THE HEIGHT OF COOL
Toronto researchers reach for the top to help city beat the heat,
From the fifth-storey roof of the University of Toronto’s new faculty of architecture building — plunk in the middle of Spadina Cres. — Robert Wright points north to the canopy of green that covers the city’s Annex neighbourhood.
Then he turns his gaze south, to Chinatown, where sprigs of more freshly planted trees are barely dotting the denuded jumble of offices, stores, streets and warehouses heading off toward Lake Ontario.
“That is what we eventually want that to look like,” the director of the faculty’s centre for landscape research says, tracing an arc with his index finger from north to south.
“We want the same kind of (green) coverage down there. Of course it will take a long time.”
But in these days of global warming, greening the city has become an urgent matter. And there’s a more immediate solution to Toronto’s carbon footprint problems growing happily at Wright’s feet.
Wright and his colleagues are now embedding the final pieces of U of T’s newest experimental green roof — a collection of 48 boxed, piped and wired plant beds that will come equipped with its own weather station.
Known as the Green Roof Innovation Testing Laboratory — or GRIT Lab — the facility is a follow-up edition to the one that’s topped the architecture school’s old College St. headquarters since 2010.
Plagued by urban heat islands, Toronto has become a leader in expanding rooftop gardens
And while it will look specifically at the use of cisternstored rainwater as a viable irrigation source for the rooftop arrays, it will also continue its predecessor’s search for ideal plant and soil combinations and investigations into the gardens’ potential to alter outdoor temperatures, moderate storm runoffs, save energy and attract pollinators.
“Every (flat) rooftop in Toronto faces south so they get maximum radiation up there,” says Wright, also dean of forestry at the school.
“So whatever we can do to make more reflective surfaces or put vegetation up there will cool the environment around them.”
Work at the College St. lab showed that temperatures at the green roof’s surface were about two degrees lower than the ambient air in summertime — a dip largely caused by the plants’ release of water vapour into the air through a process known as evapotranspiration.
But a more relevant temperature comparison would be with the standard black roof surfaces that most large buildings were fitted with throughout the city’s industrialized history.
And on bright, hot days those sun-absorbing and often vast rooftops would be about10 C hotter than their green counterparts, says Liat Margolis, a U of T landscape architect who heads the GRIT Lab projects.
Those black-top roofs, Margolis says, are major contributors to the heat-island, micro-climate effects that broil many parts of the city in summertime.
But since introducing a bylaw in 2009 mandating green roofs be installed on all new developments with floor areas greater than 2,000 square metres, Toronto now leads the continent in their deployment.
At the end of 2017, there were some 500 green roofs installed or under construction across the city, covering some 420,000 square metres — or 51 CFL football fields — worth of roof, city data shows.
And while no one knows the effect these may have on overall or even localized temperatures, Jane Welsh, a project manager in the city’s environmental planning department, says she is certain they bring cooling benefits.
“The green roofs are a whole package of benefits (and one) of the important ones is the reduction of the urban heat islands,” Welsh says.
The bylaw only covers new buildings. But some 50 older structures have been retrofitted, using a fund drawn from fines paid by developers who opt not to cover their new projects with vegetation.
(As well, the heat impact of older buildings can be mitigated by a “cool roof” strategy that places lightreflecting white membranes over dark surfaces.)
But the green roofs’ utility is becoming so apparent that the province has been looking at adjusting the building code to include technical standards for them.
That will make it easier for other municipalities to enact bylaws mandating their installation, says Hitesh Doshi, a professor in architectural science at Ryerson University.
The province amended the Municipal Act last year to allow municipalities to mandate green roof construction, housing ministry spokesperson Conrad Spezowka said in an email.
And while consultations took place last summer to make accompanying amendments to the building code, the needed changes are still in the pipe.
But Doshi says he expects them to be made soon and sees them as a validation of the expanding green roof strategy.
“There are no hard numbers,” Doshi says. “But if you are looking for progress, then that (building-code change) is progress. If any other city wants to require a green roof (program) like the City of Toronto is requiring right now, then it becomes easier for them.”
(Asked about the incoming Ford government’s plans, PC spokesperson Simon Jefferies said this week the province would have more to say after the transition.)
It was Doshi’s 2005 paper on green roofs that helped prompt the Toronto bylaw four years later. And it’s that paper that is still most often cited for estimates on their potential effects.
