Figure 2-31.-Shortening a hip rafter.
Rafter Shortening Allowance
As in the case with a common rafter, the line length
of a hip rafter does not take into account the thickness
of the ridge piece. The size of the ridge-end shortening
allowance for a hip rafter depends upon the way the
ridge end of the hip rafter is joined to the other structural
members. As shown in figure 2-31, the ridge end of the
hip rafter can be framed against the ridgeboard (view A)
or against the ridge-end common rafters (view B). To
calculate the actual length, deduct one-half the 45°
thickness of the ridge piece that fits between the rafters
from the theoretical length.
When no common rafters are placed at the ends of
the ridgeboard the hip rafters are placed directly against
the ridgeboard. They must be shortened one-half the
length of the 45° line (that is, one-half the thickness of
the ridgeboard When common rafters are placed at the
ends of the ridgeboard (view B), the hip rafter will fit
between the common rafters. The hip rafter must be
shortened one-half the length of the 45° line (that is,
one-half the thickness of the common rafter).
If the hip rafter is framed against the ridge piece, the
shortening allowance is one-half of the 45° thickness of
the ridge piece (fig. 2-31, view C). The 45° thickness of
stock is the length of a line laid at 45° across the
thickness dimension of the stock. If the hip rafter is
framed against the common rafter, the shortening
allowance is one-half of the 45° thickness of a common
rafter.
To lay off the shortening allowance, first set the
tongue of the framing square to the line length ridge cut
line. Then, measure off the shortening allowance along
the blade, set the square at the mark to the cut of the
rafter (8 inches and 17 inches), draw the actual ridge
plumb cut line. (To find the 45° thickness of a piece of
lumber, draw a 450 line across the edge, and measure
the length of the line and divide by 2.)
Rafter Projection
A hip or valley rafter overhang, like a common
rafter overhang, is figured as a separate rafter. The
projection, however, is not the same as the projection of
a common rafter overhang in the same roof. The
projection of the hip or valley rafter overhang is the
hypotenuse of a right triangle whose shorter sides are
each equal to the run of a common rafter overhang
(fig. 2-32). If the run of the common rafter overhang is
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