Roof Valley Flashing Types Open Closed Woven Difference
Roof Valley Flashing Types Open Closed Woven Difference
The valley is where two roof planes meet and water from both planes converges into a single drainage path. It is the highest-velocity, highest-volume drainage point on most residential roofs, and it is the location where the roof fails first if the valley detail is wrong. The choice between open valley flashing, closed-cut valley flashing, and woven valley flashing is one of the more consequential decisions a roofer makes during installation, and it is one that most homeowners never learn enough about to evaluate intelligently.
Each of the three valley types has a specific use case where it performs well and other situations where it performs poorly. The choice depends on the roof slope, the climate, the shingle product, the aesthetic the owner wants, and the manufacturer's warranty requirements for the shingle being installed. None of the three is universally correct.
What Each Valley Type Actually Looks Like
An open valley exposes a continuous metal flashing channel down the center of the valley with the shingles cut back several inches on either side to leave the metal visible. The metal is typically a pre-formed W-shape or V-shape in 24-gauge galvanized steel, aluminum, or copper, and the water runs down the metal channel rather than across shingle surfaces. The visual is a clean strip of metal flashing down the valley with cut shingle edges parallel to the centerline.
A closed-cut valley covers the metal flashing underneath with shingles from both roof planes, but only one plane's shingles extend across the valley centerline. The shingles from the other plane are cut back two to three inches off the centerline and the exposed shingles from the first plane carry the water down the valley. The visual is a continuous shingle surface with a single straight cut line down the valley.
A woven valley interlaces shingles from both roof planes across the valley centerline without cutting either plane. The shingles alternate between the two planes course by course, producing a basketweave appearance. The valley has no exposed cut edges and no exposed metal. The visual is the smoothest of the three options but requires the most labor at installation.
Within each broad category there are variations that affect performance. Open valleys can use a simple W-profile metal flashing or a more sophisticated battened design that adds a raised diverter rib at the centerline to prevent water from crossing the valley during high-flow events. Closed-cut valleys can be installed with a single metal flashing strip beneath the shingles or with a double-layer underlayment combination that omits the metal entirely on slopes well above the minimum. Woven valleys can be installed traditionally with course-by-course alternation or in a modified pattern that maintains the woven appearance while accommodating stiffer modern shingles. Each variation has its own performance characteristics that matter when matching the detail to the project.
Where Each Valley Type Performs Best
Open valleys handle the highest water volume and the highest drainage velocity of the three valley types. The metal channel provides a smooth, low-friction drainage path that does not absorb water and does not develop the surface granule wear that shingled valleys experience. The Asphalt Roofing Manufacturers Association has documented that open valleys typically outlast the surrounding shingle field by a significant margin in high-precipitation climates because the valley does not wear at the same rate as the shingles around it.
The open valley is the preferred detail in regions with heavy snowfall and ice damming risk. The smooth metal surface sheds snow and ice more efficiently than a shingled valley, and the metal does not trap ice the way shingle granules do. The NRCA recommends open valley construction for any roof in International Energy Conservation Code Climate Zones 5 through 8, which covers most of the northern United States and Canada.
Have you noticed that newer roofs in snowy climates almost always have visible metal valleys while older roofs in the same neighborhood have shingled valleys? That shift reflects a deliberate move in industry best practice toward open valley construction as the ice damming and water infiltration risks of shingled valleys became better documented.
Metal selection for open valleys matters more than the gauge alone suggests. Galvanized steel is the most common and least expensive option but develops rust streaks on shingles below the valley after the zinc coating wears through, which typically happens in fifteen to twenty-five years depending on coating thickness and rainfall. Aluminum lasts longer and produces no rust staining but dents more easily during installation and can react galvanically with copper flashings on the same roof. Copper is the premium option with a service life that exceeds the surrounding shingles, but the cost is roughly four times that of galvanized steel and the green patina that develops over time changes the appearance of the roof in ways some homeowners find desirable and others do not. Most production residential work uses pre-finished galvanized steel in a color that matches the shingles, which balances cost against appearance.
The Case For Closed-Cut Valleys
Closed-cut valleys produce a cleaner appearance than open valleys and use less metal, which reduces material cost. The detail also produces a slightly more weather-tight assembly at the valley centerline because the water never contacts the metal flashing directly under normal conditions. The metal is present as a backup waterproofing layer but the shingle surface carries the water.
The closed-cut detail performs well on roof slopes of 4:12 and steeper where water moves quickly enough to avoid backing up under the shingle cuts. On lower slopes, the slower water movement allows water to wick laterally under the cut edges and reach the underlying metal flashing, which becomes the primary drainage surface in those conditions. The shingle manufacturer's installation instructions typically prohibit closed-cut valleys below a specified minimum slope.
