Ice dams form when heat escapes the home through the attic, melting a build-up of snow on the roof.  The heat, combined with the insulating factor of the snow and the sun, melts the snow underneath.  The water then runs down the roof and gathers at the gutters and eaves, where it freezes.  The continual melting and thawing of snow and ice, moves up the roof and under the shingles is an ice dam and can end up causing extensive damage.

The best defence is prevention.  The three most important ways to prepare you roof for the winter months is through insulation, ventilation and waterproofing.  These three factors all play important roles, but are more effective in conjunction with each other.  Insulation prevents heat from escaping through the attic.  Ventilation removes the heat from the attic space and prevents the roof from heating up.  Finally, waterproofing prevents leaks from occurring.

It is important to talk to professionals about having any of these elements repaired, replaced or installed.  A roofing professional will know exactly what to look for and focus on to prevent ice dams.  By concentrating on improving your insulation, ventilation and waterproofing, you are ensuring that your roof will withstand the elements this winter.

Source:  http://rogersroofing.wordpress.com/2009/12/29/what-are-ice-dams-protecting-your-home-from-ice-dams/