Structured methodologies and tools for the tailored design of factories are more and more adopted by suppliers of manufacturing systems but usually discontinued after the design phase. The use of an ontology-based virtual factory, continuously synchronized with the real plant, is proposed to guarantee digital continuity and enable in situ simulation during the operating phase of a factory. This digital counterpart of the system can be used for integrated shop-floor simulations to assess future impact of production and maintenance planning decisions. An industrial application is provided in the context of roll shops, i.e., systems devoted to the grinding of cylinders for rolling mills. © 2015 CIRP.