La toiture végétalisée: How Do Green Roofs Work?
ADIT, mai 2008

     Cities worldwide are promoting environmentally "green" roofs to mitigate several urban problems. Ground cover, shrubs and other flora planted across a building's roof can reduce storm water runoff, easing the burden on local sewers and water treatment systems. And the vegetation can keep the roof cooler in summer, lowering interior air-conditioning costs and therefore peak demand on area power plants.
     Green roofs have been blossoming in Europe for more than a decade, and Tokyo now requires that at least 20% of any new roof on medium and large buildings be cultivated. Chicago is the U.S. leader. Most installations are made on newly constructed buildings, but retrofits are rising.
     In either case, the formations are built up in a series of layers that span all or part of a roof. So-called extensive roofs have fairly thin cross sections, including perhaps three inches of soil-like growing material; they weigh from 15 to 25 pounds per square foot when saturated and support low-lying plants. Intensive roofs are thicker, heavier and more costly to erect and maintain but are capable of supporting flowerbeds, shrubs, even trees. "As the plants get more demanding, the layers must become more robust, with better drainage and aeration," says Jeff Stillman, executive vice president of ZinCo USA in Newton, Mass., a division of ZinCo, Inc., the world's largest supplier of green roof components.
     Prefabricated modules of a few feet square that contain similar layers can also be assembled like puzzle pieces; this approach can be easier to install, although it can be expensive and also results in seams.
     The main drawback of all the approaches is cost. Some roofs—typically older ones—may not be strong enough to handle the weight. Standard insurance policies may construe a green roof as a structure that can create "standing water" damage, which a policy probably will not cover unless it is amended. Extensive roofs typically require minimal maintenance and an occasional dose of slow-release fertilizer; intensive roofs require more ongoing attention. Both styles, however, can turn a hot, bald roof into a pleasant space for coffee breaks, lunch, sunbathing or a simple breath of fresh air.

Did You Know...
     Installers rarely use soil for the growing medium because it is heavy and because it packs tight after repeated rains, reducing water retention and aeration for plant roots. They instead use manufactured materials. For example, granulated clay or shale may be heated until it forms air pockets; it is then cooled. Organic compost and fertilizer are added as nutrients.

     On a sunny, 80-degree-Fahrenheit day, a tar or black-painted roof can reach 180 degrees F; a white roof 120 degrees; and a plant-covered roof 85 degrees. Even if the building has ample roof insulation to retard interior heating, the intake vents for air-conditioning units are often located on the roof. Cooler incoming air lessens the system's burden, notes Jeff Stillman of ZinCo USA.

     If installed widely, green roofs could lower a city's cooling load, especially at night when bare rooftops radiate heat absorbed during the day. Since 1900 Tokyo's average temperature increase has been five times that of global warming, according to Tokyo Metropolitan University—one big reason the city is pushing such construction.

     Plants most recommended for green roofs belong to the genus Sedum. They grow low, store plentiful water in their leaves, and are bred to withstand temperature and moisture extremes. Common varieties include cape blanco, coral carpet and dragon's blood.

Rooftop garden overlooks residential Tokyo.


GREEN ROOFS generally have five layers. A root barrier prevents roots from penetrating the building. A protection mat helps to minimize punctures. The drainage layer allows heavy rain to run off to a perimeter drain yet still retains moisture for dry periods. A filter sheet stops fine particles from clogging the drainage layer. And the growing medium, typically an engineered material and not soil, provides nutrients for plants above.