When the temperatures drop below zero, and there is rain and snow, it causes ice to form which, with the increase in volume and weight, which pushes on the external infrastructure and causes parts of the building to break away and fall apart and off your building.

There's a whole freeze/thaw cycle of damage to buildings where moisture gets into the stonework, into the pores of stone or roofing material, it then freezes and expands very significantly, it then breaks up the stone and then when it thaws, bits of stone will start falling off.  The built up ice on the roof can also cause the melting ice to cause water to permiate up under the roof covering.  This is ice dammng.

Where there is severe wet snowfalls, especially one after the other, partial collapses have been reported on buildings.  There was also a complete roof collapse at a mall as a result of the weight of the ice and snow.

The biggest worries are the buildings in the historic centers, which have wooden joists and delicate roofs  Any wieght of ice and snow could render these historic structures unsafe and prone to collapse.  In attempt to save the buildings, it is suggested adding extra support to the old roof beams in an attempt to prevent any further cave-ins.

The weight of accumulated snow/ice, not the depth, is critical in assessing a roof’s vulnerability. The water content of snow may range from 3% for very dry snow to 33% for a wet, heavy snow, to nearly 100% for ice. An inch of water depth weighs 5.2 lbs. per square foot. Thus, a roof designed to carry a snow load of 20 lbs. per horizontal square foot is expected to support nearly 12 inches of wet, heavy snow.

In some cases, you should be concerned about the snow/ice loads on:
• Roof overhangs (especially large overhangs projecting several feet beyond the horizontal support), if there is substantial ice buildup
• Multilevel roofs (when the lower roof is subject to an accumulation of sliding or drifting snow or accumulation of snowmelt)
• Valleys (subject to substantial snow or ice accumulation due to drifting, sliding or melting)