Deck beam span calculator

A deck beam isn't sized by how far apart you want the posts. It's sized by the species of the lumber and by the joist span the beam carries — the tributary load. Pick the species and the joist span; get the smallest built-up beam that clears your post spacing, from AWC DCA 6 Table 3A (IRC R507.5).

01 How the table works

A deck beam span isn't a formula you run — it's a code lookup. AWC DCA 6 Table 3A, which the IRC reproduces as Table R507.5, fixes the maximum post-to-post span for each built-up beam size by two things: the species of the lumber and the joist span the beam carries. The deeper the beam, the farther it spans. The longer the joists landing on it, the shorter it spans, because more deck load bears on each foot of beam.

The joist span is the input readers get wrong. It is not the beam's own length, and it is not the post spacing you are hoping for — those are the answer, not the input. For a beam at the outer edge of a ledger-attached deck, the joist span is the distance the joists run from the house to the beam. For a beam down the middle of a deck with joists hung or lapped on both sides, it is the sum of the two joist spans the beam catches. The table already folds in that the beam carries half of each joist's load; you enter the full span and read the result.

This page carries the two species groups DCA 6 tabulates separately, the three practical built-up sizes — doubled 2×8, 2×10, and 2×12 — and the joist-span columns from 6 to 12 ft, the common single-beam range. A joist span between columns snaps up to the next one, the conservative direction. Two-ply beams, No. 2 grade:

The "2×10" on the price tag and the beam the table rates are not quite the same board. A doubled 2×10 is three inches thick and nine and a quarter deep — the span is rated on that dressed section, not on the round numbers in its name.

02 Worked example

Take a 12 ft × 16 ft (3.66 m × 4.88 m) ledger-attached deck. The joists run the 12 ft direction, from the house ledger to one beam along the outer 16 ft edge — so the beam carries a 12 ft (3.66 m) joist span. You want posts at 8 ft (2.44 m) on-center, which puts three posts along the 16 ft run and asks the beam to span 8 ft between them.

Read the 12 ft column in Southern Pine. A doubled 2×8 spans 5 ft 11 in (1.80 m) — short. A doubled 2×10 spans 7 ft 1 in (2.16 m) — still short of 8 ft. A doubled 2×12 spans 8 ft 4 in (2.54 m), so that's the smallest beam that carries posts at 8 ft. Set the widget to Southern Pine, 12 ft joists, 8 ft posts and it lands on the (2) 2×12.

Now frame the same deck in spruce-pine-fir, which is what many yards stock. At a 12 ft joist span, a doubled 2×12 in the DF/HF/SPF group spans only 6 ft 7 in (2.01 m) — it won't make 8 ft at all. The species change costs you a post: you drop the post spacing to about 6 ft 6 in and add a fourth post along the run. Same deck, same beam depth, one more footing to dig — because the lumber is weaker.

03 When this calculator is wrong

This is a scoped slice of a larger code table, and the first thing to get right is the input. The joist span is the full joist run bearing on the beam, not the tributary half and not the post spacing. Enter half the span and the beam reads a column too short — you'll think a (2) 2×10 carries a load that really needs a (2) 2×12. For a center beam with joists on both sides, add the two spans.

Where the numbers here stop applying:

• Species is the input most charts bury. A beam chart that prints one span per size is almost always Southern Pine, which spans about 25% farther than the DF/HF/SPF group. A doubled 2×10 goes from 10 ft 1 in in Southern Pine to 8 ft 1 in in SPF at a 6 ft joist span. Size off the Southern Pine row and frame with SPF and the beam is overspanned. Check the stamp on the lumber, not the chart's default.
• These are 2-ply spans only. Three-ply built-up beams and solid 4× timbers span farther and are separate rows of the table. For a post spacing a doubled 2×12 won't make at your joist span, a 3-ply 2×12 or an engineered beam (LVL, PSL rated for exterior use) is the next step — read the full table or a manufacturer's chart.
• Joist spans past 12 ft are off this page. The table runs to 18 ft; the common single-beam range stops around 12. A longer joist span needs the extended columns, and at that point a mid-span beam or a dropped-and-cantilevered layout usually makes more sense than one very deep beam.
• The 2021 IRC lets you span farther with the effective-span method. IRC Table R507.5(5) applies a joist-span factor (0.66 for a joist with no cantilever) that moves a beam to a shorter column, so a direct joist-span lookup like this one is the conservative side. If an inspector accepts the effective-span method, your beam may legally span more than the number here. This page never over-states the allowable span.
• Wet-service, No. 2, and local code all move the answer. These spans carry the wet-service reduction because a deck beam is exposed — which is why you can't borrow an interior dry-floor girder table for a deck. Higher grades span farther; a center-bearing or snow condition is different; and the span your jurisdiction's adopted code and building official sign off on is the one that passes, not a generic chart.

04 What to do with the result

Size the beam to the species you're actually buying, then build the real section. A "2×10" is 1.5 × 9.25 in dressed, not 2 × 10 — the single biggest source of estimating error in DIY framing, and the reason the beam rates on 9¼ in of depth, not ten. A doubled beam is two of those plies, so 3 in thick. Nail or through-bolt the plies together per the connector schedule, and where you stock the beam down and hang joists off its face, the beam depth has to equal or exceed the joist depth so the hangers land fully.

Set posts at or inside the span the result gives, not exactly on it — the number is a maximum, not a target. If the smallest beam that clears forces more posts than you want, the fix is usually a deeper beam or a stronger species, not stretching the span. For any case this page doesn't cover — a 3-ply beam, joists past 12 ft, a cantilever, or the effective-span method — pull the exact row from your adopted code table or have the beam confirmed by the building official or an engineer before you set the posts.

05 Common questions

How far can a double 2×10 deck beam span?

It depends on the species and the joist span it carries. Southern Pine, No. 2, 2-ply: 10 ft 1 in at a 6 ft joist span down to 7 ft 1 in at 12 ft. In the DF-L/Hem-Fir/SPF group it's 8 ft 1 in at 6 ft down to 5 ft 8 in at 12 ft (DCA 6 Table 3A / IRC R507.5). There's no single number — the joist span sets it.

Is the beam sized by post spacing or joist span?

Joist span is the input; post spacing is the answer. You tell the table the species and the joist span the beam carries, and it tells you the maximum distance allowed between posts. Entering the post spacing you want as if it were the joist span is the common mistake — it's backwards.

What joist span do I use for a beam in the middle of the deck?

Add the joist spans on both sides. A center beam with joists running 7 ft on one side and 7 ft on the other carries a 14 ft joist span, not 7 — because joists bear on it from both directions. An edge beam on a ledger-attached deck carries just the one ledger-to-beam span.

Does the species really change the beam size?

Yes, by roughly 25%. A doubled 2×10 spans 10 ft 1 in in Southern Pine but 8 ft 1 in in spruce-pine-fir at the same 6 ft joist span. Most lumber yards stock SPF, so a chart that assumes Southern Pine over-states the span for the lumber on the rack. Read the grade stamp.

Why is my deck beam span shorter than an interior floor beam of the same size?

Because a deck is exposed to weather, these spans carry the wet-service reduction. The same doubled 2×12 rated for a dry interior floor girder spans farther than it does as an uncovered deck beam. That's why you size a deck beam off the deck table (R507.5), not the interior girder table.