Installing a prehung door may be easier than installing a slab door, but still requires careful consideration to ensure a door that sits level in both its jambs and with the wall.
As buildings settle over time, inconsistencies develop that disrupt door frame and king stud alignments. One approach is to use shims as correctors of misalignments.
Measure the Door
Once you’ve taken down the old door, take some measurements before installing your prehung interior door. First measure the width and height of your rough opening to see if your prehung interior door will fit and whether there is enough room for casing around its edge.
Begin by measuring horizontally across the top of your door jamb three times from left to right and selecting the measurement that results in the narrowest door width as your target value.
Now you must level the head jamb. With a plumb bob or carpenter’s level in hand, examine both wall surface and trimmer studs to ensure they are plumb; if not, add shims under each side jamb until all are plumb.
Measure the Jambs
Once you’ve removed the old door and verified that your opening is square, level and plumb, it’s time to adjust the jambs. Use a four-foot carpenter’s level to inspect the threshold of your new opening; adjust accordingly until its bubble aligns perfectly between two lines on its edge.
Shim the head and side jambs if they’re out of plumb, filling any gaps evenly. Additionally, if casing seems to be pulling away from walls, add wood shims at every hinge location so it remains flush against framing.
Once your frame has been shimmed, use 8D finish nails through the shims into each trimmer stud and nail through them into each hinge location for installation of doors.
Shim the Jambs
Jambs should also be level and plumb for proper functioning of your door, using a pry bar to pry off trim from side jambs and measuring jamb stud height with a tape measure. If they aren’t level, shim them up until they are.
Before beginning, first stabilize the hinge side by placing shims between each hinge and wall. Next, align and plumb each latch side of the frame using a carpenter’s level and ensure all are level and plumb – repeat as necessary until achieving an even reveal is reached. When finished, nail through all shims for extra security as this will prepare your jambs for door casing installation.
Level the Jambs
Over time, buildings may settle, altering the alignment of jambs to be less than perfectly plumb and level, leading to doors not latching properly or binding upon opening them.
Employ a 6ft level to check whether or not the jack stud (interior wall framing stud on hinge side of frame) is plumb. If it is not, add wood shims between walls and casing at hinge locations before nailing them in place with 8d finish nails.
After this step is completed, slip shims beneath the trimmer stud on the latch side of the frame until its flush with the floor and repeat this process for the top striker side of the frame. Be sure to reference your door itself to check for even gaps on both sides; if these appear uneven then close and shimming until this centers around your door itself.
Install the Door
Pre-hung doors add style and security to any home while eliminating drafts, saving time by not needing to assemble frames from scratch. Before beginning installation, be sure to read all instructions and warnings on packaging thoroughly and follow all safety precautions outlined.
Start by uninstalling the existing door trim using a screwdriver and hammer. If any hinge pins are stuck, use a utility knife to unstuck them.
Installation of pre-hung doors should involve placing them so they swing freely, close with near perfect alignment, and are set at the proper height for your lockset (knob or handle). To achieve this task, an accurate level and tape measure are essential; additionally shims may need to be added between jambs if necessary – these should be created using flat 2x4s with 4d, 6d or 8d nails –