First once you have the prehung door unit, verify that the size of the rough opening is correct. With a threshold, the height should be 84" for an 80" door. Without a threshold it is 82 1/2". The width for a rough opening should be 2" larger than the prehung door. Example, for a 36" prehung door the rough opening width should be 38". Some prehung doors come complete with brick molding on the outside, which goes against the sheeting. The width of the jamb should match the width of the studs plus the thickness of the inside finish and the outside sheeting.
Apply tar paper to the perimeter of the door opening, so a good overlap of 4" can be done when the wall is papered. Install the prehung door unit into the opening, snug the brick mold into the sheeting, with the prehung door unit resting on the subfloor. With shims or wood shingles split about 1 1/2" wide, shim the sides at the top so the prehung door unit is centered in the opening, as shown:
These top shims are temporary just to position the prehung door and hold it in place. Notice the margin around the prehung door should be the same. Plumb the hinge side of the prehung door first. Shim up the low side of the jamb if the top margin above the jamb is not the same or tapered. Check the plumb on the hinge side, with the prehung door closed and shim opposite the hinges. Screw a 3" x #8 flathead wood screw in through the stop of the jamb through the shims and into the cripple, countersinking them a bit below the surface to be filled later. Do the same at the other hinges, maintaining an equal margin. Now shim and screw the latch side of the jamb into the cripple, keeping the margin the same. Three screws on each side is sufficient, none is required on the top unless the unit is a prehung double door. Shimming the jamb with the prehung door in place makes it easy.
Remove any excess shim sticking out past the jamb with a sharp utility knife or hand saw. Install the lock set as per instructions in the prehung door package.
Verify the correct rough opening for your prehung door. The width and length should be 2" more than the size of the prehung door. Add 3/4" to the height if hardwood or carpet is being installed below the prehung door.
Follow the same procedure as for an outside prehung door, except for the brick mold and tar paper. The width of jamb should be the width of stud plus the two finishes on the walls.
Hope this helps
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