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Construction Calculators

Rafter Length Calculator

Calculate the length of a roof rafter from its horizontal run and roof pitch, including overhang.

ft
Horizontal distance from the wall plate to the ridge (half the building width for a gable roof).
A 6/12 pitch rises 6in for every 12in of horizontal run.
in
Pitch angle —
Rafter length (run only) —
Rafter length with overhang —
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About the Rafter Length Calculator

This calculator finds the actual length of a sloped roof rafter from its horizontal run and pitch. Because a rafter travels diagonally rather than flat, its true length is always longer than the horizontal run it covers, and the steeper the pitch, the greater the difference.

Formula: Pitch Angle = arctan(Rise ÷ 12); Rafter Length = Run ÷ cos(Pitch Angle). For example, a 12ft run at a 6/12 pitch gives an angle of 26.57° and a rafter length of about 13.42ft.

Add the overhang (measured along the same slope) to get the total board length to cut, and always add a few extra inches for the birdsmouth notch and fascia trim when ordering lumber.

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