How Ice Dams Form On Your Roof:

As heat escapes from your roof, it melts the lower layer of snow that's collected on your shingles.  This melted snow runs down underneath the upper snow layer in the form of water.  When this water reaches the uninsulated edge of your roof, it refreezes and creates an ice dam.

This ice dam keeps additional melted snow from reaching the edge of your roof and draining into your gutters.

The water has nowhere else to go but up underneath your shingles.  It then begins to drip through your roofing material, down your walls, and into your ceiling.  This is where all the damage begins.