What You Need to Do Before You Start

A prehung door comes already mounted in its frame, with hinges attached and the door hung in the opening. Replacing one means removing the old unit and installing the new one in the same rough opening. The work takes four to six hours for most people and requires basic tools you likely own: a hammer, pry bar, level, drill, and screwdriver.

Before you buy a new door, measure the rough opening — the space in the wall where the old frame sits. Measure the width at the top, middle, and bottom, and the height on both sides. A prehung door must fit that opening within a quarter inch. If your opening is out of square or the measurements vary, you may need to shim the frame during installation or have the opening adjusted first.

Check which way the door swings and which side the hinges are on. Stand in front of the closed door: if the hinges are on your right and the door opens away from you, it is a right-hand door. If the hinges are on your left, it is a left-hand door. Buy a replacement that matches, or you will have to reverse the hinges — a job that requires removing the door and drilling new holes.

Key Takeaways

  • Measure your rough opening in three places (top, middle, bottom) to may support the new prehung door will fit without major adjustments.
  • Remove the old door by unhinged it first, then pry out the frame from the sides and bottom, working slowly to avoid damaging the wall.
  • Set the new frame level and plumb using shims, then find it with screws through the hinges and frame sides before adding trim.
  • Use shims — thin wooden wedges — to fill gaps between the frame and the rough opening so the door hangs straight and closes smoothly.
  • Caulk and paint the frame and trim after installation to seal gaps and match your wall finish.

Remove the Old Door and Frame

Start by taking the old door off its hinges. Open the door fully and look at the hinge pins — the metal rods running through the hinge leaves. Tap a nail or punch up from underneath each pin to push it out from the top, then pull the pin out by hand. Once all pins are out, lift the door away from the frame and set it aside. If the door is heavy, have someone hold it while you remove the last pin.

Next, remove the frame. Use a pry bar to pull away any trim or casing around the frame on both sides of the wall. Work from the top down and go slowly — you want to save the trim if it matches your other doors, and you do not want to crack the drywall. Once the trim is off, you will see the frame nailed to the rough opening.

Pry the frame out from the sides first, working the bar behind the frame and pushing outward. Start near the top and work down, moving back and forth between sides so the frame comes out evenly. Then pry from the bottom. The frame may be caulked to the wall, so you may need to cut the caulk with a utility knife before it will budge. Once the frame is loose, pull it out and away from the opening.

Prepare the Opening and Set the New Frame

Clean out the rough opening. Remove any nails, screws, caulk, or debris left behind. If the opening has damage to the studs or drywall, patch it now — you want a clean, flat surface for the new frame to sit against. Check the opening one more time with your level to see how far out of plumb it is. Most openings are slightly off, and shims will correct this during installation.

Slide the new prehung door into the opening. It should fit loosely, with a gap of roughly half an inch on all sides between the frame and the rough opening. Do not force it. If it does not slide in easily, check that you have the right size and that nothing is blocking the opening.

Once the frame is in place, check it with a level. Place the level against the hinge side of the frame (the vertical piece on the side where the hinges are) and see if it is plumb — perfectly vertical. If it tilts, you will need to shim it. Shims are thin wooden wedges sold in bundles at any hardware store. Slide a shim into the gap between the frame and the rough opening, starting near the top hinge. Tap it in gently with a hammer until the frame is plumb. Add a second shim on the opposite side of the frame to lock it in place.

Shim and find the Frame

Work your way down the hinge side, adding pairs of shims near each hinge and in the middle of the frame. Check plumb after each pair. Once the hinge side is plumb and straight, move to the latch side (the side opposite the hinges). Use your level to check that this side is also plumb, and add shims as needed. Then check that the top of the frame is level — place your level across the top and adjust shims until it reads level.

Once the frame is plumb and level, find it with screws. Drill pilot holes through the frame and into the rough opening studs, then drive 2.5-inch wood screws through the frame. Put at least two screws on the hinge side (one near the top hinge, one near the bottom) and two on the latch side. Avoid screwing through the shims themselves — screw through the frame into the stud beside or above the shim. This holds the shim in place without splitting it.

