Single ply roof guarantees often cover 15-20 years, but the roof 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 roof guarantees often cover 15-20 years, but the roof can last 25-40 years with the right care. Guarantees typically focus on materials, not wear and tear or maintenance