Birdsmouth Cut Calculator

This birdsmouth cut calculator works out the notch that lets a sloping rafter sit square and solid on a wall top plate. Enter the roof rise and run that set your pitch, the seat cut length, which is the flat bearing that rests on the plate and is usually the plate width, and the depth of your rafter, and the calculator returns the heel height you need to mark, the roof pitch angle, the sound timber remaining above the notch, and a check against the one third depth rule. The birdsmouth is a right-angled notch with two faces: a level seat cut that beds down on the top of the plate, and a vertical plumb cut, known as the heel cut, that bears against the outer face of the wall. The heel height is simply the seat length scaled by the roof slope, and the pitch angle is the exact setting you dial on a speed square to scribe both cuts, which sit ninety degrees apart. Getting the notch right keeps the roof plane true and stops you weakening the rafter, because cutting too deep removes timber the rafter needs to carry its load. New Zealand framing practice keeps the notch to no more than a third of the rafter depth, and the calculator flags when your numbers cross that line. The defaults describe a 190 mm rafter on a 150 in 300 pitch. Adjust them to your roof and read the worked example below.

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The 45 mm heel height is within the 63.3 mm limit, one third of the 190 mm rafter depth, so the birdsmouth meets the usual one third rule.

The pitch angle is the speed square setting for both the plumb (heel) cut and the level seat cut, which meet at ninety degrees. Check your span and load tables; estimate only.