Drain Flies: How to Get Rid of Them (and What They're Telling You About Your Pipes)

Those little fuzzy moth-like flies loitering around the plughole are drain flies, and here is the thing nobody selling you a spray wants to say out loud: the flies are not the problem. The inside of your pipe is.

Drain flies lay their eggs in the film of grease, soap scum, hair and general biological regret that coats the inside of a drain. The adults you can see are the visible tip of a production line you cannot. Which is why swatting them, spraying them, or pouring things on them is a rolling subscription rather than a fix.

What actually works (do this first)

Before anyone charges you anything, do the honest DIY version:

1. Find the breeding drain. Tape a piece of clear tape, sticky side down, over each suspect plughole overnight. The drain that decorates its tape with flies is your culprit. (Yes, this is a genuine diagnostic technique. We use it too, and we love it.)
2. Clean what you can reach. Take off the trap under the sink, or lift the shower waste, and physically scrub the first stretch of pipe with a stiff bottle brush. The film has to come off the walls. Chemicals alone slide past it.
3. Follow with an enzyme cleaner, not bleach. Enzyme or bacterial cleaners digest the organic layer; bleach mostly just annoys it on the way through.
4. Flush hot water for a minute and repeat the enzyme treatment a few days running.

Do that properly and a straightforward case ends there, for the price of a bottle brush.

When it isn’t a straightforward case

If the flies come back within a couple of weeks, the breeding layer is somewhere you cannot reach: further down the run, in a rarely used floor drain, in a guest bathroom trap that has dried out, or in a dead section of pipe left over from an old refit. This is very common in older converted buildings, and around Hounslow’s food parades the shared runs behind commercial kitchens grow the kind of grease film that a domestic brush has no chance against.

That is jetting territory. High-pressure jetting strips the pipe wall back to bare clay or plastic along the entire run, which removes the breeding ground itself. Typical domestic jetting runs £150 to £300, agreed before we start. It is the difference between evicting the flies and demolishing their housing.

One more tell worth knowing: if the flies appeared alongside a drain that has also become slow or smelly, the film has probably progressed from nuisance to early-stage blockage, and dealing with it now is cheaper than dealing with it in six months.

The prevention bit (short, because it is simple)

Flies need a stagnant film to breed in. Deny them one: run water weekly through drains you rarely use so the traps stay full, keep fat out of the kitchen sink, and give plugholes an occasional enzyme treatment. A drain that is used and rinsed is a terrible nursery, and we mean that as a compliment.