Roof Valleys: Open, Closed-Cut or Woven?
A valley is where two roof planes meet and drain into each other. It handles more water than any other part of the roof, it concentrates that water into a narrow channel, and it is where a great many roofs begin to leak.
There are three ways to build one. They are not equivalent, the choice is rarely explained to homeowners, and it is worth knowing which is on your roof.
Why valleys fail first
Two roof planes feeding one channel means the volume of water moving through a valley is far higher than on an open slope. Higher volume means faster flow, and faster flow finds imperfections. Add to that:
- Debris collects in valleys, because that is where wind and water push it. Wet debris sitting in a channel holds moisture against the roof indefinitely.
- In cold markets, valleys are where ice forms and dams.
- Every valley involves cutting shingles at an angle, which means more cut edges and more opportunity for error.
- Water in a valley is moving downhill fast enough to run past a poorly formed detail rather than dropping through it, which delays the leak and makes it harder to trace.
Whatever the method, the layer that actually matters is underneath: a valley should have ice-and-water shield membrane running its full length before any shingles go on. That is non-negotiable, and it is the first thing to ask about.
The three methods
Open valley
A metal channel is installed down the valley and the shingles are cut back on both sides to leave the metal exposed, usually a few inches wide.
Advantages: water runs on metal rather than on shingle granules, which is the fastest and cleanest path off the roof. Debris washes through more readily. Nothing abrades. It is generally the longest-lasting of the three, and it is the best performer in heavy rainfall and in ice.
Trade-offs: the metal is visible, which some homeowners dislike and others prefer. It costs more. The metal has to be the right gauge and properly hemmed at the edges, and a cheap open valley is worse than a good closed one.
Closed-cut valley
Shingles from one plane run through the valley and up the other side. Shingles from the second plane are then laid over the top and cut in a straight line a little way back from the valley center.
Advantages: it looks continuous, it is quicker and cheaper than an open valley, and done well it performs perfectly acceptably. This is the most common method on residential roofs in the United States.
Trade-offs: water runs across shingle surface rather than metal, which wears granules over time. The cut line has to be kept back from the center and the cut shingles must not be nailed too close to the channel. Both of those are easy to get wrong and neither is visible afterwards.
Woven valley
Shingles from both planes are alternated across the valley, interlacing so no cut is needed at the center.
Advantages: no exposed cut edges, and a soft appearance some people like.
Trade-offs: it is the method we would generally avoid. Weaving forces shingles to bend through the valley, and modern architectural shingles are thick and stiff enough that bending them creates bridging and stress. Many manufacturers now discourage or prohibit weaving with laminated shingles, and doing it anyway can affect warranty coverage. It is also the hardest to get right and the hardest to repair.
Which one belongs on your roof
- Heavy rainfall, Myrtle Beach, Charleston, Savannah, Jacksonville. Open metal valleys handle volume best. Worth the extra cost where storms deliver a lot of water quickly.
- Snow and ice, Denver, Parker. Open valleys, clearly. Ice slides on metal and grips shingle surface, and an open valley gives meltwater a route out rather than a place to back up.
- Hail, Denver, Parker, Oklahoma City, Springfield. Open valleys, with attention to metal gauge. Thin valley metal dents and holds water in the dents.
- Heavy tree cover anywhere. Open valleys, because debris washes through instead of lodging against a cut edge.
- Simple roofs, mild exposure, budget matters. A well-built closed-cut valley is entirely adequate and is what most houses have.
What to ask before the work starts
- Which valley method, and why that one for this roof?
- Is ice-and-water shield running the full length of every valley?
- If open: what gauge metal, what material, and how are the edges finished?
- If closed-cut: how far back from the center is the cut line, and how close will nails come to the channel?
- Are the existing valleys being fully rebuilt or worked around?
That last question matters more than it sounds. Valleys are a place where a hurried re-roof reuses what is already there.
Our roofing repair team rebuilds valleys as a standalone job where the rest of the roof is sound, and valley construction is specified on every roof replacement we quote. The underlayment guide covers the membrane that goes underneath, and a free roof inspection will tell you what you currently have.
Roof valley questions
Open metal valleys generally last longest and handle the most water, which makes them the strongest choice in heavy rain, snow, ice, hail or under trees. Closed-cut valleys are the common residential standard and perform well when built correctly. Woven valleys are the one to avoid with modern laminated shingles.
Because two roof planes drain into one narrow channel, so water volume and speed are far higher there. Valleys also collect debris, form ice in cold climates, and require angled shingle cuts that introduce more chances for error.
Yes, running the full length, regardless of which construction method is used above it. This is the single most important thing in a valley and the first item to confirm on an estimate.
With modern architectural shingles, generally yes. Laminated shingles are thick and stiff, and weaving forces them to bend through the valley, which causes bridging and stress. Many manufacturers now discourage or prohibit it with laminated products, and it can affect warranty coverage.
Yes. Where the rest of the roof is sound, a valley can be opened up, new membrane installed and the valley rebuilt as a contained repair. It is a common job on roofs that are leaking in one place but otherwise have life left.
That is a matter of taste. The metal is visible as a band down the valley, and it can be supplied in colors to suit the roof. Plenty of homeowners prefer the crisp line, and on many house styles it looks deliberate rather than utilitarian.