Green Roofs in Cities Across the United States

Imagine a city where rooftops are not just a lifeless sea of concrete, but filled with sprawling gardens and thriving ecosystems. Imagine a city with cleaner air, cooler temperatures, and more quiet and secluded places you can go to relax and destress. This is the reality of cities with green roofs. Green roofs, also known as living roofs, are simply roofs that are covered with plants. Green roofs have started popping up around cities nationally.

Simply put, a green roof is a layer of vegetation installed on top of a roof, either flat or slightly sloped  _{(Evan, 2017).}  More specifically, they are composed of engineered soil planted with drought-tolerant vegetation and are underlain by a drainage layer. The green roof substrate functions as a retention/detention basin, allowing for the eventual slow release of infiltrated rainfall to the rooftop drain, while the vegetation layer can affect rainfall infiltration and green roof runoff through  interception  and plant  transpiration , respectively.  _{(Hakimdavar, 2016).}  

The Hanging Gardens of Babylon were the first recorded green roofs, appearing 4,000 years ago in ancient Greek, Roman, and Assyrian texts (Smalls, N.D.). According to William Delong, the gardens helped keep people and buildings cool. Although there remains no physical evidence of these gardens, they are believed to have been located in modern-day Mosul, Iraq (DeLong, 2022). Vikings in the Norse era also used green roofs to survive harsh weather and insulate buildings from the cold. The oldest existing green roof today is the Guinigi Tower in Tuscany, Italy. Although it has been replanted, this green roof has been in use since the 14th or 15th century (DeLong, 2022).

In areas with a large number of impervious surfaces, stormwater runoff causes a large number of pollutants to enter local waterways. Green roofs reduce the amount of stormwater runoff from overwhelming sewers by absorbing rainfall that would have entered the sewer systems (EPA, 2018).  The excess stormwater absorbed by a green roof could have cause the sewer to overflow into local streams and lakes, carrying pollutants from the sewage system to those ecosystems. A study done by the GSA found that green roofs reduce the rate of runoff by up to 65% (GSA, 13).

Green roofs moderate building temperatures in areas with limited vegetation, specifically cities. Decreasing the temperatures of the air and surface reduces the amount of energy needed to cool buildings. This results in reduced demand for electricity, heating, and cooling, as well as fewer greenhouse emissions used for heating and cooling buildings. Additionally, the vegetation on the roof’s surface can absorb the existing ozone layer near the rooftop’s surface through the plants’ stomata, as well as some carbon dioxide emissions (EPA, 2018, 5).

Green roofs allow for a variety of plants and invertebrates to thrive in an urban environment. Succulent species such as sedum, delosperma, euphorbia, and sempervivum are commonly seen on green roofs because of their ability to store water and limit transpiration (Getter, 2015). Green roofs also provide a habitat for multiple species of birds including but not limited to herring gulls, barn swallows, and northern mockingbirds. Green roofs also provide temporary habitats for migrating birds (Green Roofs for Healthy Cities).

The 'Heat Island Effect' is the heat absorbed by buildings, which causes cities to have higher temperatures than surrounding rural areas (EPA, 2018, 3). Green Roofs help reduce the 'Heat Island Effect' and simultaneously drop temperatures within cities. A simulation of Toronto with 50% green-roof coverage evenly distributed throughout the city showed temperature reductions of as much as 2 °C  (Oberndorfer et al., 2007). Additionally, Green roofs shade buildings and increase evapotranspiration which shifts the roof’s energy balance and budget (EPA, 2018). The lower temperatures resulting from green roofs reduce the energy needed to cool buildings, which reduces energy costs in the long run. NYC Morgan Facility’s green roof is predicted to reduce the building’s energy cost by $30,000 a year (Stutz, 2010).

As mentioned before, the reduction of precipitation runoff due to Green Roof vegetation helps prevent sewer overflow, reducing maintenance costs of sewage systems. According to the EPA, Green Roofs have prevented the discharge of millions of gallons of sewage water from entering local waterways (EPA, 2021, 1), thus decreasing the amount of money needed to be spent on system maintenance as well as environmental restoration.

