Geologically, Slate is a fine-grained metamorphic rock composed of sedimentary or volcanic ash deposits which through metamorphism have been chemically and structurally changed. Slate has two defined lines of breakability – cleavage and grain. These make it possible to split the slate into thin sheets, which can then be cut and shaped into roof slates, as well as many other applications.
The geology of the slate deposit is very important, as it affects the appearance, both colour and texture, and also longevity of the finished roof. Some of the best slates in the world were laid down between 400 – 500million years ago.
Often the word “slate” is used to describe mudstone, limestone and other types of stones which are also used for roofing. These have different properties and processes involved in their make up which can result in them having very different working characteristics.
Slate has been used in building for centuries, for example the quarries in North Wales have been operating, albeit in a smaller scale, since Roman times. With slate being found on the Roman fort at Seqontium Caernarfon – circa 77AD.
There are many different producers of good quality slate around the globe – Welsh Slate, Cumbrian Slate, Cornish Slate, North American Slate, Spanish Slate and Brazilian Slate. Each one of these is producing similar but distinctly different types of slate. If you follow the thumbnails below you can investigate further.