A green roof is an engineered system for growing plants on top of buildings, explains Steven Peck, founder and president of Green Roofs for Healthy Cities, a Toronto-based advocacy group that also provides accreditation to green roof professionals.
The benefits of green roofs range from cleaner air to cost savings for building owners, making them one of the fastest-growing eco-friendly technologies on the market.
“I don’t think there is another green building technology that delivers as many benefits as green roofs do,” Peck says. “You take the benefits that nature provides and you put them into places they haven't been, like inner cities.”
Creating a functional natural system in a manmade environment is challenging. Green roofs have to be comprised of several different layers of material that protect both the building and the plants.
Those layers usually include a high-quality waterproof membrane, a root-repellent layer to protect the membrane from stray roots, a drainage layer to manage water during heavy storms, and an engineered growth medium that hosts the plants.
The plants too have to be carefully selected to withstand the harsh microclimate of an urban rooftop.
Many green roofs also have irrigation systems that can be used during extreme weather events -- for example the 30-degree heat and drought Toronto experienced in July.
“Very few plants can withstand weeks and weeks of that kind of punishment, so irrigation’s very important, both for the establishment period when it’s just starting to grow, and for the long-term,” Peck says.
Green roofs cost anywhere from $12-$50 per square foot, but when properly maintained, they can save companies and governments hundreds of thousands of dollars in the long run.
Due to the superior stormwater management capacity of rooftop gardens, a green roof can double the lifespan of a building's waterproof membrane, cutting down on costly maintenance.
Green roofs also reduce the urban heat island effect. It's estimated that even a one-degree reduction in the urban heat island effect could translate into a four per cent reduction in peak energy demand in Ontario, Peck says.
Numerous studies have been done on biophilia -- the idea that people have an innate affinity for nature and need to access natural environments to achieve better mental and physical health.
“There's a whole area of design around creating healthier urban environments -- green roofs, green walls and other biophilic elements,” Peck says. “All that stuff makes people healthier, and that saves us money big time.”
Increasingly, municipalities aren't just recommending the development of green roofs -- they're mandating it.
The City of Toronto now has a bylaw that requires new large buildings to “green” a percentage of their rooftops. Those that don't pay cash in lieu, which in turn goes towards providing financial incentives for existing buildings to convert their rooftops to gardens.
The result is that Toronto now has about 125,000 square feet of green roof currently in development, says city planner Jane Welsh.
“It's part of implementing our official plan ... to encourage more green in a very built-up urban area and make the city more attractive,” she says.