How Do Bats Get in the House?

Some people liken bats to flying rats, but bats aren't rodents at all. They are mammals that belong to the order Chiroptera, and they account for 20 percent of all mammal species. Rats and bats do have some things in common, however, including a fondness for living in human dwellings and an ability to squeeze through incredibly small openings to get in. Whereas rats and other rodents typically come through the foundation, bats prefer to enter through the attic, and, like rats, they often end up nesting in the walls.

Bats may seem scary, but they aren't all bad. In fact, they are highly beneficial in many ways. The bats found most often in North America have a diet that consists of pests that bother people, such as mosquitoes and flies. Bats have no desire to hurt people or animals and seek only solitude. Moreover, with their large ears and furry-yet-ferocious faces, they are cute in a comical sort of way. However, a bat infestation can be problematic for a number of reasons. Bats are known to carry disease, and they soil the seen and unseen parts of your house with urine and droppings (guano), which can also carry disease. And with an average lifespan of 25 to 40 years, bats can be around for a very long time.

If your house has a spacious attic, it's a target for bats searching for a place to roost. Once a colony is established, it can be difficult to eradicate. To prevent infiltration by bats, you have to deny access. Doing so will require a thorough examination of the roof, walls, and foundation of your house.

Some bats can get large. The insectivorous pallid bat (​Antrozous pallidus​), which is found west of the Rockies from Canada to Mexico, has a wingspan of 2 feet or more; it eats large insects, such as beetles and grasshoppers. Most bats that take up residence in North American homes are quite a bit smaller than this, however. Some of them aren't much larger than your thumbnail and weigh just a few ounces. It goes without saying that these bats can squeeze through very small openings. A 1/2-inch crack in a mortar joint can become a bat superhighway that leads directly to your attic or chimney.

If you want to prevent bats from entering your house, you need to conduct a thorough inspection for small, seemingly innocuous gaps through which they can enter. Here are some of the main places to look:

• The roof. Check for loose shingles. If one of these is directly over a seam between roof decking planks, bats can use it to get into the attic. Check also around the chimney for masonry cracks and gaps in the flashing that could allow access to the chimney.

• The eaves and soffits. The corners between the walls of your house and the roof overhang are common places for bats to enter the attic, especially if the tops of the walls aren't covered with trim or if the trim isn't properly caulked. Bats can also enter through any soffit or roof vent that isn't protected with a mesh barrier.

• Doors and windows. Gaps in the trim around doors and windows can give bats access to the insides of your walls. They may roost there or move into the attic or basement. The gaps are usually there because of worn or nonexistent caulking.

• The foundation. Cracks in foundations can allow bats to enter the basement, which they will happily do if it's spacious, dark and unfinished.
  

Bats are active from dusk to dawn, and if they have established a colony in your house, you'll probably see them flying out of their established passageways in the warm months just after the sun has set. Bats hibernate in the winter, so you won't see any activity when it's cold outside, but there are other telltale signs of a colony:

• Guano, or bat poop. You may see it on windowsills and outdoor ledges, on the basement or attic floor, or in insulation. It looks like mouse droppings, but if you crush a bit in the sunlight, you'll see sparkles, which are caused by the decayed bodies of insects, which are the bats' food.

• Bat urine. You may see urine deposits on walls, window sills, or on the floor. It appears milky or yellowish and may form streaks on the walls. Even if you don't see the urine, you may smell it — it has a strong odor of ammonia.

• Grease marks. Bats tend to congregate around the openings they use to enter a house, and they leave telltale discoloration, or grease marks, on the outside of the building.
  

From time to time, a stray bat may find a passageway from the attic or basement into the living area of the house, If you see one of these stragglers, it's a pretty good indication of a roosting colony. Of course, bats can also enter through an open door or window, so a solitary bat flying around your living room isn't a definitive sign that you have a bat problem. However, if a bat appears when all the doors and windows are closed, it probably came from elsewhere in the house.

Bats don't like being in places occupied by people, and if you open a door or window, the disoriented creature in your living room will probably fly away eventually. Occasionally, a bat may get tired and cling to curtains or a light fixture. The best way to catch a bat when it isn't flying around is to cover it with an empty 1-quart container, slip cardboard over the opening, and release the bat outside.

Always wear gloves when approaching a bat. It has sharp teeth and claws and will bite and scratch if it's frightened or feels provoked. There's always a good chance the bat carries a disease, and an interaction could lead to more serious consequences than the pain caused by a bite. In particular, never approach a bat that seems lethargic or is crawling around on the floor or ground, which healthy bats seldom do. A bat behaving this way might have rabies — contact a control expert immediately to remove it.