Free tool · 2021 IRC deck tables, Section R507
Deck span calculator
Pick your lumber, joist size, spacing and snow load. Get the longest joist span and cantilever, the beam span between posts, the tallest post and the smallest footing the 2021 International Residential Code allows, with the table each answer came from.
At 16 in. on center and 40 psf live load (where the ground snow load is 40 psf or less), a No. 2 2×8 joist can span 11 ft 10 in. in southern pine and 11 ft 1 in. in Douglas fir-larch, hem-fir or spruce-pine-fir. A 2×10 spans 14 ft 0 in. or 13 ft 7 in. (2021 IRC Table R507.6).
Decks are designed for 40 psf live load or your ground snow load, whichever is greater (Section R507.1). Your town lists its ground snow load in its Table R301.2 design criteria; ask the code office. Between rows, pick the next higher one. Not sure of your snow load? See who sets it for your town.
The tables assume No. 2 grade or better, pressure-treated or naturally durable (Section R507.2.1).
Span is from support to support. 12 ft 6 in. = 12.5.
Check the highlighted box: joist span 1 to 30 ft, cantilever 0 to 10 ft, post spacing 1 to 30 ft, post height 0.5 to 20 ft.
Planning information, not a survey or engineering. Call 811 and check with your local office. The numbers are read from the 2021 IRC tables as printed in ICC’s public copy; your town may enforce another edition or local changes.
How it works
Table R507.6 gives the longest joist span by design load, species group, joist size and spacing, and the longest cantilever past the beam by back span, the span behind the beam. Between back-span columns the table allows interpolation but not extrapolation. We round spans down to the inch.
The beam tables are read at the effective joist span: the actual joist span times a factor from Table R507.5(5) set by cantilever ÷ span, 0.66 with no cantilever up to 1.00 at one-fourth. The 1/12 row in ICC’s public copy prints its decimal as 0.87 instead of about 0.083, so for any cantilever up to one-tenth of the span we use the 1/10 factor, 0.80, instead of the 0.72 printed for 1/12. The larger factor gives a longer effective span and a shorter beam span, the conservative choice. A beam may cantilever up to one-fourth of its actual span at each end.
We estimate the deck area on a middle post as post spacing × (half the joist span + the cantilever). Table R507.4 gives the tallest post for that area and Table R507.3.1 the smallest footing for that area and your soil. We round footing sizes up to the inch. Where a printed thickness looks out of line we use the thicker value.
Southern pine 2×10 joists at 16 in. on center, 12 ft span, no cantilever, 40 psf
Table R507.6: 14 ft 0 in. allowed, so 12 ft is fine
Effective joist span: 12 × 0.66 = 7.92 ft (Table R507.5(5), no cantilever)
Table R507.5(1), 3-ply 2×10 southern pine: 13 ft 0 in. at 6 ft, 11 ft 2 in. at 8 ft → 11 ft 2 in. at 7.92 ft
Posts 8 ft apart: 8 × (12 ÷ 2) = 48 sq ft on each middle post
Table R507.3.1, 1,500 psf soil: 14 in. at 40 sq ft, 17 in. at 60 sq ft → 16 in. square (18 in. round), 6 in. thick
2021 IRC joist spans at 40 psf
| Joist | Species group | 12 in. o.c. | 16 in. o.c. | 24 in. o.c. |
|---|---|---|---|---|
| 2×6 | Southern pine | 9 ft 11 in. | 9 ft 0 in. | 7 ft 7 in. |
| 2×8 | Southern pine | 13 ft 1 in. | 11 ft 10 in. | 9 ft 8 in. |
| 2×10 | Southern pine | 16 ft 2 in. | 14 ft 0 in. | 11 ft 5 in. |
| 2×12 | Southern pine | 18 ft 0 in. | 16 ft 6 in. | 13 ft 6 in. |
| 2×6 | Douglas fir-larch, hem-fir, spruce-pine-fir | 9 ft 6 in. | 8 ft 4 in. | 6 ft 10 in. |
| 2×8 | Douglas fir-larch, hem-fir, spruce-pine-fir | 12 ft 6 in. | 11 ft 1 in. | 9 ft 1 in. |
| 2×10 | Douglas fir-larch, hem-fir, spruce-pine-fir | 15 ft 8 in. | 13 ft 7 in. | 11 ft 1 in. |
| 2×12 | Douglas fir-larch, hem-fir, spruce-pine-fir | 18 ft 0 in. | 15 ft 9 in. | 12 ft 10 in. |
| 2×6 | Redwood, western cedars, ponderosa pine, red pine | 8 ft 10 in. | 8 ft 0 in. | 6 ft 10 in. |
| 2×8 | Redwood, western cedars, ponderosa pine, red pine | 11 ft 8 in. | 10 ft 7 in. | 8 ft 8 in. |
| 2×10 | Redwood, western cedars, ponderosa pine, red pine | 14 ft 11 in. | 13 ft 0 in. | 10 ft 7 in. |
| 2×12 | Redwood, western cedars, ponderosa pine, red pine | 17 ft 5 in. | 15 ft 1 in. | 12 ft 4 in. |
From Table R507.6, 40 psf live load. The calculator also covers 50, 60 and 70 psf ground snow loads.
