A slate roof will likely last a lifetime. The only maintenance is to seal the roof with a 15 year sealant (minimal) and it should last forever. I am a roofer and have seen a lot of shingle roofs fail yet never seen a slate roof fail. If there is a leak then it is most likely poor workmanship or poor design by the architect or builder. There are some contractors and architects that don't know what they are doing and a slate roof is not in their scope so to speak. So if you have properly designed and installed slate roof then it will last.
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