Shower Tray Roughness Is Not a Slip Rating

Textured shower trays · Surface metrology · Slip evidence

Approx. 9 min read · Evidence reviewed 25 Sep 2026 · For buyers, laboratories, factories and technical sales teams

Short answer: Roughness can help monitor whether a production texture changes, but one Ra, Rz or Sa value is not a universal slip rating. Slip depends on the test method, contaminant, contact material, surface direction, drainage, user condition and wear. Use topography for process control and an applicable slip method for the safety claim.

Different Numbers Describe Different Things

Result	What it describes	What it cannot replace
Ra or Rz profile parameter	A line-based surface-height characteristic under defined filtering	Wet-barefoot ramp or pendulum result
Sa or other areal parameter	A three-dimensional mapped surface characteristic	Human contact, contaminant and motion in a slip method
Gloss	Reflected-light behaviour on a suitable surface	Texture depth or friction
Visual texture grade	Pattern appearance against an approved master	Instrumented topography or slip resistance
Slip test result/class	Performance under the named method and conditions	Every other method, contaminant or use condition

Specify the Roughness Measurement

“Roughness 20 microns” is incomplete. Record the parameter, instrument, stylus or optical method, filter, cut-off, evaluation length/area, direction, trace locations, specimen conditioning and data treatment. Deep stone-like texture can exceed a stylus range or cause the probe to bridge features; optical methods can struggle with translucent or glossy zones.

ISO 25178-2 defines areal surface-texture parameters, but it does not identify which parameter predicts slip for a specific shower tray. Select a small parameter set linked to the manufacturing risk—for example texture height, spacing, material ratio or local slope—then demonstrate its relationship to the relevant functional test.

Map the Tray Instead of Taking One Reading

• Centre walking field in two directions.
• Near the waste, where texture and fall may transition.
• Entry zone and primary foot placement.
• Perimeter and enclosure landing zone.
• Highest-risk mould cavities or known repair areas.

Keep the coordinates linked to fixed tray datums. Report mean, range and local outliers. A batch average can hide one polished or untextured patch exactly where a user places a foot.

Understand the Limited Role of Rz Screening

UK HSE guidance explains that Rz microroughness can indicate slipperiness for common water-wet flooring and can help monitor change. It also says roughness should supplement pendulum data and identifies surfaces where a handheld roughness meter is unsuitable, including very rough or undulating areas. That guidance is useful for thinking about screening; it is not a shower-tray certification route.

Interpretation rule: Do not lift an Rz threshold from general flooring guidance and print it as a global shower-tray pass/fail limit. First confirm surface, contaminant, geometry, instrument range and correlation to the claimed slip method.

Build a Correlation Study

1. Select production lots that span the expected texture range.
2. Map roughness/topography at fixed coordinates and directions.
3. Test representative specimens using the market-relevant slip method.
4. Include cleaning, wear or repair conditions if those claims matter.
5. Check whether the roughness metric actually separates acceptable and unacceptable slip results.
6. Set an internal process window with measurement uncertainty and guard bands.
7. Revalidate after mould, coating, raw-material or process changes.

Keep Slip Methods Separate

BS EN 16165 includes different evaluation methods, and their classes are not interchangeable. Australian AS 4586 classifies new pedestrian surfaces under its named methods. A European barefoot class, Australian pendulum class and generic coefficient of friction are not translations of one another. The sales sheet must name the method, condition, specimen and report—not only the final letter or number.

Reject the claim or request clarification if:

• the parameter name, instrument or filter is missing;
• one roughness reading represents the whole tray;
• a stylus was used outside its range on deep texture;
• roughness is converted directly into a barefoot or pendulum class;
• new-surface geometry is used to imply retained performance after wear; or
• results from different slip methods are presented in one ranking without qualification.

Related Wakebath Reading

• Textured Does Not Automatically Mean Anti-Slip
• Shower Tray Sample Approval Pack: Make the Decision Reproducible
• Shower Tray Troubleshooting: Movement, Slow Drainage or Surface Marks

Use Topography to Control the Process—Then Test the Claim

Send the texture map, proposed roughness parameter, target market and slip report. Wakebath can help separate useful production limits from unsupported safety claims.

Request a texture-evidence review

Primary References and Evidence Boundary

• ISO 25178-2:2021: current areal surface-texture terms, definitions and parameters.
• UK HSE — Surface Roughness FAQ: official explanation of Rz microroughness as an indicator and the limits of handheld roughness measurement.
• UK HSE — Assessing the Slip Resistance of Flooring: official guidance stating that microroughness supplements pendulum data and is condition-dependent.
• BS EN 16165: slip-resistance evaluation methods whose results and classes must remain method-specific.
• WorkSafe Victoria — Safe Design: regulator guidance referencing AS 4586 testing for new pedestrian surfaces.
• BS EN 14527:2016+A1:2018: shower-tray cleanability and durability product-standard scope.

Evidence boundary: Surface-texture standards define measurement language. They do not create a universal conversion from roughness to shower-tray slip safety or a cross-market equivalence between test classes.