Green Roofs Effects on Urban Environments



Green roof, in France

Isabelle Kendall, Hasan Sabri & Bailey Michell

People over 65 make up a significant portion of the United States population, and the number increases every year. By 2040, the amount of people 65 and older in our population will go from 41 million to around 80 million (Kenney, Craighead, & Alexander, 2014, p. 6). This demographic is at great risk for heat related illnesses and death due to the increasing heat indices of our planet (Conti et al., 2005). A heat index is what the combination of temperature and humidity feel like to human beings, and as temperatures rise so do indices (National Oceanic and Atmospheric Administration [NOAA], 2016). Although the elderly are the most afflicted by heat induced mortality, it can happen to anyone: young or old, rich or poor. Heat waves in Chicago, Tokyo and many other cities have caused fatalities among a variety of individuals. For instance, in the summer of 2003, over 70,000 Europeans passed away during a single heat wave (Knox, 2007). Heat waves are becoming more frequent and more devastating. During a heat wave in Chicago there were nearly 700 more heat related deaths recorded than during a heat wave one year before (Whitman et al., 1997). The increased temperatures that lead to heat related fatalities and other heat related injuries are caused by the expansion of cities across the globe, and more specifically, the materials used to construct these expansions. Materials used include gravel, cement, and asphalt. These impermeable substances that make up urban surfaces like sidewalks, roads, and traditional buildings’ roofs absorb and retain solar radiation during the day then release heat gradually at night increasing surrounding air temperatures into the next day (Knox, 2007). This temperature phenomenon is called the urban heat island (UHI) effect because it causes temperatures in urban areas to be much higher than those in the rural areas around them (Environmental Protection Agency [EPA], 2016). During summer months, the surface of a conventional roof can be as much as 50 º C (90 º F) hotter than ambient air temperatures (Liu & Baskaran, 2003). An article from the Population Reference Bureau (PRB) states that in the 1800s, only three percent of the world’s population lived in cities. By 2008, half of the global population lived in cities, and by 2050, almost 70% of the world’s population will be urbanized (Population Reference Bureau, n.d.). Since the population is continuously growing, it is plain to see that any problems facing cities now will affect a staggeringly larger proportion of people over time. Thus, finding solutions to those problems like heat waves, which occur most frequently in cities, will be an integral part of future city living.

While growing populations and increased energy consumption go hand in hand, increasing temperatures also exponentially increase power use, putting an economic and environmental strain on urban populations. Increased temperatures caused by traditional building materials lead to increases in energy consumption within urban areas by cooling systems. Regular building materials do a poor job of keeping summer heat out, leading to increased cooling system use. Environmental issues resulting from excess energy consumption include air pollution from greenhouse gases released into the atmosphere as a product of the electricity production process (McSweeney, 2015). It is also more costly for home and business owners to need to use more electricity and other energy sources to maintain comfortable and safe indoor conditions.

As the global population becomes increasingly urbanized, the problems stemming from urbanization will grow. Scientists investigating methods to reduce the effects of urbanization suggest conversion to rooftop gardens (more commonly known as a “green roof”). A green roof is a regular roof with vegetation covering it (Tam, Wang, & Le, 2016). Although there are various types of green roofs, the most common are extensive green roofs that have soil depths of as little as two inches, sturdy vegetation that requires little maintenance, and can be implemented on existing rooftops (Environmental Protection Agency [EPA], 2016). Green roofs mitigate the effects traditional building materials have on urban environments by decreasing air temperatures (Solcerova, Ven, Wang, Rijsdijk, & Giesen, 2013; MacIvor, Margolis, Perotto, & Drake, 2016; Price, Watts, Wright, Peters, & Kirby, 2011) and costly energy consumption (Squier & Davidson, 2016; Bell et al., 2016; Tam et al., 2016 ).

Green roofs produce a cooling effect on roof surface and air temperatures through the process of evapotranspiration: the transfer of water from the soil and vegetation to the air. This endothermic process consumes heat energy so the warmer the air is, the more evapotranspiration takes place, making green roofs useful even in the hottest climates (MacIvor et al., 2016). The process of cooling air temperatures depletes vegetation of water resources meaning irrigation is sometimes necessary to prevent plant deaths; however, succulent (the most commonly used being sedum) covered extensive green roofs are efficient types of green roofs requiring no additional irrigation (Solcerova et al., 2013; MacIvor et al., 2016; Price et al., 2011). In addition to plant type, the substrate those plants live in can impact cooling. In Toronto, ON researchers compared the cooling effects of a succulent, sedum, and a mix of grasses and wildflowers and found that sedum had the greatest cooling effect when paired with organic substrate. When comparing all of the vegetation-substrate combinations, sedum cooled 7º C (44.6º F) more than the worst combination which was inorganic substrate with a grass and wildflower mix (MacIvor et al., 2016). Due to green roofs decreasing air temperatures, less cooling system use is required, reducing energy expenses. During winter months, green roofs also exhibit insulator properties that result in reduced indoor heating system use while furthering energy conservation (Squier & Davidson, 2016; Bell et al., 2016; Tam et al., 2016).

