Why does my bathroom only smell in the morning?

Because nothing has run for eight hours. Traps with a marginal seal, and any outlet with biofilm in it, are at their worst after a long period without use, and the room has had no ventilation overnight. If running the taps clears it within a minute, the seal is the issue rather than a defect.

Will bleach or caustic drain cleaner fix a smelly shower?

Rarely, and it can make things worse. Caustic cleaners are formulated to clear blockages, not to remove the soft biofilm that causes the smell, and repeated use degrades plastic waste seals. Clearing the trap by hand and then using a biological treatment works better.

Can a drain smell mean there is a leak under the floor?

It can, and the tell is location. A smell that is strongest at a plughole is a trap or a biofilm problem. A smell that is strongest at a wall, a skirting or the floor, and that worsens after a shower rather than when a drain is disturbed, points at waste water discharging into the floor void.

There is a smell and the ceiling below has a stain. Is that urgent?

It should be looked at promptly. A stained ceiling below a bathroom with a drain smell usually means waste water has been entering the void for some time, and the risk is to the ceiling structure as much as to the finish. Stop using that shower or bath until it has been checked.

Who pays if the problem is in a shared drain?

Thames Water own sewers shared with neighbours, even under your garden or driveway, following the 2011 transfer, so a defect in a shared section is generally theirs to repair. Pipework within your boundary that serves only your property remains yours.

Why Your Bathroom Smells of Drains, and What Fixes It

Most bathrooms that smell of drains have a trap that has dried out or a shower waste packed with hair and soap, and both are a twenty-minute job with no tools. The version that needs an engineer is a smell that is stronger at a wall or floor than it is at any plughole.

Leak Fix LondonLast updated 19 September 2026

Flooring and plaster being made good after a leak repair

First, rule out the water itself

WaterSafe give a test that takes a minute and saves people a lot of wasted effort: fill a glass with water and smell it in a different room, away from the kitchen sink. If the smell follows the glass, it is the water and you should contact your water supplier immediately, particularly if there has recently been work on your service pipe or a new connection. If the glass smells of nothing once you are out of the bathroom, the water is fine and you are dealing with drainage.

WaterSafe describe the usual mechanism plainly: a sewage or stagnant odour from a sink or plug hole usually means there has been a blockage or a build-up of waste materials.

The six causes, in the order worth checking

1. A trap that has dried out

Every waste fitting has a U-bend holding a plug of water that seals the drain off from the room. If the fitting is not used, that water evaporates and the seal goes, and the room smells of the drain it is connected to. This is why the guest shower, the utility basin and the second WC in a rarely used cloakroom are the usual suspects, and why the smell in a flat is always worst after a fortnight away. Run every outlet for thirty seconds and flush every WC, then wait a day.

2. Biofilm in the shower waste

Hair, soap and skin build a soft grey layer inside the shower trap and the first section of waste pipe. It does not block the drain, so the shower still runs away perfectly, but it smells whenever warm water disturbs it. Lift the grille, pull out what you can reach, then flush with very hot water and a biological drain treatment rather than caustic soda, which strips the biofilm slowly but does not damage plastic waste or seals.

3. The washing machine or an unused appliance stub

A standpipe with no trap, or a trap that has dried because the machine is used weekly, vents the drain straight into the room. So does the capped stub left behind when an appliance was removed and the pipe was never sealed properly.

4. A wet-room gully with a failed or missing seal

Wet-room gullies use a shallow trap, sometimes a mechanical one rather than a water seal, and shallow traps lose their seal faster than a conventional U-bend. Some cheap gullies are fitted without the trap insert at all. If the smell is strongest at the floor drain and running water briefly fixes it, that is the answer.

5. A failed air admittance valve on a boxed-in stack

Where a soil stack is boxed in and terminated inside the building, an air admittance valve lets air in when a WC discharges and closes again to stop foul air coming out. When the valve sticks open, the bathroom smells after every flush, and you will often hear a gurgle at the same time. The valve is usually accessible behind a removable panel and is a cheap part. Worth reading alongside soil pipe leaking.

6. A broken or displaced waste pipe in the floor void

This is the one that is ours, and it is the last on the list because it is the least common. It is covered in its own section below.

What you can fix this afternoon

Run every tap, shower and gully in the property for thirty seconds and flush every WC.
Lift the shower grille, clear the trap by hand, and flush it with hot water.
Remove and clean the basin trap. Put a bucket underneath, loosen both nuts by hand, take it off, rinse it out and refit it. Check the washer sits flat when you reassemble it.
Check the washing machine standpipe has a trap and that it holds water.
Pour a little cooking oil into the trap of any outlet you genuinely never use. It floats on the water seal and slows evaporation.
If the WC smells only after flushing, check the AAV behind the boxing and the seal where the pan connector meets the stack.

