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

https://www.slideserve.com/gwyneyxcez/why-do-roof-problems-show-up-after-two-winters

Single ply roof guarantees usually cover 15-20 years, but the actual roof can often last 25-40 years. That’s because guarantees focus on materials and installation issues, not wear and tear over time