Ice dam
A ridge of ice at the roof edge that forces melt water under shingles.
An ice dam forms when heat escaping into the attic melts snow on the upper roof, which then refreezes at the cold eaves. The growing ridge of ice traps water behind it, which backs up under the shingles and leaks into the home. Good attic insulation, ventilation, and clean gutters all reduce the risk. They're a common cold-climate cause of interior water damage.