Published on 12 August 2019 12:46

Specification for ‘Non-Fragile’ and ‘Walk-On’ Rooflights

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Key Learning outcomes
  • Understand non-fragility classification criteria
  • Recognise the different classification levels of non-fragility
  • Understand rooflight application specifications
  • Identify the different installation methods relevant to rooflight categories
  • Understand different surface coatings on walk-on units, in the context of slip ratings
In construction, rooflights provide an invaluable source of daylight, for structures which may otherwise have low levels of natural illumination. Each category of rooflight has a distinct classification, in terms of fragility, loading level, installation method and surface coating. Understanding the characteristics and criteria of the various rooflight options is critical to ensuring a compliant and appropriate selection.

1.0 Understanding non-fragility classification criteria

The term ‘non-fragile’ indicates that the surface of the rooflight is durable, resilient, and suitable for walking upon. However, this is misleading, as non-fragile ratings apply to a broad range of rooflights, each with significant variation in terms of their resistance to impact.

Non-fragile classifications can be applied to polycarbonate, glass and GPR rooflights. However, there are a number of circumstances affecting impact resilience within this category, including the installation method used, and the coating applied to the surface.

The non-fragile classification is applied to rooflights which are considered resistant to impacts from falling objects (or people), and maintain an equivalent level of impact resistance to that maintained across the surface of the roof overall.

Despite the initial non-fragile classification ascribed to a particular rooflight, it’s important to recognise that certain scenarios may affect the level of impact resistance over time. A rooflight which suffers an apparently inconsequential scratch may be subject to UV damage over time, which weakens the fixture and affects resistance to subsequent impact. As a result, it is best practice to consider all rooflights to be fragile, unless they have been recently assessed as otherwise.

Rooflights with a non-fragile classification are assessed according to the roof loading capacity, as opposed to floor loadings; as a result, even non-fragile rooflight fixtures are not recommended to be walked upon. New rooflight installations should always be designed as non-fragile, and be installed using specified components and approved methodology, to maximise the longevity of the non-fragile classification.

CWCT testing for rooflights identifies a rating (Class 1, 2 or 3), according to performance under a standardised assessment. A specified weight is released in a controlled fall under gravity, at critical points, to monitor impact resistance. 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.

CWCT provides guidance for appropriate installation of rooflights. The TN66 Technical Note relates to glass roofing which is 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.
CWCT testing requires the outer pane to be broken and the test weight supported only on the inner pane for a successful pass.
CWCT testing requires the outer pane to be broken and the test weight supported only on the inner pane for a successful pass.
 
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