Sewer Smell in the Bathroom: Five Common Causes
A rotten egg or sewage smell in the bathroom is a fair reason to worry, but most sources are small and fixable. Here are five common causes, in the order worth checking, and how to tell when the problem is bigger.
Smell will not go away?
Note which fixture it comes from and when it is worst, then call.
(754) 320-541224/7 emergency lineWhy drains smell at all
Every fixture drain has a trap, a curved section of pipe that holds a small plug of water. That water seals the opening so sewer gas from the drain system cannot rise into the room. Sewer gas is a mix that includes hydrogen sulfide, which smells like rotten eggs.
When that seal fails, or when something else produces a similar odor, the smell appears. The task is to figure out which case you have. Work through the five causes below, starting with the easiest.
Cause one: a dry trap
A trap that has not seen water in a while dries out. The seal vanishes and sewer gas rises. It happens in guest baths, floor drains and tubs that nobody uses. In South Florida it happens fast in a hot house that has been closed for weeks, which is common in seasonal homes.
The fix is simple. Run water in the fixture for about a minute. Pour a cup of water down any floor drain. If the smell fades within an hour, the trap was the cause. For drains that rarely get used, a small amount of mineral oil on top of the water slows evaporation.
Cause two: buildup in the drain or overflow
Hair, soap and skin cells build a film inside a drain. Bacteria feed on it and produce an odor that smells like sewage, even though the main sewer is fine. The overflow opening in a sink can hold the same film.
- Remove the stopper and clean it by hand.
- Pour hot, not boiling, water down the drain after cleaning.
- Use a small brush to clean the overflow slot.
- Try an enzyme cleaner as the label directs, rather than a harsh chemical.
- If water drains slowly too, plan for drain cleaning.
Cause three: a toilet seal problem
A toilet sits on a wax ring or a rubber seal that joins it to the drain flange. If the ring fails, or the toilet rocks on the floor, gas and sometimes small amounts of water escape around the base. The smell is strongest next to the toilet, and the floor may feel soft or look stained.
- Check for rockingSit on the bowl and shift your weight. Any movement suggests loose bolts or a failed seal.
- Look at the baseStains, caulk gaps or dampness on the floor around the base are clues.
- Tighten gentlySnug the closet bolts, but do not overtighten, because porcelain cracks.
- Reassess in a dayIf the smell remains, the wax ring probably needs replacing.
- Call for a resetA reset or replacement is covered on the toilet repair and installation page.
Cause four: a vent problem
Drain pipes need air to move water smoothly. Vent pipes run up through the roof and let that air in. If a vent clogs with debris, a bird nest or a fallen leaf, the pipes cannot breathe. Water pulls the trap seal out as it drains, which lets gas rise.
Signs of a vent problem include gurgling sounds, slow drains in several fixtures and a smell that comes and goes with water use. Roof work is not a do-it-yourself job, so call a licensed professional. A vent that was cut short or capped during a remodel can cause the same trouble.
Cause five: a damaged or blocked sewer line
A cracked pipe under the slab, a sagging section or a root intrusion can leak gas into the house. This is the most serious cause. It tends to come with other signs, such as slow drains, gurgling toilets, wet spots in the yard or backups in the lowest fixture.
| What you notice | Likely cause | Next step |
|---|---|---|
| Smell in a rarely used bath | Dry trap | Run water and recheck |
| Smell from one sink only | Biofilm in the drain or overflow | Clean and flush |
| Smell strongest at the toilet base | Failed wax ring | Check for rocking |
| Gurgling plus smell | Vent blockage | Call for an inspection |
| Smell with multiple slow drains | Sewer line problem | Camera inspection |
A camera shows the inside of the line so the problem can be seen rather than guessed. The sewer line repair page explains what the camera reveals and how a repair is chosen.
Smells that are not sewage
Not every bad smell comes from a drain. A hot water heater with bacteria in the tank can give hot water a rotten egg smell, while cold water smells normal. A musty odor can come from mold on a wall or under a vanity. A sweet, chemical or gas smell is a different matter altogether.
If you smell natural gas
- Do not use switches, lighters or phones inside the house.
- Leave the home and get everyone out, including pets.
- Call the gas utility or 911 from outside.
- Do not go back in until responders say it is safe.
Natural gas is mixed with an odorant that smells like sulfur, so it can be confused with sewer gas.
When to call a plumber
If the quick checks do not clear the smell within a day, or if several fixtures are affected, schedule an inspection. Call right away if you see water on the floor, hear gurgling, or notice a backup. For older homes with cast iron, leak detection can find a hidden leak that lets gas escape.
What affects the fix
- Location of the source
- A trap or a toilet seal is a small job, and a line under the slab is not.
- Age of the plumbing
- Older cast iron and clay develop cracks and corrosion.
- Access
- Pipes inside finished walls need more care to reach.
- Roof height
- Vent work may need safety equipment and a second person.
Ask for a written scope that names the cause the plumber found.
Questions people ask before they call
In small amounts it is mostly an annoyance, but it can irritate the eyes and throat. Strong, persistent odors should be checked quickly, and anyone feeling unwell should get fresh air.
Cooler air and a closed door can trap odor, and a seldom used drain may dry out between uses. Run water in each fixture before bed and see if the smell changes.
Yes. A blocked vent lets water pull the trap seal out as it drains, which opens the path for gas to rise into the room.
Bleach masks the odor for a short time. It does not repair a trap, a seal or a leak, so find the source instead.
Sewer smell that keeps returning?
Describe where it is strongest and what you have tried. The dispatcher will explain what an inspection covers.