Rafter Dimensions Calculator
Calculate the length, angle, and number of rafters needed for a roof based on span, pitch, and overhang.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to Use the Rafter Dimensions Calculator
- Enter the Span – Input the horizontal distance (in feet) from one wall to the opposite wall. Must be a positive number.
- Enter the Pitch – Input the roof rise per 12 inches of horizontal run (e.g., 6 for a 6/12 pitch). Must be a positive number.
- Enter the Overhang – Input the horizontal overhang (in feet) beyond the wall. Use 0 if none. Must be a non-negative number.
- Enter the Spacing – Input the center-to-center spacing between rafters (in feet). Must be a positive number.
- Click "Calculate" – The button will validate all inputs. If any field is invalid, it will show a red border.
Results explained:
- Rafter Length (total) – The full length of each rafter in feet, including the overhang.
- Pitch Angle – The angle of the roof in degrees.
- Number of Rafters – Total rafters needed for one side of the roof (rounded up).
All inputs are in imperial units (feet). The calculator uses standard roof framing formulas.
Previous Results
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Frequently Asked Questions
A rafter is a structural beam that runs from the ridge of a roof to the wall plate, supporting the roof covering.
Rafter length is calculated as (span/2 + overhang) divided by the cosine of the pitch angle. The pitch angle is derived from the rise per 12 inches of run.
Common rafter spacing is 16 or 24 inches on center, but this calculator accepts any spacing in feet.