A Brief History of Slate

Rock Classification

Rock is a natural, geologically-created material composed of minerals. ‘How’ a particular rock was formed leads to its classification which is generally either igneous, sedimentary or metamorphic: igneous rocks are formed when molten rock solidifies; sedimentary rocks form from grains of rock or organic matter deposited by wind or water and metamorphic rocks are either igneous or sedimentary rocks that have been transformed by heat and pressure within the Earth’s crust. Typical examples of rocks within these classifications are:

Igneous granite, basalt
Sedimentary mudstone, coal, limestone, sandstone
Metamorphic marble, slate, phyllite

SSQ quarries and markets slate and phyllite, two closely related rocks that are geologically defined as ‘foliated, regional metamorphic rocks created from fine-grained sediments’.

‘Third rock from the Sun’

The Earth was formed around 4,600 million years ago. As the embryonic planet cooled its surface hardened: it now comprises a relatively thin layer of solid rock (the crust) floating on a mass of molten and semi-molten rock (the mantle) surrounding a solid core.

The crust consists of about a dozen, quite separate, irregularly-shaped ‘plates’ that float on the mantle, constantly moving because of its powerful currents. This movement results in a number of ongoing, geophysical effects: the edges of some plates slip past each other horizontally; others collide, causing one to crumple to form mountains and the other to slide beneath it, back into the mantle where it will melt and be recycled; other plates are gradually moving apart, the resulting gaps being filled with new rock welling up from the mantle and solidifying.

The Earth’s cooling also caused water vapour in the atmosphere to condense into rain to create lakes, seas and oceans. The action of wind and water began to erode these first igneous rocks into grains which settled to form the first sedimentary rocks. The continual movement of the Earth’s crust gradually transformed some of these early igneous and sedimentary rocks into metamorphic rocks.

These continual geological processes happen on a timescale measured in hundreds of millions of years and the results are profound: they create the rocks we know today and shape the geography of our planet.


The Earth was formed about 4,600 million years ago. The oldest rocks we know of are less than 4,000 million years old; the oldest rocks containing fossils are about 3,500 million years old. Just 200 million years ago, the Earth’s land masses had come together to form a vast supercontinent called Pangaea, the most recent in a series that have been formed and then broken up by plate movement: Pangaea’s break-up has resulted in the continents and oceans we recognise today. Dinosaurs became extinct 65 million years ago; the Himalayas started to form 35 million years ago (and are still growing) and modern humans appeared just 130,000 years ago.

Slate and Phyllite

The slate quarried today started to form during the Lower Palaeozoic era (a period of time defined as between 417 and 545 million years ago); phyllite is older and dates back to the Pre-Cambrian era (over 545 million years ago).

Slate is composed of fine grains of quartz and mica, either muscovite or illite, along with a range of other minerals that frequently include biotite, chlorite, hematite and pyrite. The exact mineral composition and their relative quantities affect the rock’s appearance, for example: chlorite gives slate a green colour; graphite – black; hematite – red and mica brings a silvery sheen. Phyllite’s composition is quartz, muscovite and chlorite: essentially it’s slate that has been subjected to a longer period of metamorphosis.

Slate was formed as a result of fine-grained, shale-type sediments being deposited deep on the ocean floor, the thick beds slowly compressing to form a layered sedimentary rock. This was eventually buried by movement in the Earth’s crust, the immense heat and pressure gradually transforming it into harder, stronger metamorphic rock.

This burial process often takes place in areas where neighbouring plates collide and are sometimes referred to as ‘Slate Belts’. A typical example is that covering north Wales, Cumbria, southern Scotland and Northern Ireland and marks part of the area where Laurentia (the plate containing Scotland and North America) collided with Baltica (Scandinavia) between 400 and 600 million years ago.

Mineral inclusions and pyrite

Most rocks contain ‘inclusions’ (a body or particle of another substance embedded in their structure), some are reactive, others are stable. The most potentially damaging inclusion in slate is pyrite, a shiny yellow crystal of iron disulphide (FeS2) formed from particles of organic material present in the sedimentary muds and crystallised during the rock’s metamorphic processing.

The presence of pyrite in a slate can be potentially disastrous. When exposed to water vapour in the air iron disulphide reacts, decomposing to form powdery iron sulphate and sulphuric acid. How quickly this happens, and its effect on the slate, varies:


  • It may just cause a stain around the inclusion
  • It may leave an unsightly, rust-coloured ‘run’ on both the slate and those in lower courses
  • Its decomposition will leave anything from just a pit in the surface to a hole through the slate
  • The slate may delaminate and split, or even totally disintegrate


The Thermal Cycling Test specified by EN 12326 tries to predict the effect of pyrite in the slate and offers three classifications:

T1 means there may be some discolouration around the pyrite itself, but the structure will not be affected

T2 means that although the pyrite will leach to form unsightly runs of discolouration, structural changes are unlikely

T3 means that holes may form as a result of the pyrite decomposing

Beyond this, if exfoliation, splitting or other structural changes occur, the roofing slates being tested are considered unacceptable for use.

