A roof valley is the line where two roof planes meet and drain into each other. It is the single hardest-working detail on a house.
Consider what happens during a storm. Every square foot of both roof planes sheds water, and all of it funnels into one narrow channel. A valley on a large roof can carry more water in an hour than the rest of the roof surface combined, at higher velocity, in a concentrated stream. Then it does that again for twenty-five years.
That is why valleys and flashing details account for a disproportionate share of roof leaks. Not defective shingles. The places where the roof changes direction.
This article covers the three ways a valley can be built, what the Tennessee Residential Code actually requires, and why the method a contractor chooses tells you something about the rest of the job.
The three valley methods
There are three common ways to finish a valley on an asphalt shingle roof.
Open valley. A metal lining runs the length of the valley and stays visible. Shingles from both planes stop short of the center, leaving a clean channel of exposed metal for water to run down. The metal is the drainage surface.
Closed-cut valley. Shingles from one plane run through the valley and up onto the other side. Shingles from the second plane are laid over them and trimmed in a straight line a few inches off center. Water runs on the shingle surface, not on metal.
Woven valley. Shingles from both planes alternate across the valley, interlacing. No cut line, and again the water runs on shingle surface. Woven valleys are increasingly rare with modern architectural shingles, which are too thick and stiff to weave cleanly.
All three can be code-compliant. They do not perform the same.
What Tennessee code requires
Tennessee has adopted the 2018 International Residential Code as the Tennessee Residential Code for one- and two-family dwellings and townhouses. Valley lining requirements live in Section R905.2.8.2.
For an open valley lined with metal, the code sets a minimum: the valley lining “shall be not less than 24 inches wide,” and the metal has to be one of the corrosion-resistant materials listed in Table R905.2.8.2.
That table sets minimum thicknesses by material:
- Cold-rolled copper, 0.0216 inch nominal, ASTM B370, 16 ounces per square foot
- Lead-coated copper, 0.0216 inch nominal, ASTM B101, 16 ounces per square foot
- High-yield copper, 0.0162 inch nominal, ASTM B370, 12 ounces per square foot
- Aluminum, 0.024 inch minimum thickness
- Stainless steel, 28 gage
- Galvanized steel, 0.0179 inch, 26 gage, G90 coating
- Soft lead, 3 pounds per square foot
The code permits other approaches as well. Open valleys may be lined with two plies of mineral-surfaced roll roofing, an 18-inch bottom layer under a top layer not less than 36 inches wide. Closed valleys may use one ply of smooth roll roofing not less than 36 inches wide, or the metal lining options, or a self-adhering polymer modified bitumen underlayment meeting ASTM D1970.
Two things are worth pulling out of that.
First, the 24-inch minimum applies to the metal itself, not to the visible channel. A significant portion of that width sits under the shingles on either side. If someone shows you a narrow strip of metal in a valley, ask what the full width is.
Second, code is a floor, not a target. Everything above describes the minimum a roof may legally be built to. It does not describe the best way to build one.
Why metal open valleys hold up better
Code allows several methods. Experience separates them.
Water runs on metal instead of on granules. In a closed-cut or woven valley, the concentrated stream runs directly across the surface of asphalt shingles. Asphalt shingles shed water well across a broad plane. They were not designed to be a high-velocity channel. Over years, that concentrated flow wears granules off the shingle surface in the valley, and granule loss is the beginning of asphalt degradation. Metal does not care.
Debris clears instead of catching. This matters more in the Tri-Cities than it does in a treeless subdivision. A great many homes here sit under mature hardwoods, and valleys collect what falls. A smooth open metal channel sheds leaves and needles. A woven or cut valley presents shingle edges for debris to catch on, and trapped debris holds moisture against the roof.
There is no cut line sitting in the drainage path. In a closed-cut valley, the trimmed shingle edge sits a few inches off center, inside the wet zone. That cut edge is a raw, unsealed edge. It is a manageable detail when cut cleanly and sealed properly. It is also one more thing that has to be done correctly in the wettest place on the roof.
