Deck beam span calculator and span chart (IRC 2021)

Between 4 ft 5 in and 10 ft 4 in, depending on species, load and effective joist span. At the common case, 40 psf and a 10 ft effective joist span, a southern pine double 2x10 spans 8 ft 0 in between posts, the fir group 7 ft 9 in and the redwood group 7 ft 5 in. Under 70 psf snow the southern pine figure drops to 6 ft 9 in. A single number quoted for a double 2x10 with no joist span and load beside it is not from Table R507.5.

Up to 12 ft 2 in in southern pine with 6 ft effective joists at 40 psf, and as little as 5 ft 4 in in the fir group with 18 ft joists under 70 psf snow. At a 10 ft effective joist span and 40 psf it spans 9 ft 5 in in southern pine, 8 ft 11 in in the fir group and 8 ft 7 in in the redwood group. It is the deepest two-ply beam in the table, so a flush deck needs 11 1/4 in. of framing depth; a triple 2x10 is shallower and spans almost as far.

A triple 2x12 holds the longest span in the whole table: 15 ft 3 in in southern pine with 6 ft effective joists at 40 psf. At 10 ft joists and 40 psf it spans 11 ft 10 in in southern pine, 11 ft 3 in in the fir group and 10 ft 9 in in the redwood group; with 18 ft joists under 70 psf snow the fir group figure is 7 ft 1 in. A 15 ft span means 16 ft boards, and a longer beam is spliced over a post with each ply bearing fully.

From 3 ft 5 in to 8 ft 9 in. The usual case, a low deck with 8 ft effective joists at 40 psf, gives 7 ft 7 in in southern pine, 7 ft 1 in in the fir group and 6 ft 10 in in the redwood group; at 10 ft joists those become 6 ft 9 in, 6 ft 4 in and 6 ft 1 in. That 8 ft is the effective span: 12 ft joists with no cantilever count as 7 ft 11 in and read the 8 ft column.

The effective joist span is the measured joist span multiplied by the factor in Table R507.5(5), and it is the number you read the beam table with. The factor is 0.66 with no cantilever and rises to 1.00 at a cantilever of one quarter of the span. The beam tables were built for the full-cantilever case; joists that stop at the beam deliver about two thirds of that load, so the code lets you enter the table at two thirds of the span. The calculator prints the factor, the effective span and the column it used so the step is visible.

Up, to the next printed ratio. Table R507.5(5) lists 0, 1/12, 1/10, 1/8, 1/6 and 1/4 and carries no interpolation note, so a ratio that falls between two rows takes the larger factor. A 2 ft cantilever on 12 ft joists is 0.167, exactly 1/6, factor 0.90; a 2 ft 6 in cantilever is 0.208, which rounds up to 1/4 and factor 1.00. Rounding down would enter the beam table with a shorter joist span than the deck really delivers, and the beam would come out one column too long.

Yes. R507.5 permits a beam to cantilever at each end up to one-fourth of the actual beam span, and footnote f says the same thing as the adjacent span divided by 4. With 8 ft from the end post to the next post, the beam may run 2 ft beyond the end post; with 10 ft, 2 ft 6 in. The span is measured center to center of the posts. Builders pull the posts in from the corners this way to shorten the span between them, which can drop the beam one size.

For a beam sitting on posts, yes: the span is the distance from the center of one post to the center of the next, as Figure R507.5 draws it. It is not the length of the beam. A 20 ft beam on three posts has two 10 ft spans and is read at 10 ft; on four posts it has three spans of about 6 ft 8 in. If the beam cantilevers past the end posts, the cantilever is not a span and is limited to one quarter of the span next to it. Enter the longest span between posts in the calculator.

Not as the way the beam is held up. R507.5.2 requires beam connections to posts that transfer vertical loads and resist horizontal displacement, and it points to Figures R507.5.1(1) and (2): the beam bears on top of the post through an approved post cap, or sits in a notch cut into the post so the post carries it directly. Bolts through the side of a post carry the beam in shear on two fasteners, and R507.10.2 adds that 4x4 guard posts may not be notched. Older decks were built with side-bolted beams; the current table assumes bearing.

