Under the IRC deck tables, a 2×10 Southern pine joist 16 in on center may span 14 ft 0 in, a 2×8 11 ft 10 in and a 2×12 16 ft 6 in. Cedar and redwood span about a foot less at the same size. The spans come from IRC Table R507.6 (40 psf live load plus 10 psf dead load, No. 2 grade, wet service) and hold for both the 2021 and 2024 editions.
How the joist span is checked
Joist span is the clear distance between supports: from the face of the ledger (or a beam) to the face of the next beam. The overhang past the beam, the cantilever, is checked separately against the back span it balances.
Pass when your span ≤ table span for the lumber, size and spacing, and your cantilever ≤ table cantilever at your back span
Between back spans the cantilever is interpolated, as footnote g of the table allows. The beam span comes from Tables R507.5(1) to (4): each column is headed by the joist span and cantilever pairs it covers, and the calculator uses the first column that covers yours. With no cantilever it interpolates between the "span & 0" columns (footnote a).
Worked example: 12 ft joists on a 16 ft deck
Southern pine 2×10 at 16 in, a 12 ft span and no cantilever: the table allows 14 ft 0 in, so it passes. A 2×8 (11 ft 10 in) would fail by 2 in. Along a 16 ft beam the deck needs 16 ft ÷ 16 in + 1 = 13 joists.
The double 2×10 beam under them falls in the column headed "8 + 2 / 10 + 1 / 12 + 0" and may span 9 ft 0 in, so 16 ft is 2 spans on 3 posts, 8 ft apart.
Let the joists run 2 ft past the beam instead. At a 12 ft back span a 2×10 may cantilever 3 ft 0 in, so 2 ft passes, but the beam now carries more deck: the "12 + 2" column allows only 7 ft 8 in, and the same beam needs 3 spans and 4 posts, 5 ft 4 in apart.
Deck joist span table (IRC R507.6, 40 psf)
Spans in feet-inches.
| Species | Joist | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|---|
| Southern pine | 2×6 | 9-11 | 9-0 | 7-7 |
| Southern pine | 2×8 | 13-1 | 11-10 | 9-8 |
| Southern pine | 2×10 | 16-2 | 14-0 | 11-5 |
| Southern pine | 2×12 | 18-0 | 16-6 | 13-6 |
| Douglas fir-larch, hem-fir, SPF | 2×6 | 9-6 | 8-4 | 6-10 |
| Douglas fir-larch, hem-fir, SPF | 2×8 | 12-6 | 11-1 | 9-1 |
| Douglas fir-larch, hem-fir, SPF | 2×10 | 15-8 | 13-7 | 11-1 |
| Douglas fir-larch, hem-fir, SPF | 2×12 | 18-0 | 15-9 | 12-10 |
| Redwood, western cedars, ponderosa, red pine | 2×6 | 8-10 | 8-0 | 6-10 |
| Redwood, western cedars, ponderosa, red pine | 2×8 | 11-8 | 10-7 | 8-8 |
| Redwood, western cedars, ponderosa, red pine | 2×10 | 14-11 | 13-0 | 10-7 |
| Redwood, western cedars, ponderosa, red pine | 2×12 | 17-5 | 15-1 | 12-4 |
Maximum joist cantilever
Southern pine, by back span (the joist span behind the beam). NP means not permitted. The values stop at a quarter of the back span, the limit AWC's DCA 6 also states.
| Joist, Southern pine | 4 ft | 6 ft | 8 ft | 10 ft | 12 ft | 14 ft | 16 ft | 18 ft |
|---|---|---|---|---|---|---|---|---|
| 2×6 | 1-0 | 1-6 | 1-5 | NP | NP | NP | NP | NP |
| 2×8 | 1-0 | 1-6 | 2-0 | 2-6 | 2-3 | NP | NP | NP |
| 2×10 | 1-0 | 1-6 | 2-0 | 2-6 | 3-0 | 3-4 | 3-4 | NP |
| 2×12 | 1-0 | 1-6 | 2-0 | 2-6 | 3-0 | 3-6 | 4-0 | 4-1 |
Beam span chart for decks (IRC R507.5(1), Southern pine, 40 psf)
Column heads are joist span + joist cantilever in feet; beam spans in feet-inches. A beam may overhang a post by a quarter of its span, and its plies may only be spliced over a post.
