Slate Jargon Buster

This guide will help anyone get through the construction term minefield associated with buying natural slates.

Slate Classifications

Slate is categorised in a number of ways. Different types of slates are labelled with W’s, A’s T’s, and S’ under the British Standard BS EN 12326 (2004) system. 

These categorisations are in place to help the purchaser of the slate know that the slates they are purchasing are fit for purpose by comparing the slate’s physical properties to key attributes.

In simple terms, these labels refer to the ways slates are tested:

W (Formerly A) = Water Absorption.  

Slates, like most natural materials, are susceptible to getting damaged by the elements. The W test checks to see how much water the slates absorb. Slates that absorb a lot of water are more likely to crack during a cold snap, and with the way the British weather is, having a slate that is like a sponge isn’t going to work. 

The best quality slates have a rating of W1 – meaning it absorbs minimal water (less than 0.06% of its mass in fact). 

All the slates that we provide at Ashbrook Roofing are classed as W1.

T – Terminal Cycle

As stated, slate is a natural material, and other materials are likely to appear within the slate. The frequency and whether these inclusions (known as pyrite) rust helps to determine if a slate is of good quality and to prevent unsightly rust streaks.

Here are 3 grades to look out for when selecting roofing slates:

T1, T2, and T3. 

  • T1 Slate – The best of the best where inclusions are concerned. These slates contain minimal pyrite with either no risk of rust or some surface rust, which poses no risk to the slate structure nor any rust streaks appearing.
  • T2 – These slates have inclusions that rust and cause discolouration on the slate with visual appearance change. The structure remains unaffected. 
  • T3 – Inclusions change the appearance of the slate through oxidation, and can cause structural damage by inclusion penetrating the slate forming holes. 

A T3 Slate with rusted inclusion










All Spanish slate supplied by Ashbrook Roofing is graded T1

S – Sulphur Dioxide Exposure

Poor quality slates are less resistant to pollutants present in the air, and due to the chemical makeup of natural slates, they are prone to soften and degrade over time.

There is no “pass or fail” with S testing, however, the quarries react based on the results. Similar to the Terminal Cycle, there are 3 grades with Sulphur Dioxide Exposure. 

S1 – No action is needed

S2 – The quarry must split the slates 5% thicker than standard 

S3 – Slates are at least 8mm in thickness.

Any material that is produced containing more than 20% n carbonate content is not suitable for use on external projects including roofing and cladding.

All the Spanish slates that Ashbrook Roofing source are of S1 standard.


Looking to Buy Slate

If you are looking for a slate supplier for your next project.  See our range of slate and roofing tiles.

Alternatively, if you have questions, you can see our blog, or use our live chat to ask questions.  

Brazilian Roofing Slate: Trying to seperate the good from the bad?

Much of the slate quarried in Brazil comes from ‘sedimentary mudstone’*. At the time of writing (Spring 2013), there is some debate over the future of this type of Brazilian roofing slate.

In Summer 2012, the National House Building Council (NHBC) published a technical bulletin reminding members that any roofing slate (including that sourced from Brazil) which does not comply with BS EN 12326, will not be warrantied or insured by themselves.   In February of 2013, the National Federation of Roofing Contractors (NFRC) published a warning to its members advising that Brazilian grey slate was found to be delaminating on several properties. This bulletin went on to say that Green slate from Brazil appeared not to be affected.

The current European Standard for roofing slate (BS EN 12326) applies only to slate that has been manufactured from metamorphic rock (rock that has been formed through the application of heat and pressure). It does not allow for the testing of slates manufactured from sedimentary rocks (rock formed through the application of pressure alone).

The result of these statements has ignited furious debate, the results which are confusing to all. There are clearly very good and very bad slates manufactured in every country.

There are those who point towards the high-quality Brazilian roofing slate that has been used successfully and have withstood the test of time. Others point to those suppliers who have chosen to sell cheap, low-quality Brazilian roof slates which delaminate and are not fit for purpose.

Whilst the debate for Brazilian roofing slate is likely to continue, the responsibility for solving this issue lies with the quarries and testing authorities. Between them, they must agree to the implementation of a stringent quality test that will confirm which Brazilian slate roof tiles are suitable for use.

If they fail to grab this issue, Brazilian roof slates will continue to gain a reputation for concern.


*Mudstone: A sedimentary rock that has been created through intense geological pressure being applied to sedimentary deposits. **Metamorphic Slate: A hard mudstone that has been subjected to metamorphism (intense geological pressure and geothermal heating of between 200-250degrees centigrade).

