Falling snow forms a temperature-sensitive bond to the surface of a metal roof. As that roof is warmed — whether from the sun or from building heat loss, the bond with the snow is broken and a thin film of melt water serves to lubricate the roof. This can have dramatic results as a several-ton blanket of snow suddenly slides off the roof and avalanches down upon anything in its path: Gutters, vehicles, landscape, even people. Once piled up below, that same snow bank can go on to cause additional troubles, like direct damage to the building walls, or even indirect damage caused by funneling melt water into, rather than away from, the wall. This is what makes snow retention devices desirable: They hold the snow back and allow it to leave the roof slowly, either in small amounts of snow or as melt water, avoiding the dual calamity of the avalanche.