By Tina Rosenberg
It’s spring
— time to plant your roof. Roofs, like coffee, used to be black tar. Now both
have gone gourmet: for roofs, the
choices are white, green, blue and solar-panel black.
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Chester Higgins Jr./The New York Times
A co-op in Manhattan with a roof garden.
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All are green in one sense. In different ways, each helps to solve
serious environmental problems. One
issue is air pollution, which needs no introduction.The second is the urban
heat island. Because cities have lots of dark surfaces that absorb heat and
relatively little green cover, they tend to be hotter than surrounding areas —
the average summer temperature in New York City is more than 7 degrees hotter
than in the Westchester suburbs. This
leads to heavy air-conditioning use — not good — and makes city dwellers
miserable. For a few people every year, the heat is more than a discomfort —
it’s fatal.
The other
problem is storm water runoff. In New
York, as in about a fifth of American cities, there is only one sewer system to
conduct both rainwater and wastewater. About every other rainfall in New York,
sewers flood and back up, discharging their mix of rainwater and wastewater
into the city’s waterways. It doesn’t take much to overload New York’s sewers —
it can take only 20 minutes of rainfall to start water from toilets flowing
into Brooklyn’s waterways. The water
does more than flood streets. It makes
us sick — cases of diarrhea spike when sewers overflow. When sewers back up,
polluted water runs into our lakes and oceans, closing beaches.
How can a
new roof help?
At 1:45 in
the afternoon on August 9, 2001, the temperature in Chicago was in the 90s.
Eleven stories up, on the roof of City Hall, the surface temperature of the
black tar measured 169 degrees. But Mayor Daley, environmental innovator — yes,
that Mayor Daley — had done something interesting. The year before, a section
of the City Hall roof had been painted white.
The surface temperature there was between 126 and 130 degrees. And much
of the roof of the building, and the adjacent Cook County building, had become
a garden — 20,000 plants in 150 varieties, chosen for their abilities to thrive
without irrigation and stand up to Chicago’s notorious wind. The surface
temperature of the green roof varied between 91 and 119 degrees.
So the
difference between a black tar roof and a green roof was at minimum 50 degrees.
And the green roof was able to retain 75 percent of a one-inch rainfall. The two tasks go hand in hand — green roofs
cool by capturing moisture and evaporating it.
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Matt Rourke/Associated Press
Painting a roof white in Philadelphia.
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Putting
living vegetation on the roof is not a new idea. For thousands of years people have made sod
roofs to protect and insulate their houses, keeping them cooler in summer and
warmer in winter. The modern movement
for green roofs began in the last 50 years in Europe. Germany, where about 10 percent of roofs are
green, is the leader; some parts of Germany require green roofs on all new
buildings.
Greening a
roof is not simple or cheap. Over a black roof — flat is easiest but sloped can
work — goes insulation, then a waterproof membrane, then a barrier to keep
roots from poking holes in the membrane.
On top of that there is a drainage layer, such as gravel or clay, then a
mat to prevent erosion. Next is a lightweight soil (Chicago City Hall uses a
blend of mulch, compost and spongy stuff) and finally, plants.
An extensive
roof — less than 6 inches of soil planted with hardy cover such as sedum — can cost $15 per square foot. An intensive
roof — essentially a garden, with deeper soil and plants that require watering
and weeding — can double that. But because the vegetation is thicker, it will
do a better job of cooling a building and collecting rainwater. Plants reduce
sewer discharge in two ways. They retain rainfall, and what does run off is
delayed until after the waters have peaked.
A Columbia
University study of three test roofs built by Con Edison in Queens found that
the green roof — an extensive roof, planted with sedum — cut the rate of heat
gained through the roof in summer by 84 percent, and the rate of heat lost
through the roof in winter by 34 percent.
Another
Columbia study (same researchers, same Con Ed test sites) found that green
roofs are a very cost-effective way to reduce storm water runoff. If New York has one billion square feet of
possibly greenable roof, planting it all could retain 10 to 15 billion gallons
of annual rainfall — which would cut a substantial amount of sewage overflow.
“If you add in all the other green infrastructure, such as street trees,
permeable pavement and ground collection pits, it might be possible to
eliminate the combined sewage overflow without building specialized water
detention tanks, which are hugely expensive,” said Stuart Gaffin, a research
scientist at Columbia’s Center for Climate Systems Research and the lead author
of both studies.
Green roofs
have other advantages.They scrub the air: one square meter can absorb all the
emissions from a car being driven 12,000 miles a year, said Amy Norquist, chief
executive of Greensulate, which installs green roofs.And green roofs can
provide the plants that animals, birds and bees need where parks are far apart.
White roofs
are cheap and don’t require any engineering — just a layer of special paint.
New York City is trying to coat a million square feet of roof a year. Building
owners can do the work themselves, or they can engage CoolRoofs, a city
initiative that promotes white roofs and organizes hundreds of volunteer
painters. Since 2010, about 3,000 volunteers have coated 288 buildings.
