Types of Roof Valley Flashing and How to Choose the Right One

Roof valley flashing protects where two roof planes meet and guides water away from vulnerable seams. This article explains the common types of roof valley flashing, materials, installation methods, pros and cons, and tips for selecting the best option for different roof systems. The goal is to help homeowners, contractors, and property managers make informed choices to prevent leaks and extend roof life.

Type	Best For	Longevity	Typical Cost
Open (Exposed) Valley	Asphalt shingles, metal roofs	15–30 Years	Low–Moderate
Closed (Woven) Valley	Asphalt shingles	15–25 Years	Low
Metal (Lined) Valley	High-slope roofs, heavy flow areas	30–50+ Years	Moderate–High
Roofing Underlayment Valley	Ice/snow regions	20–40 Years	Moderate

What Is Valley Flashing And Why It Matters

Valley flashing is the component installed along the internal angle between two roof slopes to channel water into gutters or off the eave. Without proper valley flashing, concentrated runoff can find leaks at fasteners, seams, or under shingles, causing water damage to roof sheathing and interior spaces.

Proper valley flashing reduces the risk of ice dam backflow in cold climates and prevents debris buildup from creating standing water, both of which shorten roof life and increase maintenance costs.

Open (Exposed) Valley Flashing

Open valley flashing, also called exposed metal valley, uses a visible metal liner—typically galvanized steel, aluminum, or copper—laid in the valley with roofing material terminated over its edges.

Materials And Characteristics

Galvanized Steel: Durable and cost-effective, but can corrode if not protected. Aluminum: Lightweight and rust-resistant, suited to coastal climates. Copper: Premium option with long life and aesthetic appeal; expensive.

Benefits

High Flow Capacity: Handles intense runoff well, making it ideal for steep roofs or large valleys. Serviceability: Easy to inspect and replace without disturbing entire roof sections.

Drawbacks

Visible Metal: Exposed metal may not match some architectural styles. Thermal Movement: Metal can expand and contract, requiring proper fastening and expansion joints to avoid buckling.

Closed (Woven) Valley Flashing

Closed or woven valleys cover the valley with shingles from both roof planes woven together so the valley liner is fully covered by the shingle surface.

Materials And Characteristics

Asphalt Shingles: Most common; woven installation requires skill to avoid weak spots. No Exposed Metal: Seamless look that matches shingle fields.

Benefits

Visual Continuity: Attractive for heritage homes and shingle-focused aesthetics. Lower Initial Cost: No metal flashing material is required, reducing material expenses.

Drawbacks

Leak Risk: Less tolerant to heavy runoff and snowmelt; not recommended for very steep or high-flow valleys. Repair Complexity: Damage often requires careful shingle matching and workmanship.

Metal Lined Valley (Hidden or Woven With Metal)

Metal lined valleys combine the aesthetics of a covered valley with the durability of a metal liner. The metal is installed under the shingles and is not visible from the roof plane.

Materials And Characteristics

Underlayment Metal: Often galvanized or aluminum strips under the shingle course. Integration: Designed to shed water while preserving a shingled look.

Benefits

Best Of Both Worlds: Offers improved water handling without a visible metal channel. Improved Durability: Reduces the risk of leaks compared to fully woven valleys.

Drawbacks

Higher Labor: Requires careful installation to ensure metal is properly lapped and sealed. Cost: More expensive than a pure woven valley.

Preformed Valley Liners And Roll Flashing

Preformed valley liners are factory-shaped metal pieces that fit standard valley widths; roll flashing uses continuous metal roll stock formed on-site.

Advantages

Consistent Quality: Factory preforms reduce on-site bending errors. Faster Installation: Roll flashing is efficient for long valleys or commercial roofs.

Considerations

Material Selection: Match metal type to roof and local climate to minimize corrosion. Seaming: Proper seams and fasteners are necessary to maintain waterproofing.

Self-Adhesive Underlayment And Waterproof Membranes

Modern self-adhesive membranes are often used under valley flashing or as a primary waterproof layer in cold climates with ice dam risk.

