How Far Can a Deck Joist Span?

A deck joist's maximum span depends on three things: the joist size, how far apart the joists sit (the on-center spacing), and the species of lumber. The building code publishes these limits in IRC Table R507.6, and every plans examiner checks your drawings against it. Here is what those numbers actually look like.

Southern Pine joist spans (No. 2 grade)

• 2x6 — 9'11" at 12" o.c., 9'0" at 16", 7'7" at 24"
• 2x8 — 13'1" at 12" o.c., 11'10" at 16", 9'8" at 24"
• 2x10 — 16'2" at 12" o.c., 14'0" at 16", 11'5" at 24"
• 2x12 — 18'0" at 12" o.c., 16'6" at 16", 13'6" at 24"

Douglas Fir-Larch and Hem-Fir span a little less than Southern Pine; cedar, redwood and the lighter pines span less again. Our calculator carries all three species groups and picks the smallest joist that clears your deck's projection automatically.

Why spacing matters so much

Closer joists share the load, so a 2x8 Southern Pine joist goes from 13'1" at 12" spacing down to 9'8" at 24". If your deck is just past a joist's limit, tightening the spacing one step is often cheaper than jumping up a lumber size — and it stiffens the deck underfoot.

A note on cantilevers

Joists can overhang the beam to form a small cantilever, but the IRC limits that overhang to one quarter of the actual back-span (R507.6.1). A deck that spans 12 ft to the beam can cantilever up to 3 ft past it — handy for stairs or a clean fascia line.