An ice dam is a condition where the temperature is just below freezing and the snow has fallen on the roof. The combination of heat from the sun and the escaping building heat causes the snow above the heated space to melt. The melted snow runs down the roof until it hits the unheated eave overhang. If the weather is cold enough, the runoff freezes and forms a small ridge. As the process continues, the ridge grows. Soon, water is backed up over the house and flows under the shingles, leaking into the home.