Deck joist span calculator

Deck joist span by size

A 2x8 deck joist spans about 11 to 12 ft at 16 in centres, a 2x10 about 13-1/2 to 14 ft and a 2x12 about 16 ft, depending on species. These are the allowable spans in AWC's deck guide, DCA 6, and IRC Table R507.6, for No. 2 treated lumber at 40 psf live and 10 psf dead load^[1][2]:

• Southern pine 2x6, 2x8, 2x10, 2x12 at 16 in: 9 ft 0 in, 11 ft 10 in, 14 ft 0 in, 16 ft 6 in.
• Douglas fir-larch, hem-fir, SPF at 16 in: 8 ft 4 in, 11 ft 1 in, 13 ft 7 in, 15 ft 9 in.
• Spacing: joists at 12 in span 10 to 15% farther than at 16 in, and joists at 24 in about 15 to 20% less; the tool's chart shows all three.

Span is measured from the face of the ledger to the face of the beam and does not include the overhang^[1].

DCA 6 deck joist spans and this calculator

The calculator reproduces DCA 6 Table 2 and the IRC R507.6 snow-load rows exactly for southern pine and the DF-L, hem-fir and SPF group:

The grouped row is the shortest span of its three species. Hem-fir governs almost every size because it has the lowest design values; Douglas fir-larch governs 2x6 joists at 16 and 24 in, where bending controls. The redwood, cedar and ponderosa pine group is not built in.

Why deck joists span less than floor joists

A deck joist spans less than the same floor joist because it is outdoors and treated: a hem-fir 2x10 at 16 in spans 15 ft 2 in indoors but 13 ft 7 in on a deck. Two adjustments cause it^[5]:

• Wet service: lumber that stays above 19% moisture loses stiffness (E × 0.9) and some strength (Fb × 0.85 when Fb × CF is above 1,150 psi; Fv × 0.97).
• Incising: the slits cut into Douglas fir, hem-fir and SPF so preservative can enter cost 20% of bending and shear strength and 5% of stiffness. DCA 6 assumes incising for these three species and not for southern pine^[1].

Indoor joists are on the joist span calculator; the deck frame itself, with joist count, hangers and blocking, is on the deck framing calculator.

Deck joist overhang

A deck joist may overhang its beam by no more than a quarter of its span, and DCA 6 lowers that limit for smaller joists so the tip stays stiff under a 220 lb point load at the edge^[1]. The calculator's overhang checks that 220 lb point load plus the full deck load on the overhang, with tip deflection limited to the overhang ÷ 180. That is a little shorter than DCA 6's figures:

Snow loads over 40 psf

Where the ground snow load is over 40 psf, DCA 6 does not apply and the 2021 and 2024 IRC Table R507.6 rows for 50, 60 and 70 psf govern^[2][3]. Those rows use the full ground snow load with a 1.15 load-duration factor; a DF-L, hem-fir or SPF 2x10 at 16 in drops to 13 ft 3 in at 50 psf and 11 ft 6 in at 70 psf. Choose the snow load in the tool above.

Rules that come with the span

• 18 ft limit. DCA 6 caps joist length at 18 ft so the footings stay within its tables, which is why long 2x12 spans stop there.
• 2x6 joists. The ledger must be at least 2x8, and joists that carry guard posts must be 2x8 or larger^[1].
• Decking. The joist spacing also has to suit the deck boards; see the deck calculator and the deck board calculator.
• Beam and posts. The joist span sets the beam size and post spacing; size them with deck post spacing.
• No hot tubs. DCA 6 decks are not designed for hot tubs or other heavy concentrated loads.