I'm putting in open metal valleys on a shingle roof.

Metal Selection & Prep

• Material: Minimum 26-gauge galvanized steel (G-90), aluminum (.032"), or copper (16 oz). Painted steel is standard. IRC R905.2.8.2 requires valley lining.
• Width: 18" minimum (W-valley) or 24" minimum (V-valley). Wider is better for cut valleys.
• Profile: W-valley (center rib) is strongly preferred over V-valley. The rib prevents cross-wash (water shooting up the opposite slope).
• Length: Max 10-ft sections. Overlap 12" minimum downhill. Seal overlaps with butyl tape or polyurethane sealant.
• Underlayment: Run ice & water shield (self-adhered) full width of valley before metal. Center it. IRC R905.2.8.2 / R905.1.2.

Installation Sequence

1. Drip Edge: Install eave drip edge first. Valley metal laps over it.
2. Valley Metal: Center metal on valley centerline. Fasten only at outer edges (1" in from edge) with 1" neoprene-washer roofing nails or ring-shank nails on 12" centers. Do not nail near center rib. Let it float for expansion.
3. Shingle Layout: Snap chalk lines 3" from center rib each side (6" total exposed metal). This is your cut line.
4. Shingling: Run shingles from both slopes toward valley. Do not nail within 6" of chalk line. Press shingles flat into valley pan.
5. Cutting (Open Cut Valley): Cut top-layer shingles after installation using a hook blade. Cut on the chalk line (3" from rib). Cut corners at 45° (dog-ear) to direct water.
6. Sealing: Dab asphalt plastic cement (ASTM D4586) under each cut shingle corner (1" diameter spot) where it overlaps metal. Do not run a continuous bead—traps water.

Critical Details

• No nails in shingles within 6" of valley center.
• No caulk/sealant on top of metal where shingles lie (creates dams).
• Clean metal of shingle granules/debris daily.
• Ventilation: Ensure ridge vent/baffles don't dump water into valley.

Code Check: IRC R905.2.8.2 (Valley flashing), R905.2.8.3 (Valley shingling), R905.1.2 (Ice barrier). Manufacturer instructions override for specific shingle warranties.

For open metal valleys on a shingle roof, follow these steps per IRC R905.2.8 and manufacturer guidelines (e.g., GAF, Owens Corning):

1. Valley Metal Selection: Use corrosion-resistant metal — minimum 26-gauge galvanized steel, 0.019" aluminum, or 16-oz copper. Width: at least 24" (wider for steep pitches or high snow loads). Lap sections 6" minimum, sealed with roofing cement.

1. Underlayment: Install self-adhering ice and water shield along the entire valley, extending at least 12" on each side of the centerline. Overlap seams 6". This is critical — metal valleys leak if underlayment fails.

1. Metal Installation: Lay the valley metal over the underlayment, centered. Secure with nails only at the top edge (to allow thermal movement). Do not nail through the flat of the valley — this causes leaks. Seal overlapping joints with asphalt roof cement.

1. Shingle Application: - Cut shingles to stop 6" from the valley centerline (3" each side for a 6" exposure). - Do not nail within 6" of the centerline. - Seal the cut edge with a 1/2" bead of roofing cement along the valley flange — this creates a water barrier and prevents wind uplift. - Weave or lace method is NOT used in open valleys — shingles are cut clean and sealed.

1. Key Detail: The shingles must not touch or cross the valley centerline. Water flows freely down the metal; shingles act as a splash guard. Improper nailing or sealing here is the #1 cause of valley leaks.

⚠️ If roof pitch < 3:12, or if using certain shingle types (e.g., some laminated), consult manufacturer — some require modified details. When in doubt, hire a licensed roofer. Metal valley work is unforgiving of errors.