Rafter Calculator
Find the common-rafter line, overhang line, roof angle, and run.
What this calculator does
A right triangle is solved for half the span and for the horizontal overhang.
Common rafter geometry
Span is outside wall to outside wall. Half the span becomes one common rafter's horizontal run.
Your plan updates instantly
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Half-span and overhang are multiplied by the pitch slope factor; cut deductions are not made.
The arithmetic uses the exact values shown above. Verify field measurements, product pack sizes, layout constraints, and local requirements before ordering or building.
How to interpret the result
A right triangle is solved for half the span and for the horizontal overhang.
How the result is calculated
Rafter = (span ÷ 2) x √(1 + (rise ÷ 12)²).
Worked example
A 24 ft span at 6:12 gives a 13.42 ft wall-to-ridge line.
Practical guidance
- Measure the project at least twice before ordering.
- Use actual product dimensions and package coverage.
- Keep a saved or printed copy of the inputs used for the estimate.
Assumptions and limitations
The result is an uncut geometric line, not structural sizing or a cut list.
Methodology
Calculations use standard geometry and normalized units. Countable products are rounded up where partial products cannot normally be purchased.
Read the complete calculation methodology and testing policy.
Frequently asked questions
Are results rounded for purchasing?
Countable materials and packages are rounded up. Measured area, length, volume, and cost remain decimal values where appropriate.
Is this a final construction specification?
No. It is a transparent planning estimate. Verify product instructions, site conditions, plans, and local requirements.