November 13, 2009
Creating Vancouver Convention Centre’s green roof no simple task
Working atop a structure built on piles and at sometimes extreme angles made life interesting for the crews working on the Vancouver Convention Centre’s new green roof.
The biggest living roof in Canada is surrounded by water on three sides, and the marine deck on which the building sits is supported by stilt-like piles.
It also features slopes of up to 53 per cent.
“There are no flat areas,” says Pete Taylor, a senior project manager with Flynn Canada, who oversaw the roof’s construction.
The idea was to integrate the project visually with the surrounding waterfront parkland and mountain backdrop so that the entire vista appears as a single, crescent-shaped green-belt.
A host of cutting-edge environmental features, including on-site water treatment, deep seawater cooling and heating, and a giant skirt designed to help restore the local fish habitat have the convention centre seeing LEED Gold certification.
But it is the roof that is most visible and that has attracted the lion’s share of attention. At 6.5 acres, it eclipses the National War Museum’s 2.6-acre green roof to rank as Canada’s biggest. It could hold 15 professional-size hockey rinks.
With more than 400,000 plants and grasses, featuring close to two dozen species native to the Pacific Northwest, the Vancouver roof is very much its own ecosystem.
It uses computers to detect leaks, and through a series of small channels and pipes funnels excess rainwater for re-use in the washrooms.
The roof even has its own colony of European honeybees.
Work crews encountered no shortage of challenges in building all of this. Ironically, water was common to all of them.
First, there was the not-so-small matter of getting materials to the site. Access for heavy vehicles was limited to a single entrance on the one side where there was dry land.
“We were sometimes bringing in three to four dump trucks of growing media a day,” Taylor says. “This was literally in the heart of the downtown, often in rush-hour traffic, and we had no room to park on-site.”
However, access paled in comparison with some structural issues. Because much of the addition and roof jut out over the water’s edge like a large pier, crews had to find innovative ways to haul materials, including plants, liners, and more than 5,000 cubic metres of growing media, to the top.
And, because the building is supported by piles, anything left on the roof had to be carefully and evenly distributed.
Crews had to use a hammerhead crane to reach the rooftop because the roadway at one end was the lone point of access. “With a regular stick crane you can get the height but you can’t get the reach,” Taylor said. “With a hammerhead you can swing in about 120 feet over the roof.”
Working cranes from the water wasn’t an option. “You’d have to put the crane on a barge, and the cost of doing that is astronomical,” Taylor says.
As soon as crews did get materials onto the roof they had to immediately move them to specific locations using conveyors and lightweight buggies instead of heavy machinery such as backhoes and bobcats.
The slopes proved particularly challenging. Taylor says crews built diverters to protect soil and vegetation from Vancouver’s rainy climate. Workers also laboured underneath rolling castor-style tents to keep materials dry.
Because of the slopes, some areas needed web-style retention systems, built using high-strength polymer matting and stainless steel cables, to hold the growing media, plants and 43 kilometres of drip irrigation piping in place.
Bruce Hemstock, of PWL Partnership, a Vancouver landscape architecture and consulting firm that worked on the project, said the roof portion of the job was one of the most technically challenging assignments his firm has taken on in its 35 years in the business.
“It was a very complicated project yet we wanted to use simple and straightforward, off-the-shelf components. It was challenging to figure out how to make it all work.”
PWL needed to ensure the construction team could easily obtain materials such as roof drains rather than having them custom built, Hemstock says. “The whole drainage system on the roof was designed so it could be ordered from any manufacturer in Canada and installed on the roof without anything special happening to it.”
With the various challenges addressed, project participants can focus on its benefits. These include the stormwater management, and the lush, green exterior is able to curb the heat-island effect which is typical with concrete and brick and is often blamed for localized warming.
As well, insulation from plants, waterproofing liner, six inches of growing medium, and four inches of extruded polystyrene increase natural temperature control and reduce the need for mechanical heating and cooling.
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