What is valley flashing?

Valley flashing is the metal or protective membrane installed in the V-shaped channel where two roof slopes intersect. As water flows down a pitched roof, it concentrates naturally in these valleys, making them high-volume runoff zones that demand reliable protection. Without proper flashing, water pools and seeps beneath shingles or roofing material, causing rot in the underlying deck and framing.

Valleys typically use either open flashing (where metal is visible in the valley channel) or closed flashing (where shingles or other roofing material covers the metal). Metal valley flashing is commonly made from aluminum, galvanized steel, or copper, each chosen for durability in Ontario's freeze-thaw cycles. Some installations use rubber or synthetic membrane systems instead, particularly on flat or low-slope applications.

Proper valley flashing must slope at least 4 percent grade to ensure continuous drainage and extend high enough up both roof slopes to capture and direct water safely to the eaves. Installation gaps, rust perforation, or separation from the deck create pathways for water intrusion. Debris buildup, ice damming, and improper overlap of shingles over valleys are common failure points. Roof repair contractors in Ontario often encounter valley leaks as a primary source of interior water damage, making accurate installation and regular inspection essential to roof longevity.