Roof valleys: open, closed and woven, and the valley lining the code calls for
A valley is the inside corner where two roof slopes meet and their water runs together. Shingle roofs do valleys three ways: open, with the metal lining showing; closed-cut, with one side's shingles cut along the valley; or woven, with both sides' shingles laced across it. The code calls for a lining under every valley, such as metal at least 24 inches wide in an open valley (IRC R905.2.8.2).
Anthony Hicks30+ years in construction. Updated November 5, 2026.
Reading as
What a valley is
A valley is the sloped inside corner where two roof planes meet: where a wing, a dormer or a cross gable joins the main roof. Both slopes drain into it, so a valley carries the water of both. The guide to the parts of a roof shows one from above.
Three ways to shingle a valley
| Valley | How it’s done | What shows |
|---|---|---|
| Open | Metal (or roll roofing) lining down the valley, shingles trimmed back from its center on both sides | A strip of metal |
| Closed-cut | One slope’s shingles run across the valley; the other slope’s are cut along a line just off its center | A clean cut line |
| Woven | Shingles from both slopes laced over each other across the valley | Interwoven shingles |
The shingle maker’s instructions say which valleys a shingle allows.
The lining under every valley
The code calls for a valley lining, put down by the maker’s instructions before the shingles go on (IRC R905.2.8.2):
- Open valleys, metal: at least 24 inches wide, in a corrosion-resistant metal from the code’s table, such as aluminum at 0.024 inch or galvanized steel at 26 gauge.
- Open valleys, roll roofing: two plies of mineral-surfaced roll roofing, the bottom 18 inches wide and the top at least 36 inches.
- Closed valleys: one ply of smooth roll roofing at least 36 inches wide, or either of the open-valley linings.
- Any valley: self-adhering polymer-modified bitumen underlayment (ASTM D1970) in place of the lining.
| Metal | Minimum |
|---|---|
| Aluminum | 0.024 inch |
| Galvanized steel | 0.0179 inch (26 gauge, G90) |
| Copper | 16 ounces a square foot (0.0216 inch) |
| High-yield copper | 12 ounces a square foot (0.0162 inch) |
| Lead | 2 1/2 pounds a square foot |
Measuring valleys
Valleys are measured by the foot along the slope. On an estimate they carry the valley lining or metal, and add waste to the shingles, since every shingle along a valley is cut. The guide to measuring a roof covers the edges, and the roofing calculator works out the roof’s area.
Flashing at walls, chimneys and pipes is in the roof flashing guide, and the ice barrier at the eaves in the ice and water shield guide.
Common questions
What is a metal valley on a roof?
An open valley lined with metal, at least 24 inches wide under the code, with the shingles trimmed back so the metal shows down the center (IRC R905.2.8.2).
Which is better, an open or a closed valley?
Both meet the code with the right lining. The choice comes down to the shingle maker’s instructions, the look, and the roof.
Do valleys need ice and water shield?
Self-adhering polymer-modified bitumen underlayment is allowed in place of the valley lining (IRC R905.2.8.2), and where ice dams form the code calls for an ice barrier at the eaves (R905.1.2).