I installed a prehung door for a closet in a playroom and the rough opening height was 83". I don't know the exact size of door because I am not posting from home right now, but when I put the door in the opening the gap from the header to the top was rather large I thought, especially for 2 1/4 inch casing to go over it. So... I shimmed the jambs up to a grand total of I believe 1 1/4" off the subfloor and then took the shims out. Several days went by (and I have yet to install the casing, just getting to it) and I kept thinking that with carpet thickness, I would be ok. However, realizing that the bottom of the baseboard will only be 1/2" off the subfloor, this still leaves me with a gap of about 3/4" between the bottom of the baseboard and the bottom of the door jambs. I know casing goes on first, but is there any other way to reconcile this gap as it will look out of place besides taking the door off and redoing it? Can the carpet people use thicker padding or should I just do what is probably a little more frustrating, but correct in the long run and take the door down and start over. Somehow, I got the door plumb. This is in an unfinished room in a 2 year old house.

ty163

 

82-1/2” is the normal height for the header. I believe the 83” header height came about because carpet would be installed afterward. When the doors are installed ahead of the carpet the jambs are shimmed 1/2” off the floor before being plumbed, affixed, and trimmed out.

Continuing this logic: the choice for the door and fame is (1. use a full length slab and allow at the header) (2. undercut the door [foreshorten it] and cut the jamb to length). It looks like whom ever did the planning chose the first method. Your slab should measure a full 80 inches, and the jamb legs should only extend a short distance past the bottom of the door if you want to duplicate the planners intention. Otherwise you need to live with whatever undercut exists. Shim the jambs 1/2” off the floor. (Thicker pad is an option).

 

If you continue adjusting for things, you are creating something that will forever need to be adjusted for. I know it is a PITA, but I would just take the door down and lower the header with a filler and replace the door. You didn't say if the wall at the top of the door is drywall or not. Can it be filled/patched if the trim doesn't cover? Good luck.