Doshi says that based on the study’s assumption that an equivalent of 8 per cent of the city’s total land area would be green-roofed — something that would require far more aggressive installation programs still — temperatures could be lowered in the city by 0.5 C to 2 C, depending on the time of year.
(The paper also showed the extra vegetation could produce significant reductions in air pollutants such as carbon monoxide and smog-causing nitrogen dioxide, and energy savings of 4.15 kilowatt hours a year in electricity costs for every square metre of roof covered.)
But the roofs’ main utility comes not from their ability to combat the heat of climate change, but the more intense rainstorms it is expected to trigger, Margolis says.
Basically, she says, buildings with green roofs are lifting the same stormwater retention capacity of the ground they cover up into the sky.
“The ability of green roofs to actually hold onto water and reduce the runoff during the peak of the storm could be monumental,” she says. “I think they can make a huge impact.”
Doshi says the roofs are rapidly being incorporated into flood plans in cities across the continent.
“I’m going to go out on a limb and say virtually every jurisdiction (in the U.S. and Canada) has recognized green roofs as part of a solution … to deal with stormwater,” Doshi adds.
And while most jurisdictions will not mandate their installation, many are giving financial credits to companies or individuals who do build them commensurate with their storm mitigation benefits, he says.
Doshi’s study said that with the green roofs at the 8-per-cent coverage level, they could potentially hold back some four millimetres of rainwater across the city from flowing into overburdened streams and sewers during storms.
“Four millimetres is fairly significant,” he says, because right now many jurisdictions are requiring that new developments ensure that no more than 25 millimetres of rainfall should go into the sewers, he says.
“So green roofs would contribute four to five millimetres of that. And today the technology is even better, and we could get even more than that.”
III
Wright hopes these sorts of improvements will be sparked by U of T’s Grit Lab projects.
“This is a lab,” he says, sweeping his hand across the top of the new Daniels Faculty of Architecture building. “We’re testing soils, we’re testing plants, we’re testing stormwater, we’re testing temperature and effects on climate.”
Each of the new roof’s 48 boxes will contain different mixtures of plants and soil substrates to determine which combinations could produce the greatest cooling and stormwater benefits, Wright says. And many will be subjected to different irrigation techniques to gauge the viability of using that rainwater — collected in a nearby cistern — to keep the beds alive and cool.
“Irrigation plays a major role in cooling,” Margolis says. “You’ve got a wet surface that’s evaporating and eva-potranspiring you’re going to have evaporative cooling.”
The trouble is, she says, the use of a municipal water source is taking one critical resource and the energy used to pipe it to do another environmentally beneficial job, Margolis says.
So much of the new roof will be watered with collected rainfall. But those stormwatered beds will be subjected to some of the contaminants that rain collects as it passes through the air and across city surfaces, she says.
“It’s the world’s first study that is this comprehensive that compares cisterncollected stormwater that is run off from the ground and gutters on the roof to (tap) water,” Margolis says.
Much of the monitoring will look at which plant and soil combinations can best tolerate those contaminants, which will include salts, metals and hydrocarbons. A charcoal-like substance know as bio-char will also be introduced at various levels into some of the soil beds to see if its filtering ability can keep contaminants away from the plants themselves.
Those plants are mostly short, succulent-based sedum mats — which resemble colourful mosses — as opposed to taller grass and meadowland plants, Margolis says.
“What we found was where the cooling happens is at the surface or right above the surface,” she says, adding that plants provide little temperature modification above the six-inch level (15 centimetres). “And if the ambient temperature is 30 C, the temperature at the surface will be an average of 2 C cooler.”
(Though lower temperatures hover close to the surface, large numbers of green tops would have a cumulative effect on city cooling, many experts say.)
That does not mean that flowering plants should not be part of the mix, however.
They play a critical role in attracting healthy populations of pollinating insects, especially bees, Margolis says.
“This is an ecological system and plants work in concert with pollinators and other insects.”
Earlier work by U of T biologist Scott MacIvor showed green roofs that lacked meadow plants allowed domesticated honeybees to outcompete the wild bees native to Ontario, which occupy a needed niche in their ecosystems.
In the end, Doshi says, the roofs have entered the mainstream of city life.
“You can’t help but notice all the green stuff on the roofs out there,” he says. “It’s just thriving.”