Does the geological age matter for roofing slate quality?

This is a recurrent question. Is “old” roofing slate better than “new”?. We know that the Earth has about 4.5 billion years (or 4.500.000.000 years). The oldest minerals retrieved have about 4.4 billion years. Compared to these ages, roofing slates are relatively young.  The oldest roofing slates quarried nowadays are from the late Precambrian, “only” 600 million years ago. Some of these roofing slates are the Brazilian low-grade slates from Minas Gerais, Brazil (lithotypes B0, G0, and R0), the phyllite from Bernardos, Spain (B2) and the slates s.s. from Wales (R1 and G1). Age has then no influence on the petrology of roofing slates, neither in the quality.

Some of the main types of roofing slataes quarried nowadays and their situation on the stratigraphical column

Some of the main types of roofing slataes quarried nowadays and their situation on the stratigraphical column

There is a lot to say about quality, but I will try to summarize that in one paragraph: Quality depends on the customer requirements. Each type of roofing slate has its own special characteristics, and it is the customer who choose which characteristics are more important for him. Usually, the customer requires advise to find out his needs.

Getting back to the title of this post, does the geological age matter for roofing slate quality?


The features that have influence on the constructive requirements of roofing slate (i.e. quality) are others: Petrology (or type of roofing slate) and customer requirements, or better: What we have and what the customer wants.

I will develop this in following posts, and I promise you won’t have to wait months to read them.



My presentation for the National Slate Association 2016 annual meeting

Hello. This is my presentation for the National Slate Association (NSA) annual meeting, which was held in Philadelphia 3-6 march. I talked about some facts involving roofing slates:
– Definition of the petrological term slate versus the commercial and constructive term roofing slate.
– Types and characteristics of roofing slates
– Oxidation
-Roofing slate standards


New version of roofing slates of the World clasiffication

Hello to everyone. I’ve been working hard in this new version of the classfication for roofing slates, I think now looks more comprenhensive.

Regards.Roofing slates of the World v_2

Publications on “roofing slate” topic

Sin título-1

This is a graphic on the number of scientific-technical publications/reports/articles/patents published under the roofing slate topic. It is not 100% accurate, since there is much literature not easily reachable, or even lost and forgotten forever. However, some interesting general conclusions can be taken from this graphic. The first point is the role of the US on roofing slate research. During the first half of the XX century, several high-quality scientific reports were published, mainly by the US Geological Survey and the Journal of the Franklin Institute. The most prominent authors were Nelson T. Dale, Edwin C. Eckel and Oliver Bowles. Personally, I have greatly enjoyed reading these papers. The conclusions of these papers are completely valid today. The US scientific production decayed in the second half on the XX century, and for the first years of the XXI century, again the number of works increased, but this time focusing on roofing techniques.

In Europe the tendency is opposed. Most of the publications are from the end of the XX century-beginning of the XXI century. Here can be deduced two tendencies: publications from the former producing countries, like France, UK, Germany and Belgium have a clear bias to heritage-related issues, while for the producing country (Spain) the publications have a strong technical component. The first tendency is especially perceptible for the UK, in which the Heritage Associations (Historic Scotland and English Stone Forum) have played an important role in preservation and documentation of ancient slate roofs.

For the new producing countries (China, Brazil and India), the number of international publications is still low. In the case of Brazil, several reports were issued during the first years of the 2000’s, related to the “Brazilian Slate – CE mark” conflict.

Some interesting publications available on the internet:


Eckel, E.C., 1904. On the chemical composition of American shales and roofing slates. The Journal of Geology 12, 25-29.

Dale, N.T., Eckel, E.C., Hilldebrand, W.F., Coons, T., 1906. Slate deposits and slate industry of the United States. USGS Bulletin #275. United States Geological Survey.

Bowles, O., 1926. Recent progress in slate technology. Journal of the Franklin Institute 202, 668-669.


Davies, D.C., 1880. A treatise on slate and slate quarrying, London.

Hughes, T., 2005. Vernacular slate and stone roofs in England, England’s Heritage in Stone. English Stone Forum, Tempest Anderson Hall, York, pp. 28-42.


Filho, C.C., 2011. As ardósias Bambuí na Marcação CE. ABIROCHAS, p. 19.


Cárdenes, V., Cnudde, V., Cnudde, J.P., 2015. Iberian roofing slate as a global heritage stone province resource. Episodes 38, 97-105.

Roofing Slates of the World, part IV

I’ve been working on a poster summarizing the main roofing slate outcrops in the World. It compiles the location of the outcrops together with a new classification of roofing slates taking into account the color and the petrology. For example, the lithotype B1 corresponds to the black-grey metamorphic slates, the most common type, while R1 would correspond to also metamorphic slates but with purple-red colors.

