Radius Edge Decking.

From the Southern Forest Products Association (SFPA).

 

IN CONCLUSION 

Now you understand why the nominal size is different from the actual size.

Nominal sizes harken back to the original rough green lumber sizes. The actual sizes are sizes after drying and surfacing the rough green lumber on all four sides.

The nominal size is the standard terminology and is applicable when for buying, selling, and specifying softwood lumber.