Is a thick roofing slate better?

Of the many questions that we are asked, the subject surrounding thickness of slate and whether a thick slate is better than a thin one, crops up most regularly.

man_making_a_roof_slateAs we discussed earlier, it is the unique layering of sediment that allows this type of rock to be split into layers. Splitting a slate thinly allows the quarry to produce more slates from every tonne of rock – and therefore make higher profits.

However there is a limit to just how thin a slate can be cut before it becomes too weak. To protect you, part of the CE / EN 12323 test ensures that the slate possesses a certain amount of strength. Every quarry is therefore limited to how thinly they cut their slate if it is to be of sufficient strength to pass the EN test.

The EN test also examines if a slate is likely to weather or degrade over time from atmospheric and pollution activity. If the test shows that the slate degrades significantly, then the quarry is forced to split their slate thicker to compensate for the expected degradation of the slate over time.

From this we can conclude that the strength of the slate lies in the quality of the rock it is cut from – not the thickness to which it is produced.

Why does the same quarry offer slates of differing thicknesses?

So why does a quarry produce slates in different thicknesses? For a quarry to be profitable it has to split the slate at the most optimum thickness – as thinly as possible to maximise profit, whilst producing slate with the strength needed to meet its purpose.

Although this varies from quarry to quarry, this optimum thickness tends to average at around 4-6mm. During the splitting process (which is still mostly done by hand), some slate will inadvertently split slightly thicker (e.g. in 6-8mm thicknesses). Because this slate cannot now be split any further (it would then be too thin), rather than waste the thick slate, it is batched together with other slates of a similar thickness and sold on as a separate grade (sometimes at a slightly cheaper price).

Regional and aesthetic preferences also influence a slates thickness. For example, home owners in Scotland almost always select heavy slates (thick slate of 8mm plus).

How to Tell Good Slate from the Bad

Quality of roofing slate varies enormously. One good slate can look identical to another which could be beset with reactive minerals that start rusting after the first rainfall. Others can fade or change colour after installation, or even start to shale (separate) and allow water to penetrate into your building.


Trying to avoid these poor quality slates whilst maximising your value for money will be your key challenge when sourcing your slate. Worryingly many people leave this down to luck or to simply use what is available at their local builder’s merchants.

Often, the country or region of a slate’s origin is often used as a badge of honour in the roofing industry.  The truth is that two slate quarries from the same country and located just a few miles apart will  produce completely different qualities (and even colours) of slate. It is vitally important that you source your slate from a single named quarry, (ideally from the same batch), to ensure a uniform appearance and quality. If you are unable to visit your supplier, always request samples and take time to inspect them.

CE / EN 12326 standards: A1-S1-T1

The starting point for protecting yourself against purchasing poor quality slate is to make sure that it is certified to the top level of the CE / EN 12326 standards. Amongst other areas, this certification tests the slate’s water absorption, carbonate content, and thermal cycle resistance.

There are three levels of pass for each part of the test. A slate that is certified to A1-S1-T1 standards has achieved the highest level of pass on each test.

Be warned that the EN 12326 tests are quite lenient. Even middling quality Chinese slate with an expected lifespan of 30-40 years can achieve the same top-tier pass as given to a Welsh slate that has a 100 year plus life expectancy. The certification should be treated as proof that the slate meets the bare minimum requirements, rather than as an indicator of high quality.

Beyond EN 12326

By looking beyond the EN 12326 standard, an educated buyer can take their appraisal of slate a step further to help ensure that they are getting the best quality and best value for their money. The following pointers will help you understand the difference between your roofing slate samples:

Grain – slate is cut from rock that was formed by the laying down of sediment in ancient seas many millions of years ago. These layers of

Welsh Blue Grey Slate
Welsh Blue Grey Slate

sediment have since been compressed (and in the case of most slate, heated by geothermic conditions) until the layers of sediment lie closely packed against one another.

Because the rock still retains these layers, it allows the quarry to split the rock into slates of a consistent thickness. Agood slate will split cleaner, be of similar thickness to one another and have minimal coddling (twisting) of the slate being evident.

You can get an indication of how well your slates will lie (and how much sorting may be required) by taking a good size handful of slates and squeezing them at each end. If the slates rock against one-another as you squeeze them, then this is a sign that there are coddled slates or slates of varying thicknesses within the sample.

