Ice dams are a huge winter problem in the Northeastern United States.

Ice build up at the eaves creates a dam that continually builds up and backs water up the roof.  Most steep roofs are designed only to shed water and are not water resistant or waterproof.

Ice weighs about 50 lbs. per cubic foot.  Snow thathas been compacted by daytime warming and nightime freezing weighs about 20 lbs. per cubic foot.  Structurally it is difficult to design for this tremendous ice or snow load at the edge of a roof.  This is a complicated by ice extending beyond the roof edge and pulling on the roofing material, fascia and siding.  A combination of two factors causes the formation of ice.  First,heat inside the building rises through the roof insulation.  Depending on the depth of snow, amount of insulation and outside temperature, this will melt the snow.  If the exterior temp is cold enough, the melting snow will turn to ice at the eaves. 
Secondly, changing outside temperatures effect ice dams.  If the outside temperature rises, the snow starts melting.  As the temperature drops, the snow begins to re-freeze from the outside.  The snow closest to the roof re-freezes more slowly.  This already melted snow water runs down the roof and freezes at the cold eaves.
The ice formation will block intended natural flow of water off the roof and this willback up under the shingles and find its way into the house.
There are three lines of defense against ice dams
1- attic insulation
2- attic ventilation
3- waterproofing shingle underlayment.  This was introduced in the mid 1980's.  It is a self adhering polymer modified asphalt sheet installed under shingles which is completely water resistant.  We use a product called Winterguard on all roofs that are ripped off before new installations.