Walk-on rooflight glass specification

An ordinary rooflight is designed to shed rain, snow and the occasional person leaning on a ladder against it. It is not designed to be stood on. Walk-on glass has a different job: it is a floor that happens to be transparent, and it has to keep carrying people for years after the day it is fitted.

Three ideas sit behind every specification. The first is load: the glass is checked against the weight it will carry, both spread across the area and concentrated at a point such as a heel. The second is failure behaviour: if a layer cracks, the rest of the assembly has to stay in place and keep supporting the person standing on it. The third is the surface: it has to stay grippy, readable and clean without scratching into a hazy patch within a couple of winters.

Glass is strong in compression and unforgiving of surface damage, which is why the build matters more than raw thickness. A thin sheet with a poor edge is more of a risk than a well-supported laminated stack. For the supports and frames that sit under the glass, walk-on rooflight sizes and supports covers how the opening is held.

Most walk-on units follow the same order, even though the details change. Reading them top to bottom tells you what each layer is for.

Wear layer

The top sheet is toughened glass, often with a treated surface for grip. It takes scuffs, grit and cleaning. It is the layer you are most likely to mark over time, which is why it is designed to be replaced as a sacrificial surface on some systems.

Structural laminate

Below the wear layer sit two or more sheets of toughened or heat-strengthened glass bonded with interlayers. This laminate carries the load across the opening. If one sheet breaks, the interlayer holds the fragments and the remaining sheets keep carrying.

Sealed cavity

A gap, sealed at the edges with a spacer, separates the load-bearing top stack from the lower pane. It is filled with an inert gas and holds the insulation value of the unit.

Room-side pane

The lowest layer is a laminated pane, often with a low-emissivity coating on one face. It adds insulation, and it is the layer you see from the room, so it also catches anything that falls through if the top stack were ever damaged.

A full unit is usually referred to as triple or quadruple layered, but the count tells you very little. What matters is what each layer is made of, how thick it is, and what it is bonded with.

Three terms turn up on every specification, and they describe different things.

Toughened glass

Annealed glass that has been heated and rapidly cooled, so the surface is held in compression. It is several times stronger than ordinary glass. When it breaks, it breaks into small blunt pieces, which is a safety benefit in a window and a drawback in a floor, because there is little left to carry weight.

Heat-strengthened glass

Treated more gently, so it is stronger than ordinary glass but not as strong as fully toughened. It breaks into larger pieces that tend to stay in the frame, which is useful for a load-bearing layer in a laminate.

Laminated glass

Two or more sheets bonded with a plastic interlayer. The interlayer holds the pieces if the glass breaks, which is the property a walk-on floor depends on. Laminated panes are also what stop a cracked layer collapsing.

The right answer is usually a combination, not one type. Fully toughened glass on the wear surface, heat-strengthened or toughened in the structural stack, and laminated throughout is a common pattern, though the final choice belongs to whoever calculates the unit. It is worth asking which glass is used in each layer, and why.

A glass floor that is smooth and polished can be slippery when wet, and a roof terrace gets wet. The surface of walk-on glass is therefore treated, and the treatment is part of the specification.

The common options are a fired ceramic frit in a pattern, a sandblasted or etched finish, and a textured or acid-etched surface. A frit pattern of dots or lines is printed onto the glass and fired in, which makes it permanent. Etched surfaces give a fine uniform grip but can show cleaning marks, and the dots reduce how much you can see through, which is a trade-off you will notice from the room below. Where the rooflight is outdoors, the slip resistance should be tested for the finish in wet conditions, and the supplier should be able to show a result for it.

How much grip you need depends on where the glass is. Inside a hallway floor, a light etch is often enough. On an exterior walkway, a roof terrace or a path to a door, a stronger pattern is worth having. We cover the choices in more depth in anti-slip glass for walk-on rooflights.

A walk-on unit sits in the roof or floor of a heated room, so it has to meet the same thermal requirements as any other rooflight. Both new and replacement rooflights are held to the limiting U-values of Approved Document L (Volume 1), and that applies to a walk-on unit just as it does to a pitched roof window.

Solar control deserves its own thought. A walk-on rooflight in a south-facing roof terrace can heat the room below in summer, and a tinted or coated layer reduces that at a cost to how much daylight comes through. It is easier to specify it at the start than to add a film later.

