How many deck joists, and how far can they span?

How many joists do I need for a deck?

Divide the width of the deck along the house by the joist spacing and add one for the end. A 16 ft wide deck with joists 16 inches apart needs 16 × 12 ÷ 16 = 12 spaces, so 13 joists — and 13 joist hangers at the ledger.

How far can a deck joist span?

Under the IRC (Table R507.6), southern pine 2 × 8s at 16 inches apart span up to 11 ft 10 in, 2 × 10s 14 ft 0 in and 2 × 12s 16 ft 6 in. Douglas fir, hem-fir and SPF span a little less, and redwood and cedar less again. The table in the result shows every size for your species and spacing.

Should deck joists be 12, 16 or 24 inches apart?

Sixteen inches is the usual spacing: the IRC (R507.7) allows 5/4 wood decking laid straight across joists 16 inches apart, but only 12 inches when it runs on a diagonal. 2 × 6 decking laid straight can span 24 inches. Composite boards have their own limits — check the maker's instructions.

How is the joist span measured?

From the face of the ledger on the house to the center of the beam, for an attached deck. Joists can also cantilever past the beam, but by less than the back span — see IRC R507.6 for the limits.

What loads are these spans for?

A 40 psf live load plus 10 psf dead load, the standard for residential decks, with No. 2 grade lumber in wet service. Hot tubs, snow loads above 40 psf and heavy planters need an engineered design.

What if my joist span is longer than the table allows?

Use larger joists, put them closer together or add a beam to shorten the span. For an 11 ft span at 16 inch spacing, a 2×8 of southern pine (11 ft 10 in) is enough; at 24 inches it is not (9 ft 8 in), and the smallest size that works is the 2×10. A joist must also be supported properly at both ends, so the allowed span is not the whole story.

Does the species change the answer?

Yes. The same 2×8 at 16 inches may span 11 ft 10 in in southern pine, 11 ft 1 in in Douglas fir, hem-fir or SPF and 10 ft 7 in in redwood or western cedar. An 11 ft span still works in the Douglas fir group.

What the calculator does

It counts the joists as the width of the deck divided by the spacing, rounded up, plus one for the end joist. Then it looks up the maximum span for your species, size and spacing in the IRC table and compares it with your joist span. The table values are for joists with no cantilever beyond the beam; cantilevers have their own limits. The ledger side of the deck needs a hanger for each joist, and the other side bears on the beam.

Worked example: a 16 ft wide deck with an 11 ft joist span

At 16 inch spacing a 16 ft wide deck needs 16 × 12 ÷ 16 = 12 spaces, so 13 joists. Southern pine 2×8s may span 11 ft 10 in, which covers 11 ft. At 12 inch spacing the count rises to 17 and the allowed span to 13 ft 1 in; at 24 inches the count falls to 9, but the 2×8 span drops to 9 ft 8 in, which is too short for 11 ft. For a 20 ft wide deck with a 12 ft span at 16 inches the table calls for a 2×10, and the count is 16 joists.

Common mistakes

Using the nominal size as the real dimension is not an issue here, but forgetting that the table is for No. 2 grade or better is: lower grades span less. Another is measuring the span to the wrong place; it is measured from the face of the ledger to the center of the beam. A third is forgetting the extra joists the code requires around stairs, a hot tub or heavy loads, none of which the table covers. The calculator also assumes the standard 40 psf live and 10 psf dead load, not snow loads above that or an engineered design.

Sources: 2021 International Residential Code, Table R507.6 (maximum deck joist spans) and Section R507.7 (decking), derived from the American Wood Council's DCA 6 Prescriptive Residential Wood Deck Construction Guide. Related: deck beam span calculator, deck calculator, joist span calculator and deck footing calculator.