Glass is slippery when wet, and a glass floor with no surface treatment is not a floor anybody should be asked to walk on. What the treatment is, and how much of it there is, is a decision with real consequences for safety, for daylight and for how the unit looks.
Why untreated glass is not acceptable
Polished glass has a very low coefficient of friction wet, and it gets worse rather than better with a thin film of water on it.
Outdoors that film is present most of the year in this country: rain, dew, condensation, sea mist on the coast. A walk-on unit set flush in an external terrace is wet a great deal of the time, and it is being walked on by people who are not thinking about it.
So a surface treatment is not an option on an external unit. It is part of the specification.
The three treatments
| Ceramic frit | Acid etch | Sandblast | |
|---|---|---|---|
| What it is | Ceramic ink printed on and fired in | Chemically etched surface | Abrasively blasted surface |
| Pattern | Dots, lines, any printed pattern | Uniform, all over | Uniform or masked |
| Durability | Fired into the glass. Excellent | Part of the surface. Excellent | Part of the surface. Good |
| Grip | High, and only where printed | Moderate, everywhere | Moderate to high |
| Light cost | Proportional to coverage | Diffuses rather than blocks | Diffuses, slightly more |
| Cleaning | Easy between the pattern | Holds dirt slightly | Holds dirt more |
Ceramic frit
The most common answer on a specified walk-on unit, and the most controllable.
A ceramic ink is screen printed onto the glass and then fired, during the toughening process, so it fuses into the surface. It is not a coating sitting on top; it is part of the glass and it does not wear off, scratch off or come away.
Because it is printed, the pattern is a design decision: dots, lines, squares, a gradient, or a full field. Coverage is chosen, which means the trade between grip and daylight is a dial rather than a switch.
The dot pattern
The familiar arrangement: a regular grid of small dots. It gives good grip because the raised ceramic breaks the water film, and it leaves the glass between the dots clear, so a substantial proportion of the light comes through and you can still see through it.
Coverage
Higher coverage means more grip and less light and less transparency. A sparse pattern is barely visible from below and gives less grip wet. Where the unit is over a habitable room, that trade is worth thinking about rather than accepting a default.
Acid etch
The whole surface treated chemically to a fine matt texture.
It is uniform, which some people prefer aesthetically to a pattern, and it gives a soft diffused appearance rather than a printed one. The grip is moderate and consistent rather than high and localised.
It diffuses light rather than blocking it, so the light transmission cost is smaller than the visual change suggests. What it does remove is transparency: you get light through it and not a view.
Sandblasting
Mechanically abrading the surface to a similar matt finish.
It achieves a comparable appearance to acid etch and a slightly coarser texture, which gives slightly more grip. The drawback is that the abraded surface is more open, so it holds dirt more readily and is harder to clean.
It is also, being an abrasion, a surface disturbance on a material whose strength depends on surface condition, which is a reason it is used with care on structural glass and why the treated ply is chosen accordingly.
How slip resistance is measured
Two systems turn up on datasheets and they measure different things.
The pendulum test gives a value representing the friction available to a foot, tested wet and dry, and it is the measure UK practice generally works to. Higher is better, and the wet figure is the one that matters on an external unit.
The ramp test gives an R rating, from a person walking on an inclined surface, and it is common on continental products and on flooring generally.
They are not directly convertible, and a supplier should be able to give the pendulum value wet for the surface being offered. That figure, rather than a description, is what to compare.
Wet is the number that matters
Worth stating plainly because dry figures are flattering and useless.
Untreated polished glass performs acceptably dry and very poorly wet. Every treatment on this page performs well dry. The whole point of the specification is the wet condition, and on an external unit in this country that is the normal condition rather than the exception.
A quotation offering an anti-slip surface without a wet slip value has not told you anything.
Which ply is treated
A detail with consequences.
The treatment goes on the top surface of the top ply, because that is where the foot is. That ply is the sacrificial one, which is convenient: if it is ever replaced, the surface treatment goes with it.
What it means practically is that scratching or damaging the top ply damages the anti-slip surface as well as the appearance, and a badly worn top surface is a serviceability issue for two reasons rather than one.
What it costs in daylight
The trade nobody quantifies at the briefing stage and everybody notices afterwards.
