Check out those icicles; this is a common scene up here in northern New England. Actually, these icicles, though a few feet long, are not even the biggest ones around. Signs warning of “Falling Ice” are common. While pretty, they can cause great damage to historic buildings (all buildings, actually) due to the pressure on the eave, the architectural details, and to the roof. If you live in the southern climate, this is likely never an issue because the icicles melt quickly; but, in colder climates, they can remain all winter long.
Ever wonder how large ice dams, like this one, form? Old House Web has a good article explaining it (and will be more technical than my iteration) – read it here. Or read this fact sheet from the Ohio State Historic Preservation Office. The short version of the story is that poor or uneven insulation causes an ice dam. If the house is not sufficiently insulated, heat will escape through the ceiling to the attic roof and cause the snow to melt. Often the lower part of the roof slope and down to the eaves will not capture as much heat as the attic, because it is exposed without an interior ceiling and heat beneath it, thus providing colder conditions. So the water that melted near the peak of the roof will drip down to the edge (the eave) and then refreeze where there is no insulation. Then the water begins to back up behind the ice dam and can cause leaks into your house.
If your house has proper insulation, then heat will not pass through the ceilings (whether you have an attic or not) and out to the roof. In that case, the roof and the eaves will be closer to the same temperature and the likelihood of ice dams decreases.
The moral of the story? Insulate your house properly. The ceilings and the roof are more important than your windows, if you ask me. Ice dams can cause moisture damage, which, leads to many problems such as rotting wood, insect infestations, mold, and more. Remove snow from your roof, too.