Birdsmouth Cut Calculator
Enter your roof pitch and rafter size to get exact seat depth, seat width, and IRC R802.7.1 compliance check.
Understanding the Birdsmouth Cut
A birdsmouth cut is a notch on the underside of a rafter where it bears on the wall's top plate. It consists of two perpendicular cuts: the heel cut (vertical, at the pitch angle) and the seat cut (horizontal, creating a level bearing surface on the plate).
Seat Width Formula
Because the rafter's bottom surface runs at the pitch slope, a deeper seat cut also means a wider one. The relationship is: Seat Width = Seat Depth × (12 ÷ Rise). For a 6:12 pitch with a 2" seat depth: 2 × (12 ÷ 6) = 4" wide.
IRC R802.7.1 Limit
The IRC limits the birdsmouth depth to no more than one-third of the rafter's actual depth. For a 2×8 (7.25" actual), the maximum is 2.417". Exceeding this weakens the rafter cross-section at the bearing point and fails the code check.
Marking with a Speed Square
Set your speed square to the pitch number (e.g., 6 for a 6:12). Place the pivot on the rafter edge and rotate until that number hits — that's your plumb line for the heel cut. Slide to your seat depth mark and draw the seat cut horizontally at 0°.
Birdsmouth Reference Table — All Pitches
Based on 2×8 rafter (7¼" actual), 2" seat depth. Max seat = 2.417" for IRC compliance.
| Pitch | Angle | Seat Width @ 2" | Speed Square | Max Seat (2×8) | IRC Status |
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Frequently Asked Questions
What is a birdsmouth cut on a rafter?
A birdsmouth is the notch cut into the bottom of a rafter so it sits flat and secure on the wall's top plate. Without it, the rafter would bear on only one edge and be prone to rolling. The notch is made with a vertical heel cut and a horizontal seat cut, and the depth is limited by IRC R802.7.1 to prevent over-weakening the rafter.
How deep should a birdsmouth cut be?
No deeper than one-third of the rafter's actual depth, per IRC R802.7.1. Many framers target 1.5" to 2" on a 2×8, which gives a solid bearing area well within the code limit. Going deeper than the maximum weakens the rafter right at its most-loaded point.
Does seat depth change the seat width?
Yes. The deeper the seat, the wider it becomes, because the rafter bottom slopes at the pitch angle. Use: Seat Width = Seat Depth × (12 ÷ Rise). On a 4:12 pitch, every 1" of seat depth produces 3" of seat width — on a 12:12, only 1" of width per inch of depth.
Is the heel cut angle the same as the roof pitch angle?
Yes. The heel cut is the plumb cut — the same angle you use to cut the ridge end of the rafter. For a 6:12 pitch it's 26.6° from vertical (or 63.4° from horizontal). Your speed square's pitch marks set this angle directly.