Cruciform welded joints with a non-weld at the weld root are investigated. It is shown that, depending on the width and shape of the non-weld tip, the fatigue crack can be initiated from the weld toe or the weld root, or simultaneously from several concentrators. The condition of equally probable initiation of fracture from the toe and the root is formulated taking into account their geometrical dimensions. For crack-shaped non-welds, an equation for calculating stress intensity factors, which takes into account the geometry of the joint, the parameters of the non-weld and the crack growth trajectory, is obtained on the basis of finite element studies. The obtained equation is compared with the known dependences.