A moulding for sealing a horizontal structure with a finishing layer of a vertical structure at a corner joint. In cross-section, the moulding has a rectangular base portion for insertion into a gap between the horizontal structure and the vertical finishing layer, and a cap portion with an upper lip for sealing against the finishing layer and a lower lip jutting downwardly for sealing against the horizontal structure. Double sided foam tape is applied only to an underside of the rectangular portion of the body. A bead of caulking at an inner face of the upper lip below an upper tip forms a flexible secondary seal of the moulding to the finishing layer of the vertical structure. While providing a secondary seal for maximum water-tightness, the caulking is protected from water exposure by the primary seal provided by the upper lip of the moulding to prevent mold build up.