Roof windows are often covered by snow in the winter. When the temperature rises, the snow starts to melt because of heat leakage through the window. The melted snow runs off the window to the colder roof surface further down. The melt water freezes into ice, because the snow below the window prevents further run off. A pool of dammed-up water collects under the ice. This can cause water to begin to get into the house.

Make sure that the melt water always has a free run-off path from the window by removing snow and ice from around the window. The easiest way to do this is by installing heating cable around the lower edge of the window.