When it comes to roofing details that separate professional work from amateur installation, valley flashing stands near the top of the list. A roof valley is where two sloping roof planes meet, and it is one of the most vulnerable points on any roof structure. Water from both slopes funnels into this narrow channel, and if the flashing is inadequate or poorly installed, leaks are almost inevitable. Many roofers default to a simple V-shaped valley flashing, but for those who want a higher level of protection, the W-shaped valley flashing offers a significant improvement. By adding a center rib that prevents water from washing across the valley and under the shingles on the opposite side, this design provides a margin of safety that standard flashing cannot match. Custom bending your own valley flashing from copper is not as difficult as it might seem, and with a metal brake and careful planning, you can fabricate flashing that fits your specific project. For a broader foundation in roof flashing installation techniques, review the principles that apply to all flashing work before tackling this advanced detail.
Why Choose a W-Shaped Valley Over a Standard V-Shaped Valley
The fundamental difference between a W-shaped valley and a V-shaped valley is the presence of a raised center rib that runs the length of the flashing. This rib changes the way water behaves at the valley intersection and provides a critical line of defense against leakage.
How the Center Rib Prevents Water Migration
In a standard V-shaped valley, water flowing down one roof slope can, under heavy rain conditions, wash across the valley floor and travel up the opposite slope. This phenomenon, called water crossover, forces water under the shingles on the far side where it finds its way into the roof deck. The center rib of a W-shaped valley acts as a dam that blocks this crossover, redirecting water back into its own channel and keeping each slope’s drainage independent.
When W-Shaped Valley Flashing Is Most Beneficial
- Low-slope roof valleys where water moves slowly and has more time to find pathways under shingles
- Roofs in heavy rainfall or snowmelt regions where valley water volume is consistently high
- Cedar shake and slate roofs where the irregular surface makes water crossover more likely
- Valleys feeding into gutter systems with limited capacity where any water diversion is critical
- Historic or premium homes where the cost of a future leak repair far outweighs the modest extra expense of better flashing
Material Selection for Custom Valley Flashing
Copper is the preferred material for custom-bent valley flashing because of its corrosion resistance, malleability, and long service life. A 16-ounce copper sheet in 24-inch width provides enough material to form the center rib, both valley pans, and hemmed edges. Galvanized steel is an alternative but requires a heavier gauge and must be protected against corrosion. Aluminum is lightweight and workable but lacks copper durability in high-traffic applications. For those working with copper, understanding copper and aluminum composite flashing options can help you decide between solid copper and bonded materials.
Setting Up the Metal Brake for W-Shaped Valley Bends
Accurate brake setup is the foundation of successful valley flashing fabrication. The W shape requires seven distinct bend operations, and each must be positioned precisely. Before cutting any metal, set up the brake and confirm the bend sequence.
Tools and Materials Required
- Metal brake capable of bending 24-inch-wide sheet metal
- 16-ounce copper sheet, 24 inches wide (or chosen alternative material)
- Aviation snips for making registration cuts
- Measuring tape and straightedge
- Permanent marker or scribe for layout lines
- Gloves and eye protection
Marking Bend Locations with Aviation Snips
Professional roofers mark bend locations with small registration cuts using aviation snips instead of pencil lines. These cuts are visible from the top of the sheet even when the metal is under the brake clamp, making alignment faster and more accurate. Make a snip cut at each of the following distances from the reference edge:
| Bend Number | Distance from Edge | Angle | Notes |
|---|---|---|---|
| 1 | 12 inches | Slightly over 90 degrees | Center rib top, first side |
| 2 | 1 inch from center | 90 degrees | Left side of center rib |
| 3 | 1 inch from center | 90 degrees | Right side of center rib (rotate and flip sheet) |
| 4 | 3/4 inch from edge A | 180 degrees | Hemmed edge, first fold |
| 5 | 3/4 inch fold | Flatten | Hemmed edge, final flatten |
| 6 | 3/4 inch from edge B | 180 degrees | Hemmed edge, other side first fold |
| 7 | 3/4 inch fold | Flatten | Hemmed edge, other side final flatten |
The registration cuts should be approximately 1/8 inch deep. Deep enough to see clearly but shallow enough to avoid weakening the material.
