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Birdsmouth Cut Calculator

Use this birdsmouth cut calculator to find the different essential dimensions and measurements on a rafter when creating a birdsmouth cut.

Clear
Plumb cut height1 3/4 in1.75 in — the vertical face of the notch
Seat cut length3 1/2 inthe level face, matching the plate width
Height above plate (HAP)4 3/8 in4.3992 in, measured plumb
Rafter depth measured plumb6.1492 in5.5 in ÷ cos 26.57°
Material left above the notch3.9348 inmeasured square to the rafter
Notch depth checkwithin the third rulethe notch should not exceed a third of the rafter depth — 1.833 in here
Pitch angle26.565°
Saw settings26.57° plumb, 63.43° level

The method

plumb cut = seat × rise ÷ run; HAP = depth ÷ cos θ − plumb cut

Never cut more than a third

The birdsmouth is a notch, and a notch removes material exactly where the rafter is carrying load. The standard limit is a third of the rafter depth — cut deeper and you have created a hinge at the wall, which is where the bending moment is highest.

The height above plate is what actually matters on site, because every rafter must have the same HAP or the roof plane will not be flat. Note that the rafter’s depth measured plumb is greater than its actual depth by a factor of 1 ÷ cos θ, which is where the arithmetic surprises people: a 2×6 at 6 in 12 measures 6.15 inches plumb, not 5.5.