Types And Uses

Peel-and-Stick Membranes: Rubberized asphalt or SBS membranes applied to the valley decking provide continuous waterproofing. Liquid Applied Membranes: Used in specialized repairs or complex intersections.

Benefits

Excellent Seal: Adhesive membranes create a monolithic waterproof layer, reducing leaks from fasteners or small gaps. Ice Barrier: Particularly valuable in freeze-thaw regions.

Drawbacks

Cost And Compatibility: Higher material cost and need for compatibility with roofing adhesives and shingles. UV Exposure: Must be covered by shingles or metal to avoid degradation.

Valley Flashing For Metal Roofs

Standing seam and screw-down metal roofs require valley solutions compatible with panel profiles, often using custom metal pans or formed flashing integrated into the panel layout.

Key Considerations

Water Tightness: Metal roofs expect rapid drainage; valleys should be continuous pans or formed systems to avoid fastener penetration in the valley bed. Thermal Movement: Allow for expansion to prevent distortion.

Common Approaches

Metal Troughs: Fabricated pans installed under or above panels. Interlocked Profiles: Panels that interlock at valleys, minimizing separate flashing.

Installation Best Practices

Slope And Flow Path: Ensure valley angle directs water to gutters or scuppers without creating ponds. Underlayment: Apply high-quality underlayment, especially in valleys, to provide backup waterproofing.

Lapping: Maintain proper lapping of flashings and shingles per manufacturer specifications to prevent capillary flow. Fasteners: Place fasteners outside the valley bed whenever possible to avoid penetration points.

Debris Management: Keep valleys clear of leaves and sediment to prevent standing water and corrosion. Regular inspection extends life and prevents surprises.

Choosing The Right Valley Flashing For A Roof

Climate: In snowy or freeze-thaw regions, prioritize robust membranes and metal-lined valleys. In dry areas, aesthetic choices like woven valleys may be acceptable.

Roof Material And Pitch: Shingle roofs often use woven or covered options; steep or large-area valleys benefit from open metal liners. Metal roofs require compatible pans or profile-integrated flashings.

Budget And Longevity: Copper or heavy-gauge aluminum warranties justify higher costs over the long term. For tight budgets, galvanized or asphalt-covered solutions are reasonable when well installed.

Maintenance, Inspection, And Repair Tips

Annual Inspection: Inspect valleys after major storms and during seasonal maintenance checks. Look for corrosion, separated seams, missing shingles, and clogs.

Minor Repairs: Seal small splits with compatible roofing sealant and replace damaged shingles promptly. Replace corroded metal flashing sections before leaks penetrate the deck.

When To Replace: Replace valley flashing when extensive corrosion, repeated leaks, or multiple patch repairs exist. Full valley replacement is often required when reroofing to ensure long-term performance.

Code, Warranty, And Professional Considerations

Building Codes: Local codes may specify underlayment types, ice barriers, and flashing practices in certain zones. Always verify local code requirements before selecting materials.

Manufacturer Warranties: Follow shingle and membrane manufacturer guidelines for valley flashing to maintain warranty compliance. Improper flashing methods can void coverage.

Hire Qualified Installers: Valley flashing requires precision; hiring experienced roofers ensures correct slopes, laps, and material compatibility to minimize future leaks.

Summary Of Pros And Cons By Type

Type	Pros	Cons
Open Metal	Durable, high flow, easy service	Visible, thermal movement issues
Closed Woven	Attractive, lower material cost	Less tolerant of heavy runoff
Metal Lined Hidden	Durable, aesthetic	Higher labor and cost
Self-Adhesive Membrane	Excellent waterproofing, ice barrier	Needs coverage, higher cost

Key Takeaways For Homeowners And Professionals

Select valley flashing based on roof material, climate, and expected water flow. Prioritize waterproof backups in cold or heavy-precipitation areas and durable metals for high-flow valleys.

Regular maintenance and professional installation preserve the flashing’s effectiveness. Proper material selection and installation reduce the risk of leaks and costly repairs over the roof’s life span.