extruded hollow thermoplastic structural components of rectilinear cross section formed for interlocking assembly with mating components for use in erecting a modular building characterized in that said components are extruded from pvc material which includes a smoke retarding agent which is covered by a skin on surfaces exposed to the environment . preferably the component is an elongated extruded hollow thermoplastic structural component of rectilinear cross section formed for interlocking assembly with mating components for use in erecting a modular building , characterized in that said component is formed as a co - extrusion of a substrate of pvc containing from about 10 % et 35 % by weight of the substrate of a reinforcing and expansion controlling agent and from about 5 % to about 35 % per weight of the substrate of smoke retarding agent , the total of said agents being less than about 45 % by weight of the substrate , and a thermoplastic skin covering surfaces of said component which remain exposed when said component is interlocked with mating components in a building structure , said substrate being substantially thicker than said skin , said skin forming a protective barrier to isolate said smoke retarding agent from exposure to the environment exterior of said component when same is incorporated into a building structure .