In short
- In the great majority of cases, a sewer smell comes from a trap: dry, clogged, or emptied by a vacuum in the pipes.
- Start with the simplest test: run water down every trap in the room, including the ones you forget (bidet, a bath that’s never used, floor drain).
- If it smells at the base of the toilet, look at the seal of the waste pipe behind the bowl.
- A stale or musty smell points more to ventilation that isn’t doing its job.
- Never mix bleach with vinegar or a descaler: the mixture releases chlorine.
You open the bathroom door and a sewer smell greets you, even though everything was cleaned the day before? Relax, the room isn’t haunted. This kind of smell almost always has a simple mechanical cause, and you can find it yourself before calling a plumber. Here is how to go about it, from the quickest check to the most technical.
Where does a sewer smell in a bathroom come from?
All the drains in the house (basin, shower, bath, toilet) are connected to the same network, which leads to the sewer or the septic tank. What stops the gases from coming back up to your nose is the trap: a bend in the pipe that permanently holds a small plug of water, called the water seal. In new construction, it must be at least 50 mm (2 in) high.
As long as this water plug is in place and clean, nothing gets through. Smells appear when:
- the trap is dry: the water has evaporated because the fixture isn’t used (guest room, holiday home, a bath neglected in favor of the shower);
- the trap is clogged: hair, soap, toothpaste and limescale form a deposit that rots and smells, even with water in it;
- the trap is emptied by suction: a toilet flush or a bath draining creates a vacuum in the pipe and sucks out the water seal of another fixture. That’s the famous “gurgle” of the basin when you flush;
- a seal leaks: at the base of the toilet or on a connection, gases get past the trap;
- air isn’t renewed: without effective ventilation, humidity stagnates and gives a musty smell.
What does it smell like, exactly?
Your nose is a good diagnostic tool. Before taking anything apart, try to put a word to the smell:
- Sewer smell near a drain: a dry or clogged trap in the great majority of cases.
- Sewer smell at the base of the toilet: the seal of the waste pipe, or a bowl that isn’t properly fixed to the floor.
- Urine or ammonia smell around the toilet: urinary scale under the rim and splashes at the base of the bowl. Leaving coarse salt in the bowl overnight is a good first response. If the limescale is old, here is how to descale a heavily scaled bowl.
- Musty or stale smell: humidity that doesn’t escape, blackened shower grout, towels that never dry.
- Smell in several rooms at once, with gurgling: a ventilation problem in the drainage network itself. Here we go beyond simple cleaning.
How do you find the source of the smell, step by step?
Follow the steps in order: the first ones fix most cases and cost nothing.
1. Find the fixture that smells the most
Close the bathroom door for a while, then go round the drains with your nose close: shower drain, basin, bath, floor drain, base of the toilet. Also note when the smell appears: after a shower, after a flush, only in the evening and morning? A smell that is stronger in the evening and early morning points more to a ventilation problem.
2. Put water back in every trap
This is the simplest test and often the most effective. Run one to two liters (about a quart) of water down each drain, including the ones that are never used. If the smell disappears within the hour, you’ve found the culprit: a dry trap.
For a rarely used fixture, get into the habit of running water down it regularly. Before a long absence, a spoonful of oil poured down the drain after the water forms a film that slows evaporation.
3. Take apart and clean the trap
If the smell comes back while the trap is full, it is probably clogged.
- Put a bowl underneath and unscrew the bottom of the basin trap by hand. It is designed for that.
- Empty it, remove the hair and the greasy deposit, and brush it with hot soapy water.
- Check the state of the seal before refitting, and replace it if it’s crushed or split.
- For a shower or bath drain, remove the grate and the cover, then catch the hair with a hook or a plastic drain snake.
For upkeep, use these products one after the other rather than mixed, because they neutralize each other: a tablespoon of baking soda in the drain against smells, rinsed with hot water, and from time to time a glass of white vinegar against limescale, left to work before rinsing. A grate on the shower drain limits the hair that ends up in the trap. The details are in our guide to vinegar, baking soda, soda crystals and percarbonate.
4. Inspect the base of the toilet
If the smell is concentrated around the toilet:
- Look behind the bowl: is the waste pipe (the pipe connecting the bowl to the drain) properly pushed in? Is its seal damp, blackened, split?
- Try to move the bowl. A bowl that moves eventually pulls the seal off the pipe.
- Wipe the base of the bowl with a dry paper towel after a flush: if it comes away damp, there is a small leak.
Replacing a waste pipe or a toilet connector is within the reach of a DIYer, but you have to shut off the water, empty the bowl and sometimes remove it. If the toilet also drains badly, you may be dealing with two problems at once.
