Natural slate - what is it and why use it?

History of slating

Natural slate is a durable and re-usable building material, that was worked on a small scale in areas of deposits in Wales and the west country in Roman times. In 1296 records show that (sclattes) slates were supplied for miners buildings at Martinstowe and John Trevisa’s translation of Ranulf Higden’s “Polychronicon” on the resources of Wales in 1387 mentions “sclattes also for hous”. 

Towards the end of the 1700’s the industrial revolution and the growth of large cities in the British Isles brought an expanding demand for building materials and the infrastructure in the form of steam engines, canals and railways to extract and transport the natural slates. By the late 1800’s the British slate industry, dominated by Welsh output reached a peak in 1882 delivering 494,100 tons. The industry declined as a result of 2 world wars recession and the introduction of cheaper man made alternatives for pitched roofing. 

There are now only 2 active quarries in Wales and small scale specialist production in Devon and the Lake district. Since the mid 1980’s there has been a resurgence in the demand for traditional building materials which has seen natural slate growing in popularity annually. Nowadays the largest source of natural slate is Spain accounting for approximately 2/3 of the UK market. There is also continuous supply from Canada and Brazil as well as other parts of the world.

Why use natural slate for your next roofing project?

Natural slate offers a fantastic combination of aesthetic appeal and exceptional durability. This durability means that natural slate can contribute considerably to a building’s whole life costs, and can also be re-used with no extra embodied energy. In fact approximately 3% of the total UK pitched roofing market is re-used natural slate. A slate roof will last the lifetime of the building, if laid correctly. Natural slate, tested to BS EN 12326 Part I and meeting the highest relevant ratings, will be unaffected by normal extremes of temperature, and highly resistant to acids, alkalis and other chemicals. This means that it is resistant to acid rain and atmospheres that contain sulphur dioxide, whereas man-made products are more liable to discolouration by acid rainfall or the growth of lichens (although surface coatings can delay this process). Aesthetically, natural slate respects the traditional or local character where it is used and, as a natural product, features variations in colours, sizes and textures. This is especially important in conservation areas, where slate selection must match existing roofing materials on surrounding buildings. 

Historically, slates were from a local source which means that usually there is a ‘type’ of roofing material particular to a location. However, nowadays with more quarries available, worldwide imported ‘varieties’ can be an excellent match, in terms of appearance and performance, for local materials.

What is natural slate made of?

In the UK almost any stone which can be split into thin sheets or slabs has been termed a slate. Some stone which can be split and used for roofing is termed slate or stone slate, some partially metamorphosed sedimentary rock is termed “mudstone”.
As all of these exhibit similar characteristics and can be installed in the same way as other natural slate common practise has lead to these materials being considered as slate in the UK.

Pre-historic oceans

Platy mica and clay minerals settled through water. This sedimentary material forms a shale which may typically be composed of clay, quartz, and mica, with smaller amounts of other minerals. The Shale may be consolidated into laminations as a result of compression from succeeding layers of sediment. Subsequent high temperatures and pressures may also alter the constituent minerals, forming new minerals.

Natural slate formed in the “Geological extruder”

When subject to the intense pressure and heat of regional metamorphism, the shale constituents reorientate to assume parallel positions perpendicular to the compressive forces – a process which produces the slaty cleavage (the plane along which the slate can be split). This fissility of a rock is its ability to be split along planes within the rock. It is independent of the original bedding, although in some cases the two may be parallel. Most slates have a secondary plane of cleavage or grain which can only be determined microscopically.

The whole process took aeons, lasting from Middle Ordovician to Middle Devonian times intense compressional forces, responsible for the mountainous areas in Scotland, Cumbria and Wales and the formation of the British Isles created the stone and the bedding planes which can be seen in quarries around the world.

The type of rock formed by metamorphism is dependent upon the composition of the original rock, and the intensity of the heat and pressure applied. The colour of slate is determined by its chemical and mineralogical composition.

Geological
system

Age
(Mill. years)

Slate 
region

Slate 
types

Devonian

345 – 395

Devon & Cornwall

Delabole (UK),Trevillet (UK)

Silurian

395 – 435

Cumbria

Burlington (UK) Greaves (UK)

Ordovician

435 – 500

North Wales
France
Spain
Canada

Ffestiniog (UK), Cwt y Bugail (UK)
Angers (FR)
most Spanish
Glendyne (CAN)

Cambrian

500 – 570

North Wales

Penrhyn (UK)

Pre-Cambrian

> 570

Brazil

Alpina (BRA)