The question arises; why not split all of them as thin as you possibly can? The solution is that they might not be strong enough. The durability of individual slates is a product of the rock?s strength and slate?s thickness. A thin slate made from solid rock will be as strong as; or stronger than; a thick slate made from weak stone. The traditional thicknesses of slates in the UK posses evolved so they is made dense sufficient in order to resist the loads they will be exposed to to a roof; so that the naturally weaker rocks are split 8 inside 12mm thick whereas their strongest is going to be as thin as 4mm.