Unlike house flies, which are small and can be found almost anywhere, big slow flies are a different matter. They are actually larger and can sneak into your home through tiny gaps and cracks. They generally live outdoors and feed on feces and decaying organic matter. In fact, they are often found near meat processing facilities.
If you notice a lot of them flying around your house, there are several possible causes. One of these causes is a leaking sewer pipe. If the sewage is leaking, the flies will be attracted to the smell. To determine if you have this problem, call your local extension office or a plumber to look at your sewer pipes.
During the summer, these flies cluster together on the warm side of buildings. As soon as the weather gets cooler, they crawl inside and hibernate. In the fall, they become active again and are found most often around windows and other areas where sunlight shines through.
Cluster flies are another common pest that enter homes through small cracks or holes. They tend to seek warmth and shelter. They generally stay in homes through the winter and emerge again in the spring as the weather warms.