What is Z-flashing?

Z-flashing gets its name from its profile: a metal piece bent into a Z shape that sits above horizontal trim, window headers, and door trim during siding installation. The upper leg of the Z extends back under the new siding, while the lower leg projects out and down, creating a path that forces water to run off and away from the wall surface behind it.

In Lake County's humid subtropical climate, where heavy rain and afternoon thunderstorms are common, proper water management is critical. When siding is installed without Z-flashing or with it improperly placed, water can get trapped behind trim boards or collect where siding meets window frames. Over time, this moisture penetrates the wall assembly, causing rot in the underlying wood framing, damage to house wrap or felt, and mold growth inside the wall cavity.

Z-flashing is typically galvanized steel or aluminum, chosen for durability and corrosion resistance. It's installed during the framing stage before new siding goes up, ensuring water sheds cleanly outward rather than being drawn inward by capillary action. Proper Z-flashing installation is a mark of quality siding work and is essential to protect the structural integrity and longevity of the exterior wall system.