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

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

Single ply roof guarantees typically cover 15-20 years, but the actual roof can last 25-40 years if looked after properly. Guarantees cover material defects, not wear and tear or poor maintenance