CSLB C-39 Roofing ContractorEstimating and PlanningMedium
A roof's flat plan (footprint) area measures 1,800 square feet, and the roof slope is 8:12. What is the actual roof surface area in squares, rounded to the nearest tenth?
- A24.0 squares
- B21.6 squares
- C19.8 squares
- D18.0 squares
Show answer & explanationAnswer & explanation
Correct answer: B. 21.6 squares
The slope factor for 8:12 is √(1 + (8/12)²) = √1.444 = 1.202. Actual area = 1,800 × 1.202 = 2,163.6 sq ft = 21.6 squares.
Why the other options are wrong
- A. Overstates the slope factor beyond the correct value.
- C. Uses an incorrect slope factor below the true value.
- D. Ignores the slope factor entirely, using only footprint area.
Slope (Pitch) Factor
A multiplier applied to a roof's flat footprint area to determine true sloped surface area, based on the rise-to-run ratio.
- Factor = √(1 + (rise/run)²)
- Footprint area × slope factor = actual roof area
- Steeper slopes require proportionally more material
Memory trick: Steeper roof, bigger factor, more shingles.