Replacing old and worn interior doors with new doors can improve the beauty of your home. Installing a door frame can be challenging and, if installed incorrectly, the door may not open and close properly. Fortunately, you can buy a pre-hung door to make the installation much easier. A pre-hung door already has the door attached to a pre-built door frame, so all you have to worry about is positioning the door frame in the opening.
Measure the dimensions of the rough doorway opening. You will need to know the height, width and thickness of the walls. Your doorway may not be completely square, so measure in several places. Buy a pre-hung door that fits these dimensions
Place a spirit level on the floor across the rough door opening to make sure the floor is even. If the floor on the hinge side of the door is lower than on the latch side of the door, place wood shims under the end of the level until the bubble is in the centre. Nail the shims to the floor.
Remove the trim insert from the main door frame. With the door closed, place the door frame into the rough opening so that the casing is against the wall. If you added any shims to the floor, make sure the bottom of the hinge side is resting on them.
Place a level against the hinge side of the door frame. Add shims between the frame and the wall until the door frame is plumb, or straight. Drive 6.25 cm (2 1/2 inch) nails through the door frame near each hinge to tack it in place. Make sure there are shims behind the nails so that you do not distort the door frame. Do not drive the nails all the way in.
Adjust the top and latch side of the door frame until they are level and plumb, using the level and shims. Drive 6.25 cm (2 1/2 inch) finish nails through the top and latch side to hold them in place, again making sure that there are shims behind the nails. Do not drive the nails all the way in.
Check the alignment of each side of the door frame with the level again. Make sure the door opens and closes properly. Make any adjustments by removing the nails and using shims to realign the door frame. Once you are satisfied with the alignment, drive the nails all the way in. Use a nail set to drive the nail heads beneath the surface without damaging the wood.
Place the trim insert into the rough door opening on the other side. Slide it into the grooves in the main door frame until the casing is against the wall.
Drive 6.25 cm (2 1/2 inch) finish nails through the sides of the door frame at the top, bottom and middle of each side to hold the two pieces together. Drive 3.75 cm (1 1/2 inch) finish nails through the casing of the door frame on both sides. Set the nails with a nail set.
Trim any protruding shims with a utility knife. Fill the nails holes around the door frame with wood putty and caulk. Use paintable caulk to fill any gaps.
Install the door knob and lock plate according to the directions on the packet.
Ask someone to help you position the door and add the shims to make it easier.