Despite the ubiquity of uncertainty at every level of archaeological data collection, analysis and interpretation, most 3D visualisations continue to convey to their audiences, as well as demand from their creators, a single concrete view of the past. Where uncertainty is conveyed it is typically in the form of visual cues that differentiate the certain from the postulated or tentative. While these may act as a qualification for the viewer, it is often at the expense of the aesthetic impact, simplicity and wholeness of the image. In this paper I will demonstrate a methodology for the visualisation of archaeological sites and buildings that incorporates the inherent uncertainty of archaeological interpretation using the procedural modelling technology of ‘shape grammars’. Shape grammar modelling allows one to create transparent and adaptable parametric procedures that act as conceptual models of our interpretation of archaeological sites, using probabilistic functions to reflect interpretive uncertainty. These procedures can be used to automatically generate large numbers of diverse, stochastically determined architectural models that as a whole reflect the range of possible interpretations of a building. The strengths of this methodology will be explored using a case study from the site of Portus, Italy, a possible ship-shed dating from the 2^nd Century AD.