Research Challenges Current Green Roof Standards

Land buried beneath rising water, torrential rain slamming into cities, shutting down roads, bridges, subways and even buildings, bacteria flowing into properties and open water systems. Headlines across Canada are awash with these images more and more, especially in older cities like Toronto where aging infrastructure, combined sewage systems and increasing rain events are a perfect storm of conditions for flood damage.

“Water management is the number one issue cities are dealing with,” says Liat Margolis, associate professor in the John H. Daniels Faculty of Architecture, Landscape and Design at the University of Toronto and director of the Green Roof Innovation Testing (GRIT) Laboratory. Her research, conducted at the GRIT Lab, examines modern green roof technologies, looking at common design standards and how they can perform better. Her team is working with the city to help inform more nuanced practices and show how “not all green roofs are created equal.”

According to Margolis’ recent study, Data-Driven Design: Research into Green Roof Performance, performance metrics of green roofs are significantly influenced by local environment conditions and the choice of growing media composition, depth, planting, the use of supplemental irrigation and other factors.

She is also finding that despite design parameters, green roofs can capture between 85 and 90 per cent of the peak volume of a storm, an “enormous contribution to alleviating pressure on urban infrastructure.”

It’s proof that also comes with a major hurdle. The City of Toronto’s green roof bylaw targets new construction with roof space above 2,000 metres, and existing buildings in the Greater Toronto Area have not been retrofitted to deal with environmental impacts. Around Pearson International Airport, Margolis says there are roughly 12,500 buildings that could be retrofitted. Even though the city adopted the Eco-Roof Incentive Program, which offers 75 cents on the dollar for insulating a green roof, since 2009, less than 30 buildings have taken advantage of that incentive.

Property owners and managers of existing properties hold off retrofits for various reasons. They face structural assessments like renovating the roof membrane to ensure no leakage. They may see this as a cost issue because the assessment process might consume the rebate.

“Also, there is probably a lack of communication on the types of solutions for property managers,” adds Margolis. “This is a way to mitigate risk and degradation to your property at large. If we manage flooding properly, it will affect everyone. While there is a bottom line approach, at the end of the day, the aggregated contribution of each building to the community eventually benefits the individual.”

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