Benefits

• Green roofs in Melbourne, Australia perform well for retention (average ~73% in total study period; average ~91% in per-event), particularly in warm and dry climates with a large proportion of small rainfall events (Zhang, 2019).
• Green roofs absorb and retain water, making them an effective strategy for the management of urban stormwater. Retention rate can be up to 100% in summer, and down to 69% at other times of the year for a 100-150 millimetre thick green roof in Portland, Oregon (Hutchinson, Abrams, Retzlaff & Liptan, 2003).
• Green roofs can delay the time to onset and peak flow, and reduce the overall volume by the substrate/layers of the system. A study by University of Auckland (Voyde, Simcock, Fassman & Wells, 2009) found 72% of cumulative rainfall volume was retained and a 92% reduction of peak flow.

A study by University of South Australia (Razzaghmanesh, 2012) showed that runoff water quality was suitable for irrigation and non-potable use. The study also showed that an extensive roof was more efficient at removing nitrogen and phosphorus than an intensive (deep) roof.

• Evapotranspiration increased by a combination of vegetation and soil
• Creating microclimates for buildings and urban canyons

Coutts AM, Daly E, Beringer J and Tapper NJ (2013) Assessing practical measures to reduce urban heat: Green and cool roofs. Building and Environment, 70, pp. 266-276.

Green roofs with a substrate of 500mm or greater provide insulation and stormwater management benefits, similar to existing ground surface.

Coutts AM, Daly E, Beringer J and Tapper NJ (2013) Assessing practical measures to reduce urban heat: Green and cool roofs. Building and Environment, 70, pp. 266-276.

In 2010, Toronto passed legislation requiring green roofs on all new developments over 2,000m². Between 2010 and 2018, over 111,000m² of green space has been added to the cityscape.

• urban wildlife habitat

• pollination of crops and other important plants