Valley Flashing: Why Roof Valleys Often Leak First

What a valley is

A valley is the V-shaped line where two roof slopes meet. Every drop of rain that falls on those two slopes eventually funnels into the valley and runs downhill together. That is a lot of water moving fast, and it is why valley details matter.

The two main valley styles

Closed woven valleys weave the shingles over the valley from both sides. They look clean but rely entirely on the shingle overlap to keep water out. When those shingles age, the valley leaks first.

Open valleys use a sheet of metal (W-profile is common) running down the center. Shingles are cut cleanly parallel to the valley line and rely on the metal underneath, not just the shingle overlap. Open valleys are more durable and hold up better to concentrated water flow.

Why valleys fail

Old closed valleys wear at the choke point where runoff is most concentrated. Closed valleys also collect debris that hold moisture against the shingles for longer. Without an ice-and-water shield underneath, even a small wear point becomes a fast leak.

Another common failure mode: nails driven through the valley line. A nail in the middle of a valley is a direct path for water into the deck.

When to rebuild a valley

If you see wear in the valley, granular loss concentrated in the V, or active leaks traced to a valley line, it is time to convert to an open W-metal valley with a full-length ice-and-water shield underneath. It outlasts the rest of the shingles.