How Thick Is Walk-On Rooflight Glass?

Standing on glass asks a lot of it. An ordinary window pane is made to resist wind and a knock, not the weight of a person concentrated on a few square centimetres of heel. Walkable glass is therefore built as a sandwich.

The top layer takes the wear and the footfall. Beneath it sit further layers, joined by tough interlayers, so that if the top surface were ever scratched or chipped, the layers below still carry the load. Because the layers are bonded, a crack in one does not run through the whole stack.

That is why the finished unit looks heavy. It is. A walkable rooflight is lifted into place by a team, not carried up by one person.

Three things decide how the glass is made, and none of them is the look of the room below.

• Span. The distance the glass has to bridge between its supports. A wide opening needs a stiffer stack than a narrow one, even for the same load.
• Load. Occasional footfall on a garden path, regular use on a roof terrace, and a spot where furniture might be dragged across are different demands. Our guide to walk on rooflight load rating sets out how those are described.
• Insulation. A walkable unit in a heated room's roof is normally double or triple glazed, which adds panes to the overall depth.

Because of that, we will not name a single thickness in millimetres and call it right for every house. A supplier's engineering team works it out from the opening dimensions we survey, and the result goes on the written quote.

It helps to think of the build-up as a recipe rather than a number. Two units of identical overall depth can behave very differently if one uses thick toughened sheets and the other thin ones. The datasheet will list each layer, the interlayer type and the load the whole assembly was tested for, and those are the lines worth reading.

For a Georgian townhouse in Bath with a narrow rear passage, there is a practical side too. A large, thick unit may be too heavy to get through the house, and the answer can be two smaller panels on a central support, each lighter and easier to handle, with a slim bar between them.

The two words crop up together on every datasheet, and they do different things.

• Toughened glass that has been heat treated so it is much stronger than ordinary glass and breaks into small blunt pieces.
• Laminated glass with a clear plastic interlayer between panes, which holds the pieces together if one breaks.
• Interlayer the thin bonding sheet between laminated panes.
• Build-up the full make-up of the unit, layer by layer.

For a floor, toughened glass supplies the strength and laminating supplies the safety net. Together they make a unit you can stand on with confidence.

Low-iron glass is the other option you may meet. Ordinary glass has a faint green edge when you look at it side-on, and a thick stack makes that more visible. Low-iron sheets are clearer and give a neutral daylight, which suits a pale kitchen or a studio below. It costs a little more, and it is a matter of taste rather than strength.

Laminated inner panes are sometimes suggested for ordinary roof windows, though Part K does not demand them in a house roof. A walkable unit is a different case altogether. Here the layering is not a recommendation, it is the whole point of the product.

Fixed walkable glass in a Victorian terrace's rear extension and a sloping roof window on the same house are therefore specified quite differently. If you want the roof window to open for air on a landing or in a loft room, an opening unit from the range described on our roof windows page suits that job, while the walk-on unit is for light and access over a floor, terrace or basement.

Surface finish adds one more consideration. A smooth top face is slippery when wet, so the outer layer is often textured or printed with a pattern for grip. Our page on walk on rooflight slip resistance goes into that.

Edge details count for as much as the face of the glass. The panes should never rest directly on bare metal or masonry, so a soft, durable setting block and a continuous seal carry the weight evenly around the frame. If a unit is bedded badly, the load concentrates at a few points, and glass is at its weakest when it is pinched.

When we visit for the survey, we measure the clear opening, check what supports the glass and ask how the unit will be used. The written quote lists the make-up the supplier has proposed, and you can ask us to explain any line. There is no obligation.

The cost follows the build-up closely, as our page on walk on rooflight cost shows. For the full range of where these units go, see the main walk-on rooflight installation page.