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Rafter calculator

Estimate one straight rafter from the full roof span, ridge-board thickness, roof rise or pitch, and horizontal eave overhang.

Geometry estimate for a straight, symmetric roof. Overhang is measured horizontally; the tool follows the same slope to extend the rafter. It does not design a birdsmouth/seat cut, tail, structural size, connections, loads or code compliance. Verify dimensions and ask a qualified professional to review construction plans. This lightweight calculation runs locally in your browser.

How do I calculate rafter length?

For a symmetric roof, the calculator halves the full span, subtracts half the ridge-board thickness, and applies the roof slope. A horizontal overhang extends the rafter along the same slope. The result is geometric and does not size a structural member.

Frequently asked questions

How do I calculate rafter length?

For a symmetric roof, the calculator halves the full span, subtracts half the ridge-board thickness, and applies the roof slope. A horizontal overhang extends the rafter along the same slope. The result is geometric and does not size a structural member.

Are my roof measurements uploaded?

No. The calculation runs in your browser; the measurements are not sent to a server or saved by this tool.

Does eave overhang increase rafter length?

Yes. Enter overhang as a horizontal distance. The calculator extends the rafter at the same roof slope, so a positive overhang produces a longer geometric rafter.

Does this calculate a birdsmouth cut?

No. A birdsmouth or seat cut depends on the wall plate, bearing length, cut depth, lumber dimensions and local requirements. This tool only reports basic roof geometry and a plumb-cut angle; have a qualified professional verify construction details.