Flat roofs represent a significant proportion of urban areas and perform a variety of functions, each of which is responsible for a corresponding variety of effects on the urban environment and its infrastructure. One of the most important functions is the collection and discharge of rainwater, especially during storms that have substantial impacts on municipal sewer systems and receiving water bodies. Both green and blue roofs are effective ways to mitigate stormwater issues, while offering other benefits that depend on the characteristics of a building and its site.
Residence for the Sisters of St. Joseph, London, ON, is a 100-suite residence that was LEED Gold Certified in 2007. Pictured is the front entrance. Roof surfaces are connected to the rainwater harvesting system. The canopy and pavements drain to the infiltration swale.
Photo: Cornerstone Architecture, Inc.
Importance of Flat Roof Impacts
Roofs typically represent over 25% of an urban area in North America, a greater proportion than pavements.1 But while flat roofs make up much of cities’ roof areas, they are generally invisible to building occupants, who are consequently unaware of the roofs’ varied functions and their impacts on the physical and natural environment. Of these impacts, the rapid and uncontrolled discharge of stormwater runoff is among the most consequential, and the discharge only increases with storm intensity. Mitigating the effects of stormwater runoff has been identified as a priority for flat roofs based on the criteria established in both the LEED and Green Globes green building rating systems.
Flat roofs can reduce stormwater discharge through the implementation of either “green” vegetated systems or “blue” detention and harvesting systems. The stormwater performance of green roofs is well-established, although research shows this is dependent on local rainfall and evapotranspiration factors that are highly variable.2 In contrast, studies of blue roofs that incorporate rainwater harvesting systems show how these can be optimized to suit local conditions.3 Both approaches also have significant potential benefits beyond stormwater management. Green roofs reduce urban heat islands, provide habitat, and offer aesthetic benefits to building occupants and their neighbors in surrounding buildings. Blue roofs can replace potable water use for irrigation or toilet flushing and are more compatible with roof-mounted solar technologies than green roofs. Yet the benefits of green and blue roofs vary considerably depending on building characteristics, urban context, and geographic location. Here we offer a framework for assessing these benefits for any particular project, illustrated by three Canadian study locations with diverse climatic conditions.
Infiltration swale. The capacity accommodates most storm events, with an overflow to the on-site stormwater management system.
Photo: Cornerstone Architecture, Inc.
Green and Blue Roofs as ‘Enhanced’ Low Impact Development SWM Strategies
Reducing the volume of stormwater entering municipal sewer systems is particularly important for those jurisdictions with combined storm and sanitary mains. In these systems, excess stormwater overwhelms the system and causes untreated sewage to bypass treatment plants and be discharged directly into surface waters. For two of the locations selected in this study, London, ON4 and Halifax, NS,5 this stormwater discharge to surface water sources is a serious risk/problem for local water quality requirements. In the third location, Calgary, AB, combined systems have been eliminated, but its storm sewer infrastructure discharges stormwater directly, and untreated, into the Bow River. Although it contains no sewage, this water does collect debris and chemical residues from hard surfaces and conveys them to the river. As observed by Sedlack:
Although a properly functioning combined sewer will burp out a mixture of stormwater runoff and household waste a few times a year, a separate [storm] sewer conveys whatever is on the impervious surfaces of the city to urban waterways during every storm.6
The realization that stormwater outflows have serious consequences, even if not part of combined sewer systems, provides an important impetus for mitigation through a collection of techniques known as Low Impact Development Stormwater Management (LID SWM). The primary purpose of LID SWM is to mitigate the effects of stormwater runoff on municipal stormwater infrastructure by redirecting rainwater discharged from roofs and other impervious surfaces away from the municipal storm sewer system.7 Table 1 (refer to PDF) summarizes the most common LID SWM strategies, which can be used individually or in combination on any particular project.
Green and blue roofs are distinct from infiltration-based strategies because they retain rainwater on the roof or collect it in a cistern to be reused. These systems divert significant volumes of water to plant evapotranspiration or to offset municipal water use, before any overflow enters an on-site stormwater management system. Because both green and blue roof systems offer benefits beyond their primary purpose of stormwater retention, they can be considered “enhanced” LID SWM strategies. Importantly, as demonstrated below, rainwater harvesting systems are capable of offsetting all the potable water that would normally be used for flushing toilets or urinals in virtually every building type across a wide range of occupancy categories.
