On a low slope (flat) roof, there are almost al-ways vertical roof projections. In many buildings these projections are mechanical equipment, ductwork and piping, with related electrical conduit. Architecturally, the most common obstructions are perimeter parapet walls, stair and elevator penthouses. All of these elements impede the smooth flow of storm winds across the flat roof. On the windward side of these projecting elements snow will drift against them. This will change the normal uniform snow loading criteria to a surcharge loading situation in the immediate vicinity. This surcharge drift load can be quite high if the projecting element is tall and the lower roof lengthy. (ASCE 7-98)Roof beams parallel with the drift surcharge load can easily be overloaded and fail if not sized to accommodate this additional drift weight. Conversely, when a flat roof steps down to a lower roof on the leeward side, a drift load will form at this step. This drift concentrated load must also be accommodated in the structural design in a similar fashion as for roof projections.

At the roof edges, the wind can gradually cantilever the snow beyond the roof edge support, causing a snow cornice. Snow cornices can grow quite large and can impose a high eccentric load on parapets and cantilevered roof structures. These cornices may pose a danger to anyone or anything that may be on the ground below when they break loose and fall. Sidewall projections, such as cantilevered surface mounted light fixtures, are highly susceptible to snow cornice damage.

If the project under design has light design snow loads, care must be taken to investigate the local history of freak storms. It is not uncommon to have the 50-year mean recurrence interval loading be substantially exceeded by freak storms within an average adult lifetime. When 1.5m (5 feet) of 25% density snow falls over a two-day freak storm, the resulting 3.81 kN/m² (78 lb/ft²) is almost four times more than an original roof design load of 0.96 kN/m² (20 lb/ft²). High loads often result from rain on snow. This overload condition can potentially collapse these lightly designed structures.

Flat or low slope roofs should always be designed to slope toward roof drains and away from the perimeter curb or parapet. The drains should be located well inboard of the perimeter walls so that building heat will keep the melt water from refreezing. The rain water pipes and the drain bottoms should be insulated to prevent condensation on the pipe's outer surface and reduce the sound of dripping water. A moderate depth snow blanket on a low slope (flat) roof will usually insulate the drain and melt water from contact with the cold outside air preventing ice formation, although ice dams can form at drains when the drain grates are exposed to the freezing air. Overflow scuppers in flat roof parapet walls serve a very useful purpose. If these scuppers are dripping or show icicle formation, then the building occupant knows that the internal drains are blocked (either by ice dams or leaves and debris) and must be unclogged. As an additional safety factor, as long as the perimeter curb or parapet height does not exceed the weight of water height equivalent to the snow load, the bathtub formed will not cause collapse of the roof structure.

In summary, low slope (flat) roofs in snow country can have distinct advantages over steep sloped roofs. The snow is retained on the roof and (with the exception of snow cornices), does not fall to the ground. Ice dams are minimized and snow cornices can be relatively easily removed (as compared to eave ice dams) as the cornices are snow and not ice. Wind stripping can keep the roof snow depth to manageable levels, especially if favorable building to wind orientation is utilized in site planning. Melt water can be internally drained.

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Roof Design in Regions of Snow and Cold
by Ian Mackinlay, FAIA; Richard S. Flood, AIA/CSI and Anke Heidrich

Hjorth-Hansen, Holand, Løset & Norem (eds.) © 2000 Balkema, Rotterdam. Proceedings of the Fourth International Conference on Snow Engineering, Trondheim, Norway; 19-21 June 2000. Rotterdam: Balkema: 213-224. ISBN 90 5809
Photographs are by Ian Mackinlay except as noted.