Birdsmouth Cut Calculator

A birdsmouth cut is a notch cut on the underside of a rafter where it sits on the wall plate. It consists of a horizontal seat cut and a vertical plumb cut (heel cut) that allows the rafter to rest securely on the wall while maintaining proper roof pitch.

The birdsmouth cut provides a stable bearing surface for the rafter on the wall plate, prevents the rafter from sliding down the roof, transfers the roof load directly to the wall, and maintains the structural integrity of the rafter system.

The heel cut depth is calculated using the roof pitch and seat cut depth. It equals the seat cut depth multiplied by the rise over run ratio (pitch/12). This ensures the cuts are perpendicular and the rafter sits properly on the wall plate.

The ridge board should be at least as deep as the rafter cut depth at the ridge. For 2x8 rafters, use at least a 2x8 ridge board. For 2x10 rafters, use at least a 2x10 ridge board to provide adequate bearing surface.

While not always required by code for smaller structures, birdsmouth cuts are highly recommended for proper load transfer and structural stability. They're essential for spans over 8 feet and required by most building codes for residential construction.

The birdsmouth cut should not exceed 1/3 of the rafter depth. For a 2x8 rafter (7.25" deep), the maximum cut depth is about 2.4 inches. Deeper cuts can significantly weaken the rafter and may violate building codes.

You'll need a circular saw or hand saw for the initial cuts, and a jigsaw or handsaw to complete the notch. A speed square helps mark accurate angles, and a measuring tape ensures precise dimensions.

Use a framing square to mark the plumb cut line at the correct angle, then measure and mark the seat cut depth perpendicular to the bottom of the rafter. The intersection of these lines defines your birdsmouth cut.