Dimensional Lumber Rules for How to Build a Deck

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Once you have decided on your deck's location, shape and size, make a 1/4- or 1/2-inch to the foot scale drawing of the deck. Before calculating how many pieces of wood or other deck material you will need based on your drawing, you need to understand the conventions for describing and ordering wood and the dimensional differences among construction lumber's nominal dimension, its dressed green dimension and its minimum dry dimension.

Thickness, Width and Length

  • Specify lumber by thickness first, width next and length last. When describing or ordering raw lumber (lumber not cut to custom length), describe thickness and width in inches and length in feet. For example, describe a piece of lumber 2 inches thick, 6 inches wide and 10 feet long as a "2x6x10." When writing out raw lumber dimensions, do not put spaces between dimensions (2 x 6 x 10), but write it as shown above, 2x6x10. Use the verbal expression "two by six by 10." When ordering lumber, state the quantity first, and then the dimensions.

Nominal and Actual Dimensions

  • A piece of raw lumber has two sets of dimensional descriptions: the nominal dimensions and the actual dimensions. The nominal length equals the actual length. However, nominal thicknesses and widths will differ from actual dimensions. A 2x6x10 has a nominal thickness of 2 inches and an actual thickness of 1-1/2 inches. It has a nominal width of 6 inches, but an actual thickness of 5-1/2 inches. See the Resources section for a convenient table of the nominal and actual thicknesses and widths of construction lumber. Order your lumber using nominal dimensions. Calculate lumber requirements using actual dimensions.

Minimum Dry and Dressed Green

  • The "actual" thicknesses and widths given in the table hold true when the wood has dried. However, construction lumber sometimes arrives on-site "dressed green," with a moisture content higher than 12 percent. At other times, the lumber will have partially dried to its "minimum dry dimension," when you get it. The table shows the differences between the two dimensions. For lumber up to 4 inches wide or thick, the difference between minimum dry and dressed green equals only 1/16 inch. For a nominal 12-inch face width, however, while the minimum dry dimension equals 11-1/4 inches, the dressed green dimension equals 11-1/2 inches.

Remeasuring

  • In most cases, the dimensional difference between dressed green and minimum dry dimensions will not pose a problem. You can calculate lumber requirements and your deck layout using the minimum dry dimensions. However, the actual horizontal decking---the surface you walk on---may require remeasurement immediately prior to installation. Over the cross-span of a 12-foot deck (the span over the widths of the decking), the total difference between minimum dry dimensions and dressed green dimensions may total more than an inch. Measure the width of decking the day you lay it down and recalculate as necessary.

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  • Photo Credit wood texture image by Daniel Gillies from Fotolia.com
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