What material does a door in your garage that leads outside need to be?
It sounds like an interior door was used. I used interior doors on a shed ; I screwed 3/8 &quot; rough sawn cedar ( exterior grade ) to the outer surface . I thought the cedar would protect them but the doors deteriorated in a few years. My other exterior doors are steel with wood frames  - no problems. Front door is solid wood - no problem ,but more expensive than steel.