Drain Flies: What Causes Them and How to Get Rid of Them
Small, furry, moth-like flies around a sink or shower are drain flies breeding in the gunk inside your pipes. How to confirm which drain they come from, how to clear them properly and when they point to a broken drain.
Drain flies, also called moth flies or filter flies, are small fuzzy flies that breed in the slimy film that builds up inside sink, shower and bath drains. To get rid of them, find which drain they come from with a tape test, then scrub out the plughole, trap and overflow with a stiff drain brush and flush it with hot water. Sprays kill the adults, but the flies keep coming back until the breeding film is removed. If they persist after every drain is clean, a cracked or leaking pipe may be the source.
Key Takeaways
- Drain flies are small, grey-brown and furry, with moth-like wings. They hop and flutter rather than fly far.
- They breed in the slime lining drains, overflows and traps, not in the water itself.
- Tape over each plughole overnight to find the source drain before cleaning.
- Physical scrubbing removes the breeding film. Pouring bleach down the plughole rarely does.
- Flies that return after thorough cleaning can point to a broken drain or a leak under the floor.
Drain flies are small, furry, moth-like flies that breed in the slimy film lining sink, shower and bath drains. If you keep seeing them resting on bathroom walls or around a plughole, they are almost certainly coming from one of your own drains. The fix is to find that drain, scrub out the slime they breed in and keep water running through it. Fly sprays only kill the adults you can see, so the flies come back until the breeding film has gone.
What drain flies are
Drain flies are also called moth flies, filter flies or sewage flies. They are small, grey or brown, and covered in fine hairs that make them look fuzzy. At rest they hold their wings in a roof shape over the body, which is why they look like tiny moths. They are weak fliers, so you usually see them sitting on a wall near a sink or shower, or making short, jerky hops when disturbed.
They are easy to confuse with two other common household flies:
- Fruit flies are smoother and tan with red eyes, and hang around fruit bowls, bins and recycling rather than plugholes.
- Fungus gnats are thin and dark with long legs, and come from damp compost in house plants.
If the flies are fuzzy, moth-shaped and gathered around a drain, they are drain flies.
Why you have them
Every drain that carries soap, hair, skin, toothpaste and food waste slowly builds up a layer of slime on the inside of the pipe and trap. Drain flies lay their eggs in that film, and the larvae live and feed in it. Standing water isn't what they need, it's the organic gunk.
The places they most often breed are:
- Bathroom basin and bath overflows, which are almost never cleaned and stay damp.
- Shower traps, especially where hair has built up. Our guide on how to unblock a shower drain shows how to open one up.
- Kitchen sink traps and waste pipes with grease build-up.
- Drains in a spare bathroom or utility room that are rarely used.
- Outside gullies close to a back door or window.
- Less obviously, a leaking waste pipe under a bath, shower tray or floor, where wet waste soaks into the boxing or subfloor.
A slow-draining sink or shower is a good hint, because a partial blockage means more slime and more still water. Drain flies often turn up alongside a bad smell. If you are also noticing a sewage smell, read our guide to a sewage smell in the house.
Find the drain they come from
Before cleaning everything, spend one night finding the source. It saves a lot of wasted effort.
Tape over each drain
Last thing at night, stick a strip of clear packing tape over each plughole and overflow in the affected rooms, sticky side down, leaving a small gap at the edge for air. Don't run water afterwards.
Check in the morning
Flies emerging from the drain get stuck to the underside of the tape. The drain with flies on its tape is the breeding site. Repeat for a couple of nights if the result isn't clear.
Check the less obvious spots
If no tape catches anything, look at the bath overflow, the washing machine standpipe, any floor drain and the outside gully. If the flies are gathering near a bath panel or skirting rather than a plughole, suspect a leak.
How to get rid of drain flies
The aim is to remove the film the larvae live in. Cleaning products poured into the plughole tend to run straight past it, so physical cleaning does most of the work.
