Single ply roofs often come with 15-20 year guarantees, but they can last 25-40...

https://www.4shared.com/office/Q9ZZDzKPge/pdf-27402-44163.html

Single ply roofs often come with 15-20 year guarantees, but they can last 25-40 years in real life. The guarantee covers basic defects early on, not wear and tear over decades