A dense frit pattern or a full etch reduces the light reaching the room below appreciably, and on a light well or a basement, where the whole point of the glass is that it is the only daylight, that matters a great deal.
The useful move is to specify the minimum coverage that achieves the wet slip value required for the situation, rather than the maximum coverage available. More grip than the situation needs is daylight given away for nothing.
Where the situation needs more
Four cases where the higher figure is worth the light.
- An external unit exposed to rain and dew, which is most of them.
- Anywhere on a slope, however slight, including a terrace laid to fall.
- Somewhere people arrive with wet feet: beside a door, near a pool, at the head of external steps.
- Anywhere with public or communal access, where the range of users is wider.
And where less will do
Internally, on a landing or a floor panel inside a dry building, the wet case largely does not arise and a lighter treatment is reasonable.
Over a light well where the daylight is precious and the surface is only walked on occasionally, a sparse pattern giving adequate rather than maximum grip is frequently the better balance.
That is a judgement about the situation and it should be a stated judgement rather than a default.
Barefoot, and the case people forget
A terrace is somewhere people walk barefoot in summer, and the surfaces behave differently under a bare foot than a shoe.
A coarse sandblast or a heavily frit-textured surface that grips a shoe well can be genuinely uncomfortable barefoot, particularly when it has been in direct sun and is hot.
Where a glass floor is part of a domestic terrace, that is worth raising, and a sample is worth standing on before deciding.
Heat, and dark patterns
A consequence of frit that is easy to miss.
A dark ceramic frit absorbs solar energy and the treated area runs hotter than clear glass beside it. In strong sun a dark patterned surface can be uncomfortably hot underfoot, and the differential heating across a panel is a thermal stress consideration for the designer.
White or light frit reflects rather than absorbs and stays cooler. On a south facing terrace it is the sensible choice for both reasons.
How they wear
All three are part of the glass rather than applied to it, so none of them wears off in the way a coating would.
What does happen is that grit underfoot abrades the surface over years, gradually polishing the high points of a texture and reducing grip. On a busy external unit, that is a slow process and it is real.
Which is why slip resistance is worth checking periodically on a unit that has been in use for a long time, rather than assumed to be a permanent property.
Applied anti-slip products
Adhesive strips and applied coatings exist and they are worth being honest about.
They work, they are cheap, and they are a retrofit answer to a unit that was specified without an adequate surface. They also sit on the glass rather than in it, they have a service life, they collect dirt at their edges, and they look like what they are.
On a unit being specified from new they are the wrong answer. On an existing unit that is slippery and cannot economically be replaced, they are considerably better than nothing.
Cleaning without destroying it
The textured surfaces hold dirt more than polished glass, and the cleaning method matters.
Fresh water and a soft brush, working with the texture rather than against it, and a mild detergent where needed. What must be avoided is anything abrasive, which damages the surface and reduces its grip, and anything that leaves a film, which is exactly the condition the treatment exists to prevent.
A silicone or water repellent product applied to an anti-slip glass floor makes it slippery, which is the worst possible outcome from a well intentioned action.
The coastal complication
On a seafront terrace, fine sand blows onto every horizontal surface and sits in the texture of an anti-slip finish.
Loose grit on a textured surface is itself a slip hazard, and it is abrasive against the surface it is sitting on. Which means the cleaning interval on a coastal walk-on unit is shorter than inland, and it is a genuine safety item rather than an appearance one.
It has its own page, and the short version is that the surface has to be kept clear for it to keep working.
What to ask for
- The wet pendulum slip value for the specific surface offered.
- What the treatment is: frit, etch or blast, and if frit, the coverage.
- The light transmission of the treated unit, not the untreated glass.
- A physical sample, to look through and to stand on.
- Whether the treatment is on the sacrificial ply.
What we specify on walk on glass
We specify the surface against the situation rather than the default, ask for the wet slip value rather than a description, and take the minimum coverage that meets it so the daylight is not given away. On an external unit we treat the surface as part of the specification rather than an option, and on a domestic terrace we get a sample somebody can stand on before it is ordered.
The grip is decided once, at ordering, on a surface people will walk on for thirty years.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.