How the lookup works
The IRC gives deck builders a prescriptive table: pick the design load, the lumber species group, the joist size and the spacing, and it lists the longest span that meets strength and stiffness limits. The span is measured from the centre of the ledger hanger (or bearing) to the centre of the beam the joist rests on.
A cantilever past that beam is limited by the back span: the table lists the longest cantilever for back spans of 4 to 18 ft. Between two columns you may interpolate; past the last permitted column (marked NP), or below 4 ft, the table gives no cantilever. The 2018 IRC instead gave one cantilever per spacing and capped it at a quarter of the span; the 2021 values never exceed that quarter either, but short joists and small sizes get less.
Formula
- Max span
- Table R507.6 [load, species, size, spacing]
- Max cantilever
- c₁ + (c₂ − c₁) × (back span − b₁) ÷ (b₂ − b₁)
b₁ and b₂ are the back-span columns on either side of your span and c₁, c₂ the cantilevers listed under them. If either is NP, no cantilever is allowed.
Worked example
2×10 southern pine at 16″ on center may span up to 14′-0″. This deck spans 12′-0″ from the ledger to the beam, so it passes. With a 12′-0″ back span the table allows a cantilever of 3′-0″, so the planned 2′-0″ cantilever is fine.
At a 13′-0″ back span, halfway between the 12 ft and 14 ft columns (3′-0″ and 3′-4″), the limit is 3′-2″.
Snow load
The table has a 40 psf live-load row and rows for ground snow loads of 50, 60 and 70 psf. Snow and people are not assumed to be on the deck at the same time, so where the ground snow load is 40 psf or less, the 40 psf live load row applies. Your building department publishes the ground snow load for your address. Above 70 psf, the table doesn't apply and the deck needs an engineered design.
Assumptions
- No. 2 grade or better, pressure-treated, wet service. Spans for the fir and spruce group include the incising factor.
- 10 psf dead load. No hot tubs, planters, or other heavy loads.
- Decking can limit the spacing (R507.7). Boards laid square to the joists: 5/4 wood allows 16″ when each board crosses three or more joists (12″ if only two), 2× wood 24″. Diagonal boards need closer joists. Composite decking follows the span on its label.
- The AWC deck guide (DCA 6) lists the same spans for a 40 psf live load, but measures its overhangs from the face of the beam and keeps them a little shorter.
Local codes vary and can be stricter than the IRC. Check with your building department before you build.
References
- ICC, International Residential Code 2021: Section R507.6 and Table R507.6, deck joist spans; R507.7 decking.
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide.