Green roof designs with major American cities

Brianna Ramsey – Environmental Science

Brandon Parker – Sustainable Horiculture

Richie Frend – BCT

Chicago city hall initiates green roof implementation within U.S. cities (McDonough, 2004).

Chicago city hall initiates green roof implementation within U.S. cities (McDonough, 2004).


The Urban Heat Island Effect is not a newly found issue, yet America has done little to combat the main causes of this problem.  The Urban Heat Island (UHI) effect is only becoming more and more evident as cities continue to grow.  Fighting this issue with real action is essential to improving the living quality in large American cities.  Yilmaz et. al. (2009) gets to the root of this problem by stating that “as cities continue to grow and develop under climate change, identifying and assessing practical approaches to mitigate high urban temperatures is critical to help provide thermally comfortable, attractive and sustainable urban environments.”  Much of the reason the Urban Heat Island effect has passed by under the radar is that during day to day activities the threat is not eminent, therefore is goes unchecked.

The urban heat island (UHI) effect is a measurable term which compares average temperatures within an urban infrastructure compared to that of a more rural one in the surrounding area (EPA, 2009).  There are many factors which contribute to this UHI issue.  One of the largest factors in this equation is excess black tar rooftops.  Areas which would normally contain high levels of vegetation have been transformed by the needs of an ever growing population.  These black roofs absorb large amounts of ultraviolet heat which helps contribute to the high average temperatures seen in the UHI effect (Couts et. al., 2011).  Another factor controlled by population growth is measured by the number of automobiles in use.  Dense populations have caused carbon dioxide emissions to skyrocket in recent years due to automobile exhaust fumes.  Both of these issues contribute to a city life which is unhealthy for both its inhabitants and the atmosphere.

These issues in current city infrastructure have been studied in great detail with regard to the effect they are having on the UHI.  It is important to first understand how much land cover rooftops consist of in major cities.


Looking at figure #1 provided by the EPA’s Reducing Urban Heat Island, compendium of strategies we can see that rooftops cover about 20-25 percent of land cover in American cities. This is huge in when it observed that conventional rooftop surfaces can exceed ambient air tempera­tures by up to 90°F (50°C) (EPA, 2008).  Population has a key part to play in measuring the Urban Heat Island.  In 1968, when the population of Columbia was only 1,000 people, the maximum warming compared with the surrounding rural area was 1°C. A small business centre with office buildings and a large parking lot was a local heat island of up to 3°C. By 1974, when the population had increased to 20,000 people, the geographic extent of the heat island had increased. Most of the town was by more than 2°C warmer than the surrounding rural land. A central commercial and residential district was 5–7°C warmer (Yilmaz 2009, pg.2). Population has played a large role in the steady increase in the Urban Heat Island Effect.  If this measable problem is to be solved we believe it is to be done in measurable ways, which is why we are proposing tax incentives are necessary to increase the installation of green roofs within major American cities to help mitigate the effects of the Urban Heat Island.

Green roof implementation will improve the air quality of the city and in doing so reduce the volume of greenhouse gases.  Greenhouse gases are gases that absorb the infrared radiation that enters earth’s atmosphere from the sun and trap excess heat (EIA, 2004).  In a healthy atmosphere, the radiation from the sun would enter the atmosphere and then be reflected back into space.  Unfortunately, inside a city there is an increase in the concentration of cars and homes per area.  A minimum of eighty percent of greenhouse gas emissions comes from the burning of fossil fuels in cars and electricity and heating of homes (EIA, 2004).  This increase in greenhouse gases assists in the localized heating of cities, or the urban heat island effect.  Not only is the volume of gas emitted in a city greater, but there is also a decrease in the area of vegetation as buildings replace green space.  Plants absorb carbon dioxide in their photosynthesis process, which is the main culprit of the greenhouse effect.  With the installation of green roofs will come an increase in photosynthesis and therefore a reduction of carbon dioxide.  There was a study that “found that 109 ha of green roofs would contribute to 7.87 metric tons of air pollution removal per year” (Bernardi, GhaffarianHoseini & GhaffarianHoseini, 2013, p.421).  On a smaller scale, for every square meter of green roof, 375 grams of carbon is sequestered (Garrison, Horowitz, & Lunghino, 2012, p.18).  The more carbon that is absorbed through plants, the more solar radiation will be reflected back into space and not be trapped within the city contributing to the urban heat island effect.  There has been a simulation that was created to explore the results of Los Angeles converting to green roof installation.  It was found that 10 to 12 percent of the smog found in LA would be absorbed in the plants and it would result in a 2.7 to 3.6 degree Fahrenheit decrease in air temperature across the city (Garrison, Horowitz, & Lunghino, 2012, p.17).  With this great reduction of air pollutants and greenhouse gases, the atmospheric temperature within the city will decrease.

Have you ever walked through a city and seen heat waves coming off the black asphalt?  You can anticipate that the air near the surface of this dark surface will be much greater than the air above a lawn.  The air surrounding a standard building roof can reach very high temperatures, since it is generally comprised of black concrete.  This contributes to the urban heat island effect, as there is a concentrated spike in temperature compared to the much lower surrounding temperatures.  Green roofs absorb this excess energy from the sun and use it productively in the photosynthesis process to create alternative energy in the form of sugars.  Plants also have an evapotranspiration cycle, which further reduces the surrounding air temperature (Garrison, Horowitz, & Lunghino, 2012, p.12).  This is the process of plants taking up water from the ground through their roots and releasing it from the pores in their leaves.  These water drops must then absorb enough sunlight energy to be converted to vapor and enter the atmosphere; this prevents the energy from becoming heat and adding to the urban heat island.  This is very similar to how humans cool down through sweating.  As a result, green roofs can reduce the close-to-surface air by 16.4 degrees Celsius on average (Moisse, 2010).  A study by an environmental engineering firm, MWH, came to the conclusion that green roofs were up to 31% cooler than any other roof top type during the hottest hours of the day which is found to be 12:30pm to 4:30pm (Taylor, 2007, p.A309).  As a result of all of the local air temperature reductions due to green roofs, a model simulation predicted a two-degree reduction of the average temperature of New York City (Bernardi, GhaffairanHoseini, GhaffarianHoseini, 2013, p.419).  As the U.S. cities heat up, green roofs can help citizens cool it down and combat the urban heat island effect.

Not only do green roofs absorb heat and convert it to alternate forms of energy, but they also reflect more of the sun radiation back into space.  Solar reflectance is the amount of solar energy that hits a surface and is reflected back (Garrison, Horowitz, & Lunghino, 2012, p.16).  This is measured with a value between 0 and 1, with the higher values being more reflective and therefore cooler surfaces.  Traditional tar or gravel that makes up most rooftops has a solar reflectance of 0.08 to 0.18 (Garrison, Horowitz, & Lunghino, 2012, p.16).  A green roof has a reflectance of 0.25 to 0.3, but with the coupling effect of evapotranspiration as mentioned above, green roofs have a reflectance equivalent to 0.7 to 0.85 (Garrison, Horowitz, & Lunghino, 2012, p.16).  With this increase in reflectance of green roofs over traditional roofs, there is a decrease in absorption of excess sunlight and heat energy, which then corresponds to a cooler temperature than traditional roofs.  A cooler temperature will bring the urban temperature down to the surrounding suburban areas and reduce the urban heat island effect.

The implementation of green roofs in major U.S. cities is an expensive project. Many people argue the efficiency of green roofs when they are so costly to install. “A 2001 report estimated that initial costs start at $10 per square foot (0.09 m2) for the simpler, extensive roof and $25 per square foot for intensive roofs”(Wong, n.d., p.5). The initial installation cost is quite expensive, which means that businesses must pay upfront costs. However green roofs provide payback incentives via cost savings from energy bills.

When dry, green roof lay­ers act as an insulator, decreasing the flow of heat through the roof, thereby reducing the cooling energy needed to reduce build­ing interior temperatures. In the winter, this insulating effect means that less heat from inside the building is lost through the roof, which reduces heating needs.(Wong, n.d., p. 5)

There was a study done in Greece where researchers measured a reduction of seven-degrees of the indoor temperature following the installation of a green roof and as a result there was a decrease of 48% in energy consumption for cooling in the building (Bernardi, GhaffairanHoseini, GhaffarianHoseini, 2013, p.417).  These costs will eventually add up and pay for the price of installation.

