What is an ice dam?
An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas. Figure 1 shows a cross section of a home with 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
An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) From draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas. FIGURE 1 shows a cross section of a home with an ice dam. FIGURE 1. Cross section of a one-and-a-half story house with an ice dam. WHAT CAUSES ICE DAMS? There is a complex interaction among the amount of heat loss from a house, snow cover, and outside temperatures that leads to ice dam formation. For ice dams to form there must be snow on the roof, and at the same time, higher portions of the roofs outside surface must be above 32 degrees, while lower surfaces are below 32 degrees. When we say temperatures above or below 32 degrees, we are talking about average temperature over sustained periods of time. The snow on a roofs surface above 32 degrees will melt. As water flows down the roof it reaches the portion of the roof that is below 32 d
