1 The garden in the city 2 Beginning construction. 3 The growing roof
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; how- ever, 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 pol- icy to encourage green roofs through urban planning and financial subsidies. 4 More and more building professionals and contractors are learning about green roof con- struction. To facilitate the learning process, the Urban Ecology Centre published two green roof reports (in French) in 2005 and 2006. The first report, Toitures vertes à 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 toit vert, documents the demonstration proj- ect from 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 mem- ber. The Canada Mortgage and Housing Cor- poration (CMHC) has also published reports relating to green roofs. Team Effort The basic construction team for a green roof includes the client, architect, structural engi- neer, 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 cor- rect 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.
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What about Leaks?
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 exist- ing flat roofs are able to support the weight of even the lightest green roof assemblies. To study residential green roof retrofit possibili- ties, the Montréal Urban Ecology Centre built a demonstration project on top of an existing 100-year-old Montréal duplex in 2005. 3 The project included the complete reconstruction of the roof structure followed by the installa- tion of a 15 cm thick extensive roof. Half of the project cost was related to the structural retrofit. Although successful, the project was expensive.
Modern green roof experience started in Germany more 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 creating the lightest technology possible so that plants are still able to survive summer droughts and, more importantly, cold Canadian winters.
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