This is how Wikipedia defines an ice dam:
An ice dam can occur when snow accumulates on the slanted roof of a house with inadequate insulation and warm air leaks into the attic at penetrations for plumbing stacks, wiring, chimneys, attic hatches, recessed lights, etc. These warm air leaks are known as attic bypasses. Heat conducted through the insufficient insulation and warm air from the attic bypasses warms the roof roof and melts the snow. Meltwater flows down the roof, under the blanket of snow, onto the eave and into the gutter, where colder conditions on the overhang cause it to freeze. Eventually, ice accumulates along the eave and in the gutter. Snow that melts later cannot drain properly through the ice on the eave and in the gutter. This can result in:
· Leaking roof (height of leak depends on extent of ice dam).
· Wet, ineffective insulation.
· Stained or cracked plaster or drywall.
· Rotting timber.
· Stained, blistered or peeling paint.
Under extreme conditions, with heavy snow and severe cold, almost any house can have an ice dam.
We currently seem to have two ice dams - one above the east window in Julie's room and one above the north window in the living room. I know this because water is drip, drip, dripping into our house in both those places. We had this problem in Julie's room last year, so that is not a surprise. The water in the living room, however, is a most unpleasant surprise.
Our living room is really a lovely room. I can honestly say this. It is a long, narrow room, with a wall of windows at either end. Ben and I have spent a lot of time and effort over the years to make this a pleasant, welcoming room. We had a hideous wood-burning stove removed from the fireplace last year. We have re-painted the walls - most recently only a couple of weeks ago. When the sun comes out, as it briefly did this morning, the living room glows, and I love to just look at it. Not today, however. The furniture is all pulled away from the windows at one end, and old towels catch the dripping water. I hate it. I hate being in there. I hate February.