How Roofing Slate is Manufactured

Natural slate comes from a sedimentary rock formed some 500 to 600 million years ago during the Cambrian, Ordovician and Silurian periods. Volcanic action, compression an movement transformed the original clay constituents into slate and gave the rock its essential characteristic – a tendency to split into sheets. Chemically, slate is composed of silica and alumina compounds with a smaller proportion of lime, magnesia and iron in the form of sulphide. The differing colours of slate available is due to the staining by the assorted products of metallic origin.

Most of the roofing slate produced in the early and mid Twentieth Century was quarried or mined in North Wales. Large boulders of rock were blasted or hewn out of the mountainside and brought to the surface. These boulders, part of naturally-occurring pillars of slate, are machine-sawn into cubes and it is the size of these pillars which determines the width of the finished product. The next stage of production if usually carried out by hand, the rock being riven by hammer and chisel down the grain until the required thickness of slate is reached. The four edges are now dressed (cut) by machine or hand to give a finished length and breadth in the largest possible dimensions. The dressed edge gives a bevelled finish to the face of the slate which is attractive in appearance since no two slates are the same. A limited number of slates are available finished with a sawn edge, giving the slates a more uniform appearance when on the roof.