The height of a common rafter, from the top of the working
point plumb line to the birdsmouth seat cut, must be determined before
the hip rafter birdsmouth can be laid out and cut. See figure 6
and "How To
Frame Gable Roofs". Because a hip rafter is 50 mm deeper than a common
rafter and its working point lands on the corner of a building, the
hip rafter will be higher above the wall plate than the common
rafter.