ABSTRACT The shear properties of unreinforced masonry are key parameters used in order to understand the behaviour of these structures under horizontal actions. These characteristics are usually determined using Destructive Testing (DT) methods. However, this typology of tests is not always suitable due to both their high impact and cost. The present work aims to design, analyze and apply an innovative Minor Destructive Testing (MDT) technique for the determination of the shear mechanical properties. To limit the impact on the structure, the Flat Jack (FJ) technique was adopted. The geometrical configuration, chosen among six different layouts, was studied in order to comprehend the role-played by different parameters and to have then a mathematical model representative of the stresses inside of the sample. Finally, four in situ tests were executed and are analyzed and discussed.