CSLB C-36 Plumbing ContractorRough-in PlumbingHard
A plumbing contractor is installing a new storm drainage system for a commercial building. The system includes several roof drains connected to a common horizontal storm drain. What is the minimum size of a horizontal storm drain pipe that serves a roof area of 1,000 square feet with a maximum rainfall rate of 4 inches per hour?
- A4 inches
- B3 inches
- C6 inches
- D5 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 4 inches
To size a horizontal storm drain, the CPC references tables based on rainfall rate and drained area. For a rainfall rate of 4 inches per hour, a 1,000 sq ft area requires a minimum 4-inch pipe. (Calculation: CPC Table 1101.8, 4" rainfall rate, 1000 sq ft, shows 4" pipe for up to 1200 sq ft).
Why the other options are wrong
- B. A 3-inch pipe is undersized for this area and rainfall rate (typically handles up to 520 sq ft at 4"/hr).
- C. A 6-inch pipe is significantly oversized for this application, leading to unnecessary cost.
- D. A 5-inch pipe would be larger than necessary but would work, however, the question asks for the 'minimum' size.
Storm Drain Sizing
The process of determining the appropriate diameter of storm drainage pipes based on the area to be drained and the local maximum rainfall intensity.
- Prevents overflow and flooding.
- Based on rainfall rate and drainage area.
- Reference plumbing code tables (e.g., CPC Table 1101.8).
Memory trick: Rain's Rate and Roof's Area 'Rule' the pipe's size.