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

https://smart-wiki.win/index.php/What_Maintenance_Records_Should_I_Keep_for_a_Commercial_Roof%3F

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