My house has an old part (the hip roof with a flat center section to the right of this photo) and a newer section (the roof with the rusty flashing to the left of this photo.) The roof of both sections is covered in slate. The new section as shown in this photo needs repair. It involves removing the slates, hoping that few of them break (as replacements are costly), and laying down a membrane & new flashing, then rehanging the slates. People like to say that a slate roof lasts forever. I have learned the hard way that it is not really true. When materials were more precious than labor the locally available slates made economic sense. However if you are not a skilled DIY type modern-day labor costs make slate roof repair pricey. We had the new section around the corner to the left of this photo repaired this spring. The section in this photo will probably be done next spring.