CSLB C-36 Plumbing ContractorService, Repair, and MaintenanceHard
A customer reports a gurgling sound coming from their shower drain after flushing the adjacent toilet. The shower drains slowly, but eventually clears. What is the most probable cause of this issue?
- AHair and soap scum buildup in the shower P-trap.
- BA partial blockage in the main vent stack or a branch vent.
- CA failing ejector pump in a basement bathroom.
- DA complete blockage in the main sewer line.
Show answer & explanationAnswer & explanation
Correct answer: B. A partial blockage in the main vent stack or a branch vent.
Gurgling drains, especially when an adjacent fixture is used, and slow drainage indicate a problem with the plumbing vent system. When the toilet flushes, it creates a vacuum that pulls air from the most accessible point—the shower drain—causing the gurgling, and also siphons water from the trap, which can contribute to slow drainage. A partial blockage in the vent prevents proper air equalization.
Why the other options are wrong
- A. Hair and soap scum in the P-trap would cause slow drainage in the shower, but not typically gurgling specifically triggered by the toilet flush, unless the blockage was upstream of the trap and affecting the vent.
- C. A failing ejector pump would affect all basement fixtures, potentially causing backups and not just gurgling in a shower drain after a toilet flush.
- D. A complete blockage in the main sewer line would cause widespread backups in lower fixtures and not just gurgling in a single shower drain.
Plumbing Vent Blockage
An obstruction in the vent system that prevents proper air circulation, leading to gurgling and slow drains.
- Causes negative pressure (vacuum) in the drain lines.
- Results in gurgling sounds as air is pulled through P-traps.
- Can lead to siphoning of P-trap water, allowing sewer gas entry.
- Requires clearing the vent pipe, often on the roof.
Memory trick: The vent's choked, the air is provoked, a gurgle is evoked.