blogTL;DR: Drain flies are small, moth-like insects that breed inside blocked or slow-running drains. You can treat a minor infestation yourself using boiling water, baking soda, and enzyme cleaners, but if they keep coming back, the drain itself is the problem. A professional drain inspection will identify the root cause and clear it properly. Call Clear Stream on 01872 222555.
If you’ve noticed tiny, fuzzy-winged insects hovering near your sink, shower, or bathroom floor, you’re dealing with drain flies. Knowing how to get rid of drain flies starts with understanding what they’re actually feeding on: the organic buildup inside your pipes. They don’t come from outside. They breed in the decomposing matter lining the inside of a slow or partially blocked drain.
Killing the visible flies is easy. Getting rid of them permanently means tackling the drain itself.
This guide covers what drain flies are, why they appear, what you can try yourself, and when a professional drain inspection is the only real fix.
What Are Drain Flies and Where Do They Come From?
Drain flies are small, hairy-winged insects, around 2 to 5mm long, that breed inside drains where organic matter has built up. They lay their eggs in the gelatinous layer of decomposing material that coats the inside of slow or partially blocked pipes. That layer is called a biofilm. It’s what the larvae feed on before emerging as adult flies.
They’re also called moth flies, filter flies, or sewer gnats. You’ll recognise them by the way they rest with their wings flat, like a tiny moth, and move in short, hesitant hops rather than flying in a straight line.
Common breeding sites include:
- Kitchen sink drains with grease and food residue
- Bathroom sink and shower drains with soap scum and hair
- Floor drains in utility rooms or garages
- Rarely used sinks where water sits stagnant in the trap
- Septic tank inlet areas in older rural properties
One thing worth knowing: drain flies don’t bite. They’re not carrying disease in the way houseflies do. But they’re a reliable sign that something is wrong with the drain they’re living in.
Why Do I Keep Getting Drain Flies?
Drain flies keep coming back because the breeding site inside the drain has not been cleared. Killing the adult flies with a spray does nothing to the larvae still developing in the biofilm below.
Most infestations follow the same pattern. You notice flies. You clean around the sink. They disappear for a few days. Then they’re back. That cycle repeats because the underlying buildup in the pipe hasn’t been addressed.
Several things create conditions that drain flies thrive in:
- Slow-draining water that allows organic matter to sit and accumulate
- Partially blocked pipes that restrict flow without stopping it completely
- Infrequently used plumbing where water stagnates in the trap and the pipe dries out
- Damaged or cracked pipe sections where waste can collect
In Cornwall, older properties with clay pipe drainage are particularly prone to this. Clay pipes develop cracks and root ingress over time, and those crevices collect debris exactly where you don’t want it. If you’re in a rural property with older drainage infrastructure, the problem may run deeper than a surface clean will reach.
How to Get Rid of Drain Flies Yourself
You can reduce a drain fly infestation yourself if it’s caught early and the drain is only mildly built up. These methods work on the biofilm layer inside the pipe rather than just killing the adult insects.
Step 1: Confirm the Breeding Site
Tape a piece of clear plastic film over the drain opening overnight. Seal the edges with tape but leave a small gap so air can still circulate. In the morning, check for flies stuck to the inside of the film. That confirms which drain is the source.
Step 2: Clear the Drain Mechanically
Use a drain snake or a zip-it tool to physically pull out as much material as possible. This is the part most people skip. Pouring liquid down a drain full of compacted buildup doesn’t work well because the liquid can’t penetrate to the pipe wall where the biofilm is thickest.
Step 3: Flush with Boiling Water
Pour a full kettle of boiling water slowly down the drain. This loosens the biofilm and kills larvae near the surface. Repeat twice, leaving a few minutes between pours. Note: do not use boiling water on plastic (PVC) pipe connections, as it can soften joints over time. Stick to very hot (not boiling) water if you’re unsure of your pipe material.
Step 4: Apply an Enzyme Drain Cleaner
Enzyme-based drain cleaners break down organic matter biologically. They’re different from chemical drain cleaners, which work on blockages but don’t necessarily clear biofilm. Apply according to the product instructions, typically at night so the cleaner has time to work without water being run. Repeat over several consecutive nights.
Step 5: Treat Adult Flies
While the drain treatment is working, sticky traps near the drain will catch adults and reduce the visible population. Insecticide sprays kill adults but don’t affect larvae, so treat them as a short-term measure only.
This process works well on a minor, early-stage infestation. If you’ve done all of this and the flies return within two to three weeks, the drain has a structural issue or blockage that a surface clean won’t fix.
What Causes Persistent Drain Fly Infestations?
A recurring drain fly problem almost always points to a persistent blockage, pipe damage, or a section of drain that simply can’t be reached with household products. The flies aren’t coming back because your cleaning routine is wrong. They’re coming back because they have a reliable breeding environment.
