The question arises; why not split all of them as thin as possible? The solution is that these might not be sturdy enough. The durability of specific slates is a product associated with the rock?s strength and the slate?s thickness. A thin slate made from strong rock will be as strong as; or stronger than; a thick slate made from weak rock. The traditional thicknesses of slates in the UK posses developed so that they are definitely made thick sufficient to resist the loads they be exposed to in a roof; so the inevitably weaker rocks are split 8 towards 12mm thick whereas that the strongest will likely be as thin as 4mm.