When you recover your roof, perhaps if the original slates are beyond repair, it is vital that you strengthen the frame within the attic that supports them.
Modern concrete tiles can he three times as heavy as natural slates. If you don’t upgrade the frame the roof will start to sag and deflect, causing no end of related structural problems.
What is not widely known is that if you are re-roofing a property then you need Building Regulation consent.