Major benefits of green roofs
“Sometimes the rainwater gets to the lake while it's still warm from the roof, and this can affect the temperature of the lake,” says Venning. “But a green roof absorbs most of it, reducing the flow and cooling down the rainwater.” Trials carried out at Trent University in Nottingham, England, found that roofs with vegetation growing on a layer of lightweight aggregate retained all the moisture in a light, three millimetre rainfall, and 73 per cent of a 43-millimetre rain that fell over two days. The excess water slowly drains into the ground or is released as vapour into the atmosphere.
Green roofs also release oxygen into the air and alleviate the “heat island” effect of the solar energy emitted as heat by row upon row of concrete or asphalt roofs. Another Trent University study showed that the temperature on a bare roof on a moderate day of 18.4°C reaches 30°C, compared to 15°C on a green roof. And because the green roof doesn't transfer heat, the inside of the building also stays cooler. By the same token, the insulating qualities of the roof keep the building warmer in winter.
Mountain Equipment Co-op's Winnipeg building, constructed from recycled materials from three structures originally on the site, is definitely more than just a pretty roof. “It's unique,” says Kula. “It has features no other building has.” It's won every major sustainable building award in Canada.
He says it operates like a living organism. “The vegetation on the roof does what humans do: It sweats, and essentially becomes an evaporative cooling system. The rainwater from the rooftop prairie garden is captured and diverted to a basement reservoir. Then it's pumped back up as needed into a drip irrigation system under the soil on the roof. In fact, water budget calculations done for the installation of the green roof demonstrated that all the building's water needs could be serviced by rainwater, including drinking water. However, Winnipeg building code officials wouldn't allow us to do this.” A solar panel installed on the roof runs the pump for the drip irrigation. But more significant is the cooling power provided by the green roof: 43 million BTUs through water evaporation alone, totally eliminating the need for a conventional air-conditioning system.
Material from two composting toilets in the building (municipal laws don't allow more) is recycled. Solid waste is composted and used as a soil amendment, and liquid waste becomes a nitrogen-rich compost tea. Kula believes this system is also adaptable to residential use. “Eventually I see homeowners becoming totally water independent by using their yards and green roofs as one big water filtration system.” Venning says another advantage of the green roof is longer life for the basic roof underneath. “A lot of people shy away from the idea because they think there'll be water leaks in the house or that the roots will grow inside, but none of these things are going to happen. In fact, your roof will last longer because it doesn't have heat baking it in summer and cold freezing it in winter,” she adds.
SOURCES
Perennial Gardens Corp., 99
Sudbury St., Toronto, Ont. M6J 3W6; 416/531-1461.
Prairie Partnership Architects, 141 Bannatyne Ave., Ste. 200, Winnipeg, Man. R3B 0R3; 204/956-0938.
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