A combination for plumbing fixtures, particularly tiled floors with drains, is disclosed. An exemplary combination includes a substantially planar shower tray bonding disk on a flooring substrate and surrounded by a prefabricated polymer foam shower tray on the flooring substrate and at the circumference of the bonding disk. The disk includes an interior rim that defines a drain opening and a plurality of slotted openings for permitting mortar to penetrate the disk. The drain opening in the disk can thus be positioned at a drain cut out in the flooring substrate.