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Calculating Rafter Lengths
Often times we are asked by our customers for rafter lengths to assist them with ordering metal roofing or calculating decking requirements. The following example and accompanying tables will enable
you to determine rafter lengths without the aid of a dimensional calculator.
Example: You have ordered trusses to be built on a 4 on 12 roof pitch that will span 23'-2" with a 12" overhang. How do you calculate the overall rafter length.
- To calculate rafter length, divide the span by 2 and add the overhang. Half the span equals 11'- 7", add the 12" overhang, and your result is 12'-7".
- Look up the corresponding value for 7" in the table below to convert it to a fraction. You can now convert 12'-7" to 12.583.
- To compute the rise of 12'-7" at a roof slope of 4 on 12, simply multiply 12.583 by the appropriate distance one foot rises at the pitch. Checking the second table, look up this multiplier for
the 4 on 12 pitch - 1.054092
Therefore, 12.538 times 1.054092 equals 13.216 or roughly 13'-3".
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