How to Build a Door Jam

Measure the wall thickness around the door opening in several places--4 1/2 inches is the most common thickness, but it may vary from slightly over to slightly under this measurement all the way around the door. The average wall thickness will be the width of the door jamb.

Measure the height and width of the door you are building the jamb for. Allow for 1/8 inch of clearance between the door jamb and the door, and an additional 1/2 inch of space at the bottom.

Add the door height plus 1/8 inch plus 1/2 inch plus 3/4 inch for the length of the two side jambs. This includes the 1/8 inch and 1/2 inch for the gaps at the top and bottom of the door and 3/4 inch for the thickness of the top jamb. Add the door width plus 3/4 inch plus 1/4 inch for the top jamb length. This includes 1/8 inch for each gap on either side of the door and 3/4 inches to allow for the joinery that holds the jambs together.

Measure and mark two pieces of 1-by-6 lumber to the height of the side jambs. Cut the side jambs to length with a miter saw. Measure and mark a third piece of 1-by-6 lumber to the length of the top jamb and cut it on the miter saw. Cut a fourth piece of 1-by-6 the width of the door plus 1/4 inch for a temporary support.

Raise the table saw blade to 1 inch. Place the rip fence to make a cut the thickness of the wall plus 1/8 inch. Rip all three jamb boards on the table saw by feeding them through the blade and holding one edge against the fence.

Lay the two side jambs side-by-side and face up on a sturdy work table. Add two pieces of scrap 1-by-6 on each side of the two jamb pieces. Clamp the jambs and scrap together with the top ends even. Set the router to make a 3/8-inch-deep cut with the rabbeting router bit.

Stand facing the ends of the side jambs and rest the router faceplate on the end of the scrap board on your left. The bit should not touch the wood. Grasp the router firmly with both hands, start the motor and feed the bit into the wood until the bearing rests against the end of the lumber. Keeping the bearing against the board end, move the router smoothly to the right into the first jamb and continue until the bit is fully into the scrap on the far side. Turn the router off.

Stand the two side jambs on edge facing each other. Clamp the temporary support piece between the two side jambs at the bottom end. Apply glue to the grooves on the top of the side jambs and on the ends of the top jamb. Place the top jamb in the grooves, flush the edges and clamp them in place. Wipe up the excess glue and let the glue set for 1 hour.

Drill four evenly spaced pilot holes through each side jamb into the top jamb and nail the side jambs to the top jamb with finish nails.