How is an ice dam formed?

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.