Drain flies: where they come from and how to get rid of them

Find which drain

1. Dry the area around each suspect drain in the evening.
2. Stick a strip of clear tape across each drain opening, sticky side down, leaving gaps for air.
3. Check in the morning. Flies stuck to the tape tell you which drain they are coming out of.
4. Repeat for a few nights. There is often more than one.

The usual suspects are drains that are rarely used: a basement floor drain, a spare shower, a laundry tub, the overflow of a bathroom sink.

Getting rid of them

1. Take off the drain cover and scrub inside the pipe with a stiff, long brush as far down as you can reach.
2. Pull out hair and sludge from the trap with a plastic drain strip.
3. Flush with plenty of hot tap water.
4. Use an enzyme or bacterial drain gel for a week. It eats the film they breed in.
5. Run water in rarely used drains every week or two to keep the traps full.

Skip the bleach and the drain cleaner. Both run straight past the film on the pipe wall, and drain cleaner causes other problems. Never mix the two.

When it is more than a dirty drain

Drain flies that keep coming back after a proper cleaning mean there is standing waste somewhere you cannot reach: a cracked drain pipe under the slab, a leaking toilet seal, a sump or sewage pit, or a blocked line holding water. Those need finding.

That is investigation, so it is hourly: $289 for the first hour on site and $189 an hour after. A camera inspection is usually how a broken pipe under a floor is confirmed.

Questions people ask