Ice damming is a problem in many homes. It occurs when the roof of a building becomes snow-covered. If the roof deck is warmed from the inside to a point where the snow on top of it melts, that melted water then flows down the roof deck. Ice dams form when that water reaches a point where the roof deck is below freezing. This causes the water to freeze on the deck. More water continues down the roof and freezes at about the same point. Over time, this freezing of the meltwater causes a dam to occur, and the meltwater builds up behind it where the deck is above the freezing point.
If the roof covering is impermeable, such as on a flat roof, then this is not a problem. However, most roofs are covered with asphalt-based shingles, or some other layered covering, like shakes or shingles. These types of coverings are only designed to shed water down the roof. They are not waterproof. Therefore, if the dam builds up high enough, the water can get in underneath the covering and leak into the building. This can cause a great deal of damage to the insulation, structure and wall finishes in the home. In severe cases, where the problem is ongoing, mould can begin to develop.
Not only can ice damming cause interior damage, but it can also damage the shingles themselves. It can also damage gutters. The weight of ice build-up can bend or twist the gutters. In extreme cases, ice build-up has been known to tear gutters right off the building.
This diagram shows typical ice dam formation: