Product answer
How thick is the glass in a walk-on rooflight?
Far thicker than a window pane, since it is several layers bonded into one slab. The final figure follows from the span, the weight it carries and any insulation job the unit has to do.
The answer
Walk-on rooflight glass is built from several layers of toughened or heat-strengthened glass, bonded with tough interlayers, so the total is a good deal thicker than ordinary window glass. The figure follows from the span, the load and the supports. The specification records it, so the survey settles it and the quote fixes it.
Why is walk-on glass made of layers?
Several panes are bonded into one. If one cracks, the others and the interlayer hold the fragments in place, so the surface keeps carrying weight instead of dropping a person through.
A single thick pane would be strong, but it could fail all at once. A laminated build-up is safer. Each layer is toughened or heat-strengthened glass, and between the layers sits a clear interlayer that bonds them together.
The parts of the stack
The top layer is the one underfoot. It takes the wear, the scratches and the grip finish. Below it come one or more structural layers that carry the main load. The interlayer, usually a tough clear plastic film, glues them into a single piece and holds broken fragments in place if something does fail.
What the interlayer buys you
Because the layers are bonded, a damaged top face does not mean a failed floor. The lower layers go on carrying the load while the damaged one is swapped, and the interlayer stops loose shards from falling into the room underneath. That is the reason laminated build-ups are the norm for glass over rooms.
| Layer | What it is | Its job |
|---|---|---|
| Top pane | Toughened glass with a grip finish | Takes wear and gives the shoe something to hold |
| Interlayer | Clear plastic film bonded under heat | Joins the panes and holds fragments |
| Structural panes | One or more heat-strengthened or toughened layers | Carry the load across the span |
| Underside pane | Laminated or toughened glass facing the room | Protects the room and can take a coating |
What decides how thick the stack ends up?
The width of the opening, the weight it must carry and how its edges are supported. A bigger span or a heavier use needs more or thicker layers, and good supports can allow slimmer glass.
There is no single thickness for walk-on glass, because every opening is different. The same three questions are asked each time.
How wide is the span?
Glass bends under load, and a longer span bends more. Doubling the width of an unsupported opening makes the glass flex far more than twice as much. A short span can get by with fewer layers; a long one needs deeper glass or a support partway across.
How heavy is the use?
The design load is part of the sum. A rooflight crossed now and then needs less than a terrace where tables and groups of people stand. The layers are chosen to meet the stated load with a margin to spare.
How are the edges held?
Glass held on all four sides is stiffer than glass held on two. A deeper rebate in the frame holds the edge better too. A good support detail lets the glass be slimmer without losing strength. The design statement should list the supports the figures assume, and the load and slip ratings guide lists the rest.
Does thicker glass keep the room warmer?
Not by itself. Warmth comes from sealed units, coatings and the gas or air gap, as with any rooflight. Thickness does matter for the height of the unit, which can affect the planning limit on a house.
People often assume heavy glass is warm glass. It is not quite so. Solid glass conducts heat well, and adding layers of it adds little insulation.
Where the warmth comes from
Insulation in a rooflight comes from a sealed cavity, filled with air or gas, between panes, and from low-emissivity coatings on the glass faces. Where the glass also has to keep a heated room warm, it is made as a sealed double-glazed unit topped with a walk-on layer, which adds depth. For an existing dwelling in England, the limiting U-value for a rooflight is 2.2 W/m²K.
What thickness does to height
A deeper build-up needs a deeper rebate in the roof or floor. If the unit sits proud of a roof, the extra depth adds to the height it stands above it. On a house, a rooflight is permitted development under Class C only if it projects no more than 150mm beyond the plane of the roof slope, so the build-up is set low into the roof wherever possible. A flat needs planning permission regardless. The terrace answer goes further into the planning side.
Next step
Our products are shown on the walk-on rooflights page. Send the quote form when you want a survey, which is free, and the build-up is then written into the fixed quote.
Build-up and weight questions
Can I swap glass thickness to save money?
Not without the supplier's engineer approving it. Thinner layers can fail the design load, and the specification is calculated as a whole. The survey gives a fixed written quote, and any change to the glass is a change to the job.
Is thicker glass heavier to install?
Yes. A large laminated unit can weigh a lot, so lifting gear or extra hands are planned for. The weight also loads the structure around the opening, which is part of what the engineer checks.
Does thick glass look green at the edges?
Standard glass has a slight green tint that shows more through a thick stack. Low-iron glass is clearer and shows less tint. It is a choice for the survey, since it affects appearance and cost.
Will the thickness limit what opens or fits in a small opening?
A small opening can take a slim build-up, but the frame and rebate use some of the width. The survey measures the clear opening you will actually see, so the glass is chosen to suit it.
Does a thicker top layer last longer underfoot?
A thicker top pane gives a deeper margin before surface wear matters, but grip comes from the finish more than the depth. Wear shows as a dulling of the pattern, so ask the maker how the finish is fixed to the glass and how it stands up to regular cleaning.