Deck Beam Span Chart — IRC 2021 Table R507.5(1)
How far a built-up deck beam can span between posts, straight from the code: 2×6 through 2×12, 1-, 2- and 3-ply, three species groups, at 40 psf live load — with the effective-joist-span step most charts skip.
This is a lookup, not a calculation: the cell already contains the dead load (10 psf), the No. 2 grade wet-service lumber and the L/360 deflection limit. Enter with the effective joist span — the actual span multiplied by the Table R507.5(5) factor — then read across. Built-up plies are fastened with two rows of 10d (3″ × 0.128″) nails at 16″ on centre along each edge, and a beam may cantilever no more than one-quarter of its own span. For the full chart see the deck beam span chart below.
What this chart answers
A deck frames like a stack of lookups. The joists are the first one — how far can this member span, from the ledger to the beam. The beam is the second: once the joists land on it, how far can the beam run between posts before it, too, sags past what the code allows. The answer lives in Table R507.5(1) of the IRC, and it is searched for in exactly the shape people search the joist table — "deck beam span chart", "how far can a 2×10 beam span", "what size beam for a deck" — because the number moves with species, ply count, beam size and the span of the joists sitting on it all at once.
This page reproduces R507.5(1) in full and spells out the step almost every reproduction leaves out: the table is entered with an effective joist span, not the actual one. Get that wrong and you read the wrong column — usually a conservative one, but the code number is a different number, and on a deck that is worth being exact about.
What the chart assumes — the conditions that travel with the numbers
A span table is a set of answers to one specific question, and the conditions live in the footnotes. R507.5(1) rates No. 2 grade lumber at wet-service with a 10 psf dead load already included, a main span held to L/360 — no more sag than span ÷ 360 — and a cantilever held to L/180. The load case is 40 psf live, the residential deck standard; the companion tables R507.5(2) through (4) step up to 50, 60 and 70 psf ground snow.
Two framing rules ride along with the table. A beam's depth must be at least the depth of the joists framing into it, so a flush beam does not step down and pinch the hanger. And beams must bear a real width on their supports — at least 1½″ on wood or metal and 3″ on concrete or masonry (R507.5.1), full width of the member.
The deck beam span chart — Table R507.5(1), cell for cell
Read down the species group, across the ply count and size, into the column for your effective joist span (6 ft through 18 ft). The cell is the maximum distance between posts, in feet-inches — 9-0 is 9 ft 0 in.
