Rafter Length Calculator

About the Rafter Length Calculator

Calculating rafter length accurately is essential for ordering the right lumber and making precise cuts. A rafter's length is determined by the horizontal run (half the building width minus half the ridge thickness), the vertical rise (set by the roof pitch), and any overhang (eave projection) you want beyond the exterior wall.

This rafter length calculator uses the Pythagorean theorem to compute the line length of a common rafter from run and rise, then adds the overhang distance adjusted for pitch. It also shows the total stock length you need to purchase and the birdsmouth cut dimensions.

Whether you're framing a simple gable roof, a shed addition, or a porch, This calculator gives you the measurements needed to lay out and cut each rafter accurately.

When This Page Helps

Rafter length calculations involve square roots and trigonometry that are error-prone with a tape measure and framing square alone. This calculator gives you the exact line length, overhang adjustment, and stock length so you can order material and set up your saw with confidence.

How to Use the Inputs

1. Enter the horizontal run (half the roof span, measured from the outside of the wall to the center of the ridge).
2. Enter the rise per foot of run (e.g., 6 for a 6:12 pitch).
3. Enter the desired overhang (horizontal projection of the eave beyond the wall).
4. Read the rafter line length, overhang length on the slope, and total stock length.
5. Use the birdsmouth dimensions to lay out your plumb and seat cuts.

Example Calculation

Tips & Best Practices

• Always measure the run from the outside face of the wall's top plate to the center of the ridge board.
• Subtract half the ridge board thickness from the run for accurate line length.
• The birdsmouth cut should not remove more than one-third of the rafter depth.
• Use a framing square or speed square to lay out plumb cuts and seat cuts directly on the rafter.
• Order rafters 2 feet longer than calculated to allow for cutting the bird's mouth and tail.
• Check that opposing rafters have equal lengths to keep the ridge straight.

Common Rafter Layout Process

Start by marking the ridge plumb cut at one end, measure the rafter line length along the top edge, mark the birdsmouth location at the wall plate, and finish with the tail cut at the eave. Use a speed square set to your pitch for consistent plumb and level cuts.

Birdsmouth Cut Guidelines

The IRC limits birdsmouth depth to one-third of the rafter depth. For a 2×8 rafter (7.25″ actual depth), the maximum seat cut is about 2.4 inches. The seat cut must be long enough to bear fully on the top plate (minimum 1.5″ for a 2× plate).

Adjusting for Different Pitches

Steeper pitches produce longer rafters for the same horizontal run. A 12:12 pitch (45°) adds 41% to the rafter length compared to the run, while a 4:12 pitch adds only 5.4%. Always recalculate when pitch changes—don't interpolate.

Frequently Asked Questions

• What is the run of a rafter?

  The run is the horizontal distance from the outside of the wall's top plate to the center of the ridge. For a symmetrical gable roof, the run equals half the building span minus half the ridge board thickness.

• How do I find roof pitch from rise and run?

  Pitch is expressed as rise per 12 inches of run. If the roof rises 6 inches for every 12 inches of run, the pitch is 6:12. You can measure this on an existing roof with a level and tape measure.

• What is a birdsmouth cut?

  A birdsmouth is a notch cut into the bottom of a rafter where it sits on the wall's top plate. It consists of a horizontal seat cut and a vertical plumb cut, allowing the rafter to bear flat on the plate.

• How long should the overhang be?

  Common overhang lengths range from 12–24 inches. Longer overhangs (18–24″) provide better weather protection and shade but may require lookout framing or outriggers for support.

• Do I need to account for the ridge board?

  Yes. Subtract half the ridge board thickness from the theoretical run. For a 1.5″ ridge board (2× lumber), subtract 0.75 inches. This shortening ensures the rafters meet the ridge correctly.

• What stock length should I buy?

  Round up to the next standard lumber length (8, 10, 12, 14, 16, 18, or 20 feet). Always buy longer than the calculated total to account for end cuts and any imperfections in the lumber.