December 5, 2009
Industry experts weigh merits of green roofs
Vancouver can be an odd place. In recent months, the municipality dug up part of city hall’s lawn for a vegetable garden.
It has also approved chickens in backyards and has been busy creating bike lanes where previously only evil autos freely roamed.
But, unlike its sibling rival, Toronto, it has not passed legislation mandating green roofs on any development, large or small.
This is in a city that is aiming to be the greenest in the world.
However, green roofs have been a controversial topic in the design and construction industry – at least until a few years ago.
Among other things, critics claim they cost too much and are ill suited to Vancouver’s rainy climate.
One by one, these criticisms and others have been brushed aside.
One of the key players shining a light on the whole world of green roofs has been the B.C. Institute of Technology (BCIT).
Maureen Connelly is director of the Centre for Architectural Ecology, which is part of BCIT’s School of Construction and the Environment.
The centre was formed in 2002 and operates a sophisticated research facility at school monitoring different types of green roofs, which are more correctly called vegetative roofs.
It also monitors a network of vegetative roofs in both the Lower Mainland and Victoria.
Critics often claim that green roofs are limited when it comes to controlling storm run-off water in the Vancouver climate.
They claim the roofs work well in climates such as those found in Alberta or southern Ontario, where long dry spells are often interrupted by dramatic downpours.
But, in Vancouver with its months of drizzle? Not so much.
Connelly said this is not correct.
She explained that determining the answer to that question was one of the main reasons that the BCIT centre was established.
In Vancouver, she pointed out, the city gets about 1,500 mm of rain per year.
Some 1,200 mm of that falls during the wet season - from October to May.
Even given those figures, she said that a vegetative roof can reduce run off by about 28 per cent.
On large projects, such as the Electronic Arts Building in Burnaby, she said she feels that the reduction could reach as high as 40 per cent.
However, she is more comfortable sticking to the 28 per cent figure.
The depth of the soil is important in the summer.
A six-inch deep roof will retain considerably more run-off from a dramatic downpour than a three-inch deep roof will.
In Vancouver, in the winter, the depth doesn’t make any difference – both a three-inch and a six-inch roof will get soaked right to the bottom.
Both, however, continue to slow run-off.
Cutting down on storm run-off water can be a real money saver in places like Toronto.
Again that city appears to be ahead of Vancouver.
Unlike Vancouver, Toronto charges building owners for storm run-off water they contribute to the city’s sewer infrastructure.
The less run-off a building creates, the smaller the bill they get from city hall.
In addition vegetative roofs can provide anywhere from seven to 10 per cent reduction in heating and cooling costs.
Besides their sheer mass, green roofs in summer provide shading from the plants on them and cooling as the plants transpire.
In addition, a green roof doesn’t absorb heat the way a traditional roof can.
Do green roofs really cost more?
It depends on how you look at it, Connelly said.
Tyrel Sutton agreed.
He is in business development in the Lower Mainland with Flynn Canada, one of the nation’s largest building envelope specialists.
Both of them pointed out that initially a green roof will cost more than a traditional roof.
Sutton, whose company provides pretty well any kind of roof you can imagine, said it can cost an extra six or seven dollars per square foot to go green.
However, he said that the cost has to be factored into the long term operating costs of a building.
She pointed out that a green roof can be expected to last at least twice as long as a standard roof.
The roof membrane, being covered with soil and vegetation isn’t subject to the dramatic daily swings in temperature that a traditional roof is subject to.
It is those variations, which even in Vancouver can regularly go from 16 degrees C to 55 degrees C daily that cause membranes to deteriorate.
Then, of course, it is necessary to factor in all those other savings such as energy costs, sound proofing advantages and – where applicable – storm water run-off costs.
The industry, said Sutton, has not done a good enough job selling the development industry on the merits of green roofs.
“We have to show developers they’re not just a cost – they are also a benefit,” he said.
“That’s something we haven’t done a good enough job of in many cases.”
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