For many centuries volcanic tuffs from western Germany have been used in the construction of important historic buildings. These suffered severe weathering damage which calls for immediate preservation actions. The effectiveness of these actions depends on the accurate knowledge of the causes and state of the damage. By mapping the building according to a classification scheme of tuff weathering forms, it was possible to make an objective and reproducible description of the damaged condition. The results of mapping a medieval minster are presented. Characteristic values relevant to weathering are also presented for Rhenish tuffs and an Easter Island tuff. The behavior of tuffs according to laboratory tests is discussed. Pore space analysis constitutes an important method for recording the state of weathering.