Bathroom Smells Like Sewage: Causes and How to Fix It
A sewage or drain smell in a bathroom means the small reserve of water in the drain trap, the water seal, no longer blocks gases from the waste pipes. The water has evaporated, it is being sucked out because the drain stack is poorly vented, or the gases are getting past a seal, a cracked trap or a faulty joint.
Does the smell keep coming back, in Paris or the inner suburbs? Call +33 1 89 70 15 36 (01 89 70 15 36 from a French phone), 7 days a week, 24 hours a day, or write to us through the contact page or WhatsApp. Many of our plumbers speak English. Our drain unclogging service also deals with drain smells, with a camera inspection if needed.
It is almost never a cleaning problem. Air fresheners hide the smell for a few hours, then it comes back, because the cause is in the plumbing. Here is how to find which one.
The trap: your barrier against smells
Under every sink, basin, shower and bath there is a trap (in French, siphon), a bend that always holds a little water. This plug of water, the water seal, physically stops sewer gases from coming up. Toilets work the same way, with the trap built into the bowl.
As long as the water seal is there, no smell gets through. So every cause comes down to one question: why is the water seal no longer doing its job?
The 6 causes of a sewage smell
1. A dry trap (the most common cause)
The water in a trap evaporates when a fixture is not used for several weeks: a guest bathroom, a spare shower, a return from holiday. Summer heat speeds this up, which explains the unpleasant surprise after the holidays.
The test: run a litre or two of water into every drain. If the smell is gone within the hour, that was it. For a rarely used fixture, run the water once a month.
2. A dirty trap
Hair, soap and limescale build up into a deposit where bacteria thrive. The smell is then more like rotten eggs, often with slower draining.
The fix: put a bucket underneath, unscrew the trap, empty and clean it. When you refit it, check the seal: a crushed seal becomes a source of leaks and smells in itself.
3. A worn seal or cracked trap
Over time, rubber seals harden and crack. A plastic trap can split, a nut can be loose. The trap is full, but the gases get around it.
The fix: look for damp or cracks on the trap and its joints, and replace the seal or the trap.
4. Poor venting of the drain stack (siphonage)
This is the least known cause, and it is common in older buildings. A waste system must be vented, usually by a pipe that runs up to the roof (the primary vent). Without it, a large discharge (a toilet flush, a bath emptying, in your flat or a neighbour's) creates suction that pulls the water out of nearby traps.
The sign: gurgling in your basin or shower when someone flushes, followed by a smell. If the smell comes back a few days after you refill the traps, the water is being sucked out, not evaporating.
The fix: a plumber checks the venting. Depending on the case, they fit an air admittance valve on the waste pipe or deal with the stack venting, which goes through the building's property manager (the syndic).
5. The seal at the base of the toilet
If the smell is concentrated around the toilet, the seal between the bowl and the waste pipe may be cracked or badly fitted. A bowl that moves slightly is a strong clue.
The fix: the bowl has to come off and a new seal goes on. Quick for a plumber, awkward to do alone.
6. A blockage further down the pipe
If the smell is strong, constant, and water drains slowly everywhere, waste water is sitting and fermenting further down. That needs professional unblocking or drain cleaning. Our guide to blocked toilets and drains covers the right steps.
Find the source in 5 steps
Work by elimination, not at random.
- Find the fixture where the smell is strongest by sniffing each drain.
- Run the water there. Smell gone? A dry trap.
- Take the trap off and clean it. Was the smell coming from the drain itself? A dirty trap.
- Listen during a flush. Gurgling elsewhere? A venting problem.
- Check the toilet. Does the bowl move, is the smell at its base? The toilet seal.
Also note whether the smell is constant or only at certain times (after draining, in hot weather): it helps the plumber a lot.
What not to do
- Cover the smell with air freshener: the cause stays.
- Pour neat bleach or harsh products: they attack seals and PVC and do nothing for a venting or water seal problem.
- Use strong acids in a basin or shower: they can damage some plastics.
- Plug a drain permanently: the smell goes, but the problem moves elsewhere in the system.
Bad drain smells elsewhere in the flat
The same mechanisms explain drain smells in the kitchen or utility area, with a few causes specific to each room:
- The kitchen sink: grease and food scraps settle in the trap and the waste pipe, then ferment. Take the trap apart and clean it, then pour in very hot water with washing-up liquid. Do not pour cooking oil down the sink.
- The washing machine and dishwasher: a drain hose pushed too far down the waste pipe, or connected without a trap, lets smells back up. The hose must be held high in a waste pipe that has a trap.
- A drain that is rarely used (floor drain in a cellar or on a balcony, basin in a spare room): the water seal evaporates. Pour in a glass of water every two or three weeks.
- A smell in several rooms at once: the cause is more often on the shared stack or its venting than in your flat. Ask your neighbours whether they notice it too and tell the building manager (syndic).
When to call a plumber
A dry trap you can fix yourself. Call a professional if:
- the smell persists when every trap is full and clean;
- you hear gurgling with every discharge, in your flat or your neighbours';
- the toilet bowl is no longer perfectly stable;
- water drains slowly throughout the bathroom;
- several flats in the building complain of the same smell.
A plumber has tools that observation cannot replace: a camera inspection of the pipes, a smoke test to find a leak in the system, a check of the primary vent. They find the failure point instead of trying things one by one.
Older buildings and co-ownership: who is responsible?
Your trap, your seals and your flat's waste pipes are your responsibility. The drain stack and its venting are common parts of the building, managed by the syndic on behalf of the co-ownership (the owners' association, called copropriété). If several flats have the same smell, or the gurgling comes from your neighbours' drains, the cause is almost always in the common parts: tell the syndic, who orders the inspection. Our article on plumbing in co-ownership buildings and the syndic's role explains who handles what.
FAQ
Why does the smell come back a few days after I refill the traps?
Because the water seal is being emptied, not just evaporating. It is an almost certain sign of poor venting of the drain stack.
Is a sewage smell dangerous?
At low concentrations, sewer gases are mainly unpleasant. A strong, persistent smell should still be dealt with quickly, because it points to a real fault in the system.
Why does my walk-in shower smell like sewage?
The extra-flat traps used under some showers hold less water. They dry out or get sucked empty faster, especially in a flat that is rarely used. Run the water regularly and clean the drain.
Should I take the trap apart to clean it?
Yes, from time to time. Put a bucket underneath, clean the inside, and check the seal when you refit it.
Get a recurring drain smell sorted
Réseau Tubulure works in Paris and the inner suburbs (92, 93, 94), 7 days a week, 24 hours a day, to find and fix drain smells, including with camera inspection. Many of our plumbers speak English.
- Phone: +33 1 89 70 15 36 (01 89 70 15 36 from a French phone)
- Form: contact us
- WhatsApp: from the button on the site
See our drain unclogging service.
