Know Your Roof · Roof Damage & Leaks
Ice Dams: What Causes Them and How to Stop Them
Quick answer
An ice dam is a ridge of ice at the cold roof edge. Winter freeze and thaw lets snowmelt refreeze at the overhang, and the ice pushes meltwater up under the shingles. The cause is usually heat escaping into the attic, so the lasting fix is in the attic, not on the roof.
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How an ice dam forms
Warm air leaking up from the house heats the attic and the roof deck. Snow on the upper roof melts, and the water runs down until it reaches the overhang, which sits outside the heated part of the house and stays cold. There it freezes. Each cycle adds to the ridge of ice, and water backs up behind it.
Why ice dams cause leaks
Shingles are made to shed water running downhill, not to hold back standing water. Water pooled behind an ice dam can work up under the shingles and past the underlayment, then into the attic, the ceiling or the walls below the eaves. That is why many roofers put ice and water barrier along the eaves.
Warning signs
Thick icicles along the gutters. A ridge of ice along the roof edge. Water stains or peeling paint on ceilings and walls near the eaves. Frost on nails in the attic. Gutters pulling away from the fascia under the weight of the ice.
How to prevent ice dams
Keep the roof deck cold. Seal the gaps that let warm air into the attic, add insulation on the attic floor, and make sure the attic has balanced ventilation, with intake at the soffit and exhaust near the ridge. Ice and water barrier at the eaves protects the roof if a dam does form.
If you have one now
Don't chip at the ice with a hammer, axe or shovel. That breaks shingles and gutters, and an icy roof is dangerous to stand on. A roof rake used from the ground can pull snow off the lower roof. For a dam that is already leaking, ice dam removal with steam clears the ice without damaging the roof.
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