Roofing slates of the world part III

Images of hand specimens and thin sections of slates from several world´s locations. Real color of the specimens may vary with respect of shown in the images.

Pizarras del Mundo03

13. Slate from Penrhyn, Wales, UK. This slate is extracted at the historic quarry of Penrhyn, and is very popular in historical buildings all over the UK. The green spots correspond to zones with reduced iron and high contents of Ca and Mg (Borradaile et al. 1990). This color change can be seen in the microphotograph of 200 microns.

14. Carbonate slate from Liguria, Italy. The Liguria slates have carbonate content (see microphotograph of 500 microns) of about 20%. However, this fact does not mean that these slates are more susceptible to weathering than other slates with carbonate contents much lower. The key factor is the specific mineralogy of the carbonate. This slate complies with the EN 12326 requirements, and constitutes a perfect material for roofing when used properly. Sample provided by Euroslate.

15. Slate from Benuza, Castilla y León, Spain. An Ordovician slate, fine-grained with some cubes of pyrite, with smooth surface and dark color. This is a classic roofing slate, i.e., a slate from the green schists facies made of quartz, chlorites and mica. Sample provided by Cupa Pizarras S.A.

16. Slate from Hubei province, China. Fine-grained slate, light colored with a marked tendency to acquire a reddish aspect which makes it very interesting for special cases, since this reddish does not seem to generate rust trails. Sample provided by the Laboratorio del Centro Tecnológico de la Pizarra.

17. Green phyllite from Lugo, Spain. This Cambrian phyllite is also a very special roofing slate, being used for some singular buildings such as the Shizuoka Convention Arts Center in Japan. It is quarried in several colors ranging from grey to green. This is the Verde Xemil variety. Sample provided by Pizarras Ipisa.

18. Slate from Villar del Rey, Badajoz, Spain. A very fine-grained slate with some pyrite cubes and a dark color, in fact this is the darkest slate quarried in Spain due to its content in graphite, up to 2%.  Sample provided by Pizarras Villar del Rey, S.A.

And please remember: There are no bad slates but bad uses. The slate should be used in accordance with the building and environment requirements, so it is critical to know and understand the rock we are dealing with.


Quality factors in slates – Part II

Grain size

The grain size of roofing slates is very small, similar to the clays. It is possible to distinguish two types of components depending on the grain size, the matrix (mica and chlorites) and the skeletal components (quartz and feldspar). The key factor is the components of the skeleton, not just the size of these grains, but their selection or uniformity in size (Figure 1). A roofing slate will have good fissility if their skeletal components have all similar size, whereas with diverse range of sizes the fissility is reduced.

Grafico ITGEeng

Figure 1. Relationship between slate components and grain size

Grain size also affects the external appearance, coarser slates have a more rough and irregular aspect, while the fine-grained slates have a more smooth and uniform aspect, and therefore brighter (Figure 2).

Figure 2. Comparision between a coarse grain slate (left) and a fine grain slate (rigth).

Figure 2. Comparision between a coarse grain slate (left) and a fine grain slate (rigth).

Textural homogeneity

By definition, a roofing slate should have a lepidoblastic texture (Figure 3). This term refers to the microscopic arrangement of the elements of the rock, which are strongly oriented along the direction of slaty cleavage or fissility. This texture must be uniform and consistent along the slate, otherwise the split process will be greatly hindered. In certain types of roofing slate, other textures can be found, but must always be homogeneous and continuous.

Figure 3. Classical lepidoblastic texture in a roofng slate (left). On the rigth, a slate with a coarser texture, which is called porphyro-lepidoblastic

Figure 3. Classical lepidoblastic texture in a roofng slate (left). On the rigth, a slate with a coarser texture, which is called porphyro-lepidoblastic

Presence of sedimentary layers

These sedimentary layers are mainly sandy levels, of thicker grain size, which were deposited when the sedimentary rock which subsequently result in the slate was formed (Figure 4), after metamorphic processes.

Figure 4. Deposition of sandy layers on the slate bulk during sedimentation.

Figure 4. Deposition of sandy layers on the slate bulk during sedimentation.

These layers can be recognized as bands of lighter colors. Since they have a grain size and texture different from the rest of the slate, they modify the homogeneity of the slate (Figure 5), so that their presence is undesirable in a good quality slate.

Figure 5. Sandy layers on a roofing slate bulk.

Figure 5. Sandy layers on a roofing slate bulk.

Summer course at Oviedo University

Ornamental rocks in construction: granite and roofing slate

This year I have organized a summer course on various technical aspects of slate and granite. The objectives and current programming are summarized as follows:

Spain is the largest producer of roofing slate and the second largest producer of granite in the world. Currently, both sectors are suffering the effects of the economic crisis, which is forcing companies to restructure looking for R & D developments that open new markets, and at the same time, incorporate the latest technologies in production processes in order to optimize operating costs.

