Bat-safe roofs
Originally posted 6th September 2022
Sustainable building isn't all about carbon reduction (though that is essential!) but also about supporting biodiversity. It's crucial to maintain and increase habitat for other species. Bats often lose out when new roofs are designed, whether in renovations or new-build. Modern breathable roofing membranes (officially Non-Bitumen Coated Roofing Membranes or NBCRM) can kill bats, as the long fibres get tangled around their claws and trap them.
Most roofs are potential roosts to bats at some point, so it's really important to consider your material choices. You should always consult with an ecologist when designing or planning anything that might affect protected species, whether that is for a retrofit or a new build project. Bats are European Protected Species and that protection has continued post-Brexit. There are strict legal licensing requirements for any work that might affect a bat roost. Check the precise requirements with the relevant statutory body for your part of the UK, and read the Bat Conservation Trust’s information here: https://www.bats.org.uk/our-work/buildings-planning-and-development/building-or-development-works. Bats can also be found behind vertical cladding if there is any way into the ventilated space behind.
Non-bitumen membranes (NBCRM) are used because they allow moisture vapour from inside the roof structure to escape (often called ‘breathable’ membranes though vapour-open is the correct term) whilst providing a water-resistant layer to stop rain getting in. This is very beneficial for any roof as it helps prevent rot, and prevents insulation becoming water-logged (wet insulation is no longer insulation…). Older bitumen-felt roofing membranes are safer for bats but trap moisture inside the roof structure, increasing condensation risk.
Progress towards bat-safe NBCRMs has been slow. Some manufacturers have made 'bat-safe' claims in the past without any evidence to back them up.
A ‘snagging propensity test’ test has now been developed to test the resistance of the membranes to snagging by bat claws, and the guidance in England has been updated to allow membranes that pass this test to be used in bat roosts. Scotland and Wales may update their guidance in a similar manner but have not yet done so. Currently there only seems to be one vapour-open roofing membrane that has passed the test, so there's not a lot of choice yet! Hopefully more manufacturers will be motivated to update their products now there's some official guidance.
Edit: It’s been rightly pointed out to me that no long term testing or in-situ monitoring has yet been done on the membrane that has passed the snagging test.
The test was initially developed Dr Stacey Waring during her PhD, and taken forward by the Bat Conservation Trust (BCT) working with Natural England, NatureScot, Natural Resources Wales, and the Universities of Leeds and Reading. Details of the test are available in their paper, here: https://www.tandfonline.com/doi/abs/10.1080/09613218.2020.1763773?journalCode=rbri20
It is possible to do away with roof membranes altogether using certain wood-fibre insulation/sarking boards that have been treated to provide a water-resistant outer surface. Many architects/builders would still use a membrane in addition to these though, in order to provide a more robust defence against water ingress in the event of rain leaking through the outer roof surface. Depending on the the level of exposure in your location and the exact nature of the outer roof covering the membrane may be unnecessary, but certainly great attention to detail is needed at joins between wood-fibre boards, with jointing tapes as necessary.
Keep an eye on the BCT page for updates and more information: https://www.bats.org.uk/our-work/buildings-planning-and-development/non-bitumen-coated-roofing-membranes
Finally, a challenge to suppliers and manufacturers of non-bitumen coated roofing membranes: are you planning to get yours tested for bat snagging propensity? You should do! And if you already know it would fail then you should be considering how to change that.
NB: I am not an expert on either bats or bat-safe construction. This is just a topic I find interesting and incredibly important.
This blog was slightly edited on 7/9/2022 to add concerns about lack of long term monitoring, along with reference to vertical cladding and further clarification of non-membrane roofs.
