One of the main surface coating quality indexes of details of mechatronic systems is surface roughness. And the surface roughness is described by surface roughness parameters. Nowadays characterization of surface roughness using three-dimension (3D) methods and parameters become more and more important. This type of surface quality characterization, unlike the two-dimension (2D) methods, provide a more complete view on the surface qualities, since the surface roughness is viewed as a spatial object. Within the last ten years intensive work is being carried out on the development of 3D roughness standards, therefore it is necessary to agree on a unified approach in the assessment of surface roughness 3D parameters. To make possible application of the ISO/DIS 25178 standard being developed one needs information on the determination of 3D surface roughness parameter precision, such as number of measurements, dimensions of measurement areas and their disposition on the measured surface. The present work provides the surface roughness assessment principles when measuring the 3D roughness of coated surfaces by means of contact-type measurement devices, based on the regularities of the theory of probability [1]. The work considers one of the basic parameters of roughness – total height of the surface St. This parameter determines the beginning of contact process. The work determines the number of measurement areas at the chosen probability of parameter precision and credibility. The result is compared with a two-dimension surface model. The experimental testing was carried out on surface coatings obtained by the help of nanotechnologies – at present very important coatings in the machine and apparatuses engineering. Experimental surface roughness measurements for steel surfaces having TiN coating were carried out to verify the previously mentioned theoretical coherences. Parameters St and Rt (maximum height of profile) were measured and their mean values were calculated for different sizes of measurement areas and profile traces. In case of St the measurement areas have the side lengths (L×L) 2, 3 and 4 cut-off lengths, in case of Rt the measurement traces L consisting of 2 to 8 cut-off lengths. 5 separate measurements were made on each surface. The measurement results were processed using mathematical statistical method. The research has resulted in the proposals for the parameter St measurement regulations.