Single ply roofs often come with 15-20 year guarantees, but their actual...

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 their actual service life can be 25-40 years or more. The guarantee usually covers specific defects, not general wear and tear