It has recently become popular in cold regions to require more and more heat resistance in roofs. Some designers advocate super-insulated roofs, where the roof insulation might exceed the wall insulation by three or even four times. This raises the first cost of the building and often cannot be justified by energy savings. In cold country, the problem with this approach is that the thickness or thermal capacity of the heat resistant material is not the only factor in effective thermal roof function. Often cracks between insulating batts allow warm, moist air to reach the outer roof surface, effectively circumventing the insulation regardless of its thickness.

The retardation of moisture is as important as thermal resistance. Warm air retains more moisture than cold air. It is desirable to keep residential building at approximately 50% RH during winter months. If the buildings are constructed without a proper vapor retarder on the warm side of the insulation, condensation may occur either inside the insulation (reducing its effectiveness) or on the under side of the roof surface. This condensation may appear as roof leaks, even though the roof surface itself is perfectly dry. The migration of moisture through the roof may actually reduce the thermal properties of the roof as the insulation increases. The higher the internal moisture (vapor pressure) and the colder the outside temperature, the better the vapor retarder must be. Vapor pressure is a powerful force and moist air can find its way through the smallest crack.
Even in cold locations where there is very little precipitation, effective control of the moisture inside a building is vital to the success of roof design.
Great internal damage can occur to buildings whose interior air is warm and moist if that air reaches the inside face of the exterior skin of the building. For these reasons the cold must be understood and be considered in the design of buildings.

III. THE INFLUENCE OF SNOW ON ROOF DESIGN

Snow adds load to roofs where it falls on them. These loads are often more like dead loads than live loads. On low-sloped (flat) roofs these loads are often thought to be some fraction of ground snow load, but this can be a dangerous assumption as will be discussed below. Most designers assume that steeply sloping (4 in 12 or greater) roofs will shed the snow and many codes allow a reduction in load for such roof. But, the author's experiences have shown (Figure 2) that even slippery-sloped roofs do not reliably shed snow. The snow can be held on the roof by projections such as vent pipes, skylights or even standing metal ribs. (Figure 1) Perhaps the most serious deterrent to sloping roofs sliding their snow blanket is the ice dams that form at roof edges and behind roof projections. Building heat or solar radiation can cause the snow on the roof to melt. The melt water runs down the roof slope under the insulating blanket of snow and refreezes when it comes in contact with cold outside air.

The ice formed adheres to the roof surface and prevents the snow from sliding as is discussed below. These ice dams can greatly increase roof loads and can cause very serious leaks.

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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.

Figure 3
Figure 2