Green roofs provide multiple physical and psychological benefits to individuals exposed to them. The physical health benefits of green roofs are glaring. The mean temperature of green roof areas in the heat of the day (between 12:30 and 4:30 pm) was up to 31% cooler than other roof types. (Taylor, 2007). Johnston–Chicago’s commissioner of the environment says these green roofs help avert heat wave–related deaths, citing how these studies show lower temperatures on green roofs compared with traditional roofs, and reduced air-conditioning use in buildings with green roofs. Moreover, there is a drastic improvement in air quality from the decreased demand for energy to cool buildings. Also, fewer pollutants are entering local waterways from excess stormwater runoff in urban areas. Additionally, green roofs absorb sound waves better than the hard surfaces of conventional roofs because the soil particles and vegetation that make up green roofs allow sound waves to interact with them more (Rowe, 2011). This noise reduction can help avoid future health problems like hearing impairment, hypertension, and ischemic heart disease and improve sleep and school performance.

The psychological benefits are just as glaring. A study led by Lee demonstrated that viewing a green roof for only 40 seconds can lead to increased attention and less moment-to-moment variability while conducting tasks, as well as a decrease in omission errors (Lee et al., 2015). Additionally, others studies indicate that the environments created by green roofs can help induce positive emotions in visitors or direct their attention away from fatigue and reduce mental exhaustion (Williams et al., 2019; Shepherd et al., 2013).

Implementing green roofs throughout urban areas can be very expensive. There are forty million acres of rooftop space in New York City alone- with a cost of $10-$30 per square foot this project can become very expensive. While it is unreasonable to think that every building in the city will install a green roof, it is still millions of dollars worth of work. However, green roofs ultimately reduce costs associated with heating and cooling buildings. In the long run, the initial investment for the green roof will prove to be beneficial.

This graph shows how Washington D.C. can save $2 billion with smart surface strategies, such as cool roofs, green roofs, solar PV, and porous pavements while enhancing health and livability, as well as cutting peak summer temperatures (Katz, 2016).

Green roof construction has disproportionately benefited wealthy communities. The high costs associated with their maintenance have the potential to displace marginalized communities (Schrieke et al., 2021). Studies done examining the distribution of green roofs in New York City found that most were not placed in heat-vulnerable areas, which also tends to coincide with disadvantaged and low-income communities (Schrieke et al., 2021; Treglia et al., 2022). However, the graphic below demonstrates how expanding green roof access with regard to areas most vulnerable to climate change could provide greater ecological and economic relief to marginalized communities.

Furthermore, Schrieke et al. suggest that the costs associated with green roof construction and maintenance could be offset by the acceptance of "weedy"-perceived species (Schrieke et al., 2021). The growth of spontaneous species on green roofs is often viewed as a failure of the original greenspace but allowing this type of coverage on green roofs has the potential to lower construction and input costs, as well as strengthen the resilience of urban communities (Schrieke et al., 2021). Many spontaneous species do not need irrigation or fertilizer to grow, reducing the need for maintenance costs or labor, and are able to thrive in a shallow substrate, lowering initial construction costs in very hot and dry climates where deep substrates are typically necessary (Schrieke et al., 2021). This type of vegetation overall has the potential to limit gentrification as it "may apply less pressure to property values and housing costs" than standard green roofs (Schrieke et al., 2021).

Green roofs can help with many of the environmental, economic, and social challenges facing cities today. They provide a multitude of benefits, including decreasing stormwater runoff, temperatures, and maintenance costs of cities and increasing biodiversity and the health of inhabitants. With an increasing need for sustainable and resilient cities, green roofs could be a critical component toward achieving these goals. Here are just some of the ways green roofs can improve cities across the US right now.

In dense cities with heat-vulnerable areas, such as NYC, New Orleans, and San Francisco, broadening green roof distribution could combat the heat island effect (Treglia et al, 2022; Climate Central, 2021).

In urban areas situated near water systems most vulnerable to pollution or at risk of overwhelming sewer systems, such as NYC and Baltimore, green roofs could help mitigate sewage water runoff by absorbing the stormwater that overwhelms sewers (Chaisson, 2017; City of Baltimore, 2018; Twohig et al., 2022).

Green roofs in urban areas prone to flooding, including many cities in Louisiana and Florida, could similarly benefit from green roof installation because of the capacity for stormwater absorption (Buxton, 2021; Twohig et al., 2022).

Green roofs are a desirable way for communities to become more sustainable. They improve human health and have measured effects toward reducing pollution and increasing biodiversity. Increasing efforts to install more green roofs across cities would positively affect the environment, economy, and community development in urban areas.

