On a spit of land in Malmö, Sweden, rises a dynamic, urbane residential and commercial district. Green roofs of moss-stonecrop sedum cover many of the houses and apartment buildings. Vegetation cools the buildings in summer, while the grass traps rainwater and releases it slowly, easing the burden on storm sewers and lessening the likelihood of flooding. The sedum grows only a few centimetres, so lawn mowers aren’t necessary. Green roofs are so big in Sweden, it’s home to the International Green Roof Institute. In fact, Sweden has become a leader in a trend that is taking root all over the world.

Don’t confuse a living roof with a garden. In most cases, the primary role of a green roof is not to provide esthetic beauty but to manage rainwater and insulate a structure. In addition to vegetation and a root barrier, it also has a retention mat, reservoir board and filter sheet. These components, plus a backup water supply, make a green roof about twice as costly as a bare concrete or asphalt top. Over the long run, however, it pays for itself. The extra insulation reduces the need for heating and cooling. And, by blocking harmful ultraviolet rays, it extends the life of the roof.

The many other environmental and social benefits of green roofs range from filtering air and water pollutants to providing horticultural therapy to green-starved urbanites. Perhaps most significantly, they reduce the so-called heat-island effect, the difference in temperature between a city’s built-up core and its suburbs. (The National Research Council Canada says that just six percent green-roof coverage in an urban area would lower the nearby air temperature by one or two degrees.)

But there are other, less tangible, benefits. For cities, installing green roofs has become a kind of civic one-upmanship, joining skyscrapers as symbols of urban progress. In an age when consumers are more eco-correct than ever, corporations have turned green roofs into marketing symbols that espouse companies’ social and environmental values. For environmentalists, the green roof is an antidote to a post-Kyoto Protocol life, creating green spaces in our increasingly polluted and paved-over world.

Chicago’s mayor, Richard Daley, has used the green roof on his city hall as his platform in a high-profile quest to make the city one of North America’s greenest. “The environmental movement seems like it’s happening somewhere else, and people forget about our own community,” he told National Geographic. “We need to be sure that we’re planning well.” Chicago has more than a million square feet of green roof, the equivalent of 15 football fields, and city officials are even researching a green roof project at O’Hare International Airport that would reduce noise pollution. Financing from the city encourages architects, contractors and developers to consider green roof initiatives for their projects.