Installation of a pre-hung door will help ensure it swings freely and closes seamlessly; simply make sure the jamb and frame are level with each wall for best results.
Add shims to the frame to achieve this result.
Measure the Opening
Before purchasing a pre-hung door, take measurements of its rough opening for the entire frame. Doing this will enable you to select an appropriately sized door and avoid potential installation difficulties.
To measure a rough opening, first remove all interior casing around the old door and frame, before taking measurements along each point along its length to measure height of stud wall at each point – taking multiple measurements from both sides and recording the lowest value as your rough opening measurement.
Verify the walls are level. If they are, your new door should open smoothly and hang evenly; otherwise it may bind or fail to latch.
Cut the Jambs
To hang the door correctly, it will require trimming the jambs. A 1/8-inch clearance between the base of casing and floor should be the norm when installing interior doors.
Use a reciprocating saw to cut through any nails or screws securing the jamb to its base, wiggling the jamb as you go to monitor tension or cracking of wood.
If the jamb is out of plumb by more than several inches, installing the door may become impossible. Shim the hinge side until it aligns flush with the stud; nail through both layers into place before closing the door to check its uniform reveal.
Shim the Jambs
Once your frame is installed, securely fasten it to the studs with screws or nails and use a level to check whether both sides of it are evenly balanced and plumb.
Shimm the strike side of the jamb starting at its stud and working up toward its hinge. Repeat this process on its latch side until there is an even gap when closing your door.
Slide shims between each wall and side jamb until they’re flush with the wall, then use your level to ensure the door is plumb. Shimming may also be required at the head jamb.
Install the Jambs
If your jambs were cut perfectly and the floor level, installing the head jamb should be no trouble at all. If not, begin by placing shims under where your hinge knuckle mounting plate will go; these should sit just behind and above it.
Shim the hinge side first and then latch side before checking them with a level or plumb bob.
Once all shims have been installed and secured, measure from each wall to a trimmer stud on both sides of your frame to check that your door opening is level. If it isn’t, trim away at either side of your door frame accordingly.
Install the Casing
Installing door casing can be completed quickly and with ease; all it requires are some special tools such as a miter saw and small finishing nails (the smaller the better).
Once the jambs are level and plumb, you can begin installing trim around the door frame. First mark cutting lines using a spirit level and hammer on head trim before marking them for removal using spirit level marks.
Now you can cut the side casing to size with your power miter saw using your reveal marks as guides. Use an adhesive of your choice to fasten it to both jambs and the wall surface while taking special care to avoid seepage.
Install the Door
An interior door should be level, plumb, and square when installed properly. A new door should swing freely when closed while aligning almost perfectly to its opening. If your opening doesn’t fit this mold then that indicates either your studs aren’t straight enough or that poor technique was employed when mounting the door.
To overcome this problem, apply a bead of caulk along the exterior trim where it meets the wall and across the floor where the threshold will sit. When installing the new unit top first so that its exterior molding slides beneath z-flashing, and use a hammer and screwdriver as necessary to shim until its head jamb is flush with both surfaces.