Birdsmouth Cut Calculator

The birdsmouth notch is the small triangular notch cut into the underside of a common rafter where it sits on the top plate of the wall — named for its resemblance to an open beak. Getting this cut right ensures the rafter bears fully and evenly on the plate, the ridge is at the correct height, and the roof planes align perfectly. Getting it wrong means gaps under the rafter, a twisted roof plane, or structural bearing problems that are costly to correct after the rafters are up. The birdsmouth cut calculator provides the exact seat depth and heel cut angle for any roof pitch and rafter lumber.

Birdsmouth Geometry: The Two Cuts

A birdsmouth consists of two saw cuts that together create a flat bearing surface on the rafter:

• Plumb cut (vertical cut): made vertically when the rafter is in its installed position; this cut is parallel to the plumb line of the building; the depth of this cut (seat depth) determines how much of the rafter sits on the plate
• Level cut (seat cut / horizontal cut): made horizontally when the rafter is installed; this cut is parallel to the top plate surface and provides the bearing surface that rests on the plate

The angle of both cuts is determined by the roof pitch. For a 6/12 pitch (6 inches of rise per 12 inches of run), the rafter angle = arctan(6/12) = 26.57°. The plumb cut angle = 90° − rafter angle = 63.43°; the level cut angle = rafter angle = 26.57° from horizontal. Use this online calculator for any pitch and rafter size. The roof pitch calculator provides the rafter angle from rise/run ratios.

Seat Depth: The Critical Code-Controlled Dimension

The seat depth (plumb cut depth) must be carefully controlled. IRC (International Residential Code) requirements:

• Maximum notch depth = one-third of the rafter depth. For a 2×8 rafter (actual depth 7.25 inches): maximum seat depth = 7.25/3 = 2.42 inches
• The remaining rafter depth above the notch (heel height) must be sufficient to carry the roof loads without failure — typically at least two-thirds of the rafter depth

Practical seat depth: for a 2×4 double top plate (actual width 3.5 inches), the seat depth should be cut to match the full plate width or slightly less, creating full bearing on the plate without excessive notching. A common starting point: seat depth = plate width × cos(rafter angle). For 6/12 pitch with 3.5-inch plate: seat depth ≈ 3.5 × cos(26.57°) = 3.5 × 0.894 = 3.13 inches — within the one-third limit for 2×10 rafters but exceeding it for 2×8. Always verify the one-third rule for your specific rafter size and pitch combination.

Heel Height and Rafter Tail

The heel height — the distance from the bottom of the notch to the bottom edge of the rafter — determines the eave detail and whether the rafter tail clears the wall below. Heel height = rafter depth − seat depth. The rafter tail extends beyond the birdsmouth to form the eave overhang; its length depends on the desired soffit width and must be calculated from the roof geometry to achieve the specified horizontal overhang.

Common Errors and How to Avoid Them

The most common birdsmouth mistakes: cutting the seat cut at a slight angle (not perfectly level) causing the rafter to rock on the plate; cutting the seat depth too deep and violating the one-third rule; measuring rafter layout from the wrong reference point at the ridge; and not accounting for the ridge board thickness when calculating rafter length. Using a framing square (rafter square) with the rise/run marks, or a speed square set to the roof angle, eliminates angle errors. Always cut a test birdsmouth on a scrap piece and test-fit on the plate before cutting the full rafter run. The roof shingle calculator and roofing calculators provide complementary roof construction tools.