Load ratings, and what a walk-on unit is rated for

Distributed against point loads

Two different things, and both are checked.

Distributed load is weight spread over the whole panel, expressed in kilonewtons per square metre. Domestic foot traffic is typically assessed at 1.5 kN per square metre; a balcony or terrace at 2.5 or more.

Point load is weight concentrated in a small area, which is the harder case. A person standing on one heel, or a planter on small feet, concentrates a lot of force into a few square centimetres.

Glass is far more sensitive to point loads than to distributed ones, which is why a panel that carries several people comfortably can be damaged by a dropped tool.

Be honest about the use

The rating is chosen from what will actually happen, not what is described at the survey. Worth mentioning:

• Furniture, and whether it has small feet
• Planters, which are heavy and permanent
• Whether anyone will stand on it to clean windows above
• Whether it forms part of a route used daily or an area crossed occasionally

It cannot be uprated afterwards. Increasing the rating means a new unit and usually new structure.

What it is not rated for

Vehicles, unless specifically designed for it, which is rare and expensive domestically. Scaffold legs or props bearing directly on the glass, which happens more often than it should during later building work.

If work is planned above a walk-on unit, protect it properly rather than trusting the rating.

Redundancy

The safety case is not that the glass will not break. It is that if the top ply breaks, the plies beneath still carry the load. That is why the build up cannot be thinned.

The figure that matters

Not the thickness of the glass and not its appearance. The manufacturer’s stated imposed load, in kilonewtons per square metre, for the specific unit at the specific size.

Everything else about a walk on rooflight, the thickness, the number of plies, the frame and the structure beneath, is a consequence of that number rather than a specification in its own right.

If a supplier cannot give you the imposed load in writing for the size you are ordering, you do not have a walk on unit; you have a thick piece of glass.

The three levels

Standard rooflight. No traffic rating at all. Designed to resist wind and snow and to keep weather out. Nobody should stand on it.

Non fragile. A safety specification rather than a traffic one, tested by dropping a weighted bag onto the assembly. It means a person falling onto it will not go through.

It is a life safety requirement and it costs a fraction of walk on glazing. It is not permission to walk on the unit: it means it will not kill you if you fall.

Walk on. Designed for repeated intentional foot traffic as a floor, to a stated imposed load, with an anti slip surface.

The gap between the second and third is where the money is, and it is why establishing what the roof is actually for matters before anything is priced.

Domestic against commercial loading

A domestic terrace is designed to a lower imposed load than a public or commercial route, and the difference is not marginal.

A private balcony or roof terrace serving one dwelling is one figure. A shared terrace serving several flats, or any route the public can use, is higher, because the density of people is higher and because the consequences of failure are different.

Where a terrace serves a flat conversion, or where a walkway crosses a rooflight in a commercial building, the domestic figure is not the right one. Getting that wrong is a serious matter rather than a specification quibble.

Tell the supplier what the space is used for and by whom. It changes the product.

Point loads, and what the rating does not include

This is the part that catches people after the installation.

An imposed load figure describes a distributed load, plus a separate point load representing a person: applied, moved and removed.

It does not describe a heavy planter full of wet soil standing in one place for five years. It does not describe a stone bench, a water feature, a barbecue or a hot tub.

A sustained point load in a fixed position is a different case entirely, and it has to be designed for rather than assumed. Where anything heavy is going to live on the terrace, say so before the unit is specified: a higher loading can be designed for, and doing it at the outset costs a fraction of the alternative.

Where a hot tub is contemplated, that is a structural design exercise in its own right regardless of what the glass can do.

Vehicles are a different product

Stated separately because it is the most serious of the confusions.

Vehicle loading imposes wheel loads an order of magnitude higher than foot traffic, concentrated on a small contact patch, applied dynamically and repeatedly. A wheel turning while stationary applies a twisting load that a rolling wheel does not, and a driveway is exactly where that happens.

Vehicle rated glazing exists and it is an engineered product with its own build up, its own frame and its own structural design.

A pedestrian walk on unit must never be used where a vehicle will cross it, even occasionally, even a light one. We will not fit one in that situation.

