Natural stone slip resistance: PTV, DCOF, R ratings and CTE

Wet polished stone is slippery. Every market measures slip resistance in its own way: a pendulum in the UK, a tribometer in the US, an inclined ramp in Germany, and classes based on the pendulum in Spain. Here is what each figure means and how the finish of the stone changes it.

First rule: the numbers do not convert

Each method uses a different device, a different slider and different conditions. A pendulum value, a DCOF and an R rating cannot be converted into one another. Specify the method that applies where the building is, and ask for a test report under that method.

United Kingdom: the pendulum (PTV)

In the UK the reference test is the pendulum, which gives a Pendulum Test Value (PTV). A rubber slider on a swinging arm sweeps across the surface, and the device measures how much it is slowed down. According to the HSE and the UK Slip Resistance Group, a PTV of 36 or more means a low slip potential.

For natural stone, the European pendulum test method is EN 14231, and you will find it on many stone data sheets.

United States: DCOF (ANSI A326.3)

In the US, hard flooring is usually specified by its dynamic coefficient of friction (DCOF), measured under ANSI A326.3. The standard sets a minimum of 0.42 for level interior floors that are expected to be walked on when wet.

Treat 0.42 as a threshold for that case, not a guarantee. Ramps, exterior areas, pool decks and wet commercial kitchens need their own assessment, and the specifier sets the requirement.

Germany: R9–R13 and A, B, C

German standards rate floors on an inclined ramp:

• DIN 51130 gives the R rating for shod foot traffic, from R9 (least slip resistant) to R13 (most).
• DIN 51097 rates barefoot areas, such as changing rooms, showers and pool surrounds, as A, B or C, with C the most slip resistant.

If a data sheet quotes an R rating or an A, B or C rating, these are the standards behind it.

Spain: CTE classes (DB SUA 1)

If the project is in Spain, the Building Code (CTE, section DB SUA 1) applies to the floors of hotels and other Residencial Público buildings, healthcare, education, retail, offices and public-assembly buildings such as restaurants. Slip resistance is expressed as an Rd value measured with the pendulum under UNE-ENV 12633:

Private homes are not covered, but the table is a sensible reference for them too. It is a summary: the project team sets each zone's class from the current version of the code.

How the finish changes grip

The same stone can perform very differently depending on its finish. As a rough guide:

• Polished: the least slip resistant finish.
• Honed: somewhat better than polished.
• Bush-hammered, flamed, sandblasted and antiqued: the most slip resistant. Flaming only works on granite and some other quartz-rich stones, never on marble.

Textured finishes such as leathered vary a lot from stone to stone, so testing matters even more. Each finish is described in our guide to finishes.

There is no fixed rating per finish. The real value depends on the stone and on how the finish was produced: two honed stones can give different results. A classic flooring marble such as Crema Valencia can be polished on a bathroom wall and may need a different finish on that bathroom's floor. The only way to know is to test it.

Getting it right on a project

1. Set the requirement for each zone in the drawings and the specification, under the method that applies locally. See how to specify natural stone.
2. Ask for a test report for the stone in the finish you will install. If there is none, have a laboratory test a sample of the same stone and finish.
3. Use one stone in several finishes. Polished on walls and, on each floor, the finish that met the requirement in testing. You keep colour continuity and comply zone by zone.
4. Plan for maintenance. Wear and some products, such as waxes and polishes, change grip over time. Include this in the building's care instructions.

Areas that need extra care

• Entrances from the street: people bring in water and grit, so treat them as wet areas.
• Showers: see marble showers.
• Terraces, pools and exterior areas: see natural stone for outdoors.
• Hotels, restaurants and shops: dry floors, wet floors and stairs within a few metres. See natural stone for hotels and retail.

Need to meet a slip resistance requirement with stone? Tell us the area, the rating required and the dimensions and we will send you photos of the available slabs with a quote.