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Ice dams
Preventing ice dams on roofs
An ice dam is a ridge of ice at the cold edge of a roof. It forms when snow on the warmer upper roof melts, the water runs down to the cold eaves, and it refreezes there. Over time the dam grows, ponds meltwater behind it, and that water can work its way under the roof covering.
Why a warm roof is the root cause
Ice dams are mostly a heat problem, not a snow problem. If the whole roof stayed at outdoor temperature, snow would not melt unevenly and meltwater would not refreeze at the eaves. The fix, therefore, focuses on keeping the roof deck uniformly cold rather than on removing ice after it forms.
A practical order of work
Most durable improvements follow the same sequence. Each step supports the next.
- Inspect the attic. Look for frost, water staining, compressed insulation, and recessed lights or ducts that leak heat.
- Seal air leaks. Close gaps at the ceiling plane so warm, moist air stops rising into the attic.
- Insulate evenly. Bring the attic floor up to a consistent depth, paying attention to the perimeter near the eaves.
- Balance ventilation. Keep soffit intake clear and confirm exhaust near the ridge so outdoor air can wash the deck.
- Verify after snowfall. Watch how snow and ice behave during the next cold spell and adjust if dams still form.
Where heat cables and membranes fit
Heat cables along the eaves can melt channels through ice so trapped water has somewhere to drain. They manage symptoms rather than the cause, and they use energy whenever they run, so they are best seen as a supplement to a cold-roof approach rather than a substitute for it.
During roof replacement, a waterproof membrane along the eaves provides a second line of defence by blocking meltwater that backs up behind a dam. It does not stop dams from forming, but it limits the damage if one does.
What to avoid
- Chipping at ice with sharp tools, which can puncture the roof covering.
- Relying only on roof-edge heat cables while ignoring attic heat loss.
- Adding more insulation over unsealed leaks, which can trap moisture in the assembly.
Safety matters: clearing snow or ice from a roof carries real risk of falls and of damage to the covering. Where the situation is unclear or unsafe, an assessment by a licensed contractor is the appropriate next step.