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Guide to Mitigation – Void Dwellers

The term “void-dweller” is used to describe bat species that often utilise small crevices for roosting. Crevices can include natural features such as stone fissures, holes in trees and under loose bark. When found in buildings, crevices can be found underneath raised tiles, within cavity walls, between a roof lining and tiles and under fascia boards. The small size of crevice dwelling bats means that the crevices they can utilise for roosting can be as small as approximately 10mm in size.

Species that are termed “void-dwellers” include (long-eared bats (Plecotus species), Natterer’s bat (Myotis nattereri), Daubenton’s bat (Myotis daubentonii), Bechstein’s bat (Myotis bechsteinii), Leisler’s bat (Nyctalus leisleri) and noctule (Nyctalus noctula))

While planning successful mitigation strategies for void-dwelling bat roosts or hibernation sites, there are a number of factors that need to be considered:

  • Temperature and humidity regimes;
  • Aspect and orientation;
  • Size;
  • Access points;
  • Lighting;
  • Materials; and
  • Vegetation linkages

brown long-eared mitigationTemperature is deemed one of the most important environmental variables affecting the success of new roosting facility. Optimum roost temperatures is a species-specific relationship.

Aspect and orientation can be manipulated in order to utilise solar energy and maintain optimum temperatures within a roost. In the northern hemisphere, a southerly or westerly facing roost site is best; particularly maternity roosts.

long-eared mitigationThe size of the roost specifically fovoid-dwelling bats should be at least 2m in height (ideally 2.8m) and 5m in length and width. The void should not be cluttered or obstructed as these species will often fly in spaces between the rafters and ceiling, and ceilings to the floor. Therefore, a typical truss design should be avoided.

Access points are key to the success of roost sites. A variety of openings can be used including larger access points which the bats can use to fly directly into the roost, or smaller access pointsBat Survey Devon whereby the bats land and crawl into the roost space.

As nocturnal mammals, bats are light-sensitive and adapted to low light conditions. Therefore, consideration must be made to reduce light spill close to the roosting area.

It is important to consider the Vegetation linkages from the roost access point/s. It may be worthwhile undertaking additional hedgerow/tree planting close to the roost access points connecting to bat commuting or foraging habitat. 


Brown long-eared bats are a common void dwelling bat. We’ve written another article which goes into greater detail regarding mitigation for this species.

More information about bats and survey techniques can be found in our articles – Bat Droppings, the Bat YearArchitectural Terms for Bat Surveyors, a Guide to Bat MitigationHorseshoe Bat Mitigation and Bat Mitigation for Crevice-dwelling bats.