Rafter Birdsmouth Cut & HAP Calculator

How to use it: enter roof pitch, actual rafter depth, wall-plate width and the notch limit you intend to check. The calculator translates pitch into seat length, vertical notch depth and height above plate (HAP).

Formula. Angle = atan(rise / run). The horizontal seat bearing equals the entered plate width. Vertical notch depth = plate width x tan(angle). HAP = rafter depth - notch depth.

Worked example. At 4 in 12 pitch with an 89 mm plate, vertical notch depth is 89 x (4/12) = 29.67 mm. A 184 mm rafter retains 154.33 mm HAP.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Pitch rise

4

Pitch run

12

Rafter depth

184 mm

Wall plate width

89 mm

Your maximum notch fraction

0.3333

Primary result: Horizontal seat bearing: 89.0 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This is cut geometry only. It does not size the rafter, account for ridge and overhang cuts, or certify a notch under any building code. Accepted notch limits and bearing details vary. Enter your own limit and have structural work checked locally.