Where the glass is part of a flat roof, condensation on the room-side pane is a sign the unit is not performing. A properly specified unit has a pane warm enough on the inside face that it does not mist in ordinary rooms.

There is no single rule that says every walk-on rooflight must use a particular glass. Instead, the glass is specified against structural guidance for glazing that people walk on, and designed by calculation for the loads that apply. Approved Document K, which covers protection from falling, collision and impact, does not set out walk-on glass as a requirement. Suppliers design to the floor loads in BS EN 1991-1-1 (Eurocode 1) and its UK National Annex, often with the Institution of Structural Engineers' guidance on structural glass alongside. The CWCT technical notes TN 66 and TN 67 are sometimes quoted, but they deal with the fragility of glazed roofs reached only for maintenance, and glass made to be walked on sits outside them.

The loads come from how the space is used. A glass panel in a private hallway sees a different load from one that guests will gather around, and an external roof terrace needs a higher allowance again, for crowding and for snow. The supplier should state the design load in writing, along with the span and support conditions it assumes. If the opening is later made larger, or supported differently, that calculation no longer applies.

Do not accept a verbal assurance that the glass is "strong enough". A specification should say what the assembly is, what load it was designed for, and under what support. If you want the load in everyday terms, how much weight can a walk-on rooflight take sets out how to think about it.

Glass is only as safe as its support. A well specified stack on a poor frame is still a poor floor, so the specification has to cover the edge and the bearing as well as the layers.

• The glass should bear on a continuous, level support, set on packers that suit the load, with no hard point-contact between glass and metal.
• The edge of every layer is polished or ground so there are no chips to start a crack.
• The glass should sit in a frame or kerb that is itself sized for the load and for the movement of the building.
• Sealant and glazing tape need to be compatible with the interlayers, since some products attack laminate edges.

The size of the opening drives the rest. A larger span needs thicker layers or a stiffer frame, so it is better to settle the opening first and the glass second. Treat the glass thickness quoted on its own with caution: it is the whole assembly and the support under it that has to work.

A walk-on rooflight is seen from underneath as well as from above, and that changes how the inside is finished. Visitors look up through the glass at your ceiling, so the lining or upstand that frames the opening shows clearly. A well-made walk-on unit can have a clean, flush reveal, but it needs planning: the plaster edge, the bead and the paint line all sit within view of the floor above.

The depth of the build matters too. A floor-level walk-on unit set into a flat roof or terrace has a build depth made up of joists, insulation and the glass itself, and the inside face of that build is part of what you see. Whether the lining runs straight or splayed depends on the depth and the daylight you want to reach the room. The finish schedule that comes with a Brightview quote records the lining shape, the plaster or board, the trim and the paint line before any opening is made, so what people see looking up from the room is agreed on paper, not left to the day.

Dust control matters here as well. Cutting a floor or ceiling opening throws fine grit, so the room is screened and cleaned before the glass goes in, and the unit is protected until the last coat of paint is dry.

In older houses, such as Georgian terraces in St Leonards or Southernhay, the ceiling the opening meets may be lime plaster on lath, which needs support during cutting and lime for making good.

Before you order, a specification for walk-on rooflight glass should answer these points in writing.

1. The overall make-up of the unit: each layer, its glass type and its thickness.
2. The design load and the standard or guidance it was calculated against.
3. The size of the opening, the span and the supports the calculation assumes.
4. The surface treatment, and the evidence for its slip resistance in wet conditions.
5. The thermal performance, the cavity gas and the spacer type.
6. How the glass bears on its frame, and what sealants and packers are used.
7. How a damaged wear layer is replaced, if the system allows it.

A new walk-on rooflight in an existing roof or floor is building work, so it needs Building Control approval, and we handle that notification for you. Cutting or trimming joists around the opening is a structural alteration and is dealt with as part of the same process. Planning is a separate matter: a rooflight in a roof slope is often permitted development, but check with the local planning authority if you are in a conservation area, in a national park or on a listed building.

If you are planning a glass floor over a roof terrace, walk-on rooflights for roof terraces and balconies looks at the extra points that apply outdoors. The full route from first look to finished room is on the walk-on rooflight installation page.

To start, ask for a quote. We survey the opening, check the structure and the ceiling around it, and send a written quote with the finish schedule.