A second grain running down the face of a slate is also evident in most slate. The very best quality and strongest slates will have a vertical grain that runs straight from top to bottom of the face of the slate. A grain that twists, swirls, or falls horizontally part of the way down the slate indicates that the slate may not be as strong or durable.

Inclusions – All slate includes amounts of non-carbonate inclusions. First clarify by looking at the test certificate to see if these inclusions are roofing_slate_containing_reactive_inclusionsreactive to water (T2 or T3) or non-reactive to water (T1).

A reactive (T2 or T3) inclusion will rust or discolour, possibly forming dirty streaks down your roof. In the case of a T3 slate, the inclusions may rust away completely leading to leaks and cracks throughout the entirety of your slate roof.

Secondly look to see if any inclusions spoil the slates ability to lie flat, and lastly appraise the overall appearance of the slate. Inclusions are normally silver or gold in colour, and can vary in size between a grain of sand and a small pea. The most desirable slates have no visible inclusions – because this is extremely rare, they command a higher price.

Colour – Slate colour ranges from green to purple, grey to black. Because Welsh slate was used to cover much or world’s roofs in the 18th and 19th century and proven to be the very best slate in the world, the two most sought after colours are particular shades that match the Welsh blue-grey and heather-purple. Unfortunately for those aspiring to purchase slates in these colours, they are only available in limited numbers from one or two quarries located in Wales, Spain and Canada. As demand often outstrips supply, they command a high price.

Sound – A good slate will ring true when tapped with the knuckle. Avoid those that when tapped, sound dull or those that rattle.

Brittleness – A brittle slate is weaker and more likely to shatter, especially during installation. During fitting, slates are cut to provide a good fit at the gable end of your roof, around chimneys, dormer windows and along your roof valleys. The most desirable slates are ones that are ‘soft’ enough to cut with slate cutters.

It is unrealistic for the inexperienced to measure how brittle a slate is. However by choosing a specialist roofing supplies merchant (especially one that is linked to the National Federation of Roofing Contractors), you are more likely to get better advice and a better slate.

Size matters:

When choosing a good slate for your roof, size too is important. Slate is disproportionally more expensive the larger size you select. However a smaller (cheaper) slate will require more time and more materials to install. If your finances are limited but time plentiful, a smaller slate may offer you better value for money.

On a final note, remember that the beauty of slate lies in the fact that it is a natural and unique material – no two slates and no two slate roofs will ever look identical. Experienced roofers expect to have to do a little sorting of the slate before they take it up to the roof and will not be surprised to find a small proportion of twisted (coddled) or cracked slates in every pallet.

If you desire every slate to be perfect and every slate to be identical to one another, a man-made slate manufactured from plastic, fibre cement or slate dust & resin, will probably meet your needs better.

Synthetic Slate Roof Tiles

Once the domain of thick, ugly, concrete ‘wedges’, synthetic slate roof tiles can now be found manufactured from a wide range of materials. New manufacturing processes have even bought improved levels of quality (especially to colour fastness) and refinement to their appearance.

Often usable at lower roof pitches, man-made slates can also be lighter and contain recycled or more eco-friendly materials that may help you gain ‘credit’ in the sustainability assessment of your planning policy statement. Prices of the cheaper man-made slates can also be significantly less than natural slates.

Due to their standard design, man-made slates can also be fixed quickly by inexperienced roofers. However, do note that many products require special fixing clips or the use of copper disc rivets to prevent wind lift.

Marley_Thrutone_Fibre_Cement_SlatesFibre cement – smooth, dark-grey and black fibre cement slates manufactured by SVK, Marley Eternit and Cembrit are the most well-known, possibly due to their low cost and ease of availability (expect to pay £10-£12 per square meter). Also available in different colours and textures, fibre cement slates have a typical lifespan of 30-50 years, although colour fastness can be considerably less. Due to the materials used in their manufacture, manufacturers recommend the use of copper fixings.

Plastic or rubber slates are becoming increasingly popular, mainly because of their eco-credentials and ease of installation. Because of their use of recycled materials and claimed lifespans of 50+ years, they can be of great interest, especially to the eco builder. Prices do tend to be expensive (higher than even the best quality natural slate), and that’s before you factor in the price of fixings.

Reconstituted stone and reconstituted slate tiles are typically made up of 60-80% recycled thick stone roofing slates and blue guttering on a country pubmaterials. Offering a more natural appearance than pure plastic or rubber slates, these weather naturally over time to provide a unique patina to every roof.

