Walk-on glass: loads and structure

Engineers describe the weight on a floor in two ways, and a walk-on unit is checked against both. The first is a spread load: people, furniture and planters across the whole surface, written in kilonewtons per square metre. The second is a point load: a single concentrated force, written in kilonewtons, such as a table leg, a ladder foot or a heel. A pane can cope with a generous spread load and still be marginal under a point load if the glass is thin or the support is far away.

Neither figure is a property of the glass alone. A manufacturer designs a unit for a stated span between supports, a stated edge bearing and a stated use. Change the span, or rest the edge on something softer, and the rating no longer applies. This is why a rating quoted on a catalogue sheet cannot be read straight across to your opening.

Who sets the figure for your job

The use of the surface sets it. A hallway floor that two adults cross is a lighter duty than a terrace where a dozen people stand for a party. We ask who and what will be on the glass, then pass the opening size, the finished surface and the supporting structure to the manufacturer and, where the structure is changing, to an engineer. The rating that comes back is written into your quote, so you can check it matches what you told us.

A walk-on unit is built up from several sheets of toughened or heat-strengthened glass, bonded with clear interlayers into one laminated panel. Each sheet carries part of the load. The interlayers share the force between them and, if the top sheet is scratched or chipped, the sheets below keep the panel working. That is why the top layer is regarded as the sacrificial one: it takes the wear so the structural layers stay intact.

Thickness goes up with span and with duty. A small panel over a short gap needs fewer or thinner layers than a long rectangle bridging a wide opening. We do not quote a thickness from memory, because it changes with each opening. If you are curious how deep a typical build-up becomes, our short answer on how thick walk-on glass is goes through it.

The surface you step on

The top face is treated so shoes grip. A fritted pattern is fired onto the glass as a grid of dots, an etched face is roughened by acid, and a textured finish is applied in manufacture. Each changes how much light passes, how the glass feels underfoot when wet and how easily it is cleaned. A finer pattern lets through more light but offers less grip, so the choice depends on whether the glass is under cover or outdoors in rain and frost.

Anything that works as a floor should be specified as slip-resistant for its location. We confirm the finish with the manufacturer at the same time as the load, so the two decisions are made together.

Glass hands its weight to a frame, and the frame hands it to whatever it sits on. In a flat roof that is usually timber joists, in a ground floor it may be a concrete slab or a masonry upstand, and in an upper floor it can be a steel beam or a timber trimmer. Each carries a point or line load differently, and each has its own limit before it moves.

The simplest mistake is to treat the opening as a hole in a surface that was already strong enough. Cutting a joist removes a piece of the structure, and the timbers either side of the gap take what it used to carry. Where the opening runs across several joists, trimmers or a beam are designed to carry it round into the walls. That design can be done by calculation from the survey measurements, and on a bigger job a structural engineer is brought in. Any strengthening shows on your quote as its own line.

Movement and tolerance

Timber shrinks and floors flex a little, and laminated glass cannot follow that movement in the same way. If the supporting edge moves more than the glass can accept, the unit takes load unevenly at the corners. For that reason the bearing is packed and the rebate sized to allow small movement, and floors that are known to bounce are stiffened before glass is fitted. We look for signs at survey: dips in the floor, cracks in plaster below, a ceiling that has sagged.

Whether the glass sits proud of the surface or level with it changes what the waterproofing and drainage have to do. A kerb lifts the frame clear of rain running across a membrane, which suits a flat roof. A flush frame leaves the glass almost level with paving or decking and removes the step, but water must be led away at the edge, and the junction between frame and surface has to stay sealed under foot traffic.

Walking on a kerb is a choice too. A low kerb is easy to step over once you know it is there, and a person who steps backwards onto it can be caught out, so on a busy terrace the flush route suits better. Over a lightwell the kerb may be the safer arrangement, since it stops debris being swept onto the glass.

Dirt, leaves and what lies on the glass

Anything that collects on the surface is extra load and extra slip risk. Leaves rot, grit scratches, and standing water freezes. A gentle fall across the glass or a drainage channel beside it keeps the surface clear. We note the fall at survey and decide the detail with the surrounding surface in mind.

Condensation gets a mention here too. A sealed double or triple unit with the right coatings limits it on the glass face, though a damp lightwell or a poorly vented void below can still show it on the underside. That is a drainage and ventilation matter for the space.

A new opening in a floor or flat roof is building work, so the structure and the energy performance have to meet the Building Regulations. Approved Document L sets a limit of 2.2 W/m²K for a rooflight on an existing dwelling. The council's building control team or a registered building control approver checks the work. We handle the Building Control notification where the work is notifiable, and the paperwork comes back to you at the end.

Planning is a different question and depends on the position. A unit set flush in a garden terrace may sit outside the rules that govern roofs altogether, whereas one in a roof is looked at against the Class C permitted development conditions. Flats, maisonettes and some converted buildings have no such rights. Our guide to walk-on rooflight Building Regulations covers the approvals in detail, and the skylight planning permission guide covers planning.

Glass as a guarded edge

When a person can stand on the glass, the edge of the drop beside it is part of the safety picture. A basement lightwell with an open edge, or a floor panel next to a stair, may need a guard or a rail. We look at the surroundings at survey and raise it before ordering.

Even a well-rated unit has a working life in daily use. Scratches build up where grit is dragged across the top layer, and a scratched surface can still be safe, but a deep chip or a visible crack in any layer is a reason to keep people off it and call us. We never advise walking on glass that is cracked, whatever its rating says.

Cleaning is straightforward. Warm water and a soft cloth or mop remove dirt. Abrasive pads, scrapers and strong solvents harm the surface and the seals, so a gentler method is better. Leave the drainage channel clear and check the frame joint once a year for gaps or lifting sealant.

What not to put on it

Think about heavy items that bring a point load: a stone planter on small feet, a barbecue, a tall ladder resting on one rail. Move heavy furniture by lifting it, never dragging it across the glass. If you plan something heavy for the surface, ask us before you buy it, and we will check it against the rating on your quote.

Lighting and heat matter a little. Strong sun through the unit on a hot day warms the room below, and a solar control coating cuts part of that. A decision about the coating is best made while the rating is being decided, since both depend on the same specification.