How far can the deck beam span?

How far can a deck beam span?

It depends on the wood, the beam size and how much deck the beam carries. From 2021 IRC Table R507.5(1): a southern pine beam of two 2×10s carrying an effective joist span of 8 ft may span 9 ft 0 in between posts; at 12 ft it may span 7 ft 4 in.

What is the effective joist span?

The actual length of the joists the beam carries times a factor from Table R507.5(5), which depends on how far the joists cantilever past the beam. With no cantilever the factor is 0.66, so 12 ft joists count as 7.92 ft — and a southern pine 2-2×10 beam may span about 9 ft 0 in.

How does a joist cantilever change the beam span?

A longer cantilever puts more load on the beam, so the factor rises: C/J of 1/12 → 0.72, 1/10 → 0.80, 1/8 → 0.84, 1/6 → 0.90 and 1/4 → 1.00, where C is the cantilever and J the joist span. The calculator uses the next row up when your ratio falls between two.

How are built-up deck beams put together?

Section R507.5 has the plies fastened with two rows of 10d (3 × 0.128 inch) nails 16 inches apart along each edge. A beam may cantilever past its end post by up to a quarter of its span.

What loads does the table assume?

A 40 psf live load and 10 psf dead load, No. 2 lumber with the wet service factor. For ground snow loads of 50, 60 or 70 psf the IRC has separate Tables R507.5(2) to (4), and your local code or a permit reviewer has the final word.

How much does a joist cantilever reduce the beam span?

For 12 ft joists, adding a 2 ft cantilever (C/J of 1/6) raises the factor from 0.66 to 0.9, so the effective joist span goes from 7.92 ft to 10.8 ft. A southern pine beam of two 2×10s then spans 7 ft 8 in instead of 9 ft 0 in between posts. The cantilever loads the beam more, so the posts must be closer together.

Is a deck beam span the same as the post spacing?

Yes: the table gives the largest distance between the centers of the posts that support the beam, with no more than the allowed cantilever past the end post. The beam is only as strong as its splices and connections, so the beam plies should be joined over a post and the post-to-beam connection made according to the code.

How the table is read

The table is indexed by wood species, by the beam (number of plies and lumber size) and by the effective joist span, the span of the joists that bear on the beam after the cantilever factor is applied. Between listed joist spans the allowed span can be interpolated, and below 6 ft the 6 ft column is used; beyond 18 ft the table gives no value. The values apply to a deck whose joists bear on the top of the beam and which carries the standard residential loads.

Worked example: 12 ft joists with a 2 ft cantilever

With a 2 ft cantilever on a 12 ft joist, C/J is 1/6, and the factor is 0.9, giving an effective span of 12 × 0.9 = 10.8 ft. From the table for southern pine, a beam of two 2×10s may span 7 ft 8 in, three 2×10s 9 ft 8 in and two 2×12s 9 ft 1 in. For Douglas fir the two-2×10 beam spans 7 ft 5 in. Without the cantilever the effective span is 7.92 ft and the two-2×10 southern pine beam spans 9 ft 0 in.

Common mistakes

Using the joist length without applying the factor overstates the load on the beam, and using a joist length that includes the cantilever as the span when the table calls for the span between supports is the opposite error. Another is the species: the same lumber size in a weaker species has a shorter allowed span. A third is reading a span for one beam size and building another; two plies of 2×10 are not three. The tables assume No. 2 grade lumber in wet service and no hot tub or heavy planters. Always check the span against the code edition your building department enforces.

Sources: 2021 International Residential Code, Section R507.5 and Tables R507.5(1) and R507.5(5) (deck beam spans and joist-span factors for cantilevers), which follow the American Wood Council's DCA 6 Prescriptive Residential Wood Deck Construction Guide. Related: deck joist calculator, deck footing calculator, deck calculator and joist span calculator.