Why slip resistance matters here
The HSE notes that most slips happen when a surface is contaminated, and outdoors the usual contaminant is plain water. Along the Redcar coast that means rain, wind-driven spray and the damp sea fret that the Met Office says easterly winds often bring in spring and early summer. Inland, in Guisborough or Loftus, shaded yards below the hills can stay damp for days. A slab that feels grippy in the showroom may behave very differently on a wet, north-facing patio in February.
How grip is measured
There are several tests, and manufacturers do not all use the same one, so it helps to know what the numbers mean.
The pendulum test (PTV)
The HSE's preferred method is the pendulum test, which swings a standard rubber "heel" across the surface and gives a pendulum test value, or PTV. It can be done dry or wet; for a patio, the wet figure is the one that matters. The HSE classifies the results as:
- 0 to 24: high slip potential
- 25 to 35: moderate slip potential
- 36 and above: low slip potential
When comparing paving, ask for the wet PTV. A product that only quotes a dry figure tells you little about a rainy afternoon.
Surface roughness (Rz)
The HSE also describes measuring surface microroughness, given as an Rz value in microns. For water-wet areas it treats below 10 as high slip potential, 10 to 20 as moderate, and over 20 as low. It is a useful check, especially on worn surfaces, but it supplements the pendulum test rather than replacing it.
"R ratings"
Many European tiles carry an R9 to R13 rating from a German ramp test. The HSE explains that these tests use oil and safety boots, or soap and bare feet, and it has reservations about using them to judge slip risk. An R rating is not the same as a PTV. If a product only has an R rating, ask whether a wet pendulum figure is available.
How the main materials compare
- Outdoor porcelain is made in finishes from smooth to heavily textured. Products sold for patios are usually tested, and many publish a wet PTV. Indoor porcelain tiles used outside are a common cause of slippery patios, so check that the product is specified for external paving. Our porcelain patios page explains thicknesses and finishes.
- Riven sandstone has a natural, uneven texture that usually grips well, though it varies slab to slab and can grow algae in shade. Sawn and honed sandstone is smoother, so it relies more on its finish. See our Indian sandstone patios page.
- Textured concrete slabs usually have good grip and are a sound, practical choice for family gardens. Our patio slabs page covers the options.
- Resin-bound gravel has a stony, open texture; some systems add a fine grit to the surface for extra grip.
Whatever the material, a darker colour does not make paving grippier, and a polished or glazed finish has no place on an outdoor patio.
Design details that matter as much as the slab
Fall
Every patio should fall gently away from the house so water runs off rather than lying in a film. Puddles are slip hazards, and in winter they turn to ice. We set the fall at the start and check it as we lay.
Steps
Steps need the most care, because a slip there means a fall. We use:
- treads deep enough for a whole foot, with consistent risers throughout a flight
- a textured or contrasting nosing so the edge of each step is easy to see
- the same grippy material on the tread as on the patio
- a handrail on longer flights and in steep gardens, where a client wants one
Our garden paths and steps page goes into building steps on sloping plots like those in Skelton and New Marske.
Slopes
Paving laid on a steep incline is harder to keep safe than level terraces joined by steps. Where a garden falls away, we usually suggest cutting level areas with retaining walls and steps between them. If a raised area would end up more than 30cm above the ground, the Planning Portal explains it may need planning permission, which is another reason to keep terraces low.
Shade, trees and drains
North-facing patios, areas under trees and spots next to downpipes stay damp the longest. We avoid smooth finishes in those places and make sure downpipes discharge into a gully or channel rather than straight onto the paving.
Keeping a patio grippy
The best-specified slab will still become slippery under a layer of algae. The RHS explains that algae, moss and lichens thrive on damp, shaded hard surfaces, and that regular brushing and better light and drainage keep them in check. In practice:
- sweep regularly, especially in autumn
- remove green growth before it builds up
- avoid sealers that leave a glossy film; if natural stone is sealed, choose a product that does not reduce grip, and test it on a spare slab first
- in icy weather, treat the main walking routes and steps
Our guide to patio aftercare by the sea covers cleaning that is kind to the becks.
Questions people ask
Is porcelain slippery when wet?
Outdoor-rated, textured porcelain is not inherently slippery. Look for a wet PTV of 36 or more and avoid indoor tiles.
What does PTV 36 mean?
It is the HSE threshold for low slip potential in the pendulum test. Values from 25 to 35 are moderate, and 24 or lower is high.
Is R11 good for outdoor paving?
An R11 rating comes from a different test using oil and safety boots. It does not convert directly to a PTV, so ask for a wet pendulum result where you can.
What is the most non-slip patio material?
Textured outdoor porcelain, riven sandstone and textured concrete can all perform well. The finish and upkeep matter more than the material name.
Sources: HSE: assessing the slip resistance of flooring; RHS: algae, lichens, liverworts and mosses on hard surfaces; Met Office: North East England climate; Planning Portal: decking and raised platforms.