CSLB C-39 Roofing ContractorLow-Slope RoofingMedium
A roof plan specifies a minimum slope of 2 percent for positive drainage on a low-slope built-up roof. What is this slope equivalent to in inches per foot?
- A1/4 inch per foot
- B1/2 inch per foot
- C1/8 inch per foot
- D1 inch per foot
Show answer & explanationAnswer & explanation
Correct answer: A. 1/4 inch per foot
A 2% slope means 2 inches of rise per 100 inches of run. Converting to inches per foot: 0.02 × 12 inches = 0.24 inches per foot, which rounds to the standard specified slope of 1/4 inch per foot.
Why the other options are wrong
- B. 1/2 inch per foot equals about 4%, double the specified slope.
- C. 1/8 inch per foot equals about 1%, half the specified slope.
- D. 1 inch per foot equals about 8.3%, far steeper than specified.
Slope Percentage Conversion
Roof slope expressed as a percentage can be converted to inches per foot by multiplying the decimal percentage by 12.
- 2% slope ≈ 1/4 inch per foot
- Formula: slope (in/ft) = percent slope × 0.12
- 1/4 in/ft is the commonly cited minimum for positive drainage
Memory trick: Multiply the percent by twelve hundredths to get inches per foot.