11 February 2009

Blue Roofs

the fifth façade – beautiful affordable eco-roofs

Owen Rose www.ecosensual.net
The human body is largely composed of water and its intimate connection to the blue planet does not stop with the flush of a toilet.
Often a source of grief; too much or not enough, water is both a visible and invisible concern for cities. not only do we have to find a source, clean it up for potable use and then distribute it, but we also have to dispose of it. Black or grey, waste water treatment is a costly and difficult task. Think of cities such as Halifax and Victoria that dump their untreated wastewater directly into the ocean. Although Montréal has a large sewage treatment plant a the east end of the island, the city still averages about 22 discharges of untreated sewage directly into the Saint Lawrence River each year. When it rains too much the combined storm and sewer system cannot process all of the water from our houses, hospitals, factories, rooftops and polluted streets. Thus the problem is passed on to the fishes, whales and other cities downstream.
Now that we have started to face the growing threat of environmental problems such as urban heat islands, air, water and noise pollution as well changing weather patterns, ecological building criteria such as the LEED (Leadership in Energy and Environmental Design) green building rating system have encouraged the construction and design of more ecologically sustainable buildings. In the six LEED categories, water management takes on several forms: one is a vegetated roof.
Instead of installing deserts of tar and gravel on our roofs, more and more institutions, businesses and homeowners are opting to grow fields, gardens and vegetable patches on them. Aside from their obvious beauty, green roofs offer natural cooling, a greater lifespan and better rain water management. They retain about 50% of fallen rain (returning water to the atmosphere bypassing the city's sewer system). The other 50% will still find its way to the drain, but the return of this water is delayed, which helps city infrastructure manage water levels during downpours and intense rainfall.

What about Leaks?
Modern green roof experience started in Germany fore than forty years ago, and the waterproof aspect of such roofs is no longer a major concern. In North America, quality waterproofing membranes are on the market, capable of withstanding the constant humid environment of green roofs. Also, earth cover protects the membranes from large day to night temperature fluctuations and the sun's ultraviolet rays, both of which break down conventional roofs over their 20-year lifespan, whereas green roofs should last about twice as long. The real challenge is in created the lightest technology possible so that plants are still able to survive summer droughts and, more importantly, cold Canadian winters.

Green roofs on New or Existing Buildings?
In the deluge of new interest in green roofs, some owners of existing flat-roofed structures have looked at retrofitting an extensive green roof on their buildings; however, not all existing flat roofs are able to support the weight of even the lightest green roof assemblies. To study residential green roof retrofit possibilities, the Montréal Urban Ecology Centre built a demonstration project on top of an existing 100-year old Montréal duplex in 2005. the project included the complete reconstruction of the roof structure followed by the installation of a 15cm thick extensive roof. Half of the project cost was related to the structural retrofit. Although successful, the project was expensive.

In the case of new construction, extensive green roofs are much more economical and usually cost two to three times the amount of normal roof standard roofing systems; however, these green roofs should also last twice as long. With the natural renewal of a city's building stock, the widespread installation of green roofs with new construction would transform a city's roofscape over time. In 2006, the City of Toronto adopted public policy to encourage green roofs through urban planning and financial subsidies [www.toronto.ca/greenroofs/index.htm].
More and more building professionals and contractors are learning about green roof construction. to facilitate the learning process the Urban Ecology Centre [www.urbanecology.net] published two green roof reports (in French) in 2005 and 2006. the first report, Toîtures vertes a la montréalaise, was a 100-page introduction to green roofs for the southern Québec climate with a survey of green roof experiences in Québec and around the world. The second report, Projet-pilote de toît vert, documents the demonstration project form its initial planning to ongoing plant maintenance. It includes many photos and illustrates the project's costs, materials, earth and plant choices, the role of each team member. the Canada Mortgage and Housing Corporation (CMHC) has also published reports relating to green roofs [www.cmhc-schl.gc.ca].

Team Effort
The basic construction team for a green roof includes the client, architect, structural engineer, green roof supplier, general contractor and roofing subcontractor. Depending on the extent of the project, a landscape architect or a horticulturalist could also be part of the team. there are more and more green roof suppliers in Canada. Technically, the construction of a green roof is not that difficult; however, due to the additional weight, a structural engineer should always be consulted. Ultimately the success of a project is determined by the correct choice of plants and substrate thickness given the roof's particular microclimate and the client's desired aesthetic. the true test of a green roof is whether the plants are able to survive three consecutive winters; so, when a new green roof technology appears, it is always a good idea to ask if it has at least three years of proven success and/or a good warranty.

Green Roof Composition
The green roof system can be installed once the roof structure has been properly designed and the appropriate roofing membranes have been applied. the system varies from one supplier to another, but usually contains a number of items such as an anti-root membrane, water drainage panels, a geotextile and specially formulated green roof light earth substrate. The weight of the earth holds the system in place by gravity.
Now the gardening begins! The most appropriate categories of plants for extensive green roofs are wild grasses, wild meadow flowers and sedums (waxy plants). the ecological goal is to favour indigenous plants, but the final choice depends on the chosen green roof technology and the roof's planned use. A number of institutional extensive projects in Montréal have used clover for its low-cost and tenacious character. Each green roof supplier is capable of furnishing information about the appropriate choice of plants and their aesthetic impacts. The installation of an irrigation system also depends on the type of roof and choice of plants. The more ecologically-oriented the roof, the less likely that it will need an irrigation system. In cities, irrigation of green roofs and gardens uses municipal water. Rain and/or grey water recuperation and indigenous landscaping strategies can help reduce summertime demand for city water resources.

It's Alive!
Unlike a normal roof that most of us tend to forget,a green roof does require regular maintenance. For an extensive ecological roof, the maintenance is minimised to about six times a year from spring through to autumn. This requires inspection of the roof drains to make sure that they are clear of debris, trimming the plants in spring and periodic weeding. If the roof does not have a built-in irrigation system, even the hardiest of ecological roofs may need additional irrigation during intense summer heat waves. Much of the roof's maintenance will depend on how the roof is used, the plants that have been chosen and the owner's gardening habits. Residential green roofs can also be used for recreational gardening.
Loft owners in Montréal have also invoked the idea of an urban cottage where the roof may include an outdoor bedroom, shower, summer kitchen and patio surrounded by a roof-top meadow and views of the city. For the building owner, green roofs increase building value and create a personal urban oasis. For the larger community, green roofs reduce the load on a city's water infrastructure and help to moderate urban temperatures. Clearly, green roofs not only benefit individuals, but they also benefit our neighbourhoods and our cities. the fifth façade has never shown so much potential.

Rose, Owen. 'Blue Roofs' On Site review, no. 17 Spring/Summer 2007
©Owen Rose and On Site review

1 comment:

hkki said...

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