A simple roof rafter has a horizontal run of 12 ft and a rise of 5 ft from plate to ridge. Ignoring overhang and ridge deductions, what rafter length follows from the right triangle?
Correct Answer
B) 13 ft
Using the Pythagorean theorem, rafter length is sqrt(12^2 + 5^2) = sqrt(169) = 13 ft. Other choices add run and rise, round without the square-root calculation, or use the wrong triangle.
Why This Is the Correct Answer
Using the Pythagorean theorem, rafter length is sqrt(12^2 + 5^2) = sqrt(169) = 13 ft. Other choices add run and rise, round without the square-root calculation, or use the wrong triangle.
Why the Other Options Are Wrong
Option A: 14 ft
rafter length, choice b: 14 ft adds an extra unstated add-on/subtraction; the numeric gap is 1.
Option C: 12.5 ft
rafter length, choice c: 12.5 ft omits a required add-on/subtraction; the numeric gap is 0.5.
Option D: 17 ft
rafter length, choice d: 12 + 5 gives 17 ft. That calculation adds those two listed numbers.
Memory Technique
Picture a tab labeled rafter length: finish every operation before accepting 13 ft.
Reference Hint
Open Carpentry and Building Construction, 2016; use the index entry or section for "Rafter length calculation by rise and run" to confirm that Using the Pythagorean theorem, rafter length is sqrt(12^2 + 5^2) = sqrt(169) = 13 ft.
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