Friction stir welding of T-joints is a promising technology for skin stiffener connections in the aerospace industry. The joints are produced from 4 mm thick 6013-T4 by plunging the tool through the skin and by stirring up the skin-stiffener interface. It is shown that the shape of the fillet between the skin and the stiffener is influenced by the backing plate. The introduction of a rounded fillet radius however increases the propensity of crack formation in the weld during FSW. These weld defects as well as their detection by the non-destructive ultrasonic testing method are discussed. Tensile tests are carried out by pulling the skin perpendicular to the stiffener and by pulling the stiffener from the skin.