A roof fitted with solar panels signals that a building is equipped with technology at the leading edge of sustainable thinking, a hard-edged surface with easily quantifiable energy and financial dividends. Green roofs elicit a different response, more emotional and somewhat ambiguous. Their benefits, though diverse, are not so easily tallied. Green roofs provide numerous payoffs for individual buildings, but their impact at the scale of the city is only beginning to be studied. While some cities and states are developing requirements or incentive packages to promote vegetated roofs, more precise tools need to be developed to address urban-scale issues like the heat island effect or storm water runoff, a major contributor to water pollution. These issues take on greater urgency as architects and planners turn to sustainable design as a means to mitigate climate change and resource scarcity.
As most architects know, green roofs consist of a watertight barrier, a growing medium, and a layer of plants, typically sedum or other drought-resistant plants (referred to as extensive green roofs), though more elaborate designs can include grasses, food crops, or even trees (called intensive green roofs). Vegetated roofs lower energy costs by reducing surface temperature in the summer and providing insulation in the winter. They also last longer than conventional roofs by blocking ultraviolet rays and rapid temperature increases from degrading roofing materials. They reduce runoff during storms, which can reduce water pollution, though it would take very significant acreage concentrated in a single area in order to have an impact. In addition, advocates argue that widespread use of the technology could reduce urban heat islands, which would have broad-based implications for energy use and air quality, such as asthma rates.
According to a 2007 report by the Toronto-based trade group Green Roofs for Healthy Cities, the industry grew by 30 percent over 2006. Chicago led the way with 517,000 square feet constructed in 2007, more than double that of its nearest competitor, tiny Wilmington, Delaware, which planted an impressive 195,600 square feet. New York placed a meager third with 123,074 square feet. “New York is very far behind Chicago. Installation costs here are much higher,” said Sarah Wayland-Smith, a landscape designer at Balmori Associates who specializes in green roofs. Wayland-Smith cites high up-front costs and an underdeveloped network of suppliers and installers, as well as, until recently, a lack of government incentives as barriers to construction in New York.
New York City government has adopted a cautious approach to green roofs, according to Rohit T. Aggarwala, director of the Mayor’s Office of Long Term Planning and Sustainability. Working with the state legislature, the mayor and the governor recently pushed through a $4.50-per-square-foot tax credit to encourage green roof construction. The mayor’s sustainability blueprint, PlaNYC, encourages green roofs but does not require them. Aggarwala, too, cites high up-front costs. “New York is already the greenest city in the United States,” he said. “We should not jeopardize the economic sustainability of the city with financially onerous requirements.” Aggarwala argues that reflective roofs can reduce cooling costs, and “blue roofs,” or simple gutter lips that slow runoff, can reduce sewage overflows, both at a fraction of the cost of green roofs. Still, he hopes the tax credits will encourage development and bring down costs. “We’ve got to get more experience. As they become better known, they become less threatening to landlords,” he said.
Since Chicago Mayor Richard Daly famously planted sedum and native grasses on City Hall in 2000, more than approximately two million square feet of green roofs on dozens of buildings have sprouted across that city. Following a brutal 1995 heat wave that killed hundreds during a blackout, the City Hall roof was conceived as a pilot project for mitigating Chicago’s urban heat island. This proliferation has been fostered by a number of incentives and requirements. Chicago’s program has also helped to bring construction costs down and increase the number of growers, suppliers, and installers in that region. While the surface temperature of City Hall and several other projects has been monitored, little research has been done on the effectiveness of green roofs at the urban scale in Chicago, according to Larry Meredith, spokesman for Chicago’s Department of Environment. Even with the impressive number of square feet planted, there may be limits to the effectiveness of the rollout, at least thus far. A map developed by architect Linda Keane and her students at the Art Institute of Chicago shows how the roofs are scattered across the city, and how modest the area of green roof coverage is at the urban scale.
