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Extending Door Jambs

Adjusting pre-hung door jambs to match the thickness of old walls


We've been renovating an old farmhouse and just discovered that the jambs on the prehung doors we ordered aren't wide enough to match the thickness of our old walls. We've looked at various moldings, hoping that with a little creativity we might be able to make something work. Do you have a better suggestion
—Rob and Missy Behmer, Chana, Ill.


Norm Abram replies: This kind of problem often comes up when installing windows and prehung doors in an old house. The solution is simple: Extend the jambs with wood boards of similar thickness.

To figure out how wide the extensions need to be, hold a straightedge against the wall so one end extends past the jamb, then measure the gap between the edge of the jamb and the straightedge. Check this gap in several places around the door. Set the rip fence on a table saw to the dimension of the widest gap, and cut extensions for each jamb.

Attach the strips with woodworking glue and finish nails. Don't try to line the edges up flush; it will look better if you offset the extensions slightly, creating a reveal. You'll probably have to plane the extensions flush with the wall, so be sure to set the nailheads deep enough that you won't nick the blade.


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