| Species group | Beam (plies × size) | 6′ eff. | 8′ eff. | 10′ eff. | 12′ eff. | 14′ eff. | 16′ eff. | 18′ eff. |
|---|---|---|---|---|---|---|---|---|
| Southern pine | 1 — 2×6 | 4-7 | 4-0 | 3-7 | 3-3 | 3-0 | 2-10 | 2-8 |
| 1 — 2×8 | 5-11 | 5-1 | 4-7 | 4-2 | 3-10 | 3-7 | 3-5 | |
| 1 — 2×10 | 7-0 | 6-0 | 5-5 | 4-11 | 4-7 | 4-3 | 4-0 | |
| 1 — 2×12 | 8-3 | 7-1 | 6-4 | 5-10 | 5-5 | 5-0 | 4-9 | |
| 2 — 2×6 | 6-11 | 5-11 | 5-4 | 4-10 | 4-6 | 4-3 | 4-0 | |
| 2 — 2×8 | 8-9 | 7-7 | 6-9 | 6-2 | 5-9 | 5-4 | 5-0 | |
| 2 — 2×10 | 10-4 | 9-0 | 8-0 | 7-4 | 6-9 | 6-4 | 6-0 | |
| 2 — 2×12 | 12-2 | 10-7 | 9-5 | 8-7 | 8-0 | 7-5 | 7-0 | |
| 3 — 2×6 | 8-6 | 7-5 | 6-8 | 6-1 | 5-8 | 5-3 | 4-11 | |
| 3 — 2×8 | 10-11 | 9-6 | 8-6 | 7-9 | 7-2 | 6-8 | 6-4 | |
| 3 — 2×10 | 13-0 | 11-2 | 10-0 | 9-2 | 8-6 | 7-11 | 7-6 | |
| 3 — 2×12 | 15-3 | 13-3 | 11-10 | 10-9 | 10-0 | 9-4 | 8-10 | |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 1 — 2×6 | 4-1 | 3-6 | 3-0 | 2-8 | 2-5 | 2-3 | 2-1 |
| 1 — 2×8 | 5-6 | 4-8 | 4-0 | 3-6 | 3-2 | 2-11 | 2-9 | |
| 1 — 2×10 | 6-8 | 5-10 | 5-1 | 4-6 | 4-1 | 3-9 | 3-6 | |
| 1 — 2×12 | 7-9 | 6-9 | 6-0 | 5-6 | 5-0 | 3-9 | 3-6 | |
| 2 — 2×6 | 6-1 | 5-3 | 4-9 | 4-4 | 3-11 | 3-7 | 3-3 | |
| 2 — 2×8 | 8-2 | 7-1 | 6-4 | 5-9 | 5-2 | 4-8 | 4-4 | |
| 2 — 2×10 | 10-0 | 8-7 | 7-9 | 7-0 | 6-6 | 6-0 | 5-6 | |
| 2 — 2×12 | 11-7 | 10-0 | 8-11 | 8-2 | 7-7 | 7-1 | 6-8 | |
| 3 — 2×6 | 7-8 | 6-8 | 6-0 | 5-6 | 5-1 | 4-9 | 4-6 | |
| 3 — 2×8 | 10-3 | 8-10 | 7-11 | 7-3 | 6-8 | 6-3 | 5-11 | |
| 3 — 2×10 | 12-6 | 10-10 | 9-8 | 8-10 | 8-2 | 7-8 | 7-2 | |
| 3 — 2×12 | 14-6 | 12-7 | 11-3 | 10-3 | 9-6 | 8-11 | 8-5 | |
| Redwood, western cedars, ponderosa pine, red pine | 1 — 2×6 | 4-2 | 3-7 | 3-1 | 2-9 | 2-6 | 2-3 | 2-2 |
| 1 — 2×8 | 5-4 | 4-7 | 4-1 | 3-7 | 3-3 | 3-0 | 2-10 | |
| 1 — 2×10 | 6-6 | 5-7 | 5-0 | 4-7 | 4-2 | 3-10 | 3-7 | |
| 1 — 2×12 | 7-6 | 6-6 | 5-10 | 5-4 | 4-11 | 4-7 | 4-4 | |
| 2 — 2×6 | 6-2 | 5-4 | 4-10 | 4-5 | 4-0 | 3-8 | 3-4 | |
| 2 — 2×8 | 7-10 | 6-10 | 6-1 | 5-7 | 5-2 | 4-10 | 4-5 | |
| 2 — 2×10 | 9-7 | 8-4 | 7-5 | 6-9 | 6-3 | 5-10 | 5-6 | |
| 2 — 2×12 | 11-1 | 9-8 | 8-7 | 7-10 | 7-3 | 6-10 | 6-5 | |
| 3 — 2×6 | 7-8 | 6-9 | 6-0 | 5-6 | 5-1 | 4-9 | 4-6 | |
| 3 — 2×8 | 9-10 | 8-6 | 7-7 | 6-11 | 6-5 | 6-0 | 5-8 | |
| 3 — 2×10 | 12-0 | 10-5 | 9-4 | 8-6 | 7-10 | 7-4 | 6-11 | |
| 3 — 2×12 | 13-11 | 12-1 | 10-9 | 9-10 | 9-1 | 8-6 | 8-1 |
⚠️ One published quirk, shown as published. In the Douglas fir-larch / hem-fir / spruce-pine-fir block, the 1-ply 2×12 row prints 3-9 and 3-6 at the 16 ft and 18 ft columns — below its own 5-0 at the 14 ft column, and identical to the 1-ply 2×10 row. That is what the code text says, and two independent reproductions render it the same way. It is not a typo this page feels free to silently correct; if you are sizing a single 2×12 beam past a 14 ft effective span, confirm against the code your authority having jurisdiction enforces.
