Have you ever seen a roof covered with plants and wondered why?
A green roof (also known as a rooftop garden or a living roof) is a vegetative layer grown on a rooftop. The popularity of green roofs has increased in the United States due to their many benefits, While pretty, green roofs serve a much greater purpose than simple beautification.
- Unlike traditional black tar roofs, green roofs reduce energy costs by absorbing heat instead of attracting it and providing natural insulation for buildings. According to a study conducted by the National Research Council of Canada, even a six-inch extensive green roof can reduce summer energy demands by more than 75 percent. By lowering air conditioning demand, green roofs decrease the production of associated air pollution and greenhouse gas emissions.
- Green roofs help reduce the Urban Heat Island Effect, a condition in which urban environments absorb and trap heat.
- A green roof’s plants remove air particulates, produce oxygen and provide shade. Additionally, this natural protection against extreme heat enables green roofs to last twice as long as traditional rooftops.
- Another important benefit of green roofs is their ability to reduce and slow stormwater runoff in urban environments. Because rooftops and streets in cities are hard surfaces, the volume and velocity of stormwater runoff increases tremendously and is a major source of flooding and pollution worldwide. Due to nonporous surfaces like pavement and rooftops, a typical city block generates more than five times more runoff than a woodland area of the same size. The ability a green roof has to absorb (and filter) water significantly lowers the risk of flash flooding and sewer overflows. In the summer, green roofs retain up to 90 percent of the precipitation that falls on them and up to 40 percent in the winter.
So, given all the benefits, why aren’t all roofs green? Outside of the United States, green roofs are much more prevalent. In Germany, for example, 14 percent of all roof area is green. Although green roofs are on the rise in the United States, they are not nearly as common. Despite their many benefits, a major obstacle is the initial expense. Green roofs typically cost two to three times more than a non-green roof. Although green roofs are not simple or cheap, many cities recognize that long-term benefits outweigh the initial cost concerns. New York City, for example, has a pilot program encouraging the installation of green roofs by offering tax abatements. Provided that other cities follow suit, you might be seeing more and more roof gardens.