There are three main causes of ice damming:
1) Poor insulation from a house ceiling. An attic with inadequate or deteriorated insulation can cause ice damming. (Be careful not to have “too much” insulation in your attic).
2) Lack of ventilation. A house with proper ventilation allows the necessary cold air to mix and cool the hot air from the house, which slows the snow melting down.
3) Leaking gaps between the living space and the attic. These spaces allow extra heat to “escape” into the attic, which warms the space and melts even more snow.
Snow on the roof will eventually melt, but if the heat from the house causes the snow closer to the roof to turn into water, then that water can either go down slowly to the gutter or travel under the shingles. There is no heat at the gutter, so snow will remain there, creating a barrier. The water that is moving down will hit that cold snow barrier and then freeze again. This creates a dam (ice damming), which will hold the remaining water on the roof.
Shingle roofs are designed to shed water; they cannot handle having it freestanding. That is why a good quality leak barrier is needed at the eaves. To help your homeowner understand the importance, you can perform a simple test with a bottle of water. This will show that every nail going through the leak barrier is automatically sealed.
The most important thing a contractor can do to help homeowners with ice dam problems is to first clean the snow off the roof (following strict safety rules) and then create exits for the trapped water.
In the long term, the contractor should:
Seal the living space.
Install adequate attic insulation.
Make sure that the attic is properly ventilated.
Preventing ice dams and icicles may not be possible at all times, but these are measures that can be taken to help stop water from entering the house.