Walk-on glass is structure that happens to be transparent

The difference in one sentence

An ordinary rooflight is designed for wind, snow and its own weight. A walk-on unit is designed for all of that plus people standing on it, and people are a concentrated, moving, unpredictable load in a way weather is not.

That changes the thickness, the number of plies, the support conditions, the deflection limits and who signs it off. Almost nothing carries over.

Glass as a structural material

Glass is strong and it is brittle, and the second word is what governs the design.

It has excellent compressive strength and good tensile strength in theory, and in practice its strength is limited by surface flaws: microscopic imperfections and edge damage where a crack can start. Once a crack starts in glass it propagates, because there is no plastic deformation to absorb the energy.

Steel bends before it breaks and gives warning. Timber creaks. Glass does neither. It is fine and then it is not, which is why the design approach is different: not “how much can it take” but “what happens when a ply fails”.

Laminated construction, and why more than two plies

A walk-on unit is a laminate of several plies of glass bonded together with structural interlayers.

The interlayer is doing two jobs. It bonds the plies so they act together rather than as separate sheets, which is what gives the assembly its stiffness. And it holds broken glass in place if a ply fails, which is what gives it post-breakage behaviour.

Ordinary overhead glazing is laminated on the inner pane for that second reason. A walk-on unit is laminated for both, with more plies and stiffer interlayers, because it has to remain safe with one ply gone.

The sacrificial top ply

The design idea that makes a glass floor sensible, and it is worth understanding.

The uppermost ply is treated as sacrificial: it is the surface people walk on, it takes the scratches, the grit, the dropped objects and the wear, and it is not counted in the structural capacity.

Which means if it cracks, the unit is still a floor. It looks alarming, and the plies below it are carrying the load as designed and the assembly has not failed. That is a serviceability problem to be dealt with rather than an emergency.

It is also why a walk-on unit that has taken damage should be assessed rather than assumed to be either fine or finished.

Post-breakage behaviour

The property that separates a safe glass floor from a dangerous one, and it is not the same as strength.

The question a designer asks is what state the unit is in after something has gone wrong, rather than how much load it takes on a good day. A build-up that fails completely when one ply breaks is unacceptable overhead, whatever its rated capacity.

A properly specified walk-on unit retains load capacity with a ply broken, so that a floor with a cracked top surface remains a floor while it is dealt with. That is what the multiple plies and the structural interlayers are for.

Heat treatment, and the trade in it

The counterintuitive part is that the strongest is not automatically the best choice for every ply. Toughened glass fails into small fragments that lose interlock, so a build-up of nothing but toughened plies can have poorer post-breakage behaviour than one including heat strengthened glass.

Which is exactly why the build-up is designed rather than selected, and why it is not a matter of asking for thicker glass.

The interlayer matters as much as the glass

Interlayers are not all the same and the difference is structural rather than optical.

A standard laminating interlayer holds broken glass together and is comparatively soft, so the plies slide relative to each other under load and the assembly is less stiff than the sum of its parts. A structural interlayer is much stiffer, so the plies act more nearly as one, and the same glass gives a considerably stiffer floor.

Interlayers also soften with temperature, which matters on a unit in direct sun. A build-up designed for a shaded position and installed on an unshaded south facing terrace is being asked something different.

What it is not

Three things it is worth being explicit about.

• Not a thicker ordinary rooflight. The build-up, the supports and the design basis all differ.
• Not something a rooflight supplier specifies from a catalogue. The loads come from the building and the build-up follows from them.
• Not retrofittable to an existing opening without checking everything under it. The structure has to carry a floor rather than a roof.

The weight, before anybody stands on it

A practical point that surprises people. A walk-on unit is several plies of thick glass. Its own dead weight is a large multiple of an ordinary rooflight of the same size, and that load is permanent, sitting on the supports and the structure beneath from the day it goes in.

So the structural question begins before the imposed load is considered at all. On a retrofit into an existing opening, the dead weight alone frequently requires the supports to be reworked.

Handling it

Follows directly, and it decides how some jobs are done.

