1. Melt-SnowIndoor heat rises through the ceiling, into the attic, and warms the roof (from the underside).  The shingles get warm.
  2. Snow that has fallen on the now-heated part(s) of the roof melts.  The melted snow then flows down the roof until it reaches the part of the roof that is lower than 32F (most commonly the overhangs).  This water refreezes and begins to form an ice dam.
  3. The dam gets bigger and bigger as more snow melts and refreezes.  Water now begins to pool behind the wall of ice.
  4. Eventually, the water works its way through the shingles (or other roofing material) and flows through the roof decking and into the attic.  From here it can seep through the ceiling, the interior walls, and into the rest of the building.