Birdsmouth Cut Calculator: Seat Cut, Heel Depth, and Rafter Length

Your details

Total exterior width of the building from wall face to wall face.
ft
Actual thickness of the ridge board (typically 1.5 in for dimensional lumber).
in
How many inches the roof rises for every 12 inches of horizontal run (e.g. 6 for 6:12).
: 12
Horizontal distance of the notch that rests on the wall plate (typically 3.5 in for a 2x4 plate).
in
Horizontal distance the rafter extends beyond the outer wall face (the eave tail).
in
Heel cut depth2x6 rafter is sufficient
1.75

Vertical depth of the notch - the cut perpendicular to the seat cut

Rafter run143.25
Rafter rise71.625
Total rafter length173.575
Pitch angle26.57deg
Height above plate5.25
Min. rafter depth (IRC)5.25
Heel cut depth1.75
Min. rafter depth (IRC)5.25
Height above plate5.25

Heel cut depth: 1.750 in - minimum rafter depth: 5.250 in

  • Your heel cut depth is 1.750 in. The seat cut is 3.5 in. These two cuts together form the birdsmouth notch.
  • The IRC building code requires the notch depth not to exceed one-third of the rafter depth. Your minimum allowable rafter depth is 5.250 in.
  • Your roof pitch angle is 26.6 degrees. The plumb cut (ridge cut) and the heel cut are both made at this angle from vertical.
  • Total rafter length (including the overhang tail) is 173.575 in. Order your lumber at least this long.

Next stepVerify your heel cut depth does not exceed one-third of your chosen rafter board depth before cutting. Use a speed square set to the pitch angle to mark the plumb and seat lines on the rafter.

 = Powered by OnlyCalculators