The Tool Desk
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Where to look for bat entry points
Inspect the entire exterior from the ground, not just the spot where you first noticed bats. Bats may use more than one opening, and the likely locations depend on the building. Check:
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- Eaves, soffits, roof edges, ridge caps and gaps under shingles
- Gable ends, dormer edges and places where roof sections meet
- Attic vents, chimneys and junctions between roofing and walls
- Loose or damaged siding, trim and wood
- Gaps around pipes, wires and other utility penetrations
Small gaps can be enough. The U.S. Fish & Wildlife Service says bats can squeeze through a crevice as small as a quarter; Michigan guidance and other state sources describe openings around half an inch. Those dimensions are not a reliable cutoff: the size and shape of a gap, and the bat using it, matter. See the U.S. Fish & Wildlife Service guidance and Michigan Department of Natural Resources guidance.
How to confirm which openings are active
Watch the building at dusk
From outside, watch the roofline and other likely openings around dusk as bats emerge. Note where each bat exits and map the locations. Repeat the observation if possible; a single evening may not show every route. New Jersey recommends observing on a warm, rain-free evening of at least 50°F, a recommendation for its local context rather than a universal threshold. Its Bats in Buildings guidance describes this observation approach.
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Look for signs inside and outside
From a safe attic location, daylight visible through a crack may reveal a gap. Illinois guidance also describes turning on a bright light inside at night and checking from outdoors for light escaping through openings. Dark rub marks can come from dirt, oils or droppings left as bats pass; fresh droppings below a suspected opening are another clue. To distinguish fresh deposits from old material, clean the area or place paper underneath and check it later. Do not enter an unsafe attic or disturb a roost if you lack the equipment and experience to do so. See the Illinois Department of Public Health’s bat-exclusion guidance.
How to exclude bats and seal the building
- Check the interior connections. Identify routes between the roost and occupied rooms. Pennsylvania guidance recommends checking these connections; sealing them can reduce the chance that bats enter living areas. Keep the exterior exits available until the exclusion is properly set up.
- Check local rules and timing before starting. Maternity seasons and winter hibernation can make exclusion unsafe: flightless pups may be trapped inside, or hibernating bats may be harmed. If bats are hibernating in a building during winter, the U.S. Fish & Wildlife Service advises leaving them in place and waiting until spring for one-way exclusion. Contact your state or local wildlife authority for current, location-specific guidance.
- Fit one-way devices to confirmed active exits. A one-way tube, netting or flap lets bats leave but prevents re-entry. Seal other confirmed openings so bats cannot switch routes, but do not block an active exit or redirect bats into occupied areas. A large colony or connected roost areas may require several devices. Monitor the exits; if bats appear blocked or confused, stop and seek professional help. Illinois and Pennsylvania describe exclusion devices in their guidance: Illinois and Pennsylvania.
- Monitor before removing the devices. Leave the devices in place for the period recommended by local authorities, and check for continued activity. Pennsylvania’s guide specifies five to seven nights for the devices it describes. Washington guidance gives five to seven days, longer in cool or rainy conditions. These are jurisdiction-specific recommendations, not a universal duration. Once you have confirmed that all bats have left, remove the temporary devices and close the openings promptly.
- Make a durable permanent repair. Choose a material suited to the opening and building surface. Official guidance identifies caulk, flashing, hardware cloth or suitable mesh, mortar, and repair or replacement of deteriorated wood. Keep vents appropriately screened and chimneys capped. Do not apply expanding foam to an active route: Pennsylvania warns that bats can become entangled as it hardens.
Why exclusion dates depend on where you live
There is no safe national calendar for bat exclusion. The dates below are examples from different state guidance—not dates to apply elsewhere. Rules and biological conditions can vary by species and location, so check with the wildlife authority responsible for your building before acting.
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| Jurisdiction and source | Guidance described |
|---|---|
| Illinois Department of Public Health, Bats and Bat Exclusion | Colony exclusion windows of March 15–May 15 and August 5–October 30, with outdoor temperatures above 50°F at dusk. |
| New Jersey Department of Environmental Protection, Bats in Buildings | Safe dates of April 1–May 14 and August 1–October 15. |
| Michigan Department of Natural Resources, Bats | Advises avoiding the typical May–mid-August maternity season. |
| Pennsylvania Game Commission, Bats in Buildings: A Guide to Safe & Humane Exclusions | Says exclusions should not take place between April and late August and notes winter hibernation concerns. |
Many bats are protected by federal, state or local provisions, and requirements depend on species and place. The U.S. Fish & Wildlife Service recommends checking with your state department of natural resources before excluding a colony.
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Consider a qualified bat-exclusion professional if the roof or attic is difficult or hazardous to reach, the building has many openings, activity suggests a large colony, you cannot confirm that bats have exited, or local timing and legal requirements are unclear. State agencies discuss wildlife specialists, licensed operators and referrals. Confirm the operator’s credentials and any required local license before hiring.
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Do not touch bats with bare hands. If a person may have been bitten or otherwise exposed, keep the bat contained if it is safe to do so and contact the state or local health department promptly for instructions on testing or medical care. If a pet had contact with a bat, call a veterinarian and local health department. The U.S. Fish & Wildlife Service and Michigan DNR provide safety guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Would a bat house help?
A bat house near the building is an optional alternate roost, not a guaranteed way to attract bats and not a replacement for properly excluding them. The U.S. Fish & Wildlife Service notes that bat houses may help where roosts are lost but are not always helpful. Consider one only as a separate habitat measure, not as a reason to leave entry points open.
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