How Thick Is Walk-On Glass?

Why one number does not exist

A window pane is a few millimetres of glass, and it is easy to assume a walk-on version is simply a fatter pane. It is not. The glass is a built-up sandwich, and the total depth moves with every project. Someone quoting a single thickness without asking about your opening is guessing.

What can be said is that the stack is far deeper than a window, and that the figure follows from a handful of decisions.

The layers that make up the depth

Each layer has a job, and the thickness of each is chosen by the maker's engineers.

1. Walking layer. Toughened or heat-strengthened glass carrying the grip finish. It takes the footfall and the scuffs.
2. Load layers. Further panes bonded beneath it with clear interlayers. These share the weight and keep carrying it if the top pane is ever cracked.
3. Insulating cavity. Where the room below is heated, a sealed gas-filled gap adds depth.
4. Underside pane. A laminated sheet on the room side that holds together if it is struck.

Add the interlayers and the spacer and the stack runs to several centimetres. The depth is then reflected in the frame, which has to be deep enough to seat all of it.

What pushes the thickness up

Wider span	More glass is needed, because larger panes flex more
Heavier use	A terrace with furniture asks more than a quiet landing
Insulation	A sealed cavity, or two, adds depth
Edge support	Glass supported on all four sides can be slimmer than on two

Span is the biggest lever

Glass bends slightly under a load, and the longer the unsupported distance, the more it bends. The design aim is to keep that movement small enough to be unnoticeable and safe. A narrow strip can be fairly slim, while a wide pane needs a deeper build-up. If a large opening is wanted, an extra supporting bar or a slightly smaller pane will often bring the thickness down.

That is why the opening is measured so carefully before the unit is ordered. Our guide to walk-on glass loading explained shows how the load and the span combine.

If someone gives you a thickness before they have seen your opening, they are quoting a brochure.

From a survey in Bishopthorpe

What thicker means for the structure

A deeper unit weighs more, and it needs a deeper rebate in the surface around it. In a timber floor the joist depth sets how much room there is. In a slab the depth is whatever it was poured at. In a flat roof the build-up of insulation and finish matters too. Some openings simply will not take a deep triple unit without altering the floor, and it is far better to learn that on the survey than on the day.

Trimmers and bearings round the opening are sized for the unit's weight, and photographs of that structure are taken before it is boxed in.

Does thicker mean safer?

Not by itself. A thicker stack carries more, but its safety also comes from the lamination: if one layer breaks, the others stay bonded and in place. A well-designed slimmer unit with the right layers is safer than an oversized one that does not suit its frame. The maker's loading figures are what to rely on, and they are given to you at handover with the other product details.

What you will notice from below

From the room under it, a deep stack shows a slight green tint along the cut edges, because thick clear glass reads that way end-on. Looking straight up it is clear. The depth shows mostly as a chunkier frame and a slightly deeper reveal in the ceiling, which we plaster neatly.

Why we do not stock sizes off the shelf

Because so many things set the thickness, walk-on units are made after the survey, to the dimensions of your opening. Nothing is held in a warehouse waiting for the right hole. The lead time reflects that, and so does the fit: the frame arrives sized to your rebate, and the glass is lifted in once the structure is ready. If the opening turns out different from the drawing, we find out during the dry fit, not once the glass is made.

Checking the depth at the survey

On the survey we probe the floor or roof beside the opening to see how deep the structure really is. A short note of that depth goes onto the written quote, so the unit is specified to fit what is actually there rather than what a drawing suggests.

Where to look next

For the product as a whole, see walk-on rooflights. If heat loss is your worry, are walk-on rooflights insulated explains how the cavity fits into the stack, and walk-on rooflight cost covers how the build-up affects the quote.