Bathtubs and shower trays, particularly those made of fiber reinforced thermoset plastic or acrylic laminate are susceptible to significant floor flexing making it necessary to provide some sort of support between the underside of the bathtub or shower tray floor and the building subfloor. Various approaches have been tried including a mortar bed, foamed in place expandable polyurethane foam and various types of filler blocks including blocks of polystyrene foam.