Abstract: A window backdam assembly includes an elongate backdam placed flat on top of a window sill near an interior end. A portion of the window sill adjacent to the exterior end is temporarily uncovered. A one-piece waterproof adhesive material is placed over the entire backdam and uncovered portion of the window sill. The elastic adhesive material is generously sized to cover the backdam, the uncovered window sill, and lower portions of the jambs of the window frame and structural panel. Thin flashing material may be applied horizontally to the structural panel beneath the sill, vertically along the two jambs of the panel, and horizontally on the top portion of panel, thereby forming a border around the panel opening.