Ice dams don't damage your roof. They damage the ceiling below it.
An ice dam forms when heat escaping from the attic melts the snow on the upper roof, water runs down to the cold eaves, refreezes, and forms a ridge. The next melt cycle can't drain past the ridge — so water backs up under the slate or shingles and finds its way inside. The roof itself often survives. The plaster ceiling and the insulation behind it do not.
In Boston and Greater Boston, ice dam season runs roughly mid-December through mid-March. Every winter, we get emergency calls from homeowners with brown stains on the ceiling and water dripping from light fixtures. The fix in February is expensive and the damage is usually already done.
A heated roof ice melt system is the only reliable preventive measure. Insulation and ventilation help — but they don't stop the cycle on Boston's steepest slate roofs, north-facing eaves, or any roof with deep valleys.
