Slate is a metamorphic rock formed from clay and mud deposits, transformed under heat and pressure so that it splits along one plane into thin, flat sheets. That cleavage — not the colour or the hardness — is the property on which every building application rests.
The original sediment was compressed within the earth’s crust, and the platy minerals in it (mainly clay minerals that recrystallised into mica and chlorite) all lined up in the same direction. This produces the slaty cleavage, which need not coincide with the original bedding of the sediment.
Characteristics
- Splits into thin sheets, typically a few millimetres to about a centimetre thick.
- Very low water absorption, which makes slate frost resistant and allows a slate roof to last for centuries.
- Colour ranges from blue-grey and black through green and purple to red-brown, depending on the minerals in the parent sediment.
- Well-known sources include Spain (Galicia), Wales, the German Eifel and Moselle regions, Thuringia, Portugal and Brazil.
Applications
- Roofing. The classic use: the roof slate. See also the article on the slate roof.
- Cladding. Slates hung vertically on a ventilated batten structure.
- Floor and wall tiles. Riven or sawn slabs, often in a thicker format.
- Window sills, thresholds and worktops.
- Historically also as writing slates, billiard table beds and gravestones.
Quality and standard
Slate for roofing is covered by the European standard EN 12326, which sets requirements for bending strength, water absorption and behaviour in thermal-cycle and sulphur dioxide tests, among others. A key selection criterion is the pyrite content: pyrite (iron sulphide) in the slate can oxidise in the presence of moisture and cause rust staining or even pitting. A high carbonate content is likewise unfavourable, because it leaches under acid rain.
Practical tips
- Do not walk on a slate roof. Slates break under point loads and are treacherously slippery when wet; work from a ladder with a ridge hook and use fall protection.
- Always order a surplus. Natural slate varies in colour and thickness between batches; reordering years later will almost certainly give a visible mismatch.
- Ask for the standard and the test report, not just the price — you cannot tell by looking whether a slate will last 30 years or 100.
- On floors: seal it. Slate is more porous than granite and takes up oil and wine stains. A colourless impregnator makes the stone stain resistant; repeat periodically.
- No acidic cleaners. Vinegar or limescale removers attack the surface. Clean with a pH-neutral product.
- Cost. As a floor or wall tile, slate sits roughly in the range of tens to well over a hundred euros per square metre in material cost, depending on source, thickness and finish; a natural slate roof is considerably more expensive than a clay tile roof, in both material and labour.
Related terms
- Roof slate
- Slate roof
- Slate hook
- Slate hammer
- Natural stone
- Plain tile
- Roof pitch
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