Properties of Slate

Durability is the most important property and the one that best characterizes Galician slate, and it is undoubtedly what sets it apart from other substitute materials used in roofing. Slate remains unchanged and constant in its technical properties in the face of many factors such as humidity, dust, salt, cold, heat, snow, or wind. Therefore, slate is resistant to attacks from atmospheric agents and corrosion caused by smoke and gases. Due to its composition, it is not affected by acid or bases, making it virtually insensitive to both natural and industrial environments. The invariability of this characteristics is partly what contributes to its greatest virtue: durability. According to a study by the University of Bath in the United Kingdom, the estimated lifespan of slate is 200 years.

Some of the material used for roofing do not provide adequate thermal insulation, which is one of the key factors that indicates the quality of the product. In fact, the Technical Building Code establishes the current “Energy Certification of Buildings”, which categorizes and differentiates building bases in their energy savings according to the thermal resistance of the elements that make up the enclosures and roof, among other. The thermal insulation of enclosures is greater the lower the thermal transmission coefficient of the elements they are made of.

In the case of roofs, slate stands out over other roofing materials, such as ceramic or concrete tiles, fiber cements, or metal roofs. This is because its thermal transmission coefficient is very low, at 0.43 Kcal/ºCmh. To understand the magnitude of this coefficient in slate, it is worth nothing that other materials like concrete tiles range from 1.5 to 1.2 Kcal/ºCmh, while ceramic tiles have an approximate coefficient of 0.80 Kcal/ºCmh, both offering lower thermal insulation than slate.

Moreover, if the usual installation of slate includes an air chamber or insulating materials, its thermal conductivity will be much lower, further improvising thermal insulation and, consequently, the energy saving of the home.

The impermeability of slate ensures low water absorption and frost susceptibility, which are closely related to its durability. Simply put, if a material absorbs a significant amount of water due to its porosity, and it is know that freezing water and the formation of ice leads to an increase in volume as t transforms from liquid to solid, it is evident that a porous material exposed to low temperatures and frost will suffer more intensely, potentially leading to cracking or breaking.

In a study conducted by the University of Santiago de Compostela, it is noted that absorption is a property of the material representing its ability to absorb water. This property is related to open porosity, or in other words, the volume of voids in contact with the atmosphere. The materials studied showed the following absorption values: Slate tiles: 0.40%; Ceramic tiles: 12%; Concrete tiles: 6%

It is clear that slate has a significantly lower absorption value compared to the other two materials.

The acoustic insulation property depends on the density of the materials that make up the enclosure, in this case, in this case, the roof. The density of slate is higher than that of other materials, which provides better acoustic insulation, both for airborne noise and for noise produced by any type of impact. The acoustic insulation property depends on the density of the materials that make up the enclosure, in this case, the roof. The density of slate is higher than that of other materials, providing better acoustic insulation, both for airborne noise and for noise produced by any type of impact. The density value of slate ranges from 2.7 – 2.85 g/cm³, while that of the main roofing materials such as ceramic tiles and fiber cement are 2.0 g/cm³ and 1.8 g/cm³, respectively. The density of slate is higher than that of other materials, providing better acoustic insulation, both for airborne noise and for noise produced by any type of impact. In case of impact noise, such as rainwater hitting the roof, slate has the advantage of being a very coherent and elastic material, so it is difficult for a well- placed slate tile to resonate, effectively absorbing the impact.

In the study conducted by the University of Santiago de Compostela, with data provided by LOEMCO, another of the most significant properties for durability, flexural strength, is analyzed. This study compares the flexural strength of different roofing materials based on laboratory test that demonstrate slate´s high flexural strength, and therefore, greater durability.

In summary, the tests assume a slate tile with a thickness of 10 mm, which is the nominal thickness of a ceramic or concrete tile, with a width of 290 mm and a length that allows it to be placed between the support bars of the bending test device, which are 370 mm apart. This slate tile can be compared to a flat ceramic tile with dimensions of 470 x 290 mm. Under these conditions, a dry test yields a breaking load value of 365 daN for the slate, which is significantly higher that the breaking load of flat ceramic tile of these dimensions, which would typically show values around 250- 300 daN. It is also higher than the flexural strength are breaking load of 150 dnN that flat concrete tiles exhibit.

To understand the magnitude of the values mentioned, the unit of force daN stands or decanewton, which is equal to 10 Newton. Or roughly1 kilogram. Therefore, it is clear that slate has a higher flexural strength than ceramic and concrete tiles, which is directly related to the greater durability of the material.

It can be affirmed that all shapes, slopes, or spaces can be covered with slate. Starting from this assertion and appreciating the unmatched aesthetic and technical quality of slate finished, it is logical to think that it can be used for various construction elements. The use of slate for roofing is the most common due to its countless characteristic that make it ideal for this purpose. However, this fact should not overshadow the diverse construction solutions that slate can provide, such as interior and exterior flooring, interior cladding, facade cladding, ventilated facades, and even decorative elements in construction.