Anti slip finishes and how they are applied

Ceramic frit

The usual answer. A ceramic ink is printed onto the glass surface in a pattern of dots, lines or squares, then fired in the toughening furnace so it fuses into the glass.

Because it is fired in, it cannot wear off, peel or be scratched away. It will outlast the installation.

Patterns vary from fine dots to bold squares. Denser patterns give more grip and less transparency, and that is the trade you are making.

Acid etching

The surface is chemically textured, giving a uniform frosted finish with grip across the whole panel rather than in a pattern.

Looks more even than frit and is generally slightly less grippy when wet. It is permanent, being a change to the glass surface itself.

Applied coatings

Anti slip coatings applied after manufacture exist. We would not specify one on a walk-on rooflight: they wear at exactly the points that get walked on most, and re-applying at height is not a small job.

Slip ratings

Manufacturers quote a pendulum test value, wet and dry. A wet value of 36 or above is generally considered low slip risk for pedestrian surfaces. Ask for the wet figure, because the dry one flatters everything.

The transparency question

This is where people push back. A fritted panel is not clear glass; you see light and shapes rather than detail.

A completely clear walk-on panel is unnerving to stand on and slippery in rain. Most people who insist on one come back to the question after the first wet day. On a light well over a basement, the frit also softens the view upward, which is usually welcome from below.

Why walk on glass needs one at all

Wet glass is dangerous underfoot. That is the whole reason the treatment exists, and it is why it is part of the specification rather than a finish option.

A walk on rooflight sits flush in a terrace surface, outdoors, exposed to rain, dew and frost. Without a slip resistant surface it would be a hazard several months of the year, and no amount of care compensates for a surface with no grip.

It also has a consequence people should know about before ordering: the treatment obscures. A walk on unit reads as a luminous panel rather than a window, and that is the trade for a floor you can stand on.

How a fired frit is made

The more durable of the two methods, and what we would specify on any route people use daily.

A ceramic based ink is screen printed onto the glass surface in a pattern, and the glass is then heated for toughening. At toughening temperature the ceramic fuses into the surface of the glass itself.

The result is not a coating sitting on top of the glass. It is part of the pane, and it has effectively the same life as the glass beneath it. It will normally outlast the sealed unit.

Because it is applied as a printed pattern, the coverage and the pattern can be varied: dots, lines, squares, or a proprietary arrangement. Higher coverage gives more grip and more obscuration.

Fritted patterns are slightly more visible from below than etched ones, which some people prefer because it makes the panel read more clearly as a floor.

Acid etching and sandblasting

Mechanical or chemical roughening of the glass surface, producing a texture rather than an applied pattern.

Also permanent in the sense that it is a modification of the glass rather than a coating, and it wears more visibly than a frit on a heavily used route, because the peaks of the texture abrade first and the surface gradually polishes.

It reads as a uniform frosted appearance rather than a defined pattern, which some people prefer aesthetically.

Applied coatings, and why we avoid them

Anti slip products exist that are applied to finished glass as a coating or a film.

They are not durable in this application. A surface being walked on outdoors, abraded by grit and exposed to ultraviolet and frost, is a hostile environment for anything sitting on top of the glass rather than in it.

They are also not the specification the manufacturer tested. A walk on unit is rated as an assembly, and altering the surface afterwards puts you outside that.

Where somebody proposes an applied anti slip treatment on an existing rooflight to make it walkable, that is a different and more serious problem: the unit either has a walk on load rating or it does not, and a surface treatment does nothing about the loading.

How slip resistance is measured

Tested and rated in the same way as flooring, with the ratings quoted for both dry and wet conditions.

Wet is the figure that matters on an external terrace. A properly specified walk on unit compares with decent external paving when wet, which is the sensible benchmark: not carpet, and not a hazard.

Ask for the rating in writing alongside the load rating. Both should be available for the specific product.

Choosing the coverage

A genuine decision rather than a default, and it trades grip against light.

Heavier coverage gives more grip and obscures more. Right where the panel sits on a route people cross in all weathers, and right where privacy from below matters.

Lighter coverage lets more light and more visual detail through, and gives less grip. Defensible where the unit is over a formal room, is rarely walked on, and the daylight matters more.

