Metamorphic rocks are formed by the recrystallisation of older rocks, when subjected to intense heat or pressure or both, within the earth’s crust. Clay is metamorphosed to slate, limestone to marble and sandstone to quartzite
Slate is derived from fine-grained sand-free clay sediments. The characteristic cleavage planes of slate were produced when the clay was metamorphosed, and frequently they do not relate to the original bedding planes. Slate may be split into thin sections (typically 4–10 mm for roofing slates) giving a natural riven finish, or it may be sawn, sanded, fine rubbed, honed, polished, flame textured or bush hammered. A range of distinctive colours is available: blue/grey, silver grey and green from the Lake District, blue, green, grey and plum red from North Wales, and grey from Cornwall.
Slate is also imported from Ireland (grey/green), Canada (blue/grey), France (blue/grey), India (blue/grey), China (blue/green/grey), Brazil (grey/green/plum) and blue/ black from Spain, which is the world’s largest producer of the material. Slate is strong, acid- and frost-resistant, lasting up to 400 years as a roofing material. The minimum recommended pitch for slate roofing is 20° under sheltered or moderate expo sure and 22.5° under severe exposure, and these situations require the use of the longest slates (460, 560 or 610 mm). Where thick slates (up to 20 mm in thickness) are used for a roof pitch of less than 25°, it should be noted that the slates lie at a significantly lower pitch than the rafters. Fixing nails should be of copper or aluminium. Slate is also used for flooring, cladding, copings, cills and stair treads. When used as a cladding material it should be fixed with non-ferrous fixings or cast directly onto concrete cladding units.
Recycled roofing slates, particularly Welsh slate, are generally available in a range of sizes and are appropriate for both conservation work and new build where an immediate weathered appearance is required. Welsh slates have a good reputation for durability, making the recycled product a viable option. Certain regional slates, such as Swithland in Leicestershire, are only
available as recycled products. This particular type of slate has a single top nail fixing and, unlike most roofing slates which are of a uniform size, is graded from large slates at the eaves to smaller units at the ridge. The standard pr EN 5534: 2013 gives the Code of Practice for slating and tiling.