How do I protect my home against ice dams?

Ice dams are formed when heat from the inside of a home escapes into the attic and warms the roof decking during the winter. This heat, combined with heat from the sun, can melt snow on the roof. Melting snow then runs down toward the eaves as water. When it reaches the cold eaves and gutters it refreezes. This continual thaw and re-freeze process creates ice dams. The result is water backing up under the roof shingles where it can soak through the roof decking or wall sheathing, causing damage to attics, ceilings and walls.
 
There are three ways to defend against the damage ice dams cause: attic insulation, attic ventilation system, and water-proofing shingle underlayment. All three work together.
 

Drawing from Certainteed Corp. literature