by Ying Yang

Created on: September 11, 2010

Green roofs are revolutionising building design and infrastructure; using the power of nature to mitigate the effects of urban development and helping to protect the environment.

First, a quick definition of a green roof. It is basically a carefully constructed garden sitting on the roof of a building. Green roofs differentiate from rooftop gardens because they aren’t just flowers in pots; they have been engineered to function in a way that helps the environment. The substrate materials and plants are carefully selected such that they will grow well in the surrounding environment. These must be placed within an impermeable layer (usually a geosynthetic textile) so that water doesn’t leak into the buildings below, and a good drainage system also needs to be installed.

Green roofs help the environment in three key ways: for storm water management; to reduce pollution; and to promote a greener, more sustainable way of living.

The main reason green roofs were originally designed was to increase rainwater infiltration in urban areas and reduce the impacts of storms.

Urban development has meant that much of what was once grass and trees are now buildings and roads. Among other things, this has led to a drastic reduction in the infiltration capacity of the land. Infiltration is when rainwater flows down through the soil into either groundwater acquifiers or underground streams, which lead eventually to the sea. This natural process is vital for recharging underground drinking water supplies, a problem many rapidly developing cities are already facing. If rainwater can not infiltrate the ground, it becomes surface run-off. High levels of surface run-off are really bad for urban areas. It increases the intensity of storms, leading to more flooding and possibly damage to pipes and stormwater catchment areas. It also poses long-term health risks. Rainwater can pick up particulates like heavy metals or pesticides and carry them through the water cycle, which could eventually contaminate drinking water supplies. Hence, the use of green roofs! The soil in the green roof is permeable so water can infiltrate it, and the plants also help stabilise the water cycle by increasing transpiration rates. Studies show that having green roofs, along with other “green infrastructure” like permeable pavements, in an urban development significantly reduces surface water run-off and prevents storms from being more damaging than they could be.

Another way in which

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