Why the glass is laminated rather than simply thick

Failure behaviour, and it is the whole safety case.

Toughened glass, when it breaks, breaks completely and instantly into fragments. A single thick toughened pane used as a floor would go from intact to gone in a moment, which is unacceptable in something people stand on.

A laminated build up of three or more toughened plies bonded with structural interlayers behaves differently. If one ply breaks, the interlayer holds the fragments and the remaining plies carry the load. The surface stays in place as something you can walk off, and the damage is visible so it can be dealt with.

That is why nobody makes a monolithic walk on rooflight, and why total thickness of 50 to 70mm is normal once the sealed cavity and inner pane are added.

Deflection, which is as important as strength

A walk on unit is designed to tighter deflection limits than a roof would need.

A structure that sags slightly under load is entirely acceptable for a roof. It is not acceptable for a glazed floor, because the glass has to sit on a support that does not move: movement at the perimeter loads the panel unevenly and stresses the seal.

That is why the engineer’s design for the structure below matters as much as the glass specification, and why it is frequently the largest line on a retrofit quotation.

Span, and why cost rises steeply

Thickness rises faster than area does.

A larger pane needs a thicker build up to carry the same load over a longer span. Thicker glass is heavier, which increases the dead load, which feeds back into the structure below and into how the unit gets onto the roof.

So doubling the size of a walk on unit more than doubles the cost, and it may change the access requirement from several people to a crane.

The anti slip surface is part of the specification

Not an optional finish. Wet glass without it is genuinely dangerous, and it is what makes the difference between a floor and a hazard.

Slip resistance is tested and rated in the same way as flooring, and a properly specified unit compares with decent external paving wet.

A ceramic frit fired into the glass during toughening is more durable than an etched pattern and will normally outlast the sealed unit. Coverage varies, and heavier coverage gives more grip and more obscuration from below.

It is a wearing surface. Grit underfoot abrades it, and it cannot be renewed in place: where it has worn to the point of being a hazard, the unit is replaced.

What to ask for in writing

The imposed load rating for the specific unit at the specific size.

Confirmation that it matches the intended use, domestic or otherwise.

The slip resistance rating and the anti slip type.

Whether any sustained point loads have been allowed for.

And the structural engineer’s design for the supporting structure, which is a separate document and one to keep with the house paperwork alongside the Building Control certificate.

Distributed and point loads are different problems

A rooflight datasheet gives a load in kilonewtons per square metre. That is a distributed load: weight spread evenly across the whole pane, which is what snow and wind pressure do.

A person is a point load. All their weight arrives through two small contact patches, and the stress in the glass beneath a heel is far higher than the same weight spread across the pane.

So the distributed figure on a standard rooflight datasheet does not answer the question of whether it can be walked on. It is answering a different question.

The stiletto case

Engineers use a small, concentrated load case deliberately, because it is the worst realistic point load a domestic floor sees.

A stiletto heel, a chair leg, a dropped tool. Each concentrates force on an area of a few square millimetres, and the glass has to survive it without cracking the walking layer.

That case frequently governs the design of the top layer, while the span and distributed load govern the structural plies beneath.

Dynamic loading

Somebody jumping and landing delivers considerably more than their static weight, for a brief moment.

Children run and jump. If children will use the terrace, that belongs in the design case rather than being discovered later, and the honest answer at design stage costs far less than the alternative.

Deflection, which is a separate requirement

Strength and stiffness are not the same thing.

A unit and its supporting structure can be strong enough and still flex noticeably underfoot. That feels unsafe, which affects whether the terrace gets used, and it works the perimeter seal every time somebody crosses.

A sealant joint flexed thousands of times fails, and that is how a structurally sound installation starts leaking.

Engineers therefore design to a deflection limit as well as a strength requirement, and it is why an apparently adequate existing structure sometimes still needs strengthening.

Non fragile is not a walk on rating

The distinction that causes accidents.

Non fragile is a fall protection classification, tested as a single accidental impact. 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. Walked on regularly, a non fragile rooflight will fail, and it will fail at the seal or the frame long before the glass does.

What to ask for

The design load case in writing, stating the distributed load, the point load, and the deflection limit, for the actual use you have described.