Birdsmouth Cut Calculator
Use this birdsmouth cut calculator to estimate seat cut, heel cut, rafter geometry, and the minimum depth guidance described on Omni.
Birdsmouth Geometry
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Quick Answer: The seat cut and heel cut are tied together by seat cut = heel cut / tan(theta), while the rafter segment geometry depends on the roof pitch angle and half the clear building width.
How to Calculate
- Enter width and ridge thickness: The clear run to one side is based on half the building width minus half the ridge thickness.
- Set the roof pitch: Use either the pitch angle directly or confirm the equivalent rise-over-12 value.
- Enter heel cut and overhang: These values define the birdsmouth notch and total rafter length.
- Review the geometry: The calculator reports seat cut, rise, run, segment length, full rafter length, and minimum member depths.
Formula
seat cut = heel cut / tan(theta), rise = tan(theta) x (W - t_ridge) / 2
| Variable | Meaning | Unit |
|---|---|---|
| theta | Roof pitch angle | degrees |
| W | Building width | length |
| t_ridge | Ridge board thickness | length |
| O | Overhang distance | length |
Worked Examples
Roof framing - Finding heel cut depth
- Seat cut: 4 in
- Pitch angle: 25 deg
Result: Heel cut = 1.865 in
This matches Omni's FAQ explanation for converting a seat cut into a heel cut depth.
Interpretation Table
| Range | Meaning | Action |
|---|---|---|
| Shallow heel cut | Less material removed from the rafter | This helps preserve section depth but still must satisfy bearing and code requirements. |
| Deeper heel cut | More material removed at the notch | Check minimum rafter depth and local code limits before construction. |
| Longer rafter | Overhang and pitch both increase member length | Confirm stock length, waste allowance, and ridge-board geometry. |
Frequently Asked Questions
A birdsmouth cut is the notch at the end of a rafter that helps it seat properly on the supporting wall plate.
Multiply the seat cut by tan(theta), where theta is the roof pitch angle.
Deep notches reduce the remaining net section of the rafter, which can affect strength and code compliance.
Note: This calculator helps with geometry, not code approval. Always confirm notch limits, bearing requirements, and rafter sizing under your local building code.
References
Last reviewed: March 14, 2026