Ice dams form when warmth from inside the home melts the bottom layer of snow on the roof, sending water trickling down to the edge. Exposed to frigid air, it immediately refreezes, clogging gutters and causing the water behind it to pool up and find a path into the home below, usually through the shingles. Ice dams are more common in older or poorly insulated homes and those with low-pitch roofs. But even in newer homes, a new roof can leak if water runs backward in between the shingles. This problem is resolved by using a high quality underlayment below the actual shingles.