Small, fuzzy flies around your basement drain, tub, or floor drain are called sewer flies or drain flies. Most of the time they're a cleaning problem. But when they keep coming back after you've cleaned every drain, they're often a sign of a cracked or broken sewer line leaking under your floor or yard.
Flies keep coming back?
A sewer camera inspection shows exactly where the line is broken. Free estimates, same-day service.
Why Sewer Flies Show Up
Sewer flies lay eggs in the slimy, organic film that builds up inside drains and sewer pipes. A few flies near one bathroom sink usually just means that drain needs a good cleaning. The trouble starts when they keep coming back no matter how much you clean — that usually means the breeding spot is somewhere you can't reach.
How a Broken Sewer Line Breeds Flies
When a sewer pipe under the basement slab or in the yard cracks, separates at a joint, or partly collapses, wastewater leaks into the soil around it. That wet, sewage-soaked soil is perfect breeding ground. The flies then work their way up through floor cracks, gaps around the pipe where it passes through the wall, a floor drain, or an uncapped cleanout. Older Bucks County homes with clay, cast iron, or Orangeburg pipe are the most likely to have this happen.
Step 1: Find Out Which Drain They're Using
Put clear tape over each drain overnight, sticky side down, with a small gap so air can move. In the morning, check which tape has flies. If one drain catches them, scrub that drain and its overflow with a stiff brush and hot water, or have it cleaned. If none of the drains catch flies but they keep showing up in the basement, the source is probably below the floor.
Step 2: Check for the Other Broken-Line Signs
Look and smell around the basement and yard: a sewer gas odor, damp spots or new cracks in the basement floor, a sunken or soggy area in the yard, extra-green grass along the line to the street, gurgling toilets, or several drains slowing down together. Flies plus any of these points strongly to a damaged sewer line.
Step 3: Get a Sewer Camera Inspection
We run a waterproof camera through your sewer line from the cleanout and watch the pipe on screen. You see exactly where it's cracked, separated, full of roots, or collapsed — and how deep and how far from the house it is. That tells us whether cleaning, a small spot repair, or a sewer line replacement is the right fix, and you get a free estimate before any work starts.
How a Broken Line Gets Fixed
- Roots or buildup, pipe still sound: hydro jetting clears the line.
- One cracked or separated section: a spot repair replaces just that piece.
- Old, collapsing, or Orangeburg pipe: a full sewer line replacement.
More warning signs: 7 Signs You Need Sewer Line Replacement.
Sewer Fly Questions
What are sewer flies?+
Sewer flies — also called drain flies, moth flies, or filter flies — are small, fuzzy, gray-black flies about 1/8 inch long with moth-like wings. They breed in the slimy film inside drains and sewer pipes and usually rest on walls near sinks, tubs, floor drains, and basement bathrooms.
Can sewer flies mean a broken sewer line?+
Yes. If you've cleaned every drain and the flies keep coming back — especially in the basement or around a floor drain — they may be breeding in sewage that is leaking from a cracked, offset, or collapsed pipe under the slab or in the soil. A sewer camera inspection is the way to confirm it.
How do I find where sewer flies are coming from?+
Tape a piece of clear packing tape over each drain overnight (sticky side down, leave a small gap). Flies stuck to the tape show which drain is the source. If no drain catches flies but they keep appearing in the basement, look at cracks in the floor, around the main cleanout, and near the sewer line's path out of the house.
Will bleach get rid of sewer flies?+
Usually not. Bleach runs straight past the slime layer where the larvae live. Scrubbing the drain with a stiff brush and hot water, or having the drain mechanically cleaned, works much better. If the source is a broken pipe, no cleaner will solve it — the pipe has to be repaired.
What other signs point to a broken sewer line?+
Sewer gas smell in the basement, damp or sunken spots in the floor or yard, extra-green strips of grass, several drains slow at once, gurgling toilets, and repeated backups. Flies plus any one of these is a strong reason to book a camera inspection.
How much does a sewer camera inspection cost?+
Every property is different, so we give free estimates. Call 215-364-0133 and we'll tell you what's involved before we come out.