Environment: Rooftop gardens can help the planet - and save money By Showwei Chu

A cooler building interior when it's hot outside is one of the more noticeable benefits of a green roof. It also absorbs rainwater, reducing storm-water runoff and stream erosion, according to studies by the National Research Council and the British Columbia Institute of Technology's Centre for the Advancement of Green Roof Technology. And it helps improve air quality, conserves energy and mitigates the urban heat island, the effect when roofs, buildings and other hard surfaces absorb heat from the sun and cause a metropolis to be hotter than the surrounding countryside.

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