Roof Valley
Also called: valley, roof valley flashing
Roof Valley is the internal angle where two roof slopes meet and drain together. It carries more water than any other part of the roof, which makes it both a leak-prone detail and a place where storm damage concentrates.
What it means
A valley is the internal angle formed where two roof planes meet. Water running down both slopes converges into it, so the valley acts as the roof’s main drainage channel, carrying the combined flow of everything above it down to the eaves.
That concentration is the whole story of the valley. Every characteristic of the detail, how it is built, why it fails, and what damage looks like there, follows from the fact that it handles far more water than any equivalent area of roof.
Open and closed
There are two common constructions and they behave differently.
An open valley leaves a metal channel exposed down the centre. Shingles from both slopes are cut back to leave a defined gap, and the water runs on the metal. It sheds efficiently and debris moves through it more readily, and the exposed metal can be inspected directly.
A closed valley runs the shingles across the intersection so that no metal is visible. It is woven or cut, and the water runs on the shingle surface. It looks cleaner and it is the more common residential detail in many markets.
Both are legitimate. They fail differently: an open valley’s vulnerability is the metal and its fastenings, while a closed valley’s is the shingle surface in a channel that is constantly wet and constantly abraded by whatever the water is carrying.
Why storm damage concentrates there
Several reasons compound.
Hail that lands anywhere above the valley eventually washes into it, which means granules stripped across the slope collect in the channel and abrade it further.
Valleys are usually protected with self-adhering membrane underneath precisely because they are the highest-risk area, and where that membrane is absent on an older roof, the valley is the place where the absence matters most.
Debris collects there for the same reason water does. A valley full of leaf litter dams the flow, holds moisture permanently against the roof, and can push water sideways under the adjacent shingles.
And when an interior leak appears after a storm, the valley is the first place to look, because it is where the volume is.
What it means for scope
Valleys are their own line item and their own work. On a replacement, valley metal and the membrane beneath it are normally renewed with the roof rather than reused, and a scope that reshingles a roof over old valley detailing has left the highest-risk area untouched.
On a repair, the question is whether the damaged area includes a valley, because a partial repair that stops short of properly reworking a valley it intersects is likely to leak at the join.
Common mistakes
Reusing old valley metal to save a small sum is a poor trade against the leak risk.
The second is inspecting valleys from a distance. The damage is in the channel and frequently under debris.
The third is treating a valley leak as a shingle problem. It is usually a detail problem.
On the call
Valleys are where leaks show up, so a homeowner reporting an interior stain after a storm is often describing a valley problem. Ask where in the house the stain is relative to the roof shape, because that detail helps the inspection start in the right place.
This is how our agents handle it on storm appointment calls and day-to-day roofing answering service intake.
Questions people ask about roof valley
- Why do valleys leak more than the rest of the roof?
- Because they carry concentrated flow. Water from two slopes converges into one narrow channel, so the volume and velocity there are far higher than anywhere else, and any weakness in the detail is tested constantly rather than occasionally.
- What is the difference between an open and a closed valley?
- An open valley leaves a metal channel exposed down the centre, with shingles cut back on both sides. A closed valley runs shingles across the intersection so no metal shows. Both are used, and they fail in different ways and are inspected differently.
- Does debris in a valley cause damage?
- It can. Leaves and grit collect in valleys because that is where the water carries them, and accumulated debris dams the flow and holds moisture against the roof surface. That accelerates deterioration and can push water sideways under the shingles.
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