Shades of Green

Green roofs have now become a standard green building technology, promoted or required 
in many municipal regions for stormwater management, thermal cooling, and ecological habitat for pollinator species, such as bees. They are a favourite among designers, policymakers and citizens not only because of this ecological multi-tasking, but also because they transform a vast and underused layer of the city—the roof scape—into a thing of beauty. 
At the University of Toronto’s Green Roof 
Innovation Testing Laboratory (GRIT Lab), our mission is to study the relation between material choices and environmental performance.

Simply put, a “green roof” is a constructed vegetative system composed of several layers: waterproof membrane, drainage layer, filter cloth, growing medium, and plants. Some green-roof installations include supplemental irrigation as well. Scientists have penned more than 30,000 papers on green roofs and dozens of green roof research labs conduct research worldwide. Why so many? What do we still have to learn about green roof systems?

green roofs, GRIT Lab, University of Toronto
The sedums blooming on the GRIT lab rooftop help researchers determine how to make more effective green roofs. Photo courtesy of GRIT Lab, University of Toronto.

The reasons we need extensive and ongoing research into green roof technologies is because there are so many different types of green roof products, materials, configurations and dimensions. Growing media and plants are in most cases locally sourced and therefore tend to vary in terms of soil-plant-climate interactions 
and performance outcomes relative to the three environmental objectives.

The City of Toronto adopted a green roof bylaw in 2009, which requires the installation of a green roof on all new buildings with 
a floor area greater than 2,000 square metres.

The University of Toronto Daniels Faculty of Architecture, Landscape, and Design established the GRIT Lab in 2010 as an interdisciplinary research facility to compare and validate industry practices and test new material innovations. Our team, having mined the data collected at the GRIT Lab as well as dozens 
of green roofs across Toronto, has reached 
a number of important conclusions about the kind of growing medium is best for water 
retention and temperature regulation—crucial factors in a green roof’s true ecological useful rather than just window dressing.

It turns out that compost-based media outperforms the aggregate media for stormwater management, thermal cooling, and biodiversity. We also test other things: irrigation versus no irrigation and the benefits of soil-moisture sensor irrigation, different depths of growing mediums and nutrient availability and their 
effect on the plants. We measure the attraction of native wild bees to different plant communities, to evaluate which works best for foraging and nesting. We research the correlation 
of building height with bee visits, particularly below the eighth storey. A partial conclusion 
is that biodiverse green roofs for bees are most suitable on mid- and low-rise roofs.

Yet after all that testing, we still need more research, and possibly always will. Not all green roofs are the same! Not all are even 
effective. But we are doing our best to find out how to make them that way.

GRIT Lab Director Liat Margolis is also Director of the Master of Landscape Architecture programme and Associate Dean of Research at the University of Toronto Daniels Faculty of Architecture, Landscape, and Design.