2×12 deck joist spans (2021 IRC)
A 2×12 Southern pine deck joist at 16-inch centers spans up to 16’6” under the 40 psf live load case (2021 IRC Table R507.6). Tighten the spacing to 12-inch centers and the same joist is listed at 18’0”; open it up to 24-inch centers and the table lists 13’6”. In the Douglas fir-larch / Hem-fir / Spruce-pine-fir group, the 16-inch figure is 15’9”.
A deck joist is not carrying a floor's loads: Table R507.6 has the wet service factor built in and runs ground snow load cases a floor table never sees, so deck framing is read from the deck table, not the floor table. For contrast, the same 2×12 Southern pine No. 2 floor joist at 16-inch centers is listed at 16’6” in 2021 IRC Table R502.3.1(2) — see 2×12 floor joist spans.
2×12 deck joist spans by load case, species group, and spacing
Rows are the table's own load cases and species groups; columns are joist spacing, on center. Every value is quoted from 2021 IRC Table R507.6 as printed.
| Species group | 12" on center | 16" on center | 24" on center |
|---|---|---|---|
| 40 psf live load | |||
| Southern pine | 18’0” | 16’6” | 13’6” |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 18’0” | 15’9” | 12’10” |
| Redwood / Western cedars / Ponderosa pine / Red pine | 17’5” | 15’1” | 12’4” |
| 50 psf ground snow load | |||
| Southern pine | 18’0” | 16’2” | 13’2” |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 17’9” | 15’5” | 12’7” |
| Redwood / Western cedars / Ponderosa pine / Red pine | 16’10” | 14’9” | 12’1” |
| 60 psf ground snow load | |||
| Southern pine | 17’3” | 15’0” | 12’3” |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 16’6” | 14’3” | 11’8” |
| Redwood / Western cedars / Ponderosa pine / Red pine | 15’9” | 13’8” | 11’2” |
| 70 psf ground snow load | |||
| Southern pine | 16’2” | 14’0” | 11’5” |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 15’5” | 13’4” | 10’11” |
| Redwood / Western cedars / Ponderosa pine / Red pine | 14’9” | 12’9” | 10’5” |
2021 IRC Table R507.6 — 40 psf live load, 50 psf ground snow load, 60 psf ground snow load, 70 psf ground snow load; 10 psf dead load, No. 2 grade, wet service factor included, deflection L/360. Table source
2×12 deck joist cantilever limits
The cantilever columns are keyed to the joist back span — the span behind the support the joist runs past. Find the back span across the top, read down to the species group and load case, and the cell is the longest cantilever Table R507.6 lists for that combination. NP means not permitted by the table. For a 2×12 Southern pine under the 40 psf live load case the table lists 1’0” at a 4-foot back span and 4’1” at an 18-foot back span, the longest back span the table covers.
Two conditions come with those columns as the table states them: a 220-pound point load at the free end of the cantilever, and deflection at the cantilever held to L/180 rather than the L/360 that applies to the back span.
| Species group | 4 ft | 6 ft | 8 ft | 10 ft | 12 ft | 14 ft | 16 ft | 18 ft |
|---|---|---|---|---|---|---|---|---|
| 40 psf live load | ||||||||
| Southern pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’6” | 4’0” | 4’1” |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’6” | 3’11” | 3’11” |
| Redwood / Western cedars / Ponderosa pine / Red pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’6” | 3’8” | NP |
| 50 psf ground snow load | ||||||||
| Southern pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’6” | 3’10” | 3’10” |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’6” | 3’8” | NP |
| Redwood / Western cedars / Ponderosa pine / Red pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’5” | 3’5” | NP |
| 60 psf ground snow load | ||||||||
| Southern pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’6” | 3’7” | NP |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’5” | 3’5” | NP |
| Redwood / Western cedars / Ponderosa pine / Red pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’2” | NP | NP |
| 70 psf ground snow load | ||||||||
| Southern pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’5” | 3’5” | NP |
| Douglas fir-larch / Hem-fir / Spruce-pine-fir | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’3” | NP | NP |
| Redwood / Western cedars / Ponderosa pine / Red pine | 1’0” | 1’6” | 2’0” | 2’6” | 3’0” | 3’0” | NP | NP |
2021 IRC Table R507.6 — maximum cantilever under the same load cases (40 psf live load, 50 psf ground snow load, 60 psf ground snow load, 70 psf ground snow load), with a 220-pound point load at the free end and deflection at the cantilever held to L/180. Table source
Reading the load case rows
The first block is the 40 psf live load case — the deck itself, with people and furniture on it. The 50 psf ground snow load, 60 psf ground snow load, 70 psf ground snow load blocks are ground snow load cases: the snow load on the ground at your address, which your building department publishes, not the depth of snow sitting on the deck. Pick the row that matches your jurisdiction's ground snow load, and where the ground snow load falls between the table's rows, the next row up is the one the table covers.
Under the 70 psf ground snow load case, a 2×12 Southern pine deck joist at 16-inch centers is listed at 14’0” against 16’6” under the 40 psf live load case — the same stick of lumber, a different row of the same table.
Point loads, cantilevers beyond the table footnotes, unusual snow loads, or anything else outside the table's stated conditions — that's an engineer's question.
Common questions
How far can a 2×12 deck joist span?
A 2×12 Southern pine deck joist at 16-inch centers spans up to 16’6” under the 40 psf live load case, per 2021 IRC Table R507.6. At 12-inch centers the same table lists 18’0”, and at 24-inch centers 13’6”. In the Douglas fir-larch / Hem-fir / Spruce-pine-fir group the 16-inch figure is 15’9”, and in the Redwood / Western cedars / Ponderosa pine / Red pine group it is 15’1”.
What cantilever does the table list for a 2×12 deck joist?
Table R507.6 lists a maximum cantilever against each joist back span. For a 2×12 Southern pine under the 40 psf live load case the table lists 1’0” at a 4-foot back span and 4’1” at an 18-foot back span, the longest back span the table covers. Those columns carry a 220-pound point load at the free end and hold deflection at the cantilever to L/180.
Do the ground snow load rows change the 2×12 numbers?
They do. Under the 70 psf ground snow load case a 2×12 Southern pine deck joist at 16-inch centers is listed at 14’0”, against 16’6” under the 40 psf live load case (2021 IRC Table R507.6). Ground snow load for your address comes from your building department, and it decides which row you read.
Other deck joist sizes and the rest of the deck
Deck joists: 2×6 · 2×8 · 2×10 · 2×12
The beam the joists land on is a different table: deck beam spans (2021 IRC Table R507.5(1)). For the same size indoors, see 2×12 floor joist spans.