Requirements for roost retention and creation

Whether you are retaining a bat roost and access points within a building or whether your only option is to create a new roost in another building on site or within a totally new structure, there are a number of considerations to take into account when works can impact on a roost.

The following elements should all be given full consideration when seeking to retain roosts or create new roosts for bats: 

 

Temperature and humidity regime within the roost

Probably the most important environmental requirement for all bat roosts is an appropriate temperature. The ability for roost sites to achieve and maintain optimum species-specific, roost temperatures is crucial for successful mitigation.

Generally, the optimum temperature range for maternity is between 30 to 40°C (keeping in mind that the larger the number of bats the greater the temperature gain from body heat). Some species have their own unique preferences and studies have shown that even a 1.5°C shift in temperature may cause bats to choose one building over another. It is always best to provide a number of different options for bats wihtin a roost so that they can choose the most appropriate temperature based on their needs.

Please see the Evidence section of this website for more detail on species-specific temperature preferences.

If at all possible, measure roost temperature continuously for a period of time (e.g. by using a data logger) for at least one season before development. This baseline data can then be used to guide the mitigation plan to help to achieve or maintain an appropriate temperature in the post development roost. For example, mitigation structures can provide a warmer or cooler roost environment than the original structure, or an environment offering more diverse temperatures to accommodate bats over a prolonged part of the year.

Aspect and orientation of the roost

The best way to influence the temperature regime of a prospective roost is by locating it with an appropriate orientation to optimise for solar gain. Summer maternity roosts in the northern hemisphere should have a southerly or westerly aspect. Hibernation sites in buildings typically should have a northerly aspect.

If possible, design building roosts to have an L-shape, or a range of internal spaces, so that bats can have a choice of aspects and temperature regimes.

Size and suitability of final roost

  • Crevice dwelling bats: the overall size of each roosting feature can be small but a minimum amount of space for bats to cluster (about 1 m2) would be useful for summer nursery roosts.
  • Roof void dwelling bats: typically need timber joists or beams on which to roost, as well as uncluttered flight space.
  • Horseshoe bats: roosting area should not be trussed to allow unobstructed flight, and should ideally be about 2.8 m in height and 5 m in length and width.
     

Access points 

  • Crevice dwelling bats and roof void dwelling bats can crawl into their roosts via small gaps in the range of 15 - 20 mm high by 20 – 50 mm wide. Access to the roost area should incorporate a crevice of this approximate size that the bats can move between or roost between. The height of entry can be from 2 metres upwards to the full height of the building.
  • Horseshoe bats fly (instead of crawl) directly into the roost. Lesser horseshoes need an access of 300 mm (w) x 200 mm (h), while greater horseshoes need an access gap of 400 mm (w) x 300 mm (h). To protect access points from disturbance, horizontal grilles can be placed over the entrance. However, ensure that the air gap between bars is at least 15 cm. Installing a porch over the entrance can also assist horseshoe bats with light sampling.
  • Locate access points lower down on a building for horseshoe bat species. In occupied buildings such as dwelling houses, bat access points should be placed away from doors and windows to avoid interference from humans or predators.
  • It is important to locate access points close to vegetation and flight lines. This allows bats to emerge earlier and forage longer. Details such as the distance between bat access points and sheltering vegetation are important to record as part of the original surveys so that appropriate conditions can be reproduced in the final mitigation provision.
  • If providing more than one access point, particularly fly-in access for horseshoe bats, be careful to design it such that it does not increase the risk of draughts or light incursion.

Lighting

  • Bats are nocturnal creatures and adapted to low light conditions. Most bat species find artificial lighting to be very disturbing.
  • Ensure that artificial light sources are not directed onto roosts, access points or flight paths.
  • Try to use lights that are low UV and thus less likely to attract insects.
  • Ensure that external lighting is pointed downwards to avoid up-spill into the environment.
  • Within a roost, the use of baffles can be used to shield against light ingress.
  • It is recommended that ecological consultants give full consideration to the presence, use and impacts of artificial lighting in survey reports and mitigation proposals.
  • Full lighting details should be included in planning proposal documents.
  • Artificial lighting should be included in the compliance checks that form part of all mitigation proposals.
  • Full consideration of artificial lighting should include reference to light levels within and external to mitigation features, known roost sites and key bat flyways.
  • Measurement of light levels should be made prior to and after mitigation and should form part of the Condition Assessment, Compliance Checks and Measures of Success.
  • Measure internal light levels using a light meter. Hold the light meter perpendicular to the most obvious source of light at all sample points and take the highest reading at each point.
  • Please consult the BCT's Bats and Lighting Guidance document and Dr Emma Stone's research page for more information.

Materials

Roof lining materials

Many modern breathable roofing membranes (BRMs), are known to abrade over time and form loose fibres, in which bats may become entangled. For this reason, mitigation projects should clearly state that no BRMs should be used in bat roosting areas, or that BRMs should be partly covered with traditional bitumastic roof felt. If modern BRMs are specified for use in mitigation sites, mitigation proposals should include a full justification for their use.

Please consult the Bats and BRMs webpage for more information.

Perching points

The availability of perching points is documented to be a limiting factor for bat species. The availability of sufficient roosting substrates is a key measure of the ecological functionality of a site, and should be given full consideration by ecological consultants in survey reports and mitigation proposals.

Perching points should provided at a range of heights in the roof space to allow the bats to take advantage of different micro-climates within the roost. Untreated battens nailed to the underside of the rafters or an untreated expanding garden trellis are simple perching options.