Deck joist span vs. spacing — how far each joist size reaches at 16″ and 24″ on-center
Span and spacing are different measurements: spacing is center-to-center across the joists; span is how far each joist reaches from ledger to beam. A bigger joist — or tighter spacing — spans farther (IRC R507.6, Southern Pine #2).
What this diagram shows
A plan view distinguishes the two deck-joist measurements homeowners confuse: spacing is the center-to-center distance across the joists (commonly 16 inches on-center), while span is the distance each joist travels from the ledger at the house to the beam. A reference table gives the 2021 IRC R507.6 maximum span for Southern Pine #2 joists at a 40 psf live plus 10 psf dead load: a 2x6 spans 9 feet at 16 inches on-center and 7 feet 7 inches at 24; a 2x8 spans 11 feet 10 inches and 9 feet 8 inches; a 2x10 spans 14 feet and 11 feet 5 inches; and a 2x12 spans 16 feet 6 inches and 13 feet 6 inches. A bigger joist or tighter spacing reaches farther.
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