I have a fair amount of experience with slate roofing. I bought a flipper home years ago with a slate roof in need of some attention as well as a multi unit I still have that had a slate roof. The reply from Texas is correct. It is a stone and therefore can last almost forever unless something breaks it. It is the nails that wear out by rusting. Slates then fall out and break. If done from scratch now, stainless steel nails would be my choice. You cannot just walk on it as it will crack and break and that makes clearing branches, leaves from valleys, painting trim and cleaning chimneys a bit more challenging. If it is the right and valuable slate, there have been times a company will remove it and pay you close to what a new shingle roof costs. I opted to repair the slate roof on the home I was renovating but even the repair was a bit costly for what seemed like minor and minimal repairs but the house, a 3 story brick Victorian, is over 100 years old, the roof looks classy, and is certainly going to last way into the future. The other building I own had a slate roof that previous roofers nailed shingles over thus punching holes in all the slate. My roofers said they had never seen that before. I had it all removed and put regular shingles back on. It didn't really cost much differently, just a little more labor to remove since there was not sheathing under it and the cost of sheathing. For the repairs, I used Lantz Slate Roofing in Lancaster and spoke to Aaron, 717-656-2620.