Prehung Interior Doors Installation In 8 Steps

“I started out using little more than a hammer, a chisel, and a screwdriver,” says This Old House general contractor Tom Silva. In those days, he’d assemble the jamb, hand-cut the hinge mortises, and hang the door separately. “It took a good long time and lots of patience,” he says.

What Is a Prehung Door?

A prehung door is a unit that comes with hardware and a frame, ready to be installed into a doorway. Purchasing one does simplify and speed up installation, but the term “prehung” is really a misnomer. These doors and jambs must still be carefully adjusted to account for shortcomings in the wall frame. “It requires accuracy to put in one of these,” Tom says. “If it’s not installed right, it won’t hang well.”

Common Mistakes to Avoid When Ordering Prehung Doors

Back when carpenters assembled the parts around a door piece by piece, they could easily customize their work, make changes, or correct problems. With a prehung door, however, most of the assembly work is done off-site, so a mistake made when you place an order can turn a perfectly good unit into worthless scrap. Here are two steps for avoiding that outcome. KNOW YOUR OPENING: Prehung doors are made to fit rough openings 2 to 2 1⁄2 inches bigger than the corresponding dimensions of the jamb. The jamb’s depth should match the thickness of the wall, including the plaster or drywall. If the opening already exists, check that the trimmers are plumb, parallel, and square to the wall and the header. If they aren’t, read “Prehung Door Swing: Getting It Perfect” (below) before you order. SPECIFY THE SWING DIRECTION: Your supplier needs to know which way you want the door to swing, but beware of the question, “You want a left-hand or a right-hand door?” Those terms don’t always mean the same thing. So rather than answering directly, say instead which side you want the knob to be on when opening the door toward you. Let that person figure out what the door’s “handedness” is. Prehung doors hang on a jamb that is “split” into two pieces. The door is attached to the main jamb, which is installed first. The split jamb has a groove under the stop so it can slide over the edge of the main jamb. Typically, both jambs are furnished with casings already attached.

How to Install a Prehung Interior Door

Parts of a Prehung Door

1. Check the rough opening

Place a 4-foot level on the floor in the doorway. If the hinge side is lower than the latch side, slip shims under the level nearest the hinge jamb. Adjust until the level’s bubble is centered. Tack the shims to the floor with a finish nail. If the latch side is lower, no shims are needed. Check the walls and the trimmer studs for plumb using a level or plumb bob. Also, check the trimmers’ faces with a framing square to see if they are square to the wall. Finally, check that the trimmers are parallel by measuring between them at the top, bottom, and middle of the opening. If the wall is out of plumb, or the trimmers are out of plumb, out of square, or not parallel, see “Prehung Door Swing: Getting It Perfect” below.

2. Shim the trimmers

On the hinge jamb, measure from the bottom of the jamb to the center of each hinge. Mark the hinge locations on the hinge-side trimmer by measuring up from the floor (or top of the shims). Tack the plumb bob to the top of the hinge-side trimmer, and measure the gap between the string and the trimmer at each hinge location. Where the gap is the smallest, place overlapping shims. Adjust the shims to 1/8 inch thick, and tack them with a finish nail. Measure the gap between the shims and the plumb bob string. Place overlapping pairs of shims at the other two hinge locations. Adjust each pair’s thickness until the gap between shims and string equals the gap at the first pair. Nail each pair to the trimmer and cut off the ends with a utility knife so they don’t protrude past the drywall.

3. Fit door into opening

Lift the door into the rough opening and push the hinge jamb tight against the shims tacked to the trimmers. Tack an 8d finish nail through the face of the hinge-side casing 3 inches below the miter, into the trimmer. Hold a level against the face of the casing and adjust the jamb in and out until plumb. If the wall is plumb and the casing rests flush against it, tack 8d finish nails through it at the other two hinge locations. If the wall is out of plumb and the casing does not rest against it, shim behind the casing at the hinge locations to make the door plumb. Nail through the casing and shims and into the trimmer. Fill any gaps between the casing and the wall with tapered wood wedges.

4. Adjust the gap between the door

Check the horizontal gap, or “reveal,” between the top of the door and the head jamb. It should be uniform from left to right and 1/8- to 3/16-inch wide. If necessary, adjust the reveal by pushing up the head casing. Set this reveal by driving an 8d nail through the face of the latch-side casing and into the trimmer, near the top of the door. Check the vertical reveal between door and jamb on the latch side. It should be about the thickness of a nickel. To adjust it, grab the casing and move the jamb by hand. Open and close the door to check that its leading edge, the one that rests against the stop, clears the jamb by a consistent 1/8 inch. Set the reveal by driving 8d finish nails every 16 inches through the latch-side casing and into the trimmer. Make sure the reveal remains consistent.

