Green Roofs

When you think of a city skyline, do you think summery expanses of prairie grasses, dotted with wildflowers and humming with birds and insects? Or maybe rooftop gardens lush with herbs and leafy greens? Terraces of green to rest your eyes on as you look up from your cubicle or down from your high-rise condo?

Such roofs are not only possible but actually in place now in Alberta. The 7,500 ft2 rooftop meadow covering Williams Engineering in downtown Edmonton was considered lovely enough to be featured in the 2014 “Greenroofs and Walls of the World” wall calendar. The green roof at Calgary’s Britannia Crossing surrounds a third-floor outdoor play area for children at the Kids and Company daycare. And the roofs of Berry Architecture and Associates in Red Deer and the Helen Schuler Nature Centre in Lethbridge were both international award winners last September in the CitiesAlive 13th Annual Green Roof and Wall Conference in Brooklyn, New York.

Such roofs are less common than those made of more conventional materials, but covering our buildings with living materials—in essence replacing the fields and farmlands our buildings have usurped—has many benefits for city life.

Roofs are literally everywhere in our urban environments, making up at least 20 per cent of a typical North American city’s surface area. They have traditionally been designed with a single purpose in mind: to enclose and protect the buildings beneath them from the elements. But cities have an effect on temperature as well. Black rooftops and the excess energy released by cars, buildings, heating and cooling systems and other mechanical activities cause the “urban heat island” effect, where a city can be up to 3°C warmer than its surrounding area in the day and up to 12°C warmer at night. On a warming planet, rising summer temperatures put an increasing strain on buildings’ cooling systems. In winter, heat loss is costly enough, but that wasted heat can also accelerate snowmelt, flooding drainage systems.

Green roofs are popular with people who demand more than shelter from their roofs.

As any homeowner knows, traditional roofs are also expensive to maintain. In Alberta’s extreme seasonal variations, protective roofing membranes are vulnerable to damage from sunlight, temperature fluctuations and hail, and need regular replacement and repair. Justin Pockar, energy and environment coordinator for the City of Calgary, explains just how much punishment a conventional roof takes: “If you have a dark roof membrane, on a sunny day in January it can often hit 15, 20, 25 degrees centigrade. That’s not uncommon, just from the solar radiation. At night that same piece of membrane is going to be –30. [You’re] cycling it through 60 degrees twice a day.”

An alternative to conventional roofing is green roofs, sometimes called living or eco-roofs. Whether retrofits on existing buildings or planned sustainability features on new builds, they’re increasingly popular with people who demand more than shelter from their roofs.

Green roofs typically consist of several layers, including waterproof and roofing membranes, thermal insulation, drainage filters, a drainage layer, and root and vapour barriers. There will of course be plants—often shallow-rooted, drought-tolerant plants such as sedums—set in a growing medium tailored to suit the plantings and the strength of the roof.

One of the key benefits of any green roof is its ability to manage water. The vegetation and soil hold water from rain and snowfall, slowing its passage into a city’s storm water system. In most modern cities the storm drains are periodically overwhelmed, causing disruptive, damaging and costly flooding. Green roofs mitigate the effects of Alberta’s typically brief intense rainstorms and episodic snow melts.

Rooftop plants give off moisture, cooling surfaces and reducing the heat island effect.

Green roofs come in many different forms and configurations. The “extensive” system may be what comes first to mind: a roof-wide expanse typically planted with low-maintenance species such as perennial grasses. The soil depth is usually between 50 and 150 mm, or 3 to 6 inches. The Meadows Community Recreation Centre in suburban Edmonton has such an installation. The extensive roofs of Calgary’s Bow Valley Square are planted with sedum mats, while the Stantec building in Edmonton sports a rooftop meadow of native plants. Native vegetation tends to be more conducive to encouraging wildlife and biodiversity but needs more initial maintenance until the plants are established.

“Intensive” and “semi-intensive” green roofs may have deeper soil to accommodate long-rooted plants. Such roofs may be used as a public space or for urban agriculture. Red Deer’s Berry Architecture and Associates green roof is an intensive garden, as is the green roof atop Lethbridge’s Helen Schuler Nature Centre. Raised beds, container gardens and specialized educational spaces such as the Community Medicine Wheel Garden on the roof of the Shaw Conference Centre in Edmonton are also intensive systems.

Whatever their configuration, green roofs have many benefits for the buildings they cover. One important economic benefit is a longer life for roofing membranes, since the layers of soil and plants protect the membrane from temperature fluctuations and ultraviolet light. Those layers also insulate the membrane, reducing a building’s heat loss in the winter and creating a cooling effect in the summer. One Carnegie Mellon University study found green roofs could reduce heat gains in summer by as much as 75 per cent—significant as our climate warms.

Adding plant life to a city’s skyline is also attractive to insects and birds, whose habitat has been lost to urbanization. Where roofs are planted with native grasses and low-growing shrubs, the biodiversity payback becomes even more significant. The roofs become part of ecological corridors across urban landscapes, which allow wildlife to travel between areas of habitat.

Although car exhaust and industrial emission standards are improving, air quality continues to suffer in cities. Reducing a building’s energy use also reduces the emissions and pollutants created by nearby power plants, further improving local air quality.

But rooftop vegetation offers some specialized benefits. Plants naturally give off moisture through their leaves, further helping to cool rooftops and reduce the heat island effect during the growing season. Their leaves trap airborne particles, which are filtered through their root systems, helping to purify the air. Plants also filter urban pollutants such as ozone, sulphur dioxide and nitrogen dioxide and convert carbon dioxide to oxygen. Cooled, filtered air flows from green rooftops down into neighbouring “building canyons,” circulating purified air to ground level.

