The question arises; why not split all of them 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 will be as strong as; or stronger than; a dense slate made from weak stone. The traditional thicknesses to slates within the UK posses developed so that they are made dense sufficient at resist the loads theyll be exposed to to a roof; so the naturally weaker rocks are split 8 in order to 12mm thick whereas that the strongest are going to be as thin as 4mm.