Drain flies are often a sign of an underlying moisture or plumbing issue rather than a random pest problem. Before learning how to get rid of drain flies permanently, it is important to understand what causes them in the first place. When the source is not addressed, drain flies tend to return again and again, even after repeated cleaning.
This article explains the most common causes of drain flies in homes and why they are especially attracted to bathrooms, kitchens, and shower drains.
Organic Buildup Inside Drains
The most common cause of drain flies is organic buildup inside drains. Over time, substances such as soap residue, hair, grease, food particles, and skin cells collect along the inner walls of pipes. This buildup forms a slimy layer known as biofilm.
Biofilm provides an ideal environment for drain flies to lay eggs. The larvae feed on the organic material inside the drain, making it possible for a full drain fly infestation to develop even when the drain appears clean from the outside.
Excess Moisture and Standing Water
According to the CDC, drain flies commonly breed in moist areas with organic buildup inside plumbing systems.
Drain flies thrive in damp environments. Any area with constant moisture or standing water increases the chances of infestation. Bathrooms, shower drains, floor drains, and unused sinks are especially vulnerable because water remains trapped for long periods.
Leaking pipes, clogged drains, and poor ventilation also contribute to excess moisture. When humidity levels stay high, drain flies can breed rapidly without disturbance.
Infrequently Used Drains
Unused or rarely used drains are a hidden cause of drain flies in many homes. When water sits in a drain trap for too long, organic matter begins to decompose. This creates a perfect breeding site that often goes unnoticed until flies start appearing.
Guest bathrooms, basements, and laundry room drains are common problem areas. Running water through these drains regularly helps prevent buildup and discourages drain flies.
Hair and Soap Residue in Shower Drains
Shower drains are one of the most common sources of drain flies. Hair, soap scum, and shampoo residue accumulate faster in shower drains than in other plumbing fixtures. This combination forms thick biofilm that supports larvae growth.
If you notice drain flies in the bathroom, the shower drain should be inspected first. Even small amounts of buildup can sustain a population of drain flies.
Cracked or Damaged Pipes
In some cases, drain flies are caused by hidden plumbing issues. Cracked pipes, loose joints, or damaged sewer lines allow organic material to collect outside the visible drain area. These hidden spaces often stay moist and undisturbed, making them ideal breeding zones.
When standard cleaning does not solve the problem, professional inspection may be required to locate and repair damaged pipes.
Poor Drain Maintenance Habits
Lack of regular drain maintenance is another major reason drain flies appear. Allowing grease, food waste, and debris to go down sinks accelerates organic buildup. Over time, this neglect leads to slow drainage and eventually attracts drain flies.
Routine cleaning and flushing drains with hot water can significantly reduce the risk of infestation.
How Understanding the Cause Helps Eliminate Drain Flies
Knowing what causes drain flies is the first step toward eliminating them permanently. Treating only the adult flies does not solve the problem. The real solution lies in removing the organic buildup and moisture that support their life cycle.
If you are dealing with an active infestation, this guide on how to get rid of drain flies explains proven treatment methods that address the root cause rather than just the symptoms.
Final Thoughts
Drain flies are not random pests; they are a clear indicator of moisture, buildup, or plumbing issues inside your home. Organic residue, standing water, and neglected drains are the primary causes behind most infestations. By identifying and fixing these issues early, you can prevent drain flies from becoming a recurring problem.
