Ice dams form when heat escaping from the attic melts roof snow, which refreezes at the cold eave and forces water back up under the shingles. In Pelham, which straddles the NH-MA border with variable winter temperatures, ice dam cycles are common from December through March. Master Roofers addresses ice dams through improved attic ventilation, enhanced ice-and-water shield membrane, and — when needed — heated cable installation in chronic trouble spots.