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The Benefits of Green Roofs in Urban Areas

As numerous heat waves passed through Los Angeles during the summer of 2016, resident Edwin Díaz suffered nose bleeds and severe migraines each time. His physician recommended him to stay cool in the 90- and 100-degree weather.[1] However, for people like Díaz who reside in poor neighborhoods without central air conditioning, that was difficult. Living in Los Angeles, a major US city, Díaz’s symptoms were partly caused and worsened by the urban heat island effect, an environmental condition in which urban areas have significantly higher average temperatures than their surrounding areas.

Urban heat results when nature and plant life is cut down and replaced with impervious structures, or structures that water cannot pass through, such as roads and buildings. The dark nature of roads and building roofs absorb heat instead of reflecting it, causing a city to be hotter than its surrounding areas. Furthermore, global warming heightens the heat island effect, leading to even higher temperatures in urban areas

Díaz’s health situation was not uncommon. Higher temperatures can cause physiological disruptions, organ damage, and even death. Fortunately, there is a solution to mitigate the heat island effect and its attending issues. This solution is green roofs, or roofs with plants placed on top of them. This innovative technology can bring back the plant life that cities replaced.

Not only do green roofs have cooling and improved environmental potential in cities, but they also have many other benefits as well. These include stormwater and related water pollution management, air pollution control, protection of roofing membranes, thermal regulation in buildings, and even long-term economic gains. These benefits all help solve the various issues involving sustainability that cities face today.

Before exploring these benefits, it is important to understand green roofs. There are two types of green roofs—intensive and extensive. Sustainability-wise, both types have similar benefits. The main difference between intensive and extensive green roofs is their function and composition. Intensive green roofs are similar to parks, since they are often for public use and are always on flat, usually concrete roof decks. They generally support a greater variety of plant species than extensive roofs and can hold trees and shrubs. Intensive roofs also need high maintenance, as they require constant irrigation, weeding, fertilization, and much more.

Extensive green roofs, on the other hand, are generally not for public use, but mainly for their environmental benefits. Extensive roofs do not need to be on a flat roof, as long as the sloped roof can carry the additional weight. Maintaining extensive roofs is much easier as well. They do not require irrigation, because natural precipitation is sufficient. The only maintenance required would be occasional weeding and a slow-release fertilizer once or twice each year.

The first main issue that green roofs can help manage is stormwater runoff. In nature, rainwater normally infiltrates into the earth or is recycled back into the atmosphere through evapotranspiration, a natural process in plants that releases water. This occurs through evaporation of water out of the soil, as well as transpiration, the release of water in plants. In natural areas such as forests, 95% of rainfall can be absorbed, compared to a measly 25% in cities.[2] Cities are unable to absorb as much water because they contain parking lots, roads, buildings, and low amounts of plant life, preventing the natural water absorption process.

This lack of absorption causes stormwater runoff, which is very harmful for cities.First, stormwater runoff can cause downstream flooding, which leads to damaged properties and buildings. Moreover, runoff often overwhelms city sewer systems. This can cause sewage overflows, which leads to waste flowing into rivers and other bodies of water, reducing the water quality. A great example of this scenario is New York City, where around half of all storms result in sewage overflow. Annually, this overflow leads to forty billion gallons of wastewater dumped into New York City’s bodies of water.[3] Additionally, runoff contains high levels of pesticides and petroleum residues. These pollutants can contaminate drinking water and destroy wildlife habitats, harming humans, plants, and animals.

Implementation of green roofs in cities can help manage stormwater and prevent the issues that stormwater runoff can cause. Plants manage rainfall in nature. Bringing plant life into cities allows them to capture precipitation and return it into the atmosphere through evaporation and transpiration. It is estimated that the widespread implementation of green roof systems can lead to reductions in stormwater runoff by 60% to 100%. Transforming just 20% of roofs in Washington, D.C. to green roofs would allow an additional 958 million liters of stormwater to be stored each year.[4] Because green roofs retain water, they also help delay runoff and sewage overflow events until green roofs are holding their max capacity of water.

In addition, green roofs are more convenient and cost-effective for stormwater management, since conventional management techniques, such as reservoirs, constructed wetlands, and sand filters, require large plots of land. These land plots are usually very expensive in cities, so conventional management techniques are difficult to utilize in urban areas. Green roofs, on the other hand, simply add to existing buildings, saving space and money.

While green roofs can help decrease water pollution from stormwater runoff, they also decrease air pollution by purifying city air. There is a lot of air pollution in urban areas, largely caused by the burning of fossil fuels. Air pollution can cause health problems in many of the major bodily systems, such as the respiratory, cardiovascular, nervous, urinary, and digestive systems. Because of these health concerns, cities should look to reduce as much air pollution as possible.

