Ice Dams are created when heat from the house gets into the attic and manages to melt the bottom of the snow pack. This melt water flows down to the edge of the roof where it is now freezing cold and you get ice dams or ice cycles. Ice dams back water up behind them and this finds it way under the shingles and into the house.
Current roofing practice is to put a protective plastic or rubber shield on the outer lip of the roof to protect against this. The real solution is continuous ventilation under the entire roof deck. No insulation touching the roof anywhere and open soffits together with some form of ventilation higher up. This air current will pick up heat escaping from the house and remove it from the attic, keeping the snow pack frozen until the sun melts it from the top down, not the house melting it from the bottom up.
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