How do you build a valley on a metal roof?
Short answer. A metal roof valley is built with a wide preformed valley pan, often with a center rib, laid over self-adhering membrane and held with cleats or fasteners only at its outer edges, then the roof panels are cut parallel to the valley, hemmed or fitted with closures, and fastened so they never pierce the pan near its center. The panels stop short of the centerline so water and debris can run freely down the open pan.
The layers, bottom to top
- Deck and membrane. Self-adhering ice and water membrane, often a high-temperature version made for metal roofs, runs down the valley, centered.
- Valley pan. A wide W or V-shaped pan, commonly with a center rib that keeps water from one slope from shooting under the panels on the other. Pieces lap upper over lower with sealant in the lap.
- Edge fastening. Cleats or fasteners only near the outer edges of the pan, so it can expand and contract and no holes sit in the water path.
- Panels. Cut at the valley angle, with ends hemmed under or set on a closure strip with sealant, and fastened only through the pan's outer zone.
Exposed-fastener vs standing seam
- Exposed-fastener panels: foam closures shaped to the panel profile, set in sealant, close off the open ribs where they meet the valley.
- Standing seam: panel ends are hemmed and hooked onto a cleat or Z closure fastened near the pan edge, leaving the pan exposed and free to move.
Common mistakes
- Narrow valley metal that lets heavy downpours overrun the panel edges.
- Screws through the pan near its center.
- Panels cut too close to the center, trapping leaves and pine needles.
- Missing closures, so wind-driven rain and insects get under the panels.
- Copper accessories on a steel roof, which corrode the steel.
Manufacturers publish valley details for each panel system, and following them is usually tied to the warranty. See metal roofing in Georgia and ice and water shield in Georgia.