Octagon Cheek Cuts. In cutting the cheeks of jack rafters for a polygonal roof, the method illustrated by b, Fig. 79, should be employed. The distance ab should be measured parallel with the plumb cut, at the top of the rafter. The length of the jack rafter, found by the method described under Topic 55, Fig. 84, is shortened the distance equal to the pitch line of one half the diagonal of the thickness of the hip rafter, on the line of the intersection of the jack, as described in Topic 55. The plumb and seat cuts may be obtained from the actual rise and run.