In almost all houses, some heat escapes through the ceiling to warm the roof from the bottom up. That can cause a phenomenon known as ice dams. See diagram.

• Rising heat from the house melts the blanket of snow and ice on the roof from the bottom up, sending water trickling down the roof.
• When water hits the cold eaves, it freezes and forms a dam, preventing drainage.
• As the ice dam grows, water is trapped behind it and backs up under the shingles.
• Water then can leak into the house through the windows or ceiling.