In addition to being a nice place to spend time outdoors, green roofs make the built environment more sustainable.
A green roof reduces storm water run-off, increases the quality of water run-off, reduces flash flooding, reduces the heat generated from reflected surfaces (which contributes to the Urban Heat Island effect), provides insulation properties to reduce energy bills (lowering global CO2 omissions), absorbs sound which in turn lowers noise in the community, traps the distribution of dust and other matter, improves air quality and increases plant life encouraging biodiversity.
If every building had a green roof the environmental benefits for the wider community would be massive. If we look at just the Urban Heat Island (UHI) effect it would be possible to lower the temperature of cities. The University of Melbourne, School of Land and Environment, said “the UHI effect is the observed trend that urban areas are 5-7 degrees warmer than non urban areas which has implications on health, comfort and energy use”. This is achieved because vegetation absorbs light that would otherwise be converted into heat energy. So a green roof can contribute to reducing green house gas emissions and help cities to adapt to the warmer climate that is our future.
The benefits of storm water management are multiple. When it rains water is stored by the substrate of a green roof and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation. The amount of water a green roof retains varies depending on the depth of the soil, the type of plants and the season. As a guide, in summer, a green roof retains 70-90% of the rain that falls onto it. And when it rains the water that eventually does go out to the storm water system is delayed. That means it does not go out at peak flow so decreases the stress on sewer systems. And the water that flows out is filtered as it passes through plants and the substrate thereby releasing good clean water into storm water catchments like Port Philip Bay.
Air quality improves because a green roof can capture airbourne pollutants. Plants also filter noxious gases. So green roofs decrease pollution.
The insulation properties of a green roof reduces daily energy demands. The National Research Council of Canada found that green roofs reduce the demand for air conditioning by over 75%.
Finally green roofs sustain a variety of plants and invertebrates and provide habitat for birds, which is great for biodiversity.