Can I use this without the roof calculator?
Yes. Switch to manual angle and enter the roof angle in degrees directly.
Birdsmouth Cut Calculator
Seat cut depth and angle for a rafter's birdsmouth notch, from wall plate width and roof angle.
Cut the birdsmouth seat at — in with a — in run.
Roof angle source
Pull from the roof calculator or override it manually.
Chained roof angle
Run the roof calculator first to prefill the birdsmouth angle with the active project roof angle.
Seat cut inputs
Wall plate width and rafter depth, in inches.
Cut the birdsmouth seat at — in with a — in run.
Cut at
Birdsmouth cut
Warnings
- INFO No roof angle is saved in the active project yet, so the manual angle is being used.
- ERROR Enter a wall plate width greater than zero.
- ERROR Enter a roof angle greater than 0 and less than 90 degrees.
- INFO This is a general reference only. Verify local code and framing practice.
Detail values
- Seat cut run
- — in
- Seat cut depth
- — in
- Seat cut angle
- — deg
Assumptions
- Seat cut angle: Equal to the roof angle (matches Roof's angleDegrees output)
Included: seat cut run, depth, and angle.
Not included: labor, verification against manufacturer or local-code specifics beyond the reference table used, material waste from cutting.
Formula details
- Seat cut run: wallPlateWidth
The seat cut spans the full wall top-plate width.
- Seat cut depth: seatCutRun x tan(angle)
Geometric identity, not a sourced rule: the birdsmouth is the right triangle formed by the level seat cut and the plumb heel cut, so the heel depth is the seat run projected at the roof angle. No external source is needed for it.
Material List
About this calculator
Pull the roof angle from the active project or override it manually when you're framing from field notes.
FAQ
What is the seat cut depth?
The vertical depth of the notch cut into the rafter so it sits flush across the top of the wall plate. It's the wall plate width projected vertically at the roof angle.
Why does the rafter depth matter?
It's optional, used only to flag a reference warning if the seat cut would remove more than roughly a third of the rafter's depth -- a common general framing guideline, not a code requirement.
Disclaimer
This is a general reference only. Verify local code and framing practice before cutting.