Since the dawn of history, man has lived in severe climates such as the Himalayas and the polar regions. He has learned techniques to conserve his body heat and to warm his habitats. Our forefathers comprehended this manipulation of nature intuitively with experience gained through generations of experimentation.

But as technology has accelerated in the past century man has lost touch with his surroundings. The remarkable relationships between man and his environment found in such traditional cold country forms as the chalet and the igloo are no longer understood. (figure 1)

The igloo is an intuitively correct design for its environment, using the natural properties of snow to the advantage of its occupants. Lisa Heschong has pointed out in Thermal Delights in Architecture that "the Eskimo essentially lives within a semitropical environment" with the help of his fur parka and his igloo. The igloo has a minimum surface area in relation to its volume, efficiently conserving heat. The blocks for the structure are cut from porous snow, and, after the igloo's erection, the inner surfaces quickly absorb the moisture produced by body heat and fire. The inside freezes, strengthening the igloo, preventing air infiltration and preserving the insulating properties of the snow.

Snow, a mixture of ice and air, is a semisolid form of water. In cold country, the most rigid constraints on design are imposed by changes in the density of water, not by freezing temperatures alone. Water expands when it freezes, and this reaction produces forces powerful enough to crack rocks, walls, and pavement, to tear shingles off roofs, and to force foundations out of the ground. It has been estimated that the force of crystallization of ice is as much as 30,000 pounds per square inch in a confined space. Few materials can resist such pressure.

Freshly fallen snow is as light and insulative as down. The plumes of snow crystals interlock with one another as they fall. The crystals entrap air and become immobile. Sooner or later, depending on temperature, humidity, and air pressure, the fine points of the snow crystals evaporate, and the air in their centers is filled with recondensed ice in a process called sublimation. The once delicate crystals can become as slippery and as unstable as a pile of ball bearings. No longer interlocked, they are mobile and can slide off roofs onto the heads of the unwary or avalanche down mountainsides into the works of man. (figure 2)

Should the temperature of the snow rise above freezing in the daytime, the melting and refreezing at night tends to glue the particles together into a living, plastic medium that can be as solid as ice or can change into avalanche-prone, unstable crystals of hoarfrost. These complex and unpredictable changes can cause a sudden fatal event in a scene of picture postcard serenity.

The downward movement of snow on a pitched roof is determined by several factors, which include: the
quantity and quality of the snow itself, the temperature of the air and the roof surface, the steepness of the slope of the roof, and the coefficient of friction of the roofing material. In general, wet or icy snow tends to stick to rough roofs of low slope, and loose dry snow tends to slide from slippery, unobstructed roofs of high slope angle.

The 1982 standards of the American National Standards Institute, whose recommendations for calculating snow loads attributable to structures are far superior to earlier building codes, permit the snow load on a roof to be neglected only when its slope exceeds 70 degrees. ANSI allows for some reduction in snow load for unobstructed slippery surfaces, but only where the slope of cold roofs exceeds 30 degrees.

Many think snow will adhere to asphalt or wood shingles and slip off metal roofing, but, in reality, depending on the angle of slope and the weather conditions, snow can stick at times to the most slippery of roofs.

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