What is surface roughness Rz testing?

The roughness of a surface has a significant impact on its contaminated slip resistance. It is a similar mechanic to that which affects slick versus grooved tyres. The Rz parameter is one of many roughness parameters used to describe surface micro-profile.

Roughness meters can be bought relatively cheaply and do not require an experienced operator, so they see considerable use as a ‘quick and easy’ measure of slip resistance. They should not be considered a slip test method in their own right however. Our own experience has shown the Rz parameter to be an extremely unreliable indicator of slip risk. The result of an erroneously classified floor may be a slip and fall claim or unnecessary replacement of the floor, both expensive ventures justifying a more accurate and reliable Pendulum test.

The Rz parameter is a measure of the average height between the peaks and troughs of a profile and is typically quoted in microns. A greater Rz value indicates a greater peak to trough height, however it is commonly, and erroneously in our opinion, thought to indicate a greater wet slip resistance. The limitation of this parameter is that a surface with a single discrete rounded peak can produce an identical Rz value to a surface with multiple sharp peaks of the same height, with the latter offering a significantly better wet slip resistance.

Other roughness parameters include Ra (roughness average), Rq (Root mean square roughness), Rt or Ry (maximum profile height), Rv or Rm (Maximum trough depth), Rp (maximum peak height), Rpm (average maximum profile peak height), Rmax (maximum roughness depth), Rc (mean height of profile irregularities); however these are used primarily in engineering applications as opposed to slip resistance measurement.