The short answer
There is no single thickness. Walk-on glass is a laminated slab of several toughened or heat-strengthened sheets bonded with clear interlayers, and the total depth is worked out for each opening from its span, its load and how its edges are supported. Small units over short gaps are noticeably slimmer than long rectangles, and a sealed double or triple unit adds to the depth of the walking layers. We take the figure from the manufacturer's calculation for your opening and write it on the quote.
- Free survey, fixed written quote
- 10-year workmanship guarantee
A stack of sheets, not one thick pane
Hold a cut sample of walk-on glass and the first surprise is the edge. It shows a stack of separate sheets, with a thin clear layer between them. Bonding them together makes one panel that behaves as a unit. When the top sheet picks up scratches from grit and shoe studs, the sheets under it carry on working, and if one layer fractures the interlayers hold the pieces in place so the panel stays in its frame.
That construction is why asking a supplier for a thickness in millimetres gets a vague reply. A rooflight that carries people is described by its layers, its span and its stated load, and the depth follows from those. Our page on walk-on glass load ratings explains the load side in detail.
What pushes the depth up or down
Span is the distance the glass crosses between supports. Glass bends a little under weight, and the longer the gap, the more material is needed to keep that bend within limits. Supports on all four sides help a pane far more than supports on two, which is why a square unit over a lightwell can be slimmer than a long strip across a hall.
Use comes next. A pane that two adults cross on the way to the garden is built differently from one under a table, chairs and a gathering of guests. Then there is the way the load meets the glass: a high heel or the foot of a ladder pushes through a tiny area, and the build-up has to cope with that as well as with a spread of people.
| If this changes | The glass tends to | Who confirms it |
|---|---|---|
| A longer gap between supports | Need more layers or deeper sheets | Manufacturer's calculation |
| Support on all four edges instead of two | Allow a slimmer build-up | Manufacturer's calculation |
| A terrace used for gatherings instead of an occasional step | Need a heavier duty specification | Agreed from your description of the use |
| Added solar control or an extra sealed pane | Add depth to the whole unit | Written on our quote |
The depth you need below the surface
The glass is only one layer of the unit. Underneath it sit the frame, the thermal break, the sealed gap of a double or triple unit and the structure that carries it all. Together they decide how much room has to exist between the finished surface and the ceiling of the space below.
This matters most in older basements. A cellar under a Victorian hall often has a low ceiling and a floor above made of boards on joists, with little depth to spare once the frame and glass take their share. A unit set on a low kerb needs less sunk depth than a flush one, but it stands up from the surface and can catch a toe. We measure the build-up of the floor or roof from above and below at the survey, because a figure from a catalogue may not fit the depth you actually have.
Weight on the day
A deep slab of laminated glass is heavy. A small unit can be carried by two installers, but a large one goes in with lifting gear, and the route to the opening, whether a hall, a side passage or a stair, has to take it. That is one reason we ask about access before we give a price. A big slab for a roof terrace wants a lift up from the garden, and the stair is the wrong way in.
Does a thicker pane let in less light?
A little, but the finish on the top face usually matters more. The anti-slip pattern, whether fired dots, an etched texture or a sandblasted band, scatters light, and a coarser pattern softens the glow more than a fine one. Extra layers cut the clarity slightly, and a solar control coating takes some warmth out of the sun on a south-facing terrace. Over a north-facing basement you can leave the glass clear, and the room gets as much sky as the opening and the surroundings allow.
For the warmth of the room below, the glass has to meet the Part L limit of 2.2 W/m²K for a rooflight, judged for the whole unit with its frame. Depth gained from an extra pane or a deeper gap is one of the ways a manufacturer reaches it, and our guide to rooflight U-values shows what the number covers.
Getting a figure for your own opening
Tell us what lies above the dark room and what will stand on the glass. We measure the opening and the structure at a free survey, pass the span and the use to the manufacturer, and the build-up is named in the fixed written quote before we fit. The wider picture of where glass underfoot suits a house is on our walk-on rooflights page, and the cost of a walk-on rooflight explains how depth feeds into the price. When you are ready, book a survey.