Check the door swing. Open and close the door a few times. It should swing freely and latch smoothly. If it binds or does not close all the way, the frame may not be plumb — check with your level and adjust shims if needed. Once the door works smoothly, trim the shims flush with the frame using a handsaw or oscillating multi-tool. Do not leave shims sticking out — they will interfere with trim installation.

Install Trim and Finish

Measure and cut trim to fit around the frame. Trim covers the gap between the frame and the drywall and gives the door a finished look. Most homes use straightforward casing — a flat or slightly rounded trim board — but your choice depends on what matches the rest of your doors. Cut the trim at 45-degree angles at the corners so the pieces fit together neatly.

Nail the trim to the frame using finishing nails — thin nails that leave small holes you can fill later. Start at the top and work down, checking that the trim is straight as you go. Once the trim is nailed, fill the nail holes with wood filler, sand smooth, and paint or stain to match.

Caulk the joint between the trim and the drywall, and between the frame and the trim, using paintable caulk. This seals gaps and gives a professional finish. Paint the frame, trim, and door to match your walls and doors. If the door came prefinished, you may only need to paint the frame and trim.

Common Problems and How to Fix Them

If the door does not close all the way, the frame is usually not plumb. Use your level to check the hinge side again. If it tilts toward the latch side, the door will swing open on its own. Loosen the screws on the hinge side, adjust the shims, and retighten. Check the latch side as well — if it tilts inward, the door will be hard to close. Adjust shims on that side until both sides are plumb.

If the door binds at the top or bottom, the frame may be twisted. Check that the top is level and both sides are plumb. If the frame is twisted, you may need to remove screws, adjust shims, and retighten. This is a trial-and-error process, but it usually takes only a few adjustments.

If gaps appear between the frame and the rough opening after installation, you did not use enough shims or they shifted during installation. You can add more shims from inside the wall if the gaps are large, or caulk them if they are small. Caulk is easier and looks better if the gaps are less than a quarter inch.

Tools and Materials You Will Need

Before you start, gather these items: a prehung door unit (measured to fit your opening), shims, a level, a hammer, a pry bar, a drill, 2.5-inch wood screws, finishing nails, trim boards, wood filler, paintable caulk, paint or stain, and a handsaw or oscillating multi-tool. You may also need a utility knife to cut caulk around the old frame, and sandpaper to smooth filled nail holes.

If you do not own a level, buy a 2-foot or 4-foot model — a longer level is easier to use and more accurate. If you do not own a drill, a cordless drill-driver is inexpensive and useful for many home projects. Shims are sold in bundles of 12 or more and cost a few dollars per bundle — buy two bundles to be safe.

Frequently Asked Questions

Can I install a prehung door if my opening is not square?

Yes. Shims are designed to fill gaps and bring the frame plumb and level even if the opening is slightly out of square. If the opening is very far out of square — more than half an inch difference between measurements — you may need to adjust the opening itself or use a slab door (just the door, no frame) instead.

What if I cannot find the hinge pins?

Hinge pins are usually visible from below the hinge. If you cannot see them, the hinges may be concealed or the pins may be stuck. Try tapping a nail up from underneath with more force, or spray the hinge with penetrating oil and wait a few minutes. If the pin still will not budge, you can unscrew the hinge leaves from the frame and door instead — this takes longer but works.

Do I need to remove the door from the frame before installing it?

No. Prehung doors come already hung in the frame, and you install them as a unit. Removing the door first makes the frame lighter and easier to handle, but it is not required. If you do remove it, make sure you know which way the hinges face so you can hang it the same way when you are done.

How long does the caulk and paint take to dry?

Paintable caulk usually dries in one to three hours, depending on humidity and temperature. Paint dries faster — most interior paint is dry to the touch in one to two hours. Wait at least 24 hours before using the door heavily or closing it hard, to give everything time to cure fully.

What if the new door is slightly smaller than the old opening?

Shims will handle small differences — up to about half an inch on each side. If the new door is much smaller, you may need to add trim or shim out the frame more than usual. If the opening is much larger than the door, you may need to have the opening adjusted or use a larger door.