Understanding the Bend Sequence
The order of bends matters because later bends can distort earlier ones. Start with the center rib. The first bend creates the rib top and should be slightly tighter than 90 degrees. Bends two and three form the vertical sides, each 1 inch from center. After the rib, move to the outer edges and fold the hemmed edges. Each hem requires two passes: a 180-degree fold followed by a flattening step. To form the second side of the center rib, rotate the sheet 180 degrees end-for-end and flip it upside down.
Step-by-Step Folding Process and Installation
With the brake set up and the copper sheet marked, the bending process proceeds systematically. Each bend builds on the previous one, and the finished piece emerges with a symmetrical W profile.
Forming the Center Rib
Begin with bend number one: the top of the center rib. Position the sheet so the brake clamp engages at the 12-inch registration cut. Raise the bending leaf to approximately 95 degrees to compensate for springback. Next, form the left side of the center rib at the 1-inch registration cut. Clamp and bend to 90 degrees. For the right side, rotate the sheet end-for-end and flip it upside down. Clamp and bend to 90 degrees. The center rib is now complete with a flat top approximately 1 inch wide and two vertical sides each 1 inch tall.
Adding Hemmed Edges
After the center rib is formed, add hemmed edges along both sides. Fold the first hem by positioning the edge with approximately 3/4 inch extending past the clamping bar. Bend to 180 degrees, then reposition and flatten the hem completely. Repeat on the opposite edge. The finished flashing should have a crisp W profile with smooth, folded edges. Inspect the flashing after each bend. The center rib should be straight and centered, and both hemmed edges parallel to the rib.
Positioning and Fastening the Flashing
Center the W-shaped valley flashing so the center rib aligns with the intersection of the two roof slopes. Shingles on both sides must cover the flat pans by at least 4 inches. Apply fasteners only along the outer edges, never through the center rib or within the water channel. Use copper or stainless steel roofing nails spaced approximately 12 inches apart. For installations over solid decking, a layer of ice and water shield for roof valleys beneath the flashing adds a valuable secondary barrier that seals around fastener penetrations.
Shingle Layout and Cutting
Trim shingles so they overlap the flat pan but leave a gap of 1/2 to 1 inch between the shingle edge and the center rib, allowing water to flow freely. Use the step method: install all shingles on one slope first, cutting at the valley centerline minus the gap, then install shingles on the other slope. This produces a clean, straight valley line. For cedar shake roofs, use a wider gap of 1 to 1.5 inches. Understanding fire-treating cedar shakes may also be relevant for code compliance in fire-prone areas.
Common Installation Mistakes
- Placing fasteners through the center rib creates holes in the critical waterproofing element
- Allowing shingles to contact the center rib can wick water around the rib and defeat its purpose
- Bending the flashing to fit an uneven valley instead of correcting the deck surface first
- Using dissimilar metals such as galvanized fasteners in copper flashing causes galvanic corrosion
- Neglecting to seal flashing splices where two sections of valley flashing overlap
Advanced Tips for Durable Valley Flashing Performance
Splicing Flashing Sections for Long Valleys
When a roof valley exceeds a single copper sheet length, splice multiple sections with at least 6 inches of overlap. Install the upper section over the lower section so water flows over the joint. Apply polyurethane sealant between the overlapping layers and stagger fastener patterns so nails from upper and lower sections do not align.
Integrating Kickout Diverter Flashing
Where a roof valley meets a wall, a kickout diverter redirects water away from the wall and into the gutter. This commonly overlooked detail is critical for preventing moisture damage to wall assemblies. The principles of bigger flashing solutions like kickout diverters show how simple design changes solve persistent water intrusion problems. Ensure the diverter sits below the valley flashing and directs water into the gutter away from the wall.
Maintenance and Inspection Checklist
- Check for debris accumulation in the valley channel and clear any blockages
- Inspect the center rib for dents or deformation that could alter water flow
- Verify that shingle edges have not shifted and begun contacting the center rib
- Look for signs of corrosion at fastener locations and splice joints
- Confirm that sealant at splices and end dams is intact and flexible
- Check gutter downspouts at valley terminations for adequate flow capacity
- Inspect the attic or ceiling below valleys for any signs of water staining
By following these fabrication, installation, and maintenance practices, W-shaped valley flashing provides exceptional protection at one of the most vulnerable points on any roof. The extra effort required to custom-bend this profile pays dividends in reduced leak risk and extended roof service life.