Section 2.4.2.2 of DIN 4760 (July 1982) gives this definition:
"Form errors of the 3rd to 5th order (roughness). Form errors of the 3rd to 5th order are regularly or irregularly recurring deviations in the actual surface of a geometrical element, where the peak-to-peak/peak-to-valley ratio generally lies between 100:1 and 5:1..." As examples of the type of deviation it mentions grooves, scratches, scales, crests (see DIN 4761) and the microstructure. Draft DIN 4762, T1 (October 1986) describes surface roughness as being "... surface irregularities with relatively small peak-to-peak distances, usually including such irregularities as are caused by the applied production process and/or other effects. … These irregularities are examined within defined limits, e.g. within the reference section. …" Roughness is determined by taking two-dimensional roughness measurements with the stylus method (exact measurement) or by drawing comparisons with surfaces of known roughness (approximate determination). With the stylus method, a probe is drawn over the surface at constant speed, producing a plot of the actual surface and enabling, for example, the following roughness parameters to be established:

• Maximum peak-to-valley height R_max : This is the greatest peak-to-valley height Z occurring over the total measuring section I_m.

• Average peak-to-valley height R_z : This is the average value of the peak-to-valley heights measured over five consecutive individual measuring sections l_e.

• Mean roughness index R_a : This is the arithmetic mean of the absolute values of all peak-to-peak distances in the roughness profile R from the centre line inside the total measuring section I_m.

Finish-machined sliding faces display the following mean roughness indexes R_a (average values) depending on their material:
tungsten carbide 0.01 mm
silicon carbide 0.04 mm
cast Cr-steel 0.20 mm
hard carbon 0.15 mm
ceramic 0.15 mm

For rotating parts, the maximum peak-to-valley height should be less than 4 mm in the area of dynamically loaded O-rings made of elastomers and PTFE, less than 16 mm in the area of statically loaded O-rings, and less than 4 mm in the area of radial shaft lip seals. Roughness can have the following effects: Rough sliding faces result in a lower breakoff torque when starting up the mechanical seal. The lower the roughness, the higher the percentage bearing area and hence the mechanical seal's loadability. By varying the roughness of finish-machined sliding faces, it is possible to influence the thickness of lubrication film between the sliding faces and hence the mechanical seal's leakage.