Figure 2-17.—Rafter length.Look at the first line of the rafter table on a framingsquare to find LENGTH COMMON RAFTERS PERFOOT RUN (also known as the bridge measure). Sincethe roof in this example has a 7-inch unit of rise, locatethe number 7 at the top of the square. Directly beneaththe number 7 is the number 13.89. This means that acommon rafter with a 7-inch unit of rise will be13.89 inches long for every unit of run. To find thelength of the rafter, multiply 13.89 inches by thenumber of feet in the total run. (The total run is alwaysone-half the span.) The total run for a roof with a 16-footspan is 8 feet; therefore, multiply 13.89 inches by 8 tofind the rafter length. Figure 2-17 is a schematic of thisprocedure.If a framing square is not available, the bridgemeasure can be found by using the Pythgorean theorumroot 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 perfoot of run (13.89 inches):Step 2. To change .12 of an inch to a fraction ofan inch, multiply by 16:The number 1 to the left of the decimal pointrepresents 1/16 inch. The number .92 to the right of thedecimal represents ninety-two hundredths of 1/16 inch.For practical purposes, 1.92 is calculated as being equalto 2 x 1/16 inch, or 1/8 inch. As a general rule in thiskind of calculation, if the number to the right of thedecimal is 5 or more, add 1/16 inch to the figure on theleft side of the decimal. The result of steps 1 and 2 is atotal common rafter length of 111 1/8 inches, or 9 feet3 1/8 inches.Example 2. A roof has a 6-inch unit of rise and aStep 1.Step 2.Step 3.25-foot span. The total run of the roofis 12 feet 6 inches. You can find therafter length in four steps.Change 6 inches to a fraction of a foot byplacing the number 6 over the number 12:(1/2 foot = 6 inches).Change the fraction to a decimal bydividing the bottom number (denomi-nator) into the top number (numerator):(.5 foot = 6 inches).Multiply the total run (12.5) by the lengthof 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 expressedin sixteenths of an inch)..75 x 16 = 12The result of these steps is a total common rafterlength of 167 3/4 inches, or 13 feet 11 3/4 inches.2-12

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