Register Friday | October 22 | 2021

Up on a Rooftop

Greening Montreal’s roofs just makes sense. So what are we waiting for?

I really, really wish I could get onto my roof. Alas, with no staircase and nowhere to prop up a ladder, I won’t be able to climb up anytime soon unless I want to risk a broken neck. Still, it’s not hard to imagine what the view would be like from four storeys up. The broad shoulders of Mount Royal would loom to the southwest; garment factories, church towers and office buildings would poke above the horizon to the east and north; and in between, an endless ocean of flat, grey roofs.

Like New York, Chicago or Philadelphia, Montreal is a flat-roofed city, a city of row houses, triplexes and apartment buildings. Unlike those cities however, Montreal has never really taken advantage of its roofs. Soon that will change. Slowly but inevitably, Montreal’s roofs are going green. It might be an unfamiliar term to many North Americans but if you’re from Europe you probably know green roofs very well. One of the first green roofs was built in Zurich in 1914, when a meadow was planted on top of a water-treatment plant’s filter tanks to keep the interior cool and bacteria-free. Since then, plant-filled roofs have been built on thousands of buildings across Europe—15 percent of all roofs in Germany are now green.

North America’s introduction to green roofs came much later, but we’ve been catching up over the past few years. Recently a green roof was built on Chicago’s City Hall, and the University of Montreal’s École polytechnique just moved to a huge new building capped with a crown of greenery. Even car companies are joining the fun—after a recent renovation, the half-million-square-foot roof of Ford’s massive River Rouge plant in Michigan went green.

But wait—what exactly does it take to make a roof “green?” First, a waterproof membrane is installed, followed by a layer of soil and then vegetation. “Extensive” green roofs, the most common, have a thin layer of soil—just a few inches—and little-to-no irrigation. These kinds of roofs are extremely flexible, easy to maintain and relatively inexpensive. “Intensive” green roofs are a lot heavier, more complex and usually require an expensive, sophisticated irrigation system—but they can be easily used for recreational purposes and can support a more dynamic ecosystem. Additionally, “soilless” green roofs can be created with hydroponic planters—in some parts of the world, notably Egypt, city-dwellers have built soilless vegetable gardens on top of their buildings to give them ready access to cheap food untainted by chemicals.

Owen Rose, an architect and urban ecologist here in Montreal, works with the Urban Ecology Centre, a group that built a green roof in the Milton-Parc neighbourhood (more commonly known as the McGill Ghetto) over the course of five days in July. I was curious to see a green roof first-hand so Rose and I met at the Urban Ecology Centre on a chilly Tuesday afternoon in October. Walking down Milton Street, we turned into a small alley and within a couple of minutes arrived at the back gate of an unremarkable duplex. After ascending to the second-floor landing, Rose hauls out a ladder. We climb.

The roof is surprisingly small, a modest-looking array of shrubs, flowers and other plants, sandwiched between the brick wall of a taller building and the gravel roofs of adjacent duplexes. Its simple appearance belies a hidden complexity however. Rose and company wanted to show Montrealers how a green roof could work for them so they picked a challenging but typical site—a two-storey, attached building in a heavily developed part of town—and created a green roof with both irrigated and non-irrigated sections, and a wide variety of plants. Beginning this fall, monitoring equipment hidden beneath the soil will report on the roof’s temperature and water retention levels.

If the experience of other green roofs is any indication, this latest one will have a distinctly positive impact. It will insulate the building—eliminating the need for air conditioning in the summer and keeping tenants toastier in cold months—clean the air and collect excess rain water that would otherwise overload the sewer system and cost the city a fortune to treat. Collectively, Rose tells me, green roofs can have a big impact on a city’s environment. “The National Research Council and Environment Canada have already done a study in Toronto,” he says, “and they found that if 6 percent of the city of Toronto was covered by green roofs, it would have about a one-to-two degree temperature difference during the summer—one-to-two degrees cooler.”

The same cooling effect can extend to buildings’ interiors. In a city like Montreal, which has hot and humid—but relatively short—summers, green roofs could not just reduce the use of air conditioning but eliminate it altogether. Heating costs could come down too—in the spring and fall, buildings with green roofs are noticeably warmer. Best of all, green roofs last far longer than ordinary ones. “There are green roofs in Germany that have been around for fifty years and still haven’t been replaced,” remarks Rose.

In new construction, building a green roof is surprisingly cheap, costing just twelve dollars more per square foot than a normal roof. “As it is, there’s nothing that should stop all new buildings from having green roofs. It’s much more cost-effective on a brand new building.” On older buildings, though, installing a green roof can be a costly endeavour. The Urban Ecology Centre’s green roof cost $40,000 and they had to scrap plans for a roof deck because they ran out of cash. “In this case the entire project was done without raising any dust on the inside. It was all done on the exterior, which made it quite a bit more expensive.” But over time, adds Rose, a city’s building stock will inevitably renew itself, through new construction and renovation, so it would make sense to subsidize or even require the construction of green roofs on new buildings.

Owen firmly believes that in fifteen years, green roofs will have become mainstream in North America. The public’s interest is already piqued—the day after I visited, a reporter from Elle Québec, a women’s lifestyle magazine, took a tour of the roof—and politicians love the idea. “For them, roofs are a neutral space. There’s no downside to supporting green roofs,” says Rose. A year ago, Montreal launched an internal committee to investigate green roofs and consider any possible subsidies or regulations.

The first step is to convince government to require green roofs on their own buildings, says Rose. In most Canadian cities, dozens of buildings are owned by the three levels of government—such a move would have a huge impact. Next would be to establish subsidies for co-operatives and individual homeowners to build green roofs. Eventually, as green roof technology becomes cheaper and more accessible, green roofs could be required on all new construction.


Currently, the only subsidy for green roofs in Canada exists in Montreal. Starting next year, Gaz Métropolitain will offer a subsidy of $5 per square foot to clients who have green roofs. For the roof atop the Urban Ecology Centre, that would have translated to an extra $5,000 in their pockets. However, most of Montreal’s buildings are powered by electricity, not natural gas, and the local hydro company has yet to offer any similar green roof subsidies.

Still, there is a lot that the average person can do, even if he or she is a penniless tenant like me. Montreal is a city of balconies, so let’s start greening our balconies. With an estimated 1.2 million balconies on the island alone, a few plants, some vegetables and a flower pot on each one would go a long way.

The Urban Ecology Centre has a detailed account of the construction of their green roof, in French, here. Documents explaining the roof’s specifications and the Centre’s goals are available in both English and French.

Christopher DeWolf wanders our streets as Maisonneuve’s urban affairs critic. His column appears every two weeks. Read more columns by Christopher DeWolf.