The question arises; why not split them all as thin as possible? The answer is that they might not be strong 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 durable rock is going to be as strong as; or stronger than; a dense slate made of weak stone. The traditional thicknesses of slates within the UK have evolved so they have always been made thick adequate inside resist the loads they will be exposed to on a roof; so the inherently weaker rocks are split 8 towards 12mm thick whereas your strongest is going to be as thin as 4mm.