If you are using a prehung door in your construction project, you’ll need to frame an opening for it as you prepare the wall where it will go.
Plan the Opening
Measure the dimensions along the outside of the pre-fabricated frame that encloses the pre-hung door. This will give you the dimensions that you will need to frame. Decide which direction you want the door to open and close.
Frame the Double Post
Once you have determined which side of the door will have the hinges, then you should create a double post at that door.
Determine if an existing 2x4 in your room framing will be adequate as the side of the doorframe for a prehung door. If not, install one at the appropriate place.
Whether you use an existing or install a new 2x4, you need to be sure that it is square and plumb. Use your square and your level to ensure that it is vertically plumb and forms a square with the top stud.
Once your first 2x4 is firmly installed, then add a second 2x4 timber to the first. Nail it securely into place, and you have your double post for the hinge side of the door.
Measure Bottom Plate
According to the instructions that came with the door, mark the width of the opening on the bottom plate. Add 1 ½ inches for the second 2x4 upright where you will be side nailing the bottom plate to the board. Mark the opening on the bottom plate.
Cut out Extra Framing
Cut out the bottom plate and remove any studs that are within the door area. Use a reciprocating saw or a circular saw and a measuring square. Set the depth of the blade at 1½ inches exactly to avoid cutting concrete.
Install Second Vertical Board
Place the second vertical framing board against the edge of the door opening. Nail through the vertical board into the bottom plate. Use the level to ensure that it is straight before nailing the top to the top plate.
Top Horizontal Board
Cut a 2x4 board the exact distance between the two vertical posts and nail it between the boards at the precise height the manufacturer lists as the height of the prehung door. If you are installing carpet or a threshold, you may need to allow extra room.
If necessary, add cripples to the space above the door. A cripple is a 2x4 that sills the space between the top of the doorframe and the top plate of the wall. Use the standard 16 inch, or 24-inch center.
After following these steps, it should be a simple process to install your prehung door.