CROSS Safety Report
Stone cladding panel concerns
A reporter is concerned about the risk of failure of stone cladding panels, in particular in respect of testing and assurance of the quality of the stone.
Key Learning Outcomes
For architects, engineers, and other specifiers of stone cladding systems:
- Give attention to the whole design of stone cladding systems and the safety-critical aspects of their materials and fixings
- Take adequate specialist advice
- Consider all good practice guidance
- Ensure adequate testing regimes are in place
- Cladding design and installation should be given the same degree of attention as the primary structure during both design and construction
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A reporter has concerns about the risk of failure associated with stone cladding panels. These systems can, says the reporter, place mechanical demands on the stone that go beyond those seen in other uses. For example, mechanical fixing systems for such panels could result in tension and shear forces within the stone which lead to failure, therefore according to the reporter, rigorous quality management systems are required to assure the quality of stone supplied.
CROSS report 467 published in July 2019 emphasises the need to address correct fixing and corrosion protection. The reporter argues that such attention must extend to the underlying nature of the stone itself and not be confined to fixing details. It is in any event, obvious that fixings are only as good as the material they are interacting with.
The issues become more acute, continues the reporter, with moves away from granite towards softer stones such as limestone and sandstone, the use of thinner panels, designers pushing technical boundaries, and possible use on buildings subject to higher wind loading.
Stone is a variable material and material extracted from the same quarry may be suitable for use or completely unsuitable. The reporter goes on to say that typically, a quarry operative makes an assessment as to the quality of the stone available at any available face as suitable or unsuitable. A block is then retrieved from the quarry face, placed on the quarry floor (along with others) to season, selected to be transferred to the stone processor to be cut into slices, squared off and (if necessary) polished. The material is then packaged, dispatched and eventually fixed into position on the building.
The reporter is concerned that insufficient action may be taken in respect of testing and assurance of quality during production and fitting processes, to check and place on record that the actual stone panels delivered to site have been assessed with regard to their suitability, particularly strength, as compared to the technical specification.
Expert Panel Comments
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Stone has been used as a cladding material for centuries and, particularly with regard to the use of indigenous stones, successfully, with many historic buildings continuing to function as originally intended. In modern times, the way in which natural stone is placed on buildings has however developed and is increasingly used in systems such as rainscreen cladding.
The reporter has concerns regarding the selection and use of natural stone and the possibility of untested material being used within cladding systems. There are also concerns about the actual fixing of the stone to the carrier systems, the implication being that risk of detachment may be present. The risk may be considered real if the stone is only assessed and selected on its visual appearance. Indeed, in the absence of appropriate quality control, unfamiliarity with statutory and good practice guidance, then failures could occur. Designers should be aware that this guidance exists and follow it.
The engineering properties of natural or quarried materials can be extremely variable and should be approached with caution in design. This is especially true for those materials which exhibit brittle failure modes, such as natural stone, and where any inherent weakness in the materials may lead to sudden and catastrophic failure in a demanding application. Recent building trends could place even greater demands on cladding systems, with the desire for more efficient and aesthetically pleasing facades pushing the boundaries of material performance.
engineering properties of natural or quarried materials can be extremely variable and should be approached with caution
Testing and quality control
Requirements for testing and declaration of the performance of natural stone elements for cladding are given in the harmonised standard BS EN 1469;2015 but a declaration to this standard may not give sufficient information for the undertaking of an adequate design, and the designer should consider whether this standard includes project-level testing with sufficient frequency as to provide adequate assurance of the as-delivered product.
The assessment of the suitability of a particular stone for a cladding system is generally set out within the BS 8298 suite of documents. Part 4 ‘Design and installation of natural stone cladding and lining – Stone cladding on rainscreen support systems – Code of practice’ provides guidance for specifying natural stone for use in rainscreen systems. The recommendations of this code of practice supplement the requirements of the harmonised standard, including in the areas of frequency of sampling and testing of the product.
The standard emphasises that stone is a natural material and that it is ‘liable to vertical and horizontal variation within its mass’. Specifiers should be aware that the nature and properties of stone can change as beds are exposed and removed. Specifiers must be aware therefore that, for instance, veins and other inclusions within the body of the stones are likely to exist. There are indigenous stones that have been used for generations within the UK on buildings and many of these have been proven with similar lengths of service. In all cases, the provenance of the stone to be used is critical in the selection process both in terms of its historic use and also the type of cladding system it is intended for. Where a particular stone has demonstrated historical performance over a number of decades then selection presents less difficulty when used in a similar application and environment.
Seeking specialist knowledge
BS 8298 Part 4, however, importantly refers to the appointment of a ‘stone specialist’ when the quality of the stone is uncertain or product control testing for a project is considered necessary; the necessity for this appointment should be considered at the concept stage of any stone rainscreen project. The standard defines the stone specialist as an individual who is ‘not affiliated to the stone fixing company’ and who is experienced in:
a) assessing the impact of geology on the likely performance of the stone;
b) the stone extraction process and the impact on the stone’s quality;
c) selecting and testing stone, and evaluating results; and
d) the engineering performance and design of natural stone cladding
The standard contains a suite of testing that the stone specialist would consider at the time of specification, also drawing in the declaration of performance and product control. The specialist will use their experience to examine the provenance of the suggested stone, its history of previous sampling and testing, assess its variability and such parameters as unit thickness. Clearly, the understanding that such a specialist brings to a project could be invaluable.
The correct selection of stone would clearly be of no benefit if the system used to attach it to the building was inadequate. For this reason, section 5 of Part 4 is dedicated to the behaviour of the primary structure and the applied metal parts. Similar to the stone, fixings are recommended to be tested in various ways and are also subject to the advice of the stone specialist.
Finally, there may be a lack of appreciation of the very high (both positive and negative) local wind pressures generated on the leading edges of buildings and roofs which must be taken into account in the design and specification of any cladding system. The pressures are impacted by the building’s geometry and other criteria unique to the building and its setting. As the reporter has suggested, when boundaries are pushed, designers must be careful to consider local pressures and other environmental effects that might shorten the life of the cladding.
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