Calculate rafter length, birdsmouth cut, ridge height and tail length for common rafters.
Step 1: Enter the building span (wall to wall, outside to outside).
Step 2: Enter the total rise from the wall plate to the ridge. Leave blank for a default 6:12 pitch.
Step 3: Enter the eave overhang (horizontal projection) if applicable.
Step 4: Optionally adjust the ridge board thickness (default 1.5"/38mm).
Results include the rafter length along the slope, tail length for the overhang, birdsmouth cut dimensions, and ridge height.
A birdsmouth (or bird's mouth) is a notch cut into the rafter where it sits on the wall plate. It consists of a seat cut (horizontal) and a plumb cut (vertical), creating a secure connection.
Rafter length depends on the building span, roof pitch, and overhang. This calculator computes the exact length on the slope plus any tail for the eave overhang.
The ridge board sits at the peak between opposing rafters. Half its thickness is subtracted from each rafter's horizontal run so the rafters meet correctly at the ridge.