Peter Wilson
Manifesto Centre for Architectural Research, Napier University

Unlike most other construction products, natural stones – even ones selected from a single quarry – can vary considerably in strength, breaking load, density, porosity, water absorption and frost resistance and, as well as colour, texture and veining. While these latter qualities relate to visual appearance, the former characteristics are crucial to the stone’s ability to perform successfully, particularly in non-loadbearing situations. Before selecting a stone type for use in cladding system, therefore, the architect or designer should give due consideration to the following points:

• Is the stone safe to use in the intended environment?

It is important to establish where the material is to be used and what it is to be used for. Only a few sandstones are suited to use as cladding, for example.

• How thinly can it be used?

It should always be remembered that stone is a natural material and may vary considerably from slab to slab. If used too thinly it can also bow and may be subject to fairly rapid deterioration.

• What is the largest size of panel or tile possible?

There is a clear relationship between the length of a stone panel and its flexural strength and it is important to establish the minimum breaking load for the slab thickness intended.

The answers to these, and the many other points highlighted in Table 5 ‘things to remember when selecting stone’ can only properly be discovered through testing, an essential process previously covered by an extensive list of British Standards and now – with an ever-expanding range of imported stones of uncertain provenance available – subject to rigorous European regulation. These European standards provide a valuable framework for assessing a stone type’s suitability, and place the responsibility for obtaining a CE marking – and the performance of the stone (although not its installation, which is still covered by British Standards) – firmly on the company converting the block into the final stone product. The supplier has a legal obligation to provide information on the lowest expected values that have been achieved by the stone type in question.

Testing should begin as early as possible in the design process and the testing of natural stone products to be used as cladding should always be supported with the results of initial type testing carried out by the quarry/factory/supplier. The quarries should also provide test data sheets which include historical information. All of the test requirements are set out in the new series of British Standard test methods which are common to CEN members. All other test methods traditionally used – and still submitted by some stone suppliers – have now been superceded.

It is important to carry out tests relevant to the particular project, however, since not all tests are appropriate in every case. Equally, although a CE marking is of immense value in ensuring that minimum standards have been met, it should not be regarded as a panacea since it will only refer to that part of the quarry that the certified stone comes from. The Stone Federation Great Britain and BRE can provide guidance on the interpretation of test results.

The list of key standards now approved at European level and published with BS EN numbers is shown below. Standards that set out test methods form part of the approvals procedure for products and have to be adopted in place of the various test methods currently used. Standards for products which have been harmonised will soon require those products to be CE marked before they can be placed on the market.

Stone supplied for use as cladding should be tested in accordance with the methods set out above. Slabs for cladding will eventually be covered by the new product standard (EN 1469 Natural stone – Slabs for cladding), the basis of CE marking. Companies supplying slabs for cladding will be required to carry out the testing and/or declare the results. For factory production control the frequency of the tests should be as fig. 2 below.

Comprehensive testing of a desired stone type at the earliest possible stage in the design process will not only help to prevent expensive mistakes being made, but will provide both designers and building owners with a reasonable degree of confidence on the future performance of the stone selected. It cannot emphasise too highly however, that visiting the quarry is an important part of the process, and that the use of proven indigenous material can provide an additional element of quality reassurance.

Fig 1: European standards