steel roofing:

This is going onto my parents garage first off.
The metal sheets do snap together, and the screws/nails go under the snap lid things.
Why will the fasteners back out, due to thermal expansion of the metal? I don’t see the metal flexing more then a small fraction of an inch over even 100 degrees…what am I missing here that causes the downfall of a metal roof?

This is material only, and not exposed fasteners. I will be doing the labor. Enlighten me to my downfalls!