A roof “valley” is the inner intersection where two sloping roof planes meet. Because it channels water from two different surfaces, it handles 10x the water volume and velocity of the rest of the roof. Standard contractors use a Closed-Cut Valley, overlapping asphalt shingles directly in the crease to save money. This causes premature granule erosion and cracking. Premium contractors mandate an Open Metal W-Valley, placing a custom-bent piece of galvanized steel or copper into the crease to handle extreme hydrodynamic flow and prevent ice damming.
If you ask a forensic roofing inspector where asphalt roofs leak first, the answer is almost always the same: penetrations and valleys. The valley is the gutter of your roof deck.
When you sign a generic contract for an “Architectural Shingle Roof,” the contractor retains the right to construct the valleys however they see fit. Unsurprisingly, they will usually choose the method that requires zero extra materials and the least amount of custom labor.
The Hydrodynamics of a Roof Valley
During a heavy storm, water falling on an open field of shingles sheds at a normal pace. However, water hitting the planes adjacent to a valley is immediately funneled inward. By the time that water reaches the bottom of a 15-foot valley, it has compounded in volume and velocity, creating a literal rushing river.
Asphalt shingles are designed to handle rain falling onto them. They are not engineered to handle high-velocity rivers running across them. The constant friction strips the ceramic granules off the asphalt, exposing the bare fiberglass mat to UV degradation. Over 10 to 12 years, the shingles in the center of the valley simply erode, dry out, and crack.
The 3 Valley Installation Methods
When obtaining estimates, you must explicitly ask the contractor which of the following three methods they plan to use. This determines the true quality of the bid.
| Method | Installation Process | Engineering Verdict |
|---|---|---|
| Woven Valley | Shingles from both sides are woven together across the center line, creating a zigzag pattern. | Obsolete. Modern architectural shingles are too thick and stiff to weave properly. Forcing them into a weave creates hollow air pockets under the crease, practically guaranteeing someone will step on it and crack it. |
| Closed-Cut Valley | Shingles from one side run fully across the valley. Shingles from the adjacent side are run over the top and cut in a straight line down the center. | Standard (Budget). Visually clean, but forces water to run directly over the raw, cut edges of the asphalt. Prone to premature granule loss and algae buildup in the crease. |
| Open Metal Valley | A 24-inch wide strip of pre-painted steel or copper is anchored in the valley. Shingles are cut 2 inches back from the center line, exposing the metal. | Premium (Bulletproof). Water hits the metal and instantly sheds without friction or erosion. Highly resistant to snow and ice dams. Lasts the entire life of the roof. |
Valley System Upgrade Estimator
Calculate the financial upgrade required to move from a budget closed-cut system to a lifetime open metal valley.
If you upgrade to an open metal valley, you must ensure the contractor orders a W-Valley profile, not a simple V-Valley.
A W-Valley features a sharp, 1-inch raised metal crimp running directly down the center line. This is a critical hydrodynamic baffle. Without it, high-velocity water racing down the steep side of a roof can wash straight across the smooth metal valley and physically shoot up underneath the shingles on the opposing side (known as cross-wash). The center crimp physically blocks the cross-wash, breaking the water’s velocity and directing it safely downward to the gutter.
Price Your Valley Upgrades Accurately
Do not leave the most vulnerable part of your roof up to chance. Use our 40-Factor Estimator to explicitly specify Open Metal Valleys, cross-referencing your linear foot requirements with local sheet metal costs.
Launch the 40-Factor Estimator →Frequently Asked Questions
Can I put Ice and Water Shield under a closed-cut valley?
You absolutely must. Building codes dictate that a 36-inch wide strip of self-adhering Ice and Water membrane must be laid down in the center of the valley before any shingles or metal are installed. If a contractor tries to use standard felt paper in a valley, they are violating code.
Do open metal valleys rust?
Not if the correct materials are used. Premium open valleys are made of heavily galvanized steel with a Kynar 500 baked-on enamel finish, aluminum, or pure copper. If a contractor uses cheap raw steel, it will eventually rust, but industry-standard pre-painted metals will easily outlast the asphalt shingles beside them.
Engineering & Testing References
- National Roofing Contractors Association (NRCA): Steep-Slope Roofing Manual – Detailed schematics on W-valley center baffle requirements to prevent cross-wash hydrodynamics.
- Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA): Architectural Sheet Metal Manual guidelines for valley metal gauge thickness and hemmed edges.
- Asphalt Roofing Manufacturers Association (ARMA): Technical bulletins deprecating woven valleys for modern laminated architectural shingles.