A roof valley is the V-shaped channel formed where two sloped sections of roof meet and run downward together. Valleys collect and carry a large volume of rainwater off the roof, which makes them one of the busiest and most leak-prone areas. Because so much water funnels through them, valleys are flashed and built with extra care, and a failed valley is a common source of leaks on an older or poorly installed roof.
Wherever two roof planes come together at an inside angle, they form a valley. Picture an L-shaped or T-shaped house: the spots where the different roof sections join create long diagonal troughs. Rain that lands on both of those slopes drains into the valley and rushes down it, so a valley handles far more water than an equal stretch of open roof.
That concentrated flow is exactly why valleys need extra protection. A well-built valley has reinforced underlayment and often a metal valley flashing or a carefully woven shingle pattern designed to keep the rushing water on top of the roof and moving toward the gutters. Done right, a valley is reliable for decades. Done poorly, with skimped flashing or sloppy shingle cuts, it becomes the first place a roof leaks.
For a homeowner, valleys are worth a careful look during any inspection. They collect leaves and debris that can dam up water, the heavy flow can wear granules off the shingles faster, and they sit at the meeting point of two slopes where movement and settling stress the materials. Many repairs that get blamed on the shingles are really about restoring a worn or failed valley.
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