5. Check that the ventilation works
In a bathroom, air must be drawn out continuously through an extractor vent. Do the toilet paper test: held against the vent, a sheet should stay stuck on its own. If it falls, the vent is clogged or the motor no longer pulls enough. Also keep in mind that the vent itself should never be blocked.
To give you an idea of scale, French regulations on home ventilation (order of 24 March 1982) provide for extraction of 15 to 45 m³/h for a bathroom depending on the size of the home, and 15 to 30 m³/h for the toilet. Without a window, leave the door ajar after the shower so air can circulate, and never block the extractor vent.
6. If it gurgles or smells in several rooms
When the water in a basin goes down with bubbles as you flush the toilet, the water seal is being sucked out by a vacuum in the network. Normally, the drain stack is open to the roof (this is called primary ventilation) to prevent this. If it is blocked, or missing on a DIY installation, the traps empty with every flush.
The usual remedy is a pressure-balancing valve (or air admittance valve): it lets air in when the pipe goes under vacuum, then closes again to block odors. It is fitted vertically, on the drain pipe, and must comply with the NF EN 12380 standard. Be careful: it is a stopgap, but it doesn’t replace proper primary ventilation. If you have any doubt about the origin of the problem, this is the time to call a plumber: a camera inspection avoids breaking things open at random.
And if the house is on a septic tank?
A full or poorly ventilated tank sends smells back up through every drain. The two things to watch:
- the sludge level: it needs emptying when the sludge reaches half the tank’s useful volume;
- the tank’s ventilation: the vent outlet (often an extractor on the roof) must not be blocked.
As for upkeep, avoid sending large amounts of bleach into it: it destroys the bacteria that make the tank work.
Which products should you use, and which should you avoid?
- Useful: white vinegar and baking soda for drain upkeep (one after the other, not mixed), coarse salt for the bowl, hot water (not boiling, to spare the ceramic and the seals) for rinsing.
- Reserve for real blockages: chemical drain cleaners. Follow the instructions, ventilate the room, and don’t use them for routine upkeep.
- Never do: mix bleach with vinegar, a descaler or another acid. The mixture releases chlorine gas. ANSES, the French health and safety agency, regularly points this out after poisonings.
- Useless in the end: air fresheners and scented blocks. They mask the smell without fixing a dry trap or a leaking seal.
How much does it cost to fix the problem?
- Putting water back in a trap, taking it apart and cleaning it: nothing at all.
- Trap seal: about €4 to €10, the seal alone or in an assorted pack (Castorama, October 2026). Note that a complete new basin trap starts at around €3 to €7 (Brico Dépôt, October 2026): when the old one is tired, you might as well replace it entirely.
- Toilet waste pipe: from €6 to €13 for a straight or angled 100 mm pipe, from €7 to €34 for a flexible or extendable pipe (Castorama and Brico Dépôt, October 2026).
- Pressure-balancing valve: about €20 to €40 for a 32 to 50 mm model, and over €60 in 100 mm diameter (Castorama partner sellers and plumbing distributors, October 2026). The big chains stock few: also look at plumbing merchants.
- Anti-odor drain or trap: an extra-flat shower drain with an anti-odor return membrane costs about €31, and a space-saving anti-odor basin trap about €22 (Brico Dépôt, October 2026).
- Plumber with camera inspection: allow about €250 for a camera run, plus about €90 for a detailed video report and €20 to €140 call-out fee (ootravaux.fr, 2026 prices). Always ask for a written quote that states whether the report and the call-out are included.
Store prices change with the shop and the period: check them before you buy.
Frequently asked questions
Why does the bathroom smell of sewage after the holidays?
Because the water in the traps evaporated while you were away, especially when it’s hot. Run water down every drain when you get back: the smell usually disappears within the hour.
Why does it smell of sewage when it rains?
Heavy rain overloads the network, which disturbs the pressure in the pipes and pushes gases back up. The problem is worse if the ventilation outlet on the roof is blocked (leaves, a bird’s nest). For a house below street level or on an old network, a non-return valve can block backflow. If it happens at every storm, have the network diagnosed by a plumber.
The shower drain smells even though it’s clean. What can I do?
Check that the water seal is really there: some extra-flat shower traps, fitted in renovations, have a reduced water seal that evaporates faster. If that’s your case, run the shower more often, or fit the drain with a mechanical anti-odor system (valve or membrane).
Is coarse salt enough against toilet smells?
Against urine smells and limescale in the bowl, it helps a lot. Against a sewer smell coming from a seal or an emptied trap, it can do nothing: you first have to fix the mechanical cause.
This article is part of our guide to cleaning your bathroom and toilet naturally, with the full daily, weekly and monthly routine.
Have you tracked down a stubborn smell at home and found an unexpected culprit? Tell us in the comments: it will help the next reader holding their nose in front of their bathroom.