What Is Roof Valley?
A valley collects the runoff of two roof planes into one channel, so it carries several times the water of any equal area of field shingles — and moves it faster. Open valleys line the channel with metal (ideally W-profile, whose center rib stops water from one plane shooting under the shingles of the other) and hold shingles back from the centerline. Closed valleys run shingles across the line, either woven or cut, over ice-and-water membrane. Valleys fail from debris dams that push water sideways under shingles, from nails driven too close to the centerline, from metal rusting through, and — on closed cut valleys — from the top layer's exposed cut edge wicking water. Homeowners spot granule loss concentrated in the valley line, exposed or rusty metal, or leaves packed in the channel. Repair beyond cleaning requires opening the valley: shingles come off both sides and the metal or membrane is replaced, not patched.
Important Distinction
Note: Open valleys expose a metal channel; closed valleys cover the line with shingles over a membrane. Closed is not inferior when membraned correctly, but debris and fast water wear closed valleys sooner — the choice trades looks against service life.
Also Known As
These are the search terms and phrases that commonly refer to this topic, so you can find it however you describe it.
How Roof Valley Relates to Other Topics
Valleys move more water than any other part of a roof; a nail placed within about six inches of the valley centerline sits in the heaviest flow on the whole roof and will eventually announce itself on a ceiling.
A true valley drains: two planes meet and the water accelerates out. A dead valley is the same meeting with no exit. The first is a normal detail handled with membrane and metal; the second is a design flaw that needs its own engineered fix.
A valley's effective slope is flatter than either roof plane feeding it, so on a low-slope roof the valley is the flattest water path on the house - and the first place the materials need upgrading.
Questions about Roof Valley?
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