Safety glass and laminated glass in a roof lantern

Laminated glass is glass made of two or more sheets with a tough interlayer between them. If it breaks, the fragments stick to the interlayer and the pane stays in the frame. That is why in a roof lantern, where the glass is above your head, the inner pane should be laminated. Safety glass is the umbrella term for glass that causes less injury when it breaks. For roof glazing, laminated glass is the most important type.

What is laminated glass?

Laminated glass consists of glass sheets bonded together under heat and pressure with one or more layers of PVB interlayer. The interlayer is tough and elastic. If something falls on the glass or it cracks under stress, it breaks in a spider's web pattern but holds together as a whole.

You can tell the build-up from a code. 33.1 means two 3 mm sheets with one 0.38 mm interlayer. 44.2 means two 4 mm sheets with two interlayers, 0.76 mm in total. More interlayer makes the glass stronger after breakage and more resistant to falling through.

Toughened glass: stronger, but safe in a different way

Toughened glass, also called thermally tempered glass, is roughly four to five times as strong as ordinary float glass. If it does break, it shatters into small, blunt granules. That is safe in a door or shower screen, where the granules fall onto the floor.

Above your head, that is exactly the problem. You do not want a shower of glass granules on your dining table. So toughened glass is often used as the outer pane of roof glazing, because it copes well with hail and temperature differences. On the inside sits laminated glass that holds everything together if it breaks.

Why the inner pane of a roof lantern is laminated

Insulating glass units, such as HR++ double glazing, consist of two panes with a cavity between them. Triple glazing has three. For roof glazing it is standard practice for the room-side pane to be laminated. If the outer pane breaks from hail or a falling branch, the inner pane holds. If the inner pane itself breaks, the fragments stay attached to the interlayer.

That applies to a small mono-pitch roof lantern just as much as to a large glass roof. A typical glass unit for a roof lantern is built up like this:

• Outer pane: often toughened glass against hail and stress
• Cavity filled with inert gas for insulation
• Coating for insulation or solar control in the right position
• Inner pane: laminated glass that stays in place if it breaks

What does the standard say about roof glazing?

In the Netherlands, NEN 3569 describes how to select glass to limit injury from breakage and the risk of falling through. Among other things, the standard looks at the position of the glass, the height, and whether someone can get on or under it. For glass above a habitable room, in practice this often means laminated glass on the inside. Exactly which build-up is needed depends on the situation.

DagLichtDesign supplies safety glass in accordance with the NEN standard. Ask in your quote about the glass build-up that applies to your situation. If a glass roof also has to be walkable, for example for maintenance, the build-up needs a separate assessment.

Laminated glass for sound and security

Laminated glass has two side benefits. The interlayer dampens sound, especially with a special acoustic interlayer. HR++ double glazing gives about 32 dB of sound reduction and triple glazing about 36 dB. If you live under a flight path or on a busy road, acoustic glass can add a little more.

Laminated glass is also much harder to break through. An extension with a flat roof is a place where someone might try to get in via the roof. A laminated pane makes that a lot more difficult. How the choice between two and three panes works out is covered in triple glazing or HR++ double glazing.

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