What's actually going on up there
Snow sits on the roof. The attic underneath is warmer than it should be, so the snow touching the upper part of the roof melts. That water runs downhill until it reaches the eave — the last few feet of roof that hangs past the wall, with no heated space beneath it. The eave is cold. The water refreezes there.
Do that for a few days and you've built a ridge of ice along the bottom edge of the roof, with meltwater pooling behind it. And here's the part that does the damage: asphalt shingles shed water that's running downhill. They don't seal against water that's sitting still. Once there's a pond behind that ice, water works under the shingles, into the decking, through the insulation, and onto a ceiling.
Which means the curtain of icicles along your eaves isn't the problem. It's the receipt.
Why this is an insulation and ventilation job
If the roof deck stays cold, the snow on it doesn't melt, and you don't get a dam. That's the whole strategy. Three things usually stand in the way:
- Insulation that's thin or uneven. Not just how much is up there, but whether it's consistent. Bare patches leak heat straight into the roof deck.
- Blocked soffit vents. Cold air is supposed to enter at the eaves and leave at the ridge. Blown-in insulation packed into the eaves stops that cold, and so does a missing baffle.
- Air leaks from the house into the attic. The attic hatch, recessed light cans, plumbing chases, the gap where interior walls meet the ceiling, and bath fans that vent into the attic instead of outside. Warm, wet air going up through those does more damage than missing insulation does.
Fix those and you've fixed ice dams. Everything else is managing symptoms.
Four things not to do
Don't chip at the ice. A hammer, a hatchet, or a shovel on a frozen roof will cost you shingles, and often the decking underneath. We've replaced more roof sections damaged by ice removal than damaged by the ice.
Don't throw salt pucks up there. They corrode flashing, gutters and fasteners, they run down onto whatever is planted below, and they do nothing about the heat that caused the problem.
Don't treat heat cable as the fix. Cable along the eave can keep a channel open in a spot that genuinely can't be solved any other way — a north-facing valley on a complicated roof, say. But it costs money every hour it runs, it fails eventually, and it doesn't make the attic one degree cooler.
Don't pressure-wash or steam it yourself. Steam done right is the safe way to take a dam off. Done wrong, or with a pressure washer, you're forcing water into the exact place you're trying to keep it out of.
If it's already happening
Watch for a stain on a ceiling near an outside wall, paint starting to bubble, damp or matted insulation in the attic, or ice more than a couple of inches thick along the eaves. Any of those means water is already getting in somewhere.
In that situation the order of operations is: get the ice off safely, find where the water is entering, dry things out, and then deal with the attic when the weather allows. Pulling a roof rake along the bottom few feet from the ground — carefully, without gouging shingles — is a reasonable thing to do in the meantime.
The fall version of this job
Before the snow: even out the attic insulation, clear the soffit vents and add baffles where they're missing, air-seal the obvious bypasses, make sure bath and kitchen fans actually vent outside, and get the gutters clear so meltwater has a path. That's a day or two of work in October, and it's the difference between a dry January and a ceiling repair.
It shows up on our fall checklist for exactly that reason.