The decision about which roof plane carries the shingles across the centerline and which plane is cut back also matters. The standard practice is to extend the shingles from the larger or steeper roof plane across the valley because that plane carries more water and benefits from the uninterrupted shingle surface. The smaller or lower-slope plane gets the cut. Reversing the convention produces a valley that drains less efficiently and can develop debris accumulation at the cut edge during fall leaf season. Experienced installers handle this convention automatically. Less experienced installers sometimes cut whichever plane is more convenient, which produces an installation that meets code on paper but underperforms in service.
Most asphalt shingle manufacturers including GAF, CertainTeed, and Owens Corning include closed-cut valley details in their installation manuals as acceptable construction for slopes above 4:12, but each manufacturer has specific requirements for the underlying ice and water shield membrane, the shingle cut-back distance, and the sealing of the cut edges. Following the manufacturer's specific detail is a warranty requirement, not a suggestion.
The cut-back distance on a closed-cut valley is more important than it sounds. The exposed shingles from one plane have to extend across the valley centerline by at least two inches so that water sheets uninterrupted into the gutter at the valley exit. The cut shingles from the other plane have to terminate two to three inches off the centerline so that the cut edge does not sit in the path of flowing water. Cuts that come too close to the centerline produce a leak source within a few years as water works under the cut edge. Cuts that come too far back from the centerline produce a wide aesthetic gap that some homeowners find objectionable. The sweet spot is narrow, and consistent cutting across the full valley length requires either skilled freehand work or a guided cutting tool.
Why Woven Valleys Have Fallen Out Of Favor
Woven valleys were the dominant residential valley detail through the 1970s and into the 1980s but have largely been displaced by closed-cut and open valley construction over the last forty years. The reason is product compatibility. Woven valleys require shingles that can be bent and conformed across the valley centerline without cracking, and modern fiberglass-mat asphalt shingles are stiffer than the organic-felt shingles common in the era when woven valleys were standard.
Most current shingle manufacturers either prohibit woven valleys outright or restrict them to specific shingle product lines designed for the application. Architectural laminated shingles, which are the most common shingle type sold today, are too thick and too rigid for proper weaving and the manufacturer warranties typically void coverage on woven valley installations. Three-tab shingles tolerate weaving better but are themselves a declining product category.
The other problem with woven valleys is the labor cost. A proper woven valley requires careful course-by-course alternation between the two roof planes, which slows the installation pace and requires more skilled labor than either of the other two detail types. The labor savings on metal flashing are typically more than offset by the labor cost premium for the weaving itself.
That said, woven valleys still appear on historic restoration projects and on some custom architectural applications where the visual is the priority. If a woven valley is required for historic accuracy, the shingle product has to be selected to match. Some manufacturers offer specific shingle product lines compatible with woven valley construction, typically marketed for restoration work, and these products carry the warranty for woven applications that standard architectural shingles do not. Specifying the right shingle for the intended valley detail is part of the planning process for any project where woven valleys are still being considered.
Underlayment And Ice And Water Shield Requirements
The valley underlayment is as important as the valley flashing itself, and the requirements differ between the three valley types. The International Residential Code (IRC) Section R905.1.2 requires self-adhering polymer-modified bitumen ice and water shield membrane in valleys in regions where the average daily temperature in January is 25 degrees Fahrenheit or lower, and most jurisdictions in those regions require the membrane regardless of valley type.
For open valleys, the ice and water shield runs the full length of the valley and extends at least 36 inches on each side of the valley centerline. The metal flashing installs over the membrane, and the shingles cut back to expose the metal. The membrane provides backup protection if the metal flashing develops a leak.
For closed-cut valleys, the ice and water shield requirements are typically more aggressive because the shingle surface is the primary water barrier and the membrane is the second line of defense. NRCA technical guidance recommends extending the ice and water shield 36 inches on each side of the valley centerline and additionally requires that the metal flashing be installed in pieces with at least 6-inch overlaps to allow for thermal movement.
For woven valleys, where the metal flashing may be omitted entirely on slopes above 4:12, the ice and water shield is the only secondary water barrier and the membrane requirements are correspondingly more rigorous. Some manufacturer details for woven valleys require a full 36 inches on each side of the centerline plus an additional layer of asphalt-saturated felt on top of the ice and water shield.
Ice and water shield product selection deserves attention too. Standard self-adhering membrane products are designed for the specific service environment of a valley, with elevated tear strength and superior self-sealing around fastener penetrations. Generic peel-and-stick underlayments without the specific valley rating may fail to seal around the shingle nails that penetrate them, which produces leak paths even when the visible flashing detail is correct. The product selection should be specified in the contractor's proposal and matched to the manufacturer's recommended underlayment list for the shingle product being installed.
