Roof Valleys: Where Water Concentrates and Leaks Begin
A roof valley carries more water than any other part of the roof, which makes it both essential and vulnerable. Here is how valleys work, the ways they are built, and why a poorly made valley is a leak waiting to happen.
In this guide
What is a roof valley?
A roof valley is the internal angle formed where two sloping roof planes meet and run down toward each other, creating a channel, like the V-shaped trough between two hills. Any roof with more than one direction of slope has valleys: wherever a dormer, a wing of the house, or an L-shaped or complex roofline brings two planes together, a valley forms along the line where they join. That channel is the natural drainage path for both roof planes, so all the water that falls on the two slopes above it funnels into the valley and rushes down it to the eave.
This makes the valley one of the hardest-working parts of a roof. While a plain field of shingles handles only the rain that falls directly on it, a valley collects and carries the combined runoff of two whole roof planes, concentrating a large volume of fast-moving water into a narrow channel. That concentration is what makes valleys both essential, they are how a complex roof drains, and vulnerable, because more water, moving faster, over a seam between two roof sections, is exactly the recipe for a leak if the valley is not built well. Valleys are a key part of the roof system covered in our parts of a roof guide.
Why valleys are the most vulnerable part of the roof
Everything that makes a valley important also makes it a weak point that demands careful construction. It carries the most water of anywhere on the roof, so any flaw is tested constantly and hard. It is a seam, the junction of two roof planes, so it is inherently a place where the continuous surface is interrupted and water could get in. It concentrates fast-moving runoff, which can carry debris, back up behind ice or leaves, and drive under shingles. And in cold climates it is a prime spot for ice to form and dam. For all these reasons, valleys are one of the most common places for roof leaks to begin, and a disproportionate share of roof leaks trace to a valley that was built or maintained poorly.
The upside is that a well-built valley handles all of this reliably for the life of the roof. The key is what goes underneath and how the valley is finished: a proper valley is reinforced with a waterproof membrane beneath it and constructed so that the heavy flow runs cleanly down the channel without finding a seam. Because the valley is where the roof is most tested, it is also where the difference between a careful installation and a careless one shows up first. A roofer who builds valleys correctly, with the right underlayment and method, produces a roof that drains its hardest-working channels without trouble.
Open, closed and woven valleys
Open valley
The shingles stop short on both sides, leaving a channel of exposed metal flashing (often color-matched) running down the center of the valley. Water flows down the smooth metal, which sheds debris and heavy flow well and is often considered the most durable and best-draining method, and many roofers prefer it, especially in heavy-rain and snow regions.
Closed (cut) valley
The shingles from one plane run through the valley and up the other side, and the shingles from the second plane are cut in a clean line just short of the center over them. It gives a continuous shingled look with no exposed metal, is popular for appearance, and works well when done correctly with proper membrane underneath.
Woven valley
The shingles from both planes are alternately laced or woven across the valley, with no cut line and no exposed metal. It creates a continuous woven appearance but is more labor-intensive, can be harder to get perfectly watertight, and is less common today than open or closed valleys.
What matters most
Any of these methods works when built correctly over a proper waterproof membrane. The method affects appearance and, somewhat, durability and drainage, but the underlayment beneath and the quality of the workmanship matter more than the style chosen for whether the valley keeps water out.
The choice among valley methods comes down to a mix of appearance, climate and roofer preference. Open metal valleys are prized for durability and drainage, especially where water and snow loads are heavy; closed valleys are popular for their clean, continuous look. What all good valleys share is a waterproof membrane, typically ice-and-water shield, laid down the full length of the valley before the finish method goes on, which is the real key to a valley that does not leak.
Why valleys leak, and how good ones do not
When a valley leaks, the cause almost always comes back to how it was built or how it has been maintained. The most common construction failure is inadequate or missing waterproofing underneath: a valley finished without ice-and-water shield laid down its length has no backup when the heavy flow finds the seam, and it is exactly where a cut-rate installer skimps because it is hidden. Other failures include shingles cut wrong in a closed valley so the cut line channels water into rather than over the seam, nails placed too close to the center of the valley where they create leak points in the path of the water, and debris allowed to build up and dam the flow. Each lets the valley’s concentrated water do what it is always trying to do, get under the roofing.
