Ice dams form for different reasons depending on the location of their formation. Glaciers flowing down valleys can block the flow of unfrozen rivers, creating ice dam lakes. Rivers in a cold climate can sometimes thaw at the upstream, which causes the water flow to be blocked where ice is deposited lower down the stream.
Ice dams, which involve the build up of water behind a blockage of ice deposits, may also refer to ice jams on a much smaller scale, though being far more relevant and destructive in this case. Ice dams occur on building rooftops when due to inadequate insulation ice melts on parts of the floor higher than the roof overhangs. The heat from the living space melts the snow near the edges, causing the water to flow down on the eaves. The snow on the roof further insulates it, maintaining the melted water, warming the attic even further and melting more snow and adding to the blocked water. As the melted water moves underneath the snow on to the eaves and into the gutter, it freezes due to a colder temperature and greater concentration of snow, creating a blockade. This blocks the downward flow of the water and it is built up.
This is particularly a serious problem pertaining maintenance of buildings as rooftop ice dams can leak to the roof space and can result in damage to the structure of the building. Ice dams cause millions of dollars of structural damage in cold climates every year.
The problem of rooftop ice dams can be fixed by improving roof insulation and ensuring proper air sealing and attic venting. Keeping the roof cool uniformly can prevent snow from melting, so that ice blockades can be avoided. One way of ensuring that is to thicken the insulation and to minimize the hot air leaks from the living space and attic to the roof, in order to stop snow from melting.