CSLB C-36 Plumbing ContractorRough-in PlumbingEasy
A plumbing contractor is roughing in a new residential drain, waste, and vent (DWV) system. What is the minimum required slope for a 3-inch horizontal drain pipe serving a water closet?
- A1/4 inch per foot
- B1/2 inch per foot
- C1/16 inch per foot
- D1/8 inch per foot
Show answer & explanationAnswer & explanation
Correct answer: A. 1/4 inch per foot
The California Plumbing Code (CPC) specifies minimum slopes for horizontal drainage piping. For pipes 3 inches or larger, the minimum slope is 1/4 inch per foot to ensure proper drainage and prevent clogging.
Why the other options are wrong
- B. While a steeper slope provides good drainage, it is not the minimum required and can sometimes lead to liquids outrunning solids, potentially causing clogs in certain situations.
- C. This slope is typically too shallow for effective drainage in most residential applications and is not CPC compliant for this pipe size.
- D. This slope is permissible for pipes 2 inches or less in diameter, but not for 3-inch or larger pipes.
Drainage Pipe Slope
The required downward angle of horizontal drain pipes to ensure gravity-assisted flow of wastewater and solids.
- Ensures proper waste removal.
- Prevents clogs and standing water.
- Minimum slopes are set by plumbing codes (e.g., CPC).
Memory trick: Slope the pipe, keep it clean, 1/4 minimum for the big drain machine.