Deck Material Calculator

A deck is three material orders stacked on top of each other: the framing (joists, beams, posts, ledger), the decking (the boards you walk on), and the fasteners (structural screws, joist hangers, deck screws). Each has its own count, and each is where inexperienced installers short the order and end up back at the yard on Saturday morning.

Deck boards: total board count equals the deck width in inches divided by the board width plus a 1/8 inch gap. For a 12 foot wide deck using 5.5 inch boards with 1/8 inch gaps, that is (144 / 5.625) = 26 boards. Each board runs the full length of the deck, so 26 boards times 16 feet is 416 linear feet of decking.

Joists: number of joists equals deck length in inches divided by on-center spacing, plus one. A 16 foot long deck with joists 16 inches on center needs (192 / 16) + 1 = 13 joists. Add doubled joists under any butt seam in the decking.

Pressure-treated 5/4 by 6 decking spans 16 inch on-center framing comfortably. Composite decking usually requires 16 inch OC for perpendicular installs and 12 inch OC for 45 degree diagonal installs. PVC and cellular composite decking often specify 12 inch OC. Read the manufacturer specification before you frame.

Cantilevers (deck boards extending beyond the outer joist) are limited by both the decking span and the framing span. Standard rule: cantilever no more than one-quarter of the interior joist span. A joist spanning 12 feet can cantilever up to 3 feet.

A single beam under a residential deck is typically a doubled or tripled 2x10 or 2x12, sized based on the joist span it supports. Two 2x10 beams work for spans up to about 8 feet between posts; three 2x10 or a doubled 2x12 gets you to 12 feet.

Footings in Texas require a minimum 30 inch depth to reach below the frost line in the north half of the state, though most jurisdictions accept 24 inches south of the DFW metro. Standard footing is a 12 inch diameter sonotube filled with concrete, with a post base bolt cast into the top.

Face-screwed pressure-treated decks: 2 screws per board per joist. For our 16 by 12 deck with 26 boards and 13 joists, that is 26 x 13 x 2 = 676 screws. Buy 5 pound boxes; you will use 700 to 800 screws by the time you account for framing.

Composite decking with hidden clip systems has its own fastener count from the manufacturer, typically one clip per joist per board. Structural connections (post to beam, joist to ledger, beam to post) require Simpson-branded galvanized or stainless structural connectors sized to the load and the local building code.

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