If you lift up on the roof near the door you will likely see the gap at the bottom of the door close in a bit. Sometimes you also have to push the wall in a bit from the outside wall near the hinges.
In essence if the roof is sagging, the walls tend to go outwards. Sort of like pressing down on a balloon.
There are a lot of threads on making a screen door and frame. Here is one of them:
My theory is the frame for a screen door if properly tied in, will help support the roof and walls on both sides of the door.
We usually travel to the opposite end of PA (NW corner).