Johnson Level & Tool Mfg. RAS-1 - Manual (Page 14)

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a tail run of 1' 9". In the same “Common” table as used
above, for an 8” rise, we find:
1' Run = 1' 2-1/2"
9" Run = 10-7/8"
1 ' 9" Tail = 2' 1-3/8"
COMMON RAFTER LUMBER LENGTH: To obtain the total
length of the rafter, an allowance for the bottom plumb cut
must now be made (see Figure 4). This allowance can be
obtained by using the tables. In our example, let us use a 2 x
6 piece of lumber which will give us a depth of 5 1/2 inches.
From the bottom allowance table (using 8" rise again), the
common and jack allowance equals 3-11/16 inches. The total
common rafter lumber length then becomes:
RAFTER LENGTH = 15' 7-1/2"
TAIL = 2' 1-3/8"
BOTTOM ALLOWANCE = 3-11/16"
TOTAL LUMBER LENGTH = 18' 0 9/16"
COMMON RAFTER LAYOUT
STEP 1. TOP PLUMB CUT: In choosing the side of your
rafter to use, put the crown (high side) up if the rafter
is not straight. Now starting at the top of the rafter, lay
our square on the face of the rafter so that the “T” bar
is draped over the top edge of the rafter (see Figure 6).
While holding the pivot point firmly against the edge of
the rafter, pivot the square so that the number 8 on the
common scale lines up with the edge of the rafter. Starting
from the pivot point, mark the top plumb cut line along
the top edge of the square.
STEP 2. SEAT NOTCH OR BIRD’S MOUTH: With your rafter
length (in our example 15' 7-1/2"), measure down along the
top edge of the rafter and mark another plumb line as you
did in Step 1. This line represents the outside wall of your