Ice and snow behave better. At elevation around Roan Mountain and on shaded north-facing slopes across the region, valleys are where snow lingers and ice builds. A smooth metal surface gives meltwater a path out. Interlaced shingles do not.
W-valley metal adds a center rib. Better valley metal is formed with a raised rib down the center, sometimes called a W-valley or a center-crimp. That rib stops water sheeting across from one plane and driving up underneath the shingles on the opposite side, which is a real failure mode on low-slope valleys and during wind-driven rain.
The trade-off is honest and worth stating: an open metal valley is visible. Some homeowners prefer the uninterrupted shingle appearance of a closed-cut valley. Valley metal is available in colors matched to common shingle lines, which narrows the gap considerably, but it remains a visible detail. That is a legitimate preference and not a wrong answer.
What should go underneath, regardless of method
The valley lining is the visible layer. Underneath it, an ice and water barrier should run the full length of the valley before any metal goes down.
This is a self-adhering membrane that seals around fasteners rather than merely covering the deck. In a valley, it is the layer that matters if anything ever gets past the top surface. Skipping it is invisible on the day the job finishes and expensive later.
Ask about it specifically. A contractor should be able to tell you what membrane goes in your valleys and how far it extends, without looking anything up.
The signs your valley is failing
Worth checking, particularly on a roof past fifteen years:
- Granule accumulation in the gutters concentrated below a valley
- A visible bare or shiny track down the center of a closed-cut valley
- Rust staining, or metal that has separated at a seam
- Shingle edges lifting along the cut line
- Debris packed in the valley that does not clear on its own
- Interior staining on a ceiling directly below where two roof planes meet
Any one of those is worth a look from someone who will actually get on the roof.
The broader point
Valleys are a reasonable proxy for how a roofing company works. They cost more to do properly, they are hidden from the street, and nobody inspects them at closing. A contractor who runs proper W-valley metal over a full-length ice and water barrier is making a decision that costs them money on a detail you would never catch.
That is usually a good sign about the parts of the job you also cannot see.
If your roof has valleys you are unsure about, a roof inspection will tell you what is actually there and how much life is left in it. If a valley is the source of an active leak, roof repair is often far less involved than homeowners expect.
Frequently Asked Questions
Do roof valleys have to have metal in them?
No. The Tennessee Residential Code permits open valleys lined with metal, open valleys lined with two plies of mineral-surfaced roll roofing, and closed valleys using roll roofing, metal, or a self-adhering polymer modified bitumen underlayment. Metal open valleys are a performance choice above the minimum, not a legal requirement.
How wide does valley metal have to be?
For an open valley lined with metal, the code requires the lining be not less than 24 inches wide. Much of that width sits concealed beneath the shingles on either side, so the visible channel is considerably narrower than the metal itself.
Is an open valley more likely to leak than a closed valley?
A properly installed open metal valley generally handles concentrated water better, because the flow runs on metal rather than across shingle surfaces and there is no cut shingle edge sitting in the drainage path. Installation quality matters more than method, but the open metal detail has fewer ways to go wrong over time.
Can you replace just a valley without replacing the roof?
Often yes. If the surrounding shingles still have serviceable life and the deck is sound, a valley can be rebuilt on its own. Whether that is the right call depends on the age of the roof, since installing new valley metal into a roof near the end of its life may not be worth the labor.
What is a W-valley?
Valley metal formed with a raised rib running down the center. The rib keeps water draining down one plane from crossing the valley and driving up under the shingles on the opposite side, which is a common failure mode during wind-driven rain and on lower-slope valleys.
Does an ice and water barrier go under valley metal?
It should. A self-adhering membrane running the full length of the valley beneath the metal is the layer that protects the deck if water ever gets past the surface. The code permits self-adhering polymer modified bitumen underlayment as a valley lining material in its own right, and using it beneath metal is standard practice on a well-built roof.