Single ply roof guarantees usually cover 15-20 years, but the roofs often last...

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

Single ply roof guarantees usually cover 15-20 years, but the roofs often last 25-40 years if well looked after. The guarantee mainly protects against manufacturing faults, not wear and tear or poor maintenance