I have a 39 and a quarter by 80" RO. I would like to buy a new door and install over a concrete threshold. I am trying to not have to cut out the concrete threshold to accommodate a standard door. Any suggestions?

 

Is that a 2x6 wall? If so, rip some 1/2" plywood down to about 6 1/2" and add two pieces to the right side to reduce the rough opening down to 38" (or install a 1x6 on the right side to reduce the opening to 38 1/4) and to move the jamb away from the wall on that side. Then get a 36x80 prehung door. They are 37 1/2" wide.

You will need to cut the door and door jamb down a little so that it will fit. Take the hinge pins out, and cut the left and right jambs down to about 79 7/8" using a skilsaw and a speed square as a guide.

If the concrete is not level, you can cut one jamb shorter or longer to compensate for the out of level floor so that when you set the door in place, the head jamb will sit nice and level.

Then install the jamb level and plumb... use pairs of tapered shims along the sides to center it in the rough opening. Those shims should go at the corners of the door and behind the hinges. If you use finish screws to pin the corners of the door, you can pull them back out as needed if you need to adjust the door after you hang it.

Once you have the jamb temporarily fastened to the rough opening, install your aluminum threshold on the floor. And them after that is in, measure how long the door needs to be. Cut the door to length... and if the door is hollow, determine if you need to plug the bottom.

You will be okay if you are only cutting off a little bit, but any more than about 3/4" and you will need to plug the bottom. I will usually reuse the piece that I cut off by running it through a tab.e saw to peel off the skin on each side of the plug. Then I glue it back in and clamp it by putting a 1x4 on edge on each side of the door, then clamp those 1x4s together to squeeze the glue joints until they are tight. After a couple hours you can take the clamps off and test fit the door.