Reducing air pollution is very important to improving the health of city residents, and green roofs, again, are a potential solution. Plants have the ability to freshen the air by taking in and absorbing air pollutants. The pollutants can also be washed into the soil by rainwater. Researchers at the University of Michigan found that covering 20% of industrial or commercial roofs in Detroit with extensive sedum green roofs would remove over 800,000 kilograms of nitrogen dioxide every year.[5] Another example is the city of Chicago. In one year, 19.8 hectares of green roofs removed 1675 kilograms of pollution, a rate of 85 kilograms per hectareIn fact, installing intensive green roofs on all of Chicago’s roofs could remove 2046.89 metric tons of pollution per year.[6] The implementation of green roofs can help improve city air quality, leading to better respiratory health and improved lung function, as well as less risk for respiratory and cardiovascular disease for all residents.

Another benefit of green roofs is the protection of roofing membranes. Roofing membranes are an important protective layer placed above roofs to control leaks. Ultraviolet radiation from the sun damages roofing membranes. Adding green roofs onto buildings would protect these roofing membranes by preventing the sun’s ultraviolet rays from hitting them. In addition, temperature fluctuations between day and night cause stress on membranes, since they expand and contract with changes in temperature. Non-green roofs tend to have temperature fluctuations of around 50 degrees Celsius, while green roofs reduce this number to just 3 degrees Celsius. One example is in Toronto, Ontario, where studies have found that membrane temperatures peaked at 70 degrees Celsius on traditional, non-green roofs, compared to a peak of 25 degrees Celsius on green roofs.[7] Because of the reduced stress and damage on roofing membranes, placing green roofs on top of roofing structures allows membranes to live two to three times longer. Numerically, this may extend membrane life by over twenty years. Many membranes underneath green roofs in Berlin, Germany have even gone 90 years without need for major reparation.[8]

The thermal regulation that green roofs provide not only protects membranes but cools entire buildings. Green roofs cool buildings by physically shading them, decreasing absorption of the sun’s heat, and by performing evapotranspiration, a plant’s natural process of returning water back into the atmosphere. Evapotranspiration cools buildings, since heat energy is released as the water evaporates. Pennsylvania State University scientists conducted studies and found that “evapotranspiration may be the most important contributor toward reducing summer building energy consumption under green roofs”.[9]

All factors combined, green roofs lead to significantly less heat gain in buildings. In Singapore, research showed that in a day, heat absorption of a building with a green roof was less than 10% than that of a building with a conventional roof. A 2004 study of Ottawa found that each year, green roofs allow a 95% reduction in heat gain in buildings.[10] In fact, green roofs have much higher albedo, the amount of light and heat reflection off of roofs, than conventional roofs and even white “cool roofs”. Higher albedos represent more light and heat reflection. Green roof albedo ranges from 0.7 to 0.85, compared to 0.05 to 0.25 in conventional roofs. White roofs have an initially higher albedo, starting at 0.8, but dust and debris buildup on white roofs cause decreases of up to 11%.[11] Green roofs are evidently the more consistent and effective way to reduce heat absorption.

Overall, decreased heat absorption, increased evapotranspiration, and higher albedo contribute to reducing the urban heat island effect and many related issues. Simulations for 50% green roof coverage in Toronto found temperature reductions of up to 2 degrees Celsius in many areas.[12] Implementation of green roofs also mitigates the dangerous health issues caused by the heat island effect.

Many urban areas struggle with stormwater runoff, air pollution, the heat island effect and the altered weathered patterns it causes, loss of plant life and greenspace, and loss of wildlife habitat. Green roofs are a solution that can solve or alleviate all of these major issues. Green roofs can hold water and manage stormwater, reduce air pollution, cool buildings and cities, save money, and most importantly, bring back plant life into urban areas. Currently, many programs have been put in place to encourage cities to plant green roofs. Such programs include Earth Pledge Green Roofs in New York City and the Ecoroof program in Portland. If efforts like this are continued to be made to promote widespread implementation of green roofs in cities, these cities will make great progress on their path to a sustainable future.

~ Eleanor Park

[1] Peterson, Molly, “Urban Heat Islands Can Be Deadly, and They're Only Getting Hotter”, Wired, 2017, para. 3

[2] Getter, Kristin L. et al., “The Role of Extensive Green Roofs in Sustainable Development”, HortScience, 2006, page 1276

[3] Ibid.

[4] Ibid, page 1278.

[5] Getter, Kristin L. et al., “The Role of Extensive Green Roofs in Sustainable Development”, HortScience, 2006, page 1279

[6] Yang, Jun et al., “Quantifying air pollution removal by green roofs in Chicago”, ScienceDirect, 2008, page 7269

[7] Getter, Kristin L. et al., “The Role of Extensive Green Roofs in Sustainable Development”, HortScience, 2006, page 1279

[8] Oberndorfer, Erica, et al., “Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services”, BioScience, 2007, page 828

[9] Ibid, page 829.

[10] Ibid, page 828.

[11] Getter, Kristin L. et al., “The Role of Extensive Green Roofs in Sustainable Development”, HortScience, 2006, page 1277

[12] Oberndorfer, Erica, et al., “Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services”, BioScience, 2007, page 829

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