Deck joist span chart

How to read the chart

Find your species group, then read across the row for your joist size to the column for your spacing. The number is the longest clear span that joist may cross unsupported — face of ledger to face of beam, not the length of the board you buy.

The three tables below are the same table, split by species group because the species changes the answer by a foot or more. Grade matters too: every value assumes No. 2, the grade most yards stock for deck framing. If you are handed No. 3, the table does not apply. Cross-check your own numbers against the span calculator.

Southern pine

The strongest of the three groups and the usual stock for pressure-treated framing east of the Rockies. A 2×8 here reaches further than a 2×10 in cedar at the same spacing.

Douglas fir-larch, hem-fir, spruce-pine-fir

The western framing group, and what most big-box dimensional lumber is. Two to six inches shy of southern pine at every size; enough to move a beam.

Redwood, western cedars, ponderosa pine, red pine

Handsome, stable, and the weakest of the three in bending. Cedar earns its place as decking and as fence stock, not as the joist under a long span.

What the chart assumes

Five things, and each of them can be wrong on your deck. A 40 psf live load with 10 psf dead — a residential deck carrying people and furniture, not a hot tub. A deflection limit of L/Δ 360, which is a stiffness rule, not a strength one: a joist can pass on strength and still feel like a trampoline. No. 2 grade. Wet service, because a deck is outdoors. And no cantilever — the moment a joist runs past the beam, the allowable back-span changes.

When it does not apply

A hot tub, a masonry planter, a built-in bench with a crowd on it — every one is a point load the table never modelled, and every one wants a designer. So does a roof over the deck, a deck more than one storey up, and any span you were planning to eke out to the last inch of the table.

The decking can also govern before the joist does. Composite boards usually specify their own maximum joist spacing, and running them on a diagonal drops that spacing again — details in joist spacing and composite vs wood. And the span you pick decides the beam under it and the footing under that, because tributary area is just half your span in each direction.

Code reference throughout: IRC 2021 R507.6.