There's several things that can cause this.
#1 the deck was built to close to the threshold. There should have been a min. of a 4" step down so water would not get under the threshold. At least 3 times a year I get calls about rotted wood right at the doorway, everytime that's the cause. The door also should have been set in a sill pan, or at least flashed under the door.
#2 If the humidy is to high in the house the moisture is going to condence on the glass and run down onto the wood and rot it out. Running a dehumidifier would sure help. I'd set it on the kitchen counter and have the drain just drain into the sink while your away.
#3 Is this house built over a crawl space? Is there a 6 mil. vaper barrier on the ground?