A walk-on unit of any size is beyond hand handling, which means lifting equipment, a crane, or glass handling plant with suction lifters. That needs access, space to set up, and sometimes a road closure.

On a terraced house in Brighton or a town centre property in Worthing, the access arrangement for the unit is frequently the largest single element of the cost, and it is established before anything is designed rather than after.

Frameless, framed and flush

Three arrangements, and they are structurally different.

A framed unit sits in a perimeter frame that carries it, in the way a rooflight does. A flush unit is set so its surface is level with the surrounding deck, which is what most people want and which puts the frame below the walking surface. A frameless unit is bedded directly onto a structural upstand with no visible frame at all.

Each has different bearing arrangements, different drainage implications and different edge details, and the appearance decision has structural consequences rather than being a finish.

Where the load actually goes

Worth following through, because every element in the chain has to be checked.

Person to glass. Glass to its bearing on all four edges. Bearing to the upstand or frame. Upstand to the structure around the opening. That structure to whatever carries it, which on a terrace deck is a floor and on a rooflight is a roof.

A unit correctly specified onto a structure that was never designed for floor loads has moved the problem rather than solved it. Which is why the design starts at the bottom of that chain rather than at the top.

Why the glass is the easy part

A useful observation about where the difficulty sits.

Specifying a glass build-up for a given load and span is a solved engineering problem, and a competent specialist does it routinely. What varies from job to job, and where the errors are, is everything else: the supports, the structure beneath, the drainage, the edge details, the access.

So a conversation that is entirely about the glass is a conversation that has not started yet.

Thermal performance, which is the other half

A walk-on unit has to be a rooflight as well as a floor, and the two requirements pull against each other.

The structural build-up wants thick laminated glass in several plies. The thermal requirement wants a sealed cavity with a low emissivity coating and a gas fill. Combining them means a sealed unit whose outer pane is the walk-on laminate, which is thick, heavy and expensive.

Some walk-on units are single glazed laminates, which is entirely appropriate over an unheated light well or an external space and is a thermal weak point over a heated room. Establishing which side of that line the unit sits on is part of the brief, because it changes the product substantially.

Condensation on a floor

A consequence of the above that catches people out.

Where a single glazed walk-on laminate sits over a heated room, its inner surface is cold, and the room’s humidity finds it. Condensation on the underside of a glass floor drips into the space below, which is precisely the room somebody put a rooflight in to make pleasant.

Over any heated space the unit is an insulated one, and the perimeter detail matters as much as it does on any other rooflight: insulation carried to the frame, and the vapour control layer sealed to it.

Optical honesty

Something to set expectations about, because it is inherent rather than a defect.

Several thick plies with interlayers between them is a considerable thickness of glass, and glass has a colour. Looked at from directly above, a walk-on unit reads slightly green or grey compared with a single pane, and the edge is visibly deep.

Low iron glass reduces the effect substantially and costs more. On a unit that people will look through at close range, particularly over a light well, it is worth specifying.

What we do and what we do not

Being clear about the line, because on this product it matters.

We install and replace rooflights, and we work to a specification for the glass build-up and the supports that has come from an engineer or from the manufacturer’s technical department against the actual loads. We do not decide the build-up ourselves, and anybody who offers to do that from a catalogue should be asked what loads they assumed.

What we bring is the installation: the opening, the supports, the bearing, the weathering, the drainage and the detailing, all of which have to be right for the specified unit to perform as designed.

Building Control

Forming an opening is notifiable, and a structural glass floor is structural work.

Which means calculations, a specification, and inspection. Where the unit forms part of a floor rather than a roof, the requirements for guarding, for the surface and for loading all come into it as well.

We notify Building Control where the work is notifiable and the completion certificate comes to you. On structural glass that certificate matters more than usual, because it is the record that the thing somebody stands on was designed rather than chosen.

What we settle first on walk on glass

We establish the loads and the structure before discussing any product, work to an engineered specification rather than a catalogue selection, and check the whole load path rather than the glass alone. We say plainly when an existing opening will not carry a walk-on unit without work beneath it, which on a retrofit is frequently the finding.

It is a structural element you can see through. Everything else follows from taking the first three words seriously.