This paper describes a modular assembly system of GFRP sandwich structures used for beam and slab applications in building construction. Sandwich assembly consists of incorporating pultruded GFRP box or I-profiles between two GFRP flat panels to form built-up modular sandwich sections. These modular sections can then be assembled in the transverse direction to form one-way or two-way spanning slab systems. Modular one-way spanning sandwich specimens were prepared via either adhesively bonding or mechanically bolting the component profiles, and were then tested under static loads. Various design parameters, such as span-to-depth ratio, connection type, and the inclusion of foam into the structure were evaluated. It was found that the type of failure mode was greatly influenced by the span-to-depth ratio, and that the insertion of foam into the structure significantly improved load carrying capacity. Moreover, adhesive bonding provided full composite action at both serviceability and ultimate loads, whereas bolted connections provided full composite action at serviceability loads but only partial composite action at ultimate loads. Finally, the incorporation of the sandwich assembly in a composite beam system is presented for building floor applications.