An aerial view of most cities will show you swathes of black asphalt or gravel-filled roofs—utilitarian spaces that have traditionally been underutilized. But some cities, like Chicago and Indianapolis, have come up with a solution to reimagine these spaces: green roofs.
What is a green roof?
According to the EPA, a green roof, also called an eco-roof or rooftop garden, is a vegetative layer grown on a rooftop. There are two main types of green roofs: intensive and extensive. Intensive green roofs are essentially elevated parks that sustain trees, walkways, and benches with their complex structural support, irrigation, drainage, and root protection layers. About a foot of growing medium is needed for an intensive green roof, creating a load of 80–150 pounds per square foot.
Extensive green roofs typically exist solely for environmental benefits—unlike intensive green roofs, extensive roofs don’t function as accessible rooftop gardens or parks. They are made up of a thin but hardy two-inch growing medium and are relatively light, weighing about 15–50 pounds per square foot.
Green roof costs and installation
The estimated cost of installing a green roof starts at $10 per square foot for extensive roofs and $25 per square foot for intensive roofs. Annual maintenance for either type of roof can range from $0.75–$1.50 per square foot. Though the initial cost of green roofs is higher than roofs with conventional materials, building owners can offset the difference through reduced energy and storm management costs.
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Why are green roofs so valuable?
According to Matthew Dillon, vice president of the World Green Infrastructure Network, “The key trends for green roofs are commercially driven by increasing realty value and improving tenancy loyalty in a competitive market.” Incorporating green roofs gives new building construction an edge over competing projects, particularly in areas where the demand for urban living is increasing and space is at a premium.
Here are a just a few other reasons why green roofs are becoming more popular in cities around the country.
The urban heat island effect is a condition in which city and suburban developments absorb and trap heat. One of the biggest benefits of a green roof is its capability to reduce the surface temperature of a roof through evapotranspiration. This is especially helpful on hot days when the surface temperature of a green roof can be cooler than the air temperature.
By reducing heat transfer through a building’s roof, green roofs can improve indoor comfort levels and lower heat stress associated with heat waves. With their natural insulating properties, they can also act as insulators for buildings, reducing the energy needed to provide cooling and heating.
Note that a single green roof will not have a great effect on reducing urban heat islands, but multiple green roofs in Air Quality Management Zones will have a noticeable effect.
Green roofs can protect the roof membrane—used to move water off of roofs—from harsh weather and ultraviolet radiation, and can last twice as long as conventional roofs. This durability is a long-term economic benefit that outweighs the start-up installation costs.
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Green roofs decrease air pollution and greenhouse gas emissions through vegetation. By adding plants and trees to the urban landscape, photosynthesis increases, reducing carbon dioxide levels produced by vehicles, industrial facilities, and mechanical systems.
The vegetation and soil on a green roof act like a sponge, absorbing and filtering water that would normally flow through gutters and down polluted streets into an overtaxed sewer system. By filtering pollutants from rainfall and reducing stormwater runoff in the urban environment, a green roof lowers the necessity for complex and expensive drainage systems in a city.
Some green roofs are merging with vertical vegetable plant walls to create a more visually appealing environment. While some cities are developing green roofs into social and recreational spaces, some designs are creating trafficable green roofs for food production and biophilic design solutions.
The rise of green roofs throughout the United States
After Chicago experienced a terrible heatwave in July 1995 that caused more than 700 heat-related deaths, Mayor Richard M. Daley and the City of Chicago constructed a 20,300-square-foot, semi-extensive green roof on top of City Hall to combat the urban heat island effect and improve urban air quality. With more than 20,000 herbaceous plants and a supplemental irrigation system, the City Hall green roof is currently seven degrees cooler than surrounding roofs, and can run as much as 30 degrees cooler in the summer.
On a similar note, the City of Indianapolis’ Office of Sustainability and United Water collaborates annually on a Green Infrastructure Grant Program to promote the installation of green roofs throughout the city. The primary goal of green infrastructure in Indianapolis is to capture stormwater so that it can be cleaned, infiltrated into the soil, and slowly released into rivers and streams. In 2010, $100,000 in funding was granted to organizations who utilized green infrastructure projects designed to improve water quality and reduce stormwater runoff.
In November 2017, Denver passed an initiative mandating that 20% of rooftop space on newly constructed buildings over 25,000 square feet must be covered by gardens or solar panels. Not only will this initiative reduce Denver’s urban heat island effect, which has made Denver five degrees hotter than surrounding areas, it will improve the city’s air quality and increase local food sustainability.
With these incentives and the many benefits that come with green roof installations, many eco-minded homeowners, businesses, and cities are finding green roofs more and more appealing and are slowly beginning to incorporate them into the local cityscape.
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