Commercial Building Inspector B2Building EnclosureMedium
An inspector is reviewing a commercial building's roof assembly that uses a fully adhered single-ply membrane. The roof slope is 1/4 inch per foot (2%). According to IBC Section 1503.2, what is the minimum required design slope for roof drainage on such a roof?
- A1/4 inch per foot (2%)
- B1/16 inch per foot (0.5%)
- C1/8 inch per foot (1%)
- D1/2 inch per foot (4%)
Show answer & explanationAnswer & explanation
Correct answer: A. 1/4 inch per foot (2%)
IBC Section 1503.2 requires roofs to be sloped a minimum of 1/4 inch per foot (2%) for drainage, unless otherwise approved by the building official. This minimum slope is crucial for ensuring positive drainage and preventing ponding water on low-slope roofs.
Why the other options are wrong
- B. 1/16 inch per foot is insufficient for positive drainage and would likely lead to ponding water.
- C. 1/8 inch per foot is also generally considered insufficient and can lead to water ponding due to construction tolerances and deflection.
- D. 1/2 inch per foot provides excellent drainage but is not the minimum required by code.
Minimum Roof Slope (Low-Slope)
The least amount of slope required for a low-slope roof to ensure positive drainage and prevent water ponding.
- Applies to roofs with slopes up to 2:12 (17%).
- Minimum 1/4 inch per foot (2%) slope.
- Crucial for preventing premature roof degradation and leaks.
Memory trick: Quarter-inch slope, keeps the water afloat.