Birdsmouth Cut Calculator

Birdsmouth Cut Calculator for Roof Rafters

A birdsmouth cut is a notch on the underside of a rafter that lets the rafter sit flat on the wall top plate while keeping the overhang intact. This calculator finds rafter run, rise, segment length, total rafter length, heel cut depth, and IRC minimum rafter depth from building width, ridge board thickness, roof pitch, seat cut length, and overhang.

Key rafter dimensions

Rafter run from half the building span minus ridge thickness:

$$X = \frac{W - t_{ridge}}{2}$$

Rise, segment length, and total rafter length:

$$Y = X \times \tan(\theta)$$

$$L_{segment} = \frac{X}{\cos(\theta)}$$

$$L_{rafter} = L_{segment} + \frac{O}{\cos(\theta)}$$

Birdsmouth cut depth

Heel cut depth from seat cut length and pitch:

$$c_h = c_s \times \tan(\theta)$$

IRC guidance requires rafter depth at least four times the heel cut depth. The calculator also reports minimum ridge board depth and height above the plate.

Related tools: Roofing Calculator, Angle Cut Calculator.