| Beam | 6+0 | 6+1.5 / 8+0 | 8+1 / 10+0 | 8+2 / 10+1 / 12+0 | 10+2.5 / 12+1 / 14+0 | 12+2 / 14+1 / 16+0 | 12+3 / 14+2 / 16+1 / 18+0 | 14+3.5 / 16+2.5 / 18+1.5 | 16+4 / 18+3 | 18+4.5 |
|---|---|---|---|---|---|---|---|---|---|---|
| Single 2×6 | 4-10 | 4-7 | 4-3 | 4-0 | 3-7 | 3-5 | 3-3 | 3-0 | 2-10 | 2-8 |
| Single 2×8 | 6-4 | 5-11 | 5-6 | 5-1 | 4-7 | 4-4 | 4-2 | 3-10 | 3-7 | 3-5 |
| Single 2×10 | 7-6 | 7-0 | 6-6 | 6-0 | 5-5 | 5-2 | 4-11 | 4-7 | 4-3 | 4-0 |
| Single 2×12 | 8-8 | 8-3 | 7-8 | 7-1 | 6-4 | 6-1 | 5-10 | 5-5 | 5-0 | 4-9 |
| Double 2×6 | 7-4 | 6-11 | 6-5 | 5-11 | 5-4 | 5-1 | 4-10 | 4-6 | 4-3 | 4-0 |
| Double 2×8 | 9-4 | 8-9 | 8-2 | 7-7 | 6-9 | 6-5 | 6-2 | 5-9 | 5-4 | 5-0 |
| Double 2×10 | 11-0 | 10-4 | 9-8 | 9-0 | 8-0 | 7-8 | 7-4 | 6-9 | 6-4 | 6-0 |
| Double 2×12 | 13-0 | 12-2 | 11-4 | 10-7 | 9-5 | 9-0 | 8-7 | 8-0 | 7-5 | 7-0 |
| Triple 2×6 | 9-0 | 8-6 | 7-11 | 7-5 | 6-8 | 6-4 | 6-1 | 5-8 | 5-3 | 4-11 |
| Triple 2×8 | 11-7 | 10-11 | 10-3 | 9-6 | 8-6 | 8-1 | 7-9 | 7-2 | 6-8 | 6-4 |
| Triple 2×10 | 13-11 | 13-0 | 12-1 | 11-2 | 10-0 | 9-7 | 9-2 | 8-6 | 7-11 | 7-6 |
| Triple 2×12 | 16-3 | 15-3 | 14-3 | 13-3 | 11-10 | 11-3 | 10-9 | 10-0 | 9-4 | 8-10 |
Hem-fir, Douglas fir and spruce-pine-fir beams span roughly 5% less, cedar and redwood a little less again; the calculator above shows the table for the lumber you pick.
Joist spacing
Spacing is limited from above by the decking, not only by the joists. IRC Table R507.7 allows 5/4 wood decking over joists at most 16 in apart (12 in on a 45° diagonal) and 2 in decking at 24 in. Trex asks for 16 in for its composite boards and 12 in on a diagonal. Closer spacing also lets a smaller joist span farther: a 2×8 Southern pine goes from 9 ft 8 in at 24 in to 13 ft 1 in at 12 in.
Snow loads
Where the ground snow load is over 40 psf, R507.1 sends you to the snow rows of the same tables. At 70 psf a hem-fir 2×10 at 16 in spans 11 ft 6 in instead of 13 ft 7 in. Pick the load in the calculator; your building department knows the ground snow load for your address.
Where to go next
The deck calculator applies these tables to a whole deck and draws the framing plan. The footing under each post is sized in the deck footing calculator. For a beam or header inside the house, use the beam span calculator; for the boards on top, the deck board calculator.