My first house roof was assembled in the same way with Rosemary tiles and lime mortar. Had to strip the whole roof, felt, new battens and used as many of the old tiles as possible. Most of the tiles had started to break up with the top surface flaking off. Fortunately I found someone who was re-tiling his house before he retired so the tiles were still good (and cheap). Every fifth row was nailed on (using copper nails) to stop the wind lifting them. I hired two scaffold towers for the job with planks between them.