Rooftop gardens a cool idea. (Built Environment).

Rooftop gardens are blooming across North America] Rooftop gardens may combat the phenomenon of city air temperatures rising 2-10[degrees]F higher than surrounding nonurban areas because of the abundance of dark, heat-absorbing surfaces--such as, well, rooftops. These "urban heat islands" increase the demand for air conditioning, which increases power plant emissions of pollutants that promote global warming. Heat islands also increase ground-level ozone, formed when heat, sunlight, and chemicals in the air mix. Ozone causes coughing and worsens symptoms of asthma, emphysema, and lung cancer.

The idea is simple--roofs are lined with a drainage layer, a waterproof membrane, a growing medium, and, of course, plants. The plants cool the air when water transpires from leaf surfaces and evaporates into the surrounding air. They help shade the soil surface, thus reducing heat buildup in the substrate materials. They also absorb air pollutants such as carbon monoxide, storing them in intracellular spaces, and provide thermal insulation.

Jessica Rio, who is public information officer for the Chicago Department of Environment, says the air on the rooftop garden atop Chicago's city hall is 15-20[degrees]F cooler than the air on the tar roof next door. "This building doesn't have to work as hard to be cool," Rio says. If reducing a heat island reduces a region's overall temperature, it could also mean less work for air-conditioning units in every building in the area.

Canada's federal National Research Council (NRC) has reached similar conclusions. In research presented at the Green Roof Workshop, held 5 March 2002 in Vancouver, British Columbia, NRC research officer Karen Liu reported that the rooftop garden at the NRC campus in Ottawa, Ontario, reduced the heat entering the building through the roof alone by as much as 85% on summer days.

Whether that decrease translates into significantly better energy efficiency overall depends on the building's construction, in particular the roof-to-wall ratio, says Liu. "In a high-rise commercial building with lots of windows and walls, the energy lost through those openings will play a bigger role in energy efficiency," she says. "But in a warehouse-type building with a large roof and few windows and walls, the energy saved could be great."

Green roots also absorb rainfall. For that reason, rainy Portland, Oregon, provides financial incentives for builders to install rooftop gardens. Tom Liptan, a city environmental specialist, says the city's experimental rooftop garden, installed on an apartment building in 1999, absorbs up to 1 inch of rainfall at a time in its 2- to 6-inch depth of soil. "That's a real nice chunk of water," he says. The garden roof also reduces runoff by as much as 90% compared with regular roofs. Slowing down drainage of stormwater into combined sewer systems reduces the amount of sewage that overflows into rivers and lakes, he says.

One drawback of rooftop gardens is that they initially cost more than traditional roofs. But the initial cost is offset by the fact that garden roofs last longer, because they are shielded from ultraviolet radiation, and because they don't experience the dramatic temperature fluctuations that can damage traditional roofing material.