Decking
Three things set how far a deck joist can go: the species, the depth of the board, and how far apart you set them. The code publishes the answer as a lookup, not a formula — there is no honest way to interpolate between the cells. Below is IRC Table R507.6 in full, the overhang table that goes with it, and the one rule almost every other chart on this subject gets wrong.
No. 2 grade, 40 psf live plus 10 psf dead, wet service, L/360 deflection. Spans are measured from face of support to face of support — not the overall length of the board, and not including any overhang past the beam.
| Species | Size | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|---|
| Southern Pine | 2x6 | 9 ft 11 in | 9 ft | 7 ft 7 in |
| 2x8 | 13 ft 1 in | 11 ft 10 in | 9 ft 8 in | |
| 2x10 | 16 ft 2 in | 14 ft | 11 ft 5 in | |
| 2x12 | 18 ft | 16 ft 6 in | 13 ft 6 in | |
| Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir | 2x6 | 9 ft 6 in | 8 ft 8 in | 7 ft 2 in |
| 2x8 | 12 ft 6 in | 11 ft 1 in | 9 ft 1 in | |
| 2x10 | 15 ft 8 in | 13 ft 7 in | 11 ft 1 in | |
| 2x12 | 18 ft | 15 ft 9 in | 12 ft 10 in | |
| Redwood, Western Cedars, Ponderosa Pine, Red Pine | 2x6 | 8 ft 10 in | 8 ft | 7 ft |
| 2x8 | 11 ft 8 in | 10 ft 7 in | 8 ft 8 in | |
| 2x10 | 14 ft 11 in | 13 ft | 10 ft 7 in | |
| 2x12 | 17 ft 5 in | 15 ft 1 in | 12 ft 4 in |
The full range across the table is 7 ft for a 2x6 in redwood or cedar at 24″ on center, up to 18 ft for a 2x12 in Southern Pine at 12″. That top figure is a prescriptive ceiling as much as a structural one — the IRC caps deck joist length at 18 feet so that the footing tables underneath it stay valid.
Ask how far a joist can cantilever past the beam and nearly every chart repeats the same sentence: one quarter of the span. That comes from IRC R507.6.1, and it is a real requirement — but it is a ceiling, not the answer. AWC DCA 6 publishes an actual overhang table, and those values are controlled by deflection: cantilever length over 180, checked with a 220 lb point load standing at the tip. The allowable overhang is whichever of the two is smaller.
On 31 of the 36 combinations in the table, the deflection limit is the smaller one. Quarter-span is the binding limit on only 5 of them, and they are all deep joists. So for most real decks, the rule everyone quotes permits an overhang the code does not.
| Species | Size | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|---|
| Southern Pine | 2x6 | 1 ft | 1 ft 1 in | 1 ft 3 in |
| 2x8 | 1 ft 10 in | 2 ft | 2 ft 4 in | |
| 2x10 | 3 ft 1 in | 3 ft 5 in | 2 ft 10 in | |
| 2x12 | 4 ft 6 in | 4 ft 2 in | 3 ft 4 in | |
| Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir | 2x6 | 11 in | 1 ft | 1 ft 2 in |
| 2x8 | 1 ft 8 in | 1 ft 10 in | 2 ft 2 in | |
| 2x10 | 2 ft 10 in | 3 ft 2 in | 2 ft 9 in | |
| 2x12 | 4 ft 4 in | 3 ft 11 in | 3 ft 3 in | |
| Redwood, Western Cedars, Ponderosa Pine, Red Pine | 2x6 | 9 in | 10 in | 11 in |
| 2x8 | 1 ft 5 in | 1 ft 7 in | 1 ft 9 in | |
| 2x10 | 2 ft 5 in | 2 ft 7 in | 2 ft 8 in | |
| 2x12 | 3 ft 7 in | 3 ft 9 in | 3 ft 1 in |
How badly quarter-span overshoots depends almost entirely on joist depth. Averaged across every species and spacing:
| Joist | L/4 overstates by | Deflection governs |
|---|---|---|
| 2x6 | 2.16× | 9 of 9 |
| 2x8 | 1.52× | 9 of 9 |
| 2x10 | 1.18× | 8 of 9 |
| 2x12 | 1.03× | 5 of 9 |
One quirk in the overhang table looks like a typo and is not: on 2x6 and 2x8 joists the allowable overhang gets longer as the spacing widens, then reverses on 2x12. AWC publishes an FAQ about it. The cantilever is checked against a fixed 220 lb point load, which does not change with spacing, while the back-span shortens as spacing widens — so the cantilever gets relatively stiffer. Deep joists are strength-driven instead, and behave the way you would expect.
