Single ply roofs often come with 15-20 year guarantees, but they can last 25-40...

https://redditchstandard.co.uk/lifestyle/how-long-does-a-single-ply-membrane-roof-really-last-a-building-owners-guide/

Single ply roofs often come with 15-20 year guarantees, but they can last 25-40 years in real life. The guarantee covers basic defects early on, not wear and tear over decades