Exterior Door is Sticking – What is causing it and how do I fix it?
I’d be checking out that threshold sooner rather than later. If you can get at it from underneath (i.e. basement) try to see if you look at the sub floor. If the floor is rotting you have a big problem that needs to be corrected ASAP. I suspect the door is warped, assuming it’s wood. You can take the door off and using a hand plane, take thin layers off of it until it clears the threshold. Ultimately it sounds like you have a water problem. You’ve got water that isn’t draining away from the door/threshold. Do you have rain gutters above the door? You need to prevent standing/splashing water around the door.