Climate-Specific Recommendations
Do you live in a region that gets significant winter snow loading? The open valley with a 24-gauge or heavier metal flashing is the industry standard recommendation. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has published research linking valley flashing type to ice dam frequency, and the data favor open valleys in cold climates by a substantial margin. The National Association of Home Builders (NAHB) Research Center concurs with that recommendation in published technical bulletins.
In hot and humid climates with high-intensity rainfall, the closed-cut valley is the more common detail and performs well when properly installed. The high water velocity from intense summer thunderstorms moves water through the valley quickly enough that the closed-cut detail does not back up, and the shingle surface tolerates the UV and thermal exposure better than exposed metal in those climates. White or light-colored metal valleys can develop heat-induced staining on the adjacent shingles in hot climates, which is one reason the closed-cut detail is preferred in the South.
In dry climates with moderate rainfall and minimal snow, all three valley types perform acceptably, and the choice typically comes down to aesthetic preference and installation cost. Open valleys with copper or pre-finished metal can serve as a deliberate architectural feature in those climates, while closed-cut valleys provide a more uniform shingle appearance for owners who prefer that look.
The wildfire-prone regions of the western United States add another consideration that affects valley detail selection. Class A fire-rated assemblies are mandatory in many wildland-urban interface zones, and the valley detail has to maintain the assembly fire rating in addition to the water-shedding requirements. Open metal valleys generally maintain Class A ratings more easily than shingled valleys because the metal does not contribute fuel to a roof fire. Closed-cut valleys with appropriate underlayments can also achieve Class A ratings but require more careful product selection. If your jurisdiction has adopted wildland-urban interface fire codes, the valley detail should be specified against those requirements as well as the standard roofing code.
What To Specify When Hiring A Roofer
When you are getting quotes for a re-roofing project, the valley detail should be specified in the written proposal, not left as an installer choice. A reputable contractor will identify the valley type they intend to install, the underlying ice and water shield specification, the metal flashing gauge and finish if applicable, and the manufacturer's installation detail they will follow. If a contractor cannot answer those questions specifically, the contractor is not the right choice for your project.
For a re-roofing project on an existing house, the valley detail should match the manufacturer's installation instructions for the shingle product being installed and the climate zone the house sits in. The contractor should be able to point to the specific page in the manufacturer's installation manual that authorizes the chosen valley type. Manufacturer warranties depend on installation according to the manual, and a valley detail outside the manual's authorized options can void coverage.
Pay attention to how the contractor describes the underlayment detail at the valley. A contractor who can identify the specific ice and water shield product, the lap width over the metal flashing, and the relationship between the underlayment and the field underlayment around the valley is demonstrating the level of detail attention that produces durable installations. A contractor who waves off questions about underlayment specifics is signaling that the work will be done by feel rather than by specification, which is rarely the better outcome.
If you are evaluating a new construction project or considering a re-roof and want a written assessment of the appropriate valley detail for your specific roof geometry and climate, contact a licensed roofing contractor or a roof consultant in your area for a written specification. A few hundred dollars spent on consulting before the work begins is significantly less expensive than discovering after installation that the valley detail does not meet the manufacturer's warranty requirements.
Inspecting an existing valley for failure signs is something every homeowner can do from inside the attic during a heavy rain. Look at the underside of the roof deck along the valley line with a flashlight and watch for any active water entry. The most common failure pattern is a thin trickle along one side of the valley centerline that appears during sustained heavy rain and stops shortly after the rain ends. That pattern indicates a sealant failure or an underlayment lap failure rather than a catastrophic flashing problem and is usually repairable without full reflashing. A widespread water entry across multiple points along the valley indicates a more comprehensive failure that may require valley reconstruction.
Final Thoughts
The valley is where the roof works hardest and where shortcuts cost the most over time. The choice between open, closed-cut, and woven valley details is not a matter of installer preference. It is a matter of matching the detail to the slope, the climate, the shingle product, and the warranty requirements. Open valleys are the preferred detail in cold and snowy climates and on lower slopes. Closed-cut valleys are the preferred detail in moderate climates on slopes of 4:12 and steeper with stiffer modern shingle products. Woven valleys are largely a legacy detail appropriate only for specific shingle products on specific roof geometries. Whichever detail your roof requires, the installation quality at the valley matters more than the specific detail type. Specify the valley detail in writing before the work starts, verify the underlayment matches the IRC requirements for your climate zone, and contact a qualified roofing contractor for a written specification if you are unsure which detail is right for your house.
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