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.

In the realm of roof and building design, the terms “non-fragile” and “walk-on” are often used interchangeably, but they represent distinct concepts. It is imperative for designers and specifiers to recognize these differences to avoid over-specifying or, more critically, under-specifying these specialized products.

Roofglaze’s Technical Team is poised to guide professionals through this nuanced landscape, ensuring that rooflights are chosen and specified with precision, aligning seamlessly with the project’s needs, safety requirements, and design aspirations. Their expertise serves as an invaluable resource in the pursuit of excellence in construction and architectural design.

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.

On the other hand, a walk-on rooflight is engineered with structural requirements in mind and is intended to bear the weight of deliberate foot traffic. These rooflights are specified in areas where regular access by individuals is part of the design, such as rooftop terraces or observation decks. Their robust construction and load-bearing capabilities make them suitable for these purposes.

Understanding these distinctions is essential for architects, designers, and specifiers to ensure the appropriate selection and placement of rooflights within a building’s design, prioritizing both safety and functionality.

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

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.

To facilitate the correct selection of glazing types for specific applications, the CWCT has developed Technical Note Number 92 (TN92). This invaluable resource serves as a guiding framework for designers and specifiers, aiding them in making informed decisions about the appropriate glazing materials for their intended use.

While non-fragile rooflights are designed to enhance safety by preventing falls or objects from penetrating them, it’s crucial to understand the distinctions within this category to ensure the utmost precision in design and specification. On the other hand, walk-on rooflights, with their robust construction and load-bearing capabilities, are the designated choice for areas where deliberate foot traffic is expected.

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

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. Experience the convenience of our professional cleaning services. Our dedicated team will leave your space spotless and fresh, so you can enjoy a clean and inviting environment.  

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.

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.

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.

In the case of Class 1 and Class 2 roofs, it becomes essential to subject the chosen glass specification to rigorous testing to determine its fragility characteristics. This critical testing process adheres to the guidelines specified in the Centre for Window and Cladding Technology’s Technical Notes 66 and 67, commonly denoted as TN66 and TN67.

However, the practical and commercial feasibility of conducting this testing on every size of glass to be used can present significant challenges and costs. Recognizing this, the CWCT has introduced a practical solution in the form of TN92, a “deemed to satisfy” document. TN92 serves as a reliable reference, indicating that certain sizes of glass conform to the testing requirements outlined in TN66 and TN67. By doing so, TN92 effectively eliminates the need for extensive and continuous testing, streamlining the process and making it more accessible for designers and specifiers.

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


Atlas Lantern Regular

Atlas Lantern Contemporary

Korniche Lanterns

Atlas Lanterns

Wendland Lanterns

Korniche Lanterns

Atlas Lanterns

Wendland Lanterns


Skyway Complete - The Total Rooflight Solution

Skyway Lid Only

Skyway Complete

Rooflight + Upstand 

Skyway Ventilation


Skyway Complete

Rooflight + Upstand 

Skyway Fixed