About the Birdsmouth Cut in Rafters
The birdsmouth cut (also called the seat cut) is a notch cut into a rafter so it can securely sit on the top plate of a wall. It ensures stability, correct alignment, and proper load distribution in roof framing. The birdsmouth cut typically consists of two parts — the seat cut (horizontal) and the heel cut (vertical).
Birdsmouth Cut Formula
To calculate the seat cut and heel cut dimensions:
Seat Cut = Wall Plate Thickness ÷ tan(Roof Pitch)
Heel Cut = Rafter Width − (Seat Cut × sin(Roof Pitch))
These formulas use trigonometry to ensure accurate angles and fit.
Step-by-Step Birdsmouth Calculation
- Measure the roof pitch in degrees (angle between rafter and horizontal).
- Measure the rafter’s width and wall plate thickness.
- Use the formulas above to compute seat and heel cuts.
- Ensure the birdsmouth cut depth is no more than 1/3 of the rafter depth to maintain structural strength.
- Mark and cut accurately using a framing square.
Example Calculation
Suppose the roof pitch is 30°, rafter width = 7.25 in, and wall plate thickness = 3.5 in:
- Seat Cut = 3.5 ÷ tan(30°) = 6.06 in
- Heel Cut = 7.25 − (6.06 × sin(30°)) = 4.22 in
- Seat Cut Length = 6.06 in, Heel Cut Depth = 4.22 in
Important Framing Tips
- Never cut deeper than 1/3 of the rafter’s total depth.
- Ensure the seat cut lies flat on the top plate for stability.
- Use a framing square and mark both the seat and heel angles clearly.
- Double-check pitch angles before making cuts to avoid misfits.
- Use treated lumber for rafters in exposed areas.
Roof Pitch Reference Table
| Pitch (°) | Rise : Run | Tangent Value |
|---|---|---|
| 15° | 1 : 3.73 | 0.268 |
| 30° | 1 : 1.73 | 0.577 |
| 45° | 1 : 1 | 1.000 |
| 60° | 1 : 0.58 | 1.732 |
Advantages of Accurate Birdsmouth Cuts
- Ensures stable roof-to-wall connection.
- Prevents rafter slipping and misalignment.
- Improves load distribution across wall plates.
- Enhances structural integrity and roof lifespan.