Heat from inside the house or more likely on the South side, the sun melts snow. The water runs down to the eaves, where with cold air top and bottom, it freezes. It can also run back under the shingles causing water damage. Best line of defense is good ventilation of the attic. On the south side of my house I have perforated metal the entire length of the eaves and continuos roof ridge vents. Good attic insulation reduces heat from the house. You can also put heating tapes on the eaves. However, besides wasting energy, they set houses on fire sometimes.
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