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Plant-Covered Roofs Ease Urban Heat


National Geographic News
January 29, 2009


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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