Roof Valley Flashing: Open vs Closed Valleys and W-Valley

A roof valley handles more water than any other part of the roof, because two slopes drain into one narrow channel. That concentration is why roof valley flashing fails more often than field shingles, and why the choice between an open metal valley and a closed shingle valley matters so much. A valley that is detailed well can outlast the shingles around it. One that is cut short, nailed in the wrong spot or missing underlayment can leak within a single storm season.

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The short answer: an open valley uses an exposed strip of metal, often a W-profile with a raised center rib, and is the most durable choice for steep roofs, heavy rain and areas with lots of leaf debris. A closed-cut valley hides the valley under shingles from one side and trims the other side in a straight line above it. A woven valley interlaces shingles from both slopes. All three need a full-width underlayment beneath, and all three depend on keeping nails out of the center of the valley.

Roof Valley Flashing Options Compared

Every roof valley flashing approach has to do the same job: carry heavy water flow downhill without letting it get under the roofing. The differences come down to what is exposed, how the valley handles debris, and how easy it is to repair later.

Metal roofs use a separate valley pan system, typically a wide formed valley with a center rib and foam closures under the panels. The principles are similar, but the components come from the panel system, so follow that system’s details rather than mixing parts.

Open Metal Valleys and the W-Valley Profile

In an open valley, a strip of metal runs the full length of the valley and the shingles stop short of the center, leaving a channel of exposed metal that widens toward the eave. That exposed channel lets water and leaves move fast without catching on shingle edges.

A w valley flashing adds a raised inverted-V rib, usually about 1 inch tall, down the center. When one slope is steeper or larger than the other, water rushing off the big slope can shoot across the valley and under the shingles on the far side. The center rib stops that crossover. The outer edges are typically hemmed (folded back) so water that reaches them gets turned back toward the channel and so cleats can grip without nailing through the metal.

Metal choices and widths

• Width: valley metal is commonly 20 to 24 inches wide, giving roughly 10 to 12 inches on each side of the center. Wider is better on long valleys and steep, high-volume roofs.
• Prefinished steel: durable and color-matched; the standard choice for most homes.
• Aluminum roll valley flashing: easy to cut and form on site and corrosion-resistant, but softer, so it dents and can move with heat more than steel.
• Copper: very long-lived, used on slate, tile and high-end shingle roofs.
• Galvanized steel: inexpensive but rusts sooner where the coating wears; look for heavier coatings.

Lengths of valley metal should lap at least 6 inches, with the upper piece over the lower, and some installers set a bead of roofing sealant in the lap. Expect the exposed channel to start around 3 inches wide at each side of center near the ridge and widen by about 1/8 inch per foot toward the eave, so the channel never pinches as water volume grows.

Closed-Cut and Woven Valleys

A closed-cut valley starts by running the shingles from the lower or smaller slope fully across the valley, extending at least 12 inches up the opposite slope. Shingles from the other slope then lap over them, and their edges are trimmed along a chalk line about 2 inches from the valley center. The upper corner of each cut shingle is clipped at an angle so water does not run along the top edge and behind the shingle, and the cut edges are bedded in a narrow bead of asphalt roofing cement.

Woven valleys interlace shingles from both slopes, each course alternating sides and extending at least 12 inches past the center. They are fast and leave no exposed edges, but thick laminated shingles do not bend tightly into the valley, leaving bridging and voids that crack. For that reason, most current shingle installations use closed-cut or open metal.

Even closed valleys benefit from metal. Some roofers lay a smooth valley metal under a closed-cut valley as added protection. Ridge and valley metals sold as a set for porch roofs and dormers make this simple on small jobs.

Underlayment: The Real Waterproofing Layer

The roof valley flashing is the first line of defense, but the underlayment beneath it is what keeps a valley dry when ice dams form, debris backs water up, or a shingle edge lifts. Current practice in most regions is a self-adhered ice and water membrane at least 36 inches wide, centered in the valley, running from eave to ridge. Synthetic underlayment from the field of the roof laps over the edges of that membrane.

Under metal, some roofers add a slip sheet or use a high-temperature membrane, because dark metal in summer sun can soften standard membranes and cause them to stick to the metal, which restricts expansion. Ask your installer what membrane they use beneath exposed metal and whether it is rated for that heat.

Why Roof Valleys Leak

Most valley leaks trace back to a short list of mistakes:

• Nails too close to the center: no fastener should be within about 6 inches of the valley centerline. Water flows highest there.
• Debris dams: leaves and needles pile up, slow the flow and push water sideways under shingles.
• Uncut upper corners: on closed-cut valleys, unclipped corners channel water behind the shingle.
• Short metal laps: laps under 6 inches, or laps installed with the lower piece on top.
• Missing membrane: older roofs often have only felt in the valley, which tears and rots.
• Ice dams: in cold climates, meltwater refreezes at the eave and backs up the valley.
• Corrosion: galvanized valley metal rusting through at the center where water and debris sit longest.

Water in a valley leak often shows up on the ceiling well below the actual failure, since it travels along the decking or rafters before dripping. Check the attic on a rainy day with a flashlight, looking for wet decking or dark streaks along the valley rafter.

Installing Valley Flashing: Steps and Safety

Valley work is steep, slippery work at the most awkward point on the roof. Before you climb, set your extension ladder on firm, level ground at about a 1-to-4 angle, extend it at least 3 feet above the eave, and tie it off. Use a roofing harness with a properly installed anchor, wear soft-soled shoes, and stay off wet, frosty or very hot roofs. Anything two stories or higher, or steeper than about 6-in-12, is a job for a roofing contractor with fall protection equipment.

For a single-story open metal valley on a reroof, the general sequence looks like this:

1. Strip and inspect. Remove old shingles and underlayment in the valley and check the decking for soft spots. Replace damaged sheathing.
2. Lay the membrane. Center a 36-inch strip of self-adhered membrane in the valley, pressing it tight into the crease without bridging.
3. Set the metal. Start at the eave, trim the bottom flush with the drip edge, and secure with cleats hooked over the hemmed edges, or nails placed at the very outer edge only.
4. Lap upward. Overlap each upper length at least 6 inches over the lower one.
5. Snap chalk lines. Mark the open channel on each side, widening toward the eave.
6. Shingle the slopes. Run shingles to the chalk lines, clip the top corners, and bed edges in roofing cement. Keep all nails at least 6 inches from center.

Cost Ranges and When to Call a Roofer

Material costs are modest. A 10-foot length of prefinished W-valley typically costs from about $20 to $60 depending on metal and width, and rolls of aluminum valley flashing run somewhat less per foot. Ice and water membrane adds a moderate amount per valley.

Labor drives the price. Replacing a single valley on an existing roof, including tearing off shingles on both sides and reshingling, commonly lands in the range of a few hundred to around $1,500, depending on length, pitch and access. During a full reroof, adding upgraded metal valleys usually costs only a modest premium over closed valleys and is often worth it under trees.

Call a licensed roofer for any two-story roof, steep pitch, tile or slate roof, recurring leaks, or rotted decking. Roofers can also tell you whether your local building department and shingle warranty require specific valley details, which affects both inspection and future warranty claims.