Johnson Level & Tool Mfg. RAS-1 - Manual (Page 11)

9
TABLE A
Change Inches to Feet
1" = .08'
7" = .58'
2" = .16' 8" = .67'
3" = .25' 9" = .75'
4" = .33' 10" = .83'
5" = .42' 11" = .92'
6" = .50'
STEP 3. CALCULATE INCH RISE: For example, with the
Run = 13' 0" and Rise = 8' 9". First convert the rise to feet
in decimal form, using TABLE A. Therefore, the Rise =
8.75'. Now, using the formula for the “Inch Rise”:
8.75' x 12
Inch Rise = = 8.08 in/ft. Run
13'
Round off the Inch Rise to the nearest inch. Since rounding
off won’t make much difference in most cases. It will make the
actual roof rise slightly higher or lower. In this example, round-
ing to 8 will make the height 8' 8" instead of 8' 9". Rounding off
this figure has more of an effect on buildings with longer rafter
runs or higher roof rises. If the Inch Rise is given on a blueprint,
this calculation is not needed, just use the one given. Now, let
us do an example of each type of rafter.
COMMON RAFTERS
COMMON RAFTER LENGTH: After the Inch Rise and Run are
found, use the tables in the back of this book to find the com-
mon rafter length. In our example an 8 Inch Rise is used, there-
fore find the 8 Inch Rise table (on page 47) and go down the left
hand column headed “Run Ft.” and locate 13'. To the right, read
the column headed “Common Rafter Length” for the rafter