Look at a roof from the street and the valleys are easy to miss. Water does not miss them. A valley is where two roof slopes meet and dump their combined runoff into one channel, which is why it carries the most water on the whole roof and, by one major manufacturer’s ranking, is the second most likely place a roof will leak. How your valleys are built, and whether they are kept clear, decides whether they hold.
What a valley is, and why it leaks
A valley is the internal angle where two sloped roof planes come together. Both planes drain into it, so it channels concentrated, fast-moving water, and it takes the most wear of anywhere on the roof. IKO ranks it the second most likely leak location, behind only penetrations. Debris and ice collect there first, and when water cannot move freely, it backs up under the shingles.
Open valleys
In an open valley, the roofer runs metal flashing down the center, then lays the shingles and cuts them back on both sides so a strip of metal stays exposed. The metal, usually galvanized or painted steel, sometimes aluminum or copper, carries the water.
- Pros: excellent at shedding water, snow, and debris, they hold up well over time, and they are often easier to repair. IKO recommends open metal valleys for best performance.
- Cons: more expensive because of the added metal, the metal is visible, and it can corrode over the decades.
Closed valleys
In a closed valley, shingles cover the valley completely and no metal shows. There are two versions. In a closed-cut valley, shingles from one slope run through and past the center, and the other slope’s shingles are cut in a straight line about two inches off center. In a woven valley, shingles from both slopes are interlaced across the valley without cutting.
- Pros: cheaper and simpler to install, with a clean continuous look and no exposed metal.
- Cons: the shingles sit in the wettest part of the roof and wear faster, water and ice shed more slowly, and debris is easier to trap.
Woven valleys are fading out. They are hard to do well, and modern architectural shingles are too thick and stiff to weave properly, so most laminated-shingle manufacturers no longer recommend them and a woven valley can void the shingle warranty.
Which is better
For steep roofs, heavy rain, or heavy snow, open metal valleys are generally the more durable choice because they move a lot of water fast and clear themselves of debris. In milder climates and on standard homes, a properly built closed-cut valley works fine and costs less. There is no single right answer, only a right answer for your roof and climate. What is not up for debate is what goes underneath.
The layer under every valley
Whatever the style, a valley should have a self-adhering ice and water membrane run down it before any metal or shingles go on. This is both best practice and code. The International Residential Code, section R905.2.8.2, requires valley lining, and for closed valleys a self-adhering polymer-modified bitumen membrane is permitted in place of the roll lining, at least 36 inches wide. You can read the code section.
A valley with no waterproof membrane under it is a leak waiting for the next hard rain. If you are reviewing an estimate, the valley lining is one of the details worth confirming is in there.
Why valleys leak
When a valley fails, it is usually one of these:
- Debris damming. Leaves and pine needles pile up, slow the water, and force it under the shingles.
- Bad or worn flashing. Corroded, cracked, or shifted metal is a direct entry point.
- No ice and water shield underneath.
- Nails driven too close to the valley center, which every manufacturer warns against.
- Cracked or worn shingles from years of concentrated flow scrubbing the granules off.
- Ice dams, since snow piles deepest in valleys and the meltwater backs up behind the ice.
A leak that shows up on the ceiling after heavy rain often traces back to a valley. Our guides on how to find a roof leak and common roof leak causes walk through tracking it down.
What it costs, and how to keep it healthy
Valley repairs commonly run a few hundred to around a thousand dollars, with valley flashing material in the range of 6 to 12 dollars per linear foot and installed flashing higher once labor is added. A full valley rebuild costs more. These are aggregator estimates, so treat them as ranges rather than quotes.
The cheapest thing you can do is keep valleys clear. Debris is the leading cause of valley leaks, and it is the one you control. Clear leaves and needles out, especially in fall, inspect the valleys after big storms, and treat any staining below a valley as a reason to look closely. A valley is often where a roof leaks first, so it is where attention pays off most.
“Valleys carry the most water and leak the soonest. Build them with metal or a proper closed cut, always lay the membrane underneath, and keep the leaves out.”
The bottom line
Open valleys with exposed metal are the durable choice for steep roofs and heavy weather. Closed valleys are cleaner and cheaper for milder climates. Either way, the waterproof membrane underneath is what actually keeps the water out, and keeping the valley clear of debris is what keeps it working. When you get quotes, ask how each roofer builds and lines your valleys.
With Hommy you post your job once and get quotes from vetted local roofers with real ratings from other homeowners, so you can see who treats the valleys as the critical detail they are.
