Where sewer flies come from and why they appear indoors
Sewer flies—also called drain flies or filter flies—breed in the buildup inside your pipes and drains. They are small (about 1/8 inch long), tan or gray, and move in short, erratic hops rather than sustained flight. They lay eggs in the organic slime that coats the inside of drains, especially where water sits or moves slowly.
They enter your house because they are already living in your plumbing. A drain fly infestation means something in your system is holding standing water or decomposing matter long enough for eggs to hatch. The most common sources are shower drains, sink traps, toilet overflow tubes, and the gaps around pipe penetrations in walls or under sinks.
You will notice them most in bathrooms and kitchens, and they often appear suddenly in large numbers because the breeding cycle is fast—eggs hatch in 48 hours and adults live 8 to 10 days. If you see them, the colony has been growing for at least a week.
Key Takeaways
- Sewer flies breed in the organic buildup inside drains, so killing adults without cleaning the drain will not stop them from returning within days.
- The fastest way to eliminate them is to pour boiling water down every drain in the house, then use a drain brush or plumbing snake to scrub the inside of the pipe.
- If boiling water and brushing do not work within a week, the colony is likely deeper in the line and you should call a plumber to snake or camera-inspect the drain.
- Keeping drains clear of hair, food, and soap buildup prevents the conditions that allow sewer flies to breed in the first place.
Boiling water and drain brushing as the first step
Start by pouring a kettle of boiling water slowly down each drain in your house—kitchen sink, bathroom sinks, shower, and toilet. The heat kills eggs and larvae in the upper part of the drain. Wait a few minutes, then pour another kettle down. This alone will not solve the problem because the colony extends deeper into the pipe where boiling water cannot reach.
Next, use a drain brush (a long, flexible brush with bristles designed to fit inside a drain pipe) to scrub the inside walls of the drain. Push it down as far as it will go, twist it, and pull it back up. Do this 5 to 10 times per drain. You are breaking up the slimy biofilm where the eggs are laid. Rinse with hot water after each scrubbing.
If you do not have a drain brush, a plumbing snake (a coiled metal cable with a handle) works similarly—feed it down the drain, crank the handle to spin it, and pull it back up. The spinning action scrapes the pipe walls. Both tools cost $10 to $30 and are available at any hardware store.
Chemical drain cleaners and why they often fail
Liquid drain cleaners (like Drano or Liquid-Plumr) are tempting because they require no physical effort, but they are unreliable against sewer flies. The chemical sits in the drain for a short time, kills some larvae, and then drains away. It does not scrub the biofilm where eggs are embedded, so new flies hatch from protected eggs within days.
If you choose to use a chemical cleaner, follow the label instructions exactly and never mix products—mixing bleach and ammonia creates toxic gas. Pour the cleaner down the drain, wait the time specified, and flush with hot water. Then when ready follow up with boiling water and a drain brush. The chemical alone will not be enough.
Enzyme-based drain cleaners (which use bacteria to break down organic matter) are safer than caustic chemicals but work slowly and are less effective for an active infestation. They are better suited to maintenance after you have eliminated the flies.
Cleaning the overflow tube and trap under the sink
The overflow tube is a small pipe that runs from the top rim of your sink down into the wall or under the cabinet. It prevents water from overflowing if the main drain clogs. Sewer flies breed inside this tube because it is dark, moist, and rarely disturbed. You can see it by looking under the sink—it is usually a 1-inch diameter pipe connected to the sink rim.
To clean it, disconnect it from the sink (you may need a wrench or pliers) and pour boiling water through it, then use a small bottle brush or pipe cleaner to scrub the inside. Reconnect it and test the sink to make sure water flows properly.
The trap under the sink (the U-shaped section of pipe) also holds standing water and organic matter. Place a bucket underneath, then unscrew the trap by hand or with a wrench. Empty the contents into the bucket, scrub the inside with a brush, and rinse it thoroughly before reinstalling. If you are not comfortable doing this, a plumber can do it in 15 minutes.
When to call a plumber for a deeper problem
If you have scrubbed every accessible drain, poured boiling water multiple times, and sewer flies are still appearing after one week, the colony is likely in a part of the line you cannot reach—deeper in the main drain, in a vent pipe, or in a section of pipe that is blocked or damaged.
A plumber can use a drain camera (a small video camera on a cable) to see inside the pipe and locate the problem. They can also use a motorized snake to reach much farther into the line than a hand snake can. If the pipe is cracked, has a low spot that holds water, or is partially blocked by tree roots or grease, the plumber will recommend repair or replacement.
Call a plumber when ready if you see sewer flies coming from a toilet, because this usually means the problem is in the main line or vent stack, which requires professional equipment to clear.
Preventing sewer flies from returning
Once you have eliminated the infestation, keep drains clear by removing hair and food waste before they enter the pipe. Use a drain strainer in every sink and shower. Pour boiling water down drains once a week to keep the biofilm thin and prevent buildup.
If you have a garbage disposal, run it with cold water for 30 seconds after each use to flush food particles down the line. Do not pour grease down the kitchen sink—let it cool and solidify, then throw it in the trash. Grease cools inside the pipe, traps food and hair, and creates the perfect breeding ground for sewer flies.
Check under sinks monthly for leaks or standing water, which can create conditions for flies to breed. If you find a slow drain, clean it when ready rather than waiting for an infestation to develop.
Frequently Asked Questions
Can I use bleach to kill sewer flies?
Bleach poured down a drain will kill some larvae, but it does not stay in the pipe long enough to eliminate the entire colony. Use it only as part of a larger cleaning routine: pour bleach, wait 30 minutes, then flush with boiling water and scrub with a drain brush. Never mix bleach with other cleaners.
Why do sewer flies keep coming back after I clean the drain?
You are likely not reaching the entire breeding area. Sewer flies can breed in sections of pipe you cannot access with a hand brush, in the overflow tube, or in the trap under the sink. Clean every accessible part of the drain, including the trap and overflow, and repeat the process weekly for two weeks to break the breeding cycle.
Are sewer flies dangerous to my health?
Sewer flies do not bite, sting, or carry disease. They are a nuisance and a sign that your drain needs cleaning, but they are not a health hazard. However, the conditions that allow them to breed—standing water and organic buildup—can lead to slow drains and odors, so eliminating them improves your plumbing's function.
What if sewer flies are coming from my toilet?
Flies in the toilet usually mean the problem is in the main drain line or the vent stack, which you cannot clean yourself. Call a plumber to inspect the line with a camera and clear any blockages. Do not delay, because this indicates a more serious drainage issue.
How long does it take to get rid of sewer flies completely?
If you clean thoroughly the first time, you should see a dramatic drop in numbers within 3 to 5 days. Complete elimination usually takes 1 to 2 weeks because some eggs may hatch after your initial cleaning. If flies are still present after two weeks of weekly cleaning, call a plumber to check for a deeper problem.