Drain Flies in the Bathroom: What Causes Them and How to Get Rid of Them

Drain flies are not coming in from outside — they are breeding inside your pipes. The small fuzzy moth-like flies resting on your bathroom wall hatched a few feet away, in a layer of gelatinous gunge coating the inside of a drain, a trap or an overflow. That is why they keep reappearing however many you kill, and it is also why the fix is simpler than most people expect once you know where to look.

How to be sure they are drain flies

Drain flies — also called moth flies, filter flies or sewer flies — are roughly 2 to 5mm, grey or tan, and covered in fine hairs that make them look like tiny moths rather than houseflies. They hold their wings flat over the body at rest.

The giveaway is how they move. They are poor fliers. They do short clumsy hops rather than circling the room, and they spend most of the day sitting motionless on a wall or ceiling near the drain they came out of. If the flies in your bathroom are zipping about the light fitting, you are probably looking at something else.

What causes them

Every waste pipe develops a soft organic film on its inner wall — soap, skin, hair, toothpaste, shower gel and the bacteria that live on all of it. In a pipe that gets used hard every day, flow keeps that film thin. In a pipe that is used lightly, or where the fall is poor and water sits, it thickens into a jelly-like sludge.

That sludge is drain fly habitat. It is where the adults lay, what the larvae eat, and the only thing they need. A drain does not have to be blocked or smelly to support them — it just has to have a film thick enough to live in.

The places we most often find the source:

• A shower or bath waste — hair and soap make the thickest film of anywhere in the house.
• A basin overflow — the small slot near the rim. Almost never cleaned, permanently damp, and a classic breeding site people never think to check.
• A guest bathroom or utility sink used once a month — low flow, thick film, dried-out trap.
• Under a floor or behind a panel — a slow leak from a cracked waste pipe, feeding a patch of permanently wet debris. This is the one that will not respond to cleaning the drain.

Drain fly larvae — what you are looking for

The larvae are thin, pale, legless and about 4 to 10mm long, with a darker breathing tube at the tail end. They live lying flat in the film rather than swimming, so you will not usually see them in standing water. Scrape the inside wall of a suspect drain with a stiff brush or an old butter knife and look at what comes out: dark jelly with fine pale threads in it is an active breeding site.

The lifecycle is quick — eggs hatch in a day or two, and the whole thing from egg to adult runs about two to three weeks. That matters practically: if you clear the film properly, the flies stop within a fortnight. If you are still seeing new ones a month later, you have missed a breeding site.

Find the right drain first

Do not treat every drain in the house. Find the one they are actually using.

Last thing at night, dry the area around each suspect drain and tape a piece of clear packing tape across the opening, sticky side down, leaving a small gap at one edge for air. Do the same over basin overflows. Check in the morning. Adults emerging overnight get caught on the underside of the tape, and whichever drain has flies stuck to it is your source. Repeat two or three nights running, because emergence is not even.

Getting rid of them properly

The rule is mechanical, not chemical. You have to physically remove the film. Anything you pour in either runs straight past it or kills the surface layer and leaves the rest.

• Brush the pipe. A long flexible bottle brush or drain brush, worked as far down the waste as it will go, then rinsed and repeated. This alone solves most bathroom cases.
• Do the overflow. Same brush, into the overflow slot. Flush it through afterwards with hot water and washing-up liquid.
• Clear the trap. If you can unscrew the bottle trap under a basin, do — empty it, scrub it, refit it. Most of the film is in there.
• Then use an enzyme or bacterial drain treatment. These digest the residue you could not reach. Used after brushing they work well; used instead of brushing they mostly do not.
• Keep the traps wet. Run water through every drain in the house weekly, including the ones you never use.

What not to bother with

Bleach is the usual first attempt and the usual disappointment. It is a liquid, the film is a water-repellent gel, and the bleach slides over it and away down the pipe within seconds. It kills what it touches on the surface and the colony rebuilds from underneath.

Boiling water has the same problem and adds a real risk of cracking a plastic waste pipe or softening a solvent-welded joint — which can turn a fly nuisance into a leak.

Caustic drain unblocker is aimed at blockages, not biofilm, and leaves the drain hazardous to open afterwards. Fly sprays deal with the adults you can see and none of the hundreds of larvae you cannot.

Are they dangerous?

They do not bite or sting and they do not damage the building. They can carry bacteria mechanically from the drain onto surfaces, so they are worth dealing with in a kitchen or a bathroom used by anyone vulnerable, and a heavy long-running infestation can shed enough fragments to irritate asthma. Mostly, though, they are a symptom rather than a hazard — what they are telling you is that something in your drainage is holding water and organic matter when it should not be.

When it is not just a dirty pipe

Call someone if you have brushed and treated the drains thoroughly and flies are still emerging after three or four weeks, if they are appearing in a room with no drain in it, or if they come with a persistent damp smell or a stain on a ceiling or wall.

Those point to a breeding site you cannot reach — most often a cracked or displaced waste pipe under a floor, behind a bath panel or in a wall void, quietly wetting the material around it. That is a drainage repair, not a cleaning job, and the flies will keep coming until the pipe is fixed. Our guide to drain repairs covers what that involves.

A dried-out or damaged trap can also let flies through from the drain proper, which is a different fix again — see why drains smell in hot weather for how trap seals fail.

Flies that will not go away in Surrey or South London? If you have cleaned the drains and they keep coming back, there is usually a wet spot somewhere you cannot see. Call 07771 200075 or message us on WhatsApp and we will help you work out where it is.