Roof Framing Tools
Birdsmouth Calculator
Calculate the exact notch depth and Height Above Plate (HAP) for your common rafters. Enter the material depth, roof pitch, and horizontal seat length (bearing on the plate) to ensure all rafters sit flat and provide a consistent roof plane.
Birdsmouth Cut Calculator
Estimate HAP and notch depth from pitch, rafter size, and seat length
Results
Enter dimensions above to calculate
Birdsmouth Cut Anatomy
The birdsmouth consists of a level seat cut and a vertical heel (plumb) cut. The leftover vertical material is the HAP (Height Above Plate).
Step-by-Step Formulas
1. Pitch to Roof Angle
Birdsmouth layout depends on the roof slope. The calculator converts pitch (rise over run) into an angle so it can translate the bearing seat length into a vertical notch depth.
2. Notch Depth from Seat Length
The seat cut length (the horizontal bearing on the plate) creates a run. Over that run, the roof rises by the tangent of the pitch angle, which determines the depth of the birdsmouth notch.
3. Height Above Plate (HAP)
HAP is the vertical height from the top of the wall plate to the top of the rafter at the outside edge of the wall. Keeping HAP consistent across all rafters ensures a flat roof plane.
4. Notch Depth Rule of Thumb
A common framing rule is to keep the birdsmouth notch shallower than one-third of the rafter depth to avoid weakening the rafter. Always follow local code and structural engineering requirements.
Worked Example
Standard framing: 2×8 rafter (184 mm depth), 6/12 pitch (~26.57°), 100 mm seat length.
💡 Pro tip
When cutting rafters, use a framing square with stair gauges to keep the birdsmouth consistent. Even a 2 mm difference in HAP can create visible dips or bumps in your roof decking. If the HAP is too small, you may need a deeper rafter or a steeper pitch.
Frequently Asked Questions
- What is a birdsmouth cut?
- A birdsmouth is a notch cut into the bottom of a rafter so it can sit flat on a wall plate. It typically includes a level (seat) cut and a plumb (heel) cut.
- How much can I notch a rafter for a birdsmouth?
- A common rule of thumb is to limit the birdsmouth notch depth to about one-third of the rafter depth. Requirements vary by jurisdiction and loading; follow local code or an engineer's guidance.
- What does HAP mean in roof framing?
- HAP stands for Height Above Plate. It is the vertical distance from the top of the wall plate to the top of the rafter at the outside edge of the wall.