Careful and patient planning will allow you to successfully install a pre-hung door in your house, but for best results it is wise to work alongside someone who has experience and an eye for level surfaces.
Before installing a doorway, place a 4-foot level on the floor inside it and adjust its bubble until its center. If one side of the hinge side of the doorway is lower than another side, add shims under each jamb to restore an equal level.
Measure the Rough Opening
Before installing your new door, it is necessary to accurately determine its rough opening in the wall. This can be accomplished by removing interior casing and revealing framing structures before taking measurements from jack stud to jack stud, header to floor and wall to wall.
A rough opening should be slightly larger than its counterpart dimensions in the head and side jambs to provide enough wiggle room to properly install frame and door in opening for perfect plumb, level, square installation.
In addition, this extra space enables you to install flooring material above the drywall and add brick mold or trim on the exterior side of the frame, as well as provide room for expansion and contraction as humidity levels fluctuate.
Measure the Jambs
Idealy, your new door should fit flush against its surrounding drywall. If it doesn’t, adjusting its jambs is necessary. First remove any trim that covers the opening; measure from stud to stud three times around it until obtaining the smallest measurement; crosscheck this measurement against prehung door sizes before taking action.
If the side jambs are too short for flooring or you require extra room for its installation, add wooden shims beneath them. First add them on hinge side of jamb and level carpenter’s level until bubble centered over shim. Secure finish nails. Now repeat on latch side and ensure there is an even 1/8-inch gap all across jamb.
Install the Jambs
Before closing and opening the door, shim the latch side jamb to make it flush with the floor. Additionally, it would be beneficial to shim out both hinge sides if your flooring hasn’t yet been installed in that area.
Use a carpenter’s level to check that the level bubble is centered over the top edge of the door jamb closest to its hinge. Shim less when there’s too much reveal between frame and door; increase shim thickness if there isn’t enough. When all jambs are level and plumb, use 4in. scrap lumber blocks on each strike-side jamb near studs as temporary supports while you nail through casing into them before taking down blocks once shims have been secured securely in their places – remove when finished nailing through casing into jambs!
Install the Door
Pre-hung doors come as complete packages with the slab door, own frame and hinges already assembled and ready for installation. While installing them may be simpler than with separate slab door and jambs units, it still requires careful adjustment for optimal fitting – use a level to determine whether they are plumb and square with both jamb sides parallel with each other and with the floor surface paralleling one another.
Help is always invaluable when shimming doors. As you do so, test that the door opens and closes freely without binding or sticking – if that isn’t the case then either chiseled down doorsill or planed off may need to happen in order to eliminate interference between doors and frames. A carpenter’s square should also be used to inspect reveal (gap between door and frame) for evenness as part of this project.
Finishing Touches
Make sure the casing rests flush against the wall. It should not exceed half-an-inch wider or taller than your prehung door frame, if that occurs use your tools to trim it and even out any discrepancies between it and its prehung counterparts by shimming if necessary to level out gaps between it and jambs.
Pre-hung doors differ from slab doors by being pre-installed with all components already assembled and in their proper places. Pre-hung doors may be preferable when changing an opening size or replacing damaged frames in existing homes.
Once your frame is secure, you can add any deadbolts or locksets that came with the door, weather stripping for improved energy efficiency, and enjoy your new door!