Roofing slates of the World

Roofing slate lithotypes


First of all: lithotype, what’s this?. A lithotype, as used in Geology, is a stone which represents the characteristics of one group. So when I talk about lithotypes I’m just referring to a set of characteristics and properties. Something like a stereotype but without negative implications.

Roofing slates are classified by the sector according to commercial terms, frequently referring to a specific brand. The geographical names are very frequent (i.e. Mosselschieffer, Brazilian Slate, Spanish slate). The market distinguish three main qualities or choices (first, second and third), but there are many other like Cofina, Historical Monuments, Scottish, etc. The characteristics of these qualities are more or less recognized by the market. This makes that the purchaser rather looks for a specific commercial brand than for a type of slate. However, it is important to define lithotypes for roofing slates, that is, to group and name the slates depending on the characteristics that clearly make this distinction. In other words, if I tell you “there is a bird” you will think in a bird, obviously, an animal with wings and two legs. But if I tell you “there is a sparrow” then you will have in mind an specific kind of bird. You might know few or a lot about the sparrow, but you know which kind of bird I’m talking about. For the slate is the same. Someone that has been working for decades in the slate world knows the slate by the name of the quarry, is like when you know a group of people just by their names, you don’t need more. But other people might do not know the names, and then needs the surnames to get the picture. This is my aim, to give general names and surnames to roofing slates, combining both geological and commercial information, and then proposing a common classification useful for everyone working with roofing slates. How?.

Dale, in 1906 proposed a classification in which the first distinguishing element was the geological origin (Sedimentary or Igneous), and then the matrix arrangement seen at the petrological microscope (Clay slates or Mica Slates). There was a third subdivision regarding the potential change in color (Fading or Unfading), which in fact is still widely used in the U.S.A. This changes in color are due to the occurrence of carbonates.

The proposed classification uses in fact the same parameters, but actualized. Two universal distinguishing features are color and petrology. Color is the reflect of the mineralogy of the roofing slate. The main minerals of slates are quartz, mica and chlorites, plus some other minerals in different proportions and occurrences. Then, there are three main families of color for roofing slates: black-grey, red-purple and green. These general colors are a reflect of the average mineralogy. Black and green slates are the result of reducing conditions, so they can contain iron sulphides and carbonates, two groups of minerals very important from a quality point of view. On the other hand, red slates never have iron sulphides, since they come from oxidizing conditions. Instead they contain abundant crystals of hematite of small size, being this one the coloring mineral. The amount of organic matter, usually present as carbon, is also a determining component for color. The organic matter gives dark tones, and is, as the iron sulphides, related with reducing conditions. Generally speaking, the contents of iron oxides versus iron sulphides and organic are inversely proportional in roofing slates. This is the first level for classification, the color, which gives then rough information about the mineralogy and geological conditions. However, there are some exceptions. In some areas, there is a especial occurrence of roofing slates known as “multicolor” or just “colored” These slates are characterized by presenting non-homogeneous red-orange-black surfaces, product of infiltration of superficial water in the cleavage planes and deposit of iron oxides. This effect is just an alteration, so the color of the roofing slate should be defined in the non-altered planes. Something different are the “variegated” slates, which are red-purple lithotypes showing areas of green lithotypes. This is typical in slates from the UK and North America, and it is due to mineralogical processes developed in the slate matrix. These type of slates are included as a sub-group of the red lithotype, which is their original lithotype.

The second level is the petrology, or the type of rock. Following the classical classification for rocks, there are sedimentary rocks (sandstones, siltstones and shales), metamorphic rocks (slates, phyllites and schists) and volcanic rocks (cinerites). Each of these groups have distinguishing features that influence their performance as a construction material. Sedimentary rocks usually have high Water Absorption (WA) and low Bending Strength (BS), while metamorphic rocks have low WA and high BS and volcanic rocks.

The proposed classification is an attempt to establish an understandable and easy way to name the different types of roofing slate used in the World. This classification has some advantages compared to the commercial terminology used nowadays. Every roofing slate in the World only matches in one category, and these categories describe the aspect and petrology of the roofing slate on a general way. From this description, some technical characteristics can be inferred, regarding to weatherable minerals, WA and BS. However, there are some minor exceptions linked to singular outcrops. For example, in the lithotype B1, which corresponds to slates s.s., the carbonate content usually is below 2%, but some outcrops from Canada and Italy have higher values of carbonate, in some cases over 20%. A complementary information to this classification might be then the geographic location. A technician from the slate market knows the regional characteristics of each slate, and a layman would know where to place it. The lithotype code would act then as the name of the roofing slate and the geographic location the surname.