Background

Green roofs are roofs that have been specially fitted or designed to support vegetation growth. Commonly used species are low-growing and drought resistant, such as sedum species or meadow grass. Green roofs attenuate and retain rainfall, emulating the behaviour of natural land. This allows them the be used as components in a Sustainable Urban Drainage System.

As population density and urban impermeable cover increase, existing sewer systems are becoming inadequate for normal flows. Climate change compounds this problem, resulting in more frequent and larger flooding events. As the average life expectancy of critical sewers in the UK is over 300 years, any methods for alleviating load on the network should be investigated.

Green roofs will reduce the volume of water that needs to be treated at a sewerage treatment plant, as well as potentially improving the quality of the roof run-off. This will reduce load on the entire network and result in lower costs and cleaner water spilling into rivers, if the sewer network does overflow. In networks where stormwater does not flow into the sewer, local flooding will be reduced.

The data presented on this website help to illustrate the effect that green roofs have on the run-off from a roof, as a result of attenuation and retention of rainfall. The performance of a green roof at any given time is influenced by a variety of climatic factors such as humidity, temperature and the time since the last rainfall event. Performance is also dependent upon physical factors relating to the green roof's configuration, such as the depth and material composition of the growing medium.

More information is available at The Green Roof Centre.