Works to Roofs, Walls and Building Services

Roof works

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.

Roofing underlays

Roofing underlays are a relatively modern innovation dating from the early 20th century. Before then roof tiles and slates were mostly left exposed on the underside, although they were sometimes 'torched' with lime mortar or laid on boarding ('sarking'). These features acted as a barrier against wind and the ingress of moisture and dust; they also helped to stabilise the roof environment and control condensation. Underlays were introduced to serve a similar purpose, and to provide a second line of defence against rain penetration. They also keep the building dry during re-roofing, thereby avoiding the need for a temporary roof. Underlays are now usually installed in old roofs when they are re-tiled or re-slated, as recommended in current good practice standards.

Until the late 1990s the underlay most widely used for tile and slate roofing was reinforced bituminous felt (commonly referred to as 'BS747 Type 1F' felt). This material works well provided the roof space is adequately ventilated. It is also good for bats as it creates a dark environment (Type 1F felt is black) with rough surfaces for them to crawl along and hang from. There have been few reports of bats being harmed in roofs where Type 1 felt underlay has been installed. But in recent years, Type 1F felt has been largely supplanted by synthetic membranes, now referred to as Non-Bitumen Coated Roofing Membranes (NBCRMs). NBCRMs are available in both vapour permeable ('breathable') and impermeable ('non-breathable') forms to suit differing applications and performance requirements, and are often used in conjunction with measures to improve energy efficiency in buildings (for example adding insulation or increasing airtightness). Unfortunately, most NBCRMs are made up of layers of non-woven, heat-bonded synthetic filaments. These filaments are long and strong, and bats can become entangled or snagged in them when their sharp claws pull them from the underlay.

In response, concern about the potential harm to bats from NBCRMs has led to the development of an industry standard test to determine the 'snagging propensity' of individual products. In addition, the statutory bodies (Natural England, NatureScot and Natural Resources Wales) responsible for overseeing licencing of works to bat roosts have published guidance on requirements for installing NBCRMs. Evidence that a product has passed the snagging propensity test must now be submitted when applying for a mitigation licence. There are also concerns about the long-term effects of changes in the roof environment on bat roosts where Type 1F underlay is replaced with a NBCRM. This is the subject of ongoing studies and monitoring.

In summary:

• The only roofing underlay allowed by the statutory nature conservation bodies without a snagging propensity test certificate is Type 1F felt. To comply with Building Regulations, however, it is important to provide appropriate vents when using Type 1F underlay to control the risk of condensation on the underside of the felt. These vents can also provide access points for bats
• NBCRMs are not permitted unless the product has passed the snagging propensity test. At present only one NBCRM has passed this test. Find out more
• As an alternative approach, traditional solutions, such as mortar torching or sarking boards, provide many of the benefits of underlay and can be effective in maintaining a benign roof environment. For further advice see our book Practical Building Conservation - Roofing

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:

• Early cavity walls 
• Insulating windows

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.