Ice dams are the accumulation of ice at the eaves and valleys of roofs, and in some winters create major problems. Insufficient attic insulation and/or attic ventilation can cause roof snow to melt and slide down to the eaves, where the roof surface is cooler and the melting snow freezes to the roof. Subsequent melting causes water to pool behind this dam and sometimes the water seeps between the shingle layers, wetting the roof deck and possibly the walls and ceilings below.
In new homes, Ice and Water Shield Underlayment is applied from the edge of the roof to a point not less than 12″ inside the inner face of the exterior wall. In some localities this eaves protection must go 10 or 12 feet up the roof (especially on low-sloped roofs), and also the full length of valleys.