This research introduces a semi-fabricated building system for the construction of floor slab. The floor slab system is a ferrocement - AAC block composite slab developed to address some of the shortcomings in existing floor slab systems. It consists of two layers; the first layer is a pre-cast inverted ribbed ferrocement layer, which acts initially as a formwork, while the second layer consists of Autoclaved Aerated Concrete (AAC) blocks and concrete ribs. Interlocking mechanism was introduced to connect the two layers to form a fully composite slab panel. The study covers the development of the composite slab and discusses its structural response under flexural load. Full-scale specimens were cast and tested under two line loads. The initial results in terms of load-deflection, crack pattern, strain distribution and failure loads, indicate that the response of the composite slab to the flexural loading is encouraging and the system can be used as a floor slab in construction sector after more comprehensive experimental and analytical investigations are curried out. © 2008 Taylor & Francis Group.