Drain Fly Guide: Why They Appear and How to Get Rid of Them

How to recognise a drain fly

Drain flies, also called moth flies or sewer flies, are only a few millimetres long with grey or brown hairy bodies and wings that fold flat like a roof. They fly weakly, in short hops, and spend most of the day sitting still on walls, tiles and the side of the bath. Fruit flies, by contrast, are smoother, often have red eyes and gather around the fruit bowl and bin rather than the plughole.

One or two in autumn may have wandered in from outside. A steady number, day after day, in the same room means they are breeding somewhere close by.

Why they breed in drains

Every waste pipe slowly builds up a slimy layer of soap, hair, skin, food and grease. Drain flies lay their eggs in that layer, and the larvae live in it, feeding on the bacteria and organic matter. Anywhere the film stays damp and undisturbed will do: the trap under a basin, a shower waste, the overflow channel on a bath or sink, a floor gully in a utility room, or a little-used toilet.

Because the whole cycle from egg to adult can take only a week or two in a warm room, numbers rise quickly once a good breeding spot exists.

Finding which drain they are coming from

Before cleaning everything, work out which outlet is the source. A simple test is to put a strip of clear tape, sticky side down, loosely across each plughole and overflow last thing at night, leaving a small gap for air. In the morning, any tape with flies stuck to the underside marks a breeding drain. Repeat over a few nights, because adults do not emerge every day.

• Basin and bath overflows, which are rarely cleaned
• Shower wastes, especially under trays with hair traps
• Floor gullies and utility room drains
• Toilets in spare rooms that are seldom flushed
• Kitchen sink traps and dishwasher connections

How to get rid of drain flies

Sprays kill the adults you can see but do nothing to the larvae, so the flies return within days. The fix is to remove the film they live in. Take out the plug or hair catcher, then scrub the inside of the waste and the overflow with a stiff, narrow pipe brush. Where you can, undo the trap under the sink, clean it out and refit it. Flush through with plenty of hot water from the tap.

Pouring bleach down the drain is popular but often disappointing. It runs straight past the film rather than scrubbing it off, and it is not good for pipe seals. Boiling water from a kettle is also best avoided on plastic wastes and older joints. Mechanical cleaning, done every few days for a couple of weeks, breaks the cycle far more reliably.

When flies point to a drainage fault

If you have cleaned every trap and overflow and the flies still appear, the breeding ground may be somewhere you cannot reach. Common causes are a cracked or broken pipe under a floor or slab, leaking onto soil that stays wet; a joint that has opened on a soil stack inside boxing; a drain run that has sagged and holds standing waste; or a blocked inspection chamber outside. Flies coming up between floor tiles, around the base of a toilet or from skirting boards are a strong hint that the pipe, not the plughole, is the problem.

In those cases the drain needs cleaning back to bare pipe and, if the flies keep returning, inspecting with a camera so the defect can be found without lifting floors on guesswork.

Keeping them away

Drain flies are a sign of a drain that is not being cleaned through. Run water in rarely used basins, showers and toilets at least weekly so traps stay full and the film is flushed. Clean overflows and hair traps as part of normal bathroom cleaning. In kitchens, keep fat out of the sink and wipe greasy pans before washing. For busier properties, or drains that foul up quickly, planned cleaning keeps the pipes clear before problems start.

Questions we get asked