Johnson Level & Tool Mfg. RAS-1 - Manual (Page 40)

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From this we can get the overhang rafter length, noting
that we are using 2 x 6 rafters giving a thickness of 1-1/2":
ROOF RUN BETWEEN A & B
– SUPPORT RAFTER RUN
– FRONT OVERHANG THICKNESS
= OVERHANG RAFTER LENGTH
63.37"
– 26.69"
– 1.50"
= 35.18"
or in feet:
= 2 ' 11 3/16"
Lastly we need to find the pitched rafter length, the rafter above
the horizontal overhang rafter. In our example, the pitched rafter
length will equal the support rafter length (i.e., 50").
STEP 2. RAFTER LAYOUTS: The support rafter bottom cut
layout (at point A on Figure 21) will be made similar to a
pitched door overhang. Drape the square “T” over the top (or
outside) edge of the support rafter and hold the square firmly
against the rafter. Then make a mark, using the angle scale,
next to the angle that is found by subtracting the roof pitch
angle (the roof inch rise in degrees) from the support rafter
inch rise angle. In our example, we have 57 3/4° (for the 19 inch
rise support) minus 33 3/4° (for the 8 inch rise roof) or 24°.
From this mark, draw a line to the pivot point and you will then
have the bottom cut layout. The support rafter top cut layout
(at point D in Figure 21) is similar to the bird’s mouth layout.
First measure up the outside of the rafter the rafter length
from the bottom cut and then draw a plumb line using the sup-
port rafter inch rise. Now, instead of measuring down the seat
depth, measure down the overhang rafter width and make a
horizontal line going to the roof edge of the rafter.
The horizontal overhang rafter layout is identical to the layout
for a flat roof extension (hence its name).