CSLB C-36 Plumbing ContractorService, Repair, and MaintenanceEasy
A homeowner reports a persistent, foul, sewage-like odor emanating from their seldom-used guest bathroom, particularly noticeable near the floor drain. They confirm no visible leaks or backups. What is the most likely cause of this issue?
- AA blockage in the main sewer line causing sewer gas to back up.
- BBacterial growth inside the drainpipe of the floor drain.
- CA dry P-trap in the floor drain allowing sewer gas to escape.
- DA ruptured drain-waste-vent (DWV) pipe behind the wall.
Show answer & explanationAnswer & explanation
Correct answer: C. A dry P-trap in the floor drain allowing sewer gas to escape.
A dry P-trap in a seldom-used drain is a common cause of sewer gas odors. The water in the trap evaporates over time, removing the barrier that prevents sewer gas from entering the living space. Adding water to the drain usually resolves the issue.
Why the other options are wrong
- A. While a main sewer line blockage can cause odors, it typically manifests with backups and odors in multiple fixtures, not just a seldom-used floor drain without visible leaks.
- B. Bacterial growth can cause odors, but they are usually more localized and less like a strong sewage smell compared to sewer gas, and less likely to be from a floor drain alone.
- D. A ruptured DWV pipe would likely cause visible leaks, water damage, and a more pervasive, often stronger, odor that might not be isolated to a single drain.
Dry P-Trap
A P-trap that has lost its water seal due to evaporation or siphoning, allowing sewer gases to enter a building.
- Common in seldom-used fixtures.
- Causes foul, sewage-like odors.
- Resolved by adding water to the drain.
Memory trick: Don't let the 'P' dry out, or the 'S' will get you!