Heat loss is the term for heat that escapes through the ceiling of the home into the attic, then eventually through the roof deck. Proper air flow or circulation will have a large effect on reducing the amount of heat that escapes through the roof deck. The only way to correct a heat loss issue is through a combination of proper ventilation and proper insulation.
In the illustration above, what is happening is that the snow on the roof is creating a “cold zone”. When the heat in the attic escapes through the roof deck it melts the bottom layer of the snow sitting on the roof. When this melt water crosses the exterior wall of the house, it hits another “cold zone” which then causes the water to freeze when it reaches the ice and builds layer upon layers of ice.
As illustrated, the “ice dam” allows the snow on the roof to build up which now traps the cold on the deck of the roof. Because there is a “cold zone” right on the roof deck, this can cause frost to form inside the roof and as this frost melts it will create a leak inside the house. It is important to realize that the roof is not leaking, but rather frost has formed inside the roof and is now melting the leak.