Decking Joist Span
Search engines mix “decking joist span” with “deck joist span.” They answer different questions. This page separates them and points you to the right table.
Joist span vs decking span
Joist span is how far a joist may run between supports (ledger/beam). That is what AWC DCA 6 Table 2—and our deck joist span tool—covers.
Decking span is how far the deck boards (2× lumber or span-rated decking) may run between joists. ALSC span-rated decking is typically rated for 16″ or 24″ joist spacing under its own policy, with a 70 psf uniform check and a 220 lb point-load check. Composite and PVC brands publish their own maximum joist spacing—often 12″ or 16″ o.c., and tighter on diagonals or stairs.
Practical workflow
- Pick decking first and note the maximum joist spacing the manufacturer allows.
- Set joist spacing to the lesser of that limit and what you want structurally.
- Size joists with the deck joist span calculator or the table so the clear span fits Table 2.
Common mix-ups
A 2x6 joist might “span” nearly 10 feet at 12″ o.c. in Southern Pine, but that does not mean a 5/4 deck board can float 10 feet. The board only spans from joist to joist. Likewise, switching to 24″ joist spacing to save lumber can void a composite decking warranty overnight.
Related pages: 16″ o.c. joist spacing, 2x8 deck joist span, assumptions.