How to read it — the effective joist span (Table R507.5(5))
The single most-missed step in the whole deck sequence: the beam table is entered with an effective deck joist span, because a joist that cantilevers past the beam pushes extra load down onto it. Table R507.5(5) scales the actual joist span by a factor tied to the ratio of joist cantilever (C) to joist span (J):
| C / J | Cantilever situation | Joist span factor |
|---|---|---|
| 0 | No cantilever | 0.66 |
| 1/12 | C/J = 1/12 | 0.72 |
| 1/10 | C/J = 1/10 | 0.80 |
| 1/8 | C/J = 1/8 | 0.84 |
| 1/6 | C/J = 1/6 | 0.90 |
| 1/4 | 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. For a plain deck with the joists ending at the beam and no overhang, C/J = 0 and the factor is 0.66: a 12 ft joist span enters the beam table as 7.92 ft, read against the 8 ft column.
Worked example — a 12 ft joist span, checked by hand
A Southern pine deck, joists running 12 ft from the ledger face to the beam, framed at 16″ on centre, with the beam flush at the outer edge — no joist cantilever. You want to know the biggest post spacing you can use with a doubled 2×10 beam.
- Find the factor. No joist cantilever, so C/J = 0 and Table R507.5(5) gives 0.66.
- Compute the effective span. 12 × 0.66 = 7.92 ft, which lands on the 8 ft column (a span between the 6 and 8 ft columns could be interpolated; 7.92 is essentially the 8 ft column).
- Look up the beam. Southern pine, 2-ply 2×10, at the 8 ft column of R507.5(1) = 9-0 (9 ft 0 in). So the posts may be up to 9 ft on centre.
- If you need more than 9 ft between posts. Step the beam up: a 2-ply 2×12 reaches 10-7 at the same column, and a 3-ply 2×10 reaches 11-2. Either clears a 10 ft post spacing with margin. If neither is enough, add a post — do not read past the table.
Every step is a single cell or a single multiplication, which is the point of a reference table over a black box: you can see the alternatives — one more ply, one size up, one more post — at a glance.
Common mistakes with the deck beam chart
- Using the actual joist span as the column. The table is keyed to the effective span from Table R507.5(5). Reading the raw span is conservative, but it is not the code number, and it can push you to a beam bigger than the code requires.
- Forgetting the joist overhang. A joist cantilever lowers the factor toward 1.00, which is the opposite of what people expect — the factor rises with cantilever, and at C/J = 1/4 it is 1.00, so the beam sees the full joist span.
- Assuming a 4-ply beam option. Table R507.5(1) stops at 3 plies. A wider beam than the table lists is an engineered design, not a table lookup.
- Reading the 40 psf table with a snow load. If your permit lists a ground snow load, R507.5(1) is the wrong table — see below.
- Ignoring the post and footing under the beam. A beam is only as good as what carries it. Post size comes from Table R507.4 and the footing from Table R507.3.1 by tributary area — the deck footing calculator runs that lookup, and the deck post spacing guide walks the post side — spacing, size, height and the post-to-footing connection.
- Cantilevering the beam like a joist. The quarter-span rule is a beam rule (R507.5), and a joist cantilever is a different column entirely — the maximum-cantilever column of Table R507.6.
Why only the 40 psf live-load table is on this page
The IRC also publishes beam spans for 50, 60 and 70 psf ground snow loads in Tables R507.5(2) through (4). This page does not reproduce them yet, and the reason is honesty rather than oversight: three cells in the published snow-load text are malformed. R507.5(2), redwood group, 3-ply 2×8 row, at the 14 ft column, prints the string 604; R507.5(3) prints 6-22 and 5-20 in the same redwood 3-ply 2×8 row at the 10 ft and 14 ft columns. None of those are valid feet-inches values, and they reproduce identically across every digital source checked — so they are not a viewer glitch, and they are not guessed at here.
The 40 psf live-load case is the one that governs the overwhelming majority of residential decks, and it is clean. If you have a ground snow load, read Tables R507.5(2)–(4) in your adopted code edition, or have the beam sized against them — the snow case is shorter than the numbers on this page, never longer.
