Natural slates – Quality and standards

BS EN 12326-1:2014

A break from the past

Until 2004 natural slates for roofing in the UK were covered by BS 680-2:1971. First written in 1944 when the only slates available in the UK were of British origin it included 3 simple pass/fail tests. There was no need for any proof of origin nor identification of samples, no requirement for regular repeat testing and no minimum number of samples to be provided. The standard existed against a background of a limited number of known sources, “craftsman” trades people with apprenticed experience of the material and decades of evidence of the performance of the rock used for roofing in the British climate. 

The Europe wide natural slate industry

Since the 1980’s there has been a rapid growth in international trade in roofing slate. Spain has become the predominant source of roofing slate in Europe. Indigenous industries still exist in the traditional European slating regions alongside the imported Spanish product. Imports of slate from further afield, the Americas and the Far East are also now established.

The European Union covers a land mass with a wide range of climates from Mediteranean to sub-arctic, maritime to continental. It covers regions with varied styles of building practice and differing politico/legal systems. In the spirit of European harmonization and with the desire not to penalize local indigenous slate industries producing stone suitable for regional climates or building traditions EN 12326 is based on a classification system where the onus is on the user to select a slate of the appropriate quality for his project and locality. 

Obligatory requirements

  1. Slate packaging must display a valid CE mark.
  2. Slate labeling must include the prescribed mechanical and performance data.
  3. Slate labeling must identify the origin of the product.
  4. Thickness of packed slates for 100 slates on each pallet.
  5. Comprehensive test data required by the standard must be available on “accompanying commercial documents”.
  6. Petrographic testing must be undertaken annually.
  7. Assessment of dimensions to ensure that no more than 4% of slates are outside dimensional tolerances.
  8. Other performance testing to be undertaken every 25,000 tonnes of production or annually, whichever is sooner.


Key performance measures mentioned in the standard

Strength - Slate should be strong enough to be used in the UK climate. The top of the range best Welsh slate between 4-6mm thick exhibit a Characteristic Modulus of Rupture (CMoR) of approximately 70 MPa, slate with a CMoR of around 35 MPa slate can be used successfully if they are correspondingly thicker, slates up to around 10mm thick are used in the UK. BEWARE OF SLATE WITH A STANDARD 4-6mm THICKNESS AND LOW STRENGTH!

Water absorption - In general, the more water a slate can absorb the sooner it will fail. In the UK a slate should absorb less than 0.6% to be reliable, W1. If it is grade W2 a slate must undergo a freeze-thaw test. If this shows no deterioration in strength after the freeze thaw test it is suitable.

Thermal cycle wetting & drying - This identifies inclusions which could cause premature failure. A T1 rating allows for no change in structure and minor discolouration. T2 allows more discolouration – sometimes runs - but no change in structure. T2 can be selected if they contain a low carbonate content i.e. less that 5% (see "Slate with a carbonate content less than 20%" below). T3 shows the presence of metallic minerals that risk forming holes in the slate. T3 are only suitable in the UK if used with a weatherproof membrane and should only be considered as a decorative cladding with no long term weathering performance. The higher the carbonate content the thicker the slate should be. In addition the higher the carbonate the more likely the discolouration. The standard recognises 3 levels of magnitude. Less than 5%, 5-20% and more than 20%. Slates in the greater than 20% category are best avoided in our climate.

Slate with a carbonate content less than 20% is categorised S1, S2 and S3. These categorisations help in determining the minimum thickness the slate should be when combined with the carbonate content, see "Thermal cycle wetting & drying" above and the strength, see "Strength" above. S1 slate is not affected by strong exposure to sulphur dioxide and is acceptable in all conditions. S2 exhibits a surface softening in the top 0.7mm of the slate and should therefore be thicker in proportion to the depth of softening according to calculation criteria in table 1 in the standard. S3 is affected by weak exposure and should be a minimum of 8mm thick upwardly adjusted according to strength and the depth of softening. Some of the best slates from Westmorland are an S2 classification but have a long proven record of performance in the UK.

Non-carbonate carbon content <2%.

BS 8000: Part 6: 2013

BS 8000: 2013 Part 6 Code of practice for slating and tiling of roofs and claddings is still in force. Section 4.3 Natural slates fittings and accessories, applies to all natural slates regardless of origin or grade. The key recommendations appear in section 4.3.1. 

“a) Sort slates into three or four groups of equal thickness.”

“c) Lay slates of equal thickness in any one course, with the thicker slates in the lower courses and the thinner slates in the upper course.” 

“Choosers” “users” and other stakeholders

BS EN 12326 is a product standard. Aspects of natural slate roofing not covered by BS EN 12326 relate primarily to brand selection and grading. Before a quality finished roof is achieved designers, specifiers, roofers, even clients and distributors have roles to fulfill. These groups have their own expectations and priorities. The phraseology they use in discussions on natural slate may be misunderstood or misused. Furthermore when slate is described generically the definition will change depending on the speaker.

Commonly used terminology

Bests, premium, firsts, seconds, thirds, mixed, thicks, clumps, heavies. These are examples of descriptions of grades of natural slate. The layman could be forgiven for thinking there is a standard rating for the quality of slates and that for example a “best” will be better than a “third”. One could also assume that durability is defined by these terms. Unfortunately this is not the case. These everyday terms do not appear in the BS EN 12326.

Quarries and importers

Each quarry and their importer will use these phrases to describe the output and amount of quality control a slate grade has undergone. That does not mean that a first from one quarry will necessarily be better than a second from another. The opposite could in fact be the case. Producers and importers use this terminology merely as an internal comparison. Furthermore not all quarries will provide a separate ACD for each grade of slate they produce. The less quality control and grading undertaken by the quarry on a particular shipment, the lower the quarries costs are likely to be. This allows the quarry and its agents to offer lower prices. It also means that tolerances on thickness and dimensional stability will be greater, putting an additional burden on the installer to sort and grade the slate. It also increases the amount of wastage.


The installer is ultimately interested in laying the slate. He is concerned with efficiency of laying, how fast the job will go, how much breakage and waste he will encounter and how much sorting is necessary on site. To the roofer consistency is vitally important. “Best” or similar terminology for a roofer will not necessarily be strongest or longest lasting, it will be the most regular in thickness and dimension. Historically sorting was undertaken by the roofer when he was counting out the slates for a particular slope (usually by weight, heavier slate will be thicker) and holing the slate (slate will be thicker at one end, the nail hole would be made at the thinner end) to receive the nail. Much natural slate nowadays is pre-holed either by the quarry/producer or the importer/distributor. The nail hole is not always correctly pre-holed.