What causes ice dams?
Roof ice dams result from the escape of heat into the attic or roof space. The cause of this is heat builds up and eventually melts the snow on the roof, which, as described above runs to the cold edge of the roof and forms an ice dam. Factors such as ventilation, insulation, solar orientation, tree coverage, home architecture, snow cover and weather conditions all interact to determine ice dam activity on every home. Because no two houses are the same here in Michigan, it is common to have neighboring homes on two ends of the ice dam spectrum; one may have major leaking, while the other is virtually unaffected.
How Do Ice Dam’s Form
Snow accumulation on the roof is the first step, and the deeper the snow, the more severe the problem is likely to be. Heat that leaks into the attic from the living space below warms the roof surface, and causes snow to melt. Melting doesn’t happen evenly or uniformly, because some parts of the roof will be warmer than others. For example, if there’s an attic hatch or drop-down stair that hasn’t been well sealed or insulated, lots of warm air will leak up in this area, causing more snow to melt on the roof surface immediately above the opening.
How To Prevent Ice Dams
The most effective long term solution is to reduce or eliminate any sources of heat in the attic and ventilate the attic space of the roof. The underside of the roof deck must be close to the temperature of the exterior side of the roof. Ideally ventilation should be installed using a continuous soffit-and-ridge vent system with baffles at the lower side of the roof. Provide at least a 2-inch space between insulation and sheathing. By providing adequate ventilation as illustrated in this diagram, the temperature of the attic will be lowered thereby lowering the underside roof deck temperature.