Around this time of the year when we have a lot of snow, roofs are susceptible to water penetration due to ice daming. This phenomenon starts with an attic that is too hot because it's not insulated or ventilated well. Warm air from our heating, cooking, and showering, rises into the attic and transfers heat to the underside of the roof. This causes the roof to be warmer in some spots relative to others. Eventually the roof can get warm enough to melt the first layer of snow, which turns to water and runs down the roof until it hits a colder area (usually at the eve edges where there is no heated space below) and refreezes forming an ice dam. This melting and refreezing goes on until finally the water backs up underneath the roof shingles and into the house. Adequate insulation in all areas of the attic and a steady flow of fresh air through the attic, provided by a combination of vents, are key to ice dam prevention.

Signs of Ice Daming
• Look for ice build up at the edge of the roof.
• Look for ice that is thicker at the roof's edge and gets thinner as it goes up
  the roof.

Cures for Ice Daming
• Make sure you have at least 6" of insulation in your attic.
• Make sure the attic side of the attic access hatch is insulated.
• Make sure the hole around the plumbing stack as the stack enters the
  attic is covered or caulked.
• Make sure all ceiling penetrations are sealed, use a combination of
  insulation and caulk as needed.
• Make sure you have adequate ventilation, usually consisting of two sets of
  vents.
• Make sure the existing soffit vents are not clogged with insulation.
• Make sure the existing roof vents are not clogged with bird's nesting
  material.