Valley Flashing Methods Compared
Why this matters
Valleys carry more water per square foot of roof surface than any other area. A 1,500 sq ft roof with one main valley dumps the entire ridge-to-eave drainage of roughly half the roof through that one valley. Get the valley wrong and the roof leaks at the heaviest-water location, which is also typically over interior space (the dining room, the master bedroom). Three valley methods are commonly used (open, closed-cut, woven). Each has specific failure modes, manufacturer warranty implications, and skill requirements. Picking the wrong method for the slope, shingle type, or climate produces a leak the customer never expected. Knowing which to use and why is the diagnostic edge.
What a valley is
A valley is the internal corner where two roof planes meet, channeling water downward. The valley shape concentrates water flow from both adjoining planes into the single drainage channel.
Valley types by geometry:
- Open valley: visible metal flashing in the valley center; shingles cut back from the metal
- Closed-cut valley: one side of shingles continues across the valley; the other side cut along a chalk line
- Woven valley: shingles from both sides woven together across the valley
- California cut (rare): variation of closed-cut with specific shingle orientation
Open valley
A continuous strip of metal (galvanized, copper, painted steel, or aluminum) runs down the valley centerline. The shingles on each side are cut along a chalk line set back 2 to 4 inches from the valley centerline, exposing the metal as a visible channel.
| Aspect | Detail |
|---|---|
| Visual | Visible metal stripe down the valley; pronounced design element |
| Water-shedding | Excellent; water flows down clean metal |
| Snow / ice tolerance | Best of the three methods; ice slides easily on metal |
| Debris tolerance | Best; leaves and debris wash through |
| Repair | Easiest; just lift the cut shingles to inspect or repair metal |
| Cost | Highest material cost; lowest labor for the install (after metal is in) |
| Skill | Moderate; sheet metal work plus shingle cutting |
| Lifespan | Matches the metal: copper 80+ years, galvanized 25 to 40 years |
| Best for | Slate, tile, premium shingle roofs; high snow load; steep pitch |
Installation requires:
- Underlayment in the valley (ice and water shield required in cold climates per IRC R905.1.2)
- Metal flashing 24 inches wide minimum (some specs 36 inches), centered on the valley
- Hemmed edges on the metal (folded back 1 inch for strength)
- Center crimp directing water down the valley center (optional but preferred)
- Shingles cut along a chalk line 2 to 4 inches back from valley centerline
- Bead of sealant under the cut shingle edge
Closed-cut valley
Shingles from the larger roof plane continue across the valley to extend at least 12 inches up the other plane. Shingles from the smaller plane are cut along a chalk line, with the cut held 2 inches back from the valley centerline. The continuous shingles form the actual valley membrane; the cut shingles overlap the continuous shingles.
| Aspect | Detail |
|---|---|
| Visual | Clean line; no visible metal |
| Water-shedding | Good in most climates; depends on shingle wash |
| Snow / ice tolerance | Moderate; ice can lift cut shingles |
| Debris tolerance | Moderate; debris can catch at the cut line |
| Repair | More difficult than open; cut shingles must be lifted carefully |
| Cost | Mid-cost material, mid-cost labor |
| Skill | Moderate; requires straight-line cutting |
| Lifespan | Matches the shingle (typically 25 to 40 years for architectural) |
| Best for | Architectural shingles in moderate climates; cost-conscious projects |
Installation:
- Underlayment in valley with ice and water shield in cold climates
- Shingle the larger plane FIRST, extending shingles across the valley to overlap the smaller plane
- Chalk a line on the smaller plane 2 inches back from valley centerline
- Shingle the smaller plane, cutting each shingle at the chalk line
- Apply sealant under the cut shingle edges
- Cut shingles edges are sealed with shingle adhesive or roofing cement
Woven valley
Shingles from each plane are alternated across the valley, "weaving" together. Each shingle from one plane is overlapped by the next shingle from the other plane and so on, producing a continuous shingle surface across the valley.
| Aspect | Detail |
|---|---|
| Visual | Continuous shingle pattern; no visible cut or metal |
| Water-shedding | Depends entirely on shingle weight and shingle interlock |
| Snow / ice tolerance | Worst of three methods; ice can lift shingles |
| Debris tolerance | Worst; debris catches at weave |
| Repair | Most difficult; have to remove woven sections to inspect |
| Cost | Lowest material; highest labor (slow weaving) |
| Skill | High; requires keeping the weave consistent |
| Lifespan | Shorter than other methods; weave-related failures |
| Best for | Limited modern application; rarely specified new |
Installation:
- Underlayment in valley
- Shingle from both planes simultaneously, alternating
- Each course from one plane overlaps the next course from the other
- Maintain consistent weave pattern down the valley
The woven valley is a legacy method. Modern shingle manufacturers generally do not warranty woven valley installations (the weave concentrates wear at the cut edges). Most current installs use open or closed-cut.
When to use which method
| Situation | Best method |
|---|---|
| Slate roof | Open valley (copper or stainless) |
| Tile roof | Open valley (copper or stainless) |
| Architectural asphalt shingle, moderate climate | Closed-cut |
| Architectural asphalt shingle, heavy snow load | Open valley (metal) |
| Premium architectural shingle, customer preference | Customer choice; both work |
| Wood shake | Open valley (metal) |
| Metal roof | Open valley (matching metal) |
| Coastal location | Open valley (stainless or copper) |
| Cost-driven re-roof | Closed-cut |
| Historic restoration | Match the original (often open with copper) |
Underlayment in the valley
Regardless of valley method, the underlayment in the valley is extra:
- IRC R905.1.2 requires ice and water shield in cold climates extending the full length of the valley
- Add at least 18 inches of width on each side of the valley centerline (36 inch total)
- Extend up to the eave per local ice-dam protection requirement
The ice and water shield is the secondary water barrier; the valley method is the primary.
Metal selection for open valleys
References
- IRC 2021, Section R905.2.8.2 (Open valleys for asphalt shingles).
- IRC 2021, Section R905.2.8.3 (Closed valleys for asphalt shingles).
- IRC 2021, Section R905.1.2 (Ice barriers).
- ASTM A653 (Steel sheet for valley flashing).
- ASTM B370 (Copper sheet).
- NRCA Roofing Manual: Steep-Slope Roof Systems, current edition.
- ARMA (Asphalt Roofing Manufacturers Association) technical guidance.
- Manufacturer installation instructions: GAF, CertainTeed, Owens Corning.
- Manuall internal: Roofing Roof Flashing Types, Roofing Repair Flashing, Roofing Copper Flashing Fabrication.