Sewer flies come from drain buildup, and you kill them by cleaning the drain itself

Sewer flies — also called drain flies or filter flies — live in the sludge and organic matter that builds up inside your pipes. You see them hovering near drains, especially in bathrooms and kitchens. They are small, fuzzy, and move slowly. The only way to stop them is to remove the material they breed in, which means cleaning the drain thoroughly, not just spraying the air.

The flies lay eggs in the biofilm layer that coats the inside of your pipes. That layer is a mix of hair, soap, grease, and bacteria. If you only kill the adult flies you see, new ones will hatch in a few days. Drain cleaning is the permanent fix.

Key Takeaways

  • Sewer flies breed in the buildup inside your pipes, so killing adults without cleaning the drain will not stop them from coming back.
  • A drain brush or plumbing snake can physically remove the biofilm layer where eggs are laid, and this is the most reliable method for most homeowners.
  • Boiling water poured slowly down the drain can kill some eggs and larvae, but works best on drains that are only lightly clogged.
  • If the infestation is severe or the drain is deeply blocked, a professional plumber can use a motorized drain auger or hydro-jet to clear the pipe completely.
  • Keeping drains clear of hair and grease prevents sewer flies from returning after you have eliminated the first generation.

Clean the drain with a plumbing snake or brush

A plumbing snake (also called a drain auger) is a coiled metal cable with a handle. You feed it down the drain, and as you turn the handle, the tip breaks up and pulls out the buildup. A manual snake costs $10 to $30 and works on most bathroom and kitchen drains. Feed it down slowly, turn the handle, pull it back out, and repeat until you feel less resistance.

A drain brush is simpler — it looks like a bottle brush on a long handle. Push it down the drain and scrub the sides of the pipe. This works well for drains that are not completely blocked. You can buy one for $5 to $15.

After you use either tool, run hot water down the drain for 30 seconds to flush out the loosened material. You may see dead flies or debris come up — that is a sign the cleaning is working. Repeat the process once a week for three weeks to break the fly lifecycle, which is about seven days from egg to adult.

Pour boiling water down the drain slowly

Boiling water kills some fly larvae and eggs, but only if it reaches them. Pour the water slowly — not all at once — so it stays hot as it moves through the pipe. A single kettle of boiling water is usually not enough on its own, but it works well after you have used a snake or brush to expose the biofilm.

Do this once a day for a week. The heat also helps dissolve some of the grease and soap buildup. If your drain is made of PVC plastic, boiling water is safe, but if you are unsure of your pipe material, use very hot tap water instead — it is still effective and carries no risk.

Use a drain cleaner if the drain is heavily clogged

Chemical drain cleaners (like Drano or Liquid-Plumr) can dissolve organic buildup, but they work best on drains that are already partially blocked. They are not the first choice for sewer flies because they do not physically remove the biofilm layer where eggs are laid — they only dissolve some of it.

If you use a chemical cleaner, follow the label instructions exactly. Pour it down the drain, wait the time it specifies, and flush with hot water. Do not mix different cleaners. Wear gloves and eye protection, and make sure the room is ventilated. Chemical cleaners are harsh and can damage old pipes or septic systems, so check your plumbing type before using one.

A better option for heavy clogs is to call a plumber. They can use a motorized drain auger or a hydro-jet (which shoots pressurized water through the pipe) to clear the drain completely without chemicals.

Prevent sewer flies by keeping drains clear

Once you have eliminated the flies, stop them from coming back by removing what they need to breed. Use a drain strainer in your sink and shower to catch hair before it goes down the pipe. Empty the strainer into the trash after each use.

Pour grease into a container and throw it away instead of rinsing it down the drain. Grease hardens as it cools and traps other debris, creating the perfect breeding ground. Run hot water down the drain once a week to keep it clear. If you have a garbage disposal, run it with cold water while grinding, then hot water after, to flush the debris out.

Clean your shower drain monthly by removing the cover and pulling out any hair buildup by hand. This takes two minutes and prevents clogs before they start.

When to call a professional plumber

Call a plumber if the drain is completely blocked and you cannot move a snake through it, if the infestation is in multiple drains throughout the house, or if you have tried cleaning and the flies return within a week. A plumber can inspect the pipe with a camera to see exactly where the buildup is and whether there is a structural problem like a crack or tree root intrusion.

If you have a septic system, be extra cautious with chemical cleaners and drain snakes — they can damage the tank. A plumber who works with septic systems knows the safe methods for your setup.

The cost of a professional drain cleaning ranges widely depending on how deep the clog is and what tools are needed. A straightforward snake-out might cost $150 to $300, while a hydro-jet can run $300 to $600 or more. It is worth the cost if the problem is severe or keeps coming back.

Frequently Asked Questions

How long does it take to get rid of sewer flies?

If you clean the drain thoroughly, you should see fewer flies within three to five days. The fly lifecycle is about seven days, so repeat the cleaning once a week for three weeks to catch each new generation as it hatches. Some flies may linger for a week or two after that, but the population should drop sharply after the first cleaning.

Can I use bleach to kill sewer flies?

Bleach poured down the drain will kill some flies and larvae, but it does not remove the biofilm where eggs are laid, so new flies will hatch. Bleach also does not stay in the pipe long enough to be fully effective. A drain snake or brush is more reliable. If you do use bleach, never mix it with other cleaners, and use it only as a supplement to physical cleaning, not as the main treatment.

Why do I still see flies after I cleaned the drain?

Adult flies can live for two to three weeks, so you may see them for a while even after you have killed the eggs and larvae. If new flies keep appearing after three weeks of weekly cleaning, the buildup may be deeper in the pipe than a manual snake can reach, or there may be a clog further down the line. This is when a professional plumber with a motorized auger or camera inspection becomes necessary.

Are sewer flies dangerous?

Sewer flies do not bite or sting, and they do not carry diseases into your home. They are a nuisance and a sign that your drain needs cleaning, but they are not a health hazard. The real concern is the clogged drain itself — if left untreated, it can back up and cause water damage or sewage overflow.

Can I prevent sewer flies without cleaning the drain?

No. Drain covers and screens stop some debris from going down, but they do not remove the buildup that is already there. You must clean the drain to eliminate the breeding ground. Prevention works only after the initial infestation is gone — then regular maintenance keeps flies from returning.