Surface Roughness Rz for Pool Surrounds

What Rz measures

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 — the Rz value.

For pool surrounds, Rz typically ranges from below 5 microns (highly polished or worn surrounds — high slip risk) to 30+ microns (heavily textured anti-slip pool tile — strong slip resistance).

HSE Rz/water slip-risk correlation

The HSE has published a correlation between Rz and wet slip risk for water-contaminated surfaces. For pools, this correlation is directly applicable because the surround contaminant is essentially water:

Rz (μm)	Water-wet slip potential
Below 10	High
10–20	Moderate
20+	Low

This sets a useful supplementary benchmark: a pool surround with Rz below 10 is in the high water-wet slip-risk band even if the pendulum result is borderline.

Why Rz complements pendulum on pool surrounds

Pendulum (PTV) measures dynamic friction. Rz measures texture geometry. They are related — surrounds with higher Rz typically achieve higher wet PTV — but the relationship is surface-dependent and not perfectly predictive.

For pools specifically, Rz adds value in:

• Long-term ageing detection — Rz changes years before they become visible in PTV deterioration; periodic Rz tracking is an early-warning indicator
• Forensic borderline cases — where pendulum is in the 25–35 moderate band, Rz adds context for interpretation
• Anti-slip treatment verification — Rz changes confirm whether texture has actually been added by treatment
• Cleaning regime impact — Rz drops over time identify cleaning chemistry that is gradually polishing the surround
• Sample testing — retained tile samples can be Rz-tested even where pendulum is not feasible on the small sample

How Rz is measured on pool surrounds

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 standard cut-off) at controlled speed. The instrument digitises the surface profile and calculates Rz internally. Multiple readings (typically five) are taken at each test point and averaged.

Pool surrounds are slightly more demanding for Rz testing than typical commercial floors because: the surround is wet (the instrument and surface need to be dried locally for measurement); falls toward drains may mean uneven contact; tile grout joints must be avoided as they produce skewed readings.

Limitations of Rz alone

Rz tells you about the texture geometry but not about its slip-relevant behaviour. Two pool surrounds with identical Rz can perform differently because:

• The texture peaks may be sharp (good for friction) or rounded (less so)
• The texture may be 'open' (drains contamination) or 'closed' (traps it)
• Surface chemistry affects how water and pool plant chemistry interact at the micro scale

For these reasons, Rz is best used as one input alongside pendulum PTV, not as a substitute. Reports combining both metrics produce stronger evidential coverage than either alone.