Given their proven effectiveness in addressing energy consumption and lowering ambient temperatures, it raises the question: why aren’t green roofs adopted more commonly? There are two major concerns voiced by those hesitant to adopt green roofs, namely concerns over water pollution and cost. Some environmentalists are concerned green roofs will contribute to the over-nutrification of surrounding aquatic ecosystems as unabsorbed water is routed from the cities into local ecosystems. In fact, there is a slight increase in nitrogen content within stormwater runoff from green roofs, but studies across various locations in southern Sweden show very similar concentrations of nitrogen runoff between green roofs and regular roof tops (Bengtsson et al., 2006, p. 61). Newly installed green roofs also leak a significant amount of phosphorus into their surroundings. However, over the first four years, the phosphorus leakage decreases by 80% (Köhler et al., 2002, p. 384). Furthermore, the concentrations of heavy metals in the stormwater runoff from green roofs correspond to reasonably polluted natural water (Berndtsson, Bengtsson, & Emilsson, 2005, p. 61). Application of fertilizer in the green roof soil is the primary cause of this nutrification and it could be reduced by utilizing better drainage and different fertilizers (Carpenter et al., 2016). Overall, the concentrations of pollutants are not a cause for major concern, especially when considering the ability of green roofs to notably decrease the amount of stormwater runoff in turn decreasing total amount of nutrification.

Due to the water-storage capacity of the substrate layer, green roofs – as compared to regular roofs- are significantly more effective at stormwater runoff reduction; green roofs can reduce runoff percentages by a degree of 45-75% (Mentes, Raes, & Hermy, 2006, p. 220). This reduction in stormwater runoff is not only beneficial because it minimizes excess nutrification, but also because it reduces heating of nearby aquatic ecosystems. The UHI effect causes stormwater runoff from traditional roofing to be too warm, creating a threat to nearby aquatic ecosystems that can be sensitive to slight temperature changes (McClure, 2012). Since green roofs provide a cooling effect, the water runoff from them is cooler than that from traditional roofing. Researchers estimate that effects related to stormwater runoff increase the cost of water treatment by half in nearly one in three cities globally, (McDonald, Weber, Padowski, Boucher, & Shemie, 2016, p. 9117) so through reducing water runoff, green roofs also save cities money. Overall, green roofs are more beneficial than detrimental to ecologically.

The next question to address is that of cost. Although green roofs are not currently cost effective for private homeowners, multifamily and commercial building green roofs are cost effective and pay back their cost within approximately five years (Blackhurst et al., 2010; Bianchini & Hewage, 2014; Mahdiyar et al. 2016). A useful smaller-scale metaphor is the LED light bulb. Even though LED light bulbs are more expensive than traditional bulbs, they are very cost effective over a long period of time. LED light bulbs are proven to pay back their price by lasting longer and saving on energy. Green roofs achieve the same effect, and with a little help from the government, they could be growing just as fast as LED bulbs.

In order to offset the initial costs of green roof installation, a combination of state and federal laws and tax incentives are needed. Several cities have already implemented successful laws and tax incentives. For example, San Francisco was the greenest city in North America of 2016 due to its various green laws (Pacific Union, 2016). San Francisco requires new buildings to devote at least 30% of roof space to green roofs and/or solar panels (Green Roofs for Healthy Cities [GRHC], 2016). While San Francisco was the greenest city in 2016, Washington, D.C and Chicago were the number one and number two cities respectively for number of installed green roofs in the year 2013 (Erlichman & Peck, 2014). Chicago offers up to 50% (up to $100,000) of any green roof’s developmental cost as long as it covers 50% or more of roof space. Meanwhile, Washington, D.C. offers from $7 up to $10 per square foot of green roof that people build. Washington’s policy of offering a certain amount of money per square foot (sq/ft) of green roof is more specific than a certain percentage of roof space, so it is preferable. We propose local laws offering multi-family and business owners at least $10 per sq/ft of green roof, varying with the complexity of the green roof. However, in order to obtain this incentive, builders are also required to cover at least 50% of their roof space with green roof. In addition to these local laws, we also propose a federal tax incentive like that of the Energy Policy Act of 2005 that credits builders up to $1.80 per sq/ft as a further incentive for people in cities to build green roofs (Plant Connection, 2017, para. 1).  

Green roofs are a costly infrastructure to install, but as a long-term investment they are cost effective. Local and federal governments will not mind paying tax incentives or passing laws for green roof development because of all of the benefits they impart. A +1 º F difference between city and rural temperatures can cause twice the amount of fatalities among the elderly in cities (Laaidi, 2012). One excess death is enough to cause worry, so this mass scale increase in mortality at higher temperatures is valid reason for the government to get involved in solving the issues of urbanization. Green roofs provide solutions to the costly effects of traditional building materials used in urban expansion by decreasing quantity of water runoff, energy consumption, and the dangerous UHI effect. All of these benefits, combined with tax incentives to diminish costs, are convincing cause for multifamily homeowners and businesses to start implementing green roofs.


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