Small, fuzzy flies hovering around a sink, shower or floor drain are common, annoying and surprisingly persistent. They are usually a sign of what is happening inside the pipe rather than a problem with the room itself.
What drain flies are
Drain flies, also known as moth flies or sewer flies, are small, dark insects with hairy wings. They breed in the slimy film of organic matter that builds up on the inside of drains, which is why they keep coming back when the underlying build-up is still there.
Why they appear
- Organic build-up. Hair, soap scum, grease and food particles form a film on pipe walls that the flies lay eggs in.
- Drains that are rarely used. Spare bathrooms and utility room floor drains can sit still for weeks, which gives the film time to develop.
- A dried out trap. If the water seal in a trap has evaporated, there is an open route between the drain and the room.
- A partial blockage further down. Slow-moving waste sitting in the pipe gives them more to feed on.
Getting rid of them
Killing the adult flies does not solve the problem, because more will hatch from eggs in the pipe. The build-up is what needs dealing with. Regularly flushing little-used drains with water, cleaning the visible parts of the drain and keeping traps topped up helps with minor cases.
When it keeps coming back
If flies return within days of cleaning, the source is probably further along the pipe than you can reach. Persistent flies, a smell, or slow drainage together suggest a build-up or blockage that needs clearing properly. A CCTV drain survey can show how far the build-up extends, and jetting can clear it from the full length of the pipe. Our page on blocked drains covers the other common causes.
If drain flies keep returning after you have cleaned the drain, the cause is likely further down the line. Call us on 0333 200 1623 or use our contact page and we will help.