Shower drain trap smells bad

Symptoms

• Musty or sewage-like smell near the drain
• Smell noticeable even when water isn't running
• Odor is worse in a rarely used shower
• No visible clog or slow drainage

Likely causes

• CommonOrganic buildup (hair, soap, skin cells) decomposing in the trapEven without a full clog, buildup in the trap can host bacteria that produce odor.
• CommonDried-out P-trap letting sewer gas escapeIf the shower isn't used regularly, the water seal in the trap evaporates and stops blocking gas from the drain line.
• PossibleBiofilm coating the inside of the drain pipeA slimy bacterial film can build up along pipe walls over time and produce a persistent smell.
• Less commonVent pipe issue allowing gas to be pulled into the drain instead of venting outsideA blocked or improperly installed vent stack can cause sewer smells to back up through drains.

What to do

1. Run water down the drain for at least a minute to refill the trap if it hasn't been used recently.
2. Remove the drain cover and clean visible buildup with a bottle brush or drain-cleaning tool.
3. Pour a half cup of baking soda followed by a half cup of vinegar down the drain, let it fizz for 10–15 minutes, then flush with hot water.
4. Repeat this weekly for a few weeks if the smell was significant, to break down biofilm gradually.
5. For rarely used drains, run water through them at least once a week going forward to keep the trap filled.
6. If the smell persists despite a filled trap and cleaning, consider that the vent stack could be blocked, which needs a plumber's inspection.
7. Note any accompanying smell of natural gas rather than sewage, which is a different and more urgent issue.

Tools and parts

• Bottle brush
• Baking soda
• White vinegar

When to call a pro

Call a plumber if the smell persists after cleaning and refilling the trap, since it could point to a vent pipe problem.

Safety