Weather related freeze and thaw cycles that are created by sunny days and cold nights can result in ice damming. Snow can melt on roofs even when the temperatures are below freezing due to building heat losses. Water from this melting snow will soak up into the snow by day freezing by night. Daily cycles create thicker ice and deeper water. Thus preventing good runoff drainage.
Water that is left standing freezes when the temperature drops again at night or from cloud cover, causing ice dams to form. Each time the cycle is repeated, the size of the dam increases until water finds its way under the roofing materials. This process provides a way for water to enter the building causing damage to the interior as well as the roof.
Once water has entered the building it follows along beams, electrical wires, plumbing, and sprinkler pipes further increasing damage to the buildings structure and its contents. Sometimes the interior damage will be several feet from where the roof leak originally occurred. Ice forming at roof drains or in roof drain pipes are also ice dams that cause roof flooding. Drains can be blocked by the initial snowfall itself or can be located in areas of the roof that are shaded from the sun and can't provide good drainage for water runoff.