What is Valley Flashing?
Valley flashing is a continuous strip of metal installed down the V-shaped junction where two sloped roof planes meet. Because a valley collects and concentrates water from both adjacent roof planes, it carries far more runoff per square foot than the rest of the roof. Valley flashing creates a waterproof channel under the shingles so this concentrated flow runs cleanly down to the gutter without finding a way under the roof deck. Valley flashing is required by the International Residential Code on all asphalt shingle roofs.
A residential roof valley where two sloped planes meet. The cyan strip running down the center is the valley flashing, creating a continuous waterproof channel for the concentrated runoff from both planes.
Where it goes and what it does
A valley is the inside corner where two sloped roof planes meet and form a V. On a typical Northern Virginia or Maryland house, you will see valleys at every place where a dormer joins the main roof, where an L-shaped house has its bend, and where two gable wings meet a hip. Every one of those valleys carries water from both adjacent planes, so the flow rate down the valley line is much higher than down a flat section of roof.
Valley flashing puts a continuous waterproof strip of metal underneath the valley line so that even if water finds its way past the shingles, it lands on the metal and continues to the gutter. Without proper valley flashing, the valley becomes the most common interior leak source on any roof, especially during ice damming and heavy summer storms.
Is valley flashing required by code?
Yes, in Virginia and Maryland. Both states adopt the International Residential Code, which under section R905.2.8.2 requires flashing of valleys on asphalt shingle roofs. The IRC also requires self-adhering polymer-modified bitumen membrane underneath the valley flashing in colder climates to provide a redundant water seal where ice dams form.
A new asphalt shingle roof without valley flashing is a code violation. A common shortcut on budget re-roofs is using a single 18-inch piece of roll roofing instead of proper W-profile metal flashing; both inspectors and any serious roofer will reject this.
Open valley vs closed valley vs woven valley
There are three different ways to install the shingles around the valley flashing. They look different and they last different lengths of time:
| Open Valley | Closed Valley | Woven Valley | |
|---|---|---|---|
| Appearance | Metal flashing visible down the center | Shingles cover the metal, valley appears as a continuous shingle surface | Shingles from both planes interweave across the valley |
| Water shedding | Best, water runs on smooth metal | Good, runs on shingle surface | Poor, water can pool at interweave joints |
| Lifespan | Longest, matches the metal | Matches the shingles | Shortest, often fails before the shingles |
| Repair | Easy, metal is exposed | Medium, must lift shingles | Hard, full valley rebuild |
| Cost | Highest material cost | Lowest labor + material | Highest labor |
Bottom line: open valley is the modern premium standard for any roof that will see heavy runoff. Closed valley is acceptable on shallower slopes and shorter valleys. Woven valley is largely obsolete and is increasingly disallowed by manufacturer installation specifications.
What DreamHome installs
DreamHome installs aluminum W-profile open valley flashing as the default on every roof replacement, sized at a minimum 24 inches wide and laid over self-adhering ice-and-water shield extending 36 inches up each side of the valley. Open valley is chosen because it sheds the concentrated valley runoff better than the closed alternative and is easier to inspect and repair. Where a homeowner specifically prefers the closed-valley aesthetic for cosmetic reasons, DH installs closed valley with extra underlayment redundancy.
Common problems with valley flashing
Almost every interior leak that comes through a roof in the first ten years originates at a valley. The usual causes:
- Valley flashing is missing entirely. Replaced by a strip of roll roofing or skipped on a budget re-roof. Common on cash flips and storm-chaser jobs.
- Valley is too narrow. Flashing under 18 inches wide allows water to overshoot the metal in heavy rain. Modern minimum is 24 inches.
- Shingles trimmed too close to the valley centerline. On open valleys, shingles should be trimmed 4 to 6 inches back from center. Trimmed too close, water can find an edge.
- Nailing through the valley flashing. The single most common installation mistake. Any nail or staple through the valley flashing leaves a hole that fails within five winters of freeze-thaw cycles.
- Missing ice-and-water shield underneath. Valley flashing alone is not enough at northern latitudes; the IRC requires a self-adhering membrane underneath as a redundant seal.
- Old valley flashing reused on a new roof. Aluminum work-hardens over 20 years of thermal cycling. Reusing it to save $150 is a corner-cut every time.