Demossing is definitely the best choises u could do, tho pressure washing ur roof could seem to come out a lot cleaner but the damage that can be done to ur roof is a lot worser then u think, when u pressure wash ur roof it will weeken the tiles and under harsh wether it can break and crack your tiles witch will give exposer to the inside of ur roof that leads to a lot more work done,that is the reason why just demossing is a a lot better answer to ur roof.