Slate is a traditional stone product that has long been used for roofing widely around the world. The presence of iron sulphides is one of the most significant factors affecting roofing slate resistance to weathering and, therefore, the quality of slates and their use in construction. Iron sulphide oxidation is the main process of roofing slate degradation. Qualitative and quantitative analyses to determine iron sulphides in roofing slates are usually conducted using transmitted and reflected light microscopy, and X-ray diffraction techniques. However, this paper presents X-ray computed micro-tomography as a new laboratory method in this field, which also allows for 3D visualization and analysis of iron sulphide distribution in roofing slates. It also discusses some technical limits of this technique and tomographic scan interpretation pitfalls. The results obtained by tomographic observations were subsequently verified by optical microscopy, X-ray powder diffraction and scanning electron microscopy, as well as energy-dispersive X-ray spectroscopy analyses. Spanish slate from the Ordovician Luarca Formation and Culm slate from the Moravice Formation in the Czech Republic are both characterized by low to extremely low iron sulphide content, and for this reason were selected for this study.