Attach the trim
Nail the trim to the door frame with No. 4 finish nails. Nail the trim to the framing with No. 6 finish nails.
In this article, we'll focus on installing standard prehung doors. These have a door jamb that's 4-9/16 in. wide and are made to fit into a 2x4 wall that's 4-1/2 in. thick. This gives just enough of a fudge factor to have the jamb a bit proud of the wall surface on each side and to make up for any irregularities in the trimmer studs of the walls. Most openings will be about 82 in. high for standard doors, so that's what we'll focus on. If your wall is thicker or your opening is shorter, you'll have other concerns we won't cover here.
Before you order your door, check the width of your opening. It should be 2 to 2-1/2 in. wider than the door. This extra space gives you room to fit the jambs and the shims into the opening to hang the door. If your rough opening is 32 in., get a 30-in. prehung door. Also check the vertical sides of the rough opening to make sure they're reasonably plumb. Openings that have a trimmer stud out of plumb more than 3/8 in. from top to bottom will make it nearly impossible to install the door. It would be somewhat like trying to put a rectangle into a parallelogram. Small variations from plumb are quite common, however. Checking both sides and getting familiar with any problems with the opening will give you an idea of how much and where to shim the jambs later.
Most installation problems occur because the floor isn't level under the doorway. If the floor slopes slightly and the jamb isn't trimmed to compensate, your latch won't line up. You must check the floor with an accurate level as shown in Photo 1.
For complete directions, just follow our step-by-step photos and instructions.
How Do You Fit the Jamb to Floors of Different Heights?
Cut a 1-ft. long strip of 1/4-in. plywood the same width as your door jamb. Drop it to the high side of the floor, tack it in place, set your scribe and mark the contour of the floor onto the plywood. Remove the plywood, cut the shape with a jigsaw and transfer the shape to the bottom of the jamb. Cut along your mark with a jigsaw. Do this for each side of the door. If your transition is more than 1/2 in., you may need to trim the bottom of the door as well.
Set the door in place
Attach temporary to the jamb
To hold the doorjamb flush with the drywall before permanently fastening it, attach temporary blocks to both sides of the jamb. Use scrap lumber to make five 4-in. to 5-in. blocks, and attach each with 2-in. 18-gauge brads. Nail three blocks on the latch side and two on the hinge side (the door slab keeps the middle of the hinge side rigid). Keep the blocks away from the hinges so they won't interfere with shimming. The casing will cover up the nail holes when the blocks are removed.