This Madison roof is showing an easy to spot example of a severe ice dam problem. Dams like this add weight to the roof, which may be more than it was designed to support and force water/ice up under the layer of shingles. The underlying felt paper begins to break down, which allows more water to seep in, making the problem even worse. The base layer of wood decking under the felt paper gradually is damaged by the water freezing and thawing. Long story short - ice dams lead to your roof leaking. Most often ice dams are caused by lack of sufficient roof ventilation.
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