Green roofs, (roofs that are deliberately vegetated), are a technology that seeks to lessen the impacts of urbanisation on people and wildlife. This study investigates their value for UK bat species within the context of the built environment. Green roofs were categorised as ‘sedum’ or ‘biodiverse’ according to their dominant vegetation type. Bat activity was monitored over 13 biodiverse, nine sedum and 17 conventional un-vegetated roofs located within the Greater London area for seven nights using Anabat SD1 detectors. Influence of roof type and environmental variables on bat activity were evaluated using generalised linear models. Pipistrellus pipistrellus were most frequently recorded followed by Pipistrellus pygmaeus, Nyctalus/Eptesicus and Pipistrellus nathusii. The mean number of call sequences per night was 5.2. Feeding events accounted for 16% (217) of call sequences. Bat activity was significantly higher over biodiverse roofs compared to conventional roofs. A greater extent of suitable habitat within the surrounding area had a positive influence on bat activity but numbers of call sequences and feeding events were significantly higher over biodiverse roofs compared to conventional roofs when suitable habitat within a 100 metre radius of the roof was below 33% cover. Other factors affecting bat activity included roof height (negative influence with increased height) and the month of survey. No significant differences were found between sedum and conventional roofs. The findings suggest that biodiverse roofs offer enhanced habitat for bats within the context of urbanised environments. Further studies are needed to predict more accurately their value as a bat conservation measure.