The term Non-Bitumen Coated Roofing Membranes (NBCRMs) refers to both breathable and non-breathable roofing membranes that are used to control for condensation damage in buildings when better sealing homes for energy efficiency. This is in line with UK Government's Net Zero Strategy requirements including reducing greenhouse gases and the use of fossil fuels to meet the UK's net zero target by 2050.

Research revealed that all non–woven roofing membranes, produced using spun-bond filaments could pose a serious threat to bats as a result of entanglement. Since 2019 the NBCRMs steering group (below) has been working to find pragmatic solutions to the use of energy efficient materials within roost spaces, including publishing a test methodology for the snagging propensity of NBCRMs. By helping companies bring to market ‘bat safe’ roofing membranes that address the climate change and energy efficiency issues in our buildings, the steering group hope to reduce the industry’s reliance on roofing membranes that can pose a serious threat to bats.

NBCRMs Steering Group


In 2020, BCT, the SNCOs (NE, NRW and SNH) and academics from the University of Reading (Dr. Emmanuel Essah) and University of Leeds (Prof. Steve Russell) published their paper on Non-Bitumen Coated Roofing Membranes methods testing (based on Stacey Waring’s (co-author) see ‘Original project overview (2013)’ below.) in Building Research and Information journal.

Titled Method for evaluating the snagging propensity of roofing membranes in buildings by roosting bats (E. Essah et al 2020) this methods paper allows the independent testing of membranes to answer the question of what constitutes a roofing membrane with minimised risk to bats where they are or could be present.

Details of the Non-Bitumen Coated Roofing Membranes steering group members, discussions on how to consider the snagging propensity testing in the process of selecting materials suitable for use in a bat roost and updates on related discussions can be viewed HERE

Current work

Work is currently ongoing to support the SNCO’s as the regulatory authorities to publish position statements on their respective websites on the issue of NBCRMs that have passed the snagging propensity test.

Please see the Natural England (NE) and NatureScot (NS) websites for their position statements in the links below:

Position statement for Natural Resources Wales (NRW) is to follow but advice is expected to be very similar, however until NRW's position has been formally changed, the use of NBCRM’s will not be licenced. For further information please contact NRW through the details below:

This guidance is supported by a FAQ document and there will be a flowchart to follow shortly to help guide industry stakeholders through this process, to protect bats and avoid offences being committed.

Please note, although BCT facilitate the steering group and are currently hosting this guidance we are not involved in assessing or approving NBCRMs, this is a technical and regulatory issue and therefore the appropriate guidance should be sought through the contact details above for SNCOs and below for technical information.

Currently the steering group is only aware of one product that has passed the snagging propensity test completed by an independent laboratory, this is TLX ‘Bat Safe’. For technical questions including building regulations in relation to TLX ‘Bat Safe’ please contact TLX Insulation on 01204 674 730 or email

Advice can also be sought from building control or the relevant manufacturer of any membrane that has passed the snagging propensity test.

Non-Bitumen Coated Roofing Membranes

Original project overview (2013)

To investigate the potential impacts that breathable roofing membranes (BRMs) may be having on bats, the Bat Conservation Trust (BCT) approached the University of Reading’s Technologies for Sustainable Built Environment (TSBE) Centre. From these initial talks a collaborative four year research partnership between the TSBE and BCT was established. Stacey Waring, the research engineer leading this work, completed her doctoral research in 2013. A summary of the main findings, with regards to bat entanglement in BRMs, can be found below.

NB - since this research was produced a number of non-breathable roofing membranes have come on the market that also pose a threat to bats, therefore BRMs are now referred to as Non-Bitumen Roofing Membranes to make this distinction.

Project aim

‘To improve understanding of the interactions between bats and BRMs, how such interactions may impact upon bat conservation and product longevity, and how this knowledge can aid mitigation in such circumstances’

BRMs are installed in many buildings to allow the roof to breathe so that traditional ventilation is not required. Research gathered over the course of the project shows that all non–woven roofing membranes, produced using spun-bond filaments pose a serious threat to bats as a result of entanglement. In addition, the functionally of the membranes is affected by the bats.


The spun-bond filaments in modern roofing membranes are exposed to abrasive forces not tested for by manufacturers when placed in a bat roost; bat claws tease filaments loose from the surface of non-woven membranes forming a ‘fluffed up’ appearance on the surface. These loose filaments can become entangled around a bats’ feet and wings, resulting in bats becoming immobilised and eventually dying.

Currently all BRMs are non-woven. This means that the risk of entanglement also extends to all other non-woven membranes currently on the market. There are also modern types of bitumen felt that contain polypropylene filaments (for example type 5U). These membranes, despite being called bitumen, still pose a risk of entanglement bats. However, traditional hessian reinforced bitumen felt remains a suitable replacement with less risk to bats.


Findings from the research indicate that the functionality and longevity of the membranes can be affected in those parts altered by the presence of bats


The research also found that the microclimatic conditions of the roof voids varied between different types of membrane.

Key Recommendations

BRMs should not be installed into a roof used by bats.

Only bituminous roofing felt that does not contain polypropylene filaments should be used. For example bitumen felt type 1F, which is hessian reinforced


Building Regulations

It is sometimes wrongly stated that the use of bitumen felt in roofs does not comply with Building Regulations. The Building Regulations that apply to this situation are Parts C and Parts L. The Building regulations that apply to existing buildings are parts L1B (domestic), L2B (non-domestic) and Part C (condensation and ventilation).

The Building Regulations state that energy performance of the whole building needs to be improved where possible for existing buildings. When considering a pitched roof this is done by increasing levels of insulation to meet the recommended minimum. The Regulations state that contractors must “assess the condensation risk within the roof space and make appropriate provisions in line with part C relating to the control of condensation”. Part C then goes on to recommend meeting the recommendations made in BS 5250:2011. In this document it is made clear that both High resistance (bitumen) and Low resistance (BRM) underlays are acceptable as long as appropriate ventilation is provided. The materials must comply with British standards, national technical certificate or another acceptable EU certificate (which bitumen felt does).

Historic Buildings / Listed building consents

Section 3.5 in part L2B of the Building Regulations states that whilst most existing buildings need to comply with energy efficiency requirements, certain classes of buildings, including listed and other historic buildings, are exempt.