Drain flies in a Korean apartment: where they come from and how to stop them

What they actually are

Drain flies are small, greyish, and fuzzy, with wings held roof-like over the body so they look more like tiny moths than flies. They are weak fliers — they tend to sit still on a wall or a tile and then make a short hop rather than circling a room. That behaviour alone is a good identification clue.

They do not breed in your bin or in your fruit bowl. They breed in the soft organic film that builds up on the inside wall of a drain pipe, just below the plughole: a layer of grease, soap residue, hair and food particles. The adults lay eggs in that film, the larvae feed on it, and the whole cycle happens inside the pipe where nothing you spray can reach it.

• Drain flies: found on bathroom walls, around floor drains, near the kitchen sink overflow, often first thing in the morning. Fuzzy, moth-shaped, sit still.
• Fruit flies: found hovering around fruit, food waste bins and drink cans. Smaller, smooth-bodied, tan or reddish, and they fly in loops rather than sitting on tiles.

The distinction matters because the treatment is the opposite. Fruit flies go away when you deal with the food. Drain flies go away when you physically clean the inside of a pipe — and they come straight back if you only deal with the adults you can see.

Why Korean apartments and villas get them so often

This is the part general advice leaves out. Three things about ordinary Korean plumbing make this problem far more common here than in a typical Western house.

1. Bathrooms have a separate floor drain that barely gets used

Korean bathrooms are wet rooms: there is a floor drain in the middle of the floor as well as the drains under the basin, the shower and the toilet. In a household that showers in a tub or uses a shower screen, that floor drain can go weeks without any water going down it. The trap below it holds a small amount of standing water — the water seal — and when that evaporates, the drain is no longer a drain. It is an open pipe connecting the sewer line directly to your bathroom, which is both a smell route and a breeding route. Dry traps are the single biggest reason we see drain flies and sewer smell in the same flat, which is why our guide on sewer smell in a Korean apartment covers exactly the same fix.

2. Several households share one vertical stack

In an apartment or a villa, your bathroom and kitchen drains do not run to their own private pipe. They join a shared vertical stack that runs down through the building, collecting the same fixtures from every floor. That means the breeding film does not have to be in your pipe at all. It can be in the branch of the flat below you, in the neighbour's kitchen line, or in the stack itself — and the adults simply walk or fly up through your dry floor drain into your bathroom. This is why people who keep an immaculate home still get them, and why cleaning your own drains sometimes only helps for a week or two.

3. Summer humidity speeds the whole cycle up

Warmth and humidity shorten the time from egg to adult, and the rainy season delivers both. It is also the time of year when unused traps dry out fastest in an air-conditioned room. That combination is why a problem that was invisible in April turns into a daily annoyance in July.

How to find which drain they are coming from

Do not start cleaning every drain in the flat. Find the source first — it takes one night and no equipment.

1. 1At night, after everyone has finished using water, take clear packing tape and cover each suspect drain: the bathroom floor drain, the shower drain, the basin, the kitchen sink, the balcony drain and the utility-room drain.
2. 2Leave a small air gap at one edge rather than sealing the drain completely airtight. A fully sealed drain can change the pressure in the pipe, and you want the flies to try to come out, not to be stopped by a vacuum.
3. 3Leave it overnight. In the morning, look at the underside of each piece of tape.
4. 4The drain with flies stuck to the tape is your breeding site. If more than one drain has flies, you have more than one source — or a shared-stack source feeding several openings.
5. 5Repeat the test after cleaning, three or four days later, to confirm the problem is actually gone rather than just quieter.

What actually works: clean the film, not the flies

The eggs and larvae live inside a sticky film clinging to the pipe wall. Anything that runs straight past that film — water, spray, most liquids — leaves it in place, and the population rebuilds within days. The film has to be physically scrubbed off.

1. 1Lift the drain cover and take out the hair strainer or basket. Most Korean floor drains and sink drains come apart by hand without tools.
2. 2Remove the trap insert if there is one. Wash both parts in the sink with detergent and a brush until there is no slippery coating left on them.
3. 3Reach into the drain opening with a long, stiff brush — a bottle brush or a drain brush — and scrub the inside wall of the pipe as far down as you can reach, all the way around. You are trying to feel the slime come off, not just rinse it.
4. 4Flush with plenty of warm water, then refit everything, making sure the trap insert seats properly and the seal holds water.
5. 5Pour a jug of water down every drain you rarely use, once a week from now on. That keeps the water seal topped up and stops the pipe becoming a doorway.
6. 6Fit a cover or a plug over the bathroom floor drain when it is not in use, so anything coming up the shared stack cannot get into the room.

Do this in the kitchen too if the tape test pointed there. Kitchen film is grease-based and much tougher than bathroom film; if it will not come off with a brush, that is a sign the layer inside the branch pipe is thick enough to need high-pressure cleaning rather than a scrub.

What not to do — including the advice you will read elsewhere

When it is not a cleaning problem

Cleaning solves the ordinary version of this. These signs mean the source is somewhere you cannot reach:

• You cleaned properly and they are back within one to two weeks
• Flies are appearing in more than one room, or in rooms with different drains
• The drains are also running slowly, or you can smell sewer gas as well
• You hear gurgling from one drain when another one empties
• Neighbours in the same line of flats have the same problem

Any of those point to a damaged or missing trap, a venting problem that keeps siphoning your water seals out, or a build-up in the shared stack that no household can clean from inside their own flat. What we do in that case is put an endoscope camera into the line and look: you see the inside of your own pipe on the monitor, and the decision about what to do next is based on that rather than on guesswork. If it is a build-up in your own branch, high-pressure cleaning strips the film off the pipe wall over the whole run, which is the difference between removing the breeding site and disturbing it.

If you are renting

If the camera shows the source is in the shared stack or the building's venting rather than inside your unit, that is normally the building's or the owner's cost rather than yours, because it is the building's fixed equipment and no single household caused it. Report it to the building management office as well as the landlord — they may already have the same complaint from the flats above and below you.

We issue a written report in English and Korean side by side, with the camera footage, so you can forward it without translating anything. Who pays for what, and the order to contact people, is covered in detail in our guide for tenants and landlords.