Large quantities of iron reinforcements, found in most Gothic monuments, are a data source for the interpretation of medieval architecture however their role both in contemporary engineering theory and the technical reality of construction yards has not yet been specified due to the difficulty of directly dating them. We present here an original radiocarbon dating methodology to date metal itself. Radiocarbon dates were measured for iron reinforcements used in specific parts of Bourges and Beauvais cathedrals, two iconic buildings in the development of French gothic architecture. Coupled with archaeometric and archaeological data, the new chronological results illuminate the major and active roles played by iron in the strategy of the building yards. At Bourges, iron was assimilated into the cathedral's construction strategy, whereas at Beauvais iron was integrated from the initial design, added to the monument following the vicissitudes of the building yard, and still used during the modern period. Thus, through decisive advances in radiocarbon dating of iron artefacts, the evolution of medieval architectural and engineering thought and action has been more reliably reconstructed.