Toughened glass explained

Toughened glass — also called tempered glass — is ordinary soda-lime float glass that has been heat-treated to increase its strength and change the way it breaks. In UK construction it is one of the most common forms of safety glass, specified for roof lights, balustrades, doors and any location where human impact is a risk.

How is toughened glass made?

The process starts with annealed glass cut to size and edge-worked while it is still in its “soft” state. The pane is heated to around 650 °C and then cooled rapidly. This creates a permanent surface compression layer balanced by tension in the core.

The result is glass that is typically four to five times stronger in bending than annealed glass of the same thickness. Manufacturing and testing in the UK generally follow BS EN 12150 (thermally toughened soda lime silicate safety glass). Compliance is demonstrated through factory production control, marking and impact performance.

What happens when toughened glass breaks?

Unlike annealed glass, which breaks into large, sharp shards, toughened glass disintegrates into small granular fragments. This behaviour is why it is classified as safety glass and is preferred where broken glass could injure people.

That fragmentation is a consequence of the internal stress pattern. It also means there is no “repair” — a broken toughened pane must be replaced in full.

Where is toughened glass used in roof lights?

In a typical insulated roof light unit, the outer pane is often toughened. It faces weather, wind load and occasional impact from debris. The inner pane — the one overhead if the outer pane fails — is frequently laminated so that glass is retained in the frame rather than falling into the room below.

Toughened glass alone does not provide post-breakage retention. That is why laminated inner panes are standard in overhead glazing for dwellings and many commercial projects.

Strength, thickness and span

Strength depends on thickness, support conditions and load case. A 6 mm toughened outer pane is common in residential roof lights; larger spans or higher loads may require thicker glass or a different build. Deflection and stress must be checked against the project wind and snow loads — your roof light manufacturer should confirm suitability for your opening size.

Impact resistance is classified under BS EN 12600 (pendulum test). Overhead glazing in critical locations must meet the class required by the building regulations and the specifier’s risk assessment.

What you cannot do after toughening

Once glass is toughened, it cannot be cut, drilled, notched or ground without destroying the heat treatment. Any holes, cut-outs or edge details must be completed before the toughening furnace cycle. This is a common source of site errors — always confirm final sizes before glass goes into production.

Toughened vs laminated — quick comparison

Property

Toughened

Laminated

Primary benefit

High strength; safe break pattern

Post-breakage retention

Typical roof light role

Outer pane

Inner pane

Can be cut on site

No

No (factory-sized)

Relevant standard

BS EN 12150

BS EN 14449

For most overhead residential roof lights, the correct specification is a toughened outer + laminated inner build within an insulating glass unit. Confirm the exact make-up with your supplier for the specific opening and regulatory context.

Vant Glass manufactures premium roof lights and glazing in Aintree, Liverpool — made in Britain, 20-year guarantee, free UK mainland delivery. Explore all products or call 03330 902 592.