A floor/ceiling 10 has downwardly extending ribs (14) separated by downwardly opening channels (16). The floor/ceiling member (10) sits down on a horizontal header (32) that is on top of a building wall that extends perpendicular to the ribs (14) and the channels (16). A preformed wall panel (24) has a lower strip portion (26) that is connected to a side flange (36) on the header (32). Preformed wall panel (24) has upwardly directed peaks (28) and upwardly opening valleys (30). The peaks (28) are sized and shaped to snuggly fit within the ceiling channels (16). The valleys (30) are sized and shaped to snuggly receive the ribs (14) when the peaks (28) are in the channels (16). A continuous lower strip portion of the preformed wall panel (24) is connected to the flange (36) of the header (32), below the ribs (14). The peaks (28) extend into and block or close off the channel openings formed in and by the channel (16) above the web (38).