Every roof has a few spots that do far more than their share of the work, and none is busier than the valley. Valley roofing is the roofing along the internal angle where two roof planes meet and slope down together, and that channel collects and funnels the combined runoff of both slopes toward the gutters. Because a valley carries so much concentrated water, it is also the single most common place for a roof to leak. Getting the valley detail right, with the correct type and proper flashing, is what separates a roof that stays dry for 30 years from one that stains your ceiling after the first hard rain.
Here is how valleys are built, the three main types, and why they fail.
What a Roof Valley Is and Why It Matters
A valley forms wherever two roof sections intersect at an inside corner, such as where a dormer or an L-shaped wing meets the main roof. Rain and snowmelt from both adjoining planes drain into that seam, so the valley moves several times the water of an equivalent stretch of open field roofing. On a large, complex roof, a single long valley can carry hundreds of gallons during a storm.
That concentration of water, plus the debris that collects there and the snow and ice that pile up, is exactly why valleys demand extra protection. A properly built valley uses a waterproof membrane underneath and a carefully lapped surface above so the fast-moving water is guided cleanly into the gutter without ever reaching the deck.
Open Valleys
In an open valley, the shingles from each slope stop short of the center, leaving a visible metal channel running down the seam. That metal, usually painted galvanized steel, aluminum, or copper, is what carries the water. Open valleys are the most durable and the fastest-draining design, and many roofers consider them the best choice for steep roofs and heavy-snow regions.
The exposed metal sheds debris well and does not trap water the way a shingled valley can. Copper open valleys can last 50 years or more and look striking, though they cost the most. The trade-off is appearance for some homeowners, who prefer not to see a metal stripe, and the need for clean, precise metal work and correct shingle clipping along the edges.
Closed-Cut Valleys
The closed-cut valley is the most common design on modern asphalt-shingle roofs. Here the shingles from one slope run all the way across the valley center, and the shingles from the other slope are cut in a straight line about 2 inches back from the centerline, overlapping the first set. The result is a clean, continuous shingle look with no exposed metal on the surface.
Closed-cut valleys install quickly and blend into the roof, which is why builders favor them. They still rely on a hidden waterproof membrane, typically ice-and-water shield, running the full length of the valley beneath the shingles. Done correctly they perform well, but the cut edge must be sealed and the underlayment flawless, because there is no metal channel as a backup.
Woven Valleys
In a woven valley, shingles from both slopes are interlaced across the center, alternating one course from each side so they weave together. This creates a continuous shingle surface with no cut line and no exposed metal. Woven valleys were traditional with old three-tab shingles and can shed water well when installed skillfully.
They have fallen out of favor for a reason. Thick architectural shingles do not weave neatly, the interlaced layers can trap water and debris in the center, and the technique is slow and hard to do well. Most contractors today reach for a closed-cut or open valley instead, reserving the woven method for repairs matching an existing woven roof.
Valley Flashing and Underlayment
No matter which type you choose, the waterproofing under the surface is what actually keeps the valley dry. Best practice is a full-width run of self-adhering ice-and-water shield membrane down the entire valley, at least 36 inches wide, before any shingles or metal go down. This peel-and-stick membrane seals around nails and provides a watertight backup layer.
Open valleys add a metal flashing over that membrane, usually a W-shaped or center-crimped profile at least 24 inches wide, with a raised rib down the middle that keeps water from washing across from one slope to the other. The metal should be hemmed at the edges so shingle water drips off cleanly, and fasteners must stay well back from the center channel, never puncturing the flow path.
Why Valleys Leak
Because valleys handle so much water, most roof leaks originate there. The usual causes are:
- Failed or missing underlayment, leaving the deck exposed once the surface wears
- Corroded or improperly lapped metal flashing that lets water sneak underneath
- Nails driven too close to the valley center, creating direct paths to the deck
- Debris dams of leaves and pine needles that back water up under the shingles
- Ice dams in cold climates that force meltwater up and beneath the roofing
- Worn or cracked shingles along the valley edges after years of heavy flow
Catch these early by checking valleys during gutter cleaning; a small flashing repair now beats a rotted deck later.
Choosing the Right Valley Type
Which valley belongs on your roof depends on climate, pitch, and budget as much as looks. In heavy-snow and ice-dam country, an open metal valley is often the smart choice, because the smooth metal sheds snow and ice fast and gives the concentrated meltwater a clear, durable path. Steep roofs also favor open valleys for the same fast-drainage reason.
If you want the shingles to run uninterrupted for a clean, monolithic look, a closed-cut valley delivers that at a lower material cost and installs quickly, which is why it dominates new construction. Reserve woven valleys for matching an existing woven roof or for traditional three-tab work, since thick architectural shingles simply do not interlace well. Whatever you choose, the decision that matters most is not the surface type but whether a full-width waterproof membrane goes underneath, because that hidden layer is what actually keeps the valley dry.
Metal Choices for Open Valleys
When you do go with an open valley, the flashing metal is a real decision with cost and lifespan tradeoffs. The common options are:
- Painted galvanized steel: the budget standard, affordable and widely used, lasting 20 to 30 years before it may rust at scratches
- Aluminum: rust-proof and lightweight, a middle-cost choice that holds up well in coastal air
- Galvalume: a zinc-aluminum coated steel with strong corrosion resistance and long life
- Copper: the premium pick at the highest cost, lasting 50 years or more and developing a distinctive patina, popular on high-end and historic homes
Match the valley metal to the roof’s expected lifespan; there is little sense pairing a 50-year slate roof with a 20-year galvanized valley you will have to tear into later. Whatever the metal, it should be at least 24 inches wide with a raised center rib and hemmed edges so shingle runoff drips off cleanly instead of wicking underneath.
Repair and Replacement Costs
A minor valley repair, such as resealing a lifted edge or clearing debris and replacing a few shingles, typically runs $150 to $500. Replacing the flashing and reworking the valley properly usually costs $300 to $1,000 per valley, depending on length, pitch, and material. Copper flashing and steep, hard-to-reach valleys push the number higher.
If a valley has leaked long enough to rot the sheathing, budget for deck replacement on top of the roofing, roughly $70 to $120 per 4-by-8 sheet installed. When you re-roof, insist on full-width ice-and-water membrane in every valley regardless of type; it is cheap insurance on the part of the roof that works the hardest. Sound valley roofing, built with the right membrane and flashing, is the best defense against the leaks that plague complex roofs.
