Ice dams and icicles: why they form and why the fix is in the attic, not on the roof edge

Every winter, homeowners pay for chipping ice off eaves, heating cables and new gutters, then see the same ice next year. NRCan's Keeping the Heat In treats ice dams as a symptom of heat escaping into the attic. Treating the symptom on the roof edge rarely solves it.

How ice dams form

NRCan describes ice dams as masses of ice at the lower edge of the roof and in gutters. Melt water backs up behind them, sometimes under the shingles, and leaks into the attic or the walls. They usually form after heavy snow: heat from a warm attic melts the snow on the roof, and the water refreezes when it reaches the cold overhang. NRCan notes that sun-heated dark siding can also drive warm air up into the eave area. Complicated roofs with valleys, dormers or large overhangs are more prone to ice dams, and uninsulated attics rarely develop them.

Why the quick fixes disappoint

NRCan lists the drawbacks. Heating cables use significant energy. Removing gutters leaves water near the foundation, which creates a basement problem. Chopping ice with axes damages the roofing. Self-sealing membranes under the shingles prevent leaks into the house, but they do not stop ice dams from forming. These measures can be part of a repair, but none removes the cause.

The real fix: keep house heat out of the attic

The main prevention, according to NRCan, is to air seal and insulate so that house heat stays out of the attic. Warm, moist air leaks into the attic around plumbing stacks, wiring, pot lights, chimney chases, bathroom and kitchen vents and the attic hatch. Sealing those leaks first, then adding insulation, keeps the roof deck cold enough that snow does not melt from below. NRCan also warns not to automatically increase ventilation; the order is sealing first.

Leak or condensation?

Water in the attic is not always from ice dams. NRCan separates leaks from outside, caused by failed roofing or flashing at hips, valleys and chimneys and by ice dams, from moisture from inside carried by air leakage. Water marks on the underside of the sheathing or along rafters point to outside leaks; discoloured insulation points to air leakage. Mould or rot means advanced condensation, and the cause must be corrected before insulation is added. Heavy frost of 10 mm (3/8 in.) or more during a cold snap is a sign to check that vents are not blocked.

Ventilation basics

For peaked roofs with an accessible attic, NRCan gives a ratio of 1 to 300 of unobstructed vent area to insulated ceiling area, split about half at the soffits and half at the gable, ridge or roof vents, with more vent area to make up for screens and baffles. It advises against electric attic exhaust fans because they can pull house air into the attic. Keep baffles at the eaves so new insulation does not block the soffit vents.

Prevention Checklist

• Look for where house air reaches the attic before buying heating cables.
• Seal stacks, wiring, pot lights, chimney chases and the hatch, then insulate.
• Keep soffit vents open with baffles when adding insulation.
• Do not add powered attic exhaust fans.
• Use a membrane at the eaves as leak protection, knowing it does not stop dams.
• Fix the cause of any mould or rot before adding insulation.

References

1. Natural Resources Canada. Keeping the Heat In - Section 5: Roofs and attics, Web page, 5.5 ice dams; assessment of leaks and condensation; 5.1.4 ventilation. Accessed 2026-11-03.

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