Wet vs Dry Pendulum Testing

The wet/dry difference

Almost every floor produces a higher PTV dry than wet. Polished tile may achieve PTV 60 dry but PTV 12 wet. Textured vinyl may achieve PTV 65 dry and PTV 50 wet. Resin-bound aggregate may achieve PTV 70 dry and PTV 55 wet.

The reduction is a function of how the surface texture interacts with a fluid film. Smooth surfaces lose the most friction when wetted because the fluid creates a hydrodynamic layer between the slider and the surface; textured surfaces lose less because the texture displaces the fluid.

Wet is usually binding

For almost any UK pedestrian environment, wet performance is the binding constraint:

• Retail entrances see rainfall transferred from shoes
• Supermarkets have refrigerated case condensation, fresh produce mist, drink and food spillage
• Hospitality has drink and food spillage, kitchen-floor moisture transfer
• Healthcare has cleaning fluids, bodily fluids, equipment fluids
• External paving has rainfall, condensation, frost-melt
• Pool surrounds and changing rooms have persistent moisture

The HSE 36+ threshold therefore applies to wet PTV in these environments, with dry PTV being a secondary measurement.

When dry is binding

Dry PTV is the relevant value for environments where wet conditions are genuinely not foreseeable:

• Internal warehouse aisles away from loading bays
• Office circulation away from external entrances and washrooms
• Climate-controlled archive and library stacks
• Internal hardware and electrical-equipment rooms
• Some sports halls and dance studios where dry use is genuinely the steady state

For these zones, dry PTV stands on its own. For everything else, wet is the test.

How testing is conducted

Standard practice is to conduct dry testing first — three directions, recorded swings — and then wet the surface and repeat. Wetting is by clean tap water at ambient temperature, applied as a continuous film across the test area. The slider runs through the wet film; some water is displaced and some remains in contact with the slider.

For specialist environments (oil-contaminated kitchen floors, soap-contaminated changing rooms), the test contaminant may be the operational contaminant rather than water — though this is non-standard and the report will state explicitly what contaminant was used.

The wet/dry delta as a diagnostic

The size of the wet-to-dry PTV reduction is itself diagnostic:

• Small delta (5–15 points) — surface texture is providing real wet-condition friction; the floor has been well selected for the environment
• Moderate delta (15–30 points) — typical for many commercial floors; acceptable if the wet value remains above 36
• Large delta (30–50 points) — the surface is heavily reliant on dry friction; wet performance may be marginal even if the absolute wet PTV is acceptable
• Very large delta (50+ points) — almost always indicates a polished surface with negligible wet friction; high slip risk in any wet condition

Reporting both values

Compliant pendulum reports show the wet and dry values for every test location, in every test direction, plus the calculated mean per slider configuration per location. This is the data the next reviewer — surveyor, insurer, expert, judge — needs to assess what the floor actually does. Reports that quote only a single PTV per location are evidentially much weaker.