How to get rid of drain flies, fast

Find the source first

Tape a piece of clear plastic sticky side down over a drain overnight. If flies are stuck to it in the morning, that drain is the nursery. Check every drain in the house, including floor drains and any trap that rarely gets used.

Clean it properly

• Pour boiling water down the drain to soften the film.
• Work a stiff drain brush down as far as it reaches and scrub the pipe walls. This is the step people skip and it is the one that matters.
• Follow with an enzyme drain cleaner, not bleach. Enzymes digest the organic layer. Bleach runs straight past it.
• Repeat for a few nights. One pass rarely gets everything.

The source everyone forgets

If the flies are around windows or the basement rather than a drain, look outside. A clogged gutter holds standing water full of decaying leaves, which is a perfect drain fly nursery sitting a few feet from your windows. Same for a blocked downspout that never fully empties.

Clear the gutter, make sure water actually runs through it, and the outdoor population dies off. If your gutters clog every season, that nursery keeps coming back until the clogging stops.

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