Single ply roofs often come with 15-20 year guarantees, but their actual...

https://blogfreely.net/rothesqlmw/what-does-a-bad-vapour-control-layer-do-to-a-flat-roof

Single ply roofs often come with 15-20 year guarantees, but their actual service life can be 25-40 years or more. The guarantee usually covers specific defects, not general wear and tear