Construction Calculator - Roofing

Birdsmouth Planning Calculator

Estimate rafter length inputs used before marking a birdsmouth cut.

Reviewed: August 26, 2026

Calculate Birdsmouth Cut

For the Birdsmouth Planning Calculator, enter horizontal run, roof rise, and overhang. It reports rafter line length and cut length from run, rise, and overhang; rafter depth and plate width are not entered, so the birdsmouth notch itself is not sized.

Construction measuring tools arranged beside a residential building project

Formula used

Rafter length = (horizontal run + horizontal overhang) × √(run² + rise²) ÷ run

Sloped rafter bearing on a wall plate with the birdsmouth seat cut, heel, plumb cut, and overhang labeled.

Worked birdsmouth planning example

Birdsmouth Planning Calculator example: The basic slope factor is √(10.44² + 5.45²) ÷ 10.44 = 1.1281. Applying it to 11.44 ft of horizontal run gives 12.9 ft (154.86 in).

Read squares and sloped area

Use the displayed length only as a starting geometry check and lay out the birdsmouth from plans, framing details, and full-size measurements.

Geometry and accessories outside the total

Ridge adjustment, heel height, seat length, plumb cut, overhang detail, lumber depth, and allowable notch limits govern an actual birdsmouth.

Questions specific to this calculation

Does this page calculate the birdsmouth notch itself?

Enter horizontal run, rise, and overhang to estimate the sloped common-rafter length. The current form does not ask for rafter depth or wall-plate width.