House Detective: Making the Most of Home Inspection

Dear Barry: We have a major problem with drain flies under our house. We’ve called a plumber and an exterminator, but both say they’ve never seen this kind of problem before. We’ve also had the septic system pumped and inspected, but this doesn’t seem to be the source of the flies. What should we do?  Tracy

Dear Tracy: Your exterminator should know about drain flies. These pests breed in the soft, organic matter that coats the insides of drainpipes. To get rid of them, you must thoroughly remove the slimy residue in the pipes. This cannot be done with common drain cleaners, boiling water, or bleach. Instead, there are special products called “drain gels” that are specifically made for this type of drain cleaning. But before using drain gel, solid residues such as hair should be purged from the drains. For this, you should hire a plumber to snake out the lines.

Keep in mind, however, that drain flies can be breeding in other locations where there is rotting organic matter, such as moldy drywall or discarded food waste. In some cases, spillage from an open waste line under a home can provide the environment needed by drain flies.

To determine if the flies are originating in your drains, there is a tape test that you can do. For several consecutive nights, place a piece of duct tape across each of your drain openings, with the sticky side down. Do not cover the entire opening with the tape. Just run a strip of tape across the center of the orifice and leave the sides open. If the flies are breeding in the drains, some of them should be stuck to the tape in the morning. Hopefully, the drain is the source. Otherwise, you’ll have the job of searching for other places where breeding might be taking place.