It must be stressed that not all inclusions are either pyrite or reactive: many are inert and may do nothing other than perhaps give the slate a ‘sparkly’ appearance. But if inclusions are present it’s vital to look at the test data accompanying the slates and check the result of the Thermal Cycling Test.

Generally, if an inclusion is ‘cubic’ and is tightly formed with ‘neat’ edges, it’s probably inert and stable.

However, the only way to confirm this is by looking at the result of the Thermal Cycling Test in the slate’s test report – this will grade the level of risk as being either T1, T2 or T3.

The effect of a pyrite dissolving and leaching to cause runs of discolouration on a roof.

The stain is permanent and cannot be removed. The slates affected will have to be removed and replaced to prevent further contamination and ensure the roof maintains its watertight integrity.

A close-up showing how a pyrite has oxidised and spread within the slate’s structure. The weakness may result in a hole or delamination.

Why natural slate

choose slate main

If you’re trying to decide between a variety of different materials for your roof, you may be wondering why Natural Slate? For those of us slate lovers, the answers are obvious. But some of you may need a little more convincing. So let us spell it out:

  • Slate is far and beyond the most beautiful and stylish roof covering material available.

Don’t just take our word for it! Check out our website and compare our gorgeous products against alternative materials on the web. We can send you samples quickly, or simply some high resolution pictures so you can make up your own mind.

  • Slate has unique and attractive design potential.

Slate is practically “engineered” by Mother Nature. Its ability to be shaped onsite and combined with other sizes and colours makes it much easier to use in complex projects. It’s VERY adaptable. As if it was meant to be that way. Standardisation is an alien concept to skilled roofers and at thicknesses starting from 3mm the possibilities are truly endless.

  • Slate will even outlast the life of the building. Once slate is installed, you’ll have very little to worry about.

We are so confident about this and we WILL guarantee this.

It is relatively inexpensive compared to other materials. Yes really! You’re not hallucinating. Over time, other materials will break down more quickly. And so they will need to be replaced and the roof “reroofed”. With good quality slate this is never an issue. Read more here.

  • Slate adds value to structures.

Good slate = high quality. High quality = more value. A recent study showed that the value of your building is influenced by the materials you use in its construction. As slate is classified as a high value material, you have nothing to lose and lots to gain by specifying natural slate. It’s a win-win situation.

  • Slate is suitable for modern architecture.


You may think of slate as being old-fashioned, but that’s where you would be wrong. The flexibility of slate goes hand-in-hand with modern architecture. Really! A sleek look is easily achievable and you are limited only by your imagination. So start thinking outside the box.


  • Good quality slate will not fade or deteriorate.


No this isn’t just a rumour. It’s a fact. And fact we are happy to guarantee in writing. Your slate will outlast the building it is on.


  • It’s low maintenance.


Slate is lower maintenance than any of the roofing materials out there over its lifespan. Our technical department would be happy to go through this with you in detail.


  • Can achieve “A+” rating in the BRE Green Guide.


Speaks for itself really! Green is the way to go.

Slate vs Alternatives

delcarmen church detailIt’s difficult for any other material to compete with natural slate. The durability of a literally ‘rock hard’ natural material means that a quality slate roof will last a lifetime (or two), while the man-made alternatives need to be replaced once, twice or even three times over the course of the building’s lifetime.

When it comes to aesthetics we’re clearly biased, but it’s hard to deny the style and finish of a quality natural slate roof. For us there simply is no comparison.

Slate vs Fibre Cement ‘Slate’

Imitation. It is the highest form of flattery. And while we do appreciate the compliment, there are some myths that need to be dispelled when it come to natural slate and fibre cement ‘slate’.

Myth #1 – Lack of availability of Natural Slate

In 2012, the estimated global slate production was over 4 million tonnes. That’s a LOT of slate – and from our experience quarrying is nowhere near capacity. So there are PLENTY of natural slates to go round. Yes, some of our best slates are rare and exclusive, but they have always been reserved for the most discerning projects anyway.

Myth #2 – Relative low cost of fibre cement ‘slate’

If you take into consideration the longevity of fibre cement compared to real natural slate – this is simply not true in a lot of cases. Studies show that over a 30 year period, a fibre cement roof will have to be replaced at least once. A natural slate roof however will last up to 100 years and require very little maintenance in 30 years. So over a 30 year period a fibre cement ‘slate’ roof will cost you on average 1.5 times that of a natural slate roof.