Where the two requirements conflict, take the grip. A slippery glass floor is a genuine hazard and a slightly darker room is not.

Where privacy also matters, the two requirements point the same way, which is convenient.

What wears it

Grit underfoot, by a wide margin, and it is not the weather or the cleaning method.

Sand, soil and gravel carried across the surface on shoes act as an abrasive under body weight. Every crossing does a small amount of damage and on a terrace used daily it accumulates.

A doormat at the door onto the terrace does more for the life of an anti slip surface than anything else available.

After that: dragged furniture, particularly metal feet; sweeping soil or gravel across the pane; and the wrong cleaning method.

Cleaning without damaging it

Warm water, a mild detergent, a soft brush or microfibre mop, rinsed thoroughly so no detergent film is left. A film is slippery and defeats the purpose.

Never wire brushes or scouring pads, which abrade the treatment directly.

Never a pressure washer at close range, which damages the surface and drives water into the perimeter seal. On a flush unit that seal is the whole defence against water ingress.

Never solvents or abrasive creams.

Algae, which no pattern compensates for

On a shaded terrace, or under trees, organic film builds up on the glass and it is genuinely slippery regardless of the treatment beneath it.

Regular cleaning removes it. Leaving it means a surface that looks perfectly acceptable and is hazardous.

On a north facing or overshadowed terrace, cleaning frequency needs to be higher than on an open sunny one, and that is worth knowing at the design stage as well as afterwards.

When it has worn out

The treatment cannot be renewed in place. Polishing out a worn area would remove the pattern, which is exactly what you do not want.

Where a surface has worn to the point where it is slippery when wet, the unit is replaced. That is a like for like swap into the existing structural opening, provided the opening and the drainage were built correctly, and it is considerably cheaper than the original installation because the structural work is already done.

The way to postpone that by twenty years is a doormat, a soft brush and a clear perimeter drainage channel.

The three methods, and how permanent each is

Fired ceramic frit. A ceramic ink is printed onto the glass surface and then fired during the toughening process, at which point it fuses into the glass itself. It cannot wear off, cannot be scraped off, and does not need renewing. This is the specification to insist on for any external or wet application.

The pattern is printed, so it can be dots, lines, a grid or a custom design, and the coverage is chosen to balance grip against light.

Acid etching or sandblasting. The surface is textured by removing material rather than adding it. Permanent, and it diffuses light and view more comprehensively than a frit pattern, which is a benefit where privacy matters and a drawback where you want the view down.

Applied coatings. A textured layer bonded to the surface after manufacture. Cheaper, and it wears under foot traffic. On a terrace that means renewing it periodically at height, which is a recurring cost and a recurring temptation to defer.

The light trade, and how to manage it

Every treatment reduces transmission and obscures the view through. On a unit that is the only daylight source for a basement or a room below, that is a real cost.

Full coverage frit gives excellent grip and passes noticeably less light. A pattern of dots or lines gives grip where feet land while leaving clear glass between, and that is usually the better balance.

Ask to see a sample rather than choosing from a specification. The difference between a dense pattern and an open one is obvious in the hand and not obvious on paper.

What the pattern does besides grip

Two further jobs, which is why it frequently earns its place three times over.

Privacy. A pattern dense enough to break up detail resolves the sightline in both directions without eliminating the light.

Vertigo. Some people find looking straight down through clear glass genuinely unpleasant, and children particularly so. Obscuring the view down makes a glass floor comfortable to use.

It also hides wear. Fine scratching from grit is far less visible on a textured surface than on clear glass.

Ratings, and what to ask for

Slip resistance is measured by recognised test methods, wet and dry, and manufacturers publish results for their treatments.

The wet figure is the one that matters outdoors. Ask for it specifically rather than accepting a general assurance that the surface is non slip.

The thing that undoes all of it

Algae. Organic film on glass is far more slippery than the glass itself, and on a shaded terrace it builds up within a season.

No treatment survives being covered in it. Regular cleaning is part of the slip resistance strategy, not separate from it.

Asking for the wet test figure

Slip resistance is measured wet and dry by recognised methods, and manufacturers publish results for their treatments.

The wet figure is the one that matters outdoors. Ask for it specifically rather than accepting a general assurance that the surface is non slip.