My parents replaced the roof on their 150-year-old slate-roofed house, as did several other people on the street.
A house that old will often have developed a few minor leaks in its lifetime. It's been subject to a lot of weathering. Probably it was put in without any felt underneath, as was normal at the time. Maybe the nails have weakened from rust. Many lofts are a bit damp, due to condensation as the roof is of course cold. So often the battens holding the slates will be a bit tired as well.
You spend a few years with strategically placed plastic tubs catching drips, then a once-a-decade storm comes along, you lose several slates at once, and it's time for a new roof.
A house that old will often have developed a few minor leaks in its lifetime. It's been subject to a lot of weathering. Probably it was put in without any felt underneath, as was normal at the time. Maybe the nails have weakened from rust. Many lofts are a bit damp, due to condensation as the roof is of course cold. So often the battens holding the slates will be a bit tired as well.
You spend a few years with strategically placed plastic tubs catching drips, then a once-a-decade storm comes along, you lose several slates at once, and it's time for a new roof.