Single ply roof guarantees usually cover 15-20 years, but these membranes often...

https://redditchstandard.co.uk/lifestyle/how-long-does-a-single-ply-membrane-roof-really-last-a-building-owners-guide/

Single ply roof guarantees usually cover 15-20 years, but these membranes often last 25-40 years when cared for properly. The guarantee sets a baseline, but real longevity depends on regular upkeep