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Buxton, R. 2021. “These Are the 20 U.S. Cities at Greatest Risk For Flooding in the Next 30 Years.” Katie Couric Media. 

California, State of. “Green Roofs and Rooftop Gardens.” CalRecycle Home Page, https://calrecycle.ca.gov/organics/compostmulch/toolbox/greenroofs/.

Chaisson, C. 2017. “When It Rains, It Pours Raw Sewage into New York City’s Waterways.” National Resources Defense Council.

Climate Central. 2021. “Urban Heat Islands.” Climate Central, Climate Matters.  https://www.climatecentral.org/climate-matters/urban-heat-islands 

Department of Planning, City of Baltimore. 2018. “Stormwater RunOff/Non-Point Pollution Prevention,” City of Baltimore Website. 

DeLong, William. 2022. “Were The Hanging Gardens Of Babylon Real Or Only A Myth?” All That’s Interesting. https://allthatsinteresting.com/hanging-gardens-of-babylon

Evan. “Green Roofs: An Analysis on Air Pollution Removal and Policy Implementation.” Debating Science, 4 Dec. 2017, https://blogs.umass.edu/natsci397a-eross/green-roofs-an-analysis-on-air-pollution-removal-and-policy-implementation/. 

Getter, Kristin L. “Selecting Plants for Extensive Green Roofs in the United States (E3047).” MSU Extension, 19 Oct. 2015, https://www.canr.msu.edu/resources/selecting_plants_for_extensive_green_roofs_in_the_united_states_e3047#:~:text=Succulent%20plants%20are%20well%2Dadapted,and%20Sempervivum%20are%20popular%20choices. 

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Katz, Greg. (2016). Achieving Urban Resilience: Washington DC.  https://coolrooftoolkit.org/knowledgebase/achieving-urban-resilience-washington-dc/ 

Lee, K. E., Williams, K. J. H., Sargent, L. D., Williams, N. S. G., & Johnson, K. A. (2015). 40-second green roof views sustain attention: The role of micro-breaks in attention restoration. Journal of Environmental Psychology, 42, 182–189. https://doi.org/10.1016/j.jenvp.2015.04.003 

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Rezaei, M., Emmanuel, N., Kim, J., & Kim, D. H. (2021). Analyzing the impact of green roof functions on the citizens' mental health in metropolitan cities. Iranian Journal of Public Health. https://doi.org/10.18502/ijph.v50i5.6107  

Rosenbluth, J. (2021, November 30). Green roofs in the time of COVID-19. National League of Cities. Retrieved January 10, 2023, from https://www.nlc.org/article/2021/11/30/green-roofs-in-the-time-of-covid-19/ 

Rowe, D. B. (2011). Green roofs as a means of pollution abatement. Environmental Pollution, 159(8-9), 2100–2110. https://doi.org/10.1016/j.envpol.2010.10.029 

Schrieke, Dean, Joel Lönnqvist, Godecke-Tobias Blecken, Nicholas S. G. Williams, and Claire Farrell. “Socio-Ecological Dimensions of Spontaneous Plants on Green Roofs.” Frontiers in Sustainable Cities 3 (2021).  https://doi.org/10.3389/frsc.2021.777128 .

Shepherd, D., Welch, D., Dirks, K., & McBride, D. (2013). Do quiet areas afford greater health-related quality of life than noisy areas? International Journal of Environmental Research and Public Health, 10(4), 1284–1303. https://doi.org/10.3390/ijerph10041284 

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Treglia, M. L., T. McPhearson, E. W. Sanderson, G. Yetman, and E. Nobel Maxwell. 2022. “Examining the distribution of green roofs in New York City through a lens of social, ecological, and technological filters.” Ecology and Society 27(3):20. https://doi.org/10.5751/ES-13303-270320

Twohig, C., Casali, Y., Aydin, N.Y. 2022. “Can green roofs help with stormwater floods? A geospatial planning approach.” Urban Forestry & Urban Greening, Volume 76, ISSN 1618-8667. https://doi.org/10.1016/j.ufug.2022.127724.

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Williams, K. J. H., Lee, K. E., Sargent, L., Johnson, K. A., Rayner, J., Farrell, C., Miller, R. E., & Williams, N. S. G. (2019). Appraising the psychological benefits of green roofs for city residents and workers. Urban Forestry & Urban Greening, 44, 126399. https://doi.org/10.1016/j.ufug.2019.126399