Birdsmouth cut calculator — seat cut, plumb line and rafter notch geometry.
A birdsmouth is defined by the roof slope, the support location and the rafter section. The calculation should describe the notch in the context of the actual rafter, not as an isolated depth value.
What defines a birdsmouth?
The rafter crosses the support at the roof angle. The birdsmouth creates a bearing surface while preserving the sloping top line of the rafter. Geometrically, the seat cut is horizontal while the heel/plumb line follows the vertical reference through the support area.
Inputs for notch geometry
- roof pitch or angle,
- rafter section depth,
- support width and position,
- selected bearing reference,
- allowed notch detail from the structural design.
Why the reference point matters
A notch dimension without a measurement origin is easy to misread. Workshop documentation should show where the notch begins along the rafter and which surface or axis is being used for the measurement.
Seat width and notch depth
The required bearing and the permitted reduction in the rafter section are structural-detail questions, not just geometry. They must follow the project and applicable requirements. A calculator can describe the cut shape, but it should not invent a structurally acceptable notch depth.
Birdsmouth in Krokwio
Krokwio keeps the notch connected to the selected rafter, its support and its workshop card. This makes it easier to move from roof geometry to a specific physical element.
Related: Rafter Calculator, Roof Pitch Calculator.
Turn roof geometry into workshop-ready information.
Use one project for geometry, 2D/3D views, element cards, cutting data, PDF documentation and QR identification.
Birdsmouth layout: seat cut versus heel depth
The horizontal seat bears on the wall plate; the plumb or heel cut locates the intersection with the rafter. The chosen reference line, support position and timber depth all affect the dimensions transferred to the workshop drawing. A long horizontal seat and a deep notch are different measurements and should never be treated as interchangeable.
Worked geometry example
At a roof pitch of 30°, a horizontal 60 mm seat corresponds to a vertical change of approximately 34.6 mm along the sloping line (60 × tan 30°). This is a geometric relationship, not an approved notch depth: the permissible cut is governed by the structural design, timber section and local rules.
Common layout errors
Check whether dimensions refer to the top or bottom edge of the rafter and whether the bearing reference is taken at the inside or outside edge of the plate. Mark the cut orientation clearly before transferring it to timber.
Explore rafter length and pitch, full roof framing geometry, and workshop cutting documentation. See Krokwio PRO.