Joist / Deck Cantilever Ratio Check (IRC R507.6)
The 1:4 cantilever rule: a joist may overhang its support by no more than a quarter of its backspan (the span back to the next support), per IRC R507.6 for decks and R502.3.3 for floors. A 10 ft backspan allows a 2.5 ft cantilever, so a 3 ft overhang EXCEEDS it and needs a 12 ft or longer backspan; a 12 ft backspan allows 3.0 ft. The prescriptive tables also cap the absolute overhang and require an uplift check, and a beam, wall, or roof bearing on the tip is a separate engineered case. This is the RATIO screen; the span tables, the tip load, the support connection, and the AHJ-adopted code govern. Distinct from the engineering cantilever-beam moment/deflection tile.
Formula and source
cantilever_max_ft = backspan_ft / 4; within_limit when overhang_ft <= cantilever_max_ft; margin_ft = cantilever_max_ft - overhang_ft.
IRC R507.6 (decks) and R502.3.3 (floors) joist-cantilever provisions, by name; the 1:4 backspan ratio. The AHJ-adopted IRC edition governs.
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