What the deck tables assume
- No. 2 grade, wet service factor included.
- 10 psf dead load; snow is not assumed to act at the same time as live load.
- Joist deflection L/360 on the main span and L/180 at the cantilever with a 220 lb point load at the end.
- Incising factor included for Douglas fir-larch, hem-fir and spruce-pine-fir (Table R507.5(1) marks only Douglas fir-larch and hem-fir beams); not included for redwood, western cedars, ponderosa pine and red pine.
- The beam tables cover beams carrying a single span of joists, with or without a cantilever, and the post table covers single-level decks.
- Beam plies nailed with two rows of 10d (3 × 0.128 in.) nails at 16 in. on center along each edge.
- Joists and beams need at least 1½ in. of bearing on wood or metal and 3 in. on concrete or masonry.
- Hangers or blocking for joist lateral restraint at least 60 percent of the joist depth.
- Posts held to footings by manufactured connectors or at least 12 in. of embedment.
- Footings at least 12 in. below undisturbed ground.
When you need a permit or an engineer
The model code, IRC Section R105.2, exempts decks not over 200 sq ft in area that are no more than 30 in. above grade at any point, are not attached to a dwelling and do not serve the exit door required by Section R311.4. In Pennsylvania, 34 Pa. Code § 403.62(c)(8) lists installing an uncovered deck where the floor of the deck is no more than 30 in. above grade among work that doesn’t need a permit, if the work doesn’t violate a law or ordinance. Pennsylvania’s Uniform Construction Code has adopted the 2021 International Residential Code, with state changes, since Jan. 1, 2026 (34 Pa. Code § 403.21). Zoning and setbacks are separate. Find who enforces the code where you live with our PA building permit lookup or building codes by state.
- Ground snow loads over 70 psf.
- Hot tubs and other heavy loads. See the hot tub weight calculator.
- Beams carrying joists from both sides, spans past the table edges, and multi-level decks for post heights.
- Materials or conditions the section doesn’t cover, which Section R507.1 sends to the design rules of Section R301.
- A house connection that can’t be verified at inspection: Section R507.8 then requires a self-supporting deck.
Related: the footing calculator, frost line depth by state, how deep footings should be, freestanding deck plans, pergola post spacing, what the permit costs in central PA and the mini excavator size chart for digging the footings.
Questions
How far can a 2×8 deck joist span?
Under 2021 IRC Table R507.6 at 40 psf live load, a No. 2 southern pine 2×8 spans 13 ft 1 in. at 12 in. on center, 11 ft 10 in. at 16 in. and 9 ft 8 in. at 24 in. In Douglas fir-larch, hem-fir or spruce-pine-fir it spans 12 ft 6 in., 11 ft 1 in. and 9 ft 1 in. Heavier snow loads shorten every span.
How far can a 2×10 deck joist span?
