How to get rid of drain flies: find the drain they come from, then scrub it out

Drain flies don't come from outside: they breed in the slimy film that coats the inside of a drain, so the fix is in the pipe, not in the air. Find the drain they come out of, scrub that film away, and the flies stop. Below, one bathroom goes from flies on the tiles to clean tape a week later, with a drawing for every step.

Part 1: Find the drain they come from

1. Look closely at the flies.

   

   Hana leans in to the tile by the bath. The fly is tiny, grey and furry, with wide wings folded like a little roof.

   Drain flies are about 4 to 5 mm long, dark and covered in fine hairs, and hold their wings roof-like over the body (Texas A&M). They are weak fliers, flutter only a short way at a time, and are most active in the evening.

   You should see: fuzzy flies resting still near sinks, baths or showers, not hovering round food.

2. Tape over every drain overnight.

   

   At bedtime, Hana sticks a strip of clear tape across the bath plughole, leaving a gap at each end. She does the same on the basin, the kitchen sink and the shower tray.

   Stick tape across each drain without covering it completely: if no air can get through, the flies won't come up (Texas A&M). UF/IFAS says the source is usually the closest drain, because these flies are weak fliers, and suggests leaving the tape for 24 hours. Our advice: tape every drain anyway. It costs nothing and shows up a second source. Don't run water into the taped drains overnight.

3. Check the tape in the morning.

   

   In the morning, the tape on the bath and the basin has tiny grey flies stuck to it. The kitchen sink and shower tapes are clean.

   Flies stuck to the tape mean that drain is a breeding site (Texas A&M). Clean every drain that caught flies. Note the clean ones too, so you can re-test them later.

   You should see: flies on one or more tapes. If every tape is clean, try the overflow, a little-used toilet and floor drains (Part 3).

Part 2: Scrub out the drain

1. Take out the plug and strainer.

   

   In rubber gloves, Hana lifts the pop-up plug out of the bath. Its stem is coated in grey slime and hair.

   Most pop-up plugs lift or unscrew straight out. Pull off the hair and slime and drop it in the bin, not back down the drain. Scrub the plug with an old toothbrush and hot soapy water.

2. Brush the inside of the pipe.

   

   Hana pushes the long brush down the plughole and twists it round and round, working it up and down.

   This is the step that matters. Scrubbing the inside of the pipe removes the slimy film the larvae feed on (Texas A&M, UF/IFAS). Push the brush as far as it goes, twist it, and pull it out and rinse it several times until it comes out clean.

   You should see: dark grey slime on the brush at first, then much less.

3. Flush the drain with hot water.

   

   Hana pours a kettle of hot water slowly down the plughole to wash away what the brush loosened.

   Texas A&M and UF/IFAS say to flush the drain with boiling water after scrubbing. Pour slowly, straight into the hole. Our advice: don't pour boiling water into a toilet bowl, which can crack, and if your waste pipes are plastic and you're unsure, let the kettle cool for a minute first.

4. Clean the overflow.

   

   At the basin, Hana pushes a small bottle brush into the overflow slot just below the tap and scrubs it, then flushes it with hot water.

   The overflow channel stays damp and is easy to forget. Scrub it the same way as the drain, then rinse it with hot water poured into the overflow hole.

5. Clear the trap under the basin, if you can reach it.

   

   Hana puts a bucket under the basin and unscrews the bottom of the white plastic trap. Dirty water and slime drop into the bucket.

   Texas A&M says to clean the traps as well as the pipes. Many UK basins have a plastic "bottle trap" whose base unscrews by hand; US sinks usually have a curved P-trap with slip nuts. Put a bucket underneath, unscrew it, scrub it in the bucket, and screw it back hand-tight with its rubber seal in place. If yours is hard to reach or the joints are sealed, skip this step (our advice).

   You should see: no drip from the trap when you run the tap afterwards.

6. Add a drain treatment, if you want one.

   

   At night, when the bath won't be used for hours, Hana pours a bacterial drain gel into the plughole, as its label says.

   Texas A&M says bacterial drain treatments that break down organic matter can help, used exactly as the label says. They work best after scrubbing, not instead of it. Use one product only, and never with bleach.

Part 3: Check the hidden places

1. Flush a toilet you rarely use.

   

   Hana remembers the little cloakroom toilet nobody uses. She brushes under its rim and flushes it.

   Texas A&M lists toilets, particularly ones that aren't used often, as breeding sites. Brush under the rim and flush it at least once a week (our advice).

2. Look for damp under loose tiles and seals.

   

   Hana checks the sealant round the bath and spots a cracked, dirty strip where water sits.

   Texas A&M lists loose floor tiles where water collects as a breeding site, and condensate lines (for example on an ice-maker). Dry and repair damp spots. For mouldy sealant, see the related guides below.

Part 4: Keep them gone

1. Repeat the brush-and-flush weekly.

   

   A few days later, Hana brushes and flushes the bath and basin drains again. Only one fly is left on the tiles.

   Eggs hatch in under 48 hours and larvae take 8 to 24 days to grow (Texas A&M), so a few more flies can appear after the first clean. Keep scrubbing each source drain weekly until no new flies turn up.

2. Re-test with tape a week later.

   

   A week on, Hana tapes the drains again. In the morning, the tapes are clean.

   Clean tape on every drain means you've found and cleared the source. Fly spray only kills adults you can see, and doesn't solve the problem (Texas A&M).

Who to ask if they won't go

This table is our general guidance on who to contact, not a statement of the law in each country.

If they keep coming back

Source: Texas A&M AgriLife Extension, Drain Flies (E-184). Rows 2 and 3 follow Texas A&M; the other rows are our advice.

Related: How to get rid of fruit flies · How to get rid of fungus gnats · How to stop condensation and mould · How to descale a shower head.