Roofconsult Website The Quality of Natural Slate: An Economic Perspective by Dr Chandra Durve of Camborne Slate
Contact Us
Industry News
Check out our web directory of the UK roofing and cladding industry

Sign up for our monthly news letter.

Over the past 15 years or so imported slates have been the subject of both controversy and misinformation, according to Dr Chandra Durve of Camborne Slate, ‘the originator of the Brazilian roofing Slate and first to mine and apply the slate on roofs’. Here he examines the issues and tries to provide answers to a number of frequently asked questions
In the last two years around two thirds of claims against the NHBC Buildmark warranty have related to pitched roofs, with those relating to slate showing an upward trend. For roofing contractors, a better understanding of issues affecting slate is critical if potential problems are to be identified. In contrast to man-made roofing products manufactured under controlled conditions by just a few predominantly European manufacturers and suppliers, natural slate used in the UK is mined from sources throughout the world under variable geological conditions and imported by a large number of suppliers. Stiff price competition and tough economic conditions have prompted the sale of low-cost material thereby prompting quality issues to arise.
Defining failure
Whilst a roofing slate is a very durable material compared to man-made products, this durability can only be achieved by careful extraction and the application of good quality control. Slates mined from the same area even within metres of each other can often vary dramatically both in terms of physical characteristics as well as quality.
Quality relates to the production of the complete slate from mining a geologically defined vein rather than randomly testing finished slates. Efficient extraction involves following veins in a rock mass with uncertain characteristics and uniformity. The highquality roofing grade material that the UK market needs requires proper quality control to ensure that, once applied to the roof, it remains completely impermeable and provides a weatherproof barrier over time. To achieve this, slates must have no interconnecting pores from one face to the other and their fabric should contain predominantly micaceous minerals with little or no substances which could adversely affect long-term performance. Failure takes place if pieces of the slate begin to break away, causing cracks or holes to appear, in contrast to aesthetic failure in which a slate either changes colour or suffers partial delamination without necessarily affecting waterproofing properties. Even so, the fact remains that a change has taken place and so it is important to understand the nature of the change in the context of failure. An example of slate failure in Manchester
So, how can failures be avoided and what quality criteria should be applied to reduce the chances of it happening? Use of the new British Standard (BS EN 12326) is a good starting point but although it provides a good benchmark it is not a guarantee of durability. This can only be achieved by understanding the key factors involved in the mining of slate and examining the supply chain or track record of the supplier. Slates which bear the genuine Camborne mark for example, as distinct from what is said to be equivalent to it, have been used throughout the UK by all the largest suppliers for the past 28 years.
The quality of slate
Slate forms over millions of years and like clay and concrete tiles, are formed from a mix of fine particles. With clay tiles, rigidity results from firing a man-made mix at high temperatures, whilst with concrete tiles a finely ground limestone mix cures in air at normal or slightly enhanced temperature. With slate, however, the naturally occurring mud particles carried by rivers or deposited by volcanic activity initially settle underwater. Largest and densest particles settle first and the smallest last with new deposits being built up to create what are known as bedding planes. When subjected to high levels of heat and pressure over millions of years these recrystalise to form new mica minerals which align themselves at right angles to the applied force. Through going pyritic growth in slate
This produces slate and it is this alignment which enables the slate to split into thin sheets. Unfortunately nature is irregular and chaotic, and in addition to carrying micas and mica-forming minerals the material also contains small quantities of other light and heavy minerals including iron, titanium, sulphur, calcium, manganese and even small amounts of gold! These settle at different rates and their concentration and interaction with the mica minerals has a major effect in determining performance. It is the ability to identify and separate the clean masses of slate veins from the surrounding heavily mineral-laden and unformed slate which determines ultimate quality and performance.
Mining variations and economics
In a quarry, the definition and selective mining of these veins requires considerable skill. The grade of a slate can be controlled by continually sampling product before and throughout the mining process and in the case of Camborne Brazilian slates, such samples are subject to a process of analysis driven by BS EN12326. Poor-quality material is left behind or removed to enable continuous mining of the vein. Experienced miners can identify good veins by colour and texture, the slate’s colour stemming from tiny amounts of chlorite, graphite and carbon which distinguish sources such as the finest Welsh ‘Old Seam’ Slate or the Camborne grey slate. Neither colour nor texture, however, determine quality although such brand names are routinely misused as generic by those seeking to sell cheaper ‘common-grade’ materials.
Flatter slate seams as in the Camborne ZPP quarries in Brazil make open cast mining far more economical. Nonetheless, opening a quarry requires considerable financial investment and geological research including diamond drilling and core sampling followed by movement of large amounts of overburden to expose the slate beds prior to mining. The stripped material usually contains a mix of low-grade slate and these tips often provide the opportunity for some to produce lowcost, deceptively nice-looking slate!
In terms of how to avoid poor-quality slate, quality is invariably therefore linked to price. You don’t have to be a rocket scientist to work out that if, for example, a standard slate from Brazil costs roughly £0.30 just to transport it, by the time importer and exporter margins and taxes have been added there is invariably very little left to produce the slate!
For further information on Camborne Slate see
Home > Articles