Maintenance of the green roof provides another cost that must be met.  “In addition to construction costs, a building owner incurs maintenance costs to care for the plants on a green roof. Al­though the level of care depends on plant selection, most of the expenses arise in the first years after installation as the plants establish themselves and mature”(Wong, n.d., p. 10). When it comes to maintenance costs, plant selection can have a significant impact. However Godfrey roofing a green roofing company out of Canada says “Drought tolerant plants are typically more resistant to weather conditions and fit well with the standard low maintenance requirements of an extensive green roof system”(Godfrey Roofing Inc). If businesses use succulent, low maintenance plants instead of perennial or annual aesthetics, energy saving benefits would still occur and there would be less maintenance costs.  Although green roofs are costly the benefits far outweigh the price not only for the reduction of the urban heat island effect but also for building energy efficiency in both the summer and winter.

Along with the federal tax incentives that go along with green roof installation, we propose further tax incentives such as tax abatements.  Following a model like New York State’s tax abatement will help initiate interest installing green roofs and will help alleviate costs of installation. The federal Energy Policy Act of 2005 already insures green efficient buildings a tax break of $1.80 per square foot that meet ASHRAE (American Society of Heating, Refrigerating,  and Air Conditioning Engineers) standards (Plant Connection Inc, 2014). Increasing the federal abatement to match New York cities standard of $4.80 per a square foot (NYC Buildings, 2010, p. 1) will help spread more interest in green roof installation and maintenance. New York City is one of the leading green roof innovative cities in the world following Washington D.C., and Chicago. However Chicago and D.C. offer grants and foundation money instead of tax abatements. To be realistic, providing each and every city in the U.S. with grants would be not sustainable for our economically struggling country.  However according to the 2012 annual green roof survey results; if tax abatements were increased this could raise interest in more businesses and cities across the nation to install green roofs. Other cities in the world such as Toronto and Vancouver are trailing U.S. cities in efficiency, they have laws requiring green roof installation, yet the results show that these are not as successful as New York City’s tax abatement or grants cities like D.C. (Erlichman, 2013). The nation needs to find a compromise to further incentivize the implementation of green roofs to reduce the urban heat island effect while considering our limited finances.  We believe that tax breaks are the most effective way to do this.

 Green roof installation can assist in mitigating the urban heat island effect through the natural qualities of the vegetation.  Through a small increase in tax incentives the United States can make a huge difference in promoting green roof installation throughout the urban nation. Offsetting the installation cost will help improve the air quality in our cities and increase our energy efficiency and ultimately assist us in combating the issue of the urban heat island effect.




Berardi, U., GhaffarianHoseini, A., & GhaffarianHoseini, A. (2013).  State-of-the-art analysis of the environmental benefits of green roofs.  Applied Energy, 115, 411-428.  Retrieved from: Elsevier.

Coutts, A. M., Daly, E., Beringer, J., & Tapper, N. J. (2013). Assessing practical measures to reduce urban heat: Green and cool roofs. Building & Environment, 70, 266-276. doi:10.1016/j.buildenv.2013.08.021

EIA (2004).  Greenhouse gases, climate change, and energy.  Retrieved from

EPA (2008) Reducing Urban Heat Islands: Compendium of Strategies. Retrieved from

Erlichman, P. (2013). Annual green roof industry survey for 2012  Retrieved from:

Garrison, N., Horowitz, C. and Lunghino, C.  (June 2012).  How green roofs and cool roofs can reduce energy use, address climate change, and protect water resources in southern California.  Emmett Center on Climate Change and the Environment.  Retrieved from


Godfrey Roofing Inc. Green roof plant selection. Retrieved 04/26, 2014, from


Moisse, K. (2010). Over the top: Data show “green” roofs could cool urban heat islands and boost water conservation. Retrieved from


NYC Buildings. (2010). NYC green roof property tax  abatement program   . Retrieved 04/26, 2014, from


Plant Connection Inc. (2014). Green rood legislation,policies,& tax incentitives. Retrieved 4/26, 2014, from


Taylor, D. A. (2007). Growing green roofs, city by city. Environmental Health Perspectives, 115(6), A306-A311. Retrieved from: J Stor.


Wong, E. Green roofs. Reducing urban heat islands: Compendium of strategies: Climate Protection Partnership Division in the U.S. Environmental Protection Agency’s Office of Atmospheric Programs


Yilmaz, Sevgi – Toy, Süleyman – Yildiz, Nalan Demircioglu – Yilmaz, Hasan. (2009). Human population growth and temperature increase along with the increase in urbanisation, motor vehicle numbers and green area amount in the sample of erzurum city, turkey. Environmental Monitoring and Assessment, 148(1-4), 205-214. doi:10.1007/s10661-007-0151-z




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