Deck Joist and Beam Span Tables, by Lumber Size

The residential code sizes a wood deck’s joists and beams with tables: how far each size and species may span, how far a joist may cantilever past its beam, and how far apart the joists may sit under the decking, measured on center: from the center of one joist to the center of the next. The span tables are for a live load of 40 psf (pounds per square foot), the code’s minimum for a deck, on top of 10 psf for the deck’s own weight. The code has a deck designed for that live load or the local ground snow load, whichever is greater. Austin adopts the 2024 International Residential Code without changing its deck section or its deck load, and its ground snow load is 5 psf, so the 40 psf tables govern there. Los Angeles adopts the California Residential Code, which carries the same deck section and tables with no California amendment in them, but California raises a deck’s live load to 60 psf, and the code’s deck tables have no row for a 60 psf live load; their 60 psf rows are for ground snow, a different load. For a Los Angeles deck, ask the city how it wants the joists, beams, posts and ledger sized before relying on these tables; its code also allows an engineered design, under the Los Angeles Building Code, for any building or structure the residential code covers. Sawn lumber for the joists, beams and posts is No. 2 grade or better, as the code requires.

Read across for the spacing: a Southern pine 2×10 at 16 inches on center may span up to 14 feet, and a Douglas fir-larch 2×8 at the same spacing up to 11 feet 1 inch. Douglas fir-larch, hem-fir and spruce-pine-fir (SPF) share one set of figures, and redwood, western cedars, ponderosa pine and red pine share another. Spans are in feet (′) and inches (″).

How far can a 2×6 deck joist span?

A 2×6 is the smallest joist in the code’s deck table. At 12 inches on center it may span up to 9 feet 11 inches in Southern pine; at 16 inches, 9 feet; at 24 inches, 7 feet 7 inches. The Douglas fir-larch group and the redwood group span less at each spacing. Joists at 24 inches on center also need decking rated for that spacing: 1¼-inch wood boards laid across the joists allow at most 16 inches, so a 24-inch layout takes 2-inch boards or a composite board labeled for it, as the next section explains.

How far apart should deck joists be?

The code caps joist spacing by the decking (Table R507.7). Under 1¼-inch (5/4) wood deck boards laid straight across the joists, perpendicular to them, joists go no more than 16 inches on center where each board crosses three or more joists and 12 inches where a board rests on only two; under boards laid diagonally, at up to 45 degrees, the limits are 12 and 8 inches. Two-inch wood boards allow 24 inches on center, the widest in the code’s wood decking table, except 18 inches where diagonal boards rest on only two joists. A span published by an accredited lumber grading or inspection agency may stand in for the 1¼-inch figures. The table does not cover stair treads.

Plastic composite deck boards, or their packaging, carry a label with their allowable load and maximum span, tested to ASTM D7032, and the joists under them go no farther apart than that span. Each wood board is fastened to every joist it crosses with at least two 8d deformed-shank nails or two No. 8 wood screws, or an approved decking or fastener system is installed to its maker’s requirements.

How far can a deck joist cantilever?

A joist may run past its beam as a cantilever, and how far depends on its back span, the length of joist behind the beam. The back span can be no longer than the joist may span at its spacing. A Southern pine 2×10 at 16 inches on center may span up to 14 feet; with 8 feet behind the beam it may cantilever up to 2 feet, and with 12 feet, up to 3 feet. At 24 inches on center the same joist may span only 11 feet 5 inches, so a 12-foot back span is too long for it at that spacing. A 2×6 may cantilever no more than 1 foot 6 inches in Southern pine, 1 foot 6 inches in the Douglas fir-larch group and 1 foot 4 inches in the redwood group, and not at all with a back span of 10 feet or more. NP in the tables below marks a back span the code allows no cantilever for. The code allows interpolating between two back spans, but not extending the table past the 4- to 18-foot back spans it lists.

How far can a deck beam span between posts?