An introduction to Natural Roof Slates from around the world

An Intro – Natural Roof Slates From Around The World: Ashbrook Roofing

Natural Roof Slate
Many of us already know that slate has been quarried from Wales and Spain for many hundreds of years.  Fewer of us realise that slate can successfully be sourced from China, Brazil and Canada too.

Negotiating your way around the bewildering varieties of slate from these different countries can be complex and time-consuming. Our ‘quick guide’ is designed to cut through some of the myths about natural roof slates, and help you hone in on a range of suitable slate for your particular project.

The first thing to remember is that no one country produces either 100% good or 100% bad slate – you will always find a range of quality (both good and not so good) wherever you look. Natural roof slate is an incredibly variable raw material that is simply not regulated by the territorial boundaries imposed by man. It is in our experience that even slate from different sections of a quarry, or a region within a country, can vary massively in quality and colour.

Whilst the above is true, slate from differing countries does seem to exhibit some general tendencies:

English Natural Roof Slate

English slate – now restricted to Westmorland and Burlington slate, both of which have been quarried from within the Lake District since the 17th century.

Offering a lifetime measured in the hundreds of years, Westmorland and Burlington slates are both incredibly hard-wearing and strikingly attractive. Westmorland slate has a distinctive blue-green appearance, while Burlington slate is smoky grey.

For new Westmorland, expect to pay around £200 per square meter, for reclaimed this price drops to around £40! Because of the complexities in installing this prestige roofing material, always make room in your budget for using experienced roofers who have worked with it in the past.

Traditional stone slate that is paler in colour (and with a slightly shorter life expectancy) is also available new from several quarries within the UK. However much of the roofing work undertaken in stone slate is now made using reclaimed stone.

Welsh Slate

Welsh slate is considered by many as the best natural slate in the world. Welsh slate is available in either heather (purple) or blue-grey.

Penrhyn slate has a beautiful soft blue/purple appearance and can be expected to last more than 100 years. In comparison to other natural slate products, it is easy to work with and requires very little maintenance. New Penrhyn slate can be pricey – it is often found cheapest at specialist roofing supplies. You may also wish to consider visiting architectural reclamation companies, as much is available second hand.

Following the closure of the Ffestiniog quarry in 2009, Welsh blue-grey slates is much harder, though not impossible to source. Currently, small amounts of slate are available from Cwt-Y-Bugail – again a specialist roofing supplier should be able to help.

Due to its quality and desirability, Welsh slate attracts a price range of anywhere between £40 and £80 per square meter.

Spanish Slate

Spain is one of the largest producers of the world’s roofing slate (the Cupa Slate company alone is said to produce almost one-quarter of the roofing slate used in the western world). Spanish slate is easy to source but be warned – the quality of Spanish slate varies massively.

At one end of the market, Benuza Blue Heavy is comparable in quality and appearance to a blue-grey Welsh slate (it is even approved for use in many of our National Parks). At the cheaper end of the market, Spanish slate can be beset with reactive (rusting) metallic inclusions and warping, and have a significantly lower life expectancy.

Chinese Slate

Chinese slate is readily available and can at first appear to offer a budget solution. From a quality perspective, slates of reasonable quality and life expectancy can be found if you look hard enough (but unlikely at a low cost). Chinese slate rarely, if ever, attains the high quality of an average Spanish slate.

Many of the Chinese slates also suffer from colour loss – if you want your slates to retain their colour specify this to your supplier. Due to its brittle nature, the Chinese slate can be slightly harder to work with.

Brazilian Slate

Much of the Brazilian slate is produced from mudrock (where the rock is sedimentary rather than a metamorphic rock that characterises European and Chinese slate). To gain the necessary strength desirable in roofing slates, Brazilian slate is cut thickly and tends to have a very pronounced riven texture.

Whilst offering the user a cheap slate, it is difficult to work with (normally requiring the use of angle grinders to cut). Because they are produced from mudrock, roofs made from Brazilian slate are currently not permitted by the National House Builders Council (NHBC).

Canadian Slate

Much of the slates available to us from Canada is of a high to very high quality. Due to their quality and colouring, they are often used as a direct replacement to Welsh slate. Until recently, Canadian slate was readily available in blue-grey, heather purple and subtle green. The closure of key quarries has however limited availability to the blue-grey varieties.

Canadian slate can be sourced at around £36.00 per square meter – cheaper than a Welsh slate but more expensive than a similar quality high-grade Spanish slate.



