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 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 of weak stone. The traditional thicknesses of slates within the UK have developed so they are definitely made dense sufficient at resist the loads theyll be subjected to at a roof; so that the inherently weaker rocks are split 8 to 12mm thick whereas your strongest is going to be as thin as 4mm.