I recall one of the Aldeer roof installers stated you need to keep an eye out on bolts that may need to be replaced with larger ones over time. Should be able to get 50 years out of the metal roof unless there is a moisture problem. I would have it checked out by a reputable long time roofing installer just as a precaution now and every 5 years for screw problem. Shouldn't charge you to come look at it.