Yes, in every cell. Southern pine out-spans Douglas fir-larch, hem-fir and spruce-pine-fir in 334 of 334 comparable cells, and those three species share one row in Table R507.5 with the incising factor included. They do not fall between southern pine and spruce-pine-fir; they are spruce-pine-fir's row. A southern pine double 2x10 at 10 ft joists and 40 psf spans 8 ft 0 in; the fir row spans 7 ft 9 in. The redwood, cedar, ponderosa and red pine row is separate and, because it is not incised, often lands between the two.

No. Table R507.5 prints 1-, 2- and 3-ply 2x6 to 2x12 beams only. Solid timbers and engineered beams are sized from other sources, covered in the beam span and size questions post.

The 50 psf table, R507.5(2). R507.1 requires a deck to be designed for the live load in R301.5, 40 psf, or the ground snow load in Table R301.2, whichever is greater, and footnote c states that snow is not assumed to act at the same time as live load, so the two are never added. A site with 45 psf ground snow also uses the 50 psf table, the next printed column up. Above 70 psf the tables stop and the deck is designed under R301. Your building department publishes the ground snow load for the site.

The code allows it; this page does not do it. Footnote a of every beam table reads "interpolation allowed, extrapolation is not allowed," so a plans examiner may accept a span worked out between the 10 ft and 12 ft columns for an 11 ft effective joist span. Older articles say interpolation is not permitted, which was the 2015 wording. The calculator and the charts here round the effective span up to the next column instead, which always gives the shorter, conservative beam span. Extrapolating past 18 ft joists or below 6 ft is never allowed.

Yes, with two conditions. Footnote e requires the beam depth to be equal to or greater than the joist depth for a flush connection, so 2x10 joists need at least a 2x10 beam. And the joist span for a flush beam is measured from the centerline of the hanger at the ledger to the centerline of the hanger at the beam, per Figure R507.6, with no cantilever possible. A dropped beam sits under the joists, the joists may cantilever past it, and the span runs from the ledger to the beam centerline. Same cell, different measurement.

No. The 2024 edition removed Table R507.5(5) and rebuilt the deck beam table so the joist cantilever is handled inside the table itself, with more columns. This page and this calculator are the 2021 IRC, which most jurisdictions enforce until they adopt 2024, and the 2021 factor method is the one in the four tables printed here. If your permit office has adopted the 2024 code, ask for its beam table; the spans differ and the factor step disappears.

Divide the beam length by the allowable span from the chart, round up to get the number of spans, and add one post. A 16 ft beam with an allowable span of 7 ft 9 in needs 16 divided by 7.75, rounded up to 3 spans, so 4 posts at 5 ft 4 in; a triple 2x10 at 9 ft 8 in needs 2 spans and 3 posts. Cantilevering the beam past the end posts shortens the spans between them. Size the posts with the deck post size calculator and the footings with the deck footing size calculator.

No. The beam tables have no spacing column; they are keyed to the effective joist span, which measures how much deck area feeds into the beam. Twelve joists at 16 in. deliver the same total load to the beam as eight joists at 24 in. over the same deck. Spacing decides the joist size in Table R507.6 and the decking span in Table R507.7, nothing more. The one thing that does move the beam is the joist cantilever, through the factor, and the joist span itself, so a deeper deck always means a shorter beam span.

Not from this table. Footnote b limits every cell to beams supporting a single span of joists, with or without a cantilever. A middle beam with joists running continuously over it from both sides picks up load from two spans and is outside Table R507.5, so it is designed under R301 or sized by an engineer. Two free-standing beams with the joists spanning between them, each carrying one span plus its own cantilever, are covered, and each beam is read with its own joist span and cantilever.

No. R507.9.1.1 says deck ledgers "shall not support concentrated loads from beams or girders." A ledger is a line of joist hangers bolted to the house band joist and its fasteners are sized in Table R507.9.1.3(1) for a spread joist load, not a point load. A beam that runs perpendicular to the house needs a post and footing at the house end, or the deck framing is turned so the joists, not the beam, meet the ledger. The same rule keeps beams off masonry veneer, since ledgers cannot bear on it either.

Deck width sets the beam length, not its size. Pick a post spacing, find the column for your effective joist span, and read the smallest beam that reaches that spacing. For a 20 ft southern pine beam with 10 ft joists at 40 psf: on three posts the spans are 10 ft and a triple 2x10 (10 ft 0 in) works; on four posts the spans are about 6 ft 8 in and a double 2x8 (6 ft 9 in) works. A 16 ft beam on three posts spans 8 ft; a double 2x10 (8 ft 0 in) covers that.