Johnson Level & Tool Mfg. RAS-1 - Manual (Page 28)

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The common scale on the square is used for all jack rafters.
Whenever a jack rafter rests against a hip or valley rafter, mark
a plumb cut, and then cut at a 45° along the mark. This will
give both the side cut and the plumb cut. Angles should be
checked for direction before cuts are made. See Figures 11 and
14 for the center common rafter at the end of the ridge.
DORMER RAFTERS
Sometimes when adding a room or remodeling, it is easier to
build a valley on top of the main roof. This saves cutting into the
main roof that could cause weakening.
STEP 1. Referring to Figure 1, mark location of valley on roof at
45° to common rafters. Then set long point of bottom end of
rafter even with the mark just made at line “A”.
STEP 2. PLUMB CUTS ON THE DORMER RAFTER: Using what-
ever inch rise has been determined, make them the same as the
common rafter.
STEP 3. RAFTER LENGTH: Once the shortest rafter is mea-
sured, the rafter length may be determined using the same
method as with jack rafters.
STEP 4. BOTTOM CUT: The heel or bottom cut is the same as
the horizontal cut of the seat notch (see Figure 7). But cut all
the way across the rafter (and without making a seat plumb
cut). The saw should be tilted at the same angle as the rise of
the roof. Thus allowing your horizontal cut to lay flat against the
roof. For example, using the common scale with an 8" rise, you
will notice that the number 8 lines up at 33 1/2° on the degree
scale. So tilt your saw at 33 1/2° before making the bottom cut.
DOOR OVERHANG
In Figure 1 is pictured a roof extension that may be found over
a doorway. To find the end cut at point “B,” hold the square so
that the “T” bar is flat against the rafter. Then using the com-
mon rafter scale, make a mark next to the inch rise that was
used for the main roof. Now draw a line from the mark just
made to the pivot point. This is then the cut required for a flat