Deck calculator: boards, joists and whether the span is legal

How far can a deck joist span?

It depends on the size of the joist, the species of timber and how far apart the joists are. The code sets the limits, and they are not guesses.

Joist	12 in centres	16 in centres	24 in centres
2x8 Southern Pine	13 ft 1 in	11 ft 10 in	9 ft 8 in
2x8 SPF / Hem-Fir	12 ft 6 in	10 ft 7 in	8 ft 8 in
2x10 Southern Pine	16 ft 2 in	14 ft 0 in	11 ft 5 in
2x10 SPF / Hem-Fir	15 ft 5 in	13 ft 0 in	10 ft 7 in

Code IRC 2021 section R507.6 joist span table, No. 2 grade, 40 psf live plus 10 psf dead load. The American Wood Council's DCA 6 restates the same figures. Snow load areas and higher live loads reduce these spans. Span is measured face of support to face of support. Checked August 2026.

Why board gaps change the board count

Because a 5½ inch board with a 3/16 inch gap occupies 5 11/16 inches of deck, not 5½. Over a 12 foot width that is the difference between 26 boards and 25, and calculators that ignore the gap consistently over-order.

The gap is not optional. Boards need it for drainage and for seasonal movement. Pressure-treated timber is usually wet when you buy it and shrinks as it dries, so it is often laid tight and opens up on its own. Composite does the opposite and expands in heat, so follow the manufacturer's gap, which is usually larger.

How far can a joist cantilever past the beam?

Up to one quarter of the actual back-span under the IRC. A joist spanning 12 feet between supports can extend 3 feet past the beam.

This is one of the more useful things the code allows, because moving the beam inboard puts the posts where they are easier to found and makes the deck edge look lighter.

Common questions

Should I run boards diagonally?

You can, and it looks good, but two things change. Joist spacing must usually reduce to 12 inches because the diagonal increases the effective span between bearing points. And waste rises from around 10% to 15% or more, because every board needs an angled cut at both ends.

What fasteners should I use?

Stainless or hot-dipped galvanised, specifically rated for contact with modern pressure-treated timber. The copper-based preservatives used since the early 2000s corrode plain steel fast. Hidden fastener systems for grooved boards use clips instead and give a cleaner surface with no visible screws.

How much does a deck weigh?

Around 10 pounds per square foot dead load for a timber deck, which is what the span tables assume alongside 40 pounds of live load. A hot tub changes this completely and needs its own structural design rather than a standard frame.

Do I need a permit?

Almost always for anything attached to the house or raised more than a low step. The ledger connection is the part inspectors look at hardest, because a failed ledger is how decks collapse, and it is the most common serious fault found.