Moisture is a major source of damage in historic solid masonry. Therefore, control of moisture movement in masonry is instrumental to the durability of masonry buildings. From research and practical experience it is known that many factors may play a role regarding permeability problems in masonry. This paper is focused on materials aspects regarding water penetration in historic fired clay masonry walls, constructed with moderate-to-high absorption bricks and lime mortars; the occurrence and influence of parameters such as brick porosity, interface leakage and mortar joint resistance are discussed. Subsequently, quantitative tests results show the effects of these parameters on leakage of solid walls of different thicknesses. The results of the investigations lead to a number of recommendations to be used in case of repair of historic solid masonry. Finally, attention is paid to the influence of workmanship on the permeability behaviour of historic solid walls.