Decking
A deck beam has no span of its own. What it can carry depends on how far the joists landing on it reach, because that is what sets the width of floor pressing down on it. So this table is read differently from the joist table: pick the row for your assembly, then the column for the joist span — and the columns are upper bounds, which is where most people get it wrong.
No. 2 grade, 40 psf live plus 10 psf dead, wet service, L/360 on the beam. Columns are the joist span the beam carries. The span you read is the distance between post centres — so this table is also your post spacing.
| Beam | 6′ joist | 8′ joist | 10′ joist | 12′ joist | 14′ joist | 16′ joist | 18′ joist |
|---|---|---|---|---|---|---|---|
| 2-2x6 | 6 ft 8 in | 5 ft 8 in | 5 ft 1 in | 4 ft 7 in | 4 ft 3 in | 4 ft | 3 ft 9 in |
| 2-2x8 | 8 ft 6 in | 7 ft 4 in | 6 ft 6 in | 5 ft 11 in | 5 ft 6 in | 5 ft 1 in | 4 ft 9 in |
| 2-2x10 | 10 ft 1 in | 8 ft 9 in | 7 ft 9 in | 7 ft 1 in | 6 ft 6 in | 6 ft 1 in | 5 ft 9 in |
| 2-2x12 | 11 ft 11 in | 10 ft 4 in | 9 ft 2 in | 8 ft 4 in | 7 ft 9 in | 7 ft 3 in | 6 ft 9 in |
| 3-2x6 | 7 ft 11 in | 7 ft 2 in | 6 ft 5 in | 5 ft 10 in | 5 ft 5 in | 5 ft | 4 ft 9 in |
| 3-2x8 | 10 ft 7 in | 9 ft 3 in | 8 ft 3 in | 7 ft 6 in | 6 ft 11 in | 6 ft 5 in | 6 ft 1 in |
| 3-2x10 | 12 ft 9 in | 11 ft | 9 ft 9 in | 8 ft 9 in | 8 ft 3 in | 7 ft 8 in | 7 ft 3 in |
| 3-2x12 | 15 ft | 13 ft | 11 ft 7 in | 10 ft 6 in | 9 ft 9 in | 9 ft 1 in | 8 ft 7 in |
Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir, Redwood, Western Cedars, Ponderosa Pine, Red Pine
| Beam | 6′ joist | 8′ joist | 10′ joist | 12′ joist | 14′ joist | 16′ joist | 18′ joist |
|---|---|---|---|---|---|---|---|
| 3x6 or 2-2x6 | 5 ft 2 in | 4 ft 5 in | 3 ft 11 in | 3 ft 7 in | 3 ft 3 in | 2 ft 10 in | 2 ft 6 in |
| 3x8 or 2-2x8 | 6 ft 7 in | 5 ft 8 in | 5 ft 1 in | 4 ft 7 in | 4 ft 3 in | 3 ft 10 in | 3 ft 5 in |
| 3x10 or 2-2x10 | 8 ft 1 in | 7 ft | 6 ft 3 in | 5 ft 8 in | 5 ft 3 in | 4 ft 10 in | 4 ft 5 in |
| 3x12 or 2-2x12 | 9 ft 5 in | 8 ft 2 in | 7 ft 3 in | 6 ft 7 in | 6 ft 1 in | 5 ft 8 in | 5 ft 4 in |
| 4x6 | 6 ft 2 in | 5 ft 3 in | 4 ft 8 in | 4 ft 3 in | 3 ft 11 in | 3 ft 8 in | 3 ft 5 in |
| 4x8 | 8 ft 2 in | 7 ft | 6 ft 3 in | 5 ft 8 in | 5 ft 3 in | 4 ft 11 in | 4 ft 7 in |
| 4x10 | 9 ft 8 in | 8 ft 4 in | 7 ft 5 in | 6 ft 9 in | 6 ft 3 in | 5 ft 10 in | 5 ft 5 in |
| 4x12 | 11 ft 2 in | 9 ft 8 in | 8 ft 7 in | 7 ft 10 in | 7 ft 3 in | 6 ft 9 in | 6 ft 4 in |
| 3-2x6 | 7 ft 1 in | 6 ft 5 in | 5 ft 9 in | 5 ft 3 in | 4 ft 10 in | 4 ft 6 in | 4 ft 3 in |
| 3-2x8 | 9 ft 5 in | 8 ft 3 in | 7 ft 4 in | 6 ft 8 in | 6 ft 2 in | 5 ft 9 in | 5 ft 5 in |
| 3-2x10 | 11 ft 9 in | 10 ft 2 in | 9 ft 1 in | 8 ft 3 in | 7 ft 7 in | 7 ft 1 in | 6 ft 8 in |
| 3-2x12 | 13 ft 8 in | 11 ft 10 in | 10 ft 6 in | 9 ft 7 in | 8 ft 10 in | 8 ft 3 in | 7 ft 10 in |
One structural quirk of the source worth flagging: DCA 6 uses three species groups for joists but only two for beams. Everything that is not Southern Pine — Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir, redwood, the western cedars, ponderosa and red pine — shares a single set of beam values, even though those species are split into two groups one table earlier. Southern Pine runs 8–12% further than that merged group on the same assembly.
DCA 6 heads these columns joist spans less than or equal to. That single phrase is the difference between a beam that passes and one that does not, because real joist spans rarely land on 6, 8, 10, 12, 14, 16 or 18 ft exactly. A 7 ft joist span is read in the 8 ft column — you round up to the next published column, never down to the nearest one.
