By Morgan Kaenzig de Denus, AMAST Content
Green roofs — also known as eco-roofs, living roofs, or vegetative roofs — are becoming more and more popular in the United States, and with good reason. Installing a vegetative roof can provide many benefits, from lessening the burden on storm drainage systems to giving shelter to migrating birds to improving air quality.
What Are Green Roofs?
Green roofs are covered with vegetation and a lightweight growing medium. Most living roofs have a protective membrane that serves as a root barrier between the roof’s membrane and the drainage system. The drainage mat will catch any excess water, while a filter fabric will help retain the soil during heavy rainfall. Some green roofs also have irrigation systems.
What Are the Different Types of Green Roofs?
Intensive living roofs can support an impressive 150 pounds of vegetation per square foot. They provide plants with deep soil and an irrigation system. Intensive roofs require maintenance and should be treated as a regular garden; the plants should be kept separate, and weeding should occur regularly.
Extensive living roofs can only support 25 pounds of vegetation per square foot. As a result, these roofs require less growing medium and often do not have an irrigation system installed. The plants on extensive living roofs are often left to grow naturally, with weeding and maintenance scheduled annually. They are less expensive, appear more natural, and are better suited to cover larger surface areas than intensive living roofs.
Semi-intensive green roofs occupy a space somewhere between intensive and extensive living roofs. Semi-intensive eco-roofs can support a wide variety of plants (like intensive roofs), but they can also support more vegetation per square foot (like extensive roofs).
What Are the Benefits of Having a Green Roof?
They Insulate Buildings
Eco-roofs are excellent insulators. They help block the flow of heat into and out of a building, which reduces the need for air conditioning and heating. The Natural Resources Defense Council (NRDC) has found that these roofs can reduce the amount of energy needed to cool the top floor by 50%.
They Improve Air Quality
Plants can remove air pollutants. According to an Environmental Protection Agency (EPA) study, a 1,000-square-foot eco-roof will eliminate 40 pounds of Particulate Matter from the atmosphere. Living roofs also increase air quality by lowering the building’s energy use, which reduces the amount of greenhouse gas released into the atmosphere.
They Increase Waste Diversion
Having a green roof can help prolong the roof membrane’s life, which will reduce the amount of material headed toward the landfill. Some growing mediums or modules also use recycled materials. Finally, living roofs can reduce the utilization of ventilation, heating, and HVAC systems, thereby extending these system’s lifespans.
They Reduce the Urban Heat Island Effect
Cities are often hotter than the surrounding rural areas because of the Urban Heat Island Effect. Cities have an abundance of concrete, asphalt, brick, and steel, which means more light energy is absorbed and converted into heat. Living roofs cover areas that absorb light energy more easily, which can help reduce the Urban Heat Island Effect.
Introducing more plants to an urban environment also helps cool cities, because plants undergo evapotranspiration. They take energy from the air to transform water into gas, which lowers the temperature.
They Reduce Noise
Additionally, installing an eco-roof can help prevent external noise from entering the building. Living roofs are particularly effective at reducing the volume of low-frequency sounds. Extensive eco-roofs can reduce outside sounds by 40 decibels, and intensive eco-roofs can reduce sounds by 46 to 50 decibels.
They Improve Stormwater Management
Having a green roof can also reduce the amount of stress placed on the sewer systems. The substrate stores water, which is then used by the plants and returned to the atmosphere. In the summer, green roofs can hold 70–90% of the precipitation they receive, and they can retain 25–40% in the winter.
The Natural Resources Defense Council (NRDC) studied green roofs and concluded that if half of all new or redeveloped projects in Southern California were outfitted with green roofs by 2035, they would experience a 20 billion gallon–reduction in rainwater runoff each year. This will help reduce erosion and pollution, as runoff often transports hazardous chemicals and pollutants to the ocean or water treatment facilities.
They Can Be a Multi-Functional Space
Green roofs can serve many purposes. Eco-roofs can serve as community hubs, community gardens, restaurant terraces, recreational spaces, and more.
They Provide a Habitat for Animals
Birds, honey bees, soil organisms, and more benefit from the presence of living roofs. They can serve as stepping-stone habitats for migrating birds, and they can help increase biodiversity.
What’s Next for Green Roofs?
While the push towards green roofs in North America has been slower than in Germany (where around 14% of roof area is green), vegetative roofs are beginning to gain popularity. Between 2015 and 2016, Green Roof for Healthy Cities asserts that the green roof industry grew by 10%, with Chicago and Toronto adding over 600,000 square feet of green roofing. Toronto was also the first city in North America to require certain buildings to have green roofs. In Toronto, any new residential, institutional, or commercial developments with at least 2,000 square meters of floor area must have a green roof.
New York City, a city with over 1.6 billion square feet of rooftop surfaces, has introduced the Green Roof Property Tax Abatement Program. This program introduced a year-long tax abatement on the construction of green roofs on residential and commercial buildings. The city has also decided to dedicate $1.5 billion to green infrastructure in the next decade. More cities are likely to follow in New York’s footsteps, due to the planet’s warming and the many long-term benefits that green roofs offer.
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