Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.
Slate can be made into roofing slates, a type of roof shingle, or more specifically a type of roof tile, which are installed by a slater. Slate has two lines of breakability – cleavage and grain – which make it possible to split the stone into thin sheets. When broken, slate retains a natural appearance while remaining relatively flat and easy to stack. A “slate boom” occurred in Europe from the 1870s until the first world war, allowed by the use of the steam engine in manufacturing slate tiles and improvements in the road and waterway transportation systems.
Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%, making the material waterproof. In fact, this natural slate, which requires only minimal processing, has the lowest embodied energy of all the roofing materials. Natural slate is used by building professionals as a result of its beauty and durability. Slate is incredibly durable and can last several hundred years, often with little or no maintenance. Its low water absorption makes it very resistant to frost damage and breakage due to freezing. Natural slate is also fire resistant and energy efficient.
Slate tiles are often used for interior and exterior flooring, stairs, walkways and wall cladding. Tiles are installed and set on mortar and grouted along the edges. Chemical sealants are often used on tiles to improve durability and appearance, increase stain resistance, reduce efflorescence, and increase or reduce surface smoothness. Tiles are often sold gauged, meaning that the back surface is ground for ease of installation.