Drain flies in the bathroom: clearing the floor trap

What drain flies are

University of Minnesota Extension describes moth flies, also called drain flies, as about 1/8th inch long and fuzzy. They live in the gelatinous film that builds up in sink, shower and bathtub drains. The same guide says they are a nuisance and do not bite, but can potentially carry diseases.

If you find moth flies in bathroom corners or resting on the wall beside the floor trap, start with the drains in that room. In a typical HDB flat or condo bathroom, that means the floor trap, the shower drain if there is a separate one, and the basin.

The flies on the wall are only the part you can see. What keeps them around is the film inside the drain, so the lasting fix is to find the drain they breed in and clean out that film, rather than chase the adults one by one.

Find the drain they come from

Knowing which drain is the source tells you where to clean, so you are not chasing tiny flies from drain to drain. University of Minnesota Extension says to tape over the drain opening, sticky side down, to see whether flies come up from it. Penn State Extension says placing a jar over the outlet is another way to find which drain is infested.

1. Note each drain. List every drain in the bathroom: the floor trap, the shower drain and the basin.
2. Cover one drain. Tape over its opening, sticky side down, or place a jar over the outlet. Leave it for a day or two.
3. Check for flies. Flies stuck to the tape or caught in the jar show that this drain is a source.
4. Move to the next. Repeat with the other drains one at a time, so you know whether more than one needs cleaning.

Clean the film out of the floor trap

HDB's floor trap guide says chokes in floor traps come from solid matter or grease. HDB's advice is to remove foreign particles, then flush and clean the floor trap.

For drain flies, the film itself has to go. University of Minnesota Extension says to remove the gunk with a stiff-bristle brush, and that a biological drain cleaner can be used. The same guide says using hot or boiling water, bleach or chemical drain cleaners is not effective.

Penn State Extension says to clean sinks, drain pipes and traps with a stiff wire brush or by steam, and that bacteria or enzyme products break down the scum.

In practice, floor trap cleaning goes like this: lift the grating, scrub both sides of it, brush inside the trap as far as you can reach, pick out any hair or debris, then flush it through with water. Clean the shower drain and basin the same way if your tape test pointed to them as well.

If water drains slowly even after a good scrub, something may be lodged further in. Our guide to a shower drain choke explains what to try next.

Smell, water seals and unused bathrooms

HDB says foul smell can come from foreign particles in the floor trap. It can also come from a toilet that has not been used for a long time, because the smell can break through the water seal. HDB's advice is to remove the particles, flush and clean the floor trap, and flush the toilet regularly.

For a bathroom that will not be used for an extended period, such as while you are away, HDB also says to temporarily seal the floor trap cover.

A spare bathroom that nobody uses is easy to forget. Flushing the toilet and running the basin tap now and then is a simple habit that keeps those drains from sitting still for weeks.

If the smell stays after cleaning, the guide to bathroom drain smell covers other things to check.

Keep them from coming back

HDB's floor trap guide says to keep the grating closed and to avoid pouring oil into the floor trap. HDB also suggests installing an anti-mosquito device at the floor trap. For other ways to stop mosquitoes breeding at home, see our cleaning guide.

Plan on more than one clean. Penn State Extension says the maggots mature in about two weeks, so keep scrubbing the trap and grating for a while, even after the flies on the wall start to thin out.

When the trap is choked

If water pools on the bathroom floor or drains slowly even after you have cleaned the trap, it may be choked deeper down. HDB says chokes in floor traps are caused by solid matter or grease, and for serious cases, HDB says to engage a licensed plumber to clear the choke.

The price depends on the job, so read the floor trap choke cost guide before you ask for quotes. When you do, say what you have already tried, mention whether other drains in the flat are slow too, and ask how the choke will be cleared.

Questions