Green Roofs in the US

By Bradley Frederick

The importance of improving the cost benefits for low impact development practices continues to grow with the urbanization of the world. It is projected 68% of the world’s population and just under 90% of Americans will live in urban environments by 2050 (United Nations, 2018). As urban areas grow, impervious areas grow as well which leads to increases in the urban heat island effect, more frequent combined sewer overflows, and lower air quality.

Green roofs, also known as living roofs, are one example of low impact development projects that can alleviate the stresses urbanization places on the environment. They are a viable option for limited space, unlike rain gardens, bioswales, and bioretention ponds. There are three types of green roofs: extensive, semi-intensive, and intensive. Extensive green roofs are the least expensive and have the lowest soil depth whereas intensive are the most expensive and have the deepest soil layer of the three. Extensive roofs can typically be installed onto existing structures since the extra load is not great enough to cause a structural failure; however, installing an intensive green roof on an existing structure usually requires renovations to increase the load capacity for that building. Green roofs are typically made up of 3 to 24 or more inches of soil and have a minimum unit load of six pounds per cubic foot when saturated with water (Köhler et al. 2001). Despite the increase in cost, deeper soil on green roofs allows for greater biodiversity in the plants and insects found (Madre et al. 2013).

While green roofs are a promising technology for reintroducing the natural environment into urban settings, the cost of installing a green roof can add up to $40 per square foot to the cost of a roof in the United States (Berghage et al. 2009). In Europe, green roofs cost the same and sometimes less than a traditional roof due to government subsidies and the widespread practice of choosing green over gray infrastructure (Berghage et al. 2009). Even though there is an initial higher installation cost in the United States, there can be direct savings seen for in heating and cooling and indirect savings seen in changes with the drainage and air quality. The city of Toronto conducted a study which found by greening 75% of the roofs in the city, there would be an initial cost savings of over $300 million with annual savings of over $37 million from reduced costs for energy, combined sewer overflow, air quality, and the urban heat island effect (Banting et al. 2005).

A green roof has an increased reflectivity of 22.2% over just soil and more than 200% over a conventional roof (Maclvor et al. 2011). The albedo of a green roof, which is the major cause for lowering surface temperature, has a direct relationship with biomass and biomass variability (Lundholm et al. 2010). Getter et al. (2008) conducted a study on the influence of substrate depth for a plant’s absolute cover, the point at which a plant has grown to its maximum size, and found depth directly influences absolute cover. For green roofs with a depth of 1.5 inches, six of the twelve species tested did not experience significant growth throughout the season, whereas only two of the twelve species did not experience significant growth for the 2.75 inch and 4 inch substrate depths (Getter et al. 2008). While the plants behaved similarly on the 2.7 inch and 4 inch substrate depths for the first growing season, the plants in 4 inch substrate depth experienced greater growth in the subsequent seasons than the plants 2.75 inch substrate depth (Getter et al. 2008). Choosing the correct plant species for the corresponding depth cannot only increase a roof’s albedo but also increases a roof’s retention capacity.

As seen from other research groups, rainwater can be retained on a green roof varies from 50 to 70% annually (Hutchinson et al., 2003; Köhler et al., 2001). Depending on the climate, this variation can range from less than 20% in cooler months to over 95% during warmer months (Berghage et al. 2009; Köhler et al. 2001). For tropical and sub-tropical climates green roof performance during the summer is equivalent to its performance in the winter since the plants are alive year-round (Voyde et al. 2010). While there are data supporting the extremity of seasonal variation for green roof performance, further research needs to be done on the effect of plant maturity. In a study focused on green roof slope on runoff delay, Getter et al. (2007) theorized plant maturity could alter the hydraulic conductivity of the growth medium and would therefore be responsible for the differences in the results found between multiple studies on roof slopes. Getter et al. (2007) also found green roofs with mature plants had a greater porosity, volume of air, organic matter, and retention capacity. This directly contradicts a study conducted by Mentens et al. (2005) which did not find a noticeable change in retention capacity based on plant maturity and instead stated the green roof’s retention depends on climate conditions and substrate depth. The above research shows that while there are benefits to green roofs, more data will be needed to adequately model the extremity of these benefits.


Sources: 

  • Banting, D., Doshi, H., Li, J., Missios, P., Au, A., Currie, B. A., Verrati, M. (2005) Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto.
  • Berghage, R. D., Beattie, D., Jarrett, A. R., Thuring, C., Razaei, F. (2009) Green Roofs for Stormwater Runoff Control.
  • Getter, K. L., Rowe, D. B., Andresen, J. A. (2007) Quantifying the effect of slope on extensive green roof stormwater retention. Ecological Engineering 31, 225-231.
  • Getter, K. L., Rowe, D. B. (2008) Media depth influences Sedum green roof establishment. Urban Ecosystems 31, 361-372. doi:10.1007/s11252-008-0052-0
  • Hutchinson, D., Abrams, P., Retzlaff, R., Liptan, T. (2003) Stormwater monitoring two ecoroofs in Portland, Oregon, USA. Greening Rooftops for Sustainable Communities.
  • Köhler, M., Schmidt, M., Grimme, F. W., Laar, M., and Gusmão, F. (2001) Urban Water Retention by Greened Roofs in Temperature and Tropical Climate. Technology Resource Management and Development – Scientific Contributions for Sustainable Development 2, 151 – 162.
  • Lundholm, J., Maclvor, J. S., MacHougall, Z., Ranalli, M. (2010) Plant Species and Functional Group Cominations Affect Green Roof Ecosystem Functions. PloS ONE 5(3) e9677. dio:10.1371/journal.pone.0009677
  • Maclvor, J. S., Lundholm, J. (2011) Performance evaluation of native plants suited to extensive green roof conditions in a maritime climate. Ecological Engineering 37, 407-417.
  • Madre, F., Vergnes, A., Machon, N., Clergeau, P. (2013) A comparison of 3 types of green roof as habitats for arthropods. Ecological Engineering 57, 109-117.
  • Mentens, J., Raes, D., Hermy, M. (2005) Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscape and Urban Planning 77, 217-226.
  • United Nations, Department of Economic and Social Affairs, Population Division (2018). World Urbanization Prospects: The 2018 Revision.
  • Voyde, E., Fassman, E., Simcock, R. (2010). Hydrology of an extensive living roof under sub-tropical conditions in Auckland, New Zealand. Journal of Hydrology 394, 384-395.

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