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Span table — all joist sizes
Maximum allowable spans per IRC R507.6 (No. 2 grade, wet service, L/360)
| Joist size | 12" OC | 16" OC | 24" OC | Status |
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Enter your deck span, decking material, and load requirements to get the right joist size, spacing, count, and lumber estimate — based on IRC span tables.
Maximum allowable spans per IRC R507.6 (No. 2 grade, wet service, L/360)
| Joist size | 12" OC | 16" OC | 24" OC | Status |
|---|
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The most common joist spacing for residential decks is 16 inches on center (OC). This works for most wood and composite decking materials. Some composite manufacturers require 12-inch OC spacing for diagonal installations, while 24-inch OC may be acceptable for thicker decking on shorter spans.
For a 16-foot span at 16 inches OC with standard 40 PSF residential loading, you would typically need 2x12 joists in SPF lumber, or 2x10 joists in Southern Pine. Always verify with local building codes, as species grade and load requirements affect the required size.
A 2x8 No. 2 grade deck joist can span up to about 11 feet 10 inches at 16 inches OC in Southern Pine, or about 11 feet 1 inch in SPF lumber, under standard 40 PSF residential loading per IRC Table R507.6. Spans decrease for wider joist spacing or heavier loads.
Yes, but hot tub areas require engineering for 100 PSF loads — more than double the standard 40 PSF residential deck load. This typically means closer joist spacing (12 inches OC), larger joists (2x10 or 2x12), additional beams, and reinforced footings. Always consult a structural engineer for hot tub installations.
The number of joists depends on your deck width and joist spacing. Divide the deck width (in inches) by the joist spacing (12, 16, or 24 inches), then add 1 for the starter joist. For a 16-foot wide deck at 16 inches OC, you need (192 / 16) + 1 = 13 joists. Add 10% extra for waste and blocking.