But less
investment buys less return. White roofs don’t catch rainwater, help
biodiversity or clean the air. Gaffin’s group found that the white portion of
the Con Ed roof averaged 43 degrees cooler than black at noon on summer days.
That’s something, but it’s a smaller cooling effect than green roofs offer.
Green roofs improve each year as vegetation becomes denser and taller.But after
a few months, a white roof tends to look like city snow — covered with soot. As
a white roof dirties, it loses a lot of its cooling ability.
The newest
roof variation is a blue roof.It’s a roof covered by a waterproof membrane and
gravel, with controlled-flow drains, and costs about $5 a square foot. Blue
roofs don’t cool anything — they help only with storm water control by
releasing water more gradually. Despite
the price, a blue roof is a hard sell — not everyone is comfortable with the
idea of a pond on the roof.
The fourth
roof option doesn’t save energy — it creates it. New Jersey has installed 500
megawatts of solar power — enough to run half a million homes. California has
installed double that. New York City? So
far, just 6.5 megawatts.
How have New
Jersey and California done it? Private
vendors install and maintain the solar panels, and are paid in future energy
savings. Scott Stringer, the Manhattan borough president, argues that New York
should use this system to put solar panels on the roof of every public
school. Stringer’s report says putting
solar roofs on all available public schools would eliminate as much carbon
emissions as planting 400,000 trees — eight times the number in Manhattan now.
Public
schools have become a testing ground for the new roofs. At the Robert Simon
complex in the East Village, which houses three schools (my children attend two
of those schools), work is beginning this summer on a farm. A committee at the
Earth School was looking for green ideas that would go beyond recycling and
create a curriculum. Abbe Futterman, the
science teacher, was already growing vegetables and fruit in sawed-off pickle
barrels right outside her classroom window, using the garden to teach plant
science and nutrition. The kids tend it, and use the produce to cook food from
around the world.
The Fifth
Street Farm will be a much larger vegetable and fruit garden in planters raised
about the roof on steel girders — not a classic green roof.The money has come
from various government offices — those of Stringer, State Senator Daniel
Squadron and City Council member Rosie Mendez.
Douglas Fountain, the architect who is implementing construction (and a
parent of a Tompkins Square Middle School student) said that it was designed to
be easily replicable by other schools.
Is a green
roof a good investment for a building owner? Perhaps, but the biggest reason
might not be reduced energy costs — lots of factors affect a building’s energy
use. More savings come from the fact
that temperature swings make a black tar roof expand and contract. The smaller
the spread, the longer the roof life.Roanoke, Va., for example, just installed
a green roof on its municipal building, at a cost of $123,000, adding anywhere
from 20 to 60 years to the life of the current roof membrane. “I personally
believe a green roof is the last roof you’ll have to put on,” said Gaffin.
But any
changes to a black tar roof are undoubtedly good investments for cities —
indeed, interest in green roofs is soaring largely because of the sewage
problem and the costs of trying to solve it the old way. New York City decided it was more cost
effective to build green infrastructure, including green roofs, than to
construct more sewer pipes or storage tanks, and it is spending $1.5 billion
over the next 20 years on green projects that will reduce rainfall runoff. The goal is to cut sewer outflows by 40
percent by 2030.
“The good
news is that this is a ‘no harm’ intervention,” said Carter Strickland, the
commissioner of the Department of Environmental Protection. “People want it; there’s a lot of other
benefits. If at the end of the day it
doesn’t do the full job, whatever you have to build on top will be much smaller
and less expensive.”
To encourage
building owners to install green roofs, New York City has a pilot program that
will end next year offering a $4.50-per-square-foot tax abatement for green
roofs that cover more than half a rooftop.
(There are also tax abatements for solar-panel roofs.)
Amy Norquist
of Greensulate said this was less attractive than it sounded.“What you have to
do to get that is quite onerous,” she said. “You need permits, filing fees,
people have to sign off — it ends up being a lot of money.”
Strickland
said that permits were required for a good reason. “It requires you to do a structural analysis
of the roof,” he said. “You’re going to
need that permit whether you build it with a tax abatement or alone.”
New York
City was not one of the first American cities to promote green roofs. “But the
city is doing quite well,” said Gaffin.
“The green infrastructure plan is very ambitious.” The problem is that the little-by-little
approach won’t produce real environmental benefits until they reach a critical
mass, and that could take a long time.
“We get biodiversity benefits from small scale greening, and individual
building owners will get an energy benefit,” said Gaffin. “ But to make a
difference to the city’s climate or hydrology we’d have to get up to 30, 40 or
50 percent coverage. What we have now is a drop in the bucket.”
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Tina
Rosenberg won a Pulitzer Prize for her book “The Haunted Land: Facing Europe’s
Ghosts After Communism.” She is a former editorial writer for The Times and now
a contributing writer for the paper’s Sunday magazine. Her new book is “Join
the Club: How Peer Pressure Can Transform the World.”
Source:
The New York Times - The Opinion Pages