The volume problem
A valley is the internal corner where two roof planes meet. Every drop of water that lands on either plane and cannot find a shorter route to the eave ends up in that line. On a house with dormers, a cross gable or an L shaped plan, a single valley can be draining a very large area into a channel a few inches wide.
Two things follow from that. The first is speed: water in a valley is moving considerably faster than water on the open field, which means it has momentum, and momentum lets water do things gravity alone would not, including running up under a lap when it meets an obstruction. The second is abrasion. Anything the water carries, grit, granules shed by the shingles above, seed pods, the sand and salt that ends up on everything in a Missouri winter, is being dragged along that line. A valley wears from the inside out.
No photograph on this page, deliberately
Two photographs in the Keys library have the word Valley in their filenames. Neither of them shows a valley. Both are hips, which is the opposite geometry: an external corner that sheds water away from itself rather than an internal one that collects it. Rather than caption a hip as a valley, this page runs without an image. The distinction matters, because a hip and a valley fail in completely different ways and for completely different reasons.
Two ways of building one, and how each fails
- Open valley
- A metal channel is laid in the valley and the shingles are cut back to leave the metal exposed down the centre. Water runs on the metal. It sheds debris well and it is easy to inspect. It fails at the fastenings, at the laps between lengths of metal, or when the metal corrodes, and it fails badly if the shingle cut is too tight or the metal is too narrow for the area being drained.
- Closed or woven valley
- Shingles from both planes are carried through the valley, either interlaced or with one plane cut over the other along a line. There is still a membrane underneath. It looks tidier and it is very common on this housing stock. It fails when the shingles in the valley wear through, when the cut line opens up, or when debris packs into it and holds water against the line.
- Both, in a hard rain
- Where a valley discharges onto a lower roof or into a gutter that cannot take the volume, water backs up at the bottom of the valley. Standing water at the foot of a valley is one of the most reliable ways to get water under a lap.
Debris is the accelerant
The metro's tree canopy is a genuine asset and a genuine roofing problem. Oak, maple, sweet gum and sycamore all produce something the roof has to move: catkins and seed in spring, leaves in autumn, twigs after every storm. All of it converges in the valley, and some of it stops there, particularly at the bottom where the valley narrows into a gutter.
Packed debris does three things. It holds water against the roof surface long after the rain has stopped, which is the condition asphalt copes with worst. It dams the flow so that water backs up and spreads sideways under the shingles either side. And it retains moisture that encourages moss and accelerates granule loss, so the surface in the valley wears faster than anything around it.
This is the one place where a homeowner's own maintenance genuinely changes the outcome. Keeping valleys and the gutters they discharge into clear, particularly after leaf fall, removes the main mechanism by which a sound valley turns into a leaking one.
How a valley leak shows up inside
- EntryUnder the lap, at the wear line or the dam Fast water meets an obstruction, or a worn line lets it through, and it goes under the shingles either side of the valley centre.
- TravelDown the valley framing A valley is framed with a rafter under it, and that rafter is a superb channel. Water that enters at the top of the valley regularly runs the whole way down the valley rafter before it leaves it.
- ShowsLow, at a wall or a corner Frequently at the outside wall or in a corner of a room well below and well away from the entry, which is why valley leaks are so often misdiagnosed as wall or window problems.
When a valley is a repair and when it is not
An open valley with sound metal and a failure at one lap or one fastening is a repair. A closed valley with a defined worn line in shingles that are otherwise in reasonable condition can be cut out and rebuilt, with new membrane and, usually, metal put in. What cannot sensibly be done is rebuilding a valley in a field of shingles too brittle to lift, because the work needed to open the valley destroys the courses either side of it.
So the test at a valley is the same as everywhere else on this site: is this a discrete fault on a roof that is otherwise working, or is it the first visible symptom of a roof that is finished. The answer is not decided by the valley, it is decided by the condition of the shingles around it. That is what the repair or replace page is about.