GROWING GREEN: Climatologist Stuart Gaffin checks the sedum plants that cover Con Edison's quarter-acre green roof.
Image: KATIE MOISSE
-
The Best Science Writing Online 2012
Showcasing more than fifty of the most provocative, original, and significant online essays from 2011, The Best Science Writing Online 2012 will change the way...
Read More »
NEW YORK—Through the rain-pocked window of his Prius heading east on the Queensboro Bridge, Stuart Gaffin sees a black, watery sea of missed opportunities.
"Look at all those. Another 100,000 square feet!"
Gaffin, a climatologist at Columbia University's Center for Climate Systems Research, is on his way to the Con Edison power plant in Queens's Long Island City neighborhood. His view from the 40-meter-high bridge is bleak, and not just because of the rain.
"Just sitting there," he sighs. "Useless black roofs."
Since 2003 Gaffin has been studying "green" roofs—those covered with vegetation growing on top of a waterproof membrane—and their potential to mitigate urban climate change.
Like most big cities, New York City is a hot spot, and not just in the cultural sense. The solar energy absorbed into the concrete buildings and asphalt street surfaces is transformed into heat, making it significantly hotter than surrounding rural areas—a phenomenon known as the urban heat-island effect.
Green roofs can cool near-surface air temperatures by an average of 16.4 degrees Celsius per unit area—slightly behind street trees in terms of heat island mitigating potential, according to a 2006 report from the New York City Regional Heat Island Initiative. And the obvious advantage to green rooftops is that they don't take up valuable ground-level space. "Roof space is just this wasted resource," Gaffin explains. "We have somewhere between 30 and 40 square miles [in New York City]—that's 22 times Central Park!"
Although cooling heat islands is an important aspect of urban climate change mitigation, an even bigger impact could come from large-scale energy conservation. That's where green roofs hold gallons of potential. Most of the rainwater dripping off black roofs mixes with pollutants in the streets and wastewater in the sewer system as it flows to 14 water treatment facilities across the city—where it costs money and energy to clean and store. During big storms, that tainted water bypasses treatment plants altogether, pouring contaminants directly into rivers.
A four-inch-thick green roof can hold a gallon of water per square foot. If all the rooftops in the city were green, they would represent a billion gallons of storage potential. Engineering tanks to store that quantity of water is "almost inconceivable," Gaffin says. "There are so many more benefits to this type of approach," he adds. "It's stopping the problem at its source."
Although many like the idea of green roofs, the installation cost ($10 to $35 per square foot, depending on roof size, accessibility for installation, and roofing and drainage components as well as the types of plants) makes them impractical for some buildings.
"There's no argument that painting a roof white is cheaper than installing a green roof," says Chris Brunner, co-founder of New York Green Roofs, LLC. But Brunner says when it comes to reducing storm water runoff and improving air quality, white roofs just don’t stack up.
That's why Gaffin is visiting Con Edison's quarter-acre green roof on this rainy afternoon. Using sensors developed for agricultural research, Gaffin is turning city roofs, like this one on top of New York City's biggest energy provider, into field sites where he can measure the energy and water absorption properties that dictate surface temperature and water storage.
Con Edison's sprawling green roof is covered with sedum plants, hardy succulents that thrive with little maintenance, if any. It is mostly green, except for a raised level that is black with a patch of white. This raised section is used as a control: Energy sensors probe the black and white surfaces to measure their surface temperatures and compare it with those of the planted area, while metered troughs collect water running off the two surfaces. Today, rainwater is cascading down the sides of the control roof.
See what we're tweeting about
14 Comments
Add CommentChicago has started doing this and the benefits are many - including reduced heating costs in winter, reduced cooling costs in summer and, of course, whatever is grown on the roof.
Reply | Report Abuse | Link to thisNothing beats going green and saving $$$.
Green roofs cost a fortune to build, and once they leak and they will they are a nightmare to repair, actually you have to remove the whole roof to repair because you just end up chasing leaks forever. I would compromise with a white roof and solar panels.
