Ice dams form along the roof edge as a result of snow melting and then refreezing at the roof edge.  If this process is allowed to continue unchecked, several inches of ice may build up and water will then begin to back up the roof behind the ice dam and leak under the roofing shingles.  Most leakage will first appear where the exterior wall and ceiling meet and around window and door openings.  It is recommended that a licensed roofing professional be contacted for removal.  Roofing contractors will have the necessary experience and insurance to accomplish this otherwise dangerous task.