Deck Joist Span Chart

Maximum span for No. 2 lumber at a 40 psf live load (and ground snow load up to 40 psf). 16 in on center, the most common spacing, is in bold.

Basis: 10 psf dead load, No. 2 grade, wet service, deflection L/360; snow not added to live load; DF-L, hem-fir and SPF values include the incising factor. Source: 2024 IRC Table R507.6, identical in the 2021 IRC. At 40 psf the spans are the same as AWC DCA 6-15 Table 2. Snow drops a southern pine 2x10 at 16 in by 25 in between the 40 and 70 psf columns.

Measure the joist span between the faces of what holds the joists up. On a typical ledger deck 12 ft deep with the beam 11 ft out, the joists span just under 11 ft and overhang 1 ft. A southern pine 2x8 at 16 in covers it (11'-10"), may overhang up to 2'-4" at that span, and with posts 8 ft apart the beam can be a 2-2x10 (8'-0" allowed).

Before choosing joists, check how far apart the deck boards let them be. The IRC sets it for wood decking; diagonal boards cross more of the gap between joists, so they need joists closer together.

The bold figures are for boards that run over three or more joists, which is almost every board on a deck. For composite boards the IRC (R507.7, R507.2.2) defers to the span printed on the label; many brands list a closer spacing for diagonal or herringbone layouts, so check before you order joists. Stair treads have their own spacing — see the deck stair calculator→.

The grade stamp on each board names the species and grade. The tables assume No. 2 or better. Treated lumber is usually southern pine in the East and South, and hem-fir or Douglas fir in the West, where it is often incised (rows of small slits that help the preservative in).

Not sure which you have? Use the lowest-spanning group that could apply. The difference is a few inches, and the calculator lets you compare.

Search results mix three sources. They agree on joist spans at 40 psf; they differ on everything around them.

Which applies depends on the edition your state or city has adopted, and decks are one of the most amended parts of the code. Many building departments hand out a DCA 6-based deck guide. The calculator uses the IRC tables. Our deck permit table lists the adopted code in 50 cities.

Maximum overhang by back span (40 psf)

2024 and 2021 IRC Table R507.6; L/180 at the overhang with a 220 lb point load at its end. Values between back spans may be interpolated; NP means no overhang is permitted. The snow-load overhangs are in the calculator.

The beam carries half the joist span plus any overhang, between posts. The 2024 IRC tables size it from the joist span and the overhang together, by species and the number of 2x plies (fastened with two rows of 10d nails at 16 in). The beam may run past an end post by up to a quarter of its span, and it must be at least as deep as the joists if they hang on its side.

* 3 printed cells across the four beam tables break the table’s own order (a heavier column or smaller beam spanning further). The calculator uses the lower neighbouring value there: 40 psf redwood 2-2x10 printed 12-2, used 11'-9"; 50 psf southern pine 3-2x10 printed 8-11, used 8'-4"; 60 psf redwood 3-2x12 printed 8-11, used 8'-4". The 2021 edition prints a different beam table, so check which edition your building department uses.

Then the footings. Each post’s footing is sized by the deck area it carries, and its depth by the frost line and whether the deck is attached. The calculator’s “Size the footings” button opens the deck footing depth and size calculator with this layout filled in.

The ledger is where decks fail. It has to be at least a 2x8, fastened to the house band joist with the bolts or lags in the IRC’s ledger table (R507.9), flashed, and never attached to brick or stone veneer. If that connection can’t be made, build the deck free-standing.

• Hot tubs, roofs, privacy screens, heavy planters or anything else beyond the uniform deck load.
• Ground snow loads over 70 psf, joists over 18 ft, or multi-level and elevated decks with braced posts.
• Engineered lumber, steel framing and composite joists — use the manufacturer’s span tables.
• Whether an existing deck is sound. Rot, corroded hangers and loose ledgers aren’t in any span table.
• Local amendments, which override the model code.

Local rules vary. These are planning figures read from the published prescriptive tables, not an engineered design. Your building department approves the plan, and its adopted code and snow load are the ones that count.

Joist spans and overhangs: 2024 IRC Table R507.6, checked cell by cell against the 2021 IRC (identical). Beams: 2024 IRC Tables R507.5(1)–(4). Decking spacing: 2024 IRC Table R507.7 (same in 2021). All read on the International Code Council’s free online view on October 23, 2026. Cross-check and history: AWC’s Prescriptive Residential Wood Deck Construction Guide (DCA 6-15), the current edition, based on the 2015 IRC. We transcribed the numbers into our own tables and drew our own diagram; the code text and the guide remain the authority.

The calculator measures the joist span as the deck’s depth minus the overhang, which slightly overstates it. Beam sizes use the printed column whose joist span and overhang are both at least yours (it doesn’t interpolate).