Ice dams form when warmth from within the home melts the bottom layer of the snow on the roof, sending water trickling down to the edge of the roof. Upon crossing the "heat wall" or an area where heat is no longer escaping the structure, the water freezes, clogging gutter and causing the water to pool. This may allow water to spider under the shingles as it freezes, eventually finding its way into the home. Installation of a bituminous waterproofing membrane referred to as "ice guard" alleviates this problem.