The Ostia Marina Project has been on-going for over ten years in Ostia antica (Rome), in the suburban district outside Porta Marina. As a result of the recent discovery of new buildings and new indepth analyses of previously excavated buildings, there is now a scientific study into the many types of Roman pavements. In this paper, some examples of these floors are presented, with a detailed study of the preparation layers. Ancient floors are often studied only as bi-dimensional surfaces, but they are fully comprehensible (and datable) only with an analysis of the third dimension, that is the preparation layers and the substrate.