How to Extend a Door Jamb

Extension jambs are found in many new homes, but more commonly in remodeled homes.

If the jamb is not flush with the inside wall after installing a door, extension jambs are necessary. The width of extension jambs are typically about 1/4 of an inch and wider. Doors are available in just about any jamb width, but if you already have a door, it must be modified to fit an existing wall. Narrow jamb extensions are nailed directly to the door jamb and wide extension jambs are nailed to the studs around the door.

Things You'll Need

• Table saw
• Miter saw
• Drill
• Power hand planer or jointer
• 1/8-inch drill bit
• Counter sink
• 1-5/8 drywall screws or 6d finish nails
• 8d finish nails
• Shims

Instructions

1. 

   Use a square or small board set against the wall to make your measurement.

   Measure the distance from the door jamb to the wall in several places. Add about 1/16 of an inch to the measurement to leave room for planing a bevel on the outer edge.

2. 

   Rip the jamb 1/16 of an inch wider to clean up the edge and make a bevel.

   Rip the two legs and header to width on a table saw.

3. Run the boards through a jointer to add a 3- to 5-degree bevel to the edge that was just cut. The bevel on the edge helps the casing lay tight to the jamb extension.

4. Cut the head and side legs to length with a miter saw. For the head length, measure between the left and right side of the jamb, adding 1/4 to 3/8 of an inch for an offset and 1-1/2 inches (3/4 + 3/4) for the thickness of both jambs. It looks much better to offset the extension on each side by 1/8 or 3/16 of an inch from the edge of the existing jamb. The length of each leg is the measurement from the floor to the head jamb plus the reveal.

5. 

   Drill pilot holes to prevent the wood from splitting.

   Assemble the head to the legs. The head sits on the legs. Drill two pilot holes with a 1/8-inch drill bit. Counter sink the holes and install 1-5/8 drywall screws. You can also nail them together with 6d finish nails.

6. Set the jamb in the opening with the bevel facing you. Shim the extension jamb every 16 inches and nail it to the studs with 8d finish nails through the shims. Drill pilot holes to prevent the wood from splitting.

Tips & Warnings

• For extension jambs less than 1-1/4 inches, simply rip a 3/4-inch board to the proper width, pre-drill the nail holes and nail them directly to the existing jamb. Clean up the edges with a sander or jointer. Insulate the space between the jamb extension and the wall.

References

• This Old House: Extending Door Jambs