My hex driver keeps stripping metal roof screws, leaving marks. What causes this?

The most common culprit is a worn hex bit. Even high-quality bits wear down, especially under the torque needed for roofing screws. When it slips, you're not just stripping the screw head; you're also scraping off the protective coating on your metal roof. That exposed metal will quickly rust, eventually leading to leaks. Always start with a fresh, high-quality hex bit that perfectly matches your screw size. Use consistent, firm downward pressure and a moderate drill speed. If you've already got significant coating damage, you'll need a licensed roofing professional to assess and repair those spots, possibly even replacing panels, to prevent costly future water damage.