Rafter Calculator
Calculate the length of rafters for a pitched roof given the span, pitch, and overhang.
Published by:
isCalculator Editorial Team
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How to Use the Rafter Calculator
This calculator computes the rafter length for a gable roof using imperial units (feet).
- Enter Building Width – The total width of the building in feet (e.g., 24).
- Enter Roof Pitch – The rise per 12 inches of run (e.g., 6 for a 6/12 pitch).
- Enter Overhang – The horizontal overhang beyond the wall in feet (default 0).
- Click "Calculate" – The rafter length appears in feet, with the formula shown below.
All inputs must be positive numbers. Overhang can be zero. The result includes the overhang.
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Frequently Asked Questions
A rafter is a structural component that runs from the ridge of a roof to the wall plate, supporting the roof covering.
Rafter length is calculated using the Pythagorean theorem: L = (span/2 + overhang) × √(1 + (pitch/12)²), where span is the building width, pitch is the rise per 12 inches of run.
This calculator uses feet for all measurements. The roof pitch is expressed as inches of rise per 12 inches of run (e.g., 6 for a 6/12 pitch).