Slate quality has had a chequered recent history. BS EN 12326 should have imposed common European requirements that are understandable to suppliers and specifiers. The NBS Technical Team look closely at the rock face.

BS EN 12326 Slate and stone products for discontinuous roofing and cladding was introduced in 2004 and comprises two parts:

  • BS EN 12326-1 Product specification
  • BS EN 12326-2 Methods of test.

Even though there are defined requirements and tests within the standard, specifiers are still finding it difficult to understand why slates that adhere to the standard can be so different in quality. Although slates may conform to the standard and carry the correct CE marking, this does not say anything about their performance or quality in use, the two main criteria looked for by specifiers.

Whilst there is a comprehensive list of slate requirements in Part 1 and 13 tests in Part 2, the standard tends to fail the specifier. The main problem is with the pass/ fail limits for each test, in some cases these are ambiguous. For example, the water absorption limit is set at < 0.6%, if a slate is lower than this it passes and is given code A1; higher than this, but with adequate freeze-thaw resistance, it can still 'pass' with code A2. In the superseded standard, BS 680-2 (1971) Specification of roofing slates, the water absorption limit was set at < 0.3%, which meant many 'good' quality slates failed.

These may now pass the BS EN 1236 test! If BS EN 12326 was amended to have a grading system like the French and USA systems, where slates are given codes/ grades on each test, this would allow specifiers and clients to choose their slates accordingly. Poor quality slates, even those that have passed tests to BS EN 12326, are likely to fail or have a shorter lifespan than say better quality slates. The main problems are:

  • Water absorption: The more slates absorb the more they are likely to fail due to cracking, increased weathering, and damage caused during freeze-thaw cycles
  • Carbonate content: Too much and the durability of slates will be affected, although at present there is no limit on carbon content, only that it 'shall conform to the manufacturer's stated value'. Slates with a carbonate content of ≤ 20% are subjected to a sulphur dioxide test and coded S1, S2 or S3 – giving criteria on their acceptability for use
  • Pyrites in slates: This can alter the stability of the slates, the Thermal Cycling Test will show whether or not the slates will oxidize with results expressed as codes T1, T2 or T3. Note that slates to code T3 'may result in water penetration'!

When specifying reroofing, it is worth noting that the standard does not take into consideration the durability of slates that are being re-used. Another problem is with cross reference of standards. BS 5534 (2003) Code of practice for slating and tiling (including shingles), still refers to BS 680-2, and has not been amended to include BS EN 12326. This causes confusion for slate suppliers, specifiers and roofing contractors alike.

When researching slates for your projects, you may note that many suppliers' websites still claim compliance to BS 680. For those that claim compliance with BS EN 12326, testing and carrying a CE marking is a legal requirement for products. But as explained above, declarations of conformity are no guarantee of a slate's suitability for a project location. The only way of guaranteeing quality of slates is to buy from a reputable supplier; one that can provide documentation of BS EN 12326 test results and who tests continuously to prove the consistency of a quarried natural material.

At present in NBS section H62 – Natural slating, general guidance 5 outlines the requirements of BS EN 12326. However, the most important NBS guidance is that to clause 280, where it states:

'It is not sufficient to specify slates to BS EN 12326-1 only, since the standard includes several different levels of conformity and the slates supplied could be at the lowest level'.

Clauses 110–150 also state that:

'The best way to obtain good quality slates is to specify a particular supplier's product that has evidence of durability in the prevailing climatic and environmental conditions.'

We will now review NBS to provide any necessary supplementary guidance based on reputable slate suppliers' experience of using BS EN 12326.