Deck joist span calculator
Maximum allowable deck joist spans per IRC Table R507.5. Pressure-treated or naturally durable lumber — 40 psf live load, 10 psf dead load.
| Size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|
| 2x6 | 10'-10" | 9'-10" | 8'-1" |
| 2x8 | 14'-4" | 13'-0" | 10'-8" |
| 2x10 | 18'-3" | 16'-7" | 13'-7" |
| 2x12 | 22'-2" | 20'-2" | 16'-6" |
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Deck joist spans are governed by IRC Section R507.5, which provides separate tables from interior floor joists. Deck joists must be pressure-treated or naturally durable species (redwood, cedar) since they're exposed to weather.
Deck joist spans are generally shorter than interior floor joist spans for the same lumber size due to the different loading conditions and environmental exposure.
Key Differences from Interior Joists
- Must use pressure-treated or naturally durable lumber
- Cantilevers limited to 1/4 of the allowable joist span
- Lateral bracing required at supports
- No 19.2" o.c. option — only 12", 16", and 24"
IRC Table R507.6 and Deck-Specific Requirements
Deck joist spans are governed by IRC Table R507.6 (referenced as R507.5 in earlier code editions), which is deliberately more conservative than interior floor joist tables. The most common species for deck framing are pressure-treated Southern Pine and Ponderosa Pine, though Douglas Fir-Larch and Hem-Fir are also used regionally. The 2018 and 2021 IRC editions introduced slightly longer allowable spans for some species-grade combinations compared to the 2015 edition — always verify which code edition your local jurisdiction enforces, as many lag behind the latest publication.
Standard span tables assume a uniform 40 psf live load plus 10 psf dead load.Hot tubs, large planters, and other concentrated loads exceed these assumptions and require an engineered design — a standard span table will not cover them. If your deck supports anything beyond typical furniture and foot traffic, consult a licensed structural engineer before finalizing joist sizing.