Drain Flies: How to Get Rid of Them for Good

Updated July 2026 · Emergency Plumber Chelsea

Drain flies, also called moth flies or sewer flies, are small, a few millimetres long, with fuzzy grey or brown wings held flat over the body like a tiny moth. They are weak fliers and tend to sit on bathroom walls, tiles and around plugholes, hopping short distances when disturbed. If you squash one it leaves a powdery smudge. They are not the same as fruit flies, which are smaller, tan with red eyes and hang around fruit bowls and bins, or fungus gnats, which are dark, long-legged and come from damp houseplant compost. Getting the identification right saves a lot of wasted effort.

They matter because of where they live. Drain flies lay their eggs in the organic sludge that coats the inside of waste pipes, traps, overflows and drains: a mix of soap, hair, skin cells, food and bacteria. The larvae feed on it, and the life cycle can complete in a couple of weeks in warm conditions, which is why a few flies can turn into dozens surprisingly quickly in summer. The flies themselves are a nuisance rather than a health risk to most people, but a lasting infestation is a reliable sign that there is standing water and organic build-up somewhere in your plumbing.

First, find out which drain they are coming from. In the evening, put a strip of clear tape sticky side down over each plughole, shower drain and overflow you suspect, leaving a gap at the edges so air can move. Check in the morning. Flies stuck to the tape show you the breeding site. Do this for the kitchen sink, basins, bath, shower, any floor drains, and the overflow holes on basins and baths, which are one of the most overlooked sources. Guest bathrooms and utility rooms that are rarely used are prime suspects.

Once you know the source, clean it mechanically. Remove the plug or drain cover, pull out any hair and debris, and scrub the inside of the plughole and the first part of the pipe with a stiff drain brush or bottle brush. The aim is to physically remove the slime layer. Then flush with plenty of hot water. For basin and bath overflows, use a thin brush through the overflow slot. If you can safely unscrew the trap under a basin or sink, clean that too, with a bucket underneath. Our guide on how to prevent blocked drains covers what to keep out of your drains afterwards.

An enzyme or bacterial drain cleaner used regularly over a week or two helps digest the film further down the pipe where a brush cannot reach. Pouring bleach or boiling water down the drain on its own rarely works, because it runs straight past the sludge coating the pipe walls. Caustic drain unblockers can damage older pipework and seals and are not a cure for flies. Sprays will kill the adults you can see, but new ones will keep hatching while the breeding site is still there.

Look at drains that are never used. Every trap under a basin, shower, bath or floor drain holds a small amount of water that blocks smells and insects from the pipe beyond. In a spare bathroom, a basement shower or a floor gully that is never used, that water evaporates and leaves an open route from the drain into the room. Running each tap for a minute every week or two keeps the traps full. If you notice a sewage smell alongside the flies, our guide to drain smells explains what it can mean.

If you have cleaned every drain, the tape test is clear, and the flies keep coming, the breeding site is somewhere hidden, and this is where a plumber or drainage engineer comes in. Common culprits are a cracked or disconnected waste pipe leaking into a floor void or boxing, a failed seal at the base of a toilet letting waste seep under the floor, a broken underground drain that has left a wet, organic pocket in the ground below a ground-floor room, or a manhole or inspection chamber with a damaged lid or benching. Flies appearing around the base of a toilet, from gaps in skirting, or from a floor with no drain at all are strong signs of a hidden leak.

Chelsea's basement flats and converted lower ground floors are especially prone to this. Many rely on pumped drainage, with a sump or pump chamber collecting waste water before it is lifted to the sewer. A sump chamber that is not sealed or has not been cleaned is an ideal breeding ground, and so is standing water left behind by a pump that is not fully emptying. Our sump pump guide covers the maintenance these systems need. Older Victorian drain runs under period terraces also crack and shift with age, and a slow leak under a floor may show up as flies long before you see damp.

Finding a hidden source usually starts with a check of the waste pipework you can reach and, where an underground drain is suspected, a CCTV drain survey, which sends a camera through the pipe to show cracks, displaced joints and build-up without digging. Heavy fat and scale build-up can be cleared with our drain clearance service, and broken sections can be put right through our drain repair service. Fixing the leak removes the breeding site, and the flies disappear within a few weeks without any need for insecticide.

If drain flies keep coming back after cleaning, or they appear alongside smells, slow drains or damp patches, call Emergency Plumber Chelsea on 020 3002 6667. We trace the source in homes and flats across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham, and explain the options before any major work starts. There is no call-out fee on jobs over 3 hours.

Quick tips

• Tape over each plughole and overflow overnight; flies stuck to the tape reveal the breeding drain.
• Scrub the slime out with a drain brush; bleach and boiling water alone run straight past it.
• Check basin and bath overflows, which are easy to miss and often the source.
• Run water in unused bathrooms and floor drains every week or two so traps do not dry out.
• Flies that return after a thorough clean usually mean a hidden leak or broken drain; get it checked.

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