The Potential of Green Roofs in London

Concerns about the cooling requirements of buildings and negative health effects from heat exposure are increasing with the public’s awareness of climate change. Green roofs have been considered a powerful mitigative and/or adaptive tool to reduce negative impacts of heat. In fact, they can reduce summer indoor-temperatures, as evapotranspiration increases latent heat-flux from a building’s roof. The cooling effect seems to be small on outdoor air temperatures, according to past studies, although green roofs have other benefits in terms of biodiversity, carbon storage, improved building thermal performance and flood management. As the need for sustainable and climate resilient building designs becomes the norm in cities, it is important to assess the status of green roofs coverage and explore potential for future implementation. Accurate information on the prevalence and characteristics of existing green roofs is indeed required to estimate any effect of green roofs on outdoor and indoor temperatures, although this information is often lacking.

Surveying Greater London, we identified existing green roofs and estimated the potential for buildings to be retrofitted with green roofs. Existing green roofs were identified using automated classification of aerial and satellite imagery. Potential for retrofit is assessed using a geospatial database of building characteristics, together with a digital surface model.

The current total green roof area in Greater London is around 1.5 square kilometres (around 0.5% of built area). We estimate that retrofitting existing suitable buildings could add another 3 square kilometres, corresponding to around 2% to the built area in Central London, and around 1% outside Central London. Existing green roofs appear mainly on new buildings rather than being retrofitted, and mainly occur on office and commercial buildings in Central London and residential blocks in redeveloped areas. Potential for retrofit may be highest in the borough of Tower Hamlets, largely on residential blocks with flat roofs.

 This work has direct relevance to sustainable planning policy, especially the London Plan Overheating and cooling policy, and will enable modelling of the building-stock and city-scale effects of green roofs.