Surface roughness (Rz) — the other half of the slip-risk story.

What Rz testing measures

The Rz parameter — measured in microns (µm) — is the average peak-to-valley height of micro-roughness across a small sample of the floor surface, using a calibrated stylus profilometer. It quantifies the surface's ability to disperse contamination: fluid sits in the valleys, allowing the peaks to maintain dry contact with a shoe sole.

Why it matters

A polished marble floor and a textured rubber tile may both score acceptable PTVs when clean and dry. But once water is introduced, the marble's near-zero Rz means there is nowhere for the water to go — the floor becomes a hydroplane. The textured tile's higher Rz disperses the contamination into the micro-valleys, allowing partial dry contact and preserving slip resistance.

This is why HSE guidance and UKSRG Issue 5 both emphasise that a slip risk assessment is incomplete without an Rz reading alongside the pendulum data, particularly in environments where contamination is foreseeable.

Indicative Rz thresholds

Approximate Rz values associated with adequate wet slip resistance, depending on contaminant:

• Clean water — Rz of 20µm or above generally adequate
• Greasy water (kitchens, food areas) — Rz of 45µm or above typically required
• Heavy contaminants (oil, food debris) — Rz of 70µm or above

These figures are indicative only and should always be considered alongside pendulum data and the specific contaminant profile of the site. We provide both readings in every UKAS-accredited report.

How the test is performed

A portable stylus profilometer is drawn across a defined contact length on the floor surface. Multiple readings are taken at each test location to ensure repeatability. The instrument is calibrated to UKAS-traceable reference standards before each site visit.