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).

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. — Source: 2021 IRC Table R507.6 (40 psf live + 10 psf dead); AWC DCA 6 Table 2

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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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    <img src="https://remodelcalculators.com/api/figure-embed/deck-joist-span-spacing" alt="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." width="800" height="510" style="width:100%;height:auto;border:0" loading="lazy">
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