Toronto likes green roofs. It pretty much has to: Since January 30, 2010, almost every new building with a footprint over 2,000 square metres has been required to have one, and the bigger the building, the greater the expanse of its roof must be green. (Until April 2012, industrial buildings were exempt from this rule, but no longer.) And then there’s U of T’s new Green Roof Innovation Testing Laboratory, or GRIT Lab. Hidden five storeys above College and Huron, on top of the John H. Daniels Faculty of Architecture, Landscape, and Design, it’s not just one green roof—it’s more like 33 of them.
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“We know in principle that these things work,” says Liat Margolis, the professor leading the lab’s green-roof research. “But can we make them work even better? And how do we know that we’re doing that?” What kind of green roof, in other words, is the very best for Toronto’s climate? That’s what GRIT Lab will try to answer over the next three years, and it’s what all these eight-by-four-foot aluminum-sided wood boxes are for. Each one has a different plant community, planting medium, and irrigation regime—a unique combination of what’s growing inside it, what kind of material surrounds it at what depth, and how it’s watered, if it’s watered at all.
If this sounds preposterously complicated, that’s because it is. Every little thing about a green roof, down to the colour of the plants, matters. When it comes to temperature, for instance, the cooler whatever’s growing on a green roof gets, the better. Cities suck up and trap hot air much more than rural areas do, creating something called an urban heat island, so if you cool down the roofs of buildings, you’ll help cool down the streets below them. When the temperature on the surface of the John H. Daniels roof hit 58.1°C one Friday afternoon in early August, the surface of one miniature green roof was 20.8° cooler. That box was filled with plants called sedum album. Often used on green roofs because of their resilience, the sedum were planted in organic soil 15 centimetres deep and watered only when required, according to a soil moisture sensor. Of the four boxes that the researchers tested that day, however, another one did even better. The plants in it were grown in the same kind of soil at the same depth as the sedum, but this one was a lusher mix of a dozen grasses and flowers native to Ontario, watered on a timer instead. The difference? 1.2°C. A small victory, sure, but it’s a start.
GREEN ROOF STATS
36,500 m2: Total surface area of green roofs in Toronto before the City’s 2010 bylaw came into effect.
238,500 m2: Total surface area that will be covered once the 180 new buildings required to have green roofs are complete. (Christie Pits could fit into that space 2.71 times.)
350 m2: Size of GRIT Lab’s green roof.
3,700 m2: Size of City Hall’s green roof, the largest publicly accessible one in Canada when it opened in May 2010.
9,250 m2: Size of the green roof planned for the TTC’s under-construction streetcar facility at Lake Shore Boulevard East and Leslie Street.