How to calculate a birdsmouth cut?
Answer
Use the roof angle and the specified seat length. Multiply the seat length by the pitch slope to get the vertical heel depth.
Compare the remaining rafter depth with the approved plans before the cut.
| Birdsmouth value | Result |
|---|---|
| Seat length | 3.500 in |
| Vertical heel depth | 1.750 in |
| Perpendicular notch depth | 1.565 in |
| Remaining depth in a 5.5 in rafter | 3.935 in |
| Height above plate | 4.399 in |
This example gives geometry. The project design gives the permitted cuts and bearing.
Calculate the cut geometry
- Confirm the roof pitch.
- Use the specified horizontal seat length.
- Multiply the seat length by rise divided by run.
- Mark the plumb cut and the level seat cut.
Notch depth
Multiply the seat length by the sine of the roof angle. The result is the notch depth perpendicular to the rafter edge.
Subtract this value from the measured rafter depth. The remaining value is the depth above the notch.
Layout check
Make a full-size test mark on scrap material. Do a check of the seat and heel against the wall plate.
The US Navy construction manual shows common-rafter and birdsmouth layout.
Use the rafter calculator to calculate the notch geometry.
Important information
- Use the approved seat length and cut limits.
- Use the rafter depth that the approved design gives.
- Do a check of one test rafter before repeated cuts.