CSLB C-39 Roofing ContractorLow-Slope RoofingHard
A roof section drains toward a single low point but currently ponds water after rainfall because the existing slope is only 1/8 inch per foot over a 24-foot run to the drain, while code minimum is 1/4 inch per foot. How much additional total rise (in inches) must tapered insulation add at the high point to meet minimum code slope over that run?
- A6 inches
- B1.5 inches
- C3 inches
- D4.5 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 3 inches
Required total drop at 1/4 in/ft over 24 ft = 0.25 × 24 = 6 inches. Existing drop at 1/8 in/ft over 24 ft = 0.125 × 24 = 3 inches. Additional rise needed = 6 − 3 = 3 inches.
Why the other options are wrong
- A. Equals the full required drop, ignoring existing slope already present.
- B. Half of the correct value; miscalculation of one slope only.
- D. Overstates the needed correction.
Ponding Correction Calculation
To eliminate ponding, additional tapered insulation must be added to increase existing slope to meet the code-minimum slope over the same run.
- Required drop = min slope × distance
- Additional rise = required drop − existing drop
- Tapered insulation systems correct low-slope ponding
Memory trick: Find the gap between what is and what should be.