This is the most common way to arrive at a span that looks right on paper and fails inspection. Interior floor joists are covered by IRC Table R502.3.1; deck joists by R507.6. The tables are not interchangeable, and the reason is in DCA 6's first footnote: deck spans assume wet service conditions. Lumber that lives outdoors carries a reduction to both its allowable bending stress and its modulus of elasticity. Same species, same No. 2 grade, same nominal board — shorter permitted span once it is exposed to weather.
Two further adjustments are baked into these numbers and are easy to miss. Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir values assume incising — the pattern of small slits cut into pressure-treated lumber so preservative can penetrate — which itself reduces design values. Redwood, western cedars, ponderosa pine and red pine are sized on northern species values with no incising assumed. If your stock is not incised, the table is conservative; it is never the other way round.
Charts on this subject cite either the IRC or the AWC deck guide, usually without saying which. For 33 of the 36 cells it makes no difference. For 3 it does — all of them 2x6, and the 2021 IRC is the more generous every time:
| Cell | 2021 IRC | DCA 6-2015 | Difference |
|---|---|---|---|
| Douglas Fir-Larch 2x6 @ 16″ | 8 ft 8 in | 8 ft 4 in | +4″ |
| Douglas Fir-Larch 2x6 @ 24″ | 7 ft 2 in | 6 ft 10 in | +4″ |
| Redwood 2x6 @ 24″ | 7 ft | 6 ft 10 in | +2″ |
DCA 6-2015 is written to the 2015 IRC, so this is an edition gap rather than a contradiction. The span table above uses the 2021 IRC values. If you are sizing a 2x6 within a few inches of the limit, it is worth knowing which edition your jurisdiction has adopted before you cut.
At 16 inches on center, 11 ft 10 in in Southern Pine and 11 ft 1 in in Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir, per IRC Table R507.6. Tighten to 12 inches on center and Southern Pine reaches 13 ft 1 in; open up to 24 inches and it drops to 9 ft 8 in. The species matters as much as the spacing here, which is why a single "2x8 spans 13 feet" answer is not usable — it is only true for one species at one spacing.
Not as far as the quarter-span rule suggests. IRC R507.6.1 caps the overhang at 25% of the adjacent span, but AWC DCA 6 publishes an actual overhang table that is controlled by deflection — cantilever length over 180, with a 220 lb point load at the tip — and on 31 of the 36 combinations that deflection limit is the smaller of the two. The allowable overhang is whichever is less. A 2x6 Redwood joist at 12 inches on center is the extreme case: quarter-span says 2 ft 3 in, the published table allows 9 in.
No, and it is the most common way to oversize a span on paper and fail inspection. Deck joists are designed under wet service conditions — DCA 6 footnote 1 states the assumption explicitly — because the lumber is exposed to weather, and wet service reduces the allowable bending and modulus of elasticity of the same board. An interior floor joist table assumes dry service. Same species, same grade, same nominal size, shorter permitted span outdoors. Use IRC Table R507.6 for decks and R502.3.1 for interior floors.
Because the overhang limit is deflection-controlled, not strength-controlled, and it is checked with a 220 lb point load at the tip rather than a uniform load. Widening the spacing does not change that point load, but it does change the tributary load on the back-span, which shortens the permitted span and therefore stiffens the cantilever relative to it. AWC publishes an FAQ on exactly this. It reads like a transcription error and is not — for a 2x6 in Southern Pine the overhang runs 1 ft at 12 inches on center and 1 ft 3 in at 24. On a 2x12 the pattern reverses.
Almost. Across all 36 cells the 2021 IRC Table R507.6 and AWC DCA 6-2015 Table 2 agree on 33 and disagree on 3, all of them in the 2x6 row: Douglas Fir-Larch 2x6 at 16 in o.c. (IRC 8 ft 8 in, DCA 6 8 ft 4 in); Douglas Fir-Larch 2x6 at 24 in o.c. (IRC 7 ft 2 in, DCA 6 6 ft 10 in); Redwood 2x6 at 24 in o.c. (IRC 7 ft, DCA 6 6 ft 10 in). The IRC is the more permissive in each case, by two to four inches. DCA 6-2015 is written to the 2015 IRC, so this is an edition gap rather than an error in either document. Build to whichever your jurisdiction has adopted, and if you are near the limit on a 2x6, check which one your inspector is holding.
No. 2 grade, 40 psf live load plus 10 psf dead load, wet service conditions, and L/360 deflection. If your deck carries a hot tub, a heavy planter run, a masonry outdoor kitchen or a roof, none of these numbers apply — that is an engineered design, not a prescriptive table lookup. The same is true if you are using a species not in the table, a grade below No. 2, or joists deeper than a 2x12.
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36 values from this document
36 values from this document
36 values from this document
IRC R507.6.1 — deck joist cantilever is limited to one-fourth of the actual adjacent joist span. This is a ceiling, not a licence: the table span must still be satisfied by the backspan itself. · verified 2026-08-01
1 value from this document
