A roof valley is the internal angle formed where two roof planes meet and slope toward each other, channeling significant water volume during rain.
Valleys handle more concentrated water flow than any other part of a roof, since they collect runoff from two roof planes at once — which makes valley flashing and underlayment especially critical, and valleys a common (though not the most common) leak point when that flashing fails or debris accumulates.
The V-shaped geometry that channels water also catches wind-blown leaves and debris, which can dam up water flow if not cleared periodically, especially near trees.
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