15 years? No part of a house should be scheduled to last 15 years. Houses are supposed to be long term. I expect a car to last 15 years.
My roof stones (not slates) were put on 198 years ago, and when the roof was recently relaid (for the first time, incidentally) most of them are re-useable. Indian stone is available for the replacements. Try stone, if the house is strongly enough built to support it.
Mind you, I haven't got solar panels on it. An east-west terrace roof in the north of England isn't best placed to take advantage!