Plant-Covered Roofs Ease Urban Heat

Donald Dawson
for National Geographic News
November 15, 2002
Like a tiny seed carried by a late summer breeze, the idea of
cultivating plants on rooftops has spread from Europe to North America
and around the world.

Toronto is the most recent city to hop on the green roof bandwagon. Last month it joined Canada's federal government and Green Roofs for Healthy Cities, a public-private lobby and research network, in announcing a $1 million (U.S. $640,000) partnership program aimed at monitoring the performance of two local green roofs over a two-year period.

Although several cities in the United States have also undertaken green roof programs, Europe is in the vanguard of the green roof experiment, with many cities having put incentive systems in place.

In Germany, for instance, more than 140 million square feet (13 million square meters) of rooftops have green roof systems thanks to incentives provided by more than 75 local governments.

Green roofs have also sparked interest in Japan, where in Tokyo the municipal government last year mandated that structures with a roof area greater than or equal to 10,765 square feet (1,000 square meters) must cultivate at least 20 percent of that area.

Combating Urban Heat Effects

The impetus behind these incentives is the hope that green roofs will reduce the "urban heat island effect"—a phenomenon created primarily by urban surfaces such as pavement and conventional roofs that absorb solar energy and re-radiate it as heat. This raises the temperature of a city several degrees higher than that of surrounding rural areas.

The urban heat island effect is further exacerbated by added heat generated by running air conditioners and machinery, and from vehicle exhaust.

The higher temperatures mean city buildings need more air conditioning, and therefore use more energy than they otherwise would. Cranking up the air conditioning provides temporary relief for those indoors, but makes the heat island effect even worse as the warm exhaust of a city's air conditioning units actually makes it hotter outside.

In addition, the increased use of fossil fuels to provide extra electricity for air conditioners also increases emissions of greenhouse gases and other pollutants. This, combined with higher summer temperatures, increases the incidence of severe smog.

Growing natural cover on a building's roof can help combat all these effects and more, scientists say.

Summer in the City

On hot days in Chicago for instance, temperatures atop the green-roofed City Hall are typically 25 to 80 degrees Fahrenheit (14 to 44 degrees Celsius) cooler than the adjacent county office building, which has a black tar roof, said Jessica Rio, spokesperson for the city's Department of Environment.

Brad Bass, a researcher who has studied the advantages of green roofs for Environment Canada, says the direct energy savings green roofs bring to buildings three stories or less are significant.

"It's hard to estimate the exact amount, but green roofs can lead to at least a 5 percent, if not even a 15 percent reduction in electricity in the summer," he said. The indirect effects of reducing the temperature over a portion of the city will lead to further reductions in energy consumption, he added.

A study by Environment Canada also suggests that if 6 percent of Toronto's roof area was converted to green roofs, greenhouse gas emissions in the city would be reduced by 2.4 megatons a year.

Trickle-Down Effects

Green roof advocates point to other indirect benefits, such as reduced stress on sewer systems.

Portland, Oregon, has seized on green roofs to curtail a problem it shares with many other North American cities: the ability of storm water to overwhelm its sewage treatment facilities during heavy storms, allowing untreated water to flow into the Willamette River.

There are two ways to reduce this problem, according to Tom Liptan, a spokesman for the city's Bureau of Environmental Services. Either expand the system's capacity with larger pipes or reduce the amount of rainwater flowing through it.

Whereas rainwater falling on a regular roof quickly flows off the roof and into a storm sewer, much of the rain falling on a green roof is absorbed by its plants and soil to later evaporate or transpire back into the air as water vapor.

The amount of water soaked up by a green roof depends on the city and the season, said Liptan. In Portland, close to 100 percent of summer rain is absorbed, while in the spring and fall, the amount varies from between 40 to 50 percent. Winter rains are absorbed least of all, with only a 10 to 20 percent retention rate. But that's still good considering conventional roofs absorb no water, he said.

"The peak flow is dramatically reduced," he said, referring to the point during the most intense part of a downpour when the combined storm and sanitary sewer systems that run through about a third of the city tend to become overloaded. "The eco-roof reduces that peak flow to about one-tenth of what it would be from a conventional roof."

Portland currently has two grant programs that offer money for green roofs and other ecologically sound projects. The city's zoning code lets developers build larger buildings if they top them with eco-roofs.

Balancing Costs

"If there is a drawback at this point, it could be the up-front cost," said Bass.

In the past, he said, green roofs have cost about twice the price of conventional roofs. These costs are now dropping, he added, but sticker shock remains a drawback.

It may ultimately come down to the benefits outweighing the costs.

If a "landscape approach" such as a green roof can achieve the same end as a sewer upgrade for a similar amount of money, said Liptan, he'd pick it any day over a bigger pipe.

"A pipe underground does nothing for air quality or urban heat island reduction, whereas the eco-roof or some of the other landscape approaches help shade hot pavement," he said, adding that aside from all their other benefits, green roofs simply look nice.

Urban rooftop gardeners concur; to them the added costs are more than made up for by the benefits of growing gardens in what was once the uncontested realm of shinglers, chimney sweeps, and Santa's reindeer.

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