I'm helping a friend re-slate the roof of his old house. When he stripped off the old tiles he was left with a feathered eaves board with the first course of half tiles and he can't remember if these were on a batten or just on the eaves board. He also can't remember how the next course of slates was fixed. He thinks these might have been nailed onto the same batten as the half tiles. Any advice on how to start would be gratefully received.