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How to prevent ice dams

Keep the roof cold. Ice dams form when heat leaking from the house melts snow on the roof and the water refreezes at the cold eaves. The fix is in the attic: seal air leaks into it, add insulation (R-38 to R-49 is the University of Missouri Extension recommendation; about 15 inches or R-50 in northern climates per NDSU), and keep soffit and ridge vents open. Heat cables and roof rakes only treat the symptom.

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How to prevent ice dams

Checklist

  • Look in the attic: daylight at the soffits, insulation depth, any frost or dark stainsFall
  • Seal gaps around plumbing vents, wiring, chimneys and recessed lightsFall
  • Weatherstrip and insulate the attic hatch or pull-down stairsFall
  • Add insulation where it is thin (mind the soffit vents)Fall
  • Install baffles so insulation doesn’t block the soffit ventsFall
  • Clean the gutters after the leaves dropLate fall
  • Rake snow off the lower few feet of roof from the groundAfter heavy snow
  • Check ceilings and exterior walls under the eaves for water stainsAfter each thaw
  • Check the roof from the ground: missing or curled shingles, damaged flashingSpring and fall

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Why ice dams form

Warm air from the house leaks into the attic and warms the roof deck. Snow on the warm part of the roof melts, runs down to the overhang, which sits outside the heated house and stays cold, and refreezes. The ridge of ice grows, water backs up behind it and gets under the shingles, into the walls and ceilings.

Big icicles along the roof edge are the usual sign. So are water stains on ceilings near outside walls, peeling paint under the eaves, or frost and damp insulation in the attic.

The three fixes, in order

Air sealing comes first, because insulation does not stop air from moving. Seal around plumbing vents, wiring holes, the chimney chase, recessed can lights and the attic hatch. Then add insulation to the depth for your climate. Then make sure the attic is ventilated so the roof stays close to the outdoor temperature: soffit vents in, ridge or gable vents out, with baffles so the insulation at the eaves doesn’t block the airflow.

The University of Missouri Extension recommends at least 2 square feet of vent for every 150 square feet of attic and about 1½ inches of air space between the roof sheathing and the insulation at the eaves. Check your local code too.

Removing snow and ice safely

Use a roof rake from the ground to pull snow off the bottom few feet of the roof, and keep it well away from power lines. Don’t climb onto an icy roof or chip at the ice with a hammer or shovel: it damages the shingles and it is a common way to fall. For an active leak, the Missouri Extension suggests melting channels through the dam with warm tap water from a hose as a short-term fix, or hire a pro with a steamer.

What you'll need

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Questions

Do heat cables prevent ice dams?

They can keep a channel open where they are installed, but NDSU Extension calls them a way to treat the symptom. Air sealing, insulation and ventilation fix the cause.

Do clean gutters prevent ice dams?

Not by themselves. Ice dams form on the roof edge because the roof above is warm. Clean gutters do help meltwater drain and reduce the weight on the gutters.

Are ice dams covered by homeowners insurance?

Water damage from ice dams is often covered, but policies differ and the dam itself usually isn’t. Check your policy and document any damage with photos.

Sources

Related guides

Fall home maintenance checklistA printable fall home maintenance checklist: furnace, gutters, outdoor faucets, sprinklers, mower storage, smoke alarms and more, with what to do and when.How to winterize a sprinkler (irrigation) systemSprinkler winterization before the first freeze: shut off the irrigation water, drain or blow out the lines zone by zone, and protect the backflow preventer.How to winterize outdoor faucetsTo winterize an outdoor faucet or hose bib: disconnect the hose, close the indoor shutoff, open the faucet to drain, and add a cover.How often to change your furnace filterCheck your furnace filter monthly and replace it at least every 3 months. Change intervals by filter type, plus pets and allergies.

General guidance for typical homes. Follow your equipment manufacturer's instructions and local codes, and hire a pro for anything you're not comfortable doing.