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Gardens in the Sky

Want to explore a new environmental frontier that can be both beautiful and beneficial? Plant a roof.

Rooftops, especially those on top of commercial buildings, tend to be three things: hot, ugly and ignored, says landscape designer and environmental writer Linda Velazquez.

Yet, they account for a remarkable amount of space in any urban area. According to estimates, about 12 percent of New York City’s total area comprises rooftops. Those numbers climb to as high as 25 percent in cities such as Houston, Texas; Chicago, Illinois; Salt Lake City, Utah; and Sacramento, California, according to U.S. government surveys.

Rooftops are no longer being dismissed so routinely. In cities around the world, architects, landscape designers, builders and public officials are finding innovative ways of converting urban and industrial rooftops from afterthoughts to valuable living resources.


The green roof of the CII-Sohrabji Godrej Green Business Centre in Hyderabad helps reduce heat ingress into the building. The center is also the first building outside the United States to earn a LEED platinum rating. Photograph courtesy CII-Sohrabji Godrej Green Business Centre

“Green roofs are vibrant and exciting alternatives to the average black-tar or concrete roofs that we see covering the world,” writes Velazquez, who is one of America’s prominent green-roof advocates.

India held its first-ever green-roof conference in Indore in 2011. Sponsored by the World Green Infrastructure Network in collaboration with a local company, Green Takniki, the symposium’s ambitious agenda was captured in its title: “Green Technology for Green Roof, Green Home and Rain Harvesting to Combat Urbanization for Sustainable Future.”

“The green-roof industry has tremendous potential in India…to save electrical energy, cool buildings and boost the economy,” says Suresh Billore, Green Takniki’s executive director.

Gardens and roofs
The idea of roofs and walls with grasses and plants is hardly revolutionary, whether dating back to the Hanging Gardens of Babylon or the sod houses of Scandinavia and the American Midwest. The modern green-roof movement, however, began in Germany in the 1960s before spreading elsewhere in Europe.

The United States didn’t seriously begin adopting green roofs until the late 1990s with the founding of the U.S. Green Building Council and Green Roofs for Healthy Cities.

In 2003, environmental architect William McDonough installed an icon of the green-roof movement: the Truck Assembly Plant at Ford Motor Company’s vast River Rouge complex in Dearborn, Michigan. At 42,000 square meters, the facility is still one of the largest free-standing green roofs in the world.

Modern green roofs aren’t simply roof gardens with potted plants. At a minimum, green roofs incorporate a covering of plants in a special growing media (or engineered soil), a drainage system, and a waterproofing and root-resistant membrane. Many newer systems are modular, composed of portable, interlocking units of growing media and plantings.

Green roofs are more costly to install than conventional ones, but they offer a remarkable range of benefits. Among them: improving water quality, decreasing the heat island effect of urban areas, saving energy, reducing both pollution and greenhouse gas emissions, and bringing biodiversity to cities through different plant varieties—besides the insects and birds they attract.

“This is a new transformative process in our cities that will reconnect us to our sense of place in the world and a sense of intimacy with nature,” declared horticulturist Ed Snodgrass at the 2011 Green Roofs and Walls Virtual Summit in September.

Water, air, energy
Many people overlook one of the chief assets of green roofs: controlling stormwater. Urban areas contain vast expanses of impermeable surfaces—roads, parking lots, sidewalks and rooftops—where rainwater rushes into sewage systems and can quickly overwhelm them.

An urban green roof slows and absorbs water, dramatically reducing the damage caused by the runoff from heavy rains. The potential savings are huge. Studies confirm that a typical green roof will retain from 60 to more than 75 percent rainwater, which is later released through evaporation.

City officials have taken note, and stormwater control is a principal driver of green roof programs throughout the United States. Philadelphia, Pennsylvania, estimates that its 32,000 square meters of green roofs can control roughly 35 to 42 inches of rain per year. That relieves the city’s water system of 3,596,000 liters of stormwater annually.

The acknowledged American leader in green roofs, however, remains Chicago, Illinois, which has more than 400 current or completed projects totaling 650,000 square meters—more than all other U.S. cities combined.

“The greatest potential of green roofs lies in their capacity to cover impervious roof surfaces with living, breathing, permeable plant material,” observes Velazquez, who publishes the blog, Sky Gardens.

The impact of green roofs on urban temperatures—the heat island phenomenon—is equally remarkable. Chicago compared summertime surface temperatures on a green roof with a neighboring building. On an August day in the early afternoon, the green roof surface temperature ranged from 33° to 48° Celsius, while the dark, conventional roof of the adjacent building was 76° Celsius. The near-surface air temperature above the green roof was about 4° Celsius cooler than that over the conventional roof.

With their insulating qualities, green roofs reverse the equation in cold weather and keep buildings warmer than normal roofs do.

Green archipelagos
Green roofs fall into two categories: extensive and intensive. An extensive installation, designed for low maintenance and limited public access, consists of groundcover as thin as 5 centimeters. In North American and European latitudes, that usually means hardy alpine plants like sedum and other succulents that can withstand temperature extremes, drought and winds that can be found on both high mountains and exposed rooftops. An excellent example is FedEx’s 16,000 square-meter facility at O’Hare International Airport in Chicago.

Intensive green roofs can almost be as varied as any conventional garden, with elaborate landscaping that incorporates trees, large shrubs and water features—often providing walkways and open space for the public. As a result, intensive green roofs—sometimes called a building’s fifth façade—can become remarkable showcases of living architecture, transforming parts of the urban landscape into an archipelago of elevated green spaces.

Consider one of the 2011 winners of the Awards of Excellence, given annually by the Green Roofs for Healthy Cities association.

The High Line project, located in New York City, has converted a disused elevated railroad into a public park that runs for 2.3 kilometers. The landscape features grasses and plants that once grew on the abandoned High Line track, along with walkways, sun deck, performance space, even a small, flowing-water channel.

“With the thousands of people that visit the High Line every day, it could be argued that the High Line receives more attention than any other green roof in the world at the present time,” says construction manager Dylan Peck.

Green Roofs for Healthy Cities also gives an annual Green Wall award for the difficult art of turning the sides of buildings into vertical gardens. The 2011 winner was the Phoenix Convention Center in Arizona for “a habitat garden that thrives on the urban structure that embraces it.”

With all these environmental and aesthetic attributes, says Velazquez, “Green roofs are fast becoming green staples in mainstream architecture and high-performance buildings.”


Howard Cincotta is a U.S. State Department writer and editor.