The most extensive modeling of the urban-scale benefits of green roofs in the United States has been done in New York. A study by the Center for Climate Systems Research (CCSR) at Columbia University’s Earth Institute estimates that in New York, fully 50 percent of all roof space would need to be greened in order to have a significant impact on the city’s heat island. The multidisciplinary study group, which relied on data and expertise from Pennsylvania State, Michigan State, and Columbia University, settled on the 50 percent baseline after deciding that 75 percent coverage was an overly ambitious figure. Their modeling indicates that 50 percent coverage would shave 1.4 degrees off the city’s heat island, which ranges from 5 to 7 degrees. What accounts for the relatively small impact even at half coverage? Remarkably, in a city as densely built as New York, roof space accounts for only 19 percent of the city’s total area (when seen from above as a single plane). While the difference between a 93- and a 94-degree day may not feel significant, it can have a massive impact on energy use. According to estimates by CCSR for the New York State Energy Research and Development Authority, every degree of temperature increase outdoors triggers demand for an additional 60 gigawatt hours of energy per day.
CCSR relied on a thermal map of the city produced by NASA, an aerial satellite image that shows hotspots in the city. Vivid in its coloring, the map includes some surprises. Midtown and Lower Manhattan, the most densely built areas of the city, are cooler than lower-scale parts of Queens and Brooklyn. “The tall buildings of Midtown and Lower Manhattan prevent solar penetration at street level,” said Stuart Gaffin, an associate research scientist for CCSR and one of the authors of the report. “They act like trees, at least in terms of shading. Parking lots, low-scale buildings, large expanses of roof space and roadways create hotspots.” Massive hotspots occur in industrial areas and at the airports, and cool spots are clearly legible in Central and Prospect parks. The map suggests that targeting certain hotspots for green roof development might be a faster way of tackling heat islands, rather than an ad hoc approach of scattering green across the city. “I believe targeting could be very effective, though I’m not sure how it could be implemented,” Gaffin said. Balmori Associates has for some years advocated such an approach for Long Island City, one of the hotspots on the NASA map, which they estimate has a roof space area equal to half the size of Central Park.Working with business owners, they have completed two extensive green roof projects on industrial buildings in the neighborhood. “There are private benefits for building owners, as well as public benefits, but the public benefits are more difficult to quantify,” Gaffin said.
PlaNYC’s Aggarwala said that the city is aware of heat island hotspots. “We’ve thought about it and talked about targeting those areas, but we haven’t identified hotspots as an urgent public concern.” He argues that the city’s MillionTreesNYC program, which calls for intensive tree planting, addresses many of the same issues and will be easier and more cost effective to implement.
Green roofs appear to be more immediately effective in controlling storm water runoff. The CCSR study found that individual green roofs retain 80 percent of storm water, and slow the release of the remaining 20 percent. During rainstorms, runoff can overwhelm the sewer system, causing raw sewage to be discharged directly into waterways, a major source of water pollution. Using the same 50-percent-coverage model, CCSR estimated that ten percent of runoff would be cut, greatly reducing the number of sewage spills. “The benefits in terms of runoff are indisputable,” Gaffin said. “They are like rooftop holding tanks.”
Assuming CCSR’s goal is a desirable one, how does New York, so far behind Chicago, even inch toward 50 percent coverage? “I don’t think it’s an impossible goal if we keep hammering away at it,” Gaffin said. “Traditional roofs provide no additional benefits.” Given New York State’s recent passage of a tax rebate for green roof construction, and pending a recovery of the building industry, there is likely to be an increase in green roof construction in the region. Gaffin points out that roofs are replaced every 20 years or so. “Of all urban infrastructure, roofs are changed most frequently,” he said.
The data suggest that green roofs are an important and effective tool in addressing urban heat islands and storm water runoff, but alone, even in great numbers, they are not likely to fix these problems using a scattershot, incremental approach. Chicago’s example shows that incentives can dramatically increase square footage of green roofs built. Columbia’s modeling shows, however, that the living system of a green roof has a fine-grained impact in the urban landscape. Precise incentive packages and deeper study could increase their effectiveness within the greater organism of the city.