Doors are precision instruments and should consistently and easily snap into their latch, clear their jamb, and swing almost effortlessly on their hinges. The fine and precise tolerances required to achieve this level of performance explains why installing a pre-hung interior door is often deemed a true measure of a carpenter’s skill.
A pre-hung door simplifies and expedites the installation process, but let me tell you that the term pre-hung is actually a misnomer. The door and jambs need to be carefully adjusted and calibrated to account for any shortcomings in a wall’s frame.
Check for the Rough Opening
Place your 6-foot level on the floor in your doorway. In case the side of the hinge is lower than the side of the latch, slip the shims under the nearest jamb. Keep adjusting until the bubble of the level is in the center. Pin these shims to the floor using a nail.
You don’t need shims in case the latch side is lower. Check the trimmer studs and your walls for plumb with a plumb bob. Also, check the face of the trimmer to ensure it is squared in reference to your wall. Lastly, by measuring between them and make sure the trimmers are parallel.
Shim the Trimmers
On the jamb, measure the distance from the jamb’s bottom to the center of every hinge. Carefully place marks on hinge locations using a trimmer by measuring down from top of the shims. Fasten your plumb bob to the top side of the hinge trimmer measuring the space between the trimmer and the string at every hinge location.
Put overlapping shims where the gap is small. Adjust the thickness of the shims to 1/8-inch, and fix them using a finishing nail. Measure the space between your plumb bob string and the shims. Place a pair of overlapping shims on the other 2 hinge locations. Adjust the thickness of each pair until the space between the string and shims is equal to the gap on the first pair. Tack each of the pairs to the trimmer and shave off its ends with a sharp knife so they do not protrude across the drywall.
Fit the Door into the Opening
Lift your door and placing it into the opening and gently push the jamb of the hinge tightly against the shims that are fastened to the trimmers. Fasten an 8d nail through the hinge-side casing exactly three inches below the level of the miter, deep into the trimmer.
Holding a level against the casing’s face, adjust the jamb out and in. In case your wall is plumb and the casing rests nicely against it, pin 8d nails through its surface at other 2 hinge locations. However, If your wall is out of plumb, which makes it difficult for the casing to rest against it, start shimming on the back of casing at the locations of the hinge to make your door plumb. Drive the nails through the shims and casing and into the trimmer. Using tapered wood wedges, fill up any gaps between the wall and the casing.
Anchor the Jamb
Place a few shims between the trimmer and main jamb on the side of the latch, located at the top of your door. When they are nearly in contact with the jamb’s back, tack them firmly to the trimmer using 8d nails, without exerting much pressure. Now drive extra pairs of shims a couple of inches below and above the strike plate and above the jamb’s base. It is important because the jamb can flex without the shims.
Adjust the Reveal
Check the reveal or horizontal gap, between the head jamb and top of your door. It must be uniform from right to left and about 1/8- to 3/16-inch wide. If required, adjust this gap by gently pushing the head casing. Fix this gap with an 8d nail; drive it near the top of your door.
Then check the thickness of the vertical gap between jamb on the latch and the door. The thickness should be about the size of a nickel. You can adjust it, by grabbing the casing and moving the jamb by your hand. To check the leading edge of the door, close and open it, pick the one that rests comfortably against the stop and clears the jamb. Fix this gap by driving nails at every 16 inches through the latch casing into the trimmer, ensuring your reveal stays consistent.
Replace Hinge Screw
Replace the center screw located at the top most hinge leaf with a long screw to fully penetrate the trimmer. This will prevent your door from binding and sagging.
Starting from the bottom, firmly push the split jamb’s edge in the groove located inside the main jamb. Tap these 2 jambs gently together with your hands. Fix the casing on each side of the miter located on the wall, at a distance of 20 inches along the casing. Hold both the jambs firmly together by driving 8d nails into the trimmers through the stop. Avoid nailing into the head jamb.
Mount the Hardware
Using the screws tack the strike plate into the mortise. If the mortise is smaller than the plate, place the plate on the surface of the jamb, highlight it using a marker, and chisel the highlighted area. Slip the bolt of the latch into the respective bore and tack the plate in the mortise on the edge of the door using the screws.
Fit a pair of knobs on each side of the latch bolt, then drive and tighten the screws that hold these knobs firmly together. Listen attentively for the latch as it slides into the strike by closing the door. Close the door again and if it rattles, slightly flex the prong located on the strike plate toward the stop. If the latch fails to catch, try bending the prong in the opposite direction and tighten the screws.
Now, your interior door has been properly installed, rest of the stuff is merely aesthetic. You can finish or paint the door however you choose. Just tape around the jambs and casing as it’s important.