Det är berget som bestämmer 

as you say . nowadays they use metal cap like in this drawing.::: www.byggogbevar.no/media/1438/legging-av-skifer.jpg?anchor=center&mode=crop&width=768&rnd=131217848700000000 but i have seen many homemade concrete top stones... and even somone using logs that was hollow on the underside. hehe yes some water can pass thru the stones if its blowing hard when it rains... but at least at modern roofs there usualy is a layer of say, tar-asfalt-ruberiod paper under the stones that drain off the water