IRC Deck Span Tables (2021)
Maximum deck joist spans (IRC R507.6) and built-up beam spans (IRC R507.5), in feet-inches. A 2×8 joist at 16″ on center spans 11'-10" in Southern Pine; a double 2×10 beam spans 9'-9" at a 4 ft tributary width. All values assume 40 psf live + 10 psf dead load and no cantilever.
Joist span — IRC R507.6
Clear span between supports, face of ledger to centre of beam. Species matters more than most people expect — the same 2×10 loses over a foot of span moving from Southern Pine to Spruce-Pine-Fir.
| Species | 2×6 | 2×8 | 2×10 | 2×12 | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| on center → | 12″ | 16″ | 24″ | 12″ | 16″ | 24″ | 12″ | 16″ | 24″ | 12″ | 16″ | 24″ |
| Southern Pine (SP)Most common pressure-treated stock in the East/South | 9'-11" | 9'-0" | 7'-7" | 13'-1" | 11'-10" | 9'-8" | 16'-2" | 14'-0" | 11'-5" | 18'-0" | 16'-6" | 13'-6" |
| Douglas Fir-Larch (DF)Common in the West | 9'-6" | 8'-4" | 6'-10" | 12'-6" | 10'-11" | 8'-11" | 15'-8" | 13'-7" | 11'-1" | 18'-0" | 15'-9" | 12'-10" |
| Hem-Fir (HF)Same tabulated spans as DF-L for deck loading | 9'-6" | 8'-4" | 6'-10" | 12'-6" | 10'-11" | 8'-11" | 15'-8" | 13'-7" | 11'-1" | 18'-0" | 15'-9" | 12'-10" |
| Spruce-Pine-Fir (SPF)Common in the North/Canada — the most conservative | 9'-1" | 7'-11" | 6'-6" | 11'-11" | 10'-5" | 8'-6" | 14'-11" | 13'-0" | 10'-7" | 17'-9" | 15'-1" | 12'-4" |
Cantilever past the beam is additional and capped at ¼ of the backspan (4 ft absolute) by IRC R507.6.1. Check a cantilever.
Built-up beam span — IRC R507.5
Post-to-post span by tributary width — half the joist span on each side of the beam, not the deck's overall size. Values are Southern Pine #2; multiply by 0.95 for Douglas Fir-Larch and 0.9 for Hem-Fir or Spruce-Pine-Fir.
| Beam | 4 ft | 6 ft | 8 ft | 10 ft | 12 ft | 14 ft | 16 ft | 18 ft |
|---|---|---|---|---|---|---|---|---|
| (2) 2×6 | 6'-6" | 5'-3" | 4'-7" | 4'-1" | 3'-9" | 3'-6" | 3'-3" | 3'-1" |
| (2) 2×8 | 8'-3" | 6'-9" | 5'-10" | 5'-3" | 4'-9" | 4'-5" | 4'-1" | 3'-10" |
| (2) 2×10 | 9'-9" | 8'-0" | 6'-11" | 6'-2" | 5'-8" | 5'-3" | 4'-11" | 4'-7" |
| (2) 2×12 | 11'-3" | 9'-3" | 8'-1" | 7'-2" | 6'-7" | 6'-1" | 5'-8" | 5'-4" |
| (3) 2×6 | 7'-9" | 6'-4" | 5'-6" | 4'-11" | 4'-5" | 4'-1" | 3'-10" | 3'-7" |
| (3) 2×8 | 10'-2" | 8'-4" | 7'-2" | 6'-5" | 5'-10" | 5'-4" | 5'-0" | 4'-8" |
| (3) 2×10 | 12'-0" | 9'-9" | 8'-6" | 7'-7" | 6'-11" | 6'-5" | 6'-0" | 5'-8" |
| (3) 2×12 | 13'-9" | 11'-3" | 9'-9" | 8'-9" | 8'-0" | 7'-5" | 6'-11" | 6'-6" |
Column headings are tributary width in feet. Between two columns, interpolate — or let the beam span calculator do it and check your posts at the same time.
When these tables do not apply
- Snow load. Tabulated values are 40 psf live + 10 psf dead. Ground-snow regions may require a derate or an engineered design.
- Concentrated loads. A hot tub, a planter wall, an outdoor kitchen or a roof over the deck is outside prescriptive scope — these need engineering.
- Grade and treatment. Values assume #2 grade or better and ground-contact-rated lumber where required. A stamp you cannot read is a stamp you cannot rely on.
- Local amendments win. Many jurisdictions amend the IRC. Your building department's table is the one that gets inspected.
Source: 2021 International Residential Code §R507.5–R507.6 and AWC DCA-6, the prescriptive deck guide the IRC references. See our methodology and sources. This page is a reference, not a substitute for a permit review or a licensed designer.
Check your own numbers
Span table questions
What is the maximum span for a 2x8 deck joist?
At 16 inches on center, a 2×8 deck joist spans 11'-10" in Southern Pine, 10'-11" in Douglas Fir-Larch or Hem-Fir, and 10'-5" in Spruce-Pine-Fir. Tighten to 12 inches on center and Southern Pine reaches 13'-1". These are IRC 2021 R507.6 values for 40 psf live plus 10 psf dead load with no cantilever.
How far can a double 2x10 deck beam span?
A 2-ply 2×10 built-up beam in Southern Pine spans 9'-9" post-to-post at a 4 ft tributary width, 6'-11" at 8 ft, and 5'-8" at 12 ft. Tributary width is half the joist span on each side of the beam, so a beam carrying 12 ft joists from one side only has a 6 ft tributary width.
What is tributary width on a deck beam?
Tributary width is the width of deck the beam actually carries: half the joist span on each side of it. A centre beam under 12 ft joists on both sides carries 12 ft. An outer beam with 12 ft joists on one side and a 2 ft cantilever on the other carries about 7 ft. Beam span tables are indexed by this number, not by the deck's overall size.
Do these span tables include snow load?
No. Published IRC deck tables assume 40 psf live load plus 10 psf dead load. In ground-snow-load regions your building department may require a derate or an engineered design, and a hot tub, planter wall or roof over the deck is an added concentrated load the prescriptive tables do not cover.
Are joist spans measured to the centre of the beam?
Span is the clear distance between supports — face of ledger to centre of beam for a typical attached deck. A cantilever past the beam is additional and is capped separately at one quarter of the backspan by IRC R507.6.1.