Those same plants, bedded in a layer of soil, absorb something else that is important to city-dwellers: sound. Traditional roofs are hard surfaces, and the ceilings may not be soundproofed against air and road traffic sounds.

Green roofs have some intangible—if unquestionably valuable—aesthetic benefits too. Workers and high-rise dwellers generally prefer green, living vistas to an endless view of concrete and artificial roof coverings. Where these roofs are accessible, they can offer stress-reducing, relaxing and even healing alternatives within the concrete jungle, as the visitors to Ted & Lois Hole Healing Garden at Edmonton’s Royal Alexandra Hospital can attest.

Pockar believes that aesthetic value may actually be quantifiable, particularly in times when office space and luxury condo prices start to slide.

“People like trees and green spaces,” he says. “The ability to put a green roof on the podium level of a condo tower is a really good selling feature. I think people are recognizing the value—not only from the building technical standpoint.”

The catch, of course, is cost. Retrofitting an existing roof can be prohibitively expensive, as the building’s structure needs to bear the additional weight of potentially saturated soil and vegetation. Engineering assessments cost money, as does installing the correct membrane, purchasing and planting appropriate vegetation, and maintaining the plants until they are established. Some installations need to budget for irrigation as well, since a roof can be a dry, wind-blown environment.

With new developments, however, a green roof is much more affordable, because the structural features can be planned for. Even so, it adds an inevitable layer of expense. Pockar, a former architectural consultant, has wrestled those figures with clients. “Green roofs are expensive. You can just imagine the cost of having to crane up all these materials and laying down the extra barriers.”

Few building plans are made to a single purpose, though. So whether it will be worth the expense depends on what you’re trying to achieve, he says. “A green roof is one of those wonderful building additions that doesn’t do any one thing fabulously but it does a whole bunch of things quite well. And that’s why it really does require a much more fulsome discussion on what it’s going to cost, and what are the benefits. Fundamentally the greatest selling feature of the green roof is occupant satisfaction. When you’re dealing with AAA office space, or condo residents, occupant satisfaction begins to be the real motivator to do these kinds of systems.”

In Toronto all new commercial and industrial buildings must have green roofs.

Landscape designer, researcher and author (Eat Up: The Inside Scoop on Rooftop Agriculture) Lauren Mandel agrees. “I spend a lot of time talking with my clients about how to create green roofs that meet their specific needs and achieve their individual goals. Sometimes that is a matter of making sure the green roof is cost-effective; sometimes it means creating a certain type of habitat; sometimes it means controlling a certain volume of stormwater or achieving certain research goals.”

If the client is interested in growing food on that roof, the discussion deepens, as does the amount of soil required. Most commercial vegetative green roofs have a fairly thin layer of soil. “Shallow root-based plants need between 50 and 100 mm of soil,” says Pockar. “It would be difficult to grow any kind of substantive food crop in only three inches of dirt.”

But there are many variations of rooftop growing. As the locavore trend expands, chef gardens are becoming more popular. Such gardens can be anything from herb planters or raised beds to rooftop greenhouses. The Fairmont hotel chain encourages rooftop gardens and beekeeping, and so you’ll find herbs on the Fairmont Banff Springs and microgreens in the Fairmont Jasper Park Lodge’s greenhouse.

Rooftop greenhouses could be a feasible option for all-season food production in Alberta. Equipped with water catchment systems, they would offer many of the same advantages as ornamental green roofs, including energy efficiency. Depending on the season, they can capture heat rising from the building or buffer heat loss by acting as an insulating layer.

The possibilities abound for growing local fresh food year round and reducing our dependence on imports. The University of Calgary’s Aquaponics Club harvested 100 fresh tomatoes from their experimental rooftop greenhouse last December. Cold-weather cities in other provinces show what’s possible: Montreal’s Lufa Farms produces 190 metric tonnes of produce annually from its greenhouses atop two large warehouses.

As ever, it depends what you wish to achieve. Hani Quan, principal planner with Edmonton’s Fresh Food and Urban Agriculture strategy, argues that pots work perfectly well for small-scale growing, and simple setups can be very effective on unused roof space. He points to Jim Hole’s rooftop tomato-growing experiments at the Enjoy Centre in St. Albert. “If you have a few beehives and some tomato plants, to me that’s a good, productive roof.”

The Enjoy Centre is also a research site, where rooftop garden modules installed by NAIT students are used to test the durability of different plant species through Alberta’s seasons. In Calgary, the Alastair Ross Technology Centre is a testing ground for different rooftop planting depths, soil types and plant species. Alberta is a young and curious market for green roofs, and many of the dozens of green roofs already installed around the province are being studied for their economic, environmental and aesthetic payback.

When rooftops are used for more than sheltering buildings—to grow food, enhance biodiversity, lessen energy use, manage storm water or just contribute to a more beautiful living and working environment—they provoke discussion in the wider world. We are approaching a tipping point where talk needs to become action. Local policymakers have welcomed the green roof concept in Alberta, but no hard rules or incentives compel developers to install them. In Toronto, meanwhile, all new commercial and industrial buildings must have green roofs.

Our cities’ man-made geography has caused the very problems green roofs help solve. We should look up, way up, for the solution.

Rhona McAdam grew up on Vancouver Island, spent the 1980s in Edmonton and 1990s in the UK, and moved to Victoria in 2002.