What Causes Ice to Form Under Snow on a Roof?

Causes

• Ice forms underneath snow on your roof when the roof is unevenly heated, allowing snow to melt and reform into ice. There are three main ways your roof is heated, including conduction, convection and radiation. Inadequate insulation will allow heat from your home to reach the roof through conduction, causing ice to form. Insulation that allows gaps for warm air to escape into the attic provide an avenue of convection that will heat your roof with the warm air from the living areas of your home. Ice formed from radiation results from heat sources inside the attic or from the sun. Chimneys can also cause ice to form on your roof since they convey heated air past the roof when they are in use.

Issues

• As melting snow runs down the roof toward your gutters, it can form an ice dam. As snow melts above the gutters, it runs down to the edge of the roof where it freezes into ice. Eventually, this creates a barrier of ice that will cause water to pool on your roof. Once the water has begun pooling it can travel underneath your shingles and into your attic, causing water damage and encouraging the growth of mold and bacteria. Ice forming on your roof also presents a safety hazard since it can give way and strike people walking below the eaves of the roof.

Temporary Fixes

• If an ice dam has already formed on your roof, you can temporarily drain the pool of water behind using warm water. Using a sprayer or water hose, direct a stream of warm water into the ice so as to cut a channel through it, allowing the water trapped behind it to escape. You can also use a brush or rake to remove snow from the roof. However, using this technique presents a severe falling hazard since you must climb on top of your icy roof to remove the snow.

Insulation

• Increasing the effectiveness of the insulation in your roof will improve the energy efficiency of your home and prevent ice dams from forming. Areas near the edge of the roof often have limited room for insulation, allowing heat to reach the roof. Installing new insulation with a higher R value will insulate these areas more effectively in these restricted spaces. In areas where your roofing insulation does not restrict the flow of air between the attic and the rest of the house, applying spray foam insulation will reduce the amount of heat reaching the roof.