Reply | Report Abuse | Link to thisDoes the addition of a green roof to an existing building result in a corresponding reduction of its safe snow loading? Just wondering whether a few inches of wet green stuff and water-saturated soil would be a problem under a winter snowfall. Wet turf is heavy.
Reply | Report Abuse | Link to thisAll of that runoff cleans the streets and flows into streams after treatment. Green Roof=Dirty City? Green Roof=Dead Fish. What do the weeds that grow when the green roof is not maintained do to asthma sufferers? Green Roof=Lawsuit?
Reply | Report Abuse | Link to thisMy roof is white. Chosen to be that color in part to reduce summer heating and in part to reduce global warming.
Reply | Report Abuse | Link to thisWhy don't building codes require that homes be oriented so that they can be efficiently retrofitted with solar collectors?
The major impediment to change is inadequately and inappropritely designed infrastructure.
It's not rocket science people.
"All of that runoff cleans the streets and flows into streams after treatment." - Skeptic
Reply | Report Abuse | Link to thisAhhh... No... It flows into streams <BEFORE> treatment.
Rainwater is used to purge sludge from sanitary sewers, and typically treatment facilities are not capable of managing all of the runoff from a rainstorm so much of it is simply dumped raw into lakes and streams.
How odd that yet another Skeptic finds himself detached from the reality of things.
I fixed many problems with my roof like leaks, heat and constant cleaning by constructing a cheap wood and plastic greenhouse. That might be a good option with the same benefits.
Reply | Report Abuse | Link to thisI understand that the embedded energy of the additional material needed to support a green roof substantially adds to the lifecycle environmental footprint of a greenroof. If that is right, it seems like the choice between them and white roofs would be between priorities, where green roofs provide better stormwater management and aesthetics with a higher environmental impact than white roofs.
Reply | Report Abuse | Link to thisiamdata,
Reply | Report Abuse | Link to thisYour statement is intriguing if not explanatory. Pray tell how the construction of a cheap greenhouse cured your house roof's shortcomings.
It's not rocket science people.
Reply | Report Abuse | Link to thisActually, it's as complex as one might like. The choices are wide - solar heating, photovoltaics, white metallic roofing panels, shading trees, roof succulents, etc, etc and mixes thereof. The variables of geographic location and zoning rules add to the complexity. Bring in the biological questions and the complexity goes way beyond rocket science.
The greenhouse was constructed on top of my house so I dont have the rain falling directly on the concrete roof. I made its roof with white translucent plastic which allows light for the plants but blocks a lot of heat. There is also a lot of shadow provided by the pots and hydroponic systems. But the biggest benefit is saving money on food.
Reply | Report Abuse | Link to this"Actually, it's as complex as one might like" - Quasimodo
Reply | Report Abuse | Link to thisAnd none of the "complexities" you mentioned have anything at all to do with orienting homes so that they can be easily be retrofitted with solar collectors. You face the largest face of the roof south, and design your subdivisions accoringly.
It's not rocket science People.
...Rainwater is used to purge sludge from sanitary sewers...
Reply | Report Abuse | Link to thisNot unless the sewer system isn't working right. Sanitary sewers carry sanitary waste, storm sewers carry runoff. Combined sewers carry both. You are describing a combined system, common in older cities and city centers here and abroad.
I've been building green roofs for 12 years a long time in the US, they have been around in Europe for over 5o years. done right, they prevent leaks and wear of the roof, weight can be controlled by the type of media used and doesn't have to add to the structural cost, they very effectively filter pollution from rain water and delay and/or eliminate run off from storm events mitigating the overwhelming of storm sewers. Cost - can be recouped over time in real terms. problems caused by paving over the environment aren't considered in building costs, if they were, green roofs would be a big savings.
Reply | Report Abuse | Link to thisThey aren't the magic pill some claim but there are quantifiable, real benefits and they look great.
If you have had problems with your green roof it probably wasn't designed and/or installed right. Call me next time!