Johnson Level & Tool Mfg. RAS-1 - Manual (Page 16)

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building. Now line up the dashed line (above the 80° incre-
ment on the square) with the plumb mark (see Figure 7A & 7B).
Draw the horizontal seat mark, a perpendicular line, along
the bottom of square. Never notch more than halfway
through the rafter and make all seat notches the same
depth.
STEP 3. TAIL OR BOTTOM PLUMB CUT: Using your tail
length (2' 1-3/8" in our example), measure down the rafter
from the seat plumb line and mark the bottom plumb line
FIG. 7A
Seat Notch or
Bird’s Mouth Layout.
FIG. 7B
Seat Notch or
Bird’s Mouth Layout.