Manufacturer data puts the service life of a quality single ply membrane at 25 to 40 years, yet the guarantee on the paperwork usually says 15 or 20. Guarantee versus service life: two different questionsThe most common confusion in flat roofing is treating the guarantee as the lifespan. A manufacturer’s guarantee is a legal promise with a safety margin built in; the service life is how long the roof actually performs. The three-decade life of a membrane roof, stage by stageYears 0 to 10: the design pays out, or doesn’t. Buy the BBA certificate, the manufacturer’s design service and the approved installer rather than the shiniest brochure, then inspect annually and fix small things while they are small.