At 40 psf live load, Table R507.6 allows a southern pine 2×10 16 ft 2 in. at 12 in. on center, 14 ft 0 in. at 16 in. and 11 ft 5 in. at 24 in. Douglas fir-larch, hem-fir or spruce-pine-fir: 15 ft 8 in., 13 ft 7 in. and 11 ft 1 in. Redwood, western cedars, ponderosa or red pine: 14 ft 11 in., 13 ft 0 in. and 10 ft 7 in.
How far can deck joists cantilever past the beam?
Table R507.6 sets the limit by joist size and back span, the span behind the beam. At 40 psf, a southern pine 2×10 with a 12 ft back span can cantilever 3 ft 0 in., and a 2×12 with an 18 ft back span 4 ft 1 in. With a 4 ft back span every size is held to 1 ft 0 in. NP in the table means not permitted.
How far can a deck beam span between posts?
It depends on the joists the beam carries. Tables R507.5(1) to R507.5(4) read beam spans from the effective deck joist span. With 12 ft joists, no cantilever and 40 psf, the effective span is 12 × 0.66 = 7.92 ft, and a 3-ply southern pine 2×10 beam can span 11 ft 2 in. between posts, a 2-ply 9 ft 0 in. A beam may also cantilever up to one-fourth of its actual span at each end.
What is the effective deck joist span?
It is the actual joist span multiplied by the joist span factor in Table R507.5(5), which depends on the joist cantilever divided by the joist span: 0.66 with no cantilever, 0.72 at one-twelfth, 0.80 at one-tenth, 0.84 at one-eighth, 0.90 at one-sixth and 1.00 at one-fourth. The beam tables are read at that effective span. Below one-tenth, this calculator uses 0.80, which gives a shorter beam span.
How big do deck footings need to be?
Table R507.3.1 sizes them by the deck area each post carries and the soil bearing value. At 40 psf on 1,500 psf soil, 40 sq ft needs a 14 in. square or 16 in. round footing 6 in. thick, and 100 sq ft needs 22 in. square or 25 in. round, 8 in. thick. Footings go at least 12 in. below undisturbed ground, and when the deck is attached to a frost-protected house they also need frost protection, usually by going below the frost line.
Can I use 4×4 posts on a deck?
Sometimes, but they get short fast as the load grows. At 40 psf, Table R507.4 lets a southern pine 4×4 stand 13 ft 8 in. carrying 40 sq ft of deck but only 8 ft 4 in. at 100 sq ft, while a 6×6 is allowed 14 ft 0 in. across the table. Redwood, western cedar, ponderosa or red pine 4×4s are not permitted above 100 sq ft. Post height is measured from the underside of the beam to the top of the footing.
Do I need a permit for a deck in Pennsylvania?
Under the Uniform Construction Code, 34 Pa. Code § 403.62(c)(8) lists installing an uncovered deck where the floor of the deck is no more than 30 inches above grade among work that does not need a permit, if the work does not violate a law or ordinance. A covered deck, or one with its floor more than 30 inches above grade, is not on that list, so plan on a permit. Zoning rules and setbacks are separate, so check with your township before you dig.
Sources: ICC, 2021 International Residential Code, Chapter 5, Section R507 Exterior Decks: R507.1, R507.2.1, R507.2.2.1, R507.3 to R507.3.3, Table R507.3.1, R507.4, Table R507.4, R507.4.1, R507.5, Tables R507.5(1) to R507.5(5), R507.5.1, R507.6, Table R507.6, R507.6.1, R507.6.2, R507.7, Table R507.7, R507.8; ICC, 2021 IRC Section R105.2, work exempt from permit (decks, item 10); 34 Pa. Code § 403.62(c)(8); 34 Pa. Code § 403.21(a)(7), International Residential Code of 2021 (amended Nov. 7, 2025, effective Jan. 1, 2026); Pennsylvania Department of Labor and Industry, Uniform Construction Code (triennial update effective Jan. 1, 2026); Londonderry Township, statewide code update effective Jan. 1, 2026 (2021 I-codes). AWC’s DCA 6 deck guide is published for the 2015 IRC, so we used the 2021 IRC tables directly. Last checked November 12, 2026.
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