CSLB C-39 Roofing ContractorLow-Slope RoofingHard
A roofing contractor is installing tapered insulation to create positive drainage on a low-slope BUR roof. The specified taper is 1/4 inch per foot, sloping from a roof drain at the low point to a high point 40 feet away. If the insulation thickness at the drain is 1.5 inches, what minimum insulation thickness is required at the high point?
- A13 inches
- B11.5 inches
- C10 inches
- D15.5 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 11.5 inches
Total rise = 1/4 in/ft × 40 ft = 10 inches. Adding this to the starting thickness at the drain (1.5 in) gives 1.5 + 10 = 11.5 inches at the high point.
Why the other options are wrong
- A. This overstates the rise by using a steeper slope than specified.
- C. This is only the rise amount, forgetting to add the starting thickness at the drain.
- D. This adds the rise twice, which does not match the given taper rate.
Tapered Insulation Slope Calculation
Tapered insulation boards are manufactured to create a designed slope toward drains by increasing thickness at a fixed rate per foot of horizontal run.
- Rise = slope rate (in/ft) × horizontal distance (ft)
- Final thickness = starting thickness + total rise
- Common taper rates are 1/8, 1/4, and 1/2 inch per foot
Memory trick: Rise equals rate times run, then add the start.