A prefabricated mantle unit has a base (36, 40) and a top plate (44) extending forwardly from the top of the base. A decorative strip (56) is oriented obliquely between the front of the top plate and the bottom of the base, with the lower edge (48) of the strip and the bottom (54) of the base forming a lower channel (50). The top element (16) of the surround (12) and the nailing flange (28) of the window box project above the top edge (24) of the window box, forming a box channel (26). The upward projection (22) of the top element is received within the lower channel (50) of the mantle unit, and a downward projecting portion (54) of the mantle base is received within the box channel (26).