Natural slate is like wine – BS EN 12326 the “appellation contrôlée”
Think of slate like wine: the better the raw material, the more the amount of refining, and the
more attention that is given to quality control procedures, random sampling and checking
throughout the production processes, the better and more expensive the final product will be.
Like wine, information about the origin and processing should appear on the label or certificating
documents. The most important consideration based on the range of tests detailed in the
standard (BS EN 12326) is the minimum slate thickness for use in our UK climate. This
minimum thickness can, indeed should be calculated based on all the individual test results
using the documented factors for the UK method of installation and UK climate. However, many
slate suppliers use the results of tests undertaken by the quarry on smaller and thinner slate
sizes used in France which, without recalculation are inappropriate for the UK.
A summary of the standard and its interpretation is listed below;
1) It is a requirement of EN12326 that an Accompanying Commercial Document (ACD)
should be with every consignment of delivered slates. The official ACD is a minimum of
3 pages long as it must include an interpretation of the tests and the results. It is
impractical to include this document in every crate of slates, however, this ACD should
be requested from the supplier and the crate label should also provide some of the more
pertinent information for the user confirming among other things its origin, dimensional
stability, strength, water absorption (i.e. A1), thermal wetting and drying performance
(i.e. T1 or T2) and sulphur dioxide exposure performance (i.e. S1).
BEWARE IF THIS INFORMATION CANNOT BE OBTAINED
2) 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, even up to around 10mm thick.
BEWARE OF SLATE WITH A STANDARD 4-6mm THICKNESS AND LOW
STRENGTH
3) 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, A1. If it is grade A2 a slate must undergo a
freeze-thaw test. If this shows no deterioration in strength after the freeze thaw test it is
suitable, although in practice this is an unrealistic expectation.
4) Thermal wetting & drying 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 5 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.
5) 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.
6) Slate with a carbonate content less than 20%, see 5 above, 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 5 above and the strength, see 1
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. 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.
BEWARE IF YOU DO NOT HAVE THE INFORMATION TO DECIDE WHAT THE
APPROPRIATE THICKNESS IS FOR THE SLATE YOU ARE CONSIDERING.
Misleadingly, many slate suppliers will use exactly the same ACD and test results for the
various grades of slate they supply from one quarry source, be they First grade, Mixed,
Standards or even Thirds. If this is the case, then why not use the lowest and cheapest
grade of slate as you should rightly expect exactly the same performance? It should be
obvious to most users that there are many reasons why a slate is categorised as a
particular grade by the quarry, some of which are performance related (a Standard
grade may be S2 rather than S1) and some are dimensionally related (more twisted,
more variance in thickness, more variance in squareness etc.).
It is well worth paying for a top quality A1, T1, S1 tested slate with a strong
characteristic modulus of rupture and supplied to the correct minimum thickness for the
UK. You then know that you will have the right product. Of course, there are other, more
subjective criteria, that make a slate ‘good’ to use, such as its ‘workability’ and ease of
installation e.g. flatness or consistency. None of these are covered by the Standard but
are nonetheless important considerations for the user. If you are buying a less
expensive lower grade product ensure that it is the appropriate thickness and be
prepared for additional time and expense spent sorting, grading and fixing. Like wine
you should look for value for money. If you buy a cheaper product you could end up with
a hangover the morning after!