Surface roughness (Rz) and slip resistance
Alongside the pendulum, the other instrument the HSE and UKSRG rely on is a roughness meter. Here’s what Rz measures, what the numbers mean, and where it helps — and where it doesn’t.
What Rz actually measures
Rz is a measure of surface microroughness — broadly, the average height from the deepest valleys to the highest peaks of a surface’s texture, measured in microns (µm, thousandths of a millimetre). It is the roughness parameter the HSE uses, and alongside the pendulum it is the second instrument the HSE and UKSRG rely on to understand slip risk.
The reason roughness matters comes down to water. When a floor is wet, a thin film of liquid forms between the foot and the surface — a “squeeze film”, much like a tyre starting to aquaplane. It is the tiny peaks in the surface texture that pierce that film and keep solid contact between foot and floor. Too smooth, and the film wins. This is precisely why a floor can grip perfectly when dry and fail when wet, and why roughness is tied so closely to wet slip resistance in particular.
What the numbers mean
Thicker, more viscous contaminants — oils and greases rather than water — need more roughness again to keep that film broken. One important practical point: Rz must be measured with the 2CR filter, not the Gaussian filter that some meters default to. The wrong filter gives different numbers for the same floor, and it is a common source of error — one of several reasons roughness is best left to someone who does it regularly.
Where roughness helps
Roughness readings earn their place where the pendulum can’t easily go: the edge of a stair nosing, the curved surface of a bath or shower tray, tight or confined spaces, and slopes steeper than about 6°. They are also a quick way to map how a large floor varies from place to place, and to monitor whether a surface is wearing smooth over time.
The limits — and why pendulum stays first
The UKSRG is clear on two points. First, Rz is complementary: it should be read alongside pendulum values, not on its own, and it must never be used to specify a floor. Second, roughness is only one facet of texture. Two floors with the same Rz can grip differently depending on the shape and spacing of the peaks, and a sharp texture can round off with wear — dropping wet grip with very little change in the Rz figure. If the pendulum and the roughness meter point to different conclusions, that is a flag to investigate further, not licence to pick the answer you prefer.
Frequently asked questions
What Rz value is considered slip-resistant?
As a guide for water-wet floors, an Rz above roughly 20µm usually means low slip potential, while below about 10µm a floor is likely to be slippery when wet. It is used alongside the pendulum, not as a standalone pass mark.
Can a roughness reading replace the pendulum test?
No. The UKSRG is explicit that Rz is complementary and should not be used in isolation, nor to specify flooring. The pendulum remains the primary measure of slip resistance.
Why does the filter setting matter?
Rz must be measured with the 2CR filter rather than the Gaussian filter. The two process the data differently and give different numbers, so using the wrong one is a genuine source of error.
When is roughness more useful than the pendulum?
Where the pendulum can’t be set up properly — stair nosings, curved bath and shower surfaces, confined spaces, and slopes steeper than about 6° — and for quickly mapping variation across a large floor or monitoring wear.
Got an awkward surface to assess?
Stair nosings, shower trays, profiled floors — we combine pendulum and roughness measurement to give you a reading you can trust.