In snow and cold country, a roof design must address snow, meltwater, and potential ice buildup in addition to the usual water-shedding conditions of temperate and tropical climates. Even roofs designed to hold snow must allow for meltwater drainage and some ice buildup.
With the exception of increased structural capacity, the functional design of a flat roof for snow and cold country is similar to that of a flat roof for a temperate climate. A flat roof that has internal roof drains and is surrounded by a curb or parapet meets the challenges of snow and cold well. It usually maintains a blanket of snow that insulates the roof and reduces interior heat loss (Figure 14). The meltwater flows over the roof and down snow-insulated drains to pipes within the warm building; since the water doesn't contact freezing air, ice doesn't form. And wind stripping helps mitigate excessive snow depths—though the design must also consider the detrimental effects of snow drifting and potential cornice formation. Also, ice water draining through pipes in warm, moist interior air can cause condensation; this can be controlled with pipe insulation.
Sloping roofs fall into one of two categories: warm roofs and cold roofs. Either type will work satisfactorily when properly designed. Warm roofs are better when it is desirable to slip snow from the roof surface and space exists for snow dump areas on the ground. Cold roofs are preferred when it is better to retain snow on the roof, although they also can be designed to shed snow.
A warm roof melts the roof snow blanket with the building's upward heat loss. The meltwater runs downslope, emerges from under the insulating snow blanket (usually at the eave), and refreezes, usually forming ice dams and icicles. As described above, these formations can cause leaks and structural overloading, and when ice dams and icicles finally break off they are likely to damage the roof edge and any part of the building they hit. Plus, it can be extremely hazardous to one's health to be standing beneath falling ice and saturated snow (Figure 15).
