Green rooftops are sprouting up across Canada, sowing the seeds of climate benefits that range from heat moderation in concrete jungles to vegetables served fresh from the roof.
In major cities, green roofs also catch and clean stormwater and absorb carbon dioxide, a key contributor to global heating.
The concept of greenery on rooftops dates as far back as 600 BC, when Nebuchadnezzar reportedly built the 20-metre-high Hanging Gardens of Babylon for his wife Amytis. Today, while Toronto is no Seventh Wonder of the World, it does have more than 1,000 green roofs.
A 10,000-square-metre green roof can turn a building from a “heat sink” into a “life-generating force,” explains green roof artist Terence McGlade. “There are so many side benefits to it besides emitting wonderful, clean air into the atmosphere from plants’ oxygenation.”
In Vancouver, a 1,850-square-metre green roof over the public library’s central branch grows drought-resistant bunchgrass and kinnikinnick. Sandra Korpan, associate director of facilities management, said this inverted roof has an elevated protective membrane letting stormwater flow under the soil into large spouts that guide it down the building into the sewer system.
Green roof soil also binds to pollutants, eating them and purifying water so it’s cleaner when it eventually drains into a lake or ocean, added Jennifer Portsmouth, business development manager for Para Space Landscaping in Burnaby, B.C.
Some people don’t realize “the green roof is actually part of the building’s cooling system,” she said, noting it can spare 20 to 50 per cent of the air conditioning in summer.
Green roofs stay substantially cooler than those covered in black asphalt. It’s crucial when rooftop temperatures of 50 C or 60 C can dwarf street-level temperatures of 25 C, according to Kerry Ross, a Calgary-based green roof professional.
They can also retain two-thirds of rainfall, sweating water through plants and filtering it down into ground-level storm systems.
“It’s not about stopping water,” Portsmouth said. “It’s about slowing the water down.”
Build costs vary based on size and greenery, but green roofs can be adapted to climate conditions anywhere in Canada with the right structure, plants and soil.
Designing a green roof
The basic foundation of a green roof has a protection layer that prevents plant roots from penetrating the infrastructure. After this, comes a drainage layer the stormwater can escape through, along with geotextile (permeable fabrics) that filters the water.
“You still want that water to be able to drain away,” said Karen Liu, a green roof specialist with Toronto-based Next Level Stormwater Management. “But you don’t want all that soil to be clogging the drainage layer.”
A 10- to 12-millimetre gap below the soil allows water to run free and drain, Liu said, but having minimal slope and no shingles on the roof is also key to an effective watershed.
“It’s not rocket science,” she said, “but there are tricks to it.”
Smaller-scale “extensive” systems that primarily grow succulents require less soil and are lighter — less than six inches of soil weighs 25 to 50 pounds per square foot. Larger-scale “intensive” systems, which can support shrubs or even trees, require deeper soil and weigh 80 to 150 pounds per square foot.
Built correctly, green infrastructure can double a roof’s lifespan, Portsmouth said, because greenery shields the membrane from damaging UV rays and the impact of humans walking on top.
For the membrane, hot tar is the most expensive but also the most effective because it ensures a green roof will survive 20 to 30 years, according to McGlade, who designed the Toronto Botanical Garden’s green roof. Two-ply modified bitumen membranes cost less but also have a relatively good track record, he said.
“You’re at the whims of how good the roof membrane is.”
Picking the right soil
When choosing soil, different options offer different benefits, said Melanie Sifton, a University of Toronto PhD candidate assessing non-potting soils (media) for green roofs.
Biochar, a charcoal-like substance, adds salt and moisture to plants and helps balance the amount of nutrients, while microbes such as yeast act as “probiotics for plants,” Sifton said.
McGlade suggested that high-organic soil can support a richer plant ecology that sweats more moisture, and high-mineral soil mainly sustains resilient succulents such as sedums.
“Succulents try to contain their own water inside their leaf system so they don’t dry out,” he said, also noting that high temperatures of 40 C stop plants from sweating.
In colder provinces, such as Alberta and Saskatchewan, plants can freeze if the soil is too shallow. For these climates, Ross recommended the soil should be at least four inches deep.
“Picking climate-adapted species is important,” she said.
But climate-adapted doesn’t always mean native. McGlade once succeeded with local wildflowers atop Alberta’s legislature, yet failed with Saskatchewan-native plants on a green roof in Regina due to the constricted growing season in -50 C.
Gauging feasibility and maintenance
Experts estimate basic green roofing costs about $15 to $20 per square foot, but Ross said average prices may have gone up with rising construction costs.
Anyone considering a green roof should hire a structural engineer to determine their building’s feasibility and ensure the roofing is installed properly, Ross added.
“With these green roofs, they have to be done right from the beginning,” Portsmouth said. “Fixing things that are high up in the air is very expensive because they’re not accessible.”
Korpan said green roof designs must also account for extra weight from wet soil — an important consideration for a city like Vancouver that can get more than two metres of rain a year.
And maintenance is crucial for longevity, Portsmouth noted. This means going out at least once a year to clear foliage.
“I would say most clients don’t maintain their roof to the standard they should.”
A new level of farming
As green roofs take root on more buildings, they’re gaining new admirers. Among them are hotel chefs who serve produce straight from their rooftops. Ross said people also want green roofs so they get easier access to gardening and nature.
“I think they’d like to increase the amount of biodiversity, attract pollinators and birds to their site, so it provides that nearby nature that people tend to crave,” she said.
In Toronto, two gardens on top of Toronto Metropolitan University are producing large amounts of food well above street level.
Arlene Throness, who manages TMU’s Urban Farm, says her team of five braves the higher winds to grow 80 different crops, including tomatoes, corn, potatoes, eggplant, squash and kale.
“We actually haven’t found anything we couldn’t grow,” she said. “Our yields are comparable to ground level.”
Harvesting 3,000 kilograms of produce per year, the farm feeds the community through markets and educates students with food justice initiatives. Throness said the gardens, five and eight storeys above ground, use 10 inches of soil that’s more organic than green roofs typically use.
“That is what holds all the life in the soil,” she said about compost. “When you have that living system, it becomes a carbon sink.”
Along with storing CO2, the farm retains almost 90 per cent of rainfall for hours after a storm, Throness said.
Since Toronto’s green roof bylaw came into effect in 2009, its eco-roof incentive program has offered $100 per square metre of green infrastructure that’s installed, to a max of $100,000 per project.
Proponents of green roofs hope to see cities introduce financial incentives similar to Toronto’s, whether to alleviate construction costs or give rebates for rooftop garden harvests.
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