Choosing your Roofing Materials

Before sitting down and wading through catalogues of roof tiles and roofing materials, it’s probably a good idea to take a step back and think about your roof, property and location as one complete picture.

hand cut forna slate installed on a conical roof
Slate Roof Tiles to Protect Your Property

For a start, it’s the roof that will be protecting your property and it needs to withstand all that British weather can throw at it for many years to come. It’s also one of the first things that people see when they look at your house. A great roof that fits in with the local surroundings may make your property easier to sell and could well add significant value.

Our series of Quick Guides are designed to help you understand a little more about the roofing materials available today, some of their pros and cons, as well as a few hints and tips to help you get best value for your money.

Popular Roofing Materials

Each type of roof material has unique properties that make it more or less suitable for particular styles of property, locations or budgets. The design you choose will depend on your own preferences, your budget, planning restrictions and the size and style of your roof. You may also wish to consider following the lead of other roofing stock in the area your property is located:

a close up of a penrhyn welsh slate roof
Beautiful Hard Wearing Penryhn Slate

Slate – slate has been a popular roofing choice since Victorian times. With the best slate offering a natural finish, low maintenance and an expected life of well over 100 years, it can offer you great value for money. What’s more, slate can be fixed to most roof designs and rarely looks out of place.

Remember that most natural slate should normaly only be installed on roof pitches of more than 27 degrees. Because it is a natural material, it also requires a little more skill to install then standard roof tiles. There are however several manmade slate-like alternative roofing materials which have a similar look and are far easier to lay, especially for the inexperienced roofer. Some of these can also be used on lower pitches than natural slate.

Whilst prices of slates vary between £10 and £150 per square meter, expect to pay somewhere around £25-£35 per meter for a good quality slate that you can expect to last upwards of 75 years or so.


Tiles – perhaps due to the wide choice of colours, shapes and quality available, roof tiles excite more conversation and debate than almost any close up of a roof made from dreadnought red clay tilesother roofing material.

As well as being able to manufacture specific sizes, shapes and colours, some tiles can be made to interlock with one another. Apart from making them easy to install, this feature allows them to be used on low roof pitches – some as low as 15 degrees (far lower than any natural slate).

Lifetime expectancy of tiles vary considerably – dependent on the type of tile and local conditions, expect them to last anything between 30 and 70 years.

If you are reading this article in Britain, then you are spoilt for choice. Many of the world’s leading tile companies with heritage dating back many hundreds of years are located here – finding your ideal tile shouldn’t be too difficult.

Budget tiles, mass produced from concrete and costing from as little as £7 per square meter are readily available in plain shapes and colours. Tiles with more interesting profiles and cambers (curves in their profiles) are available in richer colours as you head up the price scale, with the best handmade clay tiles costing anything upwards of £50 per square meter.

Thatch – thatched roofs look beautiful, are great insulators and are full of old world charm. Contrary to expectation, if treated with respect they should not present too significant a fire risk. They do take time to install and are considerably more expensive. They also require a lot of maintenance – you should budget to replace your thatched roof every 30 years or so.

Thatch is most suitable for use on steeper roofs with a pitch of at least 45°.

pretty plants make up this green roof above a classy wooden building
Green or Eco Roofs Need Careful Planning

Green roofs – Green or turfed roofs are a popular feature of many new eco buildings. As well as adding an interesting visual dimension, they have great insulating properties. Green roofs are typically built up in layers and best suit modern design buildings with a fairly shallow pitch. Designers should pay careful attention to the waterproofing system, of which many different types are available.

For the owner, it is well worth looking at the final specification prior to choosing a green roof design. You may find that the materials that make up the waterproofing layers (bitumen membranes, mastic asphalt or polymer fabrics) challenge your eco requirements – natural slate might be a greener choice.

Green roofs also require a lot of regular maintenance. Because many of the new systems have only been in use for a relatively short time, lifetime expectations are as yet unproven in the long term.

Conservation Areas and Listed Buildings

In a listed property, it is likely that you will be required to replace roofing materials like-for-like. Where original stock is not available (or are far too costly to be realistic), speak to your planning officer and try to obtain a list of modern alternative materials that are allowable. Reclaimed slate or tiles may be one alternative solution in these instances.

Even if you property is not listed or located in a conservation area, it’s always worth checking with your local authority or housing association to find out if there are constraints on the type or colour of roofing material that you may use.