The German Optical Museum, located in the heart of Jena, was built in 1923/1924 and today stands as a valuable testimony to historic building culture. As part of the ongoing modernisation measures, a strengthening solution was required for the historic steel-joint ceilings / reinforced concrete ribbed slabs, as higher imposed loads are expected in future use. The ceilings consist of ribbed elements with hollow clay blocks – a construction method typical of the period. Within the scope of a comprehensive structural engineering assessment, it was decided to increase the load-bearing capacity by means of an innovative strengthening system using CRC. The CARBOrefit® method is being applied, consisting of a 10 to 15 mm thin layer of CRC and a single layer of carbon reinforcement. The measure takes particular account of the requirements for the preservation of historical monuments as well as the partly very low tensile strength of the existing substrate. Thanks to the thin overlay and the high performance of CRC, a resource-efficient, slender and durable solution can be realised – without altering the external appearance of the building. As the museum will not be in use during construction, the strengthening work can be carried out as scheduled in spring 2025. The project clearly demonstrates how CRC enables sustainable and conservation-compliant strengthening solutions, even in challenging heritage structures. For the first time in Germany, such a strengthening approach was implemented with the result that historical ceiling structures could be preserved which otherwise would have required demolition.