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