Johnson Level & Tool Mfg. RAS-1 User Manual
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support rafter length. To begin with, we find a 2' run having
a 3' 9" length. This then leaves a 5" length which results in
an approximation of a 2-11/16" run. If needed, approximat-
ing can be done with the following equation, if desired:
Lm – Ll
( Rh x ( ) ) + Rl
Lh – Lm
Rm =
Lm – Ll
1.00 + ( )
Lh – Lm
Where R = run, L = length, l = low, m = middle, h = high.
In our example:
5.00 – 4.63
( 3.00 x ( ) ) + 2.50
5.63 – 5.00
Rm =
5.00 – 4.63
1.00 + ( )
5.63 – 5.00
= 2.69
= 2-11/16"
Thus we have for the support rafter run, 2' plus 2 11/16"
equalling 2' 2-11/16" (or 26.69"). Now to find the support
rafter rise:
Inch Rise x Run (in.)
SUPPORT RAFTER RISE (in.) =
12
19 inch rise x 26.69"
=
12
=
42.25"
Converted to feet:
= 3
' 6-1/4"
Next we need to find the horizontal overhang length
(see Figure 21). It is found by finding the roof run between
points A and B and then subtracting the support rafter run