How is an ice dam formed?

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Many factors are involved. Here are two common scenarios.

Scenario A

1- Insufficient ventilation and insulation in the attic lets heat escape and warm up the roof sheathing from underneath, melting the snow above it.

2- The resulting meltwater flows down the roof then freezes at its edge, where it is not warmed by heated space underneath.

3- The dam builds up as snow melts and freezes again. Water starts to accumulate behind the dam.

4- Water makes its way underneath the shingles or elsewhere and penetrates the house. 

Source: Canada Mortgage and Housing Corporation

Scenario B

1- Snow accumulates on roof.

2- Temperatures warm up, snow melts and meltwater flows down the roof.

3- Temperatures cool down. Ice starts to build up.

4- Temperatures warm up again. Meltwater or rain accumulates and is trapped underneath the ice. Infiltration ensues.