So if you can get your hands on quality natural slate and it requires no more investment (and often less) than fibre cement imitation ‘slate’, then why settle for anything less than the real McCoy?


Slate vs Concrete Tiles

When it comes to concrete tiles there are 2 things to remember:

#1. The short-term costs of concrete are generally less than natural slate. However over the life-cycle of the product, taking into account reroofing, the long-term costs of good-quality slate is less. Our best estimates are that over a 100-year period the cost of a natural slate roof is about 80% that of an equivalently sized concrete roof.

#2. The weight/m2 of concrete is usually at least double that of natural slate as the average thickness of a concrete tile is 10-12mm, which inevitably increases costs of construction due to higher handling costs. Longevity and environmental benefits are not comparable simply because of the relatively poor lifecycle of the concrete product (have you stumbled across any recycled concrete roofs recently?).

Slate vs Clay Tiles

why slate detailLike natural slate, clay tiles come in a variety of colours and qualities. Its short-term cost varies considerably, but overall it is similarly priced to slate. In terms of its environmental impact, however, the production of clay tiles is far more damaging to our world. The production of clay is kiln fired and during this vitrification process more energy is expended and carbon dioxide released, relative to the Natural Slate production process.

Clay tiles also need replacement more frequently, and thus become a less viable alternative to natural slate because the long-term cost becomes an issue.

In terms of aesthetics, this is subjective and depends on the demands of your projects – for us, there is no substitute for natural slate, but don’t take our word for it, have a look at our image gallery here.

The Nitty Gritty…

The tables below offer a brief view of the benefits of natural slate compared to clay tiles, concrete, and fibre cement ‘slates’.

Roofing ProductWeightDurabilityMaintenance % of initial costRe-Roofing Frequency Times
Concrete Tiles 51 30 10% 3.33
Clay Tiles 63 40 10% 2.5
Fibre Cement “Slates” 21-25 30 12% 3.33
SSQ Ultra Natural Slates 20-30 100+ 12% 1

Source: Building Magazine

The Inventory of Carbon and Energy (ICE) Chart below compares Natural Slate against Clay and Concrete Tiles below.

ProductEmbodied Energy (MJ/kg)Embodied Carbon ((kgCO2/kg)Life expectancy (Years)
Natural Roofing Slate 0.1 – 1.0 0.005 – 0.054 20 – 100+
Clay tile 6.5 0.43 40 – 65
Concrete tile 1.2 0.19 30 – 50

Source: Inventory of Carbon and Energy, University of Bath, 2006

What Makes SSQ Different?

Why SSQ 1There’s a reason thousands of architects, hundreds of roofers and countless homeowners across the world choose to rely on SSQ for a beautiful, quality roof.

When you need a breath-taking roof that is as reliable and long lasting as it is aesthetically stunning, SSQ is the only choice.

SSQ slate is not simply a material to put on your roof; its style and quality is a tradition.

We Believe a Roof is Not Just a Roof. Don’t You?

A roof is not just for keeping the elements out, and a building is not just a set of walls designed for shelter. A real building, with a real roof – with a real heart – should make your jaw drop just a little as your head tilts back and your eyes begin to dance around it.

As one of the world’s leading producers and suppliers of slate products, we’ve helped countless numbers of people get that awe-inducing finish they’ve been looking for.

Architects have come to rely on our style and unshakeable, traceable quality for the perfect roof regardless of the project.

Contractors and merchants depend on our never-under-a-million-slates stock levels and expert support service.

Homeowners choose us time and again to get a unique and enviable natural roof that is guaranteed to last a lifetime.

Whatever the project, we’ve got you covered.

SSQ in the UK

Why SSQ 2Driven by a belief in the beauty and effectiveness of high quality slate, and a passion for a level of service that makes the rest of the industry take a long, hard look at itself, our chairman forged ahead and built SSQ as the first company to import quality slates into the UK market.

Over time, other high quality sources of slate have been identified and added to SSQ’s portfolio, and today we have the largest stock of natural slate in the UK, with a minimum holding of 2 million slates at all times.

As not just suppliers, but quarry owners and producers, we have set the standard for other natural slate suppliers.

SSQ in Europe and the Rest of The World

In more recent years, SSQ has become well known and our slate sought after throughout the world… a testament to our supply of consistently high-quality slate.

With teams posted in 22 countries, a logistics office in North West Spain, and our own quarry in central Argentina, no matter where you are in the world, you don’t have to settle for anything less than SSQ quality.

18 Great Reasons To Choose SSQ

house closeupYou’re a busy person. You’ve got people to see, places to be, and multiple projects on the go. You can’t be sitting around worried about the state for your roof all day long.

