Figure 2-34.-Backing or dropping a hip rafter: A. Marking the top (plumb) cut and the seat (level) cut of a hip rafter; B. Determining
amount of backing or drop; C. Bevel line for backing the rafter; D. Deepening the bird’s-mouth for dropping the rafter.
Bird’s-Mouth
Laying out the bird’ s-mouth for a hip rafter is much
the same as for a common rafter. However, there are a
couple of things to remember. When the plumb (heel)
cut and level (seat) cut lines are laid out for a
bird’s-mouth on a hip rafter, set the body of the square
at 17 inches and the tongue to the unit of rise (for
example, 8 inches-depending on the roof pitch)
(fig. 2-34, view A). When laying out the depth of the
heel for the bird’s-mouth, measure along the heel plumb
line down from the top edge of the rafter a distance equal
to the same dimension on the common rafter. This must
be done so that the hip rafter, which is usually wider than
a common rafter, will be level with the common rafters.
If the bird’s-mouth on a hip rafter has the same depth
as the bird’s-mouth on a common rafter, the edge of the
hip rafter will extend above the upper ends of the jack
rafters. You can correct this by either backing or
dropping the hip rafter. Backing means to bevel the top
edges of the hip rafter (see fig. 2-35). The amount of
backing is taken at a right angle to the roof surface on
2-24
