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

https://wiki-wire.win/index.php/Is_BBA_Certification_Required_for_Commercial_Flat_Roofs_in_the_UK%3F

Single ply roof guarantees often cover 15-20 years, but the actual service life can stretch 25-40 years with good care. Guarantees mainly cover manufacturing defects, not wear and tear or maintenance lapses