This summer school will have a special focus on new technologies and products that have emerged in recent years, and also in the practical application of EN norms in both materials, with the aim of improving the training of technicians specialized laboratory tests. Special attention to the section of petrographic analysis and its practical applications as a qualitative indicator will be given.

The course is aimed primarily at university students of engineering and geology, architects, extractive companies and laboratories of accreditation of ornamental rocks.


  • Overview of the sectors of granite and slate roofing: history, economic, productive areas.
  • Technological advances in the sector in R & D and industrial development.
  • Geological and technical features granite and slate roofing.
  • Laboratory testing and regulations.
  • Practical aspects of implementing petrographic examination techniques in both materials.


  • Lope Calleja Escudero, PhD in Geological Sciences and Professor in the Department of Geology at the University of Oviedo.
  • Victor Cárdenes Van den Eynde, PhD in Geological Sciences and Master in Geological and Geotechnical Resources.
  • Nuria Sánchez Delgado, BA in Geological Sciences and head of the Laboratory of the Technological Center of Granite, Porrino.
  • Alvaro Ordoñez Rubio, PhD in Geological Sciences and assistant professor in the Department of Geology at the University of Oviedo.
  • Victor Pais Diz, Degree in Geology, senior geologist at Cupa Slates.

Dates and price:

15 to 19 July 2013. Enrollment period April 18 to July 8, 2013.

Price: 98,51€ for students of the Oviedo University, 140,73 € for the rest.

The course will be given in Spanish and English.

Different types of roofing slates

Definition of roofing slate after EN 12326

According to EN 12326-1:2005, from a commercial point of view, a roofing slate is a “rock which is easily split into thin sheets along a plane of cleavage resulting from a schistosity flux caused by very low or low grade metamorphism due to tectonic compression. It is distinguished from a sedimentary slate (shale, author´s note) which invariably splits along a bedding or sedimentation plane. Slate originates from clayey sedimentary rocks and belongs petrographically to a range which begins at the boundary between sedimentary and metamorphic formations and ends at the epizonal-metamorphic phyllite formations”.

This definition makes quite clear, from a petrological point of view, the range of rocks which can be considered slates. However, EN 12326-1:2005 continues defining roofing slate as a ”rock used for roofing and cladding, in which phyllosilicates are the predominant and most important components and exhibiting a prominent slaty cleavage”. Likewise, roofing carbonate slate is defined in the same way as above but with a minimum of 20% content of carbonate.

Metamorphic facies stability diagram. Modified from Spear, 1993.

Metamorphic facies stability diagram. Modified from Spear, 1993.

For the Subcomission on the Systematics of Metamorphic Rocks (SCMR), a part of the International Union of Geological Sciences (IUGS), a slate s.s. is “an ultrafine- or very fined-grained rock displaying slaty cleavage”. This slaty cleavage is also defined as “a type of continuos cleavage in which the individual grains are too small to be seen by the unaided eye”. The slaty cleavage is the most important characteristic of roofing slates, since it allows the rock to be split into large and thin tiles.

It is clear that there are two types of rocks, slates s.s., sometimes also called lutitic slates, which are low-grade metamorphic rocks (greenschist facies), and commercial slates or roofing slates, which are rocks composed mainly of phyllosilicates with an exfoliation which allows to produce tiles that may be used as roofing materials. This second group includes the slates s.s. together with other types of rocks, like shales, phyllites and schists.

A: Sedimentary slate (shale) with no develop of slaty cleavage. The planes correspond to sedimentation beds. Minas Gerais, Brazil.
B: Slate s.s., in which the planes correspond to slaty cleavage. Herbeumont, Belgium.
C: Phyllite, with a metamorphic grade slightly higher than the slate s.s., as the biotite crystals shows. Bernardos, Spain.
D: Schist, with a well developed schistosity. Finnmark, Norway.

Roofing slate deposits in the world

There are several roofing slate deposits in the world. The biggest is located in the northwest of the Iberian Peninsula, although there are other large reserves which are not yet evaluated in China and Brazil. Thus, the main producers of roofing slate are Spain, China and Brazil, in that order.

Evolution of the roofing slate trade. Data: UNSTASTS,

Evolution of the roofing slate trade. Data: UNSTASTS,

The sector has been hardly hit by the global crisis of recent years, but the production is beginning to recover in Spain, although China is gaining ground especially in volume of production. However, Chinese slate is sold at a price significantly lower than the Spanish, hindering the takeoff of a strong slate production sector in this country. First consumers of Spanish slate are France, Germany and the UK:Grafico Imp paises_ENG

From a petrological point of view, the commercial denomination “roofing slate” includes various types of rocks, with the common characteristic that can exfoliate in large and thin tiles. The specific characteristic of each type of slate depends on its petrology. These specific characteristics control the performance of a slate depending on the conditions of use and the climate.

World´s main deposits of roofing slates.

World´s main deposits of roofing slates.