Construction

Birdsmouth Cut Calculator

Find the birdsmouth (rafter notch) cut angles.

Calculated Output

Plumb cut angle (°) 26.565

Seat cut angle (°) 63.435

Seat cut depth 0.4472

Formula: plumb cut = atan(rise ÷ 12); seat = 90 − plumb

Step-by-Step Breakdown

1. Values used:

2. Pitch rise (in 12) = 6 /12

3. Rafter width = 1

5. Plumb cut angle = 26.565°

6. Seat cut angle = 63.435°

7. Seat cut depth = 0.4472

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The math behind it

The plumb (vertical) cut angle = arctan(rise ÷ 12); the seat (level) cut is its complement (90° − plumb).

6/12 pitch → plumb cut 26.6°, seat cut 63.4°.

Seat depth?

Keep the notch shallow (≤ ~1/3 of rafter depth) to preserve strength.