Deck beam span chart
Enter the joist span the beam carries and the length of the beam. You get the maximum span for every beam size from the American Wood Council's deck guide, and the posts each one needs.
Inputs
Result
| Beam | Max span | Posts |
|---|
Deck beam span chart
Maximum beam span (LB) for each joist span the beam carries, from DCA 6 Table 3A (dimension lumber) or 3B (glulam), for the species selected above.
| Beam | Joists ≤ 6 ft | Joists ≤ 8 ft | Joists ≤ 10 ft | Joists ≤ 12 ft | Joists ≤ 14 ft | Joists ≤ 16 ft | Joists ≤ 18 ft |
|---|
Values are from the American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6-15, based on the 2015 IRC). They assume a 40 psf live load and 10 psf dead load, No. 2 grade lumber and wet service. Hot tubs, snow loads above 40 psf, roofs on the deck or a local code amendment all change the answer.
Reading the table
A beam's allowed span depends on how much deck it holds up, which is set by the span of the joists resting on it. Find the column for the joist span (rounding up to the next column) and read the beam span for each size. Beam span is post centre to post centre.
The beam may run past its end posts by up to a quarter of its span, which often saves a post. The calculator counts posts both ways: with posts at the very ends, and with that overhang used.
Building the beam
- Two or three 2x plies are nailed or screwed together as DCA 6 Figure 4 shows, with joints over posts.
- Southern Pine beams in the guide are built-up 2-ply and 3-ply only. For the other species, a solid 3x or 4x member of the listed size can be used too.
- All 3-ply beams connect to their posts with a post cap. Beams sit on top of notched 6x6 posts or in a post cap; bolting a beam to the side of a post is prohibited.
- If the joists hang from the side of the beam on hangers, the beam must be at least as deep as the joists.
Glulam beams not treated with an oil-borne preservative get 89% of the table span (Table 3B, note 1). The calculator applies that unless you choose oil-borne.
Questions
How far can a double 2x10 deck beam span?
With 12 ft joists, 7 ft 1 in in Southern Pine and 5 ft 8 in in Douglas Fir-Larch, Hem-Fir, SPF, Redwood or Western Cedar, by DCA 6 Table 3A.
How far can a double 2x12 deck beam span?
With 12 ft joists, 8 ft 4 in in Southern Pine and 6 ft 7 in in the other species. With 8 ft joists, 10 ft 4 in in Southern Pine.
How far can a triple 2x12 deck beam span?
With 12 ft joists, 10 ft 6 in in Southern Pine and 9 ft 7 in in the other species. With 16 ft joists, 9 ft 1 in and 8 ft 3 in.
How many posts does a deck beam need?
Divide the beam's length, less the allowed overhang at each end, by its maximum span, round up, and add one. A 20 ft beam that may span 8 ft 4 in needs 3 posts with quarter-span overhangs, or 4 with posts at the very ends.
Can a deck beam overhang the post?
Yes, by up to a quarter of the beam's allowed span, under DCA 6 Figure 3.
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