Greening our roofs

A trend towards green roofs in cities If someone were to tell me there was something green growing on my roof, I'd probably rush to the phone to call for help. But not if I were Richard Brault, whose industrial design studio is in downtown Toronto. He and his wife, Dianne Croteau, had a garden installed on the roof of the building they own next door. From their living room window they now look out over a colourful planting of grasses, purple coneflowers, wild ginger, bee balm and blue lobelia.There's a trend toward green roofs that's slowly gathering steam in Canada and the U.S., following a much bolder European lead. “It's a modern take on the old sod roof,” says Richard Kula, an environmental scientist with Prairie Architects in Winnipeg, who oversaw the installation of a garden of plants native to the Prairies on the roof of Mountain Equipment Co-op's Winnipeg outlet in May 2002. (A wildflower garden has been growing on the roof of the company's downtown Toronto store since May 1998.)Green roofs are not balcony gardens, with potted plants artfully arranged on the concrete. They're real gardens growing in a lightweight soil mixture installed on rooftops over layers of various materials that control roots and drainage. The systems vary with the manufacturer and the roof type, but most can go right over an existing surface, as long as it's watertight and in good condition. Green roofs can be installed on flat roofs or sloped ones—“over 30 degrees and the soil could slide off,” warns Kula—and are either extensive or intensive (see “Extensive vs. Intensive”, below), depending on how you want to use them. Kula and Candy Venning—a landscape designer with Perennial Gardens Corp., which installed the Brault/Croteau roof garden, as well as several others in Toronto—are part of a new generation of enthusiastic green roof supporters, and not just because of their decorative appeal. “They have enormous environmental benefits,” says Kula, and can play a significant role in managing storm water. In a really heavy downpour, storm water can overflow gutters and rush over your average bare roof, then pick up oil and heavy metal particles from driveways and roads before running into storm drains. Storm water can also overflow into the sewage system, carrying raw sewage into lakes and streams. (Contaminated storm water is the number one cause of water pollution in most cities.)Extensive vs. IntensiveThe industry divides green roofs into two types: extensive and intensive.Extensive gardens have shallow soil (eight to 12 cm) that doesn't support deep roots. Commonly installed on sloping roofs, they also work well on flat roofs, either for their decorative effect or as accessible gardens with winding pathways, benches and barbecues, assuming the roof can withstand the weight of these accoutrements. Low maintenance plants such as hardy grasses, mosses, sedums, thymes and some wildflowers work best because they tolerate drought. Extensive gardens are often the best choice for retrofit projects on existing roofs. The Brault/Croteau garden is an extensive garden.Intensive gardens have deeper soil—15 to 35 cm—that will support perennials, shrubs and some trees; they require more maintenance and a good irrigation system. They're more often accessible spaces used for recreation and growing plants, such as vegetables. These gardens definitely need a strong roof to carry the weight of extra soil and sturdier plants, as well as garden furniture and structures such as trellises. The Mountain Equipment Co-op installation in Winnipeg is an intensive garden. Major benefits of green roofs 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.SOURCESPerennial Gardens Corp., 99Sudbury St., Toronto, Ont. M6J 3W6; 416/531-1461.Prairie Partnership Architects, 141 Bannatyne Ave., Ste. 200, Winnipeg, Man. R3B 0R3; 204/956-0938. Installing them in your home Installing a green roof on your homeShe acknowledges, however, that a green roof is, expensive to construct (see “Bringing it Home”, below). “And unfortunately, even with the environmental advantages, a green roof won't increase the resale value of your home because people still don't know how to maintain them and they can feel intimidated,” she says.But in Europe, green roofs have been developed extensively for nearly 20 years for commercial and residential buildings. Recent issues of British gardening magazines have featured planted roofs in Somerset and Saddleworth Moor (near Manchester), and at last year's Hampton Court Flower Show, designer Marney Hall won a Royal Horticultural Society award for a potter's shed roofed in various sedums. In several German cities, property taxes on commercial buildings can be reduced by up to half if the building sports a green roof, a lure that resulted in an increase of more than 70 million square metres of rooftop garden in the 1990s.In Portland, Oregon, developers are able to construct more on smaller lots as long as the buildings have green roofs. A Chicago study estimated that, if all rooftops in the city were greened over, $100 million could be saved in energy costs just through reduced use of air-conditioning units; the drop in energy consumption would also mean a reduction of carbon dioxide, better known as greenhouse gas, and sulfur dioxide, which creates acid rain. Mindful of the deadly heat waves the city is subject to, Chicago is now issuing grants to contractors who install green roofs.To their credit, many Canadian landscape architects and commercial developers are trying out green roofs, even though cities aren't handing out grants or tax incentives. Little has been done in residential applications other than a few condominium or co-operative apartment buildings. “People in single-family homes don't think the small footprint of their roof can have much effect on the environment,” says Venning. “But if one person would do it, the neighbours would notice. They'd say, ‘Well, isn't that something.' They might not do it themselves, but they'd tell others. The idea would start to seep into people's consciousness and things would start to change. If it spread to a whole neighbourhood, it might even get on the news. And the next thing you know, everybody would want a green roof.”BRINGING IT HOMEGreen roofs must be installed by experienced professionals. Be prepared to pay $10 to $15 per square foot for material and installation (depending on how accessible the roof is), including the cost of plants, which generally runs about the same as it would for a ground-level garden. Drought-resistant sedums or native plants work best. Before you call a professional, make sure your roof is:• in excellent condition, with absolutely no leakage, as installation is over the existing roof.• meets local load-bearing requirements, generally 50 to 100 pounds per square foot.• either flat or has a pitch not exceeding 30 degrees. The simplest green roofs are composed of three layers installed over the basic roof:• a drainage layer made of a springy, porous material.• filter cloth, sometimes called landscape fabric.• specially prepared soilless mix, which is lightweight, resists wind displacement, retains water and is sometimes impregnated with a slow-release fertilizer.An inexpensive soaker-hose irrigation system zig-zagged through the garden is a definite advantage if your garden isn't easily accessible for hand watering. For more information, visit the website.