Making Green Roofs Work

The effectiveness of rooftop gardens — an important element in green design — varies widely, and countries and municipalities should establish uniform standards to ensure that the roofs function properly. That’s the conclusion of a new study by an ecologist at the Zurich University of Applied Sciences, who said that many green roofs fail to retain rainwater or fully absorb solar energy and do not provide a good habitat for birds and insects, often because builders do not lay down enough soil for the gardens to thrive. The study found that more than half of the rooftop gardens in Zurich had less than 5 centimeters (2 inches) of soil, which was insufficient to support most plant life. The amount of rainwater retained in a sampling of rooftop gardens varied from 26 percent to 88 percent, depending on soil depth and drainage systems. The study recommended that green roofs feature a minimum of 10 centimeters of soil and state-of-the-art drainage systems.