Quick Answer:
If you smell a rotten-egg (sulfur) odor only in an upstairs bathroom, it’s usually a local issue: a dry or slowly flowing trap, stagnant water in a short branch line, or a fixture-specific problem. Before tearing into walls or replacing major equipment, run some simple tests and compare fixtures to narrow it down.
Why This Happens
The rotten-egg smell is hydrogen sulfide. In a house this commonly comes from one of three sources:
- Stagnant water in a p-trap or a short branch of piping where water sits and bacteria produce the odor.
- A problem in the hot-water system (water heater bacteria or corrosion) if the smell is only on hot taps.
- Occasional effects after changes to the municipal supply or disturbances in plumbing. See Sulfur smell after city water outage for that situation and Sulfur odor in laundry water for a related example in a different fixture.
Step-by-Step What to Do
1. Do a quick side-by-side cold tap check
Run the cold tap in the upstairs bathroom and, at the same time, run a cold tap downstairs. Compare smell, temperature, and clarity. If only the upstairs cold tap smells, the issue is likely isolated to that branch or fixture.
2. Flush the upstairs cold line
Run the upstairs cold water at full flow for 3–5 minutes to flush stagnant water. Smell again. If the odor disappears, the cause was likely standing water in a short pipe or trap.
3. Test hot water separately
Run hot water at the upstairs sink and then at a downstairs sink. If the smell appears only on hot taps throughout the house, suspect the water heater. If it’s only at the upstairs hot tap, the heater is less likely the source.
4. Check other fixtures in the same upstairs area
Turn on the shower, toilet flush, and any nearby sinks. If multiple upstairs fixtures show the odor, the shared branch or venting may be the problem. If only one fixture smells, focus on that fixture and its trap/aerator.
5. Inspect and clean the aerator and trap
Remove the sink aerator and sniff the flow directly. Clean the aerator if it’s gunky. If comfortable and you have the right tools, remove the p-trap under the sink, rinse it out, and reassemble. If you’re not comfortable doing this, skip to the call-a-pro step.
6. Try isolating the branch
If the bathroom has local shutoff valves for sink/shower/toilet, close them one at a time and see if the odor changes. Isolating a single fixture or a small branch helps decide whether the problem is local or shared with other plumbing.
7. Re-test after 24 hours
If the odor returned after a day, keep a note of which fixtures smell and when. Persistent return suggests bacterial growth or a plumbing configuration that allows stagnation, which may require professional help.
What Not to Do
- Do not open walls before isolating branch-only issues.
- Don’t assume the main supply is at fault without comparing taps side by side.
- Don’t mix cleaning chemicals in drains (for example, bleach with ammonia or other products).
When to Call a Professional
- The odor persists after flushing and cleaning aerators or traps.
- Multiple fixtures (upstairs and downstairs) smell and you can’t isolate the source.
- You suspect the water heater is involved (strong smell on hot water only) or there’s any sign of sewage backup or leak.
- You’re not comfortable removing traps or working on shutoffs.
Safety Notes
- A weak hydrogen sulfide odor in water is unpleasant but usually not dangerous at household levels. Strong, persistent odors or physical symptoms (headache, dizziness) warrant leaving the area and calling a professional.
- If you smell natural gas (skunky, rotten-egg-like but accompanied by hissing or the odorant used by your gas utility), evacuate immediately and contact your gas company—do not investigate further inside the house.
- Wear gloves and eye protection when removing traps or cleaning aerators. Avoid inhaling fumes from cleaning products and never mix chemicals.
Common Homeowner Questions
- Is this smell dangerous? Usually not at low levels from a bathroom trap, but strong or widespread smells should be checked by a pro.
- Will flushing the line fix it? Often yes—running cold water and cleaning aerators can remove trapped, stale water that causes the odor.
- Could the city water cause this? Yes, changes in the municipal system or outages can cause temporary odors; contact your water utility if you suspect a supply issue.
For more related articles, see the Sulfur / Rotten Egg Smell in Water hub.