Research centred around the question ‘what constitutes bat friendly breathable roofing membrane?’ is being conducted by a research engineer co-supervised by the Technologies for the Sustainable Built Environment Centre (TSBE) at the University of Reading and the Bat Conservation Trust.
Evidence found to date indicates that breathable roofing membranes are causing mass mortalities of bats in roofs roosting below the membrane (long-eared bats, horseshoes etc.) and between the membrane and the tiles (e.g. pipistrelles). The research reports incidences where 20 and 30 bats were found dead in single roofs, and 104 pipistrelles killed between tiles and Tyvek membrane beneath.
The problem with breathable membranes appears to be the fact that they degrade with bat use, becoming ‘fluffed up’ to the extent that bat claws become trapped in them. Bat mortality results from the bats’ inability to escape.
The research concludes that there are no bat-friendly breathable membranes on the market and as a result Natural England are beginning to refuse European Protected Species licence applications, which involve mitigation using roofs fitted with breathable membranes.
What does this mean for our clients?
Un-breathable bitumastic membrane is the only membrane on the market that is accepted as ‘bat friendly’ by regulators, however, it is recognised that this presents a conflict of interest with building preservation and building regulations.
Until a breathable roofing membrane is manufactured and certified ‘bat friendly’ mitigation solutions likely to be accepted by regulators are:
- For bats that roost below the membrane – fix bitumastic membrane beneath the breathable membrane and create holes throughout the bitumastic membrane that are sufficient in number and size to allow moisture transfer but small enough to prevent bat movement through the holes.
- For bats that roost between the membrane and tiles – segregate the roof to create a section lined with bitumastic membrane for use by bats enabling the remainder of the roof to be lined with breathable membrane. Note: some means of preventing bat access from the bitumastic lined roof section to the breathable membrane section would be required AND the area of roof required for use by bats would be determined by roost status and current roost area and aspect.