Deck Beam Sizing Guide — IRC R507.5 Span Tables
Pick the right built-up beam for your deck using the IRC 2021 prescriptive span table — then assemble and connect it so it passes inspection the first time.
The beam is the single member carrying the largest share of your deck's load — and the one homeowners most often undersize. The mistake is almost always the same: people size the beam to the deck's width instead of to its tributary width and post spacing. This guide walks through IRC 2021 R507.5 Table R507.5(1), the tributary-width concept that drives every number in it, how to build up the beam correctly, and the post-connection and cantilever rules that decide whether the beam passes inspection. All span figures below match DeckMath's beam-span calculator exactly.
What a beam span actually measures
Two distances define every beam, and confusing them is the #1 sizing error:
- Beam span — the clear distance between two posts (post-to-post). This is what the IRC table limits.
- Tributary width — half the joist span the beam carries. A beam under the middle of a deck whose joists span 12 ft total picks up 6 ft of load on each side, so its tributary width is 6 ft.
On a ledger-attached deck, the house ledger carries one half of the joist load and the beam carries the other half — so tributary width is half the joist span. On a freestanding deck with two beams, each beam carries half. Get tributary width right first; the beam table is meaningless without it.
IRC R507.5 — built-up beam span table
Maximum beam span (post-to-post) for built-up beams of Southern Pine #2 at 40 psf live + 10 psf dead load, per IRC Table R507.5(1) / AWC DCA-6. Read down to your beam size + ply, across to your tributary width:
| Beam (Southern Pine #2) | 6 ft trib | 8 ft trib | 10 ft trib | 12 ft trib |
|---|---|---|---|---|
| 2-ply 2×8 (two 2×8s) | 6'-9" | 5'-10" | 5'-3" | 4'-9" |
| 2-ply 2×10 | 8'-0" | 6'-11" | 6'-2" | 5'-8" |
| 2-ply 2×12 | 9'-3" | 8'-1" | 7'-2" | 6'-7" |
| 3-ply 2×8 (three 2×8s) | 8'-4" | 7'-2" | 6'-5" | 5'-10" |
| 3-ply 2×10 | 9'-9" | 8'-6" | 7'-7" | 6'-11" |
| 3-ply 2×12 | 11'-3" | 9'-9" | 8'-9" | 8'-0" |
How to read it — a worked example
You're building a 16 ft × 12 ft ledger-attached deck. Joists span the 12 ft dimension. You want posts no closer than 8 ft apart along the 16 ft beam line.
- Tributary width: joists span 12 ft, ledger carries half → beam tributary = 6 ft.
- Beam span you need: posts 8 ft apart → 8 ft beam span (with a center post; or plan posts to hit a span the beam can make).
- Look up 6 ft trib: a 2-ply 2×10 spans 8'-0" — just makes 8 ft with no margin. A 2-ply 2×12 (9'-3") or 3-ply 2×10 (9'-9") gives comfortable margin.
- Pick the 3-ply 2×10 or 2-ply 2×12, then round the actual post spacing down a few inches below the table max to leave room for moisture sag.
Building up the beam correctly
A built-up beam is two or three 2× boards fastened face-to-face to act as one member. The fastening is structural, not cosmetic — under-fastening is a common inspection failure.
- 2-ply: two rows of 10d nails at 16" o.c. (top and bottom), OR ½" through-bolts at 24" o.c. staggered. Most inspectors prefer bolts on anything carrying real load.
- 3-ply: ½" through-bolts are effectively required — you cannot reliably nail through three plies. Bolt at 16–24" o.c., staggered top and bottom.
- Crown all plies the same way (bow pointing up) before fastening so the beam cambers upward, not down.
- Use pressure-treated lumber rated for ground contact if the beam is within 6" of grade or over standing water.
Beam-to-post connection
IRC R507.5.2 requires positive connection between beam and post — the load path must be mechanical, not friction. Two accepted details:
Post cap (recommended)
The beam bears on top of the post; a Simpson BC, BCS, or AC-series cap wraps both and is fastened with structural screws or nails. This is the cleanest, strongest, easiest-to-inspect detail.
Notched 6×6 post
The 6×6 is notched so the beam sits in the notch and bears on the remaining post shoulder, then through-bolted. IRC requires a 6×6 (not 4×4) for a notched detail, and at least a 1.5" bearing shoulder. 4×4 posts cannot be notched for a structural beam.
| Post height | 4×4 allowed? | 6×6 required? |
|---|---|---|
| Up to 6 ft | Yes (light loads) | Recommended |
| 6–8 ft | No | Yes |
| Over 8 ft | No | Yes + bracing / engineering |
Cantilevering the beam
IRC R507.5.1 lets the beam overhang the end post by up to 1/4 of the allowable beam span (the back-span). This lets you push posts inboard for a cleaner look or to clear an obstruction.
