Up on the Roof

A lofty idea is blossoming in cities around the world, where acres of potential green space lie overhead.

This story appears in the May 2009 issue of National Geographic magazine.

If buildings sprang up suddenly out of the ground like mushrooms, their rooftops would be covered with a layer of soil and plants.

That’s not how humans build, of course. Instead we scrape away the earth, erect the structure itself, and cap it with a rainproof, presumably forgettable, roof. It’s tempting to say that the roofscape of every city on this planet is a man-made desert, except that a desert is a living habitat. The truth is harsher. The urban roofscape is a little like hell—a lifeless place of bituminous surfaces, violent temperature contrasts, bitter winds, and an antipathy to water.

But step out through a hatch onto the roof of the Vancouver Public Library at Library Square—nine stories above downtown—and you’ll find yourself in a prairie, not an asphalt wasteland. Sinuous bands of fescues stream across the roof, planted not in flats or containers but into a special mix of soil on the roof. It’s a grassland in the sky. At ground level, this 20,000-square-foot garden—created in 1995 by landscape architect Cornelia H. Oberlander—would be striking enough. High above Vancouver, the effect is almost disorienting. When we go to the rooftops in cities, it’s usually to look out at the view. On top of the library, however, I can’t help feeling that I’m standing on the view—this unexpected thicket of green, blue, and brown grasses in the midst of so much glass and steel and concrete.

Living roofs aren’t new. They were common among sod houses on the American prairie, and roofs of turf can still be found on log houses and sheds in northern Europe. But in recent decades, architects, builders, and city planners all across the planet have begun turning to green roofs not for their beauty—almost an afterthought—but for their practicality, their ability to mitigate the environmental extremes common on conventional roofs.

Across town from the library, the Vancouver Convention Centre is getting a new living roof. Just across the street there is a chef’s garden on the roof of the Fairmont Waterfront hotel. Across town in another direction, green roofs will go up on an Olympic village being built for the 2010 Winter Olympics. To stand on a green roof in Vancouver—or Chicago or Stuttgart or Singapore or Tokyo—is to glimpse how different the roofscapes of our cities might look and to wonder, Why haven’t we always built this way?

Technology is only partly the reason. Waterproof membranes now make it easier to design green-roof systems that capture water for irrigation, allow drainage, support the growing medium, and resist the invasion of roots. In some places, such as Portland, Oregon, builders are encouraged to use living roofs by fee reductions and other incentives. In others—such as Germany, Switzerland, and Austria—living roofs are required by law on roofs of suitable pitch.

And, increasingly, researchers such as Maureen Connelly—who runs a green-roof lab at the British Columbia Institute of Technology—are studying the practical benefits green roofs offer, helping quantify how they perform and providing an accurate measure of their ability to reduce storm-water runoff, increase energy efficiency, and enhance the urban soundscape. There is beginning to be a critical mass of green roofs around the world, each one an experiment in itself.

Another factor driving the spread of green roofs is our changing idea of the city. It’s no longer 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 conventional asphalt rooftops can be almost unbelievably high, peaking above 150°F and contributing to the overall urban heat-island effect—the tendency of cities to be warmer than the surrounding region. On green roofs the soil mixture and vegetation act as insulation, and temperatures fluctuate 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 artificial canyons into storm drains—unabsorbed, unfiltered, and nearly undeterred. A living roof works the way a meadow does, absorbing water, filtering it, slowing it down, even storing some of it for later use. That ultimately helps reduce the threat of sewer overflows, 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 flooding, and the city is considering how living roofs could moderate the threat.

Above all, living roofs are habitable. They recapture what is now essentially negative space within the city and turn it into a chain of rooftop 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. The list includes Britain’s black redstarts, a bird that colonizes the rubble of abandoned industrial sites, a habitat being lost to redevelopment. The solution fostered by Dusty Gedge, a British wildlife consultant and a driving force behind green roofs in the United Kingdom, is to create living rooftop habitat out of the same rubble.

And it’s not just a matter of making new or replacing existing habitat. In Zürich, Switzerland, 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 challenges 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 different kind of care, akin to low-maintenance gardening.

There 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 solutions, the kind of green-roof systems now being offered 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 different way of thinking altogether. A standardized green roof such as a carpet of sedums is better than a conventional roof, but it’s possible to build living roofs that are even more environmentally beneficial—locally grown, so to speak. The goal for some researchers now is to find ways to build living roofs that are ecologically and socially sound in every respect: low in environmental costs and available to as many people as possible.

Stephan Brenneisen, a Swiss scientist and a strong advocate for the biodiversity potential of living roofs, says simply, “I have to find easy, cheap solutions using materials that come from the region.” That means less reliance on plastics and other energy-intensive materials between the roof structure and the plants themselves. What matters isn’t only whether living roofs work. It’s how to make them work in the most sustainable way, using the least energy while creating the greatest benefit for the human and nonhuman habitat.

Last fall, I climbed onto the roof of the 15-story Portland Building in downtown Portland, Oregon. My guide was Tom Liptan, the city’s Ecoroof Program Manager and a self-confessed storm-water nerd, who began his experiments with green roofs by building one on his own garage in 1996. We walked to the parapet across plantings of sedums and fescues and looked down at the roof of Portland’s city hall several stories below us. It has a conventional black tar roof, the kind of roof we have taken for granted for decades. But as part of Portland’s Grey to Green project—a plan for sustainable storm-water management—that building will soon be retrofitted with a living roof. “The employees want it,” Liptan said.

In the history of that municipal building, how often had the people who worked there ever thought about that black tar roof looming over their heads? Once the living roof is completed, they may visit it only rarely, but they won’t forget that it’s there, adding habitat to the city center, filtering the rain, moderating temperatures. It reminded me of something Stephan Brenneisen said: “People feel happier in a building where we’ve given something back to nature.”

Think of the millions of acres of unnatural rooftops around the globe. And now imagine returning some of that enormous human footprint to nature—creating green spaces where there was once only asphalt and gravel. If a certain sum of human happiness is the by-product, who’s to complain?