Short Answer
Window flashing helps direct water away from the window opening and into the wall's drainage plane.
Detailed Explanation
Window flashing is a system of materials installed around a window opening to manage water. It typically includes: sill flashing (at the bottom of the opening, the most critical piece), side flashing (jamb flashing on the sides), and head flashing (at the top). The flashing is integrated with the weather-resistive barrier (house wrap) so that water that gets behind the siding is directed around the window opening and out to the exterior, rather than into the wall. Proper flashing is installed in the correct order — sill first, then sides, then head, with each layer lapped over the one below — so water sheds downward. Improperly installed window flashing is one of the most common causes of water intrusion in homes.
Why It Matters
Window flashing is the critical water-management system at window openings. Its proper installation prevents water intrusion that can cause rot, mold, and structural damage.