Canadian Consulting Engineer

Should Green Roofs Be Mandatory?

I struggled with writing this comment. Editors are supposed to take a firm position on issues, but I find myself prevaricating on whether green roofs should become a fixed requirement on all buildings...

December 1, 2009   Canadian Consulting Engineer

I struggled with writing this comment. Editors are supposed to take a firm position on issues, but I find myself prevaricating on whether green roofs should become a fixed requirement on all buildings –which is the case in Toronto thanks to a new bylaw. Word has it that Burnaby and Vancouver in B.C. are considering a similar regulation. If mandatory green roofs catch on further, the entire urban landscape of Canada could be transformed.

On October 20 at the CitiesAlive conference on green roofs held in Toronto, hardly a question about their practicality was raised. When one lone voice asked the experts on stage -all green roof designers, vendors, activists and Toronto politicians –whether anyone had compared the energy savings vs. the maintenance costs of green roofs in dollar terms, the question was abruptly dismissed by Toronto’s deputy mayor.

The problem is that “our society is geared to science,” the deputy mayor said, whereas, “we have to go with our basic instincts as a society. Nature does it best.” At that, I found myself wondering whether he would take the same cavalier approach in ordering a new roof for his own home. Surely we’re right to expect that someone has carefully measured the entire benefits vs. lifecycle costs of green roofs before they become a fixture on all our buildings?

One often cited study that found environmental benefits with green roofs was published four years ago by the National Research Council of Canada’s Institute for Research in Construction (NRCC-48204). The research found that a green roof over a Toronto community building reduced the heat flow through the roof by 70-90% in summer, and 10-30% in winter, compared to an adjacent standard bitumen roof.

Green roofs also reduce stormwater run-off, which cuts down the load flooding into city sewers. The NRC/IRC study found that run-off from the green roof was reduced by 57% on average.

The benefits of green roofs might even be global. German researchers were at the Toronto conference to promote green roofs because they say the earth’s loss of vegetation -hence loss of photosynthesis -is the prime cause of rising carbon dioxide levels in the atmosphere.

But I still have some nagging questions. All that soil has to be transported and hauled up to the roofs, for one thing. The roof structure has to be more robust, requiring more structural material. In many climates, the roofs have to be irrigated, both to keep the plants alive and for fire safety reasons. With all that water sitting up there, are the roofs more likely to leak? What about mosquitoes? Instead of requiring green roofs, why not require developers to provide more trees and permeable paving at ground level? And once a building is higher than 12 storeys, their cooling benefits are dissipated. Then there’s the biggest question of all -ongoing maintenance. Who will make sure that building owners don’t let the vegetation die after a year or two?

With all these unresolved questions, my head is spinning. Hopefully someone with a more analytic mind will come up with more complete answers soon. Bronwen Parsons


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