Green Infrastructure benefits of green roofs

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Vegetation on the Target Center Arena green roof. vegetation consisted of a pregrown Sedum mat supplemented with 22 species of plugs and 16 species of seed native to Minnesota’s bedrock bluff prairies. Image Courtesy of The Kestrel Design Group, Inc.

Green roofs occur at the beginning of treatment trains. Green roofs provide filtering of suspended solids and pollutants associated with those solids, although total suspended solid (TSS) concentrations from traditional roofs are generally low. Green roofs provide both volume and rate control, thus decreasing the stormwater volume being delivered to downstream best management practices (BMPs).

Green infrastructure and multiple benefits

Green infrastructure (GI) encompasses a wide array of practices, including stormwater management. Green stormwater infrastructure (GSI) encompasses a variety of practices primarily designed for managing stormwater runoff but that provide additional benefits such as habitat or aesthetic value.

There is no universal definition of GI or GSI (link here fore more information). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services.

Benefit	Effectiveness	Notes
Water quality	◔	Minimal wtaer quality benefits due to low pollutant concentrations. Likely to leach phosphorus during first part of lifetime.
Water quantity/supply	◕	Provides rate control (detention) and volume removal (retention) through evapotranspiration.
Energy savings	◕
Climate resiliency	◑
Air quality	◑
Habitat improvement	◑
Community livability	◔	Aesthetically pleasing but limited from public view.
Health benefits	◔
Economic savings	◑
Macroscale benefits	◔	Benefits are at microscale because of limited spatial extent of green roofs.
Level of benefit: ◯ - none; ◔; - small; ◑ - moderate; ◕ - large; ● - very high

Green Infrastructure benefits of green roofs

Because of their use of vegetation in conjunction with building design, green roofs provide multiple green infrastructure benefits.

• Water quality: Green roofs provide stormwater treatment benefits, but because pollutant concentrations are generally low, these benefits are limited. Pollutant removal mechanisms include filtering, evaporation, transpiration, biological and microbiological uptake, and soil adsorption.

Green roofs employ engineered media that is effective at removing solids, most metals, and most organic chemicals. Green roofs are generally not effective at retaining phosphorus because of the organic matter content in the media. They therefore are likely to lose phosphorus during the first years after establishment, but may gradually retain phosphorus over time.

• Water quantity and hydrology: Green roofs are effective at detaining and retaining water and provide excellent rate control, although on a small scale. The ability of a green roof to detain and retain water is a function of both the media thickness and the sorptive properties of the media.
• Climate resiliency: Green roofs provide multiple climate resiliency benefits (McCarthy and Sanchez, 2019; Santamouris, 2014)
  • They replace dark surfaces with vegetation that reflects rather than absorbs sunlight
  • Evaporation provides a cooling effect
  • For people with access to green roofs, plants can also provide shaded relief on sunny days
  • Because of the insulating value of vegetation, green roofs cool and improve heat retention during colder months. As a result, green roofs can significantly lower greenhouse gas emissions from a building.
  • Through rate control, green roofs can provide some flood mitigation, though this benefit is small
  • Vegetation on green roofs sequesters carbon, but the magnitude of this depends on the vegetation, including species and diversity, and the substrate (Whittinghill et al., 2014; Getter et al., 2009; Shafiquea et al., 2020). Deeper substrate and more complex plant communities increase sequestration over time, though initially sequestration may be slow.
• Habitat improvement:
• Community livability: Green roofs are aesthetically pleasing, though they are typically not visible to the general public. A variety of vegetation can also be used, including perennial plants, shrubs, and trees.
• Health benefits:
• Economic savings:
  • Because of the insulating value of vegetation, green roofs cool and improve heat retention during colder months. As a result, green roofs can significantly lower energy costs for a building.

Design considerations to maximize multiple benefits of green roofs

Maximizing specific green infrastructure (GI) benefits of green roofs requires design considerations prior to constructing the practice. While site limitations cannot always be overcome, the following recommendations maximize the GI benefit of green roofs. An important design consideration for many green roof benefits is vegetation. For more information, see Plant lists for green roofs.

