Drain Flies in the Bathroom or Kitchen: What They Mean and When to Worry

If you keep seeing tiny, furry, moth-like flies resting on the bathroom wall or around the kitchen sink, they are drain flies, and they are breeding in the slimy film that coats the inside of a drain, trap or overflow. They do not bite and they are not a drainage emergency. In most homes the fix is cleaning the right plughole properly. When they keep coming back after every trap in the room has been cleaned, though, they are often the first visible sign of a cracked pipe or a broken drain under the floor.

What drain flies are, and how to recognise them

Drain flies, also called moth flies or sewer flies, are a few millimetres long, grey or brown, with hairy wings held flat over the body like a tiny roof. They fly weakly, in short hops, and spend most of their time sitting still on walls near a sink, bath or shower.

They lay their eggs in the gelatinous film of soap, skin, hair and food waste that builds up where water sits in a drain. The larvae live in that film and feed on it. That matters, because it tells you where to look: not at the flies on the wall, but at whichever wet, rarely-cleaned surface inside the pipework they are coming from. Fruit flies, by contrast, hang around fruit bowls and bins, and fungus gnats around house plants.

Find the source: the tape test

Cover each plughole in the affected room overnight with a strip of clear sticky tape, sticky side down, leaving a small gap at the edge. Flies emerging from that drain get stuck on the tape, so by morning you know which one is the source.

Check the less obvious places too: the overflow slot in a basin or bath, a shower tray waste, a floor gully in a utility room or wet room, the drain under a washing machine, and any sink or shower that is rarely used. A guest bathroom that has not been used for weeks is a classic breeding site, because the water in the trap evaporates and the film dries and softens in cycles.

How to clear them yourself

Remove the film they breed in and the flies stop. Killing adults with spray does nothing while larvae are still hatching in the pipe.

• Take the trap off and empty it. Most bottle traps under basins unscrew by hand. Put a bowl underneath, remove the trap and clean it thoroughly.
• Scrub the pipe walls. A stiff, narrow drain brush pushed down the plughole and the overflow removes the film far better than any liquid. Liquids run over the top of it.
• Clean the overflow channel. Flush it with hot soapy water after brushing. It is the part everyone forgets.
• Run water in unused fittings. Run each rarely-used sink, shower and floor gully for a minute once a week so the trap stays full.
• Repeat for two weeks. Eggs already laid will hatch for a while, so the odd fly after cleaning is normal. A steady stream is not.

Enzyme or bacterial drain cleaners, used as directed over several days, help break down any film the brush could not reach.

When drain flies point to a broken drain

If every trap and overflow in the room has been cleaned, the flies are still coming, and especially if there is a sewage smell or damp patch near the floor, the source is probably outside the pipework you can see.

The usual culprits:

• A cracked or disconnected pipe under the floor. Waste water leaking from a broken joint soaks the ground or the void beneath a suspended floor, and the flies breed there instead of in the pipe.
• A broken drain under a solid floor or extension. Flies appear from gaps around the base of the toilet, along skirting boards or through cracks in the floor slab.
• A damaged soil stack or a missing connection. A pipe that was not reconnected properly after building work lets foul air and flies out into a boxed-in void.
• A neglected floor gully or chamber. In commercial kitchens and basements, a gully or interceptor that is never cleaned becomes the breeding site for the whole building.

If a smell comes with the flies, our guide to what a bad drain smell means covers the causes that overlap. A leak under the floor is not something to ignore for months, because saturated ground under a slab can settle and the leak will not fix itself.

How a hidden fault is found

A camera survey is the reliable way to find a broken pipe under a floor without lifting it. The engineer passes a camera through the pipe from the nearest access point and records the defect and its position.

That tells you whether you are dealing with a displaced joint, a crack, a root intrusion or a pipe that was never connected, and exactly where it is, so any repair is targeted instead of exploratory. The CCTV drain survey page explains what the visit involves. If you want to know whether you need one first, what a CCTV drain survey is and when you need one covers that.

Is it worth paying for an emergency visit?

No. Drain flies on their own are never a reason to pay a night rate. Clean the traps first, and if a survey is needed, book it in the standard daytime band.

The exception is if the flies are only one symptom of something more urgent, such as sewage coming up through a floor drain, a toilet that will not empty, or foul water pooling inside. Those are drainage emergencies whatever the flies are doing, and they are worth a call tonight. For everything else, the same survey in daylight hours costs less and gives the engineer a better view.