On the right, you'll see a stylized illustration of a home's roof and gutter system. There has been a recent snowfall here, and a snow pack has formed on the roof. You may also notice that the attic directly below the roof is very warm. That suggests that this home is not properly ventilated. Heat, by nature, rises; the heat generated elsewhere in the home has gone to the attic. With no venting system as a means of escape, the heat simply builds up in the attic. Consequently, the roof above will start to warm up as well, just like a skillet atop a burner.

Eventually, all that built up heat in the attic will find a way out, vents or no. Specifically, it will force its way right through the home's roofing materials. Eventually, this forced exit will show itself by the cupping and curling of shingles. Right now, however, it prematurely melts the part of the snow pack sitting on top of the warmest part of the roof. (Ever see a roof that has snow in some areas and bands of clearing in others? This is what happened.) The melting snow, of course, turns to liquid water, and flows down the slope of the roof, until it reaches the colder courses at the bottom of the roof and the home's gutter system (or simply the roof edge). But it's not as warm here as it was on the overheated section of the roof where the snow melted...

Without the overheated roof to keep it warm, the water in the gutter and atop the lower shingle courses is going to freeze again. This time, of course, it will turn into a block of ice instead of reverting to a snow pack; thus, the ice dam is formed. It looks menacing, and it is. When water freezes to become ice, it also expands. This puts stress on your gutter system; not only is the ice weighing it down, but depending on its state of repair and/or the size the dam, bursting its seams. The ice dam is also expanding upward as it freezes... back up to the lower courses of shingles.

As the ice dam freezes upward, it's not usually content to just sit on top of the shingles. Instead, the ice dam tends to wick its way underneath the lower shingle courses. As you can see to the right, the pressure of the ice intruding is lifting the lower shingles to offer an even clearer way in. Once the ice wicks far enough inside, it starts to encounter warmth again; and why not? It's reached the interior wall plates of the house. And so the portion of the ice dam that's made it inside starts to melt... right into the wall cavity. The water starts to drip down into the wall cavity and cause all sorts of problems.

The ice dam was just the catalyst for more headaches to come.