Deck board sizes and lengths

Decking board sizes: nominal and actual

Lumber is named for its size before it is dried and planed, so every board is smaller than its label. Deck boards come in three wood sizes, and the thin one — "five-quarter" — is the standard.

So the deck board actual size to plan with is 5½ in wide for every common board, wood or composite. Thickness varies more between makers of composite and PVC; the spec sheet of the board you buy gives the exact figure, and it matters for stair treads and for how far the board may span.

Decking board lengths

Treated pine and cedar deck boards come in even lengths: most yards stock 8, 10, 12, 16 and 20 ft, and some carry 14 and 18 ft as well. Composite and PVC boards come in fewer lengths — commonly 12, 16 and 20 ft.

The length to buy depends on the joists as much as on the deck. Where two boards meet end to end the joint has to land on a joist, so a length that divides into whole joist bays wastes nothing. On 16 in centres, 8, 12, 16 and 20 ft all do; a 10 ft board reaches the joist at 9 ft 4 in and the last 8 in is cut off.

Runs longer than the longest board are joined, and the joints are moved from row to row so no two in neighbouring rows share a joist. The deck material calculator plans that cut for your deck and the lengths you can buy, and counts the boards of each length.

Decking board width and coverage

Across a deck each board takes its width plus one gap. That gives the linear feet of board in every square foot of deck — the number that turns an area into an order.

That is before cuts. A deck whose boards run in one stock length has almost none; one that needs joined boards or a diagonal lay loses more, and the calculator works out how much from the actual cut plan rather than a flat percentage.

Composite decking sizes

Composite and PVC boards are made rather than milled, so their sizes are the maker's, not the sawmill's. Most sit close to 1 × 5½ in and come in 12, 16 and 20 ft lengths, with grooved edges for hidden clips or square edges for face screws. Wider boards and thinner "deck tiles" exist, but the standard board follows wood decking so the same framing works for both.

Two things differ from wood. Composite grows with heat along its length, so it wants a wider end gap and a wider side gap — usually ¼ in — and its makers set their own joist spacing rather than following the code table below.

Joist spacing for each board

A thin board spans less between joists. The 2021 IRC sets the widest joist spacing for wood decking by thickness, and a diagonal lay needs closer joists because the board crosses them at a slant.

For boards continuous over three or more joists. How far the joists themselves may span is in deck joist and beam span tables, and local codes can be stricter than the model code — the building department has the final word.

Count the boards for your deck →