Figure 1
ABSTRACT:

This paper discusses those factors the designer and builder of roofing systems must consider when working in areas where snow falls and temperatures below freezing are encountered. It analyzes the snow distribution on such roofs and the various factors that influence their design and construction. It considers conventional wood frame as well as concrete and steel construction, with normal insulation, superinsulation and ventilated (cold) roofs, with examples of each. It discusses the impact of vapor retarders, roof slope, snow arresters, and roof surface materials on roof design. It reviews roof geometry and projections through roofs on overall performance. The advantages and disadvantages of various schemes are contrasted.

I. INTRODUCTION

There are places in the world where it does not freeze and thus snow almost never falls. There are other places where it is quite cold in winter, but very little snow is found due to lack of moisture. Snow and freezing weather is very rare near sea level in the tropics, but even there, snow and cold are present at higher elevations. In North America, for example, over eighty percent of the land area has snow, ice or freezing conditions in some months of the year that require the special attention of the architect or engineer engaged in construction projects. If these factors are disregarded in the design, grave consequences will result. At the least, the function of the project will be impaired; at the most the lives of the occupants will be in jeopardy. The design of buildings in regions of snow and cold is a complex subject because freezing conditions and the build up of snow and ice is an ever-changing phenomenon hard to simplify. Snow and ice can assume the aspect of rock or water. They can be adhesive or slippery. They can change characteristics in very short periods of time, moving from benign to pernicious, from beautiful to menacing, from tranquil to terrifying. This has made the hazards of building in areas of snow and cold difficult to quantify. Building codes have often left risk assessment up to local building officials, with uneven results. Many codes consider snow as simply applying vertical loads to roofs, and with the overly optimistic assessment that steep roofs will reliably shed snow, will permit loads to be reduced as the pitch of the roof increases. Such a simplistic approach is not safe. There is no substitute for a comprehensive design approach, which takes into such diverse factors as temperature, precipitation, structural geometry, roof materials, and projections, etc. In this paper we will attempt to briefly discuss the factors that influence roof design in regions of snow and cold. It will discuss in more detail one system of reducing problems on sloping roofs. (Figure 1)

II. THE INFLUENCE OF COLD ON ROOF DESIGN

Heated buildings dissipate internal temperature through their roofs during the winter months. If the interior is at living temperature, 20°C (68°F), the difference between inside and outside might be 40°C (72°F) or more and this differential becomes greater as outside temperature drops. If the area ratio between the roof and the walls is great, as for example, in a single story residential complex, most internal heat is radiated through the roof. Many current energy codes in the USA require twice the insulation resistance in the roof as in the walls of buildings. The rational for the excess insulation at the roof is energy conservation, but as will be seen below, the design requirements are much more complex than just adding insulation to reduce the heating bill.

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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, LÝset & 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.