National Geographic : 2009 May
• Another factor driving the spread of green roofs is our changing idea of the city. It s no lon- ger wise or practical or, for that matter, ethical, to think of the city as the antithesis of nature. Finding ways to naturalize cities---even as nature itself becomes more urbanized---will make them more livable, and not only for humans. Living roofs remind us what a moderating force natural biological systems are. During the summer, daytime temperatures on convention- al asphalt roo ops can be almost unbelievably high, peaking above 150°F and contributing to the overall urban heat-island e ect---the ten- dency of cities to be warmer than the surround- ing region. On green roofs the soil mixture and vegetation act as insulation, and temperatures uctuate only mildly---hardly more than they would in a park or garden---reducing heating and cooling costs in the buildings below them by as much as 20 percent. When rain falls on a conventional roof, it sheets off the city s artificial cliffs and floods down its arti cial canyons into storm drains--- unabsorbed, un ltered, and nearly undeterred. A living roof works the way a meadow does, absorbing water, ltering it, slowing it down, even storing some of it for later use. at ul- timately helps reduce the threat of sewer over- ows, extends the life of a city s drain system, and returns cleaner water to the surrounding watershed. London, for example, is already planning for a future that may well see more street ooding, and the city is considering how living roofs could moderate the threat. Above all, living roofs are habitable. ey re- capture what is now essentially negative space within the city and turn it into a chain of roof- top islands that connect with the countryside at large. Species large and small---ants, spiders, beetles, lapwings, plovers, crows---have taken up occupancy on living roofs. e list includes Britain s black redstarts, a bird that colonizes the rubble of abandoned industrial sites, a habitat being lost to redevelopment. e solution fos- tered by Dusty Gedge, a British wildlife consul- tant and a driving force behind green roofs in the United Kingdom, is to create living roo op habitat out of the same rubble. And it s not just a matter of making new or replacing existing habitat. In Zürich, Switzer- land, the 95-year-old living roof of a water- filtration system serves as a refuge for nine species of native orchids eradicated from the surrounding countryside when their meadow habitat was converted to cropland. Proponents of living roofs argue that they have met most, if not all, of the technical chal- lenges involved in grafting a biological layer onto the top of buildings of almost any scale: everything from a vegetable stand or bus stop to the ten-acre roof of Ford s truck plant in Dearborn, Michigan. While the average cost of installing a green roof can run two or three times more than a conventional roof, it s likely to be cheaper in the long run, thanks largely to energy savings. Vegetation also shields the roof from ultraviolet radiation, extending its life. And it requires a di erent kind of care, akin to low-maintenance gardening. ere are still philosophical challenges to be met, many of them having to do with the very idea of what a roof should be and how it should perform. Clients tend to want roofs that are easy to maintain and are uniformly green year- round, perpetual lawns in the sky, not seasonal grasslands. Builders and architects tend to want interchangeable, standardized, universal solu- tions, the kind of green-roof systems now being o ered by some of the big corporate players in the living-roofs industry. A living roof, though, is not just a biological alternative to a dead roof. It requires a di er- ent way of thinking altogether. A standardized green roof such as a carpet of sedums is bet- ter than a conventional roof, but it s possible to build living roofs that are even more envi- ronmentally beneficial---locally grown, so to speak. e goal for some researchers now is to nd ways to build living roofs that are ecologi- cally and socially sound in every respect: low in Verlyn Klinkenborg is a frequent contributor. Diane Cook and Len Jenshel specialize in landscape pho- tography. All three have no fear of heights.