Accommodating bats in buildings

What do bats look for in a home?

Like us, bats are looking for shelter that is clean (free of disease and parasites), comfortable (protected from the weather) and secure (safe from predators and disturbance). However, bats also have unique requirements and may live in several different roosts throughout the year depending on their seasonal needs.
A number of factors are very important when considering roosts for bats: 

Temperature

Temperature is very important to bats. Bats are different to us in that they look for warm roosts in the summer and cool roosts in the winter.

Summer: Bats usually only have one young a year. Mother bats gather together and find somewhere warm to rear their young called a maternity roost. Males will form smaller roosts elsewhere that are often cooler than the needs of the females. Generally maternity roosts should probably be between 30 and 40°C (keeping in mind that the larger the number of bats the greater the temperature gain from body heat). Also, some species have their own unique preferences and studies have shown that even a 1.5°C difference may influence bats to choose one building over another. It is always best to provide a number of different options for bats so that they can choose the right roosts and temperature based on their needs.

Winter: British bats feed on insects which are scarce in the winter. Bats have adapted to this seasonal scarcity by going into hibernation where they lower their body temperature to the ambient temperature or just above. Hibernation roosts are often a cool space, generally between 0 and 6°C, with high humidity. Horseshoe bats prefer slightly warmer conditions of 6-10°C.

Aspect and orientation:

Summer maternity roosts in the northern hemisphere often have a southerly or westerly aspect for maximum solar heating. Male roosts and hibernation sites typically have a northerly aspect.

Creating roosts in buildings

General principles for creating new spaces for bats in buildings:

The advice below describes general principles to consider when providing new spaces for bats. It does not provide sufficient information for mitigation when a roost is already present. Work on existing roosts should be covered by an EPS licence.

Access, size of roost space and structure

• Crevice-dwelling bats can crawl into their roosts via small gaps in the range of 15 - 20mm high by 20 – 50mm wide. The roost area should maintain a crevice of this approximate size gap that the bats can roost between. The area this roost provision covers can be small but about 1m2 would be useful for summer nursery roosts. The height of entry can be from 2 - 7m.

• Roof-void dwelling bats require similar dimensions to access the roost but typically need timber joists or beams on which to roost. The height of entry can be from 2 - 7m.

• Bats needing a flying area require the same access dimension as mentioned above, 15 - 20mm (h) x 20-50mm (w) situated over 2m in height. The roosting area should not be trussed, to allow flight, and should ideally be about 2.8m in height and 5m in lengthand width.

• Horseshoe bats need a larger access so that they can fly (instead of crawl) directly into the roost. Lesser horseshoe bats need an access of 300mm (w) x 200mm (h), while greater horseshoe bats need 400mm (w) x 300mm (h).  As above, the roosting area should not be trussed, to allow flight, and should ideally be about 2.8m in height and 5m in length and width.

Materials and structure

Materials for the roosts should be rough (for grip), non-toxic or corrosive, with no risk of entanglement. The materials should also have suitable thermal properties that reduce 24 hour fluctuations but allow maximum thermal gain for summer roosts.