Walk-on against non-trafficked specification

What differs

Glass build up. A standard unit is toughened outer, cavity, laminated inner, around 28mm. A walk-on unit has a laminated stack of several toughened plies, 40mm to 60mm, designed so that if the top ply breaks the ones beneath still carry the load.

Load case. Standard units are designed for distributed wind and snow loads. Walk-on units are designed for concentrated point loads, which is a different calculation entirely.

Frame and bearing. A walk-on frame supports the glass continuously around its perimeter to much tighter tolerances.

Structure. Sized for several times the load and to tighter deflection limits.

Surface. Anti slip treatment fired into the glass.

Why it cannot be upgraded

Because the difference runs from the glass down through the frame into the structure. Fitting a walk-on panel into a frame and structure designed for a standard unit fails at whichever link is weakest, and the panel is not the weakest link.

Deciding after the structure is built means rebuilding it.

The occasional access question

“Somebody might need to stand on it once a year to clean the windows above.” That is a genuine requirement and it deserves a genuine answer.

Usually the answer is not a walk-on panel but planning how those windows get cleaned: a pole system, or leaving a walkable margin of roof beside the rooflight.

Being honest at survey

Say what will actually happen on it, including the once a year cases. It costs nothing to specify correctly at the start and a great deal to correct afterwards.

Three specifications, and they are not interchangeable

Almost every problem we see with glazing in a walkable roof comes from confusing three products that look similar from below and are engineered for completely different things.

Standard rooflight. Designed to keep weather out and let light in. It resists wind and snow. It is not designed to carry a person and nobody should stand on it, even briefly, even to reach a gutter.

Non fragile. A safety specification, tested to a recognised standard by dropping a weighted bag onto the unit. It means a person falling onto it will not go through. It is a life safety requirement, not a traffic rating, and it costs a fraction of walk on glazing.

Walk on. Designed for repeated, intentional foot traffic as a floor, to a stated imposed load, with an anti slip surface. This is what goes into a terrace over habitable space.

The gap between the second and third is the one that matters commercially. Non fragile means it will not kill you if you fall on it. It does not mean you can use it as a step.

Why non fragile is the right answer far more often

Most flat roof rooflights are on roofs nobody uses. They sit over a kitchen extension, a utility room or a hallway, on a roof that is visited twice a year for gutter clearance and otherwise ignored.

On a roof like that, the risk to manage is not somebody walking across the glazing deliberately. It is somebody working on the roof stepping backward, slipping, or putting a hand out.

Non fragile addresses exactly that risk, at a modest premium over a standard unit, and it should be the default specification for any flat roof rooflight rather than an upgrade.

Walk on glazing on the same roof would be several times the price, would need structural work the roof does not otherwise require, and would deliver nothing the situation actually calls for.

The old dome problem

Falls through fragile rooflights are a recognised cause of serious injury in roof work, and the specific hazard is almost always the same: an ageing polycarbonate dome.

Multiwall polycarbonate degrades under ultraviolet. It yellows, it becomes brittle, and it loses impact resistance progressively over fifteen or twenty years. Critically, it looks essentially the same the day before it fails as the day it was fitted. There is no warning.

Recognised guidance for roof work therefore treats older rooflights as fragile unless there is evidence to the contrary, which is a sensible default for a homeowner too.

If your roof has domes of unknown age and anybody needs to work up there, replacing them with a modern non fragile glass unit improves safety, security, thermal performance and appearance in one job, and it is among the cheaper things on a roof to put right.

What fragility is not

It is not a permanent property of a product. A unit that was non fragile when installed may not be now, because plastics degrade, seals fail and fixings corrode.

The rating describes the assembly as installed, including the frame and the fixing, not just the glazing. A non fragile pane in a poorly fixed frame is not a non fragile rooflight.

When walk on genuinely is needed

One situation, really: a terrace, balcony or roof surface that people use, over habitable space with no other route for daylight.

There it is the answer and it is worth what it costs. The alternative is a dark room below, and no amount of clever lighting substitutes for daylight in a space people occupy.

Before committing, three questions are worth asking honestly.

Will anyone actually walk on it? If the answer is only for maintenance, twice a year, that is a non fragile requirement and an access planning question rather than a glazing one.

Does the glazing need to be in the walking route? A terrace rarely needs glazing across its whole width. Moving the panel to the edge, or to a part of the terrace outside the through route, frequently turns an expensive engineered installation into an ordinary rooflight.

