How to Install an Open Roof Valley

Valleys are the primary water highways of your roof. When two roof planes meet, they create a channel that collects more water than any other part of the structure, making it the most likely spot for a leak if not properly sealed. A well-installed metal valley acts as a bridge, carrying debris and snowmelt away from the intersection before it can seep under your shingles. Getting this right requires focusing on the transitions between the deck and the metal. You want to avoid driving nails directly through the center of the valley flashing, as these become instant entry points for moisture. Instead, use cleats or clips to hold the metal down, ensuring that the shingles overlap the metal enough to create a secure, hidden drainage path.

  1. Lay Your Waterproof Defense. Apply a self-adhering membrane vertically up both sides of the valley, extending at least 24 inches on each slope. Ensure the membrane is smooth and pressed firmly against the roof deck to create a watertight base.
  2. Position Metal Precisely. Center your pre-bent metal valley flashing over the intersection. If your roof run is long, overlap the upper piece over the lower piece by at least 12 inches to ensure water flows over the seam.
  3. Clip, Don't Nail. Instead of nailing through the flashing, use metal roof cleats or hold-down clips spaced every 12 inches along the edges. Fasten the clips into the roof deck, leaving the flashing free to expand and contract with temperature changes.
  4. Create a Gasket Seal. Apply a continuous bead of high-grade roofing sealant along the outer edges of the metal flashing where it meets the roof deck. Press the flashing down into the sealant to create a gasket-like seal.
  5. Cut Shingles Straight. Lay your shingles over the valley, ensuring they overhang the metal by at least 4 inches. Use a sharp utility knife to cut the shingles in a straight line parallel to the valley center, leaving the metal exposed by about 2-3 inches.
  6. Lock Edges, Not Flow. Apply a thin bead of roofing cement under the cut edge of each shingle to keep it from lifting during high winds. Avoid excessive cement, which can dam up water flow and lead to backups.