5. Anchor the jamb

Slip a pair of shims between main jamb on the latch side and the trimmer, near the top of the door opening. When they are just touching the back of the jamb without putting any pressure on it, nail them to the trimmer with 8d finish nails. Nail additional pairs of shims a few inches above the base of this jamb, as well as just above and below the strike plate. Without these shims, the jamb could flex.

6. Replace hinge screw

On the hinge jamb, remove the center screw from the top hinge leaf and replace it with a screw that’s long enough to penetrate the trimmer. This prevents the door from sagging and binding.

Tip: If the long screws don’t match the ones that came with the hinges, install them behind the hinge leaf.

7. Attach the split jamb

Starting at the bottom, gently push the edge of the split jamb into the groove in the main jamb. Tap the two jambs together using both hands. Nail the casing to the wall on both sides of each miter, and about every 18 inches along the casing. To hold the two jambs together, drive 8d finish nails through the stop and into the trimmers: one nail at each hinge location, one through the shims near the top and the bottom of the latch jamb, and one each just above and below the striker. Do NOT nail into the head jamb.

8. Mount the latch hardware

Fasten the strike plate to the mortise in the latch jamb using the screws provided. If the plate is bigger than the mortise, put the plate on the jamb, outline it with a pencil, and chisel to the outline. Slip the latch bolt into its bore and fasten its plate into the mortise on the door’s edge with the screws provided. If the mortise is too tight, adjust its size in the same way as you did the strike plate. Fit the knobs to both sides of the latch bolt, then insert and tighten the connecting screws that hold the knobs together. Close the door and listen for the latch sliding into its strike. If the door rattles, bend the prong on the strike plate slightly toward the stop. If the latch doesn’t catch, bend the prong away from the stop. Tighten all the screws.

Prehung Door Swing: Getting It Perfect

Just as it’s easier to build a house on a level foundation, it’s simpler to hang a door that’s level, plumb, and square. While these are rare qualities in most old houses (and an unfortunate number of new ones), the fact that an opening is out of sorts doesn’t mean the door has to be. The trick is to adjust either the opening itself or your door-hanging technique. OUT-OF-PLUMB WALLS: When a wall is more than 1/8-inch out of plumb between the top of the opening to the floor, the door should be plumbed independently of the opening. Just plumb the hinge and latch jambs with a level or a bob and hold them in place with shims. There will be a gap between the casing and the wall, so cut a piece of wood to fill it. A split jamb can fit over a 2x4 stud wall up to 1/2-inch out of plumb. More than that, and you may need to modify the jamb. OUT-OF-PLUMB or NON-PARALLEL TRIMMERS: Shims can make up for out-of-plumb trimmers, unless they are so bad the the door doesn’t fit. If the fit is a bit too tight across the bottom (and the wall is newly drywalled), you may be able to gain 1/2-inch or so with a little pounding. Remove the screws that hold the drywall to the trimmers, then coax the trimmer ends into the wall with a sledgehammer. Toenail them back to the sole plate, cut the plate back flush using a reciprocating saw, and reattach the drywall screws. A sledge won’t work if the opening is too tight at the top, or if the wall is covered with old plaster. In those cases, reframe the opening or order a smaller door. OUT-OF-SQUARE TRIMMERS: A framing square can tell you whether the faces of the trimmers are square to the wall surface. If they’re not, you might end up with bound hinges or uneven gaps between the door and the jamb. To correct this, add a third shim to the standard opposed pair. Slide the third shim back and forth under the other two to change their angle with respect to the trimmer. Note: If you have done this on the hinge side, double check that all three sets of shims are plumb before installing the door.

What to Do After the Door Arrives…

Measure the length of the head and side jambs. The corresponding dimensions in a plumb and square rough opening should be 2 to 2 1⁄2 inches longer. (Worst case, a door could be installed with as little as 1/8-inch clearance, side to side.) Also check that the jamb depth equals the wall’s thickness. Check that the door will swing in the right direction after it’s installed. The clearance between the bottom of the door and the finish floor should be 3/8-inch or less. This clearance is set by trimming the ends of the hinge and latch jambs. Just be sure to allow for a threshold or thick carpeting before making these cuts. Test-fit the lockset in the holes bored in the door. Holes that are too small can be rebored. Holes that are too big will have to be plugged, sanded, and then bored again. If possible, turn this task back to the shop that did the work.

Read How to Install an Exterior Prehung Door for information on exterior door installation.

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