Deck Cantilever Calculator

Joist and beam overhang limits

Use the prescriptive one-quarter rule as a fast framing check. This tool is best for early layout review before you finalize the permit set or engineered drawings.

Actual joist backspan (ft) Supported joist span from ledger/beam to beam, not full deck projection.

Beam adjacent span between posts (ft) Use the longer adjacent beam span when checking a beam overhang.

Desired joist cantilever (in) Joist projection past the supporting beam.

Desired beam cantilever (in) Beam overhang past the end post.

Deck projection if using one beam (ft) Optional planning check for a single outer beam plus joist overhang.

Reference basis

AWC / IRC prescriptive one-quarter rule

Local review still governs; this screen does not replace member sizing or engineered design.

Rule used: maximum joist cantilever and maximum beam cantilever are each limited to one-quarter of the supported span they rely on. Heavy loads, roof loads, curved decks, or unusual framing should be engineered.

Results

-- Max joist cantilever

-- Max beam cantilever

-- Joist check

-- Beam check

Single-beam projection check

Recommended action

Interpretation

When this tool is not enough

Hot tubs, roofs, or masonry veneer

These conditions move the project out of simple prescriptive framing review.

Multi-level or irregular framing

Curved, angled, or offset framing needs a full layout review, not just ratio checks.

Member strength still matters

Passing the cantilever ratio does not prove the joist or beam size is adequate for the load.

Related tools

Joist Calculator → Footing Calculator →

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