If the building is designed to encourage snow to slide off the roof, the roof should be steep and slippery. It should be arranged so the snow will slide off cleanly, falling into areas where it will not cause harm.

A flat roof, generally defined as a roof that has a slope of half an inch in one foot or less, eliminates ice damming, sliding snow, and icicle formation and it takes maximum advantage of the insulating properties of snow. In most locations, wind stripping will control the snow buildup on the roof, and the 1982 ANSI standards permit a 20 percent reduction in snow load in windy locations. The roof should slope slightly toward interior drains, which should he brass with copper pipe. The heat of the building will keep the drains free of ice as they are protected from extreme cold by the blanket of snow on the roof. Scuppers in the outer parapet walls should be located several inches higher than the drains. These scuppers only operate if an interior drain becomes plugged or if the water volume is too great for the drains to handle, as sometimes happens when warm rain falls onto a heavy snow pack on the roof.

A flat roof will be satisfactory almost anywhere in the snow country except those rare locations without exposure to wind or sun. Even under these conditions, it may be more practical to increase the structural strength of the roof than to make the roof steep enough to shed the snow.

The roof and walls of a building retain humidity as well as heat. Warm air holds more moisture than cold air, and air at 70 degrees can hold three times the water of air at 40 degrees. Outdoors, the moisture precipitates as rain or snow as air cools: indoors, it becomes dampness on walls, windows, and ceilings.

This condensation is often mistaken for leakage. Condensation is controlled by preventing warm moist air from coming in contact with cold surfaces. At any given temperature, the air can only hold so much water. When the temperature drops, the dew point is reached and condensation will take place. A vapor barrier must be created on the inner face of the outer walls and ceilings of the building to seal in the warm moist air and prevent it from reaching cold surfaces that are below the dew point.

Condensed water vapor can become trapped in insulation in walls and roofs and freeze in cold weather. When the temperature rises above freezing, this trapped moisture melts, and the roof or walls appear to leak. The condensation may remain hidden until mildew, rot, delamination of plywood, or paint deterioration occurs. Any trapped moisture can permanently damage the building insulation. In the cold country, rainwater must not be allowed to soak the construction, as it will freeze and damage will occur, come winter.

Windows are a major challenge to the architect in the cold country. Condensation sometimes occurs even between panels of double-pane glazing. Windows with metal frames often appear to leak, as metal is an excellent conductor of temperature and the cold is conducted inside, bringing the warm inside air below the dew point. One solution is to disconnect the window frame with a piece of plastic, so there is no thermal contact between the exterior and interior of the frame. Another is to use nonmetallic window frames. Condensation on the window glass itself can often be controlled by mounting the inner face of the glass flush with the inner face of the wall.

Except in the arctic winter, sunshine should always be considered as a source of winter heat. My flat-roofed mountain house, at an altitude of 6,400 feet near Lake Tahoe, has solar collectors mounted on all south-facing walls. The vertical collectors function efficiently in the winter, catching the solar energy both from the low angle of the sun and reflections from the ground snow. The vertical collectors are never covered by snow. The structure presents its main face to the south and all major rooms are heated both actively and passively by the sun. The heat from the collectors is stored in an insulated 4.000-gallon basement tank for use on dark days. The tank usually stores enough extra heat for a hot tub on the lower deck.

One reason my mountain house (Figure 10), is so energy-efficient is that it is well insulted and tightly sealed. Ten years ago, few architects concerned themselves with indoor air pollution because buildings were so porous that outside air infiltrated into them at rates high enough to provide adequate ventilation. Now the drive to conserve energy in the cold country has cut this infiltration to one-tenth or less of what it was.

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