Actual vs. Nominal in Framing Lumber

Nominal vs. actual lumber sizes can be confusing.

Due to the expense of obtaining hardwoods, all framing lumber in modern usage is cut from soft wood trees, primarily from the pine and fir families, often referred to as white wood. For beginners, the size names of dimensional, or framing, lumber can be confusing. While the sign on the lumber bin says 2-by-4, a few seconds with a tape measure is enough to discover that this is not exactly the truth. There are two main factors contributing to this discrepancy: milling the wood for a finished surface and drying.

Lumber Standards

The lumber industry has a rough set of guidelines governing the grading of lumber. Among those are minimums for the size. Unlike extruded products, such as composite decking, lumber is cut from a living source and a number of factors make it impossible to guaranty the size of a trimmed and dried piece of lumber. The minimums ensure that a builder will have the right load-bearing strength for framing members as required by building codes.

Milling

Most boards start out slightly larger than their "nominal" or named size. A piece of rough milled 2-by-4 lumber is indeed 2-inches-thick and 4-inches-wide. All four edges of the board are then planed to smooth out the rough cuts of the mill saw for easier handling. This removes approximately 7/16 of the width and thickness from a typical board.

Drying

Green lumber is heavy, hard to cut and has a tendency to warp, twist and buckle. Commercial lumber companies place green lumber into special, heated drying sheds where they are pressed and weighted to maintain their shape. This drying process takes an additional 1/16 from the thickness and width of the lumber, leaving it ½ inch narrower and thinner than it started out. For "1-by lumber," this difference is only ¼ inch in thickness, leaving it a standard ¾ inches thick.

Standard Sizes

Framing lumber larger than 1-by is typically cut in intervals of 2 inches, where width and thickness are concerned. Two-by lumber commonly comes in 2-by-4, 2-by-6, 2-by-8, and so on up to 2-by-12. Wider pieces can be ordered, but are not commonly used. Thicknesses range from 2 inches up to 6 inches. These boards are usually cut in 2-foot intervals when it comes to length. Common sizes start at 8 feet and range as high as 20 feet.

Working with Nominal Lumber

Adjusting woodworking plans to accommodate nominal lumber sizes is simple, once you know three simple rules. The actual thickness of lumber 2-inches thick and thicker will be ½ thinner than the named thickness. The actual thickness of 1-by lumber is ¾ inches. The width of all dimensional lumber will be ½ inch narrower than the named width. You will find slight variations, up to 1/16 inch. It is best to check if small variations make a difference in your project.

References

• EZ Woodshop: Lumber Dimensions
• Wood Bin: Lumber Dimensions
• SawsNDust: Milling