Laminated vs Toughened Glass for Skylights (Safety)

Both laminated and toughened glass are types of safety glass, but they behave very differently when broken. Toughened glass shatters into small, blunt granules; laminated glass holds together because a tough interlayer bonds it. For overhead skylights, the safest specification usually pairs them, with laminated glass on the inner pane so that even if it breaks, it stays in place above your head.

Glass over your head is a different safety question to glass in a wall. This guide explains how the two safety glasses work, why the inner pane of a skylight matters most, and how a typical rooflight is specified.

What toughened glass is

Toughened, or tempered, glass is heat-treated to make it much stronger than ordinary glass, around four to five times as strong. Its defining safety feature is how it breaks: rather than forming sharp shards, it shatters into thousands of small, relatively blunt granules. That makes injury far less likely than with ordinary glass.

• Strong: resists impact and thermal stress well
• Breaks safely: crumbles into blunt granules, not sharp shards
• Does not hold together: when it breaks, the whole pane lets go and falls

That last point is the key one for overhead use. Toughened glass is excellent in a vertical position, but if a toughened pane breaks overhead, the granules can fall into the room below.

What laminated glass is

Laminated glass is two or more sheets of glass bonded together with a tough plastic interlayer. If it breaks, the interlayer holds the fragments in place, so the pane stays largely intact rather than falling away. Car windscreens use the same principle.

• Holds together: the interlayer keeps broken glass in place
• Stays overhead: a cracked pane does not drop into the room
• Extra benefits: better noise reduction and improved security

For glass above your head, this hold-together behaviour is exactly what you want.

Why overhead glazing is treated differently

In a wall, the main concern is someone walking into or falling against the glass, where toughened glass breaking into blunt granules is a safe outcome. Overhead, the concern is different: if the glass above you fails, you do not want it falling on you. That is why the inner pane of a skylight, the one facing into the room, is the critical one for safety.

The widely recommended approach for overhead glazing is to use laminated glass for the inner pane, so that even in the unlikely event of breakage, the glass is held in place by the interlayer and stays above your head rather than dropping.

How a typical skylight is specified

Most quality rooflights are double or triple-glazed units, and the panes are specified for their position:

• Outer pane: often toughened, for strength against impact, hail and thermal stress from the sun
• Inner pane: laminated, so that if it breaks it stays in place above the room

This pairing gives you the best of both: a strong, resilient outer face and a safe, hold-together inner face. The exact specification depends on the product and the situation, which we confirm for your project.

Why thermal stress matters on a rooflight

One reason the outer pane is so often toughened is thermal stress, which is a bigger issue overhead than in a wall. A rooflight in full sun heats unevenly: the centre of the pane, exposed to direct sun, expands while the edges held in the frame stay cooler and shaded. That temperature difference sets up stress in the glass, and on a large pane catching strong sun it can be enough to crack ordinary annealed glass on its own, with no impact at all. Solar control coatings and any internal shading can increase the effect by raising the heat the glass absorbs. Toughened glass, with its much higher resistance to thermal stress, is the sensible outer face for a sunlit roof for exactly this reason, quite apart from impact resistance.

Heat-soaked toughened glass

Toughened glass carries one rare but well-known quirk: a small risk of spontaneous breakage caused by microscopic nickel sulphide inclusions left over from manufacture, which can expand over time and shatter the pane without warning. The risk is low, but because a rooflight is overhead and hard to reach, it is one specifiers take seriously. The remedy is a heat-soak test, in which the toughened glass is held at high temperature for a controlled period to provoke any unstable inclusions to fail in the factory rather than on your roof. For overhead and safety-critical glazing, heat-soaked toughened glass on the outer pane, paired with a laminated inner pane, is the belt-and-braces approach, and one worth asking about for large or high rooflights.

Other safety considerations

Walk-on glass

Where a rooflight has to bear foot traffic, the glass is a heavier, engineered structural laminate designed to carry load, with an anti-slip finish. This is a step beyond standard overhead glazing.

Height and use

A rooflight over a stairwell, a child’s bedroom or a busy living space deserves particular care with the inner pane specification. We factor in how the room is used.

Beyond safety

Laminated glass also improves sound insulation and security, which can be welcome benefits in their own right alongside the safety case. The plastic interlayer damps sound vibration through the glass, which noticeably softens the drumming of heavy rain on a rooflight, and it also screens out almost all ultraviolet light, helping to protect furnishings, flooring and artwork below from fading. The same interlayer is what gives laminated glass its security advantage: even once cracked, the bonded pane resists being pushed through, so it is far harder to break into than a single toughened pane that simply falls away.

The interlayer itself

Not all laminated glass is identical, and the interlayer is where the difference lies. The standard interlayer holds fragments together and provides the UV and acoustic benefits above. A stiffer structural interlayer is used where the glass must retain residual strength after breakage or carry load, such as walk-on rooflights or very large overhead spans. For most domestic skylights the standard laminated inner pane is exactly right; the structural grade is reserved for the situations that genuinely call for it, which we identify at survey.

Which should you choose?

For overhead skylights, the practical answer is rarely one or the other in isolation. The reliable specification is a unit that uses toughened glass for outer strength and laminated glass for the inner, hold-together safety that overhead glazing needs. The exact make-up depends on the product, the size and how the room is used.

We specify the right glass for each position at a free survey, explain the reasoning, and set it out in a fixed written quote. Every installation carries our written guarantee on our workmanship.

To get the right safety glass specified for your skylight, get a quote for a free survey, or see what we install on our services page and our VELUX window installation page.