CSLB C-39 Roofing ContractorLow-Slope RoofingEasy
A contractor is designing drainage for a newly installed low-slope built-up roof to avoid chronic ponding. What is the minimum slope generally required by code and industry standards for positive drainage?
- 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
Building codes and roofing industry standards specify a minimum slope of 1/4 inch per foot (approximately 2%) for low-slope roofs to achieve positive drainage and minimize ponding water.
Why the other options are wrong
- B. 1/2 inch per foot exceeds the minimum requirement, though sometimes used for extra drainage.
- C. 1/8 inch per foot is too shallow and commonly results in ponding.
- D. 1 inch per foot is a much steeper slope than required for low-slope roofing.
Minimum Slope for Positive Drainage
The minimum recommended slope of 1/4 inch per foot for low-slope membrane roofs to prevent water ponding and promote drainage toward outlets.
- 1/4:12 slope equals approximately 2%
- Low-slope roofs are generally defined as less than 2:12
- Ponding beyond 48 hours can void membrane warranties
Memory trick: A quarter inch a foot keeps the puddles off the roof.