Warren, as usual, is on the money as it relates to how a house works.
The main culprit in the creation of an ice dam is insulation — or, as Warren points out — lack thereof. What’s happening is that the above-freezing air in your attic is melting snow under the snow-pack. Once it hits the eave or gutter — which is beyond the heat-source — it refreezes.
To add a little to Warren’s piece: As the water backs up behind the dam, the roof doesn’t leak in the sense that there’s a hole in it. It’s that the water simply finds its way up under the shingle tabs and through the tarpaper. It has nothing to do with water pressure pushing down too hard on the shingles or anything like that.
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[…] Dams. If you’ve had ice dams before, you’ll have them again. And the snowier the winter is, the more likely you’re bedroom walls will be draining water down […]
[…] If you get winter and feel like waiting for Spring, think again. Snow that can’t melt and is stuck where it can re-freeze can cause major damage like ice dams. […]