The Use of Green Roofs in Mitigating the Urban Heat Island Effect

What is a Green Roof?

While most roofs are dark and barren places lacking life, green roofs act as living ecosystems made up of various layers. These layers include waterproofing, a root repellent system, drainage system, filter cloth, a lightweight growing medium, and of course, plants (About green roofs). Green roofs can be simple, yet hearty expanses of ground cover (extensive green roof), or they may be more complex and include entire gardens and parks (intensive green roof). While there are numerous benefits and possibilities surrounding the use of green roofs, the mere presence of soil and vegetation on an otherwise lifeless surface can make all the difference in regulating nearby temperatures (Green and cool roofs, 2021).

How Green Roofs Can Help

Roofs make up 20-25% of urban surfaces (Yang et al., 2018). Depending on the size of a city, this is a huge amount of unused surface area, most of which is nothing more than desolate, impermeable surface. In New York City, for instance, it is estimated that roof space makes up approximately 1 billion square feet (NPR, 2017). Beyond just wasted space, roofs also absorb solar radiation (ie. have low albedo) leading to the heating of surrounding air, and ultimately, the creation of Urban Heat Islands.

With the implementation of green roofs, however, this heat effect is mitigated through evapotranspiration and the reflection of solar radiation. It is estimated that if all the roof space in New York City were converted to green roofs, the overall temperature of the city would decrease by 1 degree Fahrenheit (NPR, 2017). The United States Environmental Protection Agency, however, estimates that this effect can lead to overall urban temperature reductions of up to 5 degrees F and that green roofs can be 30-40 degrees F lower than conventional roofs (EPA, Using Green Roofs to Reduce Heat Islands).

The presence of soil and vegetation also act as a natural insulation (Green and cool roofs, 2021), thereby reducing the need for air conditioning (which, depending on the energy source, can lead to increased CO2 emissions and consequential increased surface temperatures, thereby creating a positive feedback loop of warming). Researchers have found that green roofs on their own can reduce indoor temperatures by 1.5 to 3 degrees Celsius (or approximately 2.7 to 5.4 degrees Fahrenheit) (Marvuglia et al., 2020), thereby reducing building energy use by 0.7% (EPA, Using Green Roofs to Reduce Heat Islands). When modeling the potential impacts this sort of temperature reduction can have amidst heatwaves, the results are dramatic. For vulnerable communities such as the elderly population, green roofs are estimated to reduce mortality by 50 to 80% (respective to the estimated 1.5 to 3 degrees C temperature reduction) (Marvuglia et al., 2020).

With the presence of climate change and consequential increased prevalence of heat waves (Levy & Patz, 2015), mitigation of Urban Heat Islands is essential in order to lower heat-related morbidity and mortality.

Implementation - Take Portland, OR as an example!

• Ecoroof Incentive from 2008-2012:

  • Constructive incentive of $5/square foot

  • Environmental Services granted nearly $2 million in incentives

  • Helped fund more than 130 projects (8 acres of ecoroofs!)

  • Initiated to manage stormwater, which managed an annual average of 4.4 million gallons of stormwater

• Portland Central City 2035 Plan (in effect as of July 2018):

  • "Ecoroof Requirement"

  • Includes buildings in the Central City over net 20,000 square feet (not rooftop parking)

  • Requires 100% of the roof space to be vegetated (except machinery)

Source: Morris, 2018

Examples beyond Oregon

• Denver, CO passed a Green Roof Ordinance in 2017 mandating green roofs on new & existing buildings (Green Roofs for Healthy Cities, 2019, p. 11)

• Toronto, Canada's Eco-Roof Incentive Program offers $100 CAD per square meter of green roof (Green Roofs for Healthy Cities, 2019, p. 7)

• Washington D.C. offers $15 USD per square foot, targeting watersheds where overflow of combined sewers are common (Green Roofs for Healthy Cities, 2019, p. 7)

• Stuttgart, Germany and Copenhagen, Denmark require green roofs on most new construction (Stutz, 2010)

Considerations & Concerns in Going Forward

• What neighborhoods will actually get these green roofs? Implementation needs to first be focused on the neighborhoods who historically have the least green space and bear the greatest burden of the urban heat island effect

  • Focus on BIPOC and low-income communities

  • Government funding for green roofs to lessen the burden of installation

• Greenwashing

  • Green roofs should not be a mere afterthought concerning nothing beyond aesthetics

  • Can be used to make large structures (e.g. apartment complex) look environmentally-friendly when in reality the roof is the only "green" aspect of the structure

• Who maintains the gardens? This is a great opportunity for employment, but if maintenance is neglected, then the green roof may deteriorate (depending on its level of self-sufficiency)

  • Especially true for the more complex intensive green roofs

• Cost of implementation: green roofs can be expensive to install, with regard to both material and labor costs

  • This may not be in the budget for some buildings

  • A great opportunity for government incentives/funding to make a large impact

• How sustainable are the green roof top materials (waterproofing, irrigation, draining materials, etc.)?

  • Studies show green roof implementation is still a net positive (Bianchini & Hewege, 2012), but more sustainable materials could improve this trade off