How Ice Dams Form?

Ice dams occur after a heavy snowfall when warm air in the attic causes the roof to warm and the snow to melt. Water running down the roof refreezes when it reaches the colder roof edge, forming a mound of ice.

The ice traps meltwater, which can seep back up under shingles and drip through the roof into your house, causing wet and stained ceiling and walls, and peeling paint and rot.

The key to preventing ice dams is simply to keep your attic and roof cold. After a snowfall, a cold roof will have a thick blanket of snow. A warmer roof, however, will soon have clear spots where the snow has melted off, and may well have icicles hanging from the eaves.

What causes different roof surface temperatures?

Heat from the house – In a house, heat moves through the ceiling and insulation by conduction through the slanted portion of the ceiling

Heat loss by air leakage – There is another type of convection that transfers heat to the attic space and warms the roof.

Exhaust systems – Exhaust systems like those in the kitchen or bathroom that terminate just above the roof may also contribute to snow melting.

Chimneys – Other sources of heat in the attic space include chimneys. Frequent use of wood stoves and fireplaces allow heat to be transferred from the chimney into the attic space.

Preventing and dealing with ice dams

Immediate action

• Remove snow from the roof. This eliminates one of the ingredients necessary for the formation of an ice dam. A “roof rake” and push broom can be used to remove snow, but may damage the roofing materials.
• In an emergency situation where water is flowing into the house structure, making channels through the ice dam allows the water behind the dam to drain off the roof. Hosing with tap water on a warm day will do this job. Work upward from the lower edge of the dam. The channel will become ineffective within days and is only a temporary solution to ice dam damage.

Long-term action

• First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space.
• After sealing air leakage paths between the house and attic space, consider increasing the ceiling/roof insulation to cut down on heat loss by conduction.