Could the room below be lit from somewhere else entirely? Worth half an hour with the plan before it is worth a quotation.

What walk on specification involves

The glass. A laminated build up, typically three or more plies of toughened glass bonded with structural interlayers, over a sealed cavity and an inner pane where the unit also has to insulate. Total thickness commonly 50 to 70mm.

Lamination rather than a single thick pane is about failure behaviour. Toughened glass, when it breaks, breaks completely and instantly. A laminated stack that loses one ply retains the others and stays in place as a surface you can walk off, with visible damage so it can be dealt with. That is the entire safety case.

The surface. An anti slip treatment, either a ceramic frit fired into the glass during toughening or an etched pattern. Fritted is more durable and slightly more visible from below. It is a wearing surface and it is what makes the difference between a floor and a hazard when wet.

Expect the pane to read as slightly textured and slightly obscured from below. That is the trade for a floor you can stand on, and if a clear view of the sky is what you want, this is the wrong product.

The structure. Designed to a stated imposed load with tighter deflection limits than a roof would need, because glass has to sit on a support that does not move. On a retrofit that means an engineer and frequently steel, and it is usually the largest line on the quotation.

The drainage. A walk on unit sits flush, because a raised kerb in the middle of a terrace is a trip hazard. That removes the usual defence against standing water, so everything depends on the surrounding surface falling away from the unit and a perimeter channel discharging into a working outlet.

The thermal detail. A flush unit over heated space is a cold bridge at the perimeter unless it is insulated and the vapour control layer is continuous and sealed to the frame. Get that wrong and you have condensation on the ceiling below in a position that is very awkward to correct.

Loadings are not one figure

A domestic terrace is one imposed load. A commercial or public route is higher, with different safety requirements. Vehicle loading is an entirely different product, with wheel loads an order of magnitude higher, applied dynamically and repeatedly, and with turning as well as rolling to account for.

A pedestrian walk on unit must never be used where a vehicle will cross it, even occasionally, even a light one.

And no walk on specification anticipates a heavy planter or a furniture leg standing permanently on one spot. A sustained point load in a fixed position is a different case from people walking across, and it has to be designed for rather than assumed.

What we will and will not do

We will specify non fragile as standard on flat roof rooflights, because it costs relatively little and removes a hazard for twenty five years.

We will fit walk on glazing where there is a genuine terrace over habitable space, and we will have the structure checked before anything is ordered.

We will not fit a standard rooflight where somebody intends to walk on it, and we will not certify an existing unit as trafficable because it looks robust. The specification is either there in writing from the manufacturer for the loading it will actually see, or it is not, and no amount of experience substitutes for that.

Where somebody asks for walk on glazing and the situation calls for non fragile, we will say so, because the difference in price is substantial and there is no reason to spend it.

Two products, not two grades of one

A non trafficked rooflight is designed for wind, snow and its own weight. Those are distributed loads spread evenly across the pane, and the datasheet figure in kilonewtons per square metre describes them.

A walk on unit is designed as a floor. Its build up comes out of a structural calculation for the specific installation: the span, the design load for the actual use, the point load case, and a deflection limit.

They are not the same product with different glass thicknesses. The structural basis is different, the frame is different, and the surrounding construction is different.

The non fragile confusion

The misunderstanding that causes accidents, and it is worth stating plainly.

Non fragile is a fall protection classification. It is tested as a single accidental impact, and it means the unit will not let a body fall through if somebody working on a roof steps back onto it.

It is not a permission to use the unit as a floor. A non fragile rooflight walked on regularly will fail, and it will fail at the seal or the frame long before the glass does.

Anyone offering a non fragile unit as an economical alternative to a walk on unit is describing a different rating.

What else changes with the specification

Weight, substantially, which drives the supporting structure and frequently the lifting arrangements.

The frame, which on a walk on unit is buried below the walking surface in a permanently wet zone and needs corrosion resistance and a thermal break accordingly.

Slip resistance, which is not a consideration on a roof nobody walks on and is mandatory on one they do.

Drainage, because a walk on unit sits in a horizontal surface where water collects rather than on a roof where it sheds.

Structure, designed to a deflection limit rather than merely a strength requirement.

How to decide

Ask whether the surface will genuinely be used. A terrace people sit on, a courtyard people cross daily, or a route to the back of the plot: walk on.

A roof accessed twice a year for maintenance: non trafficked, with the maintenance access provided by something other than the glass.

Specifying a walk on unit against a possible future terrace, without designing the structure for it, buys the cost without the capability.