Deck Beam & Joist Span Calculator

What a span table is — and why deflection rules it

A 2×10 joist will carry far more weight than these tables allow before it actually breaks. The number that governs almost every span is deflection, not strength. A joist that is strong enough but sags feels bouncy underfoot and works against the ledger connection, so the IRC caps how far a member may flex. The main span is held to L/360 — no more sag than span ÷ 360, so a 12-ft joist may drop at most 12 × 12 ÷ 360 = 0.4″. A cantilever is held to the looser L/180, but is also checked against a 220-lb point load at its tip so a railing or someone leaning on the edge can't collapse it.

How to read Table R507.6 (joists)

R507.6 is a lookup. Enter with the load case (40 psf live, or 50/60/70 psf ground snow), species group, joist size (2×6 through 2×12), and spacing (12, 16, or 24″ OC). The cell gives the max span in feet-inches — 14-0 means 14 ft 0 in. The table also carries a maximum-cantilever column keyed by back span, 4 ft through 18 ft. The joist cantilever is limited to that tabulated value; cells marked NP forbid it. Southern pine 2×10 at 16″ OC spans 14-0, with a 2-6 cantilever at a 10-ft back span.

The joist span factor almost everyone skips (Table R507.5(5))

The beam tables are not entered with the actual joist span but with an effective deck joist span. A cantilever pushes extra load onto the beam, so Table R507.5(5) scales the span by a factor tied to cantilever (C) ÷ joist span (J):

• No cantilever (C/J = 0): 0.66
• C/J = 1/12: 0.72
• C/J = 1/10: 0.80
• C/J = 1/8: 0.84
• C/J = 1/6: 0.90
• C/J = 1/4: 1.00

The factor never exceeds 1.00, so the effective span is always the same as or shorter than the actual span — a shorter effective span reads a shorter column and permits a longer beam. The calculator enters the actual span directly, which only pushes you to a more conservative column, never a less safe one.

Sizing the beam — Tables R507.5(1) through (4)

Pick the beam table for your load: R507.5(1) is 40 psf live (most decks); (2) through (4) step up to 50, 60, and 70 psf ground snow. Each has seven columns — effective spans of 6, 8, 10, 12, 14, 16, and 18 ft — and rows for the same three species groups, with 1-, 2- and 3-ply built-up beams from 2×6 to 2×12 (there is no 4-ply row). Read across to your column; the cell gives the max distance between posts. The full R507.5(1) table is also reproduced at the deck beam span chart. Built-up plies are fastened with two rows of 10d (3″ × 0.128″) nails at 16″ OC along each edge, and a beam may not cantilever more than 1/4 of its own span. Posts below are sized from Table R507.4 by tributary area.

The footnotes that change the answer

• Interpolation is permitted; extrapolation is not. Split between columns and rows; past the end of the table, add a post or call an engineer.
• Dead load is 10 psf, already in the numbers.
• Snow is not concurrent with live load. Size for the higher of the two, never both.
• No. 2 grade, wet-service lumber is assumed; a higher grade is an engineer's call, not a table swap.
• Flush connections need depth. The beam must be at least as deep as the joists framing into it.
• Bearing. Beams bear at least 1½″ on wood or metal and 3″ on concrete or masonry (R507.5.1), full width.

Deck tables are not floor tables

The numbers look close, but interior floors use R502.3 and Table R502.3.1, which splits sleeping areas (30 psf live, 10 or 20 psf dead, L/360) from other rooms (40 psf live). Decks run on R507 with a different live-load and deflection basis, wet-service lumber, and weather exposure. Pulling a size out of a floor table for a deck — or the reverse — is a framing error, not a shortcut.

Worked example — 2×10 joists, 12-ft span, no cantilever

Southern pine deck, 2×10 joists at 16″ OC, 12 ft from ledger face to beam face, beam at the outer edge with no cantilever:

• Check the joist. Table R507.6 allows 14-0 for Southern pine 2×10 at 16″ OC; 12 ft fits.
• Check the cantilever. You have no cantilever past the beam, so the R507.6 cantilever column does not constrain the framing.
• Find the factor. C/J = 0, so Table R507.5(5) gives 0.66.
• Compute the effective span. 12 × 0.66 = 7.9 ft, landing on the 8 ft column.
• Look up the beam. At the 8 ft column of R507.5(1), a doubled 2×10 reaches 9-0 and a doubled 2×12 reaches 10-7.

If the beam must span 9 ft between posts, a doubled 2×10 at 9-0 makes it with no margin; a doubled 2×12 at 10-7 clears it comfortably. That one size step is margin worth paying for on a deck holding people.

Common mistakes

• Using the actual joist span as the beam column. The table is keyed to the effective span from R507.5(5). The raw span is conservative, but it can oversize the beam the code requires.
• Mixing deck and floor tables. R507 and R502.3 use different live-load and deflection bases; a size that works in one isn't automatically good in the other.
• Extrapolating past the table. Interpolation is allowed; going beyond the last column or row is not. Over 18 ft effective span, add a post or get an engineer.
• Ignoring the cantilever limits. Joist cantilevers are capped by the Table R507.6 maximum-cantilever column for the back span; beams by 1/4 of the actual beam span.
• Assuming 24″ spacing is always fine. It's only fine if the decking allows it — 1¼″ decking tops out at 16″ on multiple-span runs; only 2″ decking spans 24″ (R507.7).

Deck beam & joist span FAQ

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