Plant-Covered Roofs Ease Urban Heat

Plant-Covered Roofs Ease Urban Heat



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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