CSLB C-39 Roofing ContractorEstimating and PlanningHard
A hip roof has a common rafter run of 15 feet with a 6:12 slope. Using the standard rafter table hip/valley factor of 16.97 inches of hip length per foot of common rafter run, what is the total hip rafter length?
- A18.75 feet
- B21.2 feet
- C17.0 feet
- D23.5 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 21.2 feet
The hip rafter factor already accounts for the diagonal geometry of the hip in three dimensions. Multiplying the common rafter run by the factor and converting to feet: 15 ft × 16.97 in/ft = 254.55 inches, then 254.55 ÷ 12 = 21.2 feet.
Why the other options are wrong
- A. Incorrect: does not correctly apply the 16.97 in/ft hip factor.
- C. Incorrect: this would be closer to the common rafter length, not the longer hip rafter.
- D. Incorrect: overstates the hip length beyond the correct factor-based result.
Hip Rafter Length via Slope Factor Table
Hip and valley rafters are longer than common rafters for the same run because they travel diagonally in three dimensions; rafter tables provide a per-foot-of-run factor to calculate their true length.
- Hip/valley factor is always greater than the common rafter factor for the same pitch
- Multiply common rafter run by the hip factor to get length in inches
- Divide by 12 to convert the result to feet
Memory trick: Hips travel farther, so their factor runs harder.