Highly porous and soft calcarenites, often called ‘calcareous tuffs’, are commonly found throughout Southern Italy and are traditionally used as building materials. The petrophysical features of some of them were determined, focusing on the microstructure (fabric and porosimetric characteristics) and on the properties related to water absorption, drying and water vapour permeability; the compressive strength was also measured. The mineralogical composition was analysed and a variable presence of clay minerals was found for the different varieties. Diverse levels of damage under a salt crystallization test were related to the mineralogical, petrographic and porosimetric features, the latter being a source of the heterogeneity of the investigated stones. Results of laboratory tests and observations of decay in the field suggest protection against water penetration as the primary requirement for slowing down the decay of such highly porous and soft stones. Some water-repellent products were tested for effectiveness in reducing water absorption and for compatibility with the original stones’ properties. Their use in the presence of salts was taken into account, including the performances of the treated stones under the salt crystallization test. Treatment issues were found to be strongly influenced by the porosimetric characteristics of the studied materials.