A good valley avoids all of this by design: a full-length waterproof membrane underneath, the finish method executed cleanly with cuts and laps in the right places, fasteners kept well back from the center channel, and a smooth path for water to run. Built this way, the valley handles its heavy load without incident for the roof’s whole life. This is why, when we or any careful roofer build a valley, the ice-and-water shield goes down first and the method is executed precisely, since the valley is the part of the roof least forgiving of shortcuts, as our underlayment guide and installation guide describe.
Caring for your roof valleys
Because valleys carry the roof’s heaviest flow and can trap what that flow carries, they are worth a little attention over the roof’s life. Their biggest maintenance enemy is debris: leaves, twigs, pine needles and granules that wash down the two planes tend to collect in the valley, and a clogged valley dams the water, forcing it to pool and back up under the shingles exactly where it is most likely to leak. Homes under trees especially benefit from keeping valleys clear, which is part of the broader roof and gutter upkeep covered in our roof maintenance guide. Keeping valleys and the gutters they feed clear lets the water run as it should.
Valleys also deserve a close look during any roof inspection, since they are both a common leak source and an early indicator of trouble, worn or lifting shingles at the valley edges, exposed or corroded metal, or staining in the attic below a valley all warrant attention before they become leaks. If a valley is leaking or failing, it can often be repaired on its own without replacing the whole roof, provided the surrounding roofing is sound, and doing so promptly protects the interior below. Treating the valleys as the hardest-working, most vulnerable part of the roof, worth keeping clear and checking regularly, is a simple habit that prevents a large share of roof leaks. We serve homeowners across our service areas with valleys built and maintained to handle everything the roof funnels into them.
Roof valley FAQs
What is a roof valley?
A roof valley is the V-shaped channel formed where two sloping roof planes meet and drain toward each other. Any roof with more than one slope direction, from a dormer, a wing, or an L-shaped or complex roofline, has valleys. The valley is the drainage path that carries the combined runoff of both planes down to the eave.
Why do roof valleys leak so often?
Because a valley carries the most water of anywhere on the roof, moving fast down a seam between two roof planes, so any flaw is heavily tested. Leaks usually come from missing or inadequate waterproof membrane underneath, shingles cut wrong in a closed valley, nails placed too close to the center, or debris damming the flow. A properly built valley over ice-and-water shield handles the load reliably.
What is the difference between an open and closed valley?
In an open valley, the shingles stop short on both sides, leaving a channel of exposed metal flashing that water runs down, durable and good at shedding debris and heavy flow. In a closed valley, the shingles continue across or are cut in a clean line over the center, giving a continuous shingled look with no exposed metal. Both work well when built correctly over a waterproof membrane.
Which type of roof valley is best?
It depends on appearance, climate and roofer preference. Open metal valleys are often favored for durability and drainage, especially in heavy rain and snow, while closed valleys are popular for their clean, continuous look. More important than the style is what is underneath, a full-length waterproof membrane, and the quality of the workmanship, which matter most for keeping the valley watertight.
Can a roof valley be repaired without replacing the whole roof?
Often yes. If a valley is leaking or failing but the surrounding roofing is sound, the valley can usually be repaired or rebuilt on its own, addressing the membrane and the shingles or metal at the valley, without a full roof replacement. Doing so promptly protects the interior below. An inspection determines whether a valley repair or broader work is the right response.
How do I maintain my roof valleys?
Keep them clear of debris. Leaves, needles and granules collect in valleys and dam the heavy flow, forcing water to back up under the shingles where it leaks, so clearing the valleys, especially on homes under trees, is the main upkeep. Have valleys inspected regularly too, since worn shingles at the edges, exposed metal or attic staining below a valley are early warnings worth addressing before they become leaks.
Valley leaking or clogged?
Valleys carry the heaviest water flow on the roof and are the least forgiving of shortcuts. We build and repair them over full ice-and-water protection so they drain cleanly for the life of the roof. Get a free assessment.
Get a Free Assessment