Rafter Birdsmouth Cut Calculator
Seat depth, plumb cut angle, heel height, and IRC notch-depth check for any rafter pitch.
What a Birdsmouth Does
The birdsmouth transfers the full vertical load of the rafter into the wall plate through bearing on the seat cut, while the plumb cut keeps the rafter aligned against the outside face of the plate. Without a birdsmouth the rafter would only bear on its corner, concentrating thousands of pounds of dead and snow load on a half-inch of wood fiber.
Seat Cut vs Plumb Cut
The seat cut is horizontal and equal to the plate width: 3½ in for a 2×4 top plate, 5½ in for a 2×6 plate. The plumb cut is vertical and equal in height to the seat cut times tan(pitch). On a steep gambrel lower rafter (65–75°) the plumb cut becomes the deeper of the two. The calculator above checks both against the IRC limit.
seat_depth = plate_width · tan(pitch)heel_height = rafter_depth / cos(pitch) − seat_depthThe IRC One-Third Rule
IRC R802.7.1.1 limits the total notch depth in any sawn rafter to one-third of the rafter actual depth. The rule exists because the notch sits at the maximum-moment point of the rafter and a notch deeper than one-third triggers tension failure across the remaining cross-section under design loads. Steep gambrel lower rafters routinely fail this check on 2×8 lumber, so bump to 2×10 or 2×12 for any lower pitch above 60°.
Common Mistakes
Three mistakes show up over and over: cutting the seat too deep "to make the rafter sit better"; setting the plumb cut to vertical instead of to the pitch angle (which leaves a gap behind the rafter); and ignoring heel height, which forces the fascia and soffit to be furred out later. Use the calculator above before the first cut.