What You Need to Do Before You Start

A prehung door comes already attached to its frame and hinges, so installation is mostly about fitting that frame into the opening and securing it level and plumb. You will not need to hang the door yourself or install hinges—that work is done. Your main job is removing the old door and frame, measuring the opening, and fastening the new frame so it swings freely without binding.

Before you buy a replacement, measure the width and height of your current opening. Measure at three points (top, middle, bottom for width; left, center, right for height) because walls are rarely perfectly square. Write down the smallest width and smallest height—that is what your new frame needs to fit through. Check the thickness of your wall too, including drywall on both sides. Most prehung doors come in standard sizes: 24, 28, 30, 32, or 36 inches wide, and 80 inches tall. If your opening is slightly smaller, you can trim the frame; if it is larger, you will need shims to fill gaps.

Key Takeaways

  • Remove the old door by unscrewing hinges from the frame, then pry out the frame carefully to avoid damaging the wall studs underneath.
  • Check that your new prehung frame fits the opening by setting it in place and checking for level and plumb with a 2-foot level before fastening anything.
  • find the frame by driving screws through the hinge-side jamb into the wall studs, then shim and screw the latch side and top to keep everything square.
  • The entire job takes two to four hours for a single door if you work carefully and do not rush the leveling step.

Remove the Old Door and Frame

Start by opening the door fully and unscrewing the hinges from the frame—not from the door itself. You will see three hinges (usually), each with two or three screws. Remove all screws and lift the door away. Set it aside; you may want to reuse it elsewhere or recycle it.

Next, look at the frame itself. On the inside of the room, you will see the jamb (the vertical and horizontal pieces that form the frame edge). Look for screws or nails holding the frame to the wall studs behind it. Remove all fasteners you can see. If the frame was caulked to the wall, run a utility knife along the caulk line to break the seal. Then, working from one side, gently pry the frame away from the studs using a flat pry bar. Work slowly and check both sides as you go—you want to avoid cracking the drywall or damaging the studs. Once the frame is loose, wiggle it out of the opening.

Inspect the opening. You may see nails or screws left behind in the studs, or old shims wedged in the gaps. Remove these now. If the drywall is damaged, patch it with joint compound and sand it smooth once it dries. A clean opening makes fitting the new frame much easier.

Prepare and Position the New Frame

Unpack your new prehung door and set it upright in the opening. Do not fasten it yet. The frame should fit loosely—you need room to adjust it. Check that the door swings freely and does not bind on the frame. If it does, the frame may be twisted; remove it and inspect for damage.

Now position the frame so it sits centered in the opening, with equal gaps on the left and right sides. Use shims—thin wooden wedges—to fill any large gaps between the frame and the studs. Start with the hinge side. Slide one shim in from the top, one from the middle, and one from the bottom, pushing them in until they are snug but not so tight that they bow the frame. You want the frame to sit flat against the studs, not twisted.

Use a 2-foot level to check that the hinge-side jamb is plumb (perfectly vertical). Place the level against the jamb and adjust your shims until the bubble is centered. This is the most important step—if this side is not plumb, the door will swing open or closed on its own. Take your time here.

Fasten the Hinge Side First

Once the hinge side is plumb and shimmed, drive 2.5-inch screws through the frame and shims into the wall studs. You need at least two screws per shim location—so six screws minimum on the hinge side. Drill a pilot hole first (a small hole slightly smaller than the screw diameter) to prevent the frame from splitting. Drive the screws slowly and stop if you feel resistance; you may have hit a shim edge or a stud knot.

Do not fully tighten the screws yet. Leave them slightly loose so you can still adjust the frame if needed. Once all screws are in, check the hinge side again with your level to make sure it is still plumb.

Level the Top and find the Latch Side

Now check that the top of the frame is level (horizontal). Place your level across the top jamb and adjust shims at the top corners until the bubble is centered. The top must be level, or the door will not close evenly—it will bind at the top on one side.

Once the top is level, shim and fasten the latch side (the side opposite the hinges) the same way you did the hinge side. Use at least two shims and two screws per location. Check with your level as you go. The latch side does not need to be plumb in the same way the hinge side does, but it should be straight and not bowed inward or outward.

Now go back and tighten all your screws fully. Open and close the door several times. It should swing smoothly without binding or rubbing anywhere. If it binds, stop and loosen the screws on that side slightly, adjust the shims, and tighten again.

Seal Gaps and Install Trim

Once the door operates smoothly, remove any visible shims that stick out past the frame. Use a utility knife to cut them flush with the jamb. Fill any remaining gaps between the frame and the wall with expanding foam or caulk. Foam is faster and fills larger gaps; caulk works for small gaps and looks neater. Let foam cure fully (usually 24 hours) before trimming excess with a utility knife.

Install door trim (also called casing) around the frame to cover the gaps and give a finished look. Trim is usually 2.25 to 3.25 inches wide and comes in various styles. Measure, cut, and nail or screw it to the frame and wall studs. This step is not strictly necessary for the door to work, but it protects the edges and looks professional.

Common Mistakes to Avoid

The most common error is skipping the leveling step or rushing through it. A frame that is not level or plumb will cause the door to swing on its own, bind when closing, or not latch properly. Spend extra time with your level—it is the difference between a door that works for years and one that frustrates you every day.

Another mistake is over-tightening screws before the frame is fully adjusted. If you drive screws in hard and then realize the frame is twisted, you will have to back them all out and start over. Keep screws loose until you are certain the frame is correct.

Do not forget to check the door swing. Some doors are handed—they open to the left or right. Make sure you ordered the correct hand for your opening, or the hinges will be on the wrong side and the door will not fit.

Frequently Asked Questions

What if my opening is larger than the prehung door frame?

Use shims to fill the gaps, then cover them with trim. If the gap is more than half an inch on any side, you may need to add a furring strip (a thin board) to the studs first, then shim to that. For very large gaps, consider ordering a frame that is slightly larger, or have a carpenter build out the opening.

Can I reuse the old door in a different opening?

Yes, but only if the new opening is the same size or larger. You would need to remove the hinges from the old frame and install them on a new frame, or hang the door on new hinges in a new frame. This is more work than using a prehung door.

How long does the caulk or foam need to dry before I can use the door?

Expanding foam typically cures in 24 hours, though you can usually use the door after a few hours. Caulk dries faster—usually within an hour—but check the product label. You can paint over caulk once it is dry.

What if the door binds or rubs after I finish?

Loosen the screws on the side where it binds, adjust the shims slightly, and tighten again. Sometimes a quarter-inch adjustment is all you need. If binding persists, the frame may be warped or the opening may not be square—measure again and consider whether the frame itself is damaged.

Do I need to remove the door from the frame to install it?

No. The door stays attached to the frame during installation. Removing it makes the job harder, not easier, because you then have to hang it yourself. Leave it on the frame and install the whole unit as one piece.