Figure 2-17.Rafter length.
Look at the first line of the rafter table on a framing
square to find LENGTH COMMON RAFTERS PER
FOOT RUN (also known as the bridge measure). Since
the roof in this example has a 7-inch unit of rise, locate
the number 7 at the top of the square. Directly beneath
the number 7 is the number 13.89. This means that a
common rafter with a 7-inch unit of rise will be
13.89 inches long for every unit of run. To find the
length of the rafter, multiply 13.89 inches by the
number of feet in the total run. (The total run is always
one-half the span.) The total run for a roof with a 16-foot
span is 8 feet; therefore, multiply 13.89 inches by 8 to
find the rafter length. Figure 2-17 is a schematic of this
If a framing square is not available, the bridge
measure can be found by using the Pythgorean theorum
root of 193 is 13.89.
Two steps remain to complete the procedure.
Step 1. Multiply the number of feet in the total run
(8) by the length of the common rafter per
foot of run (13.89 inches):
Step 2. To change .12 of an inch to a fraction of
an inch, multiply by 16:
The number 1 to the left of the decimal point
represents 1/16 inch. The number .92 to the right of the
decimal represents ninety-two hundredths of 1/16 inch.
For practical purposes, 1.92 is calculated as being equal
to 2 x 1/16 inch, or 1/8 inch. As a general rule in this
kind of calculation, if the number to the right of the
decimal is 5 or more, add 1/16 inch to the figure on the
left side of the decimal. The result of steps 1 and 2 is a
total common rafter length of 111 1/8 inches, or 9 feet
3 1/8 inches.
Example 2. A roof has a 6-inch unit of rise and a
25-foot span. The total run of the roof
is 12 feet 6 inches. You can find the
rafter length in four steps.
Change 6 inches to a fraction of a foot by
placing the number 6 over the number 12:
(1/2 foot = 6 inches).
Change the fraction to a decimal by
dividing the bottom number (denomi-
nator) into the top number (numerator):
(.5 foot = 6 inches).
Multiply the total run (12.5) by the length
of the common rafter per foot of run
(13.42 inches) (fig. 2-16):
Step 4. To change .75 inch to a fraction of an inch,
multiply by 16 (for an answer expressed
in sixteenths of an inch).
.75 x 16 = 12
The result of these steps is a total common rafter
length of 167 3/4 inches, or 13 feet 11 3/4 inches.