Which is why we’re here. We want to make YOUR life easier. Our services ensure that once you choose a product, we’re there to take care of the rest. We won’t leave you fretting over your specification. No, once you come along with us your life will be smooth sailing. At least when it comes to your roof. Here’s what you’ll be getting:

1. A dedicated support team available at a moment’s notice. You’ll be surprised at how involved we would like to be. Your success is our success because our mission is to deliver stunning slate roofs!

2. A CPD seminar tailored to meet your project’s needs. You can decide what you want presented before the actual presentation. You’re always in the driver’s seat.

3. Product guarantees. We promise that you’ll be 100% satisfied with your project. How many other companies can promise that?

4. On-site survey service and support at installation. We will be there to walk you through everything step by step. We won’t compromise our level of support once the material is specified. We’re there as and when needed.

5. A huge reference list going back thirty years. We like to think that we know everyone and everything related to slate.

6. A selection of products large enough to cover all your requirements. No matter what you’re looking for we have it covered.

7. We have the best slate technical department in the UK and you’ll have personal access to it. We have a particular passion for COMPLEX design roofs.

8. A state-of-the-art top-of-the-line London showroom  that gives you the most perfect opportunity to choose the exact look and feel that your heart (and wallet) desires. No more guessing from a catalogue.

Visit our state of the art showroom in London

18 Reasons 2OK so hopefully by now you know we put our customers first when it comes to service. So how about the performance of the slate products we sell? Simply put, our slates are tested to the highest standards in the industry, so you can rest assured your roof will last:

9. Our best products are tested to NF 228. This is undeniably the highest roofing standard available anywhere in the world. Find out more here.

10. Our other products are tested to ASTM and Belgium Standards. Themselves pretty demanding standards.

11.  All of our products are tested to CE. This will be a legal requirement from July 2013 onwards at SSQ. But we’re one step ahead. It’s already company policy.

We’re also convenient.  Our stock profile has been engineered to suit the industry’s requirements:

12. We can deliver any sized project even at short notice. Try us – you won’t be disappointed.

13. We’re always consistent with our product offering so you can trust that multiple projects can have the same type of slate if necessary.

14. We have NF 228 grade products in UK sizes stocked at all times. 24/7. 365 days a year. You get the picture.

For a little bit of panache, our product profile contains exclusive products ONLY available through SSQ:

15. We are the only company in the world to offer the UK market roofing phyllite. If you like slate you will fall head over heels in LOVE with phyllite. Trust us on this one.

16. Our portfolio contains slates that have been accepted for use on heritage and listed buildings, and approved for use in certain conservation areas, where choice of material is limited. You know how picky those old buildings can be!

18 Reasons QuarryLast but not least, we know that traceability of slates and quality control are seriously important issues. That’s why you need to know where your slate is coming from and whether they have been quality-controlled at source.

The last two reasons address this head-on:

17. We have a representative office in Spain and Argentina (where the majority of our slates comes from), so that what you buy is what you get EVERY SINGLE TIME. Visits to our quarries are always welcome and in fact encouraged. So if you want to tour our quarry, don’t hesitate to contact us. Find out more here.

18. We are the only UK-based company that employs quality controllers at source for consistency of supply. You’ll never find lower-quality slate slipped in with your order. Find out why this is important here.

What is ‘Phyllite’ Slate?

Welcome to the Rolls Royce of Natural Stone.phyllite rolls royce 6

Phyllite slate is:

  1. Harder
  2. Denser
  3. Stronger

After a few hundred million years of heat and pressure, sediments become shale, and shale becomes slate.  Let the process continue, and slate becomes phyllite.  At each stage, the stone gets harder, denser and stronger.

This progression is summarised below:

Sediments → shale → slate → phyllite

Phyllite represents a slightly higher temperature and pressure environment than slate.  Within the phyllite, the clay breaks down and recrystallizes to produce stable minerals under these new conditions. Specifically, the clay of the slate breaks down and undergoes a reorganization to produce mica.  The mica grains are very small, too small to be seen by the unaided eye, but large enough to effect the luster of the rock.  Phyllites have what is known as a “phyllitic sheen”.  This is a satin-like luster helps to distinguish phyllite from slate in the field.

What is Phyllite 2

Natural Phyllite’s satin like luster helps to distinguish it from slate

On its way to becoming phyllite, “phyllite slate” represents a rare ideal: It combines slate’s distinctive cleavability and riven texture, with phyllite’s superior performance.

Commercially accessible phyllite is rare — there are only a handful of sourced known worldwide.

SSQ’s Riverstone is one of those few available.

Suitable for roofing, flooring, cladding and slabs, it is a product that will add natural beauty to any structure. For more information on this product we have dedicated an entire section to this exciting product.

Akmens Klasika