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

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

Single ply roof guarantees typically last 15-20 years, but the actual roof can often go 25-40 years with proper care. The guarantee covers materials and workmanship but not wear and tear or poor maintenance