Types of Slate

Types of Slate

The geology of slate is important, as it affects the appearance and longevity of the finished roof. Often the word “slate” has been used to describe mudstone, limestone and other types of roofing stone. However, their properties are significantly different, requiring a different method of use.

The most common, and strongest, type of slate comes from metamorphic rock, which has been exposed to immense heat and pressure over millions of years. This gives good slate some useful properties:

  • It can be split thinly, reducing the weight of the roof. Some slates distributed by SIG in France are a consistent 2.8mm thick
  • Low water absorption: reducing the risk of frost damage, permitting an extremely long service life and excellent durability
  • Hardness: reducing the mass of material required. Compared with natural slates, concrete or clay tiles are substantially thicker. A flawless slate has a distinctive ring when struck.
  • Workability: Slate can be cut, holed and trimmed using hand tools, making for fast, safe work when completing the roof.

Slates from the UK, Spain, Canada and China are produced from metamorphic deposits. These seams can contain twists produced by geological activity, and part of the skill of production is in making the best finished slates from that seam. SIGA slates in the Excellence and Specification ranges are hand-sorted to ensure only the flattest and straightest slates are packed at the quarry.

Buying Brazilian Slate

Brazilian slate is actually a sedimentary rock or mudstone, with significantly different properties. It cleaves very cleanly and all rock deposits are naturally very flat. This gives a very consistent appearance, but mudstone is significantly more brittle than metamorphic rock. This means Brazilian slate can only be cut or holed with power tools and can suffer higher breakage rates in use, particularly if the roof is trafficked after installation. Water absorbency can also be higher than with metamorphic rock, so there can be a long-term risk of frost damage. However, Brazilian material continues to be seen as an economical slate despite the differences in use.

If you choose to use Brazilian slate, we recommend that the slate is split no thinner than 7mm, and you ensure you obtain valid test results. While SIG Roofing can supply Brazilian slate to fulfil customer requirements, there are currently none in the SIGA range, or guaranteed by SIG, as valid BS EN12326 test certification is not always available. Brazilian slate is available in graphite or grey-green colours. If you require a certified, good quality grey-green slate, with an SIG-backed guarantee, please contact us to discuss the options.