The heartland of rag slating is north Cornwall
Rag slates are nailed directly to the rafters. Usually this is not a problem because they are very wide so they overlie several rafters and can be easily nailed. Narrower slates though only lie on one rafter and have to be fixed with two nails above each other.
Typically rafters are about two inch square and the slates are torched to keep out draughts and snow.