Deck Joist Span Calculator
Deck joist spans depend on lumber size, species, and spacing. This calculator looks up allowable spans from IRC Table R507.6, which applies to Douglas fir-larch and southern pine at 40 psf live load plus 10 psf dead load. Select the joist size and spacing, then enter your proposed span. The tool tells you whether your span is within the allowable limit. Always verify with a licensed engineer or building inspector for your specific conditions, species, and local code amendments.
IRC Table R507.6 joist span data
Lookup: Allowable span = spanTable[size][spacing]
Status: PASS if proposed span <= allowable span, FAIL otherwise
The spans in this table assume Douglas fir-larch or southern pine lumber graded No. 2 or better, 40 psf live load, and 10 psf dead load. For other species (hem-fir, spruce-pine-fir) the allowable span is reduced by approximately 10 to 15 percent. Verify with your local building department.
| Joist Size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|
| 2x6 | 10'-0" | 9'-1" | 7'-11" |
| 2x8 | 13'-2" | 11'-11" | 10'-5" |
| 2x10 | 16'-8" | 15'-2" | 13'-2" |
| 2x12 | 19'-9" | 17'-11" | 15'-7" |
Deck framing best practices
- Use pressure-treated lumber rated for ground contact (UC4B) where joists contact concrete or sit within 6 inches of the ground.
- Use joist hangers at the ledger and beam connections to meet code requirements.
- Crown all joists (arch facing up) before installation so they do not sag under load.
- Block between joists at mid-span for spans over 8 feet to prevent lateral buckling.
Deck joist span calculator: frequently asked questions
What determines the allowable span for deck joists?
Allowable span depends on lumber species, size (2x6, 2x8, 2x10, 2x12), joist spacing (12, 16, or 24 inches on center), and the assumed live and dead load. IRC Table R507.6 uses 40 psf live load plus 10 psf dead load for decks.
What is the maximum span for a 2x8 deck joist at 16 inches on center?
Per IRC Table R507.6, a 2x8 Douglas fir or southern pine joist at 16 inches on center can span approximately 12 feet 6 inches under standard 40 psf live load plus 10 psf dead load conditions.
What live load is used for deck joist spans?
Residential decks are designed for a 40 pounds per square foot (psf) live load per IRC Section R301.5, plus a 10 psf dead load for the decking and framing weight.
How does joist spacing affect allowable span?
Closer spacing (12 inches on center) allows longer spans than wider spacing (24 inches o.c.) because each joist carries less tributary load. Doubling the spacing roughly reduces the allowable span by 15 to 20 percent.
Do I need to check beam spans as well?
Yes. Joists span from beam to beam (or beam to ledger). Beams in turn must span from post to post. Both the joist span and the beam span must satisfy the applicable IRC span tables independently.
Official sources
- International Code Council: IRC Table R507.6 Deck Joist Spans.
- American Wood Council: DCA6 Prescriptive Residential Wood Deck Construction Guide.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.