Researchers from the University of Toronto's John H. Daniels Faculty of Architecture, Landscape, and Design have conducted an in-depth analysis of green rooftops in Toronto, providing a framework for future of green roof planning and design in urban settings.
The interdisciplinary team examined images gathered using aerial remote sensing technology to track vegetation health across nearly 1,500 roofs in Toronto between 2011 and 2018.
Their findings, published in Nature Cities , reveal an overall improvement in vegetation health and a reduction in vegetation patchiness as green roofs age.
"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 Sean C. Thomas, a professor at the Institute of Forestry & Conservation in the Daniels Faculty.
The team - which included lead author Wenxi Liao, a U of T doctoral graduate in forestry conservation and civil engineering, and researchers from U of T's Faculty of Arts & Science, Toronto Metropolitan University and Carleton University - analyzed multispectral airborne images captured by the City of Toronto. These images are extremely high resolution, approximately seven centimetres per pixel.
Toronto became the first city in North America to implement a green roof bylaw in 2009. It requires new developments larger than 2,000 square metres in gross floor area to dedicate part of their rooftop space to green roofs - areas designed exclusively for vegetation and not intended for public access.
While high wind exposure and shallow soils on exposed rooftops can be challenging for plant growth, Thomas says visible and infrared data showed most of green roofs in Toronto are becoming greener.
There were exceptions, however.
"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."
Most green roofs are planted with low mat-forming species such as sedum because of their extreme drought tolerance - and the study suggested that roofs with sedum performed the best. Sedum stores atmospheric carbon dioxide at night that is then used for photosynthesis the following day, helping to reduce water loss.
Thomas says the study's findings can help inform future green roof designs, helping to improve their overall sustainability .
"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's (NSERC) Collaborative Research and Training Experience (CREATE) program.