Common causes of persistent infestations:
- A partial blockage that slows water flow without stopping it completely, creating ideal conditions for biofilm to accumulate
- A collapsed or offset pipe section where waste collects in a low spot
- Root ingress into clay or older earthenware pipes, creating a catch point for organic debris
- A damaged or missing rodding eye cover allowing waste to back up in an inaccessible section
- A septic tank or treatment plant inlet that needs servicing
In these situations, you need to see inside the pipe to understand what’s happening. That means a CCTV inspection.
When Should You Call a Drainage Professional?
Call a professional if the infestation has returned after a thorough DIY clean, if the flies are appearing in more than one location, or if you can smell an odour from the drain even after cleaning.
These are all signs of a problem inside the pipe rather than on the surface. They’re also signs you could make worse by continuing to pour chemicals down the drain.
Other situations that need a professional:
- You’ve identified a slow drain that hasn’t improved with cleaning
- You’re in an older property with clay or earthenware drainage
- Flies are appearing near a floor drain, inspection chamber, or outdoor gully as well as inside
- You’re managing a rental property or commercial premises and need documentation that the issue has been properly resolved
Blocked or slow drains often sit at the root of recurring drain fly problems. A professional inspection will identify whether the issue is a surface buildup, a partial blockage, or a structural defect in the pipe.
How Clear Stream Resolves Drain Fly Problems
Clear Stream tackles drain fly infestations by diagnosing and clearing the root cause inside the pipe, not just treating the symptom.
Here’s how we approach it:
CCTV Drain Survey: A small camera is passed through the drain to inspect the interior. This shows exactly where buildup is concentrated, whether there’s a blockage, any pipe damage, root ingress, or joint displacement that’s creating a catch point for debris. It gives a clear picture before any work begins, so there are no surprises.
High-Pressure Jetting: Where biofilm and blockage buildup is confirmed, high-pressure jetting clears the pipe interior thoroughly, reaching sections that enzyme cleaners and mechanical tools can’t. Jetting removes compacted grease, scale, root intrusion, and debris from the pipe wall rather than just breaking it up and pushing it further along.
Drain Repairs Where Needed: If the survey reveals a structural defect, a cracked joint, or pipe damage creating the problem, we’ll advise on the repair options. Depending on the location and access, that may include No-Dig Repair methods to avoid excavation.
Clear, Fixed-Price Quotes: Before any work begins, you’ll receive a fixed-price quote. No call-out fees, no hidden costs, and no pressure to commit before you’re ready.
We cover the whole of Cornwall and Devon, and we’re on-site within 1 to 2 hours in an emergency. All drain repairs come with a 5-Year Guarantee.
Contact Clear Stream or call 01872 222555 to book an inspection.
Conclusion
Drain flies are a symptom, not a standalone problem. They breed inside drains because there’s something inside that drain they can live on: a biofilm, a blockage, or a structural fault that’s allowing waste to collect.
A few things to take away:
- DIY methods work for mild, early-stage infestations if you clean the pipe mechanically as well as chemically
- If the flies return after a thorough clean, the drain has a deeper issue that needs inspection
- A CCTV survey is the only way to confirm what’s happening inside the pipe
If drain flies keep coming back, it’s time to look inside the drain. Clear Stream covers all of Cornwall and Devon, with no call-out fees and fixed-price quotes before any work begins. Call us on 01872 222555 or get in touch online to arrange a survey.
Frequently Asked Questions
Are drain flies harmful to health?
Drain flies don’t bite and don’t carry disease in the way houseflies do. They’re not a direct health risk, but they’re a reliable sign of a drain that contains decaying organic matter, which can harbour bacteria. If they’re appearing in a kitchen or food preparation area, that’s a hygiene concern worth addressing promptly.
How long does it take to get rid of drain flies?
With consistent DIY treatment targeting the biofilm in the pipe, a mild infestation typically clears within two to three weeks. If flies are still present after that period, the drain needs a professional inspection to identify why the breeding site persists.
Can drain flies come from outside?
Drain flies breed inside drains, not outdoors. If you’re seeing them enter from outside, they’re likely breeding in a drain gully, inspection chamber, or external drain close to the property rather than coming from a natural environment. The source is still a drain.
Do drain fly killers and chemical drain cleaners work?
Insecticide sprays kill adult flies but don’t reach the larvae in the biofilm. Standard chemical drain cleaners can help with a partial blockage but don’t reliably clear the biofilm layer where larvae develop. Enzyme-based cleaners are more effective for this, but they won’t resolve a structural drain problem.
Why are drain flies worse in summer?
Warmer temperatures accelerate the drain fly breeding cycle. Larvae develop faster and emerge sooner, which means populations can grow quickly during summer months. If you’ve had a slow drain all year without flies, summer is often when an infestation first becomes noticeable.
Can drain flies breed in a clean-looking drain?
Yes. The biofilm they breed in coats the inside of the pipe, below the visible surface of the drain opening. A drain can look perfectly clean at the top and still have significant buildup further down. This is why surface cleaning with disinfectant rarely solves a drain fly problem on its own.
