About Deck Beam Span & Footing Size Calculator (IRC R507.5)
The deck beam span and footing calculator reads 2021 IRC Table R507.5(1) — the beam companion to the R507.6 joist table — to give the maximum allowable span of a multi-ply wood deck beam between posts. The table assumes a 40 psf live load, 10 psf dead load, L/360 deflection, No. 2 grade lumber, and wet service conditions, and covers three species groups: southern pine, Douglas fir-larch / hem-fir / spruce-pine-fir, and redwood / western cedars / ponderosa pine / red pine. Per R507.5, the joist span the beam supports is rounded up to the next table column (a deck with 11 ft joists uses the 12 ft column), and spans beyond 18 ft fall outside the prescriptive table.
The same inputs then size the post and footing: the beam carries half the joist span plus the full cantilever, so each post supports that tributary width times the post spacing at 50 psf total load. Dividing the post load by the allowable soil bearing pressure (1500 to 3000 psf presets per IRC Table R401.4.1) gives the minimum bearing area, reported as both a round footing diameter and an equivalent square side, each rounded up to a whole inch. Finally, pick a sonotube diameter and pour depth to get the concrete volume with 10% waste and the number of 60-lb or 80-lb premix bags to buy.
How It Works
- Select the lumber species group and beam build-up — two or three plies of 2x8, 2x10, or 2x12 nailed per R507.5. Southern pine spans the farthest; the redwood/cedar group the least.
- Enter the deck joist span the beam supports. The tool rounds it UP to the next R507.5(1) column (6, 8, 10, 12, 14, 16, or 18 ft) and reads the maximum allowable beam span between posts — for example, southern pine (2) 2x10 supporting 12 ft joists may span 7 ft 4 in.
- Enter the joist cantilever and post spacing. The tributary width is half the joist span plus the cantilever, and the post load is tributary width x post spacing x 50 psf (40 live + 10 dead).
- Pick the allowable soil bearing pressure. The required bearing area is the post load divided by the soil pressure; the tool reports the minimum round footing diameter and the equivalent square side, both rounded up to the next whole inch, with a 6 in minimum-thickness advisory per Table R507.3.1.
- Choose a sonotube diameter (8 to 16 in), pour depth, and tube count: the tool returns the concrete volume in cubic feet and cubic yards, adds 10% waste, and rounds up the 60-lb and 80-lb premix bag counts.
Worked Example
A dropped beam supports deck joists spanning 12 ft with no cantilever, on posts spaced 6 ft apart over 1500 psf soil. The tributary width is 12 / 2 = 6 ft, so each post carries 6 x 6 = 36 sq ft of deck at 50 psf — a 1800 lb post load. The required bearing area is 1800 / 1500 = 1.2 sq ft, which needs a round footing of 2 x sqrt(1.2 / pi) = 1.24 ft = 14.83 in, rounded up to 15 in diameter (or a 14 in square). On 2000 psf soil the same post needs only a 13 in round footing. In southern pine, Table R507.5(1) allows a (2) 2x10 beam supporting the 12 ft joist span to run 7 ft 4 in between posts, so the 6 ft post spacing works at 81.8% utilization. Filling one 12 in sonotube 36 in deep takes pi x 0.5^2 x 3 = 2.36 cu ft of concrete — 2.59 cu ft with 10% waste — which is five 80-lb bags or six 60-lb bags.
Formulas
- Allowable beam span (table lookup)
L_B = TableR507.5(1)[species][plies-size][ceil(L_J -> column)]- Post load from tributary area
P = (L_J / 2 + L_C) x S_P x (LL + DL)- Required footing size (R507.3.1 bearing basis)
A = P / q ; d = 2 x sqrt(A / pi) ; s = sqrt(A)- Sonotube concrete and bags
V = pi x (D / 24)^2 x (H / 12) x n ; bags = ceil(1.1 x V / yield)
Standards & References
- IRC 2021 R507.5 / Table R507.5(1) -- maximum deck beam spans
- IRC 2021 R507.3 / Table R507.3.1 -- deck footing size and bearing
- IRC 2021 Table R401.4.1 -- presumptive soil load-bearing values
- AWC DCA 6 -- Prescriptive Residential Wood Deck Construction Guide
Frequently Asked Questions
How far can a double 2x10 deck beam span?
Per 2021 IRC Table R507.5(1) at 40 psf live load, a southern pine (2) 2x10 beam spans 10 ft 4 in when the joists span 6 ft, 8 ft 0 in at a 10 ft joist span, and 7 ft 4 in at a 12 ft joist span. The Douglas fir-larch / hem-fir / SPF group spans 10 ft 0 in, 7 ft 9 in, and 7 ft 0 in at the same columns, and the redwood/cedar group slightly less. The longer your joists, the shorter the allowable beam span — add a ply or a deeper member to stretch it.
What if my joist span falls between table columns?
Round it up. Table R507.5(1) has columns at 6, 8, 10, 12, 14, 16, and 18 ft of joist span, and the conservative prescriptive read uses the next column at or above your actual span — an 11 ft joist span is sized from the 12 ft column. (Footnote a of the table permits interpolation, but never extrapolation beyond 18 ft; longer joist spans need an engineered beam.)
How big does a deck footing need to be?
Big enough that the post load divided by its bearing area stays below the allowable soil pressure. Each post carries a tributary area of deck — half the joist span plus the cantilever, times the post spacing — at 40 psf live + 10 psf dead. A 12 ft joist span with posts 6 ft apart puts 1800 lb on each post, which on default 1500 psf soil needs 1.2 sq ft of bearing: a 15 in round or 14 in square footing. IRC Table R507.3.1 tabulates the same result from tributary area, and footings must extend below frost depth per R507.3.3.
What soil bearing value should I assume?
IRC Table R401.4.1 presumes 1500 psf for clay and silt mixes, 2000 psf for sand, 3000 psf for gravel, and more for rock. Unless a soils report or the local building department says otherwise, 1500 psf is the safe default — footnote a of R401.4.1 requires it where soil is not classified. Stronger soil shrinks the footing: the 1800 lb example needs 15 in of diameter at 1500 psf but only 13 in at 2000 psf.
How many bags of concrete does a sonotube footing take?
A tube holds pi x radius^2 x depth of concrete: a 12 in diameter tube filled 36 in deep is pi x 0.5^2 x 3 = 2.36 cu ft, or about 2.59 cu ft once you add 10% waste for spillage and over-excavation. At typical yields of 0.60 cu ft per 80-lb bag and 0.45 cu ft per 60-lb bag, that is five 80-lb bags or six 60-lb bags per tube. For more than roughly a dozen tubes, ready-mix by the cubic yard is usually cheaper than bags.
How far can a deck beam cantilever past its post?
Table R507.5(1) footnote f limits the beam cantilever to one-fourth of the adjacent beam span — a beam spanning 7 ft 4 in between posts may overhang the end post by up to 22 in. The joists themselves may also cantilever past the beam by up to one-fourth of the joist span per R507.6, which is why this tool adds the joist cantilever to the tributary width when sizing the post and footing.