The question arises; why not split all of them as thin as you possibly can? The answer is that these might not be sturdy enough. The strength of individual slates is a product of the rock?s strength and the slate?s thickness. A thin slate made from solid rock is going to be as strong as; or stronger than; a thick slate made of weak stone. The traditional thicknesses of slates within the UK come with developed so that they are definitely made dense adequate at resist the loads they be exposed to in a roof; so the naturally weaker rocks are split 8 inside 12mm thick whereas your strongest will be as thin as 4mm.