How Much Lumber to Build a Backyard Deck

Start with the deck surface. Multiply length by width for square feet, then divide by the coverage of one board. A 5/4×6 deck board is 5.5 in wide and needs a 1/8 in gap, so each board covers 5.625 in — 0.469 ft — of deck width per running foot. A 12 × 16 deck is 192 ft², and 192 ÷ 0.469 = 410 linear feet of decking. Buying 16 ft boards, that is 26 boards; add 10% for cuts and defects and order 29.

Joists come next. Joists run the short way in most decks and are counted with the spacing formula: (deck width ÷ spacing) + 1. At 16 in on centre across 16 ft that is (192 ÷ 16) + 1 = 13 joists, each 12 ft long. Use 16 in on centre for standard decking, and tighten to 12 in if you are running boards diagonally or using thin composite.

Then the structure below. A ledger board the full length of the house side, a rim joist on the other three sides, one or two beams sized to the span, and posts at 6–8 ft along each beam. A 12 × 16 deck typically lands on 4 to 6 posts, each in a footing sized for the local frost depth. Beams are commonly doubled or tripled 2×10s for spans up to about 10 ft.

Do not forget the hardware, which is often 15–20% of the bill: joist hangers at every joist end, structural screws for the ledger, hurricane ties, post bases, and either deck screws at about 350 per 100 ft² or hidden fasteners at roughly the same count for grooved boards.

Finally, add railings and stairs. Railing posts sit at maximum 6 ft centres, balusters at no more than 4 in clear spacing, and each stair tread takes two stringers minimum plus one for wide stairs. Pressure-treated southern yellow pine is the value option; cedar and composite cost more per foot but change nothing about the quantities above.