The structure of the concrete floor presented in this paper consists of a thin funicular vault, quadrilateral in plan, supported on its four corners, and stiffened by a system of “ribs” walls on its extrados. The structural prototype is completed with tension ties, which link the supports and absorb the horizontal thrusts of the funicular shell. It is a prototype for the NEST-HiLo project to be realized late 2015 in Dübendorf, Switzerland. The solution is inspired by built examples in tile vaulting in which thin vaults are stiffened by diaphragms, also called spandrel walls. In the presented structure, this structural system is implemented and constructed in high-performance concrete to achieve an extreme thinness (2 cm in the case of this prototype for both vault and ribs) and to resist asymmetrical loading. This floor would thus save more than 70% of weight compared to traditional, 25-30 cm thick concrete floor slabs used in the construction of framed buildings. This directly lowers the requirements for the foundations (often a dominant resource and cost factor), but also enables lightweight building extensions and a reduction of total floor height, offering possibilities to address, among other issues, the vertical densification of cities. A form-finding and analysis procedure for the design of such floor systems is presented, which consists of consecutive topology, shape and size optimizations.