Searching for consistently good-quality slate

Michael Hallé, commercial director for Welsh Slate, advises specifiers and contractors to ask for third party testing.

Natural roofing slate is, by its very nature, not produced from a perfectly consistent raw material. Stone which has an ability to be split is quite common, and as well as slate includes mud stone and some sand stones. What is not common is consistently good-quality roofing slate.

The formation process of slate is similar wherever it occurs in the world. It is a metamorphic rock which means in simple terms it started its life as mud, clay, silt or ash layers that have undergone a low-grade metamorphism (change) due to heat and pressure.

This heat and pressure changes the molecular structure of the rock, and in the case of slate gives it the ability to be split into thin sheets. This is by no means a quick process. In the case of Penrhyn Welsh Slate, the process started more than 500 million years ago (as long as any other slate in the world) when clay minerals were washed down a river and deposited in a sea.

Fast forward about 100 million years or so and the volcanic activity that eventually formed what we know now as Snowdonia applied the necessary heat and pressure to change the now mud stone into the slate that is extracted today.

The regions of the world where slate is found tend to be mountainous as in most cases the massive tectonic forces that formed the mountains also formed the slate. What is different is the quality of the slate that can be extracted.

Differences in the quality of natural slate were detailed in a Royal Commission as early as the mid-1800s but not formalised in the United Kingdom until the issuing of the London Counties Standard in 1938 which became, to all intents and purposes, the British Standard BS 680 in 1939. Both of these standards were set up in light of premature failures of some slates after as little as five years.

The testing element within the standard, which was written by (or with the influence of) the larger slate quarries, set a very high threshold for wetting and drying, acid emersion and water absorption, based on testing of slates with a proven durability in the U.K. environment. Only a few quarries in production (such as Penrhyn, Cwt-y-Bugail, Ffestiniog, Greaves, Dinorwic, Delabole, Kirby and Elterwater) could meet this.

But the testing element was only a small part of the whole standard. The standard also detailed the requirement for longitudinal grain and consistent thickness as well as the permitted sizes.

The British Standard was superseded in 2003 by the often maligned European Standard BSEN 12326. This is an inclusive standard whereas BS 680 was exclusive. That is to say the European Standard includes classification for all roofing slates produced in Europe – good, bad and indifferent. Some suggest that because there is no “fail” within the standard that it is weak but it just needs the reader to look a little bit closer to see what the resulting test certificate means.

There are three main testing criteria to the standard – water absorption (W), thermal cycling (T) and sulphur dioxide exposure (S) – and each test has three separate classifications. The top performing slates would be classified as W1, T1 and S1.

The water absorption test is one of the most critical with there being a statistical correlation between water absorption and durability – the lower the water absorption, the more chemically durable the slate.

Poorer performing slates which have a tendency to rust would be classified as T2 or T3. Slates with high calcium carbonate content, which can lead to the premature fading often associated with Chinese imports, would have a S2 or S3 classification. Welsh slate from both the Penrhyn and Cwt-y-Bugail quarries achieves the top classification.

Further afield, while the European Standard only relates to slate produced in Europe, slates from other countries such as China, Brazil, Canada and America also need to show compliance with it to be able to be CE marked and therefore legally sold in Europe. Even when, or if, the UK is out of the EU, the SEN Group which oversees European Standards will still recognise the standard in the UK as it is not directly related to the EU.

Most importantly, the standard states it is beholden on all manufacturers to test their slates to the BSEN standard at least annually and whenever there is likely to be significant differences in the properties of the slate, as when slate is extracted from an area which has not been worked and tested in recent times. Every crate of roofing slate sold must have the details of the testing results and copies of test certificates should be readily available.

Many quarries “self-certify” the testing, and while this is allowed, some may argue it does not carry the credence of independent testing. While Welsh Slate test internally throughout the year we also use an accredited third party and proudly hold the BSI Kitemark as proof of this.

It is not uncommon for us to record slight variances from test to test (although they are always within the top grade) but that is exactly what you would expect from a material that was not formed in a consistent factory environment.

But we have also viewed historical test data from other manufacturers and note with interest that some test results do not change from year to year, the conclusion being that their stone deposit is either the most consistent natural material known to man or their testing regime is not up to standard.

We would advise specifiers and contractors to ask for the most recent test for the slates they want to use but to also not be afraid to also ask for tests from the past few years for comparison.