A green roof is a roof of a building
that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems. (The use of “green” refers to the growing trend of environmentally friendly and does not refer to roofs which are merely colored green, as with green roof tiles or roof shingles.)
Container gardens
on roofs, where plants are maintained in pots, are not generally considered to be true green roofs, although this is an area of debate. Rooftop ponds are another form of green roofs which are used to treat greywater.
Also known as “living roofs”, green roofs serve several purposes for a building, such as absorbing rainwater, providing insulation, creating a habitat for wildlife, and helping to lower urban air temperatures and combat the heat island effect. There are two types of green roofs: intensive roofs, which are thicker and can support a wider variety of plants but are heavier and require more maintenance, and extensive roofs, which are covered in a light layer of vegetation and are lighter than an intensive green roof.
The term green roof may also be used to indicate roofs that use some form of "green" technology, such as a cool roof, a roof with solar thermal collectors or photovoltaic modules. Green roofs are also referred to as eco-roofs, oikosteges, vegetated roofs, living roofs, and greenroofs.
It all started in ancient Mesopotamia. That's how old the idea of a "green" roofs is. From the Ziggurat of Nanna to the fabled hanging gardens of Babylon, humans have been growing plants on roofs. Turf and sod have topped an array of human dwellings — but the emergence of a bona fide green roof industry is fairly recent.
Here in the United States, that industry is just a few years old. But green roofs are being touted as the answer to a number of environmental problems — and they're showing up all over the country. NPR's Ketzel Levine reports.
Commercial green roofs are not roof gardens; many of them can't take foot traffic. Instead, they're like green skins, layers of vegetative matter that grow directly on rooftops. They are far less romantic than they sound.
Green roofs are tools for dealing with stormwater runoff and reducing urban heat islands. Other industry claims include their ability to reduce energy use by insulating buildings from extreme temperatures. The scientific data to support these and other benefits are still being collected, but based on how they've performed — for decades — in Germany and the Netherlands, green-roof specialists are confident in their curative powers.
A growing number of architects, engineers, urban ecologists and city planners agree. Increasingly high-profile green roof projects have been built in the United States in the last five years. Among the best-known green roofs are the ones atop Chicago's City Hall and a Ford Motor Co. facility in Dearborn, Mich. Some of the newer roofs making the news include a residential high-rise in New York City, a prairie-covered library in Evansville, Ind., and the top of the Multnomah County Building in Portland, Ore.