This article discusses the application of so-called ultrasonic quasi-Rayleigh waves to detect surface defects of mechanical constructions, namely plate structures. The application of quasi-Rayleigh waves allows the extension of the scope of detection using conventional ultrasonic methods that are based on bulk waves. This extension means larger distances as well as higher sensitivity of the detection of surface defects such as fatigue or corrosion cracks. An advantage of this method is the transfer of wave energy from one side of a plate to another, which helps to overcome one-sided obstacles (such as cross-pieces, reinforcement elements). The article describes characteristic properties of quasi-Rayleigh waves that are important for the proper (in terms of frequency in particular) design of the excitation of waves towards the structure. FEM simulation results then provide information regarding the sensitivity of the wave response to the presence and sizes of surface defects (perpendicular slots) in an isotropic material with the properties of steel. The theoretical knowledge is set against experimental measurements obtained with the use of a steel plate with cross-pieces welded to it.