To keep the destructive potential of ice dams and icicles at manageable levels, sloping warm roofs require careful attention to detail in design and construction. Carrying building heat to the roof's edges can reduce ice formation. Drop zones—areas around the buildings that can receive falling snow and ice — are a must in warm-roof design. These zones must be located away from windows and door openings.
Most builder/developer-designed housing and light commercial projects in snow and cold country use warm roof types because they are the most economical to build. However, this can cost more in the long run because minimal drawings and specifications, plus a lack of proper snow and cold design and a speedy construction process, often result in a roof that is prone to damaging ice dams and icicles.
A cold roof is an upper roof that acts as an umbrella over the insulated structural roof below. It provides a ventilated air space, or cavity, between the two roofs. When the building loses heat through the lower warm roof, the heat is released into the air cavity, where it is ventilated and cooled by outside air before it can melt the snow pack on the upper roof (Figure 16). Cold roofs work best where midwinter air temperatures are low.
The ventilated cavity must allow a large volume of outside air to flow through it from the eaves to the roof ridge and "wash away" any heat that rises through the warm roof below. Air is circulated through cold roof cavities by convection and air pressure (wind) on the building walls. If the airflow cavity is too small or the airflow is restricted, the air temperature in the cold roof will rise above freezing in the vent cavity and melt the snow cover, resulting in ice dams and icicles at the down slope of the roof as in a warm roof.
A roof that meets normal building code ventilation requirements falls into the "insufficient for cold roof ventilation" category. Although more costly, a properly designed cold roof can eliminate most roof snow melting (and the resultant ice dams and icicles) stemming from building heat loss. Cold roofs may experience some minor ice dam and icicle formation, but this is due to unusual climatic conditions. Because a properly functioning cold roof allows roof snow to melt only in warm weather or under direct sun, it largely eliminates damaging ice problems. When the roof is sloped and it is desirable to hold the snow on it, a properly designed cold roof is essential. Snow retention can be assisted by snow arresters or snow clips designed to resist the fill weight of the snow pack above them (Figure 17).
