How to use the Roof Rafter Calculator

Calculate a gable roof rafter’s theoretical slope length including horizontal overhang.

Calculation formula

One rafter’s horizontal run = span/2 + overhang; roof rafter length = horizontal run × sqrt(1 + (pitch/12)^2).

Recommended steps

1. Enter values using the units shown beside each field.
2. Review whether the example assumptions match your situation.
3. Run the calculator and compare the key metrics.
4. Use the source data or official rates when precision matters.

Known limitations

Basic symmetrical gable roof with no ridge-board deduction, seat cut, heel height or structural framing requirements.