Recent advances in the design of multi-storey timber buildings have led to viable structural systems that allow open floor plans with large spans between frames and/or walls. Timber-concrete composite (TCC) flooring can achieve the spans required but have the potential to be flexible under diaphragm actions, which can significantly alter the seismic response of a building. In-plane experimental tests on a one-third scale TCC floor were performed using quasi-static earthquake loading simulation. The experimental results indicate that the deformation between the floor and lateral load resisting systems (LLRS) is much greater than the in plane deformation of the floor diaphragm itself for the square aspect ratio considered. Hence, a floor system with similar aspect ratio can be modelled as a single-degree-of-freedom for future structural analyses. Different diaphragm connections were considered between the floor unit and lateral restraints, which simulate the lateral load resisting system. The connection was either timber-to-timber or concrete-to-timber, incorporated screws or nails acting as dowels or inclined at 45 degrees. Each connection type performed differently in terms of stiffness, strength, ductility capacity and induced damage. Screws that were orientated at 45 degrees to the connection interface were significantly stiffer than fasteners aligned orthogonal to the interface. There was little difference in the initial stiffness for the concrete-to-timber connection compared to the timber-to-timber connection. The testing indicated that a timber-to-timber interface is more desirable because of construction ease and reparability.