A seat cut is a cut made in a rafter, specifically at the bottom of the rafter which will allow it to fit squarely on the top plate.
Why is a Seat Cut Necessary?
A seat cut is performed for the reason of allowing an even and flush mounting between a rafter and the truss that the rafter is mounted to. This allows the rafter to maintain an even and equal balance on the truss which gives the rafter a great amount of strength and balance. This leads to the rafter being able to support a greater amount of overall weight and insures that the rafter will maintain the position that it is set at to hold the roof line up evenly.
This seat cut process is done by making a diagonal cut along the end of the rafter directly proportional to the angle the rafter should have. This angle of the seat cut will depend on the slop line of the roof and varies widely from structure to structure. The higher the roof line the greater the angle of the seat cut will be.