Sewer flies come from drain buildup and need both cleaning and drain sealing to stop returning

Sewer flies — also called drain flies or moth flies — live in the organic sludge that builds up inside drains. They breed in that sludge, not in the sewer itself. To get rid of them, you need to clean out the drain material where they lay eggs, then seal or cover any opening where they can enter your home. If you only kill the adults you see, new ones will hatch from the buildup within days.

The flies are small, dark, and fuzzy, about the size of a grain of rice. You will see them around sink drains, shower drains, or floor drains in basements. They move slowly and do not bite. The real problem is that their presence means your drain is clogged with organic matter — hair, soap, food particles, and bacteria — which is also a breeding ground for other pests and can lead to slow drainage or backups.

Key Takeaways

  • Sewer flies breed in the sludge inside drains, so you must clean the drain itself, not just spray for insects.
  • A drain brush, boiling water, and a commercial drain cleaner or enzyme product can remove the buildup where flies lay eggs.
  • After cleaning, cover or seal the drain opening with a plug, strainer, or drain cap to stop new flies from entering.
  • If flies return after cleaning, the drain may need professional rodding or the problem may be in a floor drain or vent pipe you have not yet treated.

Clean the drain with a brush and boiling water

Start by removing any visible debris from the drain opening. Use a drain brush — a long, thin brush with bristles designed to fit inside a pipe — to scrub the sides of the drain. Push it down as far as it will go and twist it to loosen the sludge buildup. Pull it out and repeat several times. You will see dark, slimy material come up; that is the breeding ground.

After brushing, pour boiling water down the drain slowly. The heat kills fly larvae and eggs in the upper part of the drain and helps flush loosened material downward. Let the water sit for a few minutes, then flush with cold water. Do this once a day for three to five days. Boiling water alone will not remove all the buildup, but it is a safe first step and works well for light infestations.

Use a drain cleaner or enzyme product for heavy buildup

If brushing and boiling water do not stop the flies within a week, the buildup is too thick. You have two options: chemical drain cleaners or enzyme-based products. Chemical cleaners like Drano or Liquid-Plumr dissolve organic material quickly but are caustic and can damage old pipes. Enzyme products like Green Gobbler or Bio-Clean work more slowly — usually overnight — but are safer for pipes and septic systems.

Follow the product instructions exactly. Most require you to pour the product down the drain, wait several hours or overnight, then flush with hot water. Do not mix products. If you use a chemical cleaner, do not follow it with an enzyme product or vice versa. Repeat the treatment once a week until the flies are gone and the drain drains quickly. Once the drain is clean, the flies will have nowhere to breed.

Seal the drain opening to prevent new flies from entering

Even after cleaning, flies can re-enter through an open drain. Cover the drain with a plug, strainer, or drain cap when you are not using it. For a sink, a straightforward rubber drain plug or a mesh strainer works. For a shower, a drain cover or plug fits over the opening. For a floor drain in a basement, use a drain cap or a weighted cover that sits flush with the floor.

If you use the drain regularly, a strainer catches hair and debris and also blocks flies. If you rarely use the drain, a plug or cap is simpler. Make sure the seal is tight — flies can squeeze through small gaps. If the drain is in a basement or crawl space and you do not use it, you can also pour a small amount of mineral oil or bleach down it once a month to kill any larvae that may have entered.

Check for other entry points if flies return

If sewer flies come back after you have cleaned and sealed the main drain, they may be entering from a different source. Check for floor drains in the basement, laundry room, or garage. These drains often sit unused and can become breeding grounds. Clean and seal them the same way you did the sink or shower drain.

Also check vent pipes on the roof or outside walls. These pipes allow sewer gas to escape but can also let flies in. If you see flies near a vent pipe, the problem may be inside the vent itself. You can install a vent cap or screen to block flies while still allowing air to flow. If the infestation is severe or you cannot locate the source, a plumber can use a camera to inspect the pipes and find where flies are entering.

Prevent sewer flies from returning

Once you have cleared the infestation, keep drains clean by running hot water down them daily and removing hair and debris regularly. Use a drain strainer in sinks and showers. Once a month, pour boiling water or a commercial drain cleaner down drains you use frequently to prevent buildup.

If you have a floor drain that you do not use, cover it with a cap or pour a small amount of mineral oil down it monthly. This creates a barrier that blocks flies and other pests. If you notice slow drainage or a sewer smell, clean the drain when ready — these are signs that buildup is starting again.

When to call a plumber

If you have cleaned the drain thoroughly and sealed all openings but flies still appear after two weeks, the problem may be deeper in the plumbing system. A plumber can use a drain snake or high-pressure water jet to clear buildup in pipes you cannot reach. They can also inspect the entire drain line with a camera to find cracks, breaks, or other entry points where flies are getting in.

Call a plumber when ready if you see flies coming from a vent pipe, if multiple drains in your home have flies, or if you also notice sewage backing up or a strong sewer smell. These signs point to a problem in the main drain line or septic system that requires professional attention.

Frequently Asked Questions

Can I use insect spray to kill sewer flies?

Spray kills the adult flies you see, but new ones will hatch from eggs in the drain within days. Spray is only useful after you have cleaned the drain and removed the breeding material. Even then, it is not necessary — sealing the drain stops new flies from entering.

What if I have a septic system instead of a sewer line?

The same steps work. Clean the drain with a brush and boiling water or an enzyme-based drain cleaner. Avoid harsh chemical cleaners, which can kill the bacteria in your septic tank. Seal the drain opening the same way. If flies persist, have the septic tank inspected for cracks or leaks.

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

If you clean the drain thoroughly and seal it, flies should be gone within one to two weeks. If you only brush and use boiling water, it may take three to four weeks. If you do not seal the drain, new flies will keep entering and the problem will not end.

Can sewer flies spread disease?

Sewer flies do not bite or sting and do not carry disease. They are a nuisance and a sign of drain buildup, but they are not a health hazard. The real concern is the clogged drain itself, which can lead to slow drainage, backups, or mold growth.

Why do I have sewer flies if my drains seem to drain fine?

Drains can drain slowly enough to allow buildup without being completely clogged. Hair, soap, and organic matter accumulate on the drain walls even if water still flows. This buildup is invisible but provides the perfect breeding ground for flies. Cleaning the drain removes this hidden material.