When temperatures drop an ice dam may form as snow melts off of your roof and refreezes in your gutters. This blockage can cause excessive unwanted water runoff, damage to your shingles or eaves, and even leaks in your ceiling. Ice dams may be prevented by keeping your entire roof the same temperature as your eaves with proper ventilation, additional insulation, and professionally sealed air ducts. Of course, if an ice dam has already formed, rock salt can be used to melt the ice and reestablish water flow.