For roofs to withstand the normal environmental forces they are exposed to without leaking, it is wise to apply a fully adhered underlayment in areas prone to problems.
The problem begins when snow accumulates on the roof. Heat from the interior rises through the attic space, causing the snow to melt and water flowing toward the eave under the snow pack. This occurs no matter how well the building is insulated. The water refreezes once it reaches the eave because the eave is no longer over heated interior space. This phenomenon continues, forming a large ice dam at the eave with a pool of standing water behind it.