- Beam span 8 ft → max overhang 2 ft beyond the end post
- Beam span 10 ft → max overhang 2.5 ft
- Beam span 12 ft → max overhang 3 ft
Don't confuse beam cantilever (the beam past the post, R507.5.1) with joist cantilever (the joist past the beam, R507.6.1). They're independent limits and a deck can use both at once. Anything beyond the prescriptive 1/4 ratio needs a stamped engineering design.
Snow and heavier loads
The table above is 40 psf live load — the IRC residential default for most of the US. In snow country the design load rises, and every span in the table shrinks proportionally:
- 50 psf (light snow, most of the US) — derate spans ~10%
- 60 psf (Northeast / Great Lakes) — derate ~18%
- 70 psf (Northern New England, Rockies) — derate ~25%
Point loads change everything: a hot tub, outdoor kitchen, or planter wall is not a uniform load. Those sit on a dedicated beam-and-post assembly directly under the load, sized by the actual filled weight — never on a beam sized for general deck live load.
The three most common beam mistakes
- Sizing to deck width, not tributary width + post span. The beam doesn't care how wide the deck is — only how far between posts and how much joist it carries.
- Side-mounting the beam to posts with bolts only. The beam must bear on top of the post (post cap) or in a notched 6×6. Bolts-through-the-side is not a prescriptive load path.
- Forgetting the species derate. Big-box SPF spans ~10% less than the Southern Pine numbers in the table — designing an SPF beam to SP spans is an overstress.
Frequently asked questions
How far can a deck beam span between posts?
It depends on beam size, ply count, species, and tributary width — not deck width. A 2-ply 2×10 Southern Pine beam carrying 6 ft of tributary spans 8'-0" between posts; the same beam at 12 ft tributary drops to 5'-8". A 3-ply 2×12 at 6 ft tributary reaches 11'-3". Use the IRC R507.5 table above or DeckMath's beam-span calculator for your exact combination.
What is tributary width on a deck beam?
Tributary width is the portion of joist length whose load lands on that beam — usually half the joist span. If joists span 12 ft and the house ledger carries one end, the beam carries the other 6 ft, so tributary width is 6 ft. Every number in the IRC R507.5 beam table is indexed to tributary width, so you must calculate it before sizing the beam.
Should a deck beam be 2-ply or 3-ply?
Use the fewest plies that pass at your span — it's the cheapest option. Go 3-ply when a 2-ply of the same size doesn't reach your post spacing. For example, at 8 ft tributary a 2-ply 2×10 spans 6'-11" but a 3-ply 2×10 reaches 8'-6" — the third ply buys ~1.5 ft. Three-ply beams should be through-bolted, not nailed.
Can I bolt a deck beam to the side of the posts?
Not as a prescriptive design. IRC R507.5.2 requires the beam to bear on top of the post through a post cap, or sit in a notched 6×6 post with a bearing shoulder. A beam hanging off the side of a 4×4 or 6×6 on bolts alone is one of the most common inspection rejections. Use a Simpson post cap — it's cheap, strong, and obvious to the inspector.
Do I need a 6×6 post or is 4×4 enough?
4×4 is acceptable only for short, lightly loaded posts (roughly under 6 ft). Anything taller, anything carrying a notched beam, and anything in snow country should be 6×6. A notched beam connection always requires a 6×6 — you cannot notch a 4×4 and keep enough bearing material.
How much can a deck beam cantilever past the end post?
IRC R507.5.1 allows the beam to overhang the end post by up to 1/4 of the allowable beam span. So an 8-ft beam span permits a 2-ft overhang, a 12-ft span permits 3 ft. This is separate from the joist cantilever limit (1/4 of joist back-span, R507.6.1) — a deck can use both at once.
Does snow load change the beam size?
Yes — significantly. The standard table is 40 psf. In snow regions the design load rises to 50–70 psf and beam spans shrink roughly 10–25%. A beam that passes at 40 psf in a mild climate can fail at 60 psf in the Northeast. Always size beams to your local design load, not the national default — DeckMath's beam-span and snow-load calculators handle this automatically.
What species is my pressure-treated lumber?
Check the grade stamp: 'SP' or 'SYP' = Southern Pine (strongest, standard east of the Mississippi), 'DF' or 'DFL' = Douglas Fir-Larch (West Coast), 'HF' = Hem-Fir, 'SPF' = Spruce-Pine-Fir (common in big-box stores in the Midwest/Northeast). SPF and Hem-Fir span about 10% less than Southern Pine, so don't design SPF to SP numbers.
What if no prescriptive beam passes my span?
If even a 3-ply 2×12 won't reach your post spacing at your tributary width, you have three options: (1) add a post to shorten the beam span — usually the cheapest fix; (2) reduce tributary width by adding an intermediate beam; or (3) move to an engineered beam (LVL, PSL, or steel) with a stamped design. The IRC prescriptive table tops out around a 14-ft span — beyond that, engineering is required.
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