green roof

Research challenges current green roof standards

Old infrastructure, combined sewage systems, rain events, a ‘perfect storm’ for flood damage
Tuesday, May 30, 2017
By Rebecca Melnyk

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.”

Green roof solutions under the radar

Solutions for better green roof configurations will be further probed once GRIT Lab moves into its brand new headquarters at One Spadina Crescent. There, Margolis’ team will be looking at how green infrastructure technologies work in tandem with underground rainwater cisterns. Toronto’s green roof bylaw targets new construction, which includes installing cisterns. There are “opportunities to synergistically design the two technologies as a closed-loop system.” This could reduce runoff, achieve water conservation and thermal cooling and provide a more biodiverse habitat for pollinators.

“A major move forward would be finding positive benefits of water filtration, and improving water management on a site could be a major contribution to the city,” she emphasizes. “We are urging the City of Toronto to set performance targets, rather than specify materials.”

Materials currently favoured by the industry and city are actually not performing as well as newer materials Margolis is studying, further pointing to how green roof practices in Toronto need to be re-examined.

For example, biologically-derived growing media retains more water and nutrients and sustains more plant cover and diversity than mineral-based media, which is favoured for its hardiness. Biologically-derived media (with a large proportion of wood-based compost) also did a better job in sustaining grass and herbaceous plants over the past five years at the GRIT Lab. This directly affects the ability to grow pollinator-friendly plant species, is locally sourced and 100 per cent recyclable, not mined and transported from far away like aggregate.

For thermal regulation, vegetation also provides shade to hot surfaces below. The majority of surfaces across the GTA, from asphalt parking lots to roofing tiles, are dark surfaces that radiate heat back. Roof membranes on a hot day can reach 60 degrees, adding to the urban heat island effect. Green roofs contribute to the business of solving a larger issue – climate change, while also acting as an insulating layer by protecting a costly roof membrane from the elements, while adding social benefits for tenants.

As a first step, Margolis emphasizes the significance of planting trees around properties. A 2014 TD Economics study on the value of trees in Toronto valued the city’s urban canopy at $80 million per year in ecosystem services like flood reduction, cooling, air quality and biodiversity.

“Urban vegetation, particularly trees, is extremely important for reducing ambient temperature in cities, which then contributes to the reduction of energy consumption for cooling.”

Photo courtesy of John H. Daniels Faculty of Architecture, Landscape and Design at the University of Toronto. Liat Margolis leading a Doors Open tour.

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