Single ply roof guarantees often cover 15-20 years, but the actual roof can...

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

Single ply roof guarantees often cover 15-20 years, but the actual roof can last 25-40 years with proper care. The guarantee usually covers materials and workmanship, not wear and tear or maintenance issues