Math July 13, 2026 · 8 Min Read

Rafter Length Calculator – Guide & Formulas

Calculate rafter length from run, rise, pitch, and overhang. Includes collar tie and birdsmouth adjustments.

Calculate precise common rafter lengths for sloped roofs. Account for run, rise, pitch, birdsmouth cut depth, and overhang dimensions to plan your roofing materials with high carpentry precision.

Key Takeaway

Use the free Rafter Length Calculator to calculate rafter length from run, rise, pitch, and overhang. includes collar tie and birdsmouth adjustments. Get instant results with step-by-step explanations.

How to Use the Rafter Length Calculator

  1. Enter the horizontal run of the roof (half the span of the building).
  2. Choose whether to specify by vertical rise or roof pitch ratio.
  3. Input overhang length and birdsmouth cut details to compute full rafter stock length.

The Formula

Rafter Length = sqrt(Run^2 + Rise^2) + Overhang - (Ridge / 2)

Variable Definitions

  • Run: Horizontal distance from wall to ridge centerline
  • Rise: Vertical distance from top plate to ridge centerline
  • OverhangLength: Extra sloped length extended past the outer wall

Common Rafter for 20ft Span House

You are framing a gable roof over a 20-foot wide house. The horizontal run is 10 feet (half span), the roof pitch is 6/12, and you want a 12-inch horizontal overhang.

  1. Step 1: Calculate rise: Pitch is 6/12, so for a 10 ft run, rise is 10 × 6" = 60 inches or 5 feet.
  2. Step 2: Find base rafter length: √(10² + 5²) = √125 ≈ 11.18 feet (11 ft 2-3/16 inches).
  3. Step 3: Calculate slope multiplier for 12-inch horizontal overhang: Multiplier is 1.118, so overhang length on slope = 1.118 × 1 ft = 1.118 ft.
  4. Step 4: Combine lengths: 11.18 ft + 1.118 ft = 12.30 feet. Order 14-foot lumber for stock cuts.

Frequently Asked Questions

How does a birdsmouth cut affect rafter length?

A birdsmouth cut is a notch cut into the rafter so it sits flat on the top wall plate. It slightly lowers the roof line but does not change the centerline mathematical rafter length.

Should I subtract the ridge beam thickness?

Yes. Since the rafter meets the center ridge board, you must subtract half the thickness of the ridge board (typically 0.75 inches for 2x lumber) from the final length.

What is the standard spacing for roof rafters?

Most structural codes recommend spacing rafters either 16 inches or 24 inches on-center depending on the lumber size and expected snow/wind load.