Natural Slate Roof Buyer's Guide
– your essential guide to slate roofing materials

Your roof is a crucial part of your building, and it’s vital to choose the right slate to keep it watertight for generations. With numerous slate options available, selecting the best one for your needs can be overwhelming.

Welcome to our Ultimate Slate Buying Guide. You’ve chosen slate for its renowned quality, and you’re considering it for your roofing project. However, deciding which slate is right for your specific roof can be challenging. This comprehensive guide provides everything you need to confidently purchase, install, and maintain your new natural slate roof.

What is Natural Slate?

Slate is a natural stone formed over hundreds of millions of years. It originates from layers of mud that, over time, were subjected to intense geological forces. These layers were compacted and compressed, transforming into sedimentary mudstone.

This process created distinct layers within the stone, known as the “bedding plain”, corresponding to the original mudstone layers.

In the case of true metamorphic slate, the stone underwent an additional transformation. Immense geological pressure from tectonic plate movements and volcanic activity, combined with extreme temperatures, caused the molecules and elements in the stone to re-align in a single direction. This re-alignment formed what is known as the cleavage plain, and this is what gives slate its unique strength and allows it to be split into thin sheets.

Why Choose Natural Slate

There are a wide variety of different types of roofing materials available, so why choose natural slate? There are a number of reasons why natural slate can make a great roofing material.

A Brief History of Slate

Slate has been used a building material for centuries. The earliest recorded use of slate in this region dates back to the 2nd century AD, when the Romans used locally quarried Welsh slate to build the fort of Segontium (modern Caernarfon).

Over the centuries, hundreds of small slate quarries in Ireland, England, Scotland and Wales provided roofing slates for the buildings of the past. The availability of slate, combined with its non-combustible properties, made it an ideal choice, allowing houses to be built closer together with a reduced risk of fire.

Before the advent of road transport, slate was shipped by boat and distributed throughout the country via canals. As time progressed, the slate industry in Wales became the dominant force.

The slate industry in North Wales reached its peak during the industrial revolution in the late 1800s and employed more than 10,000 people. The largest quarries were Penrhyn, Dinworig and Ffestiniog which dominated the world’s slate production. However, after the turn of the century, and with the emergence of cheaper man-made roofing materials, the slate industry went into decline.

In the 1980s, the Spanish slate industry in Galicia and Castile y Leon began to develop as a global force that has now surpassed Wales as the world’s dominant producer.

Types of Natural Slate Roof Tiles

There are a large variety of slates available on the market from numerous countries all over the world in a range of colours, textures, sizes etc..

Some of the more popular slates are:

Welsh Slate

The most famous slate quarry in the world is the Penrhyn quarry in North Wales. The slates from this quarry known as ‘Bangor Blues’ have been produced for over 500 years and cover many of the world’s most iconic buildings. These slates are generally regarded the best quality slates in the world.

Penrhyn Bangor Blue slates are known for their exceptional quality and are instantly recognisable by their Heather Blue / Purple colour. They remain a popular choice today and are distributed throughout Ireland and the UK by Lagan Building Solutions (LBS).

Canadian Slate

Canada is another source of high-end slates, and the Glendyne quarry in Quebec is the country’s main producer. The Glendyne quarry produces a beautiful blue grey slate which is specified worldwide, including in North America, Australia, mainland Europe, Ireland and the UK.

The Glendyne slate is known for its exceptional quality and is distributed throughout the UK and Ireland by Lagan Building Solutions (LBS).

Spanish Slate

Approximately 80% of worlds natural slate comes from north-west Spain. With numerous quarries and producers, Spanish slates vary in quality, grade and thicknesses.

There are some fantastic roofing slates available from Spain but it is important to do your research there is a lot of sub-standard Spanish slate on the market as well. We recommend that you stick to the premium grade selection when choosing a Spanish slate.

Slates are still predominantly split by hand and, as it is a natural product, there will be different quality selections – the perfect, uniform grades are sold as ultra or prime grades and anything that does not meet this quality criteria can be sold off as a lower grade. Lower grade slate tends to be more inconsistent and can have higher levels of wastage.

The most common sizes for Spanish slates are 50x25cm (which gives a coverage of 20 slates per m2) and 60x30cm (13 slates per m2).

Brazilian Slate

Around 10% of the slates on the market are sourced from the Minas Gerais region of Brazil. Brazilian slates are generally more affordable than their Spanish and Welsh counterparts, but it is important to recognize their geological differences.

Unlike true metamorphic slates, Brazilian slates do not have a natural cleavage plane and split along the bedding plane, resulting in significant differences. Due to their geological composition, Brazilian slates tend to be more brittle, making them more prone to breaking if not handled properly. Slates must be sawn rather than cut, and on roofs, they require both nail fixing and additional support with slate hooks.

