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Deck Joist Span Calculator (IRC R507.6)

Find the maximum allowable span for deck joists by lumber species, size and spacing from the IRC deck joist span table, check your planned span, see the allowed cantilever, the smallest joist that works, and the number of joists for the deck width.

Lumber species

Joist size

Joist spacing

Planned joist span (ledger/beam to beam)

Deck width (length of ledger / beam)

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How it works

Span from IRC Table R507.6 (species × size × spacing); cantilever ≤ span ÷ 4; joist count = floor(width × 12 ÷ spacing) + 1

Deck joists are sized from prescriptive tables that already contain the engineering — bending strength, deflection limited to span/360 and the wet-service reduction for exterior lumber — so a builder only needs species, size and spacing. Southern pine is the strongest common species and spruce-pine-fir the weakest, a difference of about a foot of span in 2×10s; closing the spacing from 16 to 12 inches buys another foot or so, and cantilevering the joists up to a quarter of their back-span over the beam is a cheap way to gain deck depth without a longer span. The tables are for the standard 40 psf residential live load; heavy snow country, hot tubs and planters need an engineered design.

Worked example

A 2×8 southern pine joist at 16″ on centre may span 11′-10″, so a planned 12 ft span is 2 inches too long — go to 2×10 (14′-0″), tighten to 12″ o.c. (13′-1″), or move the beam in and cantilever. A 16 ft-wide deck at 16″ o.c. needs 13 joists plus the doubled outer members.

Frequently asked questions

Does the span include the cantilever?

No — span is measured between the ledger (or beam) and the beam; the overhang beyond the beam is counted separately and limited to one quarter of that span, also capped by the table.

Can I use pressure-treated "SPF" from the store?

Treated lumber in the eastern US is usually southern pine; in the west it is often hem-fir or Douglas fir. Read the grade stamp — species and grade (#2 or better) must match the table row you use.

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Span values follow the IRC 2021 deck provisions (R507) and AWC DCA 6 for #2 grade lumber at 40 psf live + 10 psf dead load, wet-service, simplified to the common cases. Local amendments, snow loads, hot tubs, cantilevers and other species/grades change the numbers — confirm with the local building department or an engineer.