How to tell if rooflight glass is laminated

Every safety-rated pane in a rooflight unit should carry a small permanent mark, and it is the quickest answer you will get without a tool.

Stand under the rooflight in good light and look at the extreme corner of the inner pane, close to the frame or the spacer bar. You are looking for a few millimetres of etched text, sometimes with a manufacturer's name or logo. A toughened pane is typically marked with a British Standard number for toughened safety glass, often with the word toughened or a T. A laminated pane usually says laminated, or shows the laminated standard number, sometimes with a thickness such as 6.4 or 8.8.

Those thickness codes are worth understanding. A figure like 6.4 describes two layers of glass bonded by a thin interlayer, so the total is larger than either sheet. An 8.8 pane is the same idea with thicker sheets. A plain figure with no laminated mark, such as 4 or 6, usually describes a single sheet of annealed or toughened glass.

A corner stamp can be worn, painted over or hidden under a frame bead. If you cannot find one, do not assume the glass is plain. Move on to the edge.

Where a pane is visible at its edge, laminated glass gives itself away.

Laminated glass is a sandwich: two sheets of glass with a clear plastic interlayer bonded between them. Seen from the side, the edge shows a faint horizontal line running through the middle of the thickness, usually slightly milky or slightly green. Toughened glass has a clean, solid edge with no line, and it often shows a slight green tint end-on.

In a sealed rooflight unit the edges are normally hidden by the gasket, the spacer and the frame, so this check is most useful on a pane that has been removed, on a spare, or on a unit visible from a loft hatch or a flat-roof upstand. A torch held flat to the glass helps, because the interlayer catches the light.

1. Find a visible edge. A corner at a cut-out, a pane in a lantern where the gasket has been trimmed back, or a spare unit in the garage.
2. Shine a torch along the glass. A thin bright line through the middle means a laminated build.
3. Compare the two panes. Note which one is inner and which is outer, since units often pair different glass.

A sealed unit holds two panes, and they are often different. Only one of them is directly above your head.

The outer pane faces the weather and is often toughened, or toughened with a self-cleaning or solar control coating on it. The inner pane faces the room. When it is laminated, a pane that breaks stays in the unit, held by the interlayer, instead of breaking into pieces and dropping. That is why laminated inner glass is the usual choice for glazing overhead, and why we write it into the specification for a replacement unit. Building Regulations Part K covers safety glazing in locations where people may be struck by glass, and overhead glass is a place where the laminated answer is the careful one.

The stamp on the outer pane tells you only half of the story, so look at both corners if you can see them. On many units the marking sits at the same corner of each pane, but it can be on the room side of the outer pane, which you would see only from outside or from a ladder. If the roof is high, sloping or wet, leave the outside to us.

Not everything in a roof that looks like glass is glass, and not all older glass behaves the way modern units do.

Domes and some older lantern and conservatory panels are polycarbonate or acrylic. They are plastic, they do not shatter like glass, and they carry a different mark or none at all. A dome that yellows, crazes or scratches easily is plastic. Our page on failed polycarbonate dome rooflights and replacing them like for like deals with that type.

Some rooflights installed before modern safety standards were common hold wired glass or single sheets of plain annealed glass. Annealed glass breaks into large sharp shards, which is the worst case overhead. If the stamp reads nothing, or the glass has a wire mesh running through it, treat the pane as a candidate for renewal rather than something to leave in place.

Finding out what the old unit held helps us build the better one, which is the useful end of this whole exercise.

If the inner pane is laminated, a replacement can match it or add an acoustic interlayer, and you keep the overhead safety you already had. If it is toughened, you have a good opportunity to change it when the unit next fails or mists. Should a failed rooflight unit be replaced with laminated glass explains the thinking, and upgrading the glazing when a rooflight unit fails sets out the other changes worth making at the same time.

At the survey we look at the stamps and write down what is in the old unit. The new build then goes into the quiet spec with the panes, interlayer, spacer and seals named, so nothing is left to guess. If the unit has also clouded, is a misted rooflight dangerous covers that side, and our misted and failed rooflight replacement page covers the full service.

Can I tell by how the glass breaks?

Yes, but it is not a test to run. Toughened glass crumbles into small blunt pieces, while laminated glass cracks in a web and stays in one piece. Please check the stamp instead of finding out the hard way.

Is a coated pane the same as laminated?

No. A coating is a thin metal-oxide layer on the surface, chosen for heat or self-cleaning. It changes how the glass looks, sometimes with a faint blue or green reflection, but it does not hold a broken pane together.

Does laminated glass block noise better?

The interlayer damps vibration, so a laminated pane passes less rain drumming and traffic noise than a plain pane of the same thickness. An acoustic interlayer goes further, and will a replacement glass unit make my rooflight quieter explains how the build is chosen.

Is my VELUX glass laminated?

The stamp will tell you. Many roof windows carry a laminated inner pane, but the marking on your own window is the only reliable answer, since it varies with the model and its age.

If you would like us to check the glass in person, get a quote and we will record what is in the old unit before we specify the new one.