Drain flies in the bathroom: where they breed and how to stop them

How to recognise drain flies

Drain flies, also called moth flies or sewer gnats, are a few millimetres long with hairy, rounded wings held flat or roof-like over the body. They fly weakly, tend to rest on walls and ceilings near sinks, baths and showers, and leave a small smudge when squashed. They are most noticeable in the evening and after the bathroom has been warm and humid.

Fruit flies look similar at a glance but have smooth wings, often red eyes, and cluster around fruit bins and kitchen waste. Fungus gnats are usually found near houseplants. Getting the identification right matters, because the fix for drain flies is in the plumbing.

Find the breeding site before you treat anything

Each outlet in the room is a possible source, and so is any spot where water sits. A simple test narrows it down. Clean and dry the outlets, then cover each one overnight with a strip of clear tape, sticky side down, leaving a small gap for air. Flies emerging from that waste will be stuck to the tape in the morning. Check every outlet: basin, bath, shower, bidet, floor gully and any kitchen or utility waste nearby.

If none of the outlets catch flies, look further. A sealed bath panel hiding a slow leak, a damp void under a shower tray, a toilet with a failed seal at the pan connector, or a standpipe behind a washing machine can all hold enough wet organic matter to support a colony.

• Outlets rarely used: guest basins, spare showers, floor drains
• Overflows on basins and baths, which are rarely cleaned
• Around the base of the toilet and behind boxing
• Under the bath or shower tray, if water has been escaping
• External gullies and inspection chambers close to the bathroom

Clearing the slime from traps and wastes

The eggs and larvae live in the gelatinous film that coats the inside of the trap and waste pipe, made up of soap, hair, skin and toothpaste residue. Bleach poured down the plughole tends to run straight past it. The effective approach is mechanical: remove and clean the trap where it is accessible, use a stiff pipe brush through the waste and overflow, and flush thoroughly with hot water.

Bottle traps under basins and most bath traps can be unscrewed by hand. Shower traps usually have a removable insert or cup under the grille. Refit washers carefully and run water afterwards to check that nothing leaks, because a disturbed trap seal can cause a slow drip that feeds the next generation of flies. Repeat the cleaning for a couple of weeks, since new flies can still emerge from eggs already laid.

Do not mix drain cleaning chemicals, or use them in a trap you are about to dismantle. Tell a plumber what has already been put down the drain before they work on it.

Dry traps and unused outlets

Every waste outlet has a trap that holds a small plug of water to keep drain air out of the room. When an outlet is not used for weeks, that water evaporates. The trap then becomes an open route from the drain into the bathroom, and flies breeding further down the pipe can come straight up. A sewer smell in the same room is a strong clue that this is happening.

Running each tap for a minute every week or so keeps the seal topped up. If a basin or shower has been permanently abandoned, it is better to have the waste properly capped off than to rely on remembering. Floor gullies in wet rooms and utility rooms dry out especially quickly and are a common source in otherwise clean bathrooms.

When flies point to a leak or broken drain

Flies that return after every outlet has been cleaned and every trap refilled are a signal worth taking seriously. Behind a bath panel or under a shower tray, a slow leak from a waste joint can soak the subfloor for months and grow exactly the kind of film drain flies need. Around a toilet, a failed pan connector seal can leak both water and drain air into the floor void.

Below ground, a cracked or displaced drain under a ground-floor bathroom can let waste soak into the soil and fill, giving flies a breeding site you cannot reach from inside. In that case you may also notice smells that come and go, damp at the base of walls or slow-draining outlets. A drain camera survey can show whether the pipe is intact without lifting floors on a guess.

Getting help from a plumber

If you have cleaned every waste, kept the traps topped up and still have flies after two or three weeks, the source is probably somewhere you cannot see. Loyal Plumbers investigates drain smells, leaking wastes and damaged drains across London and inside the M25. We can check traps and seals, look behind panels where access allows, and arrange a CCTV drain survey when the evidence points below the floor.

When you call or enquire, tell us which room the flies are in, whether there is a smell, which outlets are rarely used, and whether the bathroom is on the ground floor or above another room. That lets us bring the right equipment on the first visit rather than starting from scratch.