Surface roughness — specifically the Rz value, measured in microns — describes the micro-texture of a floor surface. It is a useful complement to pendulum testing, particularly in forensic work, because Rz captures surface texture at a finer scale than the pendulum can directly resolve. HSE guidance correlates Rz to wet slip risk for water-contaminated surfaces.
Rz is the average peak-to-valley height of the surface profile over a defined sampling length, in microns (millionths of a metre). A precision diamond stylus is drawn across the surface, recording vertical movement as it crosses peaks and troughs. The instrument processes the trace to extract the average peak-to-valley height — that is the Rz value.
For floor surfaces, Rz values typically range from 1 micron (highly polished glass-like surfaces) to 80 microns or more (coarsely textured industrial flooring).
The HSE has published a correlation between Rz and wet slip risk for surfaces contaminated with clean water:
| Rz (μm) | Wet slip potential |
|---|---|
| Below 10 | High |
| 10–20 | Moderate |
| 20+ | Low |
This correlation is specifically for water contamination. With other contaminants — oil, soap, sugar solution — different Rz values may produce different slip outcomes.
PTV (from pendulum testing) measures dynamic friction. Rz measures surface texture. They are related — surfaces with higher Rz typically achieve higher wet PTV — but the relationship is surface-dependent and is not perfectly predictive.
Where the two measurements diverge, the divergence is itself useful information. A surface with high Rz but low wet PTV may have texture that is not connected (deep but disconnected pits hold water but do not displace fluid from under a slider). A surface with low Rz but acceptable wet PTV may have hydrophobic surface chemistry that resists fluid film formation.
A portable stylus roughness meter is placed on the surface and triggered. The stylus traverses a defined sampling length (typically 12.5 mm at the most common cut-off setting) at controlled speed. The instrument digitises the surface profile and calculates Rz internally.
Multiple readings are taken at each test point — typically five — and averaged. Different cut-off settings (Lc) capture roughness at different scales; the standard cut-off for floor surfaces is 2.5 mm.
Rz tells you about the texture geometry but not about its slip-relevant behaviour. Two surfaces with identical Rz can perform differently because:
Rz is therefore best used as one input alongside PTV, not as a substitute for it.
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