Warm sloping roofs (i.e. sloping roofs with little or no attic or rafter ventilation) will melt the roof snow faster and more continually than a "cold roof" for the same installed roof insulation. This continual melt water generation must be constantly intercepted and drained away before it is exposed to the freezing air at the eaves. If not, the melt water will refreeze and form ice dams and icicles. Icicles may be exquisitely beautiful but other than that, neither ice dams nor icicles have any redeeming social value. Ice dams cause a concentrated unbalanced loading condition at eaves. Ice dams cause melt water to pond and back up behind the dam. This in turn can easily lead to leaks if the roof substrate is not adequately waterproofed. Icicles and ice dams are deadly when they fall and destructive to the roof when they tear away. Ice dams usually take shingles, flashings, and other materials along when they fall. Shoveling off roof snow to mitigate ice dams is usually a case of wasted time and money with a hefty roof repair cost thrown in for good measure. Chipping away at ice dams with shovels, picks, etc. more often than not damages the roofing. Shoveling from the eave partially up the roof is a waste of time and energy as an ice dam will form just down slope of where the shoveling stopped (melt water hits the freezing air at that point). Only if the entire roof slope from eave to ridge is cleared of snow will the ice dam/icicle formation cease.

Cold sloping roofs (i.e. sloping roofs where a great deal of natural ventilation is provided at the attic or rafter space) work well to mitigate ice dam formation. (Figure 5) With a properly designed cold roof, the large outside air ventilation air flow below the roof surface washes away the building heat loss migrating into the cold roof cavity. When this "washing away" effect is substantial, little or no interior heat warms the underside of the roof, and therefore the sloping roof snow cover is not melted by building heat. No melt water, no ice dam/icicle formation.

A cold sloping roof and its ice dam mitigation performance will be discussed in section VI.


VI. ANALYSIS OF A COLD ROOF DURING THE WINTER OF 1998/1999

The journal article "Attic Ventilation Guidelines to Minimize Icings at Eaves" (Tobiasson, Buska, Greatorex, 1998) presents a case study of ice damming and formation of icicles at four identically built buildings in upstate New York. Outside temperatures and attic temperatures were measured. The conclusion of this paper is that icing problems at eaves occur only when outside temperatures are below -5.6°C (22°F), and attic temperatures are above -1.1°C (30°F) at the same time. Tobiasson (1998) refers to this situation as "Icing Envelope". Later mechanical ventilation in the attic space was introduced in 56 buildings with former icing problems. This ventilation was designed to keep the attic temperature under the critical -1.1°C (30°F). Maintaining these low temperatures, icing problems were avoided or significantly reduced. The article reports that "there were no reports of problematic ice dams or icings on any of the modified buildings". Tobiasson, Buska, Greatorex, (1998).

This test building is at Sugar Bowl, Norden, CA (near Donner Summit) in the Northern Sierra Mountains. (Figure 6) This cabin is the now constructed Third Case Study in the article "The Impact of Ice Dams on Buildings in Snow Country" Mackinlay, Flood (1996).

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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 5
Figure 6