rafter length calculations

Rafter Length Calculations

Rafter length is horizontal run multiplied by the pitch factor, with adjustments for overhang, ridge thickness, and any field cut allowance.

Inputs

Results

Immediate estimate

Rafter length14.46 ft173.6 in along the rafter

Pitch factor1.118

Main rafter length13.35 ft

Tail length1.12 ft

Plumb cut angle26.6 degrees

This is a geometry calculator. It does not check rafter span, lumber size, bearing, thrust, collar ties, ridge beams, or local code.

Diagram

Rafter geometry

Horizontal run and roof pitch are converted to sloped rafter length.

Run
Pitch
Rafter length

How this calculator works

Enter horizontal run, roof pitch, overhang, ridge thickness, and cut allowance to calculate common rafter length.

Pitch factor

The calculator derives a pitch factor from rise over 12 inches using the Pythagorean theorem.

Length model

Main rafter length is adjusted for half the ridge thickness, then tail length and optional cut allowance are added.

Geometry only

This page calculates layout length. Use the rafter span calculator or adopted tables for structural span checks.

Pitch factorsqrt(12 squared + pitch rise squared) / 12

Main length(horizontal run - half ridge thickness) x pitch factor

Total lengthmain length + overhang x pitch factor + cut allowance

Assumptions

Run: Horizontal run is from outside wall or bearing line toward the ridge before ridge-thickness deduction.
Cut allowance: Optional allowance is added for field trimming, birdsmouth layout, or preference.

Worked examples

6:12 common rafter

Common gable roof rafter geometry.

14.46 ftRafter length

Steeper roof with trimming allowance

Includes overhang and cut allowance.

20.26 ftRafter length

FAQ

Is rafter length the same as rafter span?

No. Rafter length follows the slope; rafter span is the horizontal projection between supports.

Why subtract half the ridge thickness?

A common layout convention subtracts half the ridge board thickness from the horizontal run for opposing rafters.

Does this include birdsmouth cuts?

It includes an optional cut allowance, but actual birdsmouth layout depends on bearing details and local practice.

Sources and review

Roof measurement and shingle planning, GAF. Accessed 2026-07-16.
2021 International Residential Code, International Code Council, 2021. Accessed 2026-07-16.

Reviewed 2026-07-20. Planning estimates only; confirm project-specific requirements.