Green roofs are gaining popularity across the globe. As urban development ramps up, green roofs offer unexpected room for nature. They hold all the benefits of green open spaces while mitigating numerous environmental issues; an inevitable result of city buildout. These benefits are, increasingly, making them a staple of green building and of sustainable neighborhood development.
Green roofs have a positive impact on a building’s energy consumption. By providing an additional layer of insulation, they significantly reduce a building’s need for cooling and heating, making it more energy efficient, saving energy costs, and decreasing its carbon emissions. Buildings are responsible for more than 40 percent of the energy consumed in cities. Heating and cooling alone account for 35 to 60 percent of energy use in buildings resulting in nearly 40 percent of urban emissions, globally, on average.
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But green roofs have more benefits. They play an important role in mitigating the effects of impervious surfaces in our cities. One of their greatest uses is in fact their ability to reduce the volume of surface runoff which decreases the risk of floods in case of heavy precipitation. Depending on the season and the type of roofing system, green roofs are able to retain 40 to 100 percent of stormwater. Once the soil media is saturated, the water is slowly released into the sewage system preventing overflowing. By infiltrating the different layers of the green roof composite, the stormwater runoff is partially filtered and is less polluted when discharged into natural streams.
The vegetation planted on green roofs has proved to have an effective impact on urban heat islands by regulating fluctuating ambient temperatures and improving micro-climates. Vegetated roofs also improve air quality and provide a home for wildlife, preserving biodiversity in urban environments. These green spaces on top of buildings also offer room for social and economic activities. Rooftop urban agriculture projects have been sprouting up in cities like New York, Rotterdam, Copenhagen, Tokyo and Montreal. Like other urban farming initiatives, rooftop farms promote sustainable living by contributing to the local food production system and to recycling urban organic waste. In dense cities and neighborhoods, green roofs can act as additional public spaces, albeit usually owned and managed by private parties. When accessible, these sky gardens offer an oasis of luscious greenery and great vantage points for visitors to discover or rediscover the city.
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Despite all these benefits, only a few cities in the U.S. provide incentives for developers and building owners to install green roofs, whether in new developments or as part of building retrofitting projects. With a few exceptions, green roofs are often not listed as an independent item in environmental and sustainability documents, but go under stormwater management improvement measures or as part of a green building certification program.
Chicago, Illinois is leading the way with its Green Roof Grants Program (no longer accepting applications) and a Floor-Area Ratio (FAR) bonus. To-date the city counts more than 500 vegetated rooftops, totaling a surface of more than 5.5 million square feet. Its City Hall rooftop continues to be a flagship project of green roofs’ numerous benefits. Other cities like Portland, Oregon, Austin, Texas, Philadelphia, Pennsylvania, Washington, D.C. and more recently San Francisco, California and Denver, Colorado offer grant programs, tax credits or abatements or have passed bills to support greening more of their roofs.
In many parts of Europe, green roofs are more common and are even becoming mandatory. In Germany, a pioneering country in the development and implementation of green roofs technology, 14% of all flat roofs are green and one third of its cities have regulations that support vegetated roofs. In 2010, Copenhagen, Denmark enacted a law that requires the installation of green roofs in most of its new local plans. London, UK and Basel, Switzerland both have laws and requirements that mandate greening their roofs.
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Installing a green roof remains, however, an expensive enterprise. The costs start at $10 per square foot for extensive roofs, and $25 per square foot for intensive roofs. Annual maintenance costs should also be accounted for. On average, they range between $0.75 and $1.50 per square foot. In comparison, a conventional flat roof costs around $1.50 to $10.00 per square foot. Without subsidies and despite green roofs many benefits and the long-term savings they may offer, the financial burden might detract many developers and building owners from taking this additional green step. Especially with the rapid development of new composites and materials offering more energy savings and more comfort at lower costs, a focus on green roof benefits at the building-level might not be enough.
Sustainability sits at the top of the priorities of many cities across the world, especially since the adoption of the Sustainable Development Goals (SDG) and the Paris Climate Agreement by the United Nations in 2015. It is important that the impact of different sustainability measures and solutions, including green roofs, become measurable. More research and data should be collected, at the urban and metropolitan scale, to quantify the contribution of green roofs to achieving city sustainability goals. Measuring green roofs impact on stormwater management, air, and water quality would especially allow cities to strategically allocate resources for the installation of green roofs and make them part of city sustainability agendas.
Does your city have green roofs? How do green roofs contribute to a city’s livability? And what impact do they have on a city’s overall sustainability?
Let us know what you think in the comments section below and join us at #TheGlobalGrid Twitter chat to answer these questions and more, with our co-host, Greenroofs.com. Linda Velazquez, ASLA, LEED™ AP, GRP, publisher, and editor of Greenroofs.com will be on our experts' panel along with Tessa Duste (Moss Amsterdam), Ben Flanner (Brooklyn Grange) and Dusty Gedge (Living Roofs). Save the date: Wednesday, April 18th at 12:00 p.m PT.