Pre-hung doors make installing new house doors much simpler. These come complete with frames and casings already attached for effortless installation.
Installing a pre-hung door requires taking care to ensure its opening is level and plumb. For this, use a level and shim the jambs until the bubble reads dead center on both jambs.
Measure the Rough Opening
For optimal results, ensure your rough opening is slightly larger than the dimensions of the door and jambs. This extra room will enable you to shim the frame and door as necessary to achieve a perfectly plumb, level, and square installation.
To determine the rough opening size, remove interior casing (also called molding) from around a doorway to expose frame studs beneath. Take accurate measurements using a framing square and plumb bob to check their alignment to the wall.
Record the width, height and depth of the rough opening for your new door and jambs, noting its dimensions in inches, centimeters or millimeters. Also take note of whether or not your new door will open into or out from its room of origin (inswing opens into room while outswing opens to outside). These measurements will come in handy when purchasing prehung doors as this way you can ensure one fits precisely into its designated spot in your opening.
Shim the Jamb
Shims help ensure your door fits the opening perfectly in its frame wall opening. Use a carpenter’s level to verify the alignment of side jambs and head jambs (top) against their trimmer stud. Slide shims beneath side jambs into space between frame and floor until frame is level and top has uniform reveal from one hinge to another.
Shimming can also help even out an uneven floor surface. To check this, place a carpenter’s level horizontally against both the doorframe and floor; if its bubble reads level, that means your floor is plumb; otherwise you will need to shorten one of its jambs for correct alignment.
Start by placing shims between the header and wall. Next, repeat this procedure on both latch-side jamb and head-side jamb until both are plumb with uniform reveal at their tops. Finally, use 2-1/2 inch finish nails to anchor any loose shims that may remain.
Cut the Jambs
Once the door frame is in place, use shims to ensure a 1/8-inch gap exists between each hinge location and its casing – this should remain consistent throughout its entirety.
Use a level to assess and mark the reveal on both sides of the frame, and keep this measurement handy when painting or carpeting the room later on – an incorrect reveal could cause mismatching colors and bulging carpet.
Shim the latch side of the frame as well, to ensure it is even with the hinge side. Now your prehung door should open and close without gaps; now install longer screws in each hinge if desired but be mindful not to pull out too far and cause misalignment with its alignment. Finally, nail through all shims into the stud framing using matching putty; this way the nail heads won’t show through your trim finish.
Shim the Door
Shimming the doorframe serves to ensure that, once it has been secured, the gap between door and frame remains even on both sides. Most pre-hung doors come equipped with an 1/8 inch gap at their hinge side or top; this can easily be solved by adding additional shims as necessary.
Hold the frame while adding wooden shims to the trimmer studs on the hinge-side. A level is helpful here as it’s easy to forget that frames aren’t always perfectly square.
Use tapered shims with one side thicker than the other to level your frame. Start by adding pairs of tapered shims on either side of each hinge to level your frame from top to bottom hinge. Shim directly behind each top, middle, and bottom hinge as needed until level reading appears on level indicator. Checking level may require further adjustments until readings appear even.