Abstract: Apparatus is provided for cleaning horizontal surfaces, such as the grated flooring of paint workshops, by hydrodynamic action. The apparatus includes a carriage which can be moved back and forth over the surface to be cleaned, and a winding drum which is carried by the carriage and is arranged to pay out and wind in a high-pressure water supply hose as the carriage moves back and forth. The hose feeds water under pressure to nozzles carried at the ends of rotary arms mounted at the front and to both sides of the carriage. Water under pressure is also supplied to nozzles provided on two transverse arms of the carriage. These latter arms are angularly orientable about their longitudinal axes.