Although the UK Building Regulations permit the use of toughened glass overhead, it is not best practice.
It is the inner pane of a double or triple layer glass unit, that protects the building occupants in the event of a breakage or failure of the glass unit. This is a particularly important consideration where glass could fall if broken, such as in overhead skylight glazing.
Laminated glass is a type of safety glass that typically consists of two (or more) layers of glass separated by an “interlayer”, such as polyvinyl butyral (PVB). It is the PVB interlayer that holds the glass together when broken, and prevents any broken pieces from falling inside and potentially causing injury.
Toughened glass is another type of safety glass, which has undergone a process of tempering that gives it increased strength when compared to normal float/annealed glass. However, the “break characteristics” of toughened glass are very different from laminated glass. When toughened glass breaks it shatters into lots of small granular pieces and if used overhead, these pieces will fall inside.
Pilkington Glass state on their website:
“When toughened glass breaks it breaks safe into small particles. When it is overhead the particles can come down locked together forming a mass of glass that may cause injury”.
Saint Gobain strongly recommend using laminated glass as the inner pane, this is from their website:
“Overhead glazing is more susceptible to impact from falling objects and wind-borne debris than vertical glazing.
Glass is also more likely to fall out of the frame when broken, therefore careful consideration must be given to the breakage characteristics of the specified glass.
In recognition of the breakage characteristics of toughened glass, we strongly recommend the use of a laminated glass in single-glazed or as the inner pane of a double-glazed unit where it is necessary to minimise the risk of glass falling after breakage.”
Whilst there are glass roof light suppliers that do use a toughened glass inner pane, we would only ever recommend using laminated safety glass – so there is never any risk of injury from falling glass. This safety consideration is relevant regardless of the building type, but we feel that it is particularly important for buildings used by children, such as schools.
We suggest that potential suppliers should be asked for confirmation and evidence of the glass specification used in their products. The minimum glass specification used in our modular glass skylights is: 8mm toughened outer pane over an 8.8mm laminated safety glass inner pane.
If you would like expert rooflight specification advice for your project, please contact us now.
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