Green Roofs Grow Over Toronto
By Contributing Writer Brian J. Barth
Thanks to a 2010 bylaw, the city’s rooftops are now the lushest in Canada
According to a 2005 report, 21 percent of Toronto’s land area consists of rooftops, a figure which is similar in any other dense metropolitan area. For the most part, rooftops throughout human history have served a single purpose: to protect the structure from the elements. But as urban land grows increasingly invaluable, and the negative impacts of the built environment on both people and the planet are more widely understood, planners, designers, homeowners, developers, and city builders of all stripes are looking to get more value from this underutilized space covering a fifth of the urban environment.
Roofing materials are traditionally dark in color, which absorbs sunlight throughout the day and radiates it at night, leading to the “heat island” effect—where urban areas are several degrees warmer than surrounding rural areas. This causes cooling systems to work harder, thus consuming more electricity, which not only contributes extra greenhouse gases to the atmosphere, but inflates the hydro bill.
Another problem: rooftops, by definition, are impermeable. This is necessary for keeping the elements out, but it means that rainfall, rather than soaking gently into the ground, rushes off in stormwater drains, causing massive erosion in urban waterways and increasing the likelihood of flooding during high rainfall events. Roof water runoff can also pick up contaminants (sometimes from the roof, but mostly in situations where it runs across paved surfaces on its way to a storm drain), further degrading urban stream quality.
All of which has led to the growth of green, or “living,” roofs in recent years as a remedy for the problems associated with conventional roofing materials. The idea is to cover the roof structure with a waterproof membrane (basically a pond liner), add a shallow layer of growing medium, and plant the space with tough, resilient species that can hold up to the hot, windy conditions found on most rooftops.
These aerial gardens become oases for birds and native insects, helping to offset the biodiversity lost to urban development. The soil and plants shield the structure from the sun and act as a layer of insulation, cutting down on hydro costs in both summer and winter, and reducing the urban temperatures by roughly 2 degrees Celsius. When torrential rains come, the garden acts as a sponge, soaking up the water much like the natural environment does and preventing excessive runoff.
Green Roofs in Toronto
In 2010, Toronto became the first city in North America to pass a green roof bylaw, a move emulated by San Francisco in 2016. Many municipalities around the world have developed incentive programs for green roofs, as well. Within five years of passing the bylaw, more than 300 new green roofs with a total area greater than 250,000 square metres have been built, placing the city second in North America for green roof coverage.
Washington, D.C., first on the list, has nearly three times the green roof space of Toronto, largely as a result of a rebate on stormwater fees, which ranges from $7 to $15 per square foot of green roof, among other incentives. City leaders have set a goal of installing green roofs on 20 percent of D.C. buildings by 2020.
The Toronto bylaw requires green roofs on new construction (whether residential, commercial, or industrial) above 2000 square metres in gross floor area, though any residential building less than six stories in height is exempt. The green roof requirement starts at 20 percent of the “available roof space”—defined as the total roof area minus areas covered by renewable energy installations, as well as a small exemption for outdoor amenity space—on smaller buildings and goes up to 60 percent on structures with a gross floor area of 20,000 square metres or greater. Reductions to the required green roof size are granted on a case-by-case basis, with a $200 per square meter cash-in-lieu payment.
Owners of buildings that are exempt from Toronto’s green roof bylaw—existing structures and new construction under 2000 square metres in gross floor area—are eligible for incentives to add a green roof in the form of a grant calculated at $100 per square metre.
Green roofs are typically hidden from view at street level, but Toronto’s skyline has grown quite lush from a bird’s eye view. The city maintains a list of some of the more notable green roofs in the city, including publicly accessible locations like the 4,000 square metre installation on the podium roof at city hall.
Green roofs are great for the birds and bees, but civic leaders in Toronto and elsewhere have financial rewards in mind, as well. Toronto’s 2005 report found that if 75 percent of rooftops that were suitable for a green roof were converted, an area equivalent to roughly 5000 hectares, a staggering number of economic benefits would accrue:
· Erosion control savings worth $25 million
· Pollution control cost avoidance worth $14 million
· Three additional "beach open" days per year worth $750,000
· Total infrastructure savings worth up to $79 million
· Total energy savings of $181 million per year
Green Roof Design Options
Green roofs fall into two broad categories: intensive and extensive.
Extensive installations are typically larger and cheaper per square metre. The growing medium is shallow (5 to 10 centimetres) and supports a limited palette of plants, typically small succulents and hardy grasses that can survive with little soil or irrigation. Extensive green roofs are often light enough to install on top of conventional roof assemblies.
Intensive roofs are characterized by a deeper growing medium (anywhere from 10 to 100 centimetres) and often require structural upgrades to support the additional weight. They are more common where the roof is accessible and intended as a beautiful outdoor amenity. Intensive roofs can support landscaping similar to ground level plantings, including small trees for shade, shrubbery for wind protection and privacy, and flower beds for seasonal interest; and they are often enhanced with benches, pergolas, outdoor kitchens and other features. Rooftop food gardens, such as the one atop Toronto’s 401 Richmond St. building, are an increasingly popular variation on green roof theme.