A beam carries the joists to the posts, and the longer the joists it carries, the shorter it may span. The tables below give the code’s maximum beam spans for joists that do not cantilever past the beam, and they are for a beam carrying a single span of joists; a beam with joists spanning to it from both sides is outside them, so ask the city how to size one. A Southern pine beam of two 2×10s carrying joists that span 12 feet may span up to 9 feet between supports, and one of three 2×10s up to 11 feet 2 inches. Where the joists’ span falls between two columns, the beam span may be worked out in proportion between them; reading the next longer joist span’s column is a shortcut that gives a shorter beam span. The code allows no reading beyond its table’s 6- to 18-foot joist spans. Its table also covers single, double and triple 2×6 beams, 6- and 18-foot joist spans and joists that cantilever past the beam, which shorten the beam’s span; those are left out here.

A double or triple beam is two or three boards, or plies, fastened together with at least two rows of 10d nails at 16 inches on center along each edge, and each ply runs unbroken from support to support, and across any support the beam cantilevers past. A beam may cantilever past a support up to a quarter of the actual span beside it. A flush beam, one with the joists framing into its side rather than resting on top, is at least as deep as the joists. Each end of a beam has at least 1½ inches of bearing on wood or metal, or 3 inches on concrete or masonry, across the full width of the beam.

What do the span tables assume?

The tables assume No. 2 grade lumber or better, with the wet service factor built in: the strength reduction for wood that stays damp outdoors. They assume a dead load of 10 psf and a live load of 40 psf, with no snow at the same time. And they build in stiffness: members are sized so a span bends under load no more than 1/360 of its length, and a cantilever no more than 1/180 of its length; for a joist’s cantilever, that is with a 220-pound load at its tip. For conditions this section of the code does not prescribe, such as another material or a heavier load, it sends the design to its general design section, R301. Its tables for 50, 60 and 70 psf ground snow loads, Tables R507.5(2) to R507.5(4) and the snow rows of Table R507.6, are a separate set, not given here.

How are deck posts and footings sized?

The same section of the code sizes posts and footings. For a single-level deck, a wood post’s maximum height depends on the deck’s load, the post’s size and species, and the deck area it carries (Table R507.4), and a footing’s minimum size depends on that area and the soil’s allowable bearing pressure (Table R507.3.1). The deck footing and post height guide gives both tables’ 40 psf rows and how deep the footings go. The model code waives footings for a free-standing deck in two cases only: when its joists rest directly on the ground over their entire length, or when all of these hold: its joists bear directly on precast concrete pier blocks at grade, meaning at ground level, without support by beams or posts; the deck is 200 square feet or less; and its walking surface is no more than 20 inches above grade at any point within 36 inches of its edge, measured horizontally.

How is a deck attached to the house?

A deck attached to the house is positively anchored to it and designed for both vertical and lateral (sideways) loads, never with toenails or nails that can pull out; where the framing cantilevers, the connection resists the uplift of the code’s full deck live load on the cantilevered part, 40 psf under the model code (R507.8). Where that connection cannot be verified at inspection, the deck must be self-supporting. The code carries the deck’s weight to the house’s band joist, the board that rims the house’s floor framing, through a ledger: at least a nominal 2×8, No. 2 grade or better, in pressure-treated Southern pine, incised pressure-treated hem-fir (slit so the preservative soaks in) or a naturally durable, decay-resistant wood. The ledger carries no concentrated load from a beam or girder and is never supported on stone or masonry veneer (R507.9.1.1). Table R507.9.1.3(1) spaces the ½-inch lag screws or bolts that fasten it by the deck’s load and joist span. Lateral loads go to the ground or to a structure that carries them there. Where the code’s hold-down details are used, the hold-downs, metal tie-down connectors, go at no fewer than two points, within 24 inches of each end of the deck and each rated for at least 1,500 pounds, or at no fewer than four, each rated for at least 750 pounds, depending on the detail (R507.9.2).

Whether the deck needs a permit at all is a separate question: the deck permit brief gives Austin’s and Los Angeles’s lines, and the deck railing and stair guide covers guards and stairs.