Despite these differences, Brazilian slate is a more economical roofing option and has become increasingly popular with customers.

Understanding the Costs of Natural Slate

The cost of natural slate can vary widely depending on the type, quality, and source. As with most things in life, the saying "you get what you pay for" often applies.

Welsh Bangor Blues Costs

Welsh Bangor Blues are always positioned at the premium end of the market, reflecting their top-tier quality and historical prestige.

Canadian Slate Costs

Canadian Slate offers an excellent balance between quality and affordability. While still considered a high-end option, Canadian slates are generally more accessible and less expensive than their Welsh counterparts.

Spanish Slate Costs

Spanish Slate presents a broad spectrum of options due to the variety of quarries, grades, and qualities available. It's important to approach Spanish slate purchases carefully; opting for the cheapest option can be risky. Lower-priced Spanish slates may suffer from lower quality or contain higher levels of iron pyrites, which are metallic inclusions that can be detrimental over time. Iron pyrites can cause the slate to rust and, in severe cases, break down.

Key Testing Standards for Natural Roofing Slates

The current standard for natural roofing slates is BS EN 12326-1:2014. This is a European standard which tests a slate under 12 criteria.

Most of these criteria are either a ‘pass’ or ‘fail’ with the exception of:

Water absorption test

This test will test how water the slate absorbs. A good slate should be non-porous but all slate will, to some extent, absorb water and this is what will ultimately cause the slate to break down.

There are 2 possible results here:

• A1 (<0.6) best
• A2 (>0.6) this would indicate a highly porous slate

Thermal cycling test

The thermal cycling test looks at any iron pyrities in the slate and tests how it reacts with the atmosphere.  There are 3 possible results which are T1 (best), T2 or T3.

• T1 classification means that the pyritie in the slate was tested and there were no changes in appearance.  
• T2 classification means that any pyrities in the slate was tested and there was reaction or discolouration but without any structural changes.
• T3 classification means that there were changes to the appearance of the slate which penetrated the slate and caused holes in the slate.

Sulphur Dioxide test

This is a measure of the amount of calcium carbonate within a slate. Higher amounts of calcium carbonate can cause problems and there are 3 possible results here:

• S1 classification means the calcium carbonate content is less than 5%
• S2 classification means that the calcium carbonate content is between 5% and 20%
• S3 classification means that the calcium carbonate content is higher than 20%

Design Specifications

There are a number of technical design specifications you should consider before choosing which slate you should go for.

• What is your roof pitch?

• What is your rafter length?

• How exposed is your site?

What style of house are you building? Different slate may be better suited to different house designs. For example, Georgian, modern, contemporary, arts and craft, farmhouse, sustainable / eco houses, cottage style, barn conversion, regency revival, gothic revival and tudor revival.

The Slating Process

Natural slates have no interlock and are fitted by placing the slates on top of each other in a double lap pattern (unlike concrete tiles, there is no inter-lock in natural slates). The slates are waterproofed by a combination of their head-lap and side-lap as shown below.

The head-lap is the area where 3 slates over-lap. Most natural slate are pre-holed to give a head-lap of 80 – 100mm.

For fitting natural slates, there are codes of practice:

• BS 5534 (British standard for slating and tiling)
• Irish Code of Practice 2 (ICP 2)

How to Choose The Right Slate Supplier

When selecting a slate supplier, it's important not to be swayed by unrealistic claims, such as 100-year warranties. Instead, consider the following factors to ensure you're making a reliable choice:

• Company Reputation: What is the reputation of the company you are dealing with? Check reviews, testimonials, and industry standing.

• Expertise in Slate: Are they specialists in slate, and how long have they been working with this material? A company with extensive experience in slate is more likely to provide quality products and advice.

• Business Longevity and Stability: How long has the company been in business, and are they financially stable? A long-standing, financially secure company is more likely to be around in the future if any issues arise.

• Post-Sale Support: In the event of a problem down the line, how likely is the company to still be in operation and available to assist?

Choosing a slating contractor

Installing slate requires a higher level of skill compared to other roofing materials due to its natural characteristics. It's crucial not only to select the right slate and supplier but also to choose a qualified slating contractor for the installation.

• Specialised Expertise: Slate is a specialist product and needs to be fitted correctly.  Avoid general roofing tilers or joiners who claim they can handle slate roofing. We recommend that you go for bona fide slaters who specialize in this field and have a proven track record.

• Quality of Work: A poor installation can compromise even the highest quality slate. Always review examples of the contractor’s previous work to assess their expertise and attention to detail.

• Reputation and Endorsements: Check the contractor’s reputation and standing with your slate supplier. A reputable contractor will have a positive relationship with suppliers and a history of successful slate installations.

Choosing a skilled and experienced contractor is essential to ensure that your slate roof is installed correctly and performs well over time.