Are flies in my drain a sign it’s blocked?

Very likely, yes. When a drain empties slowly, water and gunk sit in the pipe and a slimy film builds up on the walls. That film is exactly where drain flies lay their eggs. So a slow drain and a cloud of small, fuzzy flies on the bathroom wall usually go together.

Most of the time the blockage is hair and soap in the first part of the pipe, which you can clear yourself. A few signs mean it is worth getting a plumber, and they are below.

How to tell the drain is the source

• It drains slowly, or water stands in the shower tray or basin for a while.
• It gurgles or smells, especially after you run water.
• The flies gather on the wall nearest that drain and sit flat with fuzzy, moth-like wings. Not sure they are drain flies? See how to tell the small flies apart.
• The tape test catches them. Lay clear tape over the plughole at night, sticky side down, with a small gap for air. Flies on it in the morning mean that drain is breeding them.

Clear it yourself first

1. Take out the plug, stopper and hair catcher and pull out what is caught underneath. A cheap plastic hair-grabber strip reaches further down than fingers.
2. Use a plunger. Cover the overflow hole with a wet cloth so the push goes down the pipe, not out the overflow.
3. Clean the trap under the sink if you are comfortable doing it. Put a bucket underneath, unscrew it by hand, rinse it out, screw it back hand tight and check for drips.
4. Brush the pipe with a long, narrow drain brush to scrape off the film the flies breed in.
5. Finish with an enzyme or foaming drain cleaner at night, following the label. It breaks down what the brush loosened. Our answer on why bleach does not clear drain flies explains why this beats bleach.

Safety: if you have already poured a chemical unblocker down, do not plunge straight after, because it can splash back. Never mix drain products with each other or with bleach. Wear gloves, open a window, follow each label, and keep products where children and pets cannot reach them.

When to call a plumber

• More than one drain is slow at once, or water bubbles up in the bath when you use the basin or flush the toilet. That points to a blockage further down, shared by several fittings.
• A sewage smell that does not go after cleaning.
• Water backing up outside, in a yard drain or inspection cover.
• Flies keep coming when every drain is clean and fast. The film may be somewhere you cannot reach: a cracked pipe, a leak under a bath or shower tray, or damp under the floor. Ask whether the plumber can look inside the pipe with a camera.

Tell the plumber what you have seen and when: which drains are slow, how long the flies have been around, and what you have already tried. A few dated photos of flies on the tape help them see it is the pipe, not the room.

If you rent, report a slow drain and the flies to your landlord or agent in writing, with dates. Who fixes what depends on your tenancy and where you live. If nothing happens, see what to do when a landlord will not act.

Once it is clear, keep it that way

• Fit a hair catcher on shower and bath drains and empty it often.
• Use a drain cover or the plug at night while the last adults die off. It stops flies coming out of the pipe and new ones getting in to lay eggs. A cover helps once the pipe is clean, not before.
• Brush the pipe now and then and run water through drains you rarely use, so the trap stays full.

Expect a few flies for some days after the drain runs freely. If you are still catching them on tape a couple of weeks later, go back to the plumber list above. Gaps where pipes come through walls can also let flies in from a damp void, so seal those while you are at it.