Greener cities: U of T study advances future green roof design
In 2009, Toronto became the first city in North America to implement a mandatory Green Roof Bylaw, requiring new developments larger than 2,000 sq. m to dedicate part of their rooftop space to green roofs, areas designed exclusively for vegetation and not intended for public access under the bylaw’s criteria.
An interdisciplinary team of researchers from the University of Toronto, Carleton University and Toronto Metropolitan University have published their novel analysis of green rooftops in Toronto, providing a framework for future of green roof planning and design in urban settings.
Their paper, entitled “Remote Sensing for Healthy Vegetation on Green Roofs,” was published this week in Nature Cities. Bringing together expertise from diverse fields including civil engineering and U of T’s department of geography and planning in the Faculty of Arts & Science, it also synthesizes contributions from forestry and landscape architecture at U of T’s John H. Daniels Faculty of Architecture, Landscape, and Design.
The team includes U of T doctoral graduate in forestry conservation and civil engineering Wenxi Liao; U of T master of landscape architecture graduates Madison Appleby and Howard Rosenblat; U of T forestry doctoral graduate Md. Abdul Halim; Assistant Professor Cheryl A. Rogers from the department of geography and environmental studies at TMU; Professor Jing M. Chen from U of T’s department of geography and planning within the Faculty of Arts & Science; U of T landscape architecture Professor Liat Margolis, director of the Green Roof Innovation Testing Laboratory (GRIT Lab) at the Daniels Faculty; Jennifer Drake, an associate professor and Canada Research Chair in Stormwater and Low Impact Development at the department of civil and environmental engineering, Carleton University; and Professor Sean C. Thomas of the Institute of Forestry & Conservation at the Daniels Faculty.
Daniels building at the University of Toronto, St. George campus, 1 Spadina Cres. (photo by David Lee)
“There’s clear potential for using remote sensing tools to monitor green roofs, since they’re visible from above,” says one of the paper’s authors, Thomas. “However, satellite images are generally too coarse to distinguish individual green roofs, and urban environments pose additional challenges due to complex geometry and shading.”
The City of Toronto had previously captured multispectral airborne images with extremely high resolution – approximately seven centimetres per pixel. These images allowed the team to analyze a set of 1,380 green roof units over time, from 2011 to 2018.
“Doing technical processing on a large scale, solving problems, like image correction, and being able to follow the green roofs over time gives us some real insights and guidance on how to do a better job with green roofs,” says Thomas.
High wind exposure and shallow soils might be expected to result in declining vegetation health, says Thomas, however, the researchers were able to conclude that most of the green roofs in Toronto are doing well and becoming greener. Thomas says among the many indices used from images, the normalized difference vegetation index (NDVI) did best at quantifying the total amount of vegetation, by using both visible and infrared data.
“Given all the data, we were able to look where there’s a break point. It’s clear that green roofs on top of tall buildings did much worse than ones at four stories or lower. Green roofs that are long and skinny also tend to have more degradation,” he says.
Most of the green roofs are made of low mat-forming plants, like sedum, explains Thomas, because of their extreme drought tolerance. At night, sedum stores atmospheric carbon dioxide (CO₂), which is then used for photosynthesis the following day, helping reduce water loss. Thomas says, as expected, sedum-dominated roofs showed overall the best performance.
He adds the study’s results are expected to better enhance the sustainability benefits of expanding green roofs even further.
“With a conventional flat or a slanted roof, water drains quickly off the structure. In an extreme rainfall event that can overwhelm infrastructure. Green roofs are an effective way of storing water and delaying the peak of the flood event,” he says.
“Green roofs also offer a cooling effect that helps mitigate what is known as the ‘urban heat island’, along with reducing noise pollution, improving air quality and enhancing urban biodiversity by supporting insects and birds.”
This research was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Research and Training Experience (CREATE) program.
