Ice Dams on your roof are NOT a fault of poor shingle installation.

Ice Dams are created by melting ice that is impeded from running off the roof, and forms a puddle in the middle of your shingles.

Ice melts from heat.

Your roof becomes hot enough to melt the ice typically from the hot air touching the roof decking below increasing the temperature to where the ice begins to melt.

There are two very important factors at play here: Ventilation, and Insulation

Scenario One: Poor ventilation.

In this scenario, the air in the attic is not able to exit, or exits too slowly. Assume it is static.

What do attics do when the roof above is heated? They warm up. The areas of the roof without snow heat up, thermally transfering the heat to the backside of the roof sheating, heating up the air.

This hot air in turn heats up all areas of the attic, including the backside of the roof decking where there is ice. (snow is ice).

The water runs down, all the way to the edge of the roof.

The edge of the roof is typically sticking out, the roof is likely still frozen, and the water refreezes.

The next day, things start to melt, and now there is a frozen dam.

The water collects, and starts backing up underneath the shingles.

Remember, a shingle is water proof. But shingles are layered, and as a group are water shedding, not water proof.

Scenario Two: Poor Insulation

In this scenario, the air in the attic is either heated from the house – or even more frequently, the air FROM the house comes into the attic.

The same process as poor ventilation – the underside of the roof decking is heated, and the whole process begins.