How an Open Valley Works
A valley is the V-shaped channel formed wherever two roof slopes come together. It carries a large volume of water, snowmelt, and debris, so it is one of the most failure-prone areas on any roof.
In an open valley, a continuous strip of metal flashing (often pre-finished steel or aluminum, frequently with a raised center rib) is installed down the center of the channel. The shingles on each side are trimmed back and stop short of the centerline, leaving the metal exposed. This contrasts with a closed valley, where shingles run across or weave through the valley and hide it completely.
Why It Matters in the Inland Northwest
Spokane and North Idaho roofs face heavy snow loads, repeated freeze-thaw cycles, and stubborn ice dams. Valleys concentrate all of that runoff into a narrow path, and a slow-draining valley is exactly where ice dams form and water backs up under the roofing.
- Faster drainage: The smooth metal surface sheds snowmelt and rain quicker than a shingled channel, reducing standing water and ice buildup.
- Sheds debris: Pine needles and grit slide off metal instead of collecting and damming the flow.
- Better freeze-thaw durability: Granule-covered shingles in a closed valley wear from constant water flow and ice movement; a metal channel holds up far better over our winters.
Open vs. Closed Valley
Both styles can perform well when installed correctly, but they behave differently in snow country.
- Open valley: Exposed metal, fastest drainage, longest service life in heavy-snow climates, and a clean defined look. Typically costs a bit more in materials and labor.
- Closed valley: Shingles cover the channel for a seamless appearance, but the flow path is rougher and the shingles take more wear where water and ice concentrate.
For our climate, a properly flashed open valley with ice-and-water shield underneath is a smart, durable choice. If you are planning a roof replacement, ask which valley type your contractor recommends and why.
Common Open Valley Problems
Even a good design fails if the details are wrong. Watch for these issues in older or poorly installed roofs:
- Exposed fasteners through the metal, which create leak points right in the wettest part of the roof.
- Shingles cut too close to the centerline, allowing wind-driven water and ice to creep under them.
- Missing ice-and-water shield beneath the metal, leaving no backup if water gets past the flashing.
- Corroded or dented metal from age, debris, or improper foot traffic during other work.