What is an ice dam?

Like the name suggests, an ice dam is a dam made of ice along the edge of the roof line, and as most dams do, an ice dam prevents melting snow from finding its way to the gutters and downspouts. Instead, the snow melting into water finds the dam of ice and refreezes forming even more ice. Sometimes the water backs up behind the dam and will leak into the house and cause damage to walls, ceilings, insulation, and other areas.

Figure 1 shows a cross section of a home with an ice dam.

What causes different roof surface temperatures?

Ice dams form at the edge of the roof; this is due to a heat source higher up toward the peak of the roof causing the snow cover to begin to melt. This heat source is most always due to a lack of properly insulated attic space. The next time you drive through your neighborhood, look at the homes that have a full roof of snow – more than likely, these homes are properly insulated and vented… When you spot a home that has “vertical stripes” of visible roof (see picture # 1), these are the homes that are more than likely not insulated and vented properly so the heat from the home is warming the roof line between each of the ceiling rafters.

Picture # 1 shows a single story house with an ice dam. The points of heat loss can be clearly seen as those areas with no snow. The ceiling below this area needs to be examined for air leakage and missing or inadequate insulation.

Photograph 2 illustrates unusually high heat loss from the roof. There is very little snow left on the roof and at its edge is both an ice dam and a "beautiful" row of icicles.

Dealing with ice dams

Immediate action:

• Have the snow removed from the roof. This eradicates one of the ingredients necessary for the formation of an ice dam. A "roof rake" can be used to remove snow, they are available at most home improvement stores.
• In an emergency situation where water is flowing into the house, a channel must be made through the ice dam that will allow the water behind the dam to drain off the roof. Hosing with tap water on a warm day will do this job.

• Long-term action:

• Increase the ceiling/roof insulation to cut down on heat loss. The R-value of the insulation should be at least 30 to be completely effective.