Occasionally, a door jamb will need to be replaced because it has become damaged with use. It can be broken during a break-in or just dented and gouged as a result of moving furniture. Whatever the reason, it's important that you replace it quickly so the door is secure. This is a simple home improvement project, but it requires a few carpentry skills and a little knowledge.
TIP: You'll have to remove the door and install it again for this DIY job. You can technically do that yourself, it's all too easy to drop a door and damage things (or other people). Consider getting a friend to help with this task to make sure the door doesn't move until you want it to move and then help you carry it.
Step 1 - Remove Existing Door
Remove the existing door by pulling out the hinge pins and lifting it off of the hinges. Set the door aside in a safe place, and place the pins in a plastic bag so you don't have to worry about losing anything important. Leave the other side of the hinges on the door jamb, as you will use this piece later.
Step 2 - Remove Trim Casing
Carefully trim casing from the doorway next. Use a flathead screwdriver or small pry bar to work between the casing and the drywall. Start at one place along the trim, and then begin slowly loosening the rest of the casing until you have all of the trim free. Set these pieces aside so that you can reuse them once the new door jamb is installed.
Step 3 - Pull Existing Door Jamb Free
Now it's time to pull the existing door jamb loose with the pry bar or screwdriver. Look closely at how the jamb is attached to the door framing. In older homes, it is usually attached with nails, but newer homes often use wood screws. Pull a piece up until you can see how it is attached. Then, either use a power drill to unscrew wood screws, or just pull the jamb free with the pry bar. Make sure to preserve the side of the door jamb with the hinges attached.
Step 4 - Measure Door Opening for New Jamb
Measure the door opening, including the width and height of the door. Go from stud to side for the width of the opening and from the bottom of the floor to the height of the bottom of the header for the door height. Also, note the location of the hinges on the opening by taking the piece of jamb that you preserved with the hinges on it. Measure to where the center of each hinge is located starting at the top of the jamb piece.
You can purchase a prefabricated door jamb at a local home improvement center or you can have one custom-made for very little expense. A custom one is more likely to fit perfectly for your door opening, but you will probably need to wait at least 24 hours for it to be constructed from the time you place your order. If you decide to have one fabricated, take one of the hinges with you to help in the construction. A fabricated door jamb will be in three pieces that you will have to assemble before you can put it in place.
Step 5 - Temporarily Install New Jamb
Once you have the replacement jamb in one piece, put in into place in the door opening. Temporarily screw the jamb into place on the hinge side first. Then, slide shims into place along the space between the jamb and the wall.
Step 6 - Level and Plumb
Take the level and check your jamb so you can adjust the shims as needed. Also, take the carpenter square to make sure that it is plumb as well.
Step 7 - Permanently Affix Jamb
After you've made any necessary adjustments, you can attach the other pieces of the jamb to the door opening, and screw them into place.
Step 8 - Reattach Hinges and Door
Finally, unscrew the hinges from the old door jamb piece, and fit them into place where you marked previously. Screw them in with an electric screwdriver. Then (with help from a friend or assistant) hold your old door up correctly and simply slide the hinge pins back into place. Tap them lightly on the ends with a rubber mallet if the pins are too tight to slide in completely.