I've got (I think) concrete roof tiles. They are about 35 years old I would guess. It looks like the coating is coming off. I can see a lot of similarly colour gunge in the guttering being washed down and my drains seem to be always full of the stuff. I am wondering about trying to coating them or replacing them. In principle they look perfectly ok, apart from the loss of the coating. Do coatings work or should I just bite the bullet and have them all replaced next time I'm doing some work on my roof?