- Remove what you can see. Lift out the plug, pop-up waste or shower trap cover and pull out hair and debris.
- Scrub the pipe. Use a long, flexible drain brush, pushed down the plughole and worked up and down the pipe and into the overflow. The black or grey slime that comes out is where the flies breed.
- Clean the trap. Under a sink, put a bowl beneath the U-bend or bottle trap, unscrew it by hand and wash it out. Refit it hand-tight and check for drips.
- Flush with hot water. Run hot water through the drain for a few minutes. On plastic pipework use hot tap water or water just off the boil, not repeated kettlefuls of boiling water, which can soften plastic traps and joints.
- Follow up with an enzyme or bacterial drain cleaner. These are designed to break down organic build-up and are gentler on pipes than caustic products. Follow the instructions and leave it overnight.
- Repeat for a week. Clean again after a few days. Adults that hatched before you started will live on briefly, so expect some stragglers.
To stop them coming back, run water through every drain at least once a week, including spare bathrooms, clean the overflows every few weeks, and fit hair catchers on showers. If a drain is slow, clear it properly. Our guide to unblocking a drain covers plungers, rods and traps.
Why they keep coming back
If you have cleaned every drain properly and the flies are still appearing after a couple of weeks, the source is probably somewhere you can't reach with a brush:
- A leaking waste pipe. A loose joint under a bath or shower tray can drip dirty water into the boxing or floor void for months. Damp, dirty material there is ideal for drain flies. Look for staining, a musty smell or soft flooring.
- A cracked or displaced drain. Underground drains can crack or shift at the joints, often from ground movement or tree roots, leaving pockets of standing waste. Our article on tree roots in drains explains the signs.
- A partly blocked soil stack or outside gully that keeps waste sitting close to the house.
- A dry trap on a drain nobody uses, which also lets sewer smells in. See drain smells from unused bathrooms.
When to call a plumber
Call a plumber if the flies keep returning after thorough cleaning, a drain is slow or gurgling, there is a sewage smell, or you can see damp around a bath, shower or kitchen unit. We can take traps and waste pipes apart properly, find and repair leaking joints, clear blockages with rods or drain jetting, and where the problem is underground, carry out a CCTV drain survey to see what is happening inside the pipe.
When you call, tell us which drain the flies come from, what you've already tried and whether anything is draining slowly. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before any work starts.
Flies still coming back from a drain?
It usually means a hidden blockage or leak. Tell us what's happening and we'll confirm the price before starting.
📞 Call 020 7870 3200Or see our blocked drain service and pricing.
Frequently Asked Questions
Are drain flies harmful?
Drain flies don't bite or sting and aren't generally considered a health risk, but they come from dirty drain slime, so you don't want them landing on food or surfaces. A large number is also a sign that a drain is dirty, partly blocked or damaged.
Does boiling water kill drain flies?
Hot water helps loosen the film they breed in, but on its own it rarely clears an infestation because it runs straight past most of the slime. It works best after the trap and pipe have been scrubbed. Use hot rather than boiling water on plastic waste pipes and traps, as repeated boiling water can soften plastic and loosen joints.
How long does it take to get rid of drain flies?
Once the breeding drain has been properly cleaned, you should see far fewer flies within a few days. Adults that have already hatched can hang around for a week or two. If numbers aren't falling after a week, you've probably missed the source.
Why do I have drain flies in a bathroom I rarely use?
Unused drains are a common source. Water in the traps sits still, slime builds up, and nothing flushes the eggs and larvae away. Run each tap and shower for a minute every week or so and clean the overflow on the basin and bath.
Can drain flies mean a broken drain?
Yes. If every drain has been cleaned and the flies keep returning, they may be breeding in a leak or a cracked pipe under the floor or behind a bath panel, where wet waste has soaked into the ground or boxing. A plumber can check the waste pipes and, if needed, carry out a CCTV drain survey.