Roof measurements become easier to check when pitch and rafter length are treated as geometry first. Measure a level run and vertical rise, then add overhang as a horizontal projection; leave structural sizing and cuts to the drawings and responsible professional.
Measure rise and run safely
A level run is horizontal distance, not the distance along the roof surface. Use a level and a vertical reference from a safe location, or measure from plans. Rise and run can be inches, feet, or metric units as long as each pair matches.
Pitch notation such as 6:12 means six units of rise for every twelve horizontal units. The same ratio can be expressed as angle or slope percentage.
From pitch to rafter length
The rafter calculator applies the Pythagorean relationship to the horizontal run and pitch. If an eave projects beyond the wall, enter that horizontal projection too. The result does not subtract a ridge board or account for a seat cut, birdsmouth, fascia, or field tolerance.
Use geometry for planning
Geometry can help estimate sheathing area, compare options, or order a rough length. It does not determine member size, spacing, connections, wind/snow resistance, drainage, or local code compliance.
Check the roof in context
Confirm the roof plan, wall bearing points, ridge detail, overhang detail, and material manufacturer instructions. A qualified professional should review structural changes and any roof access or fall-protection plan.
Worked examples
6:12 roof
- Measure 12 in run and 6 in rise.
- Pitch is 6:12, angle about 26.57°, slope 50%.
- For a 12 ft run, multiply by √(1 + 0.5²).
Result: The geometric rafter length is about 13.42 ft before cuts.
4:12 roof with eave
- Use a 10 ft wall-to-ridge run and 1.5 ft horizontal overhang.
- Total horizontal run is 11.5 ft.
- Apply the 4:12 slope factor.
Result: The estimate is about 12.14 ft; confirm cut lines and ridge deductions in the framing plan.
Safety and responsibility
Roof work has fall and structural hazards. Use safe access and qualified review for framing or changes to an existing roof.