Cities are key social and economic organizing units, bringing together concentrations of talented and creative people, jobs, and all the inputs required for growth. Nonetheless, continued rapid development of cities out of what had been untouched natural landscapes raises important questions about the environmental impact. Green roofing is a technique that can help mitigate some of the negative effects urban growth has caused by reintroducing the natural landscape.
Green roofing involves growing plants on rooftops, thus replacing some of the vegetated footprint that was destroyed when the building was constructed. Green roofs, sometimes referred to as vegetated roofs or eco-roofs, consist of a waterproof membrane, soil as a growing medium, and a variety of plants. Green roofs offer many private, public, and design-based benefits, including structural enhancements and maintenance of waterproofing functions.
The benefits of green roofing can be achieved through virtually all types of green roof systems and designs. In a traditional black roof, the waterproofing material will degrade over time primarily due to sun exposure and temperature extremes. However, a green roof has the ability to minimize both of these effects. Since a green roof prevents sunlight from reaching the membrane beneath, temperature extremes are greatly moderated. A black roof can undergo a temperature extreme of well over 100°F, while a green roof generally experiences a swing of less than 10°F. Compared with conventional roofs, this protection can more than double the number of years typically needed before a roof must be replaced.
In general, roofs are the point where the greatest heat is lost in the winter and the hottest temperatures emanate from in the summer due to the amount of surface area exposed to the elements. Green roofs offer much greater insulation, allowing for a significant decrease in the amount of energy needed to moderate the temperature of a building. This vastly improves energy conservation, which translates into fewer greenhouse gas emissions. As an added bonus, this increased insulation gives green roofs excellent noise attenuation and greatly reduces outside noise levels, which is particularly important in areas with heavy air or automotive traffic.
In a typical urban area, rain that falls onto a conventional roof quickly flows off into storm sewers and out into nearby bodies of water. This excess runoff can cause numerous environmental problems such as sweeping pollutants into the water, eroding river banks, and even flooding. Green roofs can form a key part of stormwater management, reducing the flow of water that runs off the roof by retaining a portion of the rainfall. This retained water is stored by the substrate and taken up by the plants and returned to the atmosphere through transpiration and evaporation. On hot days during the summer, this process is beneficial to the building and its occupants because it cools the ambient air, leading to reduced air conditioning needs and less energy consumption.
Additionally, highly developed areas that are heavily paved with clusters of buildings tend to be much warmer than surrounding rural areas due to the absorption of solar radiation from the manmade surfaces. These areas typically hold in more heat and release it much slower than rural areas, resulting in an overall increase in temperature. This effect can also lead to an increase in energy consumption, rates of heat-related illnesses and deaths, and air pollution.
Green roofs are able to cover some of the hottest surfaces in the urban environment and still absorb less sunlight than black rooftops. The vegetation on a green roof is the main factor to this ability as it releases dew and completes an evaporation cycle each day, cooling cities during hot summer months, providing shade, and reducing this urban heat island effect.
Green roofs also provide a new habitat for plants and animals in urban areas, which helps increase biodiversity. Increased biodiversity can help rebuild ecosystems and allow for the plants and animals to function as they did before they were disturbed by the building’s development. Diverse and stable ecosystems not only have positive impacts on the environment but also create psychological well-being for the people living and working in the community.
Another health benefit green roofs provide is improving air quality because the plants filter noxious gases and decrease the amount of airborne pollutants. On top of that, the cooler temperatures, beautiful carpets of flowers, and butterflies they attract prove that green roofs are valuable community spaces for people working or living in the building and can potentially increase employee productivity.
Urban greening, which is another term for green roofing, can also increase a building’s marketability and property value due to the increased energy efficiency. The green roof market itself offers potential long-term job opportunities for both skilled and unskilled workers related to manufacturing, plant growth, design, installation, and maintenance.
Green roofs provide numerous social, environmental, and economic benefits to not only the individual building but also the wider urban environment. While they are increasingly prominent as an environmental solution, green roofs are not a new idea. Architects and planners have been using green roofs for decades to help solve environmental problems by bringing nature back to the city in key ways. As we look to the future, it is important to continue building and installing green roofs. With cities and urban islands continuing to be built around the globe, the financial and environmental benefits of green roofs only increase for the community.
Green roofs and stormwater management
According to a 2011 General Services Administration report, The Benefits and Challenges of Green Roofs on Public and Commercial Buildings (www.gsa.gov/portal/mediaId/158783/), research has identified green roofs as one of the best ways to address wet weather flows in urban areas with high-density development. Key findings of research include the following:
- Green roofs can reduce the frequency of combined sewer overflows.
- Green roofs can reduce the rate of runoff from a roof by up to 65 percent.
- Green roofs can add three hours to the time it takes runoff to leave a roof.
- Green roofs can catch and permanently retain the first 1/2 inch to 3/4 inch of rainfall in a storm.
- Green roofs’ ability to buffer acid rain can be a significant benefit in areas where acid rain is common.
Sandesh Joshi is the president and co-founder of Cary, N.C.-based Indovance Inc. (www.indovance.com), a CAD and drafting outsourcing service. Prior to founding Indovance, he worked at SolidWorks Corporation as a senior research and development member.