Figure 2-17.Rafter length.

Gable

Figure 2-19.-Steel square used to lay out plumb and seat cuts.

Builder 3&2 Volume 02 - Construction manual for building structures
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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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