Deck beam span FAQ
How far can a 2×10 deck beam span?
A doubled (2-ply) Southern pine 2×10 spans 10-4 between posts at a 6 ft effective joist span, 9-0 at 8 ft, 8-0 at 10 ft, 7-4 at 12 ft, 6-9 at 14 ft, 6-4 at 16 ft and 6-0 at 18 ft — straight from Table R507.5(1). Tripling the plies buys roughly two more feet at every column: a 3-ply 2×10 reaches 13-0 at 6 ft and 8-6 at 14 ft. The species group changes the number, so a Douglas fir-larch or spruce-pine-fir 2-ply 2×10 is lower than the Southern pine figure at the longer columns.
What is the effective deck joist span?
The beam tables are entered not with the actual joist span but with an effective span: the actual joist span multiplied by a factor from Table R507.5(5) that depends on how much the joists cantilever past the beam. With a flush beam and no joist overhang the factor is 0.66, so a 12 ft joist span enters the table as 12 × 0.66 = 7.92 ft, read as the 8 ft column. The factor is 1.00 only when the joist cantilever is one quarter of the joist span. Using the raw span instead is conservative — it reads a longer column and a shorter allowed beam.
How many plies does a deck beam need?
Pick the smallest beam that clears your span. Table R507.5(1) lists 1-, 2- and 3-ply beams from 2×6 to 2×12 (there is no 4-ply row). A doubled beam is the common choice because it is easy to assemble from two long boards, but a 3-ply beam spans further, and a single 2×12 can be enough on a small, closely-posted deck. Move up a size or add a ply before you move a post, and remember that plies must be fastened with two rows of 10d nails at 16″ on centre along each edge.
Does the deck beam span change with snow load?
Yes. Table R507.5(1) is the 40 psf live-load case. Tables R507.5(2), (3) and (4) carry 50, 60 and 70 psf ground snow loads and give shorter spans. If your jurisdiction lists a ground snow load on the permit, the 40 psf table does not govern — and snow is never assumed to act at the same time as the live load, so you size for whichever is higher, never both.
Can a deck beam cantilever past the post?
Only up to one-quarter of the beam's own span. The footnote to Tables R507.5(1)–(4) reads: "Beam cantilevers are limited to the adjacent beam's span divided by 4." That is a beam rule — it is not the rule for joists, whose cantilever comes from the separate maximum-cantilever column of Table R507.6.
Can I use this beam table to size a floor beam?
No. Table R507.5 is for exterior decks with a 40 psf live load, a 10 psf dead load and a wet-service, weather-exposed basis. Interior floors are sized from R502.3 and its own tables, which use different live-load and deflection assumptions. Crossing a size from one to the other is a framing error, not a shortcut.
Where these numbers came from
Table R507.5(1) and Table R507.5(5) were fetched on 2026-10-09 from the up.codes viewer for the 2021 IRC as adopted in Connecticut (chapter 5, Floors, table_R507.5-1), parsed cell by cell, and diffed against a second, ICC-embedded reproduction at advantagelumber.com — the two agree on all 36 rows. The footnotes quoted above (10 psf dead load, No. 2 grade wet-service, L/360 main span, L/180 at a cantilever, the one-quarter beam-cantilever rule) are read verbatim from the same page. The IRC is adopted state by state: check your own edition and amendments, and the authority having jurisdiction always wins.
This is a working reference, not engineering. A table lookup is the right answer only while your situation sits inside the table — inside the span, inside the load, inside the conditions above. Anything outside it, or any deck carrying unusual loads, needs a designer or an engineer.
Related pages
- Deck joist span table — IRC Table R507.6 — the joist half of the same lookup, with the maximum-cantilever column
- Deck beam & joist span calculator — both tables applied to one layout, including the R507.5(5) factor
- Deck footing size calculator — the posts and pads that carry whatever the beam carries
- Deck board spacing guide — the decking and its gaps, which set the maximum joist spacing
- Deck stairs guide — step count, stringer spacing and the landing rule
- Reference tables — every code table, with its source and verification date