My slate roof is 100+ years old. Generally, the slates are fine…but the nails holding them in are rotting. (The flashing is iffy too.) I have replaced a few fallen slates with synthetic slates, but I expect more to fall. Unfortunately, I will probably replace my roof with a premium architectural shingle that imitates the look as best as possible (for half the cost). Most of my neighboring attached houses have replaced their slates with asphalt already.