Doorjambs, also known as door frames, are the pieces of wood on both sides and on top of the door. These jambs span from one side of the wall to the other. The door casing is nailed to the jamb and to the wall and covers the space between the doorjamb and the wall. Doorjambs are designed to fit standard width walls made out of 2-by-4 or 2-by-6 lumber plus a layer of 1/2-inch drywall on each side of the wall. Sometimes walls are thicker than standard size or may differ because they have additional or different sized cladding layers. In these cases it is necessary to extend the width of the doorjamb. This requires the addition of jamb extensions, or strips of wood, to one side of the doorjamb.
Measure the distance from the edge of the doorjamb to the face of the drywall to determine the thickness of the jamb extension. Measure at several spots on the doorjamb and use the largest dimension.
Use a 1-by-4 and cut the jamb extension. Set the saw fence on the table saw to the desired dimension. Don't rely on the scale on the saw. Use a tape measure to set the distance from the edge of the saw blade to the side of the fence.
Cut the extensions to length. Measure the height of the side jambs and the width of the top jamb to determine the proper lengths.
Apply glue to the back of the extension jamb where it will meet the doorjamb and put the extension in place. The side face of the extension jamb should be flush with the side of the doorjamb.
Nail through the face of the extension jamb into the doorjamb to secure it, using finish nails that are long enough to pass through the extension and penetrate the doorjamb by at least an inch. Repeat this for all three extension jambs.