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Calculate the number of joists required by spacing.
With these inputs:
| Result | Value |
|---|---|
| Joists required | 10 joists |
| Total linear footage | 160 ft |
| Estimated lumber cost | $512 |
| Joist length each | 16 ft |
| Joist hangers | 10 at the ledger (20 if the beam is flush) |
| Joist tape (incl. 10% laps) | 190 ft — 4 × 50 ft rolls |
Plus 2 rim joists and a ledger board, typically purchased separately. Hangers: one per joist at the ledger; a joist that sits on top of a dropped beam needs no hanger there, only a hurricane tie. Tape covers the top edge of every joist and the rim.
Joists carry everything on the deck and hand it to the beam. Size them from the span and spacing, not from what looks sturdy — a joist that is one size small does not fail dramatically, it just bounces underfoot for twenty years and sags where the furniture sits. This works out the count, the lumber total and the blocking from your deck's dimensions.
It depends on the span and the spacing. Using the AWC DCA 6 tables for Southern Yellow Pine at 16 inches on centre, a 2x6 reaches about 9 ft, a 2x8 about 11 ft 11 in, a 2x10 about 14 ft and a 2x12 about 16 ft 6 in. Douglas Fir-Larch is slightly less, Hem-Fir and cedar less again. Tightening to 12 inches on centre buys about a foot on most sizes.
Sixteen inches on centre is the standard and suits nearly everything. Twelve inches is required for most composite and PVC decking laid diagonally, and by some brands for straight installation too. Twenty-four inches is only permitted with 2x thick wood decking — 5/4 boards and composite both deflect noticeably at that spacing.
Doubling a joist roughly doubles its load capacity but does very little for its span, because span is governed by depth. Two 2x8s side by side still reach about the same distance as one. Doubling is the right answer under a point load — a hot tub, a planter, a stair landing — and the wrong answer when you simply need to reach further. For that, go deeper.
Perpendicular to the decking boards, and usually perpendicular to the house so they can hang off a ledger and land on a beam at the outer edge. That also sets your board direction: boards run parallel to the house in the usual layout. If you want the boards running the other way, the frame rotates with them and the beam moves to the side.
Add up the top edge of every joist and rim, allow 10 percent for laps, and divide by the roll length — usually 50 ft. A 16 ft joist run on a 12 ft wide deck at 16 inches on centre has 10 joists, or 160 ft, plus a 12 ft rim board: 172 ft, 189 ft with the allowance, so four 50 ft rolls of 2 inch tape. Tape the beam with 4 to 6 inch tape, and the ledger with flashing rather than tape alone.
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