Figure 2-37.Intersecting roof with unequal spans.
formed by the intersecting ridges. Valley jack rafters run
roofs, but they are quite rare and require special framing
from the valley rafters to both ridges. Hip-valley cripple
jack rafters are placed between the valley and hip rafters.
An intersecting roof with unequal spans requires a
supporting valley rafter to run from the inside corner
formed by the two sections of the building to the main
ridge (fig. 2-37). A shortened valley rafter runs from the
other inside comer of the building to the supporting
valley rafter. Like an intersecting roof with equal spans,
one with unequal spans also requires valley jack rafters
and hip-valley cripple jack rafters. In addition, a valley
cripple jack rafter is placed between the supporting and
shortened valley rafters. Note that the ridgeboard is
lower on the section with the shorter span.
Valley rafters run at a 45° angle to the outside walls
of the building. This places them parallel 10 the hip
rafters. Consequently, they are the same length as the
A valley rafter follows the line of intersection
between a main-roof surface and a gable-roof addition
or a gable-roof dormer surface. Most roofs having
valley rafters are equal-pitch roofs, in which the pitch
of the addition or dormer roof is the same as the pitch
of the main roof. There are unequal-pitch valley-rafter
In the discussion of valley rafter layout, it is
assumed that the roof is equal pitch. Also, the unit of run
and unit of rise of an addition or dormer common rafter
are assumed to be the same as the unit of run and rise of
a main-roof common rafter. In an equal-pitch roof, the
valley rafters always run at 45° to the building lines and
the ridge pieces.
Figure 2-38 shows an equal-span framing situation,
in which the span of the addition is the same as the span
of the main roof. Since the pitch of the addition roof is
the same as the pitch of the main roof, equal spans bring
the ridge pieces to equal heights.
Looking at the roof framing diagram in the figure,
you can see the total run of a valley rafter (indicated by
AB and AC in the diagram) is the hypotenuse of a right
triangle with the altitude and base equal to the total run
of a common rafter in the main roof. The unit of run of
a valley rafter is therefore 16.97, the same as the unit of
run for a hip rafter. It follows that figuring the length of
an equal-span valley rafter is the same as figuring the
length of an equal-pitch hip roof hip rafter.
A valley rafter, however, does not require backing
or dropping. The projection, if any, is figured just as it
is for a hip rafter. Side cuts are laid out as they are for a