Ice dams usually occur after a heavy snowfall followed by sunny days or above-freezing temperatures. Often caused by poor attic insulation or ventilation, the ice dam is formed when the warm air inside your home leaks into the attic and warms the underside of the roof causing snow and ice on the roof to melt. Ice dams can also form if the radiant heat from the sun melts the snow on your roof, despite the ambient temperature being below freezing.