Last time we talked about framing the rough opening for a door. Now it’s time to finish the opening. If you’re building the frame from scratch (as opposed to hanging a pre-hung door), you’ll need to install the jambs. These should be wide enough to cover the rough framing and the wallcovering on either side of the wall. You can buy jambs, or make your own by ripping finished lumber to the right width. Measure and cut so the finished opening will be about 1/4″ wider than the door (1/8″ either side) and about 3/4″ higher.
Rabbet the top of the side jambs to provide a place for the head jamb to sit, then glue and nail the pieces together.
Now place the finished jamb in the opening and insert shims at the hinge locations. By inserting two shims opposite to each other, you provide a firm base, and it’s easy to adjust the width by moving them closer together or farther apart.
Make sure the hinge jamb is plumb and nail it at the shims. Then shim the head jamb so it’s level and square to the hinge jamb, and nail it in place. Finish up by shimming and nailing the latch jamb plumb. If all went well, there’s room for the door with clearance all round.
Cut off the shims. Your finish casing trim will cover up the gap here.