Short Answer
Proper attic ventilation can help reduce conditions that contribute to ice dams, but ventilation alone is not a guarantee against ice dams. Ice dams involve heat transfer, snow, roof temperature, insulation, air leakage, ventilation, and weather conditions.
Detailed Explanation
Ice dams form when warm air from the living space enters the attic and warms the roof deck, melting snow on the upper roof. The melted water runs down to the colder eave edge, where it refreezes, creating a dam that backs up water under the shingles. Ventilation helps by keeping the attic — and therefore the roof deck — closer to the outside temperature, which reduces the melting that starts the ice dam cycle. But ventilation alone isn't enough if warm air is leaking into the attic through gaps in the ceiling (air leakage) or if insulation is inadequate. The most effective ice dam prevention combines proper ventilation, adequate insulation, and air sealing to minimize heat transfer from the living space to the attic.
Why It Matters
Relying on ventilation alone to prevent ice dams is insufficient. A comprehensive approach including insulation and air sealing is more effective.