Deck Joist Spacing and Span Table (2021 IRC)
Short answer: most decks use joists 16 inches on center. Your decking sets the limit: under the 2021 International Residential Code (IRC), 1¼ in (5/4) wood deck boards need joists no more than 16 in apart when the boards run straight across, or 12 in when they run on a diagonal. 2 in thick wood decking can span 24 in. For composite boards, follow the manufacturer’s span rating.
How far each joist can reach between supports depends on its size, the wood species and the spacing. A No. 2 southern pine 2×10 at 16 in can span 14 ft 0 in, for example.
To count the joists and boards for your deck, use the deck calculator.
Maximum joist spacing by decking (IRC Table R507.7)
| Decking | Boards straight across | Boards on a diagonal |
|---|---|---|
| 1¼ in (5/4) wood | 16 in | 12 in |
| 2 in wood | 24 in | 24 in |
These figures assume each board crosses three or more joists. A board resting on only two joists counts as a single span, which allows 12 in (straight) or 8 in (diagonal) for 5/4 boards and 24 in or 18 in for 2 in boards. Diagonal means up to 45 degrees.
Deck joist span table (IRC Table R507.6)
Maximum joist span for 40 psf live load and No. 2 grade lumber (9′ 11″ means 9 feet 11 inches):
Southern pine
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2×6 | 9′ 11″ | 9′ 0″ | 7′ 7″ |
| 2×8 | 13′ 1″ | 11′ 10″ | 9′ 8″ |
| 2×10 | 16′ 2″ | 14′ 0″ | 11′ 5″ |
| 2×12 | 18′ 0″ | 16′ 6″ | 13′ 6″ |
Douglas fir-larch, hem-fir, spruce-pine-fir
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2×6 | 9′ 6″ | 8′ 4″ | 6′ 10″ |
| 2×8 | 12′ 6″ | 11′ 1″ | 9′ 1″ |
| 2×10 | 15′ 8″ | 13′ 7″ | 11′ 1″ |
| 2×12 | 18′ 0″ | 15′ 9″ | 12′ 10″ |
Redwood, western cedars, ponderosa pine, red pine
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2×6 | 8′ 10″ | 8′ 0″ | 6′ 10″ |
| 2×8 | 11′ 8″ | 10′ 7″ | 8′ 8″ |
| 2×10 | 14′ 11″ | 13′ 0″ | 10′ 7″ |
| 2×12 | 17′ 5″ | 15′ 1″ | 12′ 4″ |
The table assumes wet-service lumber and 10 psf of dead load. In snow country, use the row for your ground snow load if it is above 40 psf. Spans get shorter: a southern pine 2×10 at 16 in drops to 13 ft 9 in at 50 psf and 11 ft 11 in at 70 psf. You may interpolate between the values in the code table, but not go beyond them.
Cantilevers: how far joists can overhang
A joist can run past its beam (a cantilever) only a limited distance, and the limit depends on the back span (the length on the other side of the beam). At 40 psf, a southern pine 2×10 with a 10 ft back span can overhang up to 2 ft 6 in. A 2×6 can overhang at most 1 ft 6 in, and is not permitted to cantilever at all with a back span of 10 ft or more. See Table R507.6 for every size.
How many joists do you need?
Divide the deck length (in inches) by the spacing, round up, and add one for the end. A 16 ft deck at 16 in on center needs 192 ÷ 16 = 12 spaces, so 13 joists. Add rim joists and any doubled joists your plan calls for.
Common questions
Can deck joists be 24 inches apart?
Only with decking rated for it, such as 2 in thick wood boards, or a composite rated for 24 in. 5/4 wood boards need 16 in (12 in on a diagonal).
How far can a 2×8 deck joist span?
At 16 in on center and 40 psf: 11 ft 10 in in southern pine, 11 ft 1 in in Douglas fir-larch, hem-fir or spruce-pine-fir, and 10 ft 7 in in redwood or cedar.
How far can a 2×10 deck joist span?
At 16 in on center and 40 psf: 14 ft 0 in in southern pine, 13 ft 7 in in the fir and spruce group, and 13 ft 0 in in redwood or cedar.
Do I need a permit?
Usually, yes. Most towns adopt the IRC with local changes, and decks attached to a house almost always need a permit and inspection. Check with your building department before you build.
Sources
- International Code Council: 2021 International Residential Code, section R507 (decks): Table R507.6 joist spans and cantilevers, Table R507.7 decking spans.
- American Wood Council: DCA 6, Prescriptive Residential Wood Deck Construction Guide: the companion guide many building departments hand out for ledgers, posts and footings.