Comparing Green and Blue Roof Performance
In this research, we compared the performance of green and blue roofs across a range of building types using the 15 examples summarized in Table 2 (refer to PDF). Typical size and occupancy data for each example were taken from the U.S. EPA Data Trends database,8 with the exception of data for educational buildings that were taken from a review of schools by the National Renewable Energy Lab.9
To examine the effects of different climatic conditions on the performance of rainwater harvesting, each building type was modeled using rainfall data for London, ON, Calgary, AB, and Halifax, NS. London, ON is in a humid continental zone typical of eastern North America, having moderate winters with significant snowfall and warm summers. Calgary, AB is in a climate zone typical of northwestern North America, having cold winters with limited snowfall as well as cool dry summers. Halifax, NS is in a northeastern coastal climate zone with moderate winter and summer temperatures as well as significant precipitation throughout the year. Together, these locations provide a diverse range of climate conditions representative of Canada and the northern United States. Comparative data for green roofs were taken from an empirical study by Sims and others that measured performance of identical installations using 150 mm of growing media in the same three cities over two years, 2013 and 2014.10
Green roof at the Residence for the Sisters of St. Joseph. This is located on a projecting portion of the building where it is visible from the residential suites and common areas above.
Photo: Cornerstone Architecture, Inc.
The performance of rainwater harvesting systems in each location was modeled by constructing a spreadsheet to calculate water demand. The water demand was determined by the building size and occupancy data from the EPA8 and NREL,9 combined with statistics on the frequency of toilet or urinal use from the LEED rating system.11 The total water demand is divided by the volume of rainwater available according to monthly precipitation data from Environment and Climate Change Canada.12 This generates an annual percentage of rainwater harvested for each type of building in each location, which is compared to the average percentage retained by green roofs in each location,10 summarized in Table 3 (refer to PDF).
The London, Calgary, and Halifax results are presented in Figures 1, 2, and 3, respectively. Each figure compares the demand from fixtures with the supply from rainwater (in liters, y axis, left scale) for the 15 different building types (x axis). The percentage of rain retained by the harvesting system in each building type is represented by blue dots (right scale). This enables a comparison with the performance of a green roof from Sims et al10 as represented by a horizontal green line in the figures. All results are normalized per square meter (10 ft2) of building area.
These results illustrate the effectiveness of both green and blue roofs in retaining rainwater, particularly in urban locations with low to moderate precipitation like Calgary and London. The results also show that rainwater harvesting is more effective in buildings with higher and more constant occupancy, particularly those in the educational and institutional categories. The following discussion develops a rationale for considering these and other benefits of green and blue roofs as part of identifying the most appropriate opportunities for implementing them.
Why Use a Green Roof?
Beyond their stormwater mitigation and other benefits, an attribute commonly associated with green roofs is increased thermal resistance. However, as explained in ASHRAE Journal by Lstiburek,13 the modest insulating effect provided by a green roof can be replicated on a conventional roof with a small increase in the roof insulation itself, at an insignificant cost and weight of material compared to the growing medium required to support plants. Likewise, the contributions made by a green roof to reducing urban heating can also be accomplished by incorporating a high albedo roof membrane as part of a conventional roof, as recognized in both the LEED and Green Globes rating systems.
Given these considerations, combined with the substantial capital and maintenance costs associated with green roofs, it is remarkable that they have continued to increase in prevalence. While some of this could be attributed to effective marketing by green roof system manufacturers or enthusiastic endorsement by building designers, the increasing implementation of green roofs belies other powerful motivations that go beyond either hyperbole or the quantifiable biophysical factors discussed above. Advocates like Green Roofs for Healthy Cities identify the contributions green roofs make to biodiversity where this is otherwise limited in an urban setting, as well as their aesthetic value when they are visible or accessible either to building occupants or their neighbors.14 Rick Fedrizzi, co-founder of the U.S. Green Building Council, even makes a case for ascribing increased real estate values for buildings with green roofs, and for neighboring buildings that overlook them.15 Although less amenable to quantification, none of these attributes is trivial, considering the increasing importance society is placing on both the environmental health and visual quality of the urban environment.
Why Use a Blue Roof?
The simplest blue roof technique for rainwater management in buildings consists of flow control roof drains, which detain rainwater and gradually release it over a period of time, typically 24 hours. Because these can be implemented on any roof, they are compatible with other sustainable roof technologies such as solar photovoltaic panels or solar thermal systems. Perhaps because they are so common, they can be overlooked as LID SWM measures, as evidenced by their absence from Table 1 (refer to PDF).
View of front entrance of Earl’s Court Village, London, ON. All roof surfaces are connected to the rainwater harvesting system, with the exception of the porch in the foreground that is covered by a green roof.
From the point of view of stormwater management, both flow control roof drains and rainwater harvesting systems have predicable performance. Our results show that beyond their contribution to stormwater management, rainwater harvesting systems can significantly offset the normal demand for potable water: when used inside buildings, harvested rainwater can offset all of the potable water normally used for flushing toilets and urinals for the wide range of building types and for the locations examined in this study.
Creating a Decision Tool
Combining our results for rainwater detention with the other attributes of green and blue roofs discussed above produces criteria that can be applied in selecting the most appropriate system for a particular project. Specifically, these criteria are: retaining rainwater, providing natural habitat, improving roof appearance, accommodating other sustainable rooftop technologies, and offsetting potable water use. These criteria are illustrated in Figure 4, organized into a flowchart to guide decision-making.
