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Rafter Shortening Allowance | Up
Builder 3&2 Volume 02 - Construction manual for building structures | Next
Bird’s-Mouth |
Figure 2-32.—Run of hip rafter projection.
18 inches for a roof with an 8-inch unit of rise, the length
of the hip or valley rafter tail is figured as follows:
1.
2.
Find the bridge measure of the hip or valley
rafter on the framing square (refer to figure
2-16). For this roof, it is 18.76 inches.
Multiply the bridge measure (in inches) of the
hip or valley rafter by the projection (in feet) of
the common rafter overhang:
3. Add this product to the theoretical rafter length.
The overhang may also be stepped off as described
earlier for a common rafter. When stepping off the
length of the overhang, set the 17-inch mark on the blade
of the square even with the edge of the rafter. Set the
unit of rise, whatever it might be, on the tongue even
with the same rafter edge.
Rafter Side Cuts
Since a common rafter runs at 90° to the ridge, the
ridge end of a common rafter is cut square, or at 90° to
the lengthwise line of the rafter. A hip rafter, however,
joins the ridge, or the ridge ends of the common rafter,
at other than a 90° angle, and the ridge end of a hip rafter
must therefore be cut to a corresponding angle, called a
side cut. The angle of the side cut is more acute for a
high rise than it is for a low one.
The angle of the side cut is laid out as shown in
figure 2-33. Place the tongue of the framing square
along
Figure 2-33.—Laying out hip rafter side cut.
the ridge cut line, as shown, and measure off
one-half the thickness of the hip rafter along the blade.
Shift the tongue to the mark, set the square to the cut of
the rafter (17 inches and 8 inches), and draw the plumb
line marked “A” in the figure. Then, turn the rafter
edge-up, draw an edge centerline, and draw in the angle
of the side cut, as indicated in the lower view of figure
2-33. For a hip rafter to be framed against the ridge, there
will be only a single side cut, as indicated by the dotted
line in the figure. For one to be framed against the ridge
ends of the common rafters, there will be a double side
cut, as shown in the figure. The tail of the rafter must
have a double side cut at the same angle, but in the
reverse direction.
The angle of the side cut on a hip rafter may also be
laid out by referring to the unit length rafter table on the
framing square. (Look ahead to figure 2-41.) You will
see that the bottom line in the table is headed SIDE CUT
HIP OR VALLEY USE. If you follow this line over to
the column headed by the figure 8 (for a unit of rise of
8), you will find the figure 10 7/8. If you place the
framing square faceup on the rafter edge with the tongue
on the ridge-end cut line, and set the square to a cut of
10 7/8 inches on the blade and 12 inches on the tongue,
you can draw the correct side-cut angle along the
tongue.
2-23
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