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Commercial Nov 3, 2019 Lowering City Temps with Rooftop Gardens  
Commercial Nov 3, 2019 Lowering City Temps with Rooftop Gardens  

Rooftop gardens have been around for more than two thousand years, when the Hanging Gardens of Babylon were built. With their groomed terraces featuring trees and plants, this Wonder of the Ancient World offered shelter from the desert heat. Throughout the centuries, people have sought out the open, fresh air of a rooftop escape, particularly in large cities. Before air conditioning, the roof was a welcome reprieve from stifling apartments. Today’s city dwellers still seek out rooftops, no longer as just a place to cool off, but as a garden venue. Behind this trend is a completely different set of incentives.  

In environmental terms, a large metropolitan area is called an “urban heat island.” This means that the city is a lot warmer than the rural areas surrounding it. The difference in temperature has to do with how well the surfaces in each environment absorb and hold heat. According to the Environmental Protection Agency (EPA), the annual air temperature of a city with 1 million people can be 1.8–5.4 degrees Fahrenheit (1–3 degrees Celsius) warmer than its surroundings.

Green Relief
A “green roof” provides natural insulation, absorbs storm water, improves air quality and can moderate the heat island effect, especially during the day. Green roof temperatures can be 30–40°F lower than those of conventional roofs; even city-wide temperatures can be reduced. A rooftop garden is an element of a green roof, which might also include renewable energy features like solar panels, trees, shrubs and other elements of a park, contemporary drainage systems, or sections of “living roof,” in which actual roofing material has been replaced with drought-tolerant plant cover. Green roofs can reduce a building’s greenhouse gas emissions, energy use and heat absorption.

Rooftop gardens have been common throughout Europe for more than 40 years. In North America, Toronto has led the way, passing a green roof law ten years ago. Since the law was put into place, approximately 640 green roofs representing five million square feet of roof covering have been built. In the U.S., New York City has been active in the movement; today, there are for-profit, community, and private farms in all five of New York City’s boroughs. One of the most successful is the massive garden situated atop three rooftops in Brooklyn and Queens. Together they produce more than 80,000 pounds of organic vegetables per year.

Taking Action
In January, 2017, San Francisco launched their Better Roofs Ordinance, requiring that 15-30 percent of most new construction have either living or solar roofs. According to the city’s Planning Department website, rooftops represent more than 30 percent of San Francisco's land area. The Ecoroof requirement in Portland, Oregon requires new buildings of 20,000 square feet or larger built within the central city to have a complete living roof (partially or completely covered with vegetation and a growing medium).

In 2001, Chicago was one of the first large cities to develop a living roof - on a 38,800-square-foot rooftop. It covers an entire city block. Since 2015, Chicago’s cool roof law, offering a floor-to-area ratio bonus written into building ordinances, has spread to other states. Last year, Denver voters passed the nation’s strictest green roof requirement. The city’s urban heat island effect is the country’s third worst, after Las Vegas and Albuquerque. The ordinance requires new buildings and re-roofing projects larger than 25,000 square feet to install a cool roof – a light colored roof that reflects heat – and then select other sustainable green roof options, including enrollment in a flexible energy program, achieving a green building certification, or payment to the Green Building Fund. 

As our cities continue to set higher goals to meet the world’s energy needs, rooftop gardens, living roofs, solar panels and other options for updating and building urban buildings will be in more demand than ever. Over the long run, businesses’ energy consumption and costs are reduced, city denizens have a green refuge, and everyone has quality food to share. The day your favorite restaurant can harvest the finest ingredients for your dinner salad right outside their door is a reality.

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