Calculate birdsmouth seat cut depth, plumb cut angle, and notch dimensions for common roof rafters. IRC R802.6 compliance check.
Enter the actual rafter depth in inches (7.25 for a 2×8, 9.25 for a 2×10), the roof pitch angle in degrees, and the wall plate width — typically 3.5 inches for a 2×4 top plate. The seat cut is made equal to the plate width so the rafter bears fully on the wall.
The plumb cut height follows from the geometry: seat width × tan(pitch angle). At 26.6° (a 6/12 pitch) a 3.5 inch seat produces a plumb cut of about 1.75 inches, leaving a Height Above Plate (HAP) of 5.5 inches on a 2×8 rafter.
The notch check follows IRC R802.6: the cut depth must not exceed one-third of the rafter depth. For a 2×8 the limit is about 2.42 inches; the calculator flags a warning whenever the plumb cut exceeds that limit, which happens on steep pitches or wide plates.
HAP is the rafter depth remaining above the seat after the notch is cut: rafter depth minus the plumb-cut height. It is the structural depth that actually carries bending across the wall plate, so framers mark it on every rafter to keep the roof plane consistent.
The calculator applies the one-third rule from IRC R802.6: notch depth may not exceed rafter depth ÷ 3. That allows roughly 2.42 inches on a 2×8 (7.25 in deep) and 3.08 inches on a 2×10 (9.25 in deep).
Notch depth equals plate width × tan(angle), and the tangent grows quickly with pitch. At 45° a full 3.5 inch seat produces a 3.5 inch notch — more than one-third of even a 2×10 — so a shallower seat or a deeper rafter is required.
Use the width of the bearing surface, normally the top plate: 3.5 inches for 2×4 framing or 5.5 inches for 2×6 framing. The calculator sets the seat cut equal to this value, so a wider plate directly increases the plumb-cut depth.