Quantcast Figure 2-17.Rafter length.

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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 procedure. 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 Step 1. Step 2. Step 3. 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. 2-12

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