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How far joists or a beam may overhang their support — the one-quarter rule, checked against the joist span table.
With these inputs:
| Result | Value |
|---|---|
| Maximum joist overhang | 3 ft |
| Overhang you want | 2 ft |
| Verdict | Backspan is over the 2x8 span limit (11 ft 11 in) — go deeper or add a beam |
| Backspan needed for that overhang | 8 ft |
| 2x8 span limit at 16" (pine) | 11 ft 11 in |
| One-quarter rule | 12 ft ÷ 4 = 3 ft |
| Total joist length | 14 ft |
IRC R507.6 lets a joist cantilever up to one quarter of its actual backspan, and no further than the cantilever column of the code table for that size and spacing — deeper joists and closer spacing allow more. The backspan itself must be within the span table. Always block the joists over the beam and fix them with hurricane ties; a heavy load at the tip (hot tub, planter, privacy wall) needs an engineer.
A cantilever is the part of a joist or beam that overhangs its last support. The code rule is simple: the overhang may be no more than one quarter of the span behind it, and the whole joist — span plus overhang — must still work in the span table. Enter the backspan and joist size and this gives the maximum overhang, or enter the overhang you want and it tells you the backspan you need.
Joists may overhang the beam by no more than one quarter of the span behind it: a 12 ft backspan allows up to 3 ft, an 8 ft backspan 2 ft. The code table also caps the overhang by joist size and spacing, so a small joist at wide spacing gets less.
Yes — a beam may overhang its end post by up to one quarter of the beam span between posts. With posts 8 ft apart, the beam can run up to 2 ft past the last post.
It hides the beam and posts under the deck, gives a cleaner edge, and lets the deck clear obstacles such as a slope, a buried pipe or a window well without extra footings.
Block between the joists over the supporting beam and at the ends, and fasten each joist to the beam with a tie. Blocking stops the joists rolling and spreads a point load at the edge across several of them.
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