If the smell has gone after all that, you are done and you do not need us.

The version that is a leak

A waste pipe under a bathroom floor runs at a shallow fall inside a shallow void. On a tiled floor over chipboard or a screed, there is almost nothing between the pipe and the tiles. Two things go wrong. A push-fit joint creeps apart under thermal movement, or the pipe gets stood on during a bathroom refit and cracks along the top, where it will not leak until the pipe runs full.

Either way, a small amount of waste water discharges into the void on every shower. There is no puddle, because the void absorbs it. What you get instead is a smell that behaves differently from all six causes above:

It is stronger at a wall, a skirting or the floor than at any plughole.
It gets noticeably worse in the half hour after someone showers, rather than when a drain is disturbed.
It has not improved after every trap in the property has been cleaned and refilled.
There is a stain, a bulge or mould on the ceiling of the room below, which is worth cross-checking against a damp patch on a ceiling.
Drain flies appear, which need standing organic matter to breed in.

Finding it does not mean taking the bathroom out. Each appliance is run in isolation with the void monitored, so the shower, the bath, the basin and the WC are eliminated one at a time. Moisture mapping across the floor and the ceiling below locates the wettest point rather than the most visible one, dye traced into a suspect waste confirms which pipe is discharging, and a camera through an existing access point or a single small opening confirms the fault before anything is cut. Then the pipe is repaired or replaced and the floor is put back: screed, tile, grout and seal. That is pipework repair and replacement, and the making good is done by our own people rather than left to you.

Who owns which pipe

If the problem turns out to be outside, the boundary matters. Thames Water state that you are responsible for all waste drainage pipes, such as gullies, gutters and drains, within your property boundary, so long as they serve just your property. Public sewers under roads and footpaths are theirs, and so are sewers shared with neighbours, even where those run under your garden or driveway. Following the transfer of private sewers in October 2011, the shared sections that used to be the householder's problem became the company's.

In practice, a smell confined to one bathroom is inside the building and is yours. A smell across a whole ground floor, or one that comes with slow drainage everywhere and a surcharging manhole, is a drainage problem and often the water company's to attend.

Where the design rules sit

Internal waste pipework in the UK is designed to BS EN 12056-2, which covers gravity drainage systems inside buildings and sets out principles for both the layout and the calculation of sanitary pipework in dwellings and commercial buildings. Most of the recurring smell problems we see in London flats are a consequence of pipework that was rerouted during a refit without the fall or the venting being reconsidered, which is exactly what that standard exists to prevent.

What it costs

If the fix is one of the six self-service causes, it costs you an afternoon. If you want it investigated, detection is £150 per hour, agreed before we attend, and if we do not find the fault you do not pay for the time spent looking. Repair and reinstatement are quoted once the cause is known. See pricing.

Frequently asked

Why does my bathroom only smell in the morning?: Because nothing has run for eight hours. Traps with a marginal seal, and any outlet with biofilm in it, are at their worst after a long period without use, and the room has had no ventilation overnight. If running the taps clears it within a minute, the seal is the issue rather than a defect.
Will bleach or caustic drain cleaner fix a smelly shower?: Rarely, and it can make things worse. Caustic cleaners are formulated to clear blockages, not to remove the soft biofilm that causes the smell, and repeated use degrades plastic waste seals. Clearing the trap by hand and then using a biological treatment works better.
Can a drain smell mean there is a leak under the floor?: It can, and the tell is location. A smell that is strongest at a plughole is a trap or a biofilm problem. A smell that is strongest at a wall, a skirting or the floor, and that worsens after a shower rather than when a drain is disturbed, points at waste water discharging into the floor void.
There is a smell and the ceiling below has a stain. Is that urgent?: It should be looked at promptly. A stained ceiling below a bathroom with a drain smell usually means waste water has been entering the void for some time, and the risk is to the ceiling structure as much as to the finish. Stop using that shower or bath until it has been checked.
Who pays if the problem is in a shared drain?: Thames Water own sewers shared with neighbours, even under your garden or driveway, following the 2011 transfer, so a defect in a shared section is generally theirs to repair. Pipework within your boundary that serves only your property remains yours.

Sources

No find, no fee · £150 per hour. The rate is fixed before we attend and does not rise for a difficult property, an awkward access or an evening visit. The total depends on how long the job takes; the rate cannot change.