• Water quality
  • Because of low pollutant concentrations, green roofs have limited impact on reducing pollutant loads in stormwater. However, the engineered media for green roofs may leach phosphorus. Low organic matter media, media that does not leach phosphorus (e.g. peat), or amendments (e.g. iron filings) may minimize or eliminate phosphorus losses from green roofs.
• Water quantity/supply (Bollman et al., 2019)
  • Increase media depth to extent economically feasible
  • Maximize the sorptive and retention properties of the media. Increasing the organic fraction can increase water retention but may result in phosphorus export and other concerns if drying occurs. Additives such as perlite and pumice increase water retention. See Bollman et al., 2019)
• Climate resiliency (Dvorak, 2021; Shafique et al., 2020)
  • Select a variety of plants adapted to different microclimates. This increases likelihood that plants will remain established in situations where the climate or microclimate changes. Include at least 15-20 taxa of plants that are native to a variety of microclimate conditions, including a mix of annuals and perennials. Leaf area index is a good indicator of sequestration potential.
  • Increase plant diversity, growing a mix of types (e.g. perennials, annuals, bulbs, etc.) and forms (e.g. height, coverage). Include prostrate and upright forms of plants to allow vegetation to compete for its preferred niche on a green roof.
  • Utilize nature-based approaches similar to the ecoregion in which the green roof exists
  • Incorporate grasses into the landscape as they are deep-rooting and drought tolerant
  • Select vegetation that has high root biomass
  • Use light-colored mulch to avoid excessive heat gain to the substrate and help retain moisture in the substrate
  • If irrigation is used, moisture sensors should be included in zones to help conserve water and prevent overwatering of the substrate.
• Habitat (Dvorak, 2021)
  • Incorporate annuals into plant selection. Many annuals are important pollinators that can self-sow into bare areas. Since many annuals complete their life-cycle during one season (spring, or summer), it may be necessary to include several species of annuals to have plants actively growing in multiple seasons. Examples include bluebonnets (Lupinus texensis), Indian paintbrush (Castilleja), Firewheel (Gaulliardia pulchella), and black-eyed Susans (Rudbeckia hirta).
• Community livability
• Health benefits
• Economic benefits (Sailor et al., 2011)
  • Increasing media depth and use of light-colored reflective material in areas lacking vegetation results in greater energy savings

References

• Bollman, M.A., G.E. DeSantis, R.M. DuChanois, M. Etten-Bohm, D.M. Olszyk, J.G. Lambrinos, P.M. Mayera. 2019. A framework for optimizing hydrologic performance of green roof media. Ecol Eng. 140:1–105589. doi: 10.1016/j.ecoleng.2019.105589.
• Dvorak, B. 2021. Designing and Planting Climate Resilient Green Roofs. IN Living Architecture Monitor, accessed September 13, 2022.
• Getter, K.L., D. Bradley Rowe, G. Philip Robertson, Bert M. Cregg, and Jeffrey A. Andresen. 2009. Carbon Sequestration Potential of Extensive Green Roofs. Environ. Sci. Technol. 43:19:7564–7570. https://doi.org/10.1021/es901539x.
• McCarthy, J. and E. Sánchez. 2019. 6 Ways Green Roofs Protect Cities From Climate Change. Global Citizen website, accessed September 12, 2022.
• Sailor, D. T.B. Elley, and M.Gibson. 2011. Exploring the building energy impacts of green roof design decisions – a modeling study of buildings in four distinct climates. Journal of Building Physics. Volume 35, Issue 4. https://doi.org/10.1177/1744259111420076.
• Santamouris, M. 2014. Cooling the cities – A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Solar Energy. Volume 103:682-703.
• Shafique, M., X. Xuec, and X. Luoab. 2020. An overview of carbon sequestration of green roofs in urban areas. Urban Forestry & Urban Greening Volume 47.
• Whittinghill, L.J., D. Bradley Rowe, R. Schutzkic, and B.M. Cregg. 2014. Quantifying carbon sequestration of various green roof and ornamental landscape systems. Landscape and Urban Planning Volume 123:41-48. https://doi.org/10.1016/j.landurbplan.2013.11.015.