The question arises; why not split them all as thin as you possibly can? The answer is that these 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 sturdy rock is going to be as strong as; or stronger than; a dense slate made from weak rock. The traditional thicknesses of slates within the UK need evolved so they tend to be made dense needed in order to resist the loads they be subjected to at a roof; so that the inevitably weaker rocks are split 8 to 12mm thick whereas your strongest will likely be as thin as 4mm.

Split slate as thin as possible