An innovative experimental procedure is described that is used as a preliminary design stage for the pre-qualification of strengthening mortars applied to historical masonry buildings. In the analysis of the behavior of masonry structures and their constituent materials, increasing importance has been assumed by the study of the long-term evolution of deformation and mechanical characteristics, which may be affected by both loading and environmental conditions. Through static and fatigue tests conducted in the laboratory on different dimensional reinforced masonry specimens, it has been possible to investigate the durability of strengthening materials. This methodology is useful to select, from a range of alternatives, the most suitable strengthening mortar for historical masonry, and to avoid the errors associated with materials that are not mechanically compatible. Special attention was devoted to fatigue and thermo-hygrometric aspects, the effects of which on masonry are often significant enough to hinder the validity of repair works. The experimental procedure has been effectively used in the biggest restoration building site in Europe, the Royal Palace of Venaria.