This decision flowchart is based on our assessment of the attributes of green and blue roofs using both the available literature and the empirical examples. It offers decision-makers an approach to guide deliberations on whether to use either a green roof or a rainwater harvesting system in lieu of conventional flow-control roof drains or infiltration-based measures. The initial decision depends on whether the proponents are willing to commit financial resources to consider alternative approaches. From that point, much depends on the characteristics of the particular project and the value the proponents place on the less tangible but important attributes of green roofs versus the more objective attributes of rainwater harvesting systems. The key factors identified in this study in selecting green versus blue roofs (visibility of the roof surface, importance of habitat creation, and presence of other rooftop systems) are noted as cumulative, which means they should be considered together in determining whether or not the project conditions favor a green roof.
For the large building types considered above, rainwater harvesting systems have the potential to make a substantial difference in reducing stormwater impacts on downstream water treatment infrastructure while also reducing demand from upstream municipal water supply systems. As more buildings employ these systems, the cumulative benefits for cities will become more significant. Like the development of green roofs, greater awareness of appropriate applications for rainwater harvesting systems and less uncertainty over their design and operation will hopefully lead to their wider acceptance, despite their lack of cachet and visibility.
A Hidden Urban Resource
While flat roofs are normally invisible and inaccessible both to building occupants and pedestrians, they nonetheless are a major feature of the urban landscape. Roof surfaces also have significant effects on the urban environment, particularly in the rapid discharge of rainwater during storms. Out of the collection of low impact development techniques available to manage stormwater, green and blue roofs offer significant benefits beyond the control of excess rainfall, most importantly habitat creation and water conservation. Green and blue roof systems, appropriately applied and combined with other strategies that conserve energy, enable deployment of solar PV, reduce emissions, and mitigate urban heating, enable the roofs of large buildings to be transformed from a hidden opportunity to become an important urban resource for cities across a wide range of climate zones.
- Jacobsen, M. & Ten Hoeve, J. 2012. “Effects of urban surfaces and white roofs on global and regional climate.” Journal of Climate 25: 1028-1047.
- Viola, F., Hellies, M., Deidda, R. 2017. “Retention performance of green roofs in representative climates worldwide.” Journal of Hydrology 553: 763-772.
- Sample, D. & Liu, J. 2014. “Optimizing rainwater harvesting systems for the dual purposes of water supply and runoff capture.” Journal of Cleaner Production 75: 174-194.
- Ecojustice. 2013. “The great lakes sewage report card.” Retrieved 18 November 2016 from: http://www.ecojustice.ca/wp-content/uploads/2014/08/FINAL-The-Great-Lakes-Sewage-Report-Card-2013.pdf.
- Sierra Legal Defence Fund. “The national sewage report card: grading the sewage treatment of 22 Canadian cities.” Retrieved 18 November 2016 from: http://www.bucksuzuki.org/images/uploads/docs/sewage_report_card_III.pdf.
- Sedlack, D. 2014. Water 4.0: The Past, Present, and Future of the World’s Most Vital Resource. New Haven: Yale University Press, p. 128.
- Toronto and Region Conservation Authority. “Low impact development stormwater management planning and design guide version 1.0.” Retrieved 18 November 2016 from: http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/01/LID-SWM-Guide-v1.0_2010_1_no-appendices.pdf.
- EPA. “Energy Star portfolio manager data trends: combined commercial/institutional data series.” U.S. Environmental Protection Agency. Retrieved 19 December 2016 from: https://www.energystar.gov/sites/default/files/tools/DataTrends_All_20150129_508.compressed.pdf.
- NREL. “Technical support document: development of the advanced energy design guide for K-12 schools.” National Renewable Energy Lab. Retrieved 27 November 2016 from: http://www.nrel.gov/docs/fy07osti/42114.pdf.
- Sims, A., et al. 2016. “Retention performance of green roofs in three difference climate regions.” Journal of Hydrology 542: 115-124.
- CGBC. 2009. LEED Canada for new construction and major renovations 2009. Ottawa: Canada Green Building Council.
- Historical data (2016), Environment and Climate Change Canada. Retrieved 12 November 2016 from http://climate.weather.gc.ca/historical_data/search_historic_data_e.html.
- Lstiburek, J. 2011. “Seeing red over green roofs.” ASHRAE Journal, June 2011: 68-71.
- “About green roofs. Green Roofs for Healthy Cities.” Retrieved 18 October 2016 from: https://greenroofs.org/about-green-roofs/.
- Fedrizzi, R. 2015. Green Think: How Profit Can Save The Planet. New York: Disruption Books, p. 452. •
About the Authors
Richard W. Hammond, OAA, is a principal architect at Cornerstone Architecture Incorporated, based in London, ON. Geoffrey M. Lewis, Ph.D., is a research specialist with the Center for Sustainable Systems at the University of Michigan, Ann Arbor, Mich. Sarah Elizabeth Wolfe, Ph.D., is a professor in the School of Environment, Resources and Sustainability at the University of Waterloo, Waterloo, ON.