In the upper roofs of the Courtyard on Bear Street build- ing, like in many alpine precedents, the incline of the roofs balances the requirement to shed snow and rain with a need to provide as much useable space as possible beneath it. Its low slope allows snow to accumulate and provide an insulat- ing blanket against the winter chill.
u rban development in Canada is intrinsically associated with surviv- ing and adapting to winter. Banff,Alberta sits at an elevation of 1,384m above sea level, latitude is 52°N, it is surrounded by mountains and is subject to unexpected winter warming ; all of which mean it is both dry and cold, with heavy snow that can melt quickly on short days in a long winter. The Land-Use Bylaw of Banff and the Banff Design Guidelines dictate a material palette for roofing of cedar shakes, heavy asphalt shingles, concrete tiles or slate tiles. For the low slope roof on this project, our first choice of cementitious corrugated roof panels presented problems with jointing, torsion, drainage and ice-jamming due to the effect of freeze-thaw cycles on lapped material. We developed instead a composite system of cedar decking affixed to a layer of strapping above a full SBS roof membrane. Snow Sliding of snow occurs at the roof-snow interface. If snow stays on a roof, it either melts slowly because of building heat loss through the roof assembly, or rapidly because of high outside air temperatures, rain or sun. The cohesion and friction forces between snow and a roof surface also vary with the roughness of the roof itself. Normally, snow slides when heat loss through the roof causes the 0°C temperature line to move up into the insulating snow layer and melt the bottom surface of the snow, lubricating the roof surface and destroying cohesion and friction forces. An insulating layer of snow cover can allow significant melting at the roof surface, producing a layer of slush which allows the snow to slide. 1 Ice Melt water will freeze on roof eaves, forming icicles, ice on walkways below and ice dams where water from the melting snow freezes in the drains. 2 Detail: snow/surface/water Kerry Ross
The project here is a green, sustainable mixed use development in Banff, and the detail looked at here is a double roof skin, designed to deal with snow on roofs.
References: Taylor, D.A. ‘Danger: Falling Snow’. Construction Practice. Ottawa: National Research Council of Canada, 1990. Taylor, D.A. ‘Sliding Snow of Sloping Roofs’. Canadian Building Digest no. 228. Ottawa: Division of Building Research, National Research Council of Canada, 1983. Baker, M.C. ‘Ice on Roofs’. Canadian Building Digest no. 89 . Ottawa: Division of Building Research, National Research Council of Canada,1967. The roof system used on the Bear Street project allows melt water from snow accumulation to drain through gaps in a top surface of cedar decking. Like a cascading stream in a mountain crevasse, drain water is channeled by waterproof strapping to a roof valley and down to water basins where it is guided through piping inside wood columns to an underground cistern beneath the parking ramp. The snow remains dry above the cedar cladding, reducing the weight of the snow and preventing the formation of ice-jams.The low slope design combined with the ability to drain any melt water eliminates the need for snow fences.
project: BEAR STREET PROJECT location: Banff Alberta client: Arctos & Bird Enterprises Ltd. date of completion: Oct. 2004
architects of record: Zeidler Carruthers & Associates Architects design architects: William McDonough + Partners specifications: Susan Morris Specifications structural engineers: Read Jones Christoffersen mechanical engineers: MechWave Engineering electrical engineers: Stebnicki Robertson & Associates landscape architects: Scatliff Miller Murray (roof)
VDMO Landscape Architects (courtyard)
energy consultant: GF Shymko & Associates builders: PCL Maxam Construction Management
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O n S ite review
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I ssue 9 2003
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