Gutter and downspout cleaning will help keep this from happening.  Ice dams form when water accumulates in the gutters and freezes. Additions snow comes along and we have a freeze thaw cycle but not enough thaw to allow the gutters to carry melting snow away.  Melting snow begins then to back up underneath the roof shingles.  If we have enough snow, this water can back up far enough to get into the house and cause extensive water damage to the interior. (In some particularly bad cases heat tape can be installed on the roof, in gutters, and down spouts to keep ice melted, and water moving).  All this to say a little preventive maintenance goes a long way.