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Non-fragility

As a leading UK manufacturer of cutting-edge Flatglass Rooflights, the Roofglaze Technical Team have a full and comprehensive understanding of non-fragility, the legal requirements surrounding specifying non-fragile rooflights and the issues that should be considered when designing rooflights into buildings.

When designing a roof or building, the terms “non-fragile” and “walk-on” are often used interchangeably when, in reality, they are quite different and the designer / specifier should ensure that they understand the difference so as not to over-specify, or more crucially under-specify, these types of products.

The Basics

A non-fragile rooflight is not designed to be walked on. It is designed to save lives by preventing people or objects accidentally falling through it when accessing roof areas that are not designed for regular foot traffic.

A walk-on rooflight is designed to take structural requirements into consideration and should be specified where deliberate foot traffic will occur.


Roofglaze Non Fragility Impact Report
St Edmunds Terrace 1

The Detail

Non-fragile rooflights have several classifications that have been determined by the Centre for Window and Cladding Technology (CWCT) who are the publisher of industry standards and guidance on building envelopes.

The CWCT have produced Technical Note Number 92 (TN92) to assist designers and specifiers ensure that the correct glazing type is used for the required application.

CWCT TN92 states that:

  • Class 1 are roofs which will be walked on for occasional cleaning or maintenance activities, and which will therefore need to support both the weight of people on the glass and their equipment.
  • Class 2 are roofs where people are not intended to walk on the glass, but which are required to be non-fragile to protect people in the following circumstances:
    • Where maintenance personnel walking adjacent to the glass roof could trip or fall onto the glass surface.
    • Where maintenance personnel working on the glass roof could fall onto the glass surface from crawler boards or other access equipment.
  • Class 3 are roofs which are fragile.
  • Class 0 roofs are outside the scope of TN92 as they are designed as structural elements.

Roofglaze Achieves CWCT TN66 & TN67 Class 1 Non-fragility Rating
Roofglaze Rooflights Shop

For Class 1 and Class 2 roofs, testing is required on the chosen glass specification to ascertain whether the glass is non-fragile or not. This testing is to be carried out in accordance with the CWCT Technical Notes 66 and 67 (TN66 & TN67). However, as the test must be carried out on every size of glass that is to be used, practically and commercially, this type of continuous testing is prohibitive. With this in mind, the CWCT devised TN92 which is a “deemed to satisfy” document for glass that will satisfy the testing requirements set out TN66 & TN67 up to certain sizes, thus negating the need for testing.

“Table 2” within TN92 gives the size criteria and glass specification for a Class 2 non-fragile rooflight:

Cambridge Mosque

The 9.5mm laminate and the 11.5mm laminate relate to the inner skin. The outer skin will usually be a 6mm or 8mm heat soak tested toughened pane and is always required to be broken in the TN66/67 testing.

To achieve Class 1 classification, the rooflight still needs to achieve Class 2 on the inner skin but the outer must be deemed robust enough to be occasionally walked upon.

In this circumstance, TN92 states:

“Where robustness is required, the requirements of TN66 would normally be satisfied by a pane of 10mm toughened glass for pane sizes up to an area of 4.5 m².”

Walk-on rooflights are required to be designed to structural floor loadings in accordance with BS EN 1991-1-1. The designer / specifier will need to understand the required loadings so the glazed unit can be manufactured to meet those demands; the rooflight can then be designated Class 0.


CWCT Test Report
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Further Information

More details can be obtained from the ACR (Advisory Committee for Roof Safety), or from NARM (National Association of Rooflight Manufacturers).

Please feel free to Contact Roofglaze to discuss your project requirements with our expert team.

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