A sewage smell in a bathroom is usually caused by a dry P-trap, a clogged drain, or a vent problem. Run water in unused drains and call a plumber if it persists.
Common causes
- Dry P-trap: fill by running water.
- Clogged drain or buildup.
- Blocked vent stack.
- Cracked pipe or wax ring problem on the toilet.
- Sewer line problems.
Quick fixes
- Run water in all drains for a minute.
- Clean the drain.
- Check the wax ring.
Safety
Sewer gas contains hydrogen sulfide and methane. Ventilate and call a plumber if the smell persists.
How drains normally block sewer gas
Every sink, shower and floor drain has a P-trap, a U-shaped pipe that holds water and forms a seal against sewer gas. If the water evaporates or is siphoned out, gases enter the room.
Dry trap
Floor drains, guest bathrooms and rarely used showers dry out in weeks. Pouring a quart of water into the drain, followed by a spoonful of cooking oil to slow evaporation, can restore the seal.
Other causes
- A cracked or loose toilet wax ring.
- Clogged or blocked vent pipe on the roof, which causes slow drains and gurgling.
- A cracked drain pipe or leaking joint behind the wall.
- Dirty sink overflow holes and drains with biofilm.
- Sump pump or septic system issues.
- Mold or mildew.
Gas involved
Sewer gas contains hydrogen sulfide (rotten egg smell), methane, ammonia and carbon dioxide. Low levels cause nausea and headaches. Higher concentrations of hydrogen sulfide are dangerous, and methane can be explosive in enclosed spaces.
Quick fixes
- Run water in every drain for a minute.
- Clean drains with baking soda followed by vinegar, then flush with hot water.
- Check the toilet base for movement or leaks.
- Check under sinks for dry or missing traps.
- Clean the overflow hole in the sink.
Septic homes
Smells near drains or the yard may mean a full tank or failed drain field. Pump the tank every 3 to 5 years.
Call a plumber if
The smell persists, drains gurgle, multiple fixtures are slow or you suspect a vent blockage or broken pipe.
Safety
Leave the area and call the gas utility if you smell rotten eggs along with other gas odors, since natural gas has added sulfur-like odorant.
Why drain traps matter
Every sink, shower and floor drain has a U shaped pipe called a P trap that holds water and blocks sewer gas from entering the room. If a drain is rarely used, the water in the trap can evaporate within weeks, letting gas rise. Pour a quart of water down the drain, followed by a spoonful of cooking oil to slow evaporation. Other causes include a dry floor drain, a blocked plumbing vent on the roof, a leaking drain pipe, a failed wax ring under the toilet, a broken sewer line or a full septic tank.
Safety and when to call a plumber
Sewer gas contains hydrogen sulfide, which smells like rotten eggs, plus methane and ammonia. Low levels cause headaches and nausea, high levels can be dangerous, and methane is flammable. Ventilate the room and avoid open flames if the smell is strong. Call a plumber if the smell persists after refilling traps, if multiple drains gurgle, if you see sewage backing up, or if you have a septic system with slow drains. Use bathroom fans and clean drains periodically. If anyone has symptoms such as dizziness or nausea, leave the area.
Based on data from the EPA and plumbing industry guidance
