Birdsmouth Cut Calculator

A birdsmouth is the notch cut near the bottom of a rafter so it sits flat on the top plate of the wall instead of teetering on a point. It has two cuts: a horizontal seat cut that bears on the plate, and a vertical plumb cut, sometimes called the heel cut, that faces the wall. Get the two right and the rafter drops onto the wall and sits tight. Get them wrong and the roof line wanders.

Both cuts come straight from the roof pitch. Pitch is given as a rise over a 12-inch run, so a 6/12 roof climbs 6 inches for every 12 it travels. The pitch angle is the arctangent of rise over run. The plumb cut height is the seat length times the pitch, so a 3.5-inch seat on a 6/12 roof gives a heel of 1.75 inches. The seat length is usually the width of the wall plate the rafter lands on, 3.5 inches for a 2x4 wall.

There is one rule that keeps the rafter strong. The notch should not cut away more than a third of the rafter’s depth, measured square to the rafter edge. This calculator works out that notch depth and checks it against the one-third limit, so you know before you saw whether a deeper seat would weaken the rafter. It also gives the height above plate, the amount of rafter standing above the seat, which sets where your overhang and fascia land.

Enter the pitch, the seat length, and the rafter depth, and you get the plumb cut, the seat cut, the pitch angle, the notch depth with its limit check, and the height above plate. Mark the cuts with a framing square set to the same rise and run.