Materials / Valley Flashing

Valley flashing lines the trough where two asphalt-shingle roof planes meet, directing concentrated runoff away from the seam. IRC R905.2.8.2 governs it and distinguishes open valleys, where the lining stays exposed, from closed (cut or woven) valleys, where shingles are carried across the center. Open valleys require corrosion-resistant metal at least 24 inches wide or two-ply mineral-surfaced roll roofing complying with ASTM D3909 or ASTM D6380 Class M. Closed valleys permit one ply of smooth roll roofing at least 36 inches wide complying with ASTM D6380, the same metal or two-ply options, or self-adhering polymer-modified bitumen underlayment complying with ASTM D1970 in place of a separate lining.