Joist / Deck Cantilever Ratio Check (IRC R507.6)
The 1:4 deck rule: a joist may overhang its support by a quarter of its backspan.
Example
You enter
- Backspan (ft, support to next support) 10
- Cantilever / overhang (ft) 3
- Deck joist size 2x10
- Joist species (Table R507.6 group) southern_pine
You get
- Max cantilever (Table R507.6, at most backspan / 4) 2.50 ft (30.0 in)
- Margin (ft) -0.5
Checked against published worked examples from: Journal of Light Construction (reposted by Marvel Builders).
Details, formula, and sources
ScopeIRC Table R507.6 holds a deck joist to no more than a quarter of its backspan (the span back to the next support), then caps or forbids the cantilever by joist size and species. A 10 ft backspan of 2x10 southern pine allows 2.5 ft, so a 3 ft overhang EXCEEDS it; the same backspan of 2x6 allows none. A floor cantilever is limited to the joist depth under R502.3.3. The tables also require an uplift check, and a beam, wall, or roof bearing on the tip is a separate engineered case. This reads the Table R507.6 cantilever limits (capped at backspan / 4); the joist span itself, the tip load, the support connection, and the AHJ-adopted code govern. Distinct from a cantilever-beam moment and deflection analysis.
Formula or tabletable_limit_ft = IRC 2024 Table R507.6 (identical to 2021) maximum cantilever for the joist size and species at the tabulated backspan column at or above backspan_ft (NP = not permitted); ratio_limit_ft = backspan_ft / 4; cantilever_max_ft = min(table_limit_ft, ratio_limit_ft); within_limit when overhang_ft <= cantilever_max_ft; margin_ft = cantilever_max_ft - overhang_ft.
Source and editionIRC 2024 Table R507.6 (deck joist spans and cantilevers, 40 psf live load; every cantilever value identical to IRC 2021, checked 2026-10-17), by name; backspan columns 4 to 18 ft. The AHJ-adopted IRC edition governs.
Free accessIRC Table R507.6 is a prescriptive table (many jurisdictions post the adopted IRC); the tile reads its cantilever columns for the entered joist size, species, and backspan.
What governsEstimate. AHJ and licensed professional govern.
API field namesbackspan_ft, overhang_ft, joist_size, species, cantilever_max_ft, margin_ft
- Table R507.6 cantilever maximum cantilever by joist size, species, and backspan column (4-18 ft), never more than backspan / 4; NP where the table forbids oneIRC 2024 Table R507.6 (identical in 2021)
- Tip load is engineered a beam / wall / roof bearing on the cantilever tip is an engineered condition, not prescriptiveIRC / engineering