Ice Dam - Alloy Gutter

Roof shingles are warmed by heat from the sun and heat escaping from inside the home.
Snow begins to melt upon contact with the warmed roof shingles.
Melted snow travels along the roof’s slope to the gutter.
Melted snow refreezes when it touches cold eaves and gutters. The ridge of ice is called an ice dam. Icicles form along the dam.
Once the melted water backs up past the lower row of shingles, the water flows down through the roof sheathing and into ceilings and walls.

In the winter your attic should be cold and dry. Here are some simple tips to prevent warm air from getting into your attic:

Bring insulation up to R-49.
Seal gaps around ductwork, recessed lights, skylight enclosures, the tops of walls and partitions, pipe penetrations, attic access, drop ceilings, and the chimney.
Never exhaust bathroom or kitchen vents into the attic. Those vents should always discharge outside.
Insulate all ductwork in the attic.
Make sure your attic is vented properly and adequately.
Don’t cover attic vents with insulation.
Inspect the vent screening periodically. It can tear, deteriorate, or become obstructed.