My name is Víctor Cárdenes, from Spain. I am geologist, and have a PhD on roofing slates. I´ve been working on research on this exceptional material for the last 20 years. I´ve published more than 30 research papers on roofing slate, covering many aspects such as history, weathering behavior, geology, standards… During this time, I´ve found some things about roofing slate you might find interesting. In this section I will give you some facts about roofing slate.
But first, let me make clear what is a roofing slate. There is a rock, named slate, which is “a fine-grained metamorphic rock displaying slaty cleavage”. This slaty cleavage is the distinctive property, and gives the slate its characteristic ability to be split along regular surfaces. Slates are rocks rather common in the Earth´s crust. They can be found in many places, and because of their slaty cleavage, have been used in construction since the dawn of human civilization. Think about it, nature gives to you a rock which is already preformatted, with little modifications you can obtain rather regular blocks for construction. Because of this, slate was used profusely in ancient buildings in areas where it was available.
There are many types of slates on the world. Some are rough and coarse-grained, others are soft and fine-grained, but usually all of them are crushed by the tectonic stress. Our planet is in motion, and this motion creates stresses that originate mountain, earthquakes and volcanos. So when you find slate, it will be most likely affected by these forces. For general construction, this is not a big deal. You can still get more or less regular blocks. However, sometimes it is possible to find fine-grained, smooth and compact slate which can be split into thin and regular tiles. This exceptional type of slate can be used then for roofing. This is roofing slate, a premium natural rock with unique characteristics. But be aware, not all roofing slates are slates. There are other metamorphic rocks which can be used for roofing. These rocks are closely related to slate, in fact they form a sequence of increase in metamorphic degree. Generally speaking, metamorphism is the increase of temperature and pressure that forms new rocks. This sequence, in terms of increasing metamorphism, is
shale → low grade slate → slate → phyllite → mica schist
Most of roofing slate are indeed slates, but not all slates can be used for roofing. My estimations, based on the analysis global roofing slate market, show that about 90% of the roofing slates traded in the world are slates. These types of roofing slates have different properties. Along metamorphism, there is an increase in crystallinity, in other words, the minerals that form the rock grow due to the increase in temperature and pressure. Because of this, pores in the rock disappear, which leads to a decrease in water absorption and an increase in stiffness.
Well, this was a general overview to the roofing slate world. In future posts I will review what has science said about roofing slates, and how we can use this information to enhance our knowledge on this exclusive groups of rocks, roofing slates.