In the roofing industry we all refer to slates and tiles and know which is which. But do other sections of the construction industry define the terms roofing slate and roof tile in the same way and who is right?
     If we look at roofing products that are called slates it is hard to decide if they are a roofing slate, or a roof tile that looks like a slate. If being flat and laid to a double lapped pattern defines a roofing product as a roofing slate, then shingles and shakes should be wooden roofing slates. If slates have no nibs and tiles do, then Kent peg tiles would be clay roofing slates.

Natural slate
In its purest form roofing slates are dark coloured pieces of metamorphic rock. The origin of the rock is clay, silt, shale or volcanic ash that has been heated and compressed over millions of years to form what we know as slate. The rock is smooth and strong and can be split into sheets 4-15mm thick, before being cut into a rectangular shape. Premier roofing slates come from the West Coast of the UK from Cornwall, through North Wales, Cumbria and into Scotland. Whilst some imported slate is metamorphic rock, the rest is sedimentary rock and may not have the same qualities as metamorphic rock. Each piece of rock laps over the piece of rock below in a pattern called double lapping. On a sloping roof, water draining down the roof is not prevented from draining into the vertical joints, but any water that does will be guided out again.

Stone slate 
Stone slates are formed of sedimentary rock, such as limestone or sandstone, which is naturally found, or dressed, into slabs of a thickness of between 19-35mm. The slabs vary in surface texture, being rougher and lighter in colour than natural slate. Being rectangular they are laid double lapped like natural slates, with the exception that they are not center nailed; being very heavy they only require head nailing.

Copies
All other roofing materials that are called roofing slates are copies of the natural or stone slate, either in true size and texture or in the laid finished appearance.

Double lap slates
Fibre cement: the ability to manufacture fibre cement into sheets 4mm thick provides a material that can be cut into rectangular pieces very close in appearance to natural slate. However, the rigidity of the material is lower and therefore requires the tail of the slates to be held down with a copper rivet. In all other respects it can be used as a direct replacement for natural slate, but at a lower cost and shorter life expectancy.
Concrete: some concrete roofing slates are designed to look like thick natural or stone slates. They are produced in various colours and sizes both with and without nibs but tend to weather differently due to the cement content.
Resin slate: resin slates developed from the need to manage the vast quantities of slate waste by grinding and mixing it with resin before moulding back into an exact replica of a natural slate.