Drain flies, also called sewer flies or moth flies, are the small fuzzy insects that sit on bathroom walls and rise from a sink when the tap runs. They breed in the gelatinous organic film that lines drains, overflow passages, floor drain traps and the inside of rarely used fixtures, and they are notoriously hard to get rid of because the problem is never the flies, it is the film, and the film is somewhere nobody can see. An inspection camera small enough to enter a 1.5 in drain turns a guessing game into a short search, because the breeding film has a distinctive look and the camera can check each drain in minutes.
Quick answer: drain fly larvae live in a thick, dark, gelatinous biofilm that builds up in slow or rarely used drains, overflow channels, floor drain traps, under loose tiles and in the gap between a sink and its drain flange. On camera the breeding site shows as a thick, wet, dark layer, often with visible movement or small pale larvae in it, in a drain that is otherwise lightly used. Finding it means checking every drain in the affected room with a small head, including the ones that seem clean. Clearing it means physically removing the film, not pouring bleach into it.
Where they breed
Bathroom sink overflow channels, which are never flushed. Tub and shower drains with hair and soap film. Floor drains in basements, laundry rooms and garages whose traps dry out and refill with scum. Rarely used fixtures: a guest bath, a basement shower, a utility sink. The gap between a sink drain flange and the basin, where water seeps and sits. Condensate drains and air handler pans. Under a loose floor tile or a cracked shower pan where water has collected. And, in a house with a broken drain under the slab, the soil around the break, which is the one case where the camera finds a structural problem behind the flies. The biofilm guide describes the film in general.
What the camera shows
A normal drain wall has a thin dark film. A breeding site has a thick one: a wet, gelatinous layer that may be grey, brown or black, that fills the trap arm or the overflow passage, and that in a good light shows small pale larvae moving in it. Standing water with a scum on top is another sign. A trap that has been dry and refilled shows a ring of film at the old water line. The camera head will usually come out coated, which is itself confirmation. A 17 or 18 mm head on a thin rod is the right tool, and the 17 mm camera guide covers the options.
The search
Start in the room with the most flies. Check the sink drain and, separately, the overflow by feeding the camera into the overflow opening. Check the tub or shower drain and overflow. Check any floor drain, including under appliances. Check fixtures that are rarely used. Then check the adjacent rooms, because the flies travel. Tape a cup or a piece of card over each drain overnight and look for flies in the morning if the camera is inconclusive; the drain that produces flies is the breeder. If every drain is clean and the flies persist, the source is outside the plumbing: a cracked pan, a leak under a floor, or a break under the slab, and under-slab inspection is the next step.
Clearing the film
Remove it. A stiff brush on a flexible rod, a drain-cleaning brush, or pulling the trap and scrubbing it. Scrub the overflow channel with a bottle brush. Flush with hot water. Then an enzyme or bacterial product to digest what the brush missed, used repeatedly over a week or two. Bleach and boiling water kill some larvae on the surface and leave the film they live in, which is why they fail. Refill dry traps and keep them filled by running the fixture weekly or adding a trap primer. Recheck with the camera after two weeks. Enzyme drain treatments covers what to expect afterward.
When flies mean a bigger problem
Flies in a room with no drain, flies from a crack in a floor, or flies that return after every drain has been cleaned, point to water collecting somewhere it should not: a broken drain under the slab, a leaking shower pan, a failed wax ring under a toilet. The small camera has done its job by eliminating the drains; the next tool is a full sewer camera from a cleanout to check the building drain, and the under-slab cast iron guide describes the usual culprit in older houses.
Choosing the camera
The drains involved are 1.25 to 2 in with traps at the start, so the head has to be 17 or 18 mm on a 5 mm rod, and a short reel is better than a long one because the search is in the room, not down the lateral. A detachable or well-placed screen helps when the work is under a sink. None of the full-size 23 mm reels will enter a sink or overflow, and pushing one into a tub trap is how heads get stuck.
Common questions
Why do drain flies keep coming back?
Because the film they breed in is still there. Killing flies and pouring bleach do not remove it.
Can a sewer camera find drain flies?
A small-head inspection camera can find the breeding film in a drain. A full-size reel is too large for the fixtures involved.
Do drain flies mean a broken sewer pipe?
Usually not. If every drain is clean and the flies persist, a break under the floor becomes a possibility worth a camera inspection from the cleanout.
Related guides
See finding a sewer smell with a camera, the best sewer cameras for 1.5 in pipe, and camera inspection of a basement floor drain.