Installing Door Jambs

Doors are held in their rough openings by a frame called a jamb. If you plan to install a ready-made door, choose a prehung unit, which is supplied with a factory-built jamb. When you build a door to fit an odd-size opening, you will have to make the jamb yourself.

Materials: For interior doors, purchase 3/4-inch jamb stock; use 1-or 1 1/2-inch wood for exterior doors. In both cases, the stock should be as wide as the thickness of the wall. Special stock with grooves along the back face will prevent the wood from warping badly with humidity changes.

For the stops, choose flat stock since you can simply miter it to fit at the corners. 

Installing the Jambs: A complete jamb assembly consists of a top and two side pieces joined with special joints called dadoes (right). Here, grooves are cut into the side pieces to accommodate the top piece. You can make these grooves with a router whose bit matches the width of the stock, or you can use a table saw fitted with a dado head. Plan the interior dimensions of the jamb 1 inch wider and higher than the door itself.

For the door to hang properly, the jamb must be perfectly square and plumb — this is achieved by wedging shims between the jamb and the rough opening and making adjustments to them as necessary. If the finish floor is not yet in place, you will also need spacers under the side jambs to hold them until the unit is nailed in place.

Making and Fastening the Frame

1. Assembling the Pieces.

Cut the side jambs slightly shorter than the distance between the finish floor and the top of the rough opening. Make the head jamb 1/2 inch longer than the desired inside width of the jambs.

Measure along one side jamb a distance equal to the desired height of the jamb opening and make a mark. With a router or table saw, cut a dado 3/4 inch wide and 1/4 inch deep across the inside face of the jamb at and above this point. Mark and cut a dado on the other side jamb in the same way.

Fit the head jamb into the dado in one of the side jambs and drill pilot holes for two 1 1/2-inch No.8 wood screws. Fasten the jambs together with wood glue and screws. Fasten the head jamb to the other side jamb in the same way (above).

2. Positioning the Jamb.

Tack a brace to the wall diagonally across each top corner of the door opening.

Position the jamb in the opening, propping it against the braces. Cut a 1-by-4 spreader to fit between the side jambs and place it on the floor between the jambs to keep them apart.

Insert pairs of shims on both sides of the door between the side jambs and the rough framing at both ends of the head jamb, adjusting them to center the jamb assembly in the opening.

With a carpenter’s level, check the head jamb for level (above); if necessary, shift the assembly slightly and adjust the shims.

At the top of each side jamb, drive a 3 1/2-inch finishing nail through the jambs and shims into the rough framing.

3. Checking for Plumb.

Tap pairs of shims between the side jambs and the wall at both ends of the spreader.

Mark the center of the head jamb on its edge and the center of the spreader on its face.

Tack a nail into the edge of the head jamb at the center mark. Hang a plumb bob from the nail so the point of the bob is just above the spreader.

Tap the shims flanking the spreader in or out to align the center mark on the spreader directly under the bob (left).

At the bottom of each side jamb, drive a nail through the jamb and shims into the rough framing, then remove the plumb bob, nail, and spreader.

Drive additional pairs of shims at the hinge side of the jamb, locating one pair at each planned hinge location.

Drive two pairs of shims on the lockset side, locating them just above and below the latch location.

To ensure that the side jambs are straight, press a straight board or a long carpenter’s level against one jamb to flatten it and nail through the jamb and shims into the rough framing (right). Secure the opposite side jamb in the same way.

Cut off the shims one at a time by holding the end of the shim and slicing across it repeatedly with a utility knife (inset) until you can break off the waste piece easily.

With a nail set, sink all the nails; at each hinge location, bury the nails deeper than the thickness of the hinge leaves. If you will be painting the jambs, fill the holes with spackling compound; otherwise, apply wood putty.

Adding Stop Molding

1. Laying Out the Door Stops.

Adjust a combination square to the thickness of the door.

On the side of the jamb that the door will sit flush with when closed, place the handle of the square against the edge of the lockset-side jamb. Set the tip of a pencil against the end of the ruler and run the square down the length of the jamb to mark a guideline for the door stop (left).

Mark a guideline on the hinge-side jamb in the same way, but add 1/16 inch to the measurement to prevent the door from binding when it is closed.

2. Installing the Head-Jamb Door Stop.

Cut a length of door stop to fit along the head jamb, mitering both ends at 45 degrees with a power or manual miter saw.

Position the stop on the head jamb, aligning the front edge with the guidelines on the side jambs. Tack the stop to the head jamb with 1 1/2-inch finishing nails spaced every 10 to 12 inches and driven only partway in (right).

3. Fitting in the Side-Jamb Stops.

Cut two lengths of door stop to fit along the side jambs, mitering the top ends.

Align one stop with the guideline on the lockset-side jamb, holding the end tightly against the head stop. Tack it in place as you did the head-jamb stop (left). Do not drive the nails home or install the hinge-side stop until you have hung the door.