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A non-fragile classification is applied to rooflights which are considered resistant to impacts from falling objects (or people). Importantly a non-fragile rooflight maintains the equivalent level of impact resistance as that across the surface of the roof overall. New rooflight installations should always be designed as non-fragile.
CWCT testing for rooflights identifies a non-fragility rating of Class 1, 2 or 3, according to performance under a standardised impact assessment.
Class 1 is categorised as suitable to walk on for brief maintenance or cleaning.
Class 2 is unsuitable for walking upon, but resistant to breakage in the event of surface impact.
Class 3 is a ‘fragile’ classification, requiring additional safety considerations. The TN66 Technical Note relates to glass rooflights which are not publicly accessible but may be accessed for maintenance purposes.
TN67 outlines the required assessment process to ensure glass roofs are compliant with CWCT classifications.
TN92 details the criteria required, for glass used in Class 2 roofs, to achieve a ‘deemed to satisfy’ rating.
Overall a Class 1 certified rooflight will ensure an optimum level of safety performance and mitigate overall risk.
Laminated glass and toughened glass are two separate specifications (although commonly confused). The ideal rooflight designation to ensure safety performance is a double-glazed unit (at a minimum) with an outer toughened pane and laminated glass inner panel.
Toughened glass has a high resistance to breakage but if broken ‘crumbles’ into granular fragments, rather than jagged shards.
Laminated glass consists of two pieces of glass held together by an extremely tough PVB plastic interlayer. If smashed, laminated glass holds its structure and does not break up into shards or fragments – thereby remaining in place and ensuring occupant safety below.
Many rooflights are manufactured using aluminium frames but, because metal conducts heat, it can cause heat from inside to escape through the frame. A thermal break is basically a piece of material that’s ‘sandwiched’ in the frame of the rooflight to minimise heat loss – and ensure optimal energy efficiency. However, if the additional material makes the frame section larger, it will also increase the exposed surface area and the risk of condensation forming (especially in rooms with high humidity, such as a kitchen or bathroom). Condensation appears when warmer air makes contact with the frame, if the aluminium frame is colder than the humid air inside (referred to as ‘cold bridging’). A high-quality rooflight will feature a well-designed thermal break to prevent both heat escaping and eliminate cold bridging. Thermal break materials can also help boost the rigidity and structural integrity of the frame.
This information is provided as a high-level overview. For detailed advice, please visit: https://www.narm.org.uk/