I recently bought a house and the roof needs some attention. However, I don't know if it needs to be patched up a bit or fully retiled. As far as I can see it does not leak (I looked inside the attic). Thing is, the house is very tall so it's difficult to get up there and have a look. Are there any tell-tale signs that would indicate whether it needs to be retiled? How long would a slate roof normally last? Thanks in advance