Technical Guidance


Before the publication of the first edition of ACR[M]001 by the Advisory Committee for Roof Safety in 2000, there was no clear guidance of how manufacturers and installers of systems could demonstrate that the requirements for non-fragility of roofs were being met…

Now ACR[M]001:2014 Test for Non-Fragility of Large Element Roofing Assemblies [fifth edition], often referred to as the ‘Red Book’, prescribes how a representative roof assembly should be tested to demonstrate its ability to resist the impact of a person falling onto it, and then supporting their weight.

According to the ACR document, an assembly can be classed as either fragile or non-fragile. The test covers all components including all of the fasteners, sealant tapes etc – that are required to be representative of the finished installation.

The application of the test and classification for in-plane rooflights is further described and defined in The Rooflight Association (RA) Technical Document NTD03.

The Test Procedure

The test should be carried out by a competent person. It involves an ‘impactor’ comprising a 300mm diameter cotton weave sandbag filled with 45kg of soft sand, being dropped from a height of 1.2 metres onto the roofing assembly.

The test sample is built on a standard roof rig representing a typical metal roof structure one metre high from the ground.

The impactor should be dropped in specific worst-case scenario zones, depending upon the assembly, where are generally:

  • Within 150mm of the centre of the sample.
  • Within 300mm of a support point.
  • Within 150mm of the assembly edge, adjacent to the underlap with the next sheet.
  • Where ever the competent person deems the worst-case scenario to be.

Classification of Results

If the assembly fails to survive the impact, and the impactor passes through, then the assembly is classed as fragile.

If the impactor is retained by the assembly after the first drop for a minimum of five minutes, it can be classified as non-fragile and may be used for, or within roof construction.

If, after the second impact at the same location, the impactor is retained for a further five minutes, the assembly can then be rated as Class B non-fragile.

If the impactor is not retained for at least five minutes after the second impact, then the assembly is rated as Class C non-fragile.

Class A non-fragile is only achieved where no part or component of the assembly suffers any damage or detriment because of the test, that might affect the long-term strength or weatherability of the assembly. Such is the destructive nature of the test; no known self-supporting lightweight metal roof assembly can achieve an ‘A’ rating irrespective of the performance of the rooflight. Most metal cladding system manufacturers design their systems to achieve Class B. It should be noted that where any part of a roofing assembly is unfixed or partially fixed, it should always be treated as fragile.

Period of non-fragility

A number of factors will impact on the period of non-fragility which will be achieved by a roofing assembly including the quality and durability of the roof materials, and the quality of installation of the whole roof assembly.

Manufacturers can only indicate the expected period of non-fragility from extensive testing carried out and cannot offer performance guarantees for considerations such as installation quality, which are out of their direct control.

Further guidance is available from The Rooflight Association and The Metal Cladding and Roofing Manufacturers Association (MCRMA).

Imported rooflights claiming the same or similar specifications and not manufactured to BS EN1013, and the UK Annex to this Standard, cannot be assumed to achieve the same non-fragility ratings, or for the same periods of use.  In such cases, proof of conformant non-fragility testing should be requested by the Specifier/Contractor.

For more information on the different Zenon rooflight configurations and the fragility classifications given, you can download the section from our technical manual here.

The next section of the technical guide looks at fire performance.