How thick is walk-on rooflight glass?

Why walk on glass thickness is not one number

People ask for walk on glass thickness as if every panel were cut from the same sheet. It is not. A walk-on panel is built up from several layers of glass bonded together, and each manufacturer tests its own build-up against its own published loads. The thickness you end up with is the total of those plies and the interlayers between them.

Two things drive the total. The first is span: the longer the clear opening the glass has to cross, the more it bends under load, so the stack gets deeper. The second is the load class the unit is rated for, which for domestic use makers commonly quote as 1.5 kN/m² across the area and 2.0 kN concentrated at one point. A commercial or vehicle-adjacent rating needs a different build altogether.

Support matters as much as span. A panel resting on a kerb all the way round shares its load in two directions, while one carried on two opposite edges works harder in one direction only. The manufacturer's tables assume a stated support arrangement, so the kerb detail and the glass are chosen together, never one after the other.

That is why a quote that simply says 'toughened glass' tells you very little. The useful question is which build-up, from which maker, at which size.

What is inside a laminated walk-on panel

Walk-on glass is always laminated. Several plies of toughened or heat-strengthened glass are bonded with tough interlayers, so that if the top ply is scratched or even cracked, the plies beneath keep carrying the load and the fragments stay held in place. A single pane of toughened glass would not be used for this job.

Manufacturers describe their build-ups as a count of plies plus the interlayers, and the top ply is often treated as the wearing surface, with the lower plies doing the structural work. Some makers publish worked examples for particular spans and loads. Treat any example as that manufacturer's own and as relevant only to that product, size and rating.

The upper face can carry an etched or fritted pattern for grip. Our guide to anti-slip walk-on glass covers the finishes and how they behave when wet.

• Several glass plies bonded by interlayers
• Toughened or heat-strengthened glass, chosen by the maker
• Top face finished for grip where it is a walking surface
• Edges supported all round on a prepared bearing

The insulating unit underneath

A walk-on rooflight over a heated room is normally a double or triple glazed arrangement. The laminated walking layer sits on top, and a sealed insulating unit sits below it to deal with heat loss and condensation. So the figure you see in a brochure may describe the walking layer only, the whole assembly, or both, and it is worth asking which.

For the thermal side, a rooflight in an existing dwelling meets the 2.2 W/m²K limit in Part L 2021, Table 4.2, and the product details in the job plan state the unit's own figure. The lower inner pane is usually laminated as well, which is a recommendation rather than a Part K requirement for a dwelling roof.

How the stack changes with size

The table shows the direction of travel, not measurements. Each row is a general relationship, and the actual build-up for your panel comes from the manufacturer's tables once the clear span and the load class are fixed.

Who decides, and what we do with the answer

The thickness is a manufacturer's decision, taken from their own testing, and it is not something to adjust on site. A fitter who thins the glass to save weight, or swaps in another maker's panel on an existing kerb, has changed the rated product. We order the build-up the maker specifies for the measured opening and the load class you need.

Our free survey takes the clear opening between the supports, notes what will stand or pass over the glass, and looks at the structure beneath. The written quote then names the unit, its size and its rating. The reasoning behind load classes is in the walk-on rooflight load ratings guide, and what will sit on the panel is covered in putting furniture on walk-on glass.

Questions to put to a supplier about the build-up

When you are comparing walk-on panels, a short list of questions sorts the properly specified unit from the vague one. None of them needs technical knowledge, and a good maker will have the answers in a data sheet.

Ask for the ply count and interlayer type, the rated loads and how they were tested, the span the build-up was tested at, and whether the figure covers the walking layer alone or the whole insulating assembly. Ask too whether the edge support the maker assumes matches the kerb you have, since a panel tested on four sides does not behave the same on two.

• How many plies, and what interlayer
• Rated spread load and point load
• The span the rating applies to
• Walking layer only, or the full assembly
• The edge support the rating assumes

Where the build-up appears in the job plan

A written job plan comes with each Ridgeway Skylights quote. Its unit row records the maker, the size, the load class and the build-up, so the glass thickness is settled on paper while nothing has yet been paid. Its Building Control row states how the structure and the fitting will be notified, where the work is notifiable.

If two quotes arrive side by side, compare those rows before comparing the totals. Two panels with the same plan size can hide very different stacks, and that difference is worth as much attention as the figure at the bottom. For the wider picture of fitting a walk-on unit, start from the walk-on rooflight installation page.