Water is prevented from entering the wall interior of a building from in and around a wall component, such as a window, mounted in the building wall, by positioning a diverter directly beneath the wall component. The diverter includes an upper surface that slopes toward the wall exterior, a plurality of spaced, parallel ribs extending upwardly from the upper surface and beyond the exterior wall to form a plurality of downwardly sloping channels, a cover over the outer ends of the ribs to form drainage openings at the ends of the channels. A downwardly extending non-linear pathway from the drainage openings to the exterior of the diverter prevents water from entering the wall interior through the channels.