A “prehung” door is sold attached to its jambs. Anybody who has hung a door to existing jambs recognizes the miracle that is the prehung door. Instead of worrying about an error of 1⁄16" here or 1⁄8" there, prehung doors come perfectly fitted. Your main challenge is to avoid ruining that perfect alignment during installation.
Prehungs have one major limitation: You must remove and replace the casing molding on both sides of the doorway. Sometimes, like near the end of a remodeling project, there's no molding to remove. Sometimes, as with an exterior door, the advantages of a tight fit outweigh the molding work. One caution: Due to the many combinations of size, finish, and style, your particular door may well be a special-order item. Plan ahead.
Step 1: Prepare the Opening
Prehung doors are generally sold by the rough opening—the size of the hole in your wall, as described later. To measure the rough opening accurately, strip off the old door. Remove the trim (molding).
Remove the jambs starting from the bottom. Slip a prybar behind the jambs and pry against the stud. If the jamb is stuck behind some flooring, pry at the middle and saw through the jamb. Any damage to the nearby wall should be covered by trim.
Although it's hard to enlarge an opening, it's easy to narrow or shorten one. Nail 2 × 4 spacers against the stud or header, and then nail the jambs to the spacers. Plan on some drywall repair when you cover these spacers at the end.
Step 2: Buy the Right Door
To buy a door, you need to know the swing and the size.
Which way should your door swing? Stand where the door should swing toward you. If the hinges should be on the left, you need a “hinge-left” door. If they should be on the right, you need a “hinge-right” door. Simple as that—so long as you're pulling the imaginary door toward you!
Check the Doorway
Place a long level (or a short level taped to a straight board) against the inside of the studs. If the studs are more than 1⁄4" out of plumb, compensate by ordering a door for a smaller rough opening, as described in the next section.
Place the level against the wall, near the doorway, to check if the wall is plumb. It's hard to correct a wall that's tilted; but at least you'll understand why the door swings on its own!
Sizing the Door
Most interior doors are 1 3⁄8" thick; exterior doors are usually 1 3⁄4" thick. The most common door size is 30" × 80" or 2'6" × 6'8" in lumber jargon.
As I mentioned, more important than door size is rough opening size. To find the rough opening width, measure the narrowest point between the studs and subtract 1⁄2" for shims. To find the rough opening height, measure from the top (head) jamb to the floor and subtract 1⁄4" for shims.
Measure wall thickness. If it's more than 4 1⁄2", order wider jambs or buy jamb extenders (narrow strips of wood you nail to the edge of the jambs to make them flush with the wall).
You may find that a previous owner (like the professor who once lived in this house) did some carpentry-butchery. With the framing exposed, it's easy to make repairs so the new door can have a long, sturdy lifetime.
To strengthen the door jamb, we pried off scraps of wood and nailed 2 × 4 blocking to the stud. A single 2 × 4 would be stronger, but harder to slip into place.