Planning & EstimatingQuestion 533 of 700
A gable roof has a total span of 24 ft and a slope of 6 in 12 (a 6-inch rise for every 12 inches of run). Ignoring overhang, what is the approximate theoretical length of a common rafter from ridge to top plate?
a.About 13.4 ft
b.About 12.0 ft
c.About 15.0 ft
d.About 18.0 ft
Explanation
The run is half the span: 24 / 2 = 12 ft. A 6-in-12 slope rises 6 inches per 12 inches of run, so over a 12 ft run the rise is 6 ft. The rafter is the hypotenuse: length = square root of (12 squared + 6 squared) = square root of (144 + 36) = square root of 180 = about 13.4 ft. Option b uses only the run. Option c and d overstate the rise. Use the Pythagorean theorem with run and rise, not the full span.
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