Valley flashing: what it does and how it fails
Valley flashing carries more water than any other joint on the roof, so it fails early.
Valley flashing is the strip of metal laid in the crease where two roof slopes meet. It catches the runoff from both sides and sends it down to the gutter. That one channel can carry the water of half the roof, which is why it wears faster than almost anything else up there.
What valley flashing does
Water from two slopes lands in the same narrow line. The flashing is the waterproof floor of that line. Shingles overlap its edges, but the metal does the real work. If it cracks or lifts, water goes under the shingles and onto the deck.
Open, closed and woven
In an open valley the metal stays visible down the middle. In a closed valley shingles cover it. A woven valley interlaces the shingles from both sides and has no visible metal. Each style hides or shows different evidence from the ground. We cover the trade-offs in open vs. closed valley.
Signs it is failing
- Rust, pinholes or a dull, flaking surface on an open valley.
- Shingle edges along the valley curling, cracking or missing granules.
- Leaves, pine needles or shingle grit packed in the crease.
- A ceiling stain that appears in heavy rain, usually below the line of the valley.
Water that enters at a valley can travel along a rafter before it drops. The stain is rarely under the entry point.
Why debris matters
A valley full of leaves holds water against the metal and pushes it sideways under the shingles. A clean valley sheds it. Keep overhanging branches trimmed from the ground, and take it as a warning sign if the valley looks packed.
Why we stop at looking
We do not publish steps for clearing a valley, resealing it or replacing the metal. Valleys sit in the middle of the roof, on a slope, often wet with debris. That is the worst place to stand. Look with binoculars from the yard, check the attic below the valley with a flashlight, and give a roofer your photos.
For the leak itself, read roof valley leaks. For the general parts of the system, see what roof flashing is.
Frequently asked questions
What is valley flashing?
It is the metal channel laid where two roof slopes meet. It catches water from both slopes and carries it to the gutter.
Why do roof valleys leak first?
They carry the runoff of two slopes in one narrow line, and they collect leaves and grit. That load wears the metal and the shingle edges faster than the open roof.
Can I clear a valley myself?
We do not give steps for it. A valley is on the slope, often wet, and clearing it means standing on the roof. A roofer should do it.
How do I check a valley from the ground?
Use binoculars. Look for rust, curled shingle edges and packed debris. Inside, check the ceiling and the attic framing below the valley.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)