Whenever I install an interior door over a floor that has already been laid, there’s a good chance that the floor is not perfectly level. To accommodate an uneven floor and achieve an even reveal at the top, I trim one side of the jamb.
I set up my laser level so that it casts its beam across the rough opening. (Alternatively, I could use a spirit level and make a pencil mark on each side of the opening.) Starting at one side of the jamb, I make a pencil mark on a story pole at the line. Moving to the other side, I make another mark. The distance between the two marks is the amount I need to trim off the high side of the jamb.
I align the lower mark on the story pole with the bottom edge of the jamb and transfer the upper mark to the jamb, which becomes my cutline. If I haven’t assembled the jambs yet, it’s simple to trim the piece on a miter saw. On prehung units, I simply square a line and trim the end with my jigsaw.
Mike Patterson, Gaithersburg, MD