Drain flies in the bathroom: how to get rid of them
Tiny grey flies on the bathroom tiles, around the toilet or above the kitchen sink are almost always drain flies, also called sewer gnats or moth flies. They do not come in from outside but from a drain in your home. Find that drain and clean it thoroughly, and they are usually gone within a few weeks. Below you can read how to recognise them, how to find the right drain and what does and does not help.
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What are drain flies or sewer gnats?
Drain flies are small, fuzzy flies of 2 to 5 millimetres that live in and around drains. They are also called sewer gnats, drain gnats, sewer flies or moth flies. The name moth fly comes from their broad, hairy wings, which make them look like a very small moth.
The larvae live in the layer of slime on the inside of a drain. The adult flies sit on the wall or the tiles close to that drain.
What do drain flies look like?
A drain fly has a grey or grey-brown, fuzzy body and broad wings that it holds over itself like a little roof when it rests. It is a poor flyer: it makes short, fluttering hops and soon settles again.
This is how to tell it apart from other small flies:
- Drain fly (sewer gnat): grey and fuzzy, broad wings, sits still on the wall near a drain in the bathroom, toilet or kitchen.
- Fruit fly: light brown with red eyes, hovers around fruit, food waste and empty bottles.
- Fungus gnat: black and slender with long legs, walks over the compost of houseplants.
Still not sure which fly you have? Read small black flies in the house.
Where do drain flies come from?
Drain flies come from a drain with a layer of slime made of soap residue, hair and grease. The females lay their eggs in that layer and the larvae live in it.
The usual places:
- the shower drain and the bath waste
- the overflow of the basin and the bath
- the trap under the basin or kitchen sink
- a drain you rarely use, for example in a guest bathroom, utility room or cellar
A drain that is rarely used is especially vulnerable: the water in the trap evaporates and the slime is left undisturbed.
Can drain flies come out of the toilet?
Yes, but drain flies usually come from a drain next to the toilet and not from the bowl itself. A toilet that is flushed every day gives the larvae little chance.
If they do come from the toilet, it is often one that is rarely used, or there is build-up under the rim or a leaking connection behind the pan. If you also see them in places without a drain, or you notice a sewer smell, a waste pipe may be leaking or broken. Have that checked by a plumber or a drainage company.
How do you find out which drain the flies are coming from?
You test it with a strip of clear sticky tape. In the evening, stick a strip over the drain with the sticky side facing down and leave a little room for air at the edges. If there are flies stuck to the underside the next morning, they are coming from that drain.
Test every drain in the house this way, including the ones you rarely use, and repeat it for a few nights. Look at what is stuck to the tape as well: a fly with red eyes is a fruit fly, because fruit flies can also breed in a dirty drain. In that case our guide on how to get rid of fruit flies will help you further.
How do you get rid of drain flies?
You get rid of drain flies by removing the slime the larvae live in. We do not have a product for this; thorough cleaning is the solution here.
- Take out the grate or cover and remove hair and dirt.
- Scrub the inside of the waste pipe with a flexible drain brush, as far as you can reach.
- Rinse with a kettle of hot water. With plastic pipes, hot is better than boiling.
- Brush out the overflow of the basin and the bath and rinse that too.
- Unscrew the trap and rinse it clean.
- Repeat this a few times a week, until the tape stays empty for a few nights in a row.
Swat or vacuum up the adult flies that are already flying around. Without new flies emerging from the drain, the problem stops by itself.
Will bleach, vinegar or bicarbonate of soda get rid of drain flies?
Not well enough. Bleach, vinegar, bicarbonate of soda and boiling water run past the slime and are gone again in seconds, while the larvae sit deep inside that layer. Scrubbing with a brush does more than any liquid.
Never pour bleach into a drain together with vinegar or a drain unblocker: this releases harmful fumes.
Are drain flies harmful?
Drain flies do not bite or sting. They are mostly a nuisance, and if you see a lot of them, it is a sign that dirt or water is standing somewhere.
How long does it take to get rid of drain flies?
Allow two to three weeks after the first thorough clean. Eggs and pupae that the brush missed will still hatch in that time, which is why you repeat the cleaning a few times.
If they keep coming back after a month, the source is somewhere else: a forgotten drain, a leak under the floor or a damaged pipe.
How do you stop drain flies from coming back?
You stop drain flies from coming back by keeping drains clean and in use. Run water through rarely used drains every week, so the trap stays full.
Remove hair from the shower drain regularly, brush the drain through once a month and do not leave water standing in buckets or plant saucers. Fix a dripping tap or leaking connection quickly.
