Works to Roofs, Walls and Building Services
Almost all UK bat species have been found roosting in roofs and many roost or hibernate in gaps and cracks in mortar. Any work carried out may affect bats or a bat roost. Building owners, managers, advisers and contractors need to ensure the bats and roost are protected at all times.
A weather-tight roof is, of course, essential to both buildings and their interiors. Regular inspection and maintenance of the roof covering, structure and rainwater goods will increase their lifespan, reduce the loss of historic fabric and the frequency of more costly and major works. Not surprisingly, such working practices also benefit bats so long as access points are not blocked.
If you are considering repairs, a major re-roofing or works on the surrounding structure, you will need to get specialist advice. This will ensure works can be planned in and around the bats’ period(s) of greatest vulnerability. Your bat specialist may be able to work out how to phase the work by compartmenting roof sections but you will need a bat licence for any such scheme.
Case study – The Vyne roof project
A good example is the National Trust’s The Vyne roof project. Their case study: How were bats protected during The Vyne roof project? explains how the trust planned this major project.
Roof felts and linings
Roofing underlays are normally installed in old roofs when they are re-tiled or re-slated. The main purpose of the underlay is to reduce wind uplift, and a second line of defence against rain. They also help to keep the roof dry during re-roofing thereby avoiding the need for a temporary roof.
For over 70 years the material commonly used for underlay was impermeable bituminous felt (conforming to BS747; Type 1F). Type 1F bituminous felt worked well if the roof space was adequately ventilated, and it was good for bats as it created a dark environment and a rough surface to hang from or crawl along.
However, in recent years Type 1F felt has been largely supplanted by low vapour resistance (‘breathable’) underlays. These are meant to let excess moisture vapour escape from unventilated roof voids, while acting as a barrier to liquid water penetration from outside. These membranes also need ventilation to work effectively.
Most low vapour resistance underlays (and their non-permeable (non-breathable) counterparts) are made up of layers of non-woven, heat-bonded synthetic filaments. These filaments are long and strong and bats can become entangled in them when their sharp claws pull the filaments from the underlay. In contrast, there have been few reports of bats being harmed in roofs where Type 1F underlay has been used. There are also concerns about the long term effects on bat roosts of changes in the roof environment where Type 1F underlay is replaced with a ‘breathable’ membrane.
At present, the only roofing underlay that is allowed by Natural England and the other UK statutory nature conservation organisations to be used in a bat roost is hessian-reinforced bitumen Type 1F felt.
The Bat Conservation Trust is actively involved in ongoing research into the safety of roofing membranes.
When using Type 1F underlay, to comply with Building Regulations it is important to provide appropriate vents to control the risk of condensation on the underside of the felt. These vents can also provide access points for bats. If you are using traditional mortar torching instead of an underlay, consult with a bat specialist to incorporate openings so bats can continue to enter the roof void from outside.
Walls, shingles and weatherboards, and other gaps
Many bat species roost or hibernate in gaps and cracks in mortar, and tiles, shingles and weatherboarding or use these gaps to gain access to spaces behind.
Mortar gaps and cracks often indicate that repair work is necessary to prevent further deterioration. Although the priority must be to safeguard the structural integrity of the building, many bat species roost or hibernate in cracks and gaps, or use them to gain entry into spaces behind. You will need to get advice from a bat specialist.
If you need to treat shingles and weatherboards, see our advice on timber treatments.
Insulating walls and roofs
Historic England supports the government’s aims of improving the energy efficiency of buildings. Our guidance on insulating older homes includes advice on insulating walls and roofs. Bats also need to be considered so you'll need to get advice from a bat specialist.
Insulating walls in historic buildings will alter the performance of the masonry and requires careful design, correct choice of materials, good detailing and high standards of workmanship. Buildings may be exempt from energy efficiency requirements if listed or in a conservation area. Others may not have to meet the requirement if it is not economically or technically feasible to do so.
Cavity wall insulation
The Bat Conservation Trust has highlighted that cavity wall insulation schemes may pose a risk for bats and has identified mitigation measures that comply with Energy Performance Certificate (EPC) requirements.
Cavity walls only became a widespread form of construction after the First World War although there are early-19th-century examples. These early ‘hollow walls’ have two separate leaves of masonry and were developed to provide protection from the elements, especially from driving rain in exposed locations. As these early forms of cavity wall tended to have very narrow cavities and bridging elements they are generally unsuitable for cavity fill insulation.
Find more of Historic England's advice about:
Rewiring, plumbing and other service work
Some areas used by bats as roosting places, like roof voids and cellars, may need to be entered by maintenance staff, plumbers, electricians and others to carry out repairs, servicing and tests, and building surveys. Although vital to running a serviceable building, all of these operations could disturb roosting bats; get advice from a bat specialist on the best way to proceed.