Deck Cantilever Calculator
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.
What you'll need
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What a deck cantilever is
A cantilever is the overhang: the length of a joist that runs past its last support, or the length of a beam that runs past its end post. Designers use it to tuck the beam and posts back under the deck, to clear a slope or an obstacle, and to get a clean floating edge. The overhang is carried by the length of joist or beam behind the support — the backspan — acting as a lever, which is why the rule links the two.
The one-quarter rule
Under IRC R507.6 and the AWC DCA 6 guide, a deck joist may cantilever no more than one quarter of its actual backspan, and no more than the cantilever limit printed in the code table for that joist size, species and spacing. Beams follow the same one-quarter rule measured against the span between the last two posts (IRC R507.5). The whole joist — backspan plus overhang — must also fit the span table.
| Backspan | Maximum overhang (¼) | Total joist length |
|---|---|---|
| 6 ft | 1 ft 6 in | 7 ft 6 in |
| 8 ft | 2 ft | 10 ft |
| 10 ft | 2 ft 6 in | 12 ft 6 in |
| 12 ft | 3 ft | 15 ft |
| 14 ft | 3 ft 6 in | 17 ft 6 in |
Those are ceilings, not targets. The code table can cut the overhang shorter for small joists at wide spacing, and a heavy load at the edge — a planter, a privacy wall, a hot tub — needs an engineered design whatever the ratio.
Working backwards from the overhang you want
If you know you want a 2 ft overhang, the backspan has to be at least four times that: 8 ft. The calculator shows that figure for whatever overhang you enter, and checks that the backspan itself is within the span for your joist size and spacing. If it is not, go up a joist size or add a beam — the deck joist span calculator shows the smallest joist that works.
Building a cantilever properly
- Block over the beam. Solid blocking between the joists where they cross the beam stops them rolling.
- Tie every joist down. A hurricane tie or similar connector at each joist resists the uplift a lever creates at the back.
- Keep the rim honest. The rim joist at the end of a cantilever carries the railing posts; through-bolt them with blocking so they resist the required 200 lb outward load.
- Never cantilever a ledger deck from the ledger. The overhang is always past a beam, never past the house.
Related tools
Size the beam and find post spacing with the deck beam span calculator and the deck post spacing calculator. The joist span chart and beam span chart have the tables on one page.
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Frequently asked questions
- How far can a deck cantilever?
- 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.
- Can a deck beam cantilever past the post?
- 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.
- Why cantilever a deck at all?
- 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.
- Do I need to block cantilevered joists?
- 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.