Rounding down feels like the smaller error. It is not, because you are then claiming post spacing the table never gave you:
| Actual joist span | Correct | Rounded down | False gain |
|---|---|---|---|
| 7 ft | 8 ft 9 in (8′ col) | 10 ft 1 in (6′ col) | +1 ft 4 in |
| 9 ft | 7 ft 9 in (10′ col) | 8 ft 9 in (8′ col) | +1 ft |
| 11 ft | 7 ft 1 in (12′ col) | 7 ft 9 in (10′ col) | +8 in |
| 13 ft | 6 ft 6 in (14′ col) | 7 ft 1 in (12′ col) | +7 in |
Worked on a Southern Pine 2-2x10, the assembly most likely to be sitting under a mid-size deck. The error is largest at the low end — 1 ft 4 in at a 7 ft joist span — because the columns are closest together there.
Adding a third member to a built-up beam is 50% more lumber and 50% more nailing. Across every depth and every joist span in the table it buys almost exactly the same thing:
| Depth | 2-ply | 3-ply | Mean gain |
|---|---|---|---|
| 2x6 | 6 ft 8 in | 7 ft 11 in | 25.4% |
| 2x8 | 8 ft 6 in | 10 ft 7 in | 26.3% |
| 2x10 | 10 ft 1 in | 12 ft 9 in | 25.8% |
| 2x12 | 11 ft 11 in | 15 ft | 26.0% |
Two-ply figures are at a 6 ft joist span; the mean is across all seven columns. The consistency is the useful part — a third ply is worth about a quarter more span whatever you are building with, so you can make that call at the lumberyard without re-reading the table.
A common assumption on site is that two 2x members nailed together are at least as good as one solid timber of the same nominal size. The table says otherwise, and by a wide margin:
| Built up | Solid | Solid gains |
|---|---|---|
| 3x6 or 2-2x6 — 5 ft 2 in | 4x6 — 6 ft 2 in | +1 ft (19%) |
| 3x8 or 2-2x8 — 6 ft 7 in | 4x8 — 8 ft 2 in | +1 ft 7 in (24%) |
| 3x10 or 2-2x10 — 8 ft 1 in | 4x10 — 9 ft 8 in | +1 ft 7 in (20%) |
| 3x12 or 2-2x12 — 9 ft 5 in | 4x12 — 11 ft 2 in | +1 ft 9 in (19%) |
The reason is thickness, not magic. A 4x is a full 3½ in through; two 2x members give you 3 in, and the nails between them do not make the pair behave as one solid section. DCA 6 makes the same point silently in its row labels — it groups a solid 3x with the 2-ply built-up version and gives them one shared row, then gives the 4x its own. If you are buying, a solid 4x8 does the work of a beam a size up.
A deck has two separate cantilever allowances and they are easy to conflate. Joists may run past the beam, and the beam may run past its end post by up to 25% of the beam span — so a beam spanning 10 ft between posts can carry 2 ft 6 in beyond the last one. They stack, which is how a deck corner ends up further from its nearest post than anyone intended.
Note the asymmetry in how the two are written. The beam overhang is a flat proportion of the span. The joist overhang is not — DCA 6 publishes a separate deflection-controlled table for it, and that table is the smaller limit on 31 of its 36 cells. If you are cantilevering joists as well as the beam, the quarter-span rule is the wrong tool for the joist half; the numbers are in the deck joist span chart.
Between 2 ft 6 in and 15 ft, depending on the assembly and — this is the part most charts bury — on how far the joists it carries span. A beam is not rated in isolation. Three 2x12s in Southern Pine reach 15 ft when the joists landing on them span 6 ft, and only 8 ft 7 in when those joists span 18 ft. Same beam, 6 ft 5 in of difference, because the joist span sets how much floor the beam is carrying.
For posts 8 ft apart in Southern Pine, a 2-2x12 — its published span is 9 ft 2 in, leaving 1 ft 2 in of margin. Drop to a 2-2x10 and you get 7 ft 9 in, which is short of 8 ft, so the posts would have to come closer together. The two knobs are always the same: a deeper or thicker beam, or more posts.
The next one up, always. DCA 6 heads the columns "joist spans less than or equal to", so a 7 ft joist span is read in the 8 ft column, not the 6 ft one. Rounding the wrong way is the most common error on this table and it is not a rounding-error-sized mistake — on a 2-2x10 it hands you 1 ft 4 in of post spacing the code does not allow.
Not according to this table. At a 6 ft joist span, a solid 4x8 spans 8 ft 2 in where two 2x8s nailed together span 6 ft 7 in — 24% further. The same gap holds across every depth, and the reason is thickness: a 4x is a full 3½ in, while two 2x members give you 3 in. DCA 6 groups the solid 3x with the 2-ply, not with the 4x, which tells you the same thing.
Deeper, if you have the headroom — but the honest answer is that three of the shallower board usually beats two of the deeper one. Three 2x10s outspan two 2x12s at every published joist span, by 10 in at a 6 ft joist span. And a third ply is worth a remarkably steady 25–26% of extra span for 50% more lumber, whichever depth you are working in.
Yes — DCA 6 allows the beam to run past the post face by up to 25% of the beam span, so a beam spanning 10 ft between posts can cantilever 2 ft 6 in beyond the last one. That is a separate allowance from the joist overhang past the beam, and the two stack: joists can run past the beam and the beam can run past the post. Do not confuse the two limits, and note this one is a straight proportion rather than the deflection-controlled table the joists get.
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AWC DCA 6-2015, Prescriptive Residential Wood Deck Construction Guide, BEAM SIZE & ASSEMBLY REQUIREMENTS: deck beams may extend past the post face up to LB/4 · verified 2026-08-31
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