Protecting Heritage Roofs and Beneath Them: Why Type 1F Felt Matters
Roofs of older and historic buildings are often home to more than just slate, timber, and tiles. They frequently provide critical habitat for UK-protected species like bats. When refurbishing or reroofing such properties, material choice isn’t just a matter of weatherproofing or regulations—it’s about ecology, heritage and legal compliance too.
The Heritage Material Advantage
Using materials that are sympathetic to the original construction of a building “heritage materials”brings multiple benefits:
- They preserve historic character and the building’s performance.
- They maintain the roofing assembly in a way that earlier occupants used and that nature adapted to.
- In the case of underlay, choosing the “traditional” option may mean fewer surprises when dealing with ecology and protected species.
One such heritage-friendly material is Type 1F bituminous felt (also referred to as BS 747 Type 1F or hessian-reinforced bitumen felt). According to Historic England:
“Until the late 1990s the underlay most widely used … 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.” Historic England
Why Bats and Roofing Underlays Should Be Considered Together
All UK bat species and their roosts are legally protected. Under the Bat Conservation Trust / statutory nature conservation organisation guidance, any work on roofs must consider the presence or potential presence of bats. Bat Conservation Trust+2Arbtech+2
Modern roofing membranes (often referred to as “breathable roof membranes” or non-bitumen-coated roofing membranes, NBCRMs) have been found problematic in bat roost scenarios. Why?
- Many contain long spun-bond fibres (polypropylene or polyethylene). Bats crawling or hanging underneath may become entangled. The Ecology Co-op+1
- These membranes can alter micro-climates (temperature, humidity) within the roof void, potentially making it less suitable for bats. Acer Ecology
- Because of these risks, the only underlay that the SNCOs currently allow without a snagging-propensity test certificate in a known bat roost is Type 1F bituminous felt. Historic England+1
Why Type 1F Felt is the Safe Choice
Here are the specific reasons why Type 1F remains the preferred option when bats may be present:
- Surface texture and darkness: The black bitumen-saturated hessian surface offers a rough, dark substrate that bats can grip and feel secure in—something they depend on.
- Low entanglement risk: Unlike synthetic membranes with long fibres, Type 1F uses short fibre or felt construction without loose filaments, so there’s minimal risk of entrapment. Bat Conservation Trust
- Proven track record: Historic England notes that there have been “few reports of bats being harmed in roofs where Type 1 felt underlay has been installed.” Historic England
- Regulatory trust: When a roof is used by bats, using Type 1F avoids triggering the more onerous snag-propensity testing required for alternative membranes.
Integrating Heritage & Ecology into Your Roofing Specification
When you’re specifying or recommending roof materials on a heritage building (or any building with bat potential), keep these best-practice steps in mind:
- Assume bats may be present unless a recent survey (within last 2 years) confirms otherwise. Ashbrook Roofing Supplies LTD+1
- Choose roofing underlay accordingly: If bats are known/suspected, go with Type 1F bituminous felt or alternative heritage methods (e.g., sarking boards + torch lime-mortar).
- Ensure ventilation is maintained: Even with Type 1F you must provide appropriate vents to control condensation—this also supports bat access and a breathable environment. Historic England
- Avoid modern NBCRMs unless certified: If you’re using a breathable membrane, ensure it has passed the snagging-propensity test and is approved for bat roosts. Otherwise, the risk is greater. Bat Conservation Trust+1
- Detail access for bats: Ensure your design allows for bat ingress/egress where needed. Blocking or sealing roof voids without specialist approval is likely illegal. Ashbrook Roofing Services
- Document in your specification: Make sure the roofing specification states that “Type 1F bituminous felt to BS747 (or equivalent) shall be used in any roof void where the presence of bats is confirmed or reasonably suspected.”
Final Thought
When heritage buildings are reroofed, the instinct may often be to specify the latest high-performance membranes or modern breathable underlays. But when the roof void doubles as wildlife habitat, that decision matters. Opting for an appropriate heritage underlay like Type 1F felt is not just a nod to traditional building practice—it’s a responsible ecological choice.
Using materials that align both with the building’s character and nature’s needs ensures your project achieves the best of both worlds: healthy, long-lasting roofs and secure habitats for our protected bat species.