A modular curb assembly of pre-cast pre-finished components forming a base for supporting sidewalls of a shower stall or other enclosure attached to a floor. The curb is made from a plurality of pre-cast pre-finished components of substantially uniform shaped cross section joined in end to end relationship to each other and attached to a floor in a predetermined contour around the perimeter of the enclosure. The components may be straight, curved or in the shape of angular corners and are cast in various configurations which provide a choice of different end angles so that the contour of the curb can be varied depending upon which different combination of component configurations are used in any particular curb design. The pre-cast pre-finished components of this invention are made from a cast polymer composite material similar to or like Solid Surface or Cultured Marble, including the group of granite, onyx, alabaster or the like, as well as cement based, or concrete gypsum cement, composite materials which form a ready to use waterproof outer surface requiring no additional coating or layer added thereto. The walls may be of glass block, plate glass or other materials.