CSLB C-39 Roofing ContractorEstimating and PlanningHard
A rectangular building measures 30 feet wide (eave to eave) and 50 feet long, with a roof slope of 5:12. Using a rafter length factor of 1.083 for a 5:12 slope, what is the total roof surface area (both roof planes combined), in square feet?
- A1,500 square feet
- B1,624.5 square feet
- C1,687 square feet
- D1,562 square feet
Show answer & explanationAnswer & explanation
Correct answer: B. 1,624.5 square feet
The horizontal run to the ridge is half the building width: 30 ÷ 2 = 15 feet. Rafter length equals run × slope factor: 15 × 1.083 = 16.245 feet. Each roof plane's area equals rafter length × building length: 16.245 × 50 = 812.25 sq ft. Doubling for both planes: 812.25 × 2 = 1,624.5 square feet.
Why the other options are wrong
- A. This is the flat plan area (30 × 50) without applying the slope factor.
- C. Uses a slope factor higher than the correct 1.083 value.
- D. Uses an incorrect slope factor slightly lower than 1.083.
Gable Roof Area from Ridge/Eave Dimensions
Total gable roof area is found by computing the sloped rafter length from the run and slope factor, then multiplying by building length and doubling for both roof planes.
- Run to ridge equals half the building width.
- Rafter length = run × slope factor from a rafter table.
- Multiply rafter length by building length, then double for two planes.
Memory trick: Half the width, stretch it up the slope, then double the plane.