Lofty ideas – Bat mitigation and bat loft design
Understanding the requirements, purpose and design options for bat lofts is an important consideration for architects, following the discovery of a bat roost during the development process.
Discovery of a bat roost
The client has commissioned you to redesign a building into their dream property. Whether it is an old farm building or a more modern building that needs a face lift, you set to work on the design to impress and inspire. For many of the planning applications put forward, there is often a requirement for an ecological survey for bats. Bats are present in all locations and are not restricted to rural areas. Peter Hacker, Wildwood ecologist reports “we have found bats roosting under PVC fascia boards on a 1960’s housing estate, under lifted roofing felt on a flat roofed school building, in the pitched roof of an outside toilet as well as in the loft of a modern bungalow”.
If a bat roost is discovered during a Preliminary Roost Assessment (PRA) or subsequent emergence survey, there will be a need to accommodate roosting bats, post construction. It is worth considering when starting to draw up the new design, that bats may be using the property as a roost. Compensation for the loss of a roost could be as simple as a new built-in bat box or the need for a bat loft. A loft could be incorporated in the early stages of the design process; an area Wildwood ecology could assist with as we have a wealth of experience.
Wildwood Ecology worked alongside the architects and builders on the restoration of Llwyn Celyn, a Grade 1 listed heritage building in the Llanthony Valley. Involved from the early stages of this complex project, a bat loft was designed and incorporated into the structure of the restored buildings. The loft has been a success and is regularly monitored. Increasing numbers of lesser horseshoe bats are being recorded year on year. The loft is a two-storey structure, with a monitoring hatch and internal features for lesser horseshoe and brown long-eared bats. The design also incorporated a built-in heating device to maintain perfect ambient temperatures.
Research into mitigation and compensation for the loss of a bat roost has highlighted the most successful approach is to retain the roost insitu. Unfortunately, in some cases, the retention of the roost could also result in a decline in use and the numbers of bats using the roost. This decline could be down to the change of the features around the roost or changes to the structure which causes a change in the ambient temperature and the temperature regimes within the roost.
Bats in some ways resemble the behaviours of humans. Some like a small bijou space (common pipistrelle), others prefer a large spacious area (greater horseshoe). Humans like to have a view or access to amenities, so do bats. These mammals like to be in an area of treelines and hedgerows for food and commuting routes. Close resources mean the bats do not have to waste precious energy searching for food. Our houses are heated in the winter, and in the summer months we want our rooms cooler. Bats also like the option to move around the roost as temperatures change.
For some developments or projects, retaining the roost in its original undisturbed position is not possible, therefore the loss of the roost has to be accommodated. However, attempting to replicate a roost that has been specifically chosen by a bat is difficult and not always successful.
Bats are a protected species, and this legal protection covers the disturbance and/or destruction of a roost and every effort must be made to provide a roosting opportunity with equal or greater qualities of that lost.
Brown long-eared, lesser and greater horseshoe bats are species that require an open space in their roost. They like to fly around the roost in order to warm up prior to exiting. Unlike horseshoe species, the brown long-eared bat has the ability to crawl through holes, cracks and crevices to reach the open space of the roost. Horseshoe species require free flight access to the roost, meaning no obstacles or barriers in their way for entering or exiting the roost. Although, there is anecdotal information from experienced ecologists who have encountered the phenomenon of horseshoe bats going against the grain and crawling.
Bat loft design
Should a roost for a species that needs flight access be lost, there is every likelihood that a bat loft will be required to compensate for the loss. There are no hard and fast rules regarding the design of bat lofts. However, there are a set of requirements for the design to make it a suitable place to roost. Ideally the design should take into account additional bat species that could use the loft, not just the species whose roost is being lost. The bat loft can be a stand-alone structure or incorporated into the design of the development.
Wildwood Ecology has recently been involved in the design of bat lofts for a variety of properties, including an internal loft within an old coach house near Marlborough, forming part of a new roof and a new loft in a Grade II listed farm building near Bath, that is being repurposed. Discussions with the architect throughout the design process enabled us to come up with a suitable bat loft that not only met the needs of the client, but also the Heritage Officer.
Ten top tips for bat loft construction
- The loft needs to have a large void space to allow the bats to fly around.
- For horseshoe bats this void space should be unobstructed by constructional timbers.
- The void space should be located within the roof of a building that has an apex.
- If the loft is standalone, an apex roof should be built.
- The recommended height from floor to the top of the apex is in excess of 2.8 metres with a recommended width and length of five metres or more.
- Ideally one of the roof pitches should face in a southerly direction.
- The entrance of the loft should be positioned as close as possible to the original entrance to the roost.
- There should be no external lighting around the entrance at a height of 3-4 metres.
- It is worth considering the use of a hopper at the entrance. This is a 45-degree metal lined shoot that deters birds from entering the roost, but still allowing access to bats.
- The loft should be built with a lockable panel into the loft to allow access by a licenced ecologist for monitoring.
The roof of the loft, where possible should be constructed of the original or similar tiles but incorporating a variety of bat tiles. Raised bats tiles provide at roost space for crevice dwelling bats such as common and soprano pipistrelles.
The lining of the roof must be made of traditional hessian bitumen roofing felt or wooden sarking. Non-bitumen coated roofing membranes (formerly known as Breathable Roofing Membranes (BRMs) should not be used, as bat claws get caught in the fibres the bats get trapped.
The internal lining of the loft should be constructed from rough untreated timber. This is the preferred material for bats and allows bats to cling the rough surface. The internal design of the loft space should incorporate a variety of different features that creates opportunities for hanging, crevices to crawl into and areas that create different thermal regimes.
Ground floor and access
For the stand-alone bat loft or if space is available, as part of the development, a ground floor area should be created. This space can be constructed of breeze block/ brick or stone and ideally rendered on the inside. An opening roughly one metre square should be cut into the ceiling of the ground floor to allow bats free flight access from the loft area to the ground floor area. As the bat loft is likely to have been created because of a granted bat licence, the loft will be required to be monitored. In order to do this, a door should be built into one of the ground floor walls and a lockable hatch, two metres square built into the ceiling of the ground floor, opposite the free flight access opening.
Features can be added to this lower section of the loft, such as horizontal crevices or hanging points created from expandable flower trellises.
Finally, a north facing wall could be purposed as a hibernation wall. This is created by building a second internal wall 50cm away from the northern wall. Within the cavity of these two walls, small mortar blocks are put in place to create crevices within this cavity. Access holes are left in the exterior of the northern wall to allow bats into the cavity.
Whilst there is no guarantee of the new bat loft being successful, recent studies (Collins et al, 2020) found that new lofts in adapted buildings and retained lofts were more successful than new lofts in new buildings. Additionally, the higher the number of small internal cavity voids within a loft showed a higher positive relationship with the number of bats using a loft.
Bat loft versus bat box
Bat lofts are significantly more excessive than bat boxes. However, if we are to continue to prevent further loss to these wonderful, clever creatures then every effort should be taken to ensure their survival where they cohabit with humans.
Whilst bat boxes fulfil a purpose, a study by Stone et al, 2013, showed that bat lofts had a 74% occupancy rate compared to a 13% occupancy rate of boxes. The success of bat boxes is being studied and the initial indications show that built in bat boxes are more successful than externally mounted boxes. The overall message is that a bat loft has a greater potential of being used or reused by bats than other commercially available alternatives. If would like further advice about bat loft design, please do contact us.
Ian Weller, Ecologist