A building construction assembly composed of factory-manufactured thermoplastic structural modules which when interconnected at a building site then create the entire shell of an edifice having side walls and a roof. Each module which is formed of a pair of parallel panels bridged by transverse webs is provided on either side along its length with a pair of flexible catch flanges projecting from the panels. When assembling these modules to create the shell of the edifice, adjacent modules are interconnected in coplanar relation by a double-faced joiner on each face of which is a pair of teeth. When one face of the joiner is pressed against a side of a module, its catch flanges then flex to snap into and behind the teeth to connect the joiner to this module. The other face of the joiner is similarly connected to the side of an adjacent module whereby the modules are then interconnected in coplanar relation. When it becomes necessary to interconnect four modules in a cross formation, use is then made of a square, quad-faced joiner on each face of which are a pair of teeth which are engageable by the catch flanges projecting from the side of the module to be joined to this face.