Having been there done that on many occasions, it’s likely a lot of you are in the same predicament. Then let us get started on removing and replacing a single door jamb leg. Door jambs consist of 3 parts, the head or header piece of jamb and the 2 door jamb legs. The hinge side jamb leg is where all the strength typically is and the header section rarely gets much abuse.
But the jamb leg where the lockset strike plate exist, is effectively where the brunt of the damage usually occurs. Interior door jambs will differ from exterior door jambs. Note: images at left show both interior and exterior door jamb varieties. An exterior door jamb typically will be thicker and milled for actual door placement. Interior door jambs typically are one flat solid stock of 3/4″ pine, but will also come in finger joint paint grade materials for cost effectiveness per lineal foot, for the end buyer/user.
The replacement process is generally the same for any wood framed door jamb, remove both sides of the doors trim [casing] on the applicable door jamb leg. If you get these off without damaging you should be able to reuse them. Pull the nails through the back side of the casing so you won’t be harming the casing face, for appearance reasons. Now, before the original installer sets the jamb, they are cut to size and pre-assembled, meaning there should be nails or screws where the head meets the legs. Knowing this will save you time and further repairs, thus they must be cut, prior to removal. Using a reciprocating saw or [sawsall] with a metal blade to cut the hidden fasteners.
Once this is out and away, you can now cut your new jamb leg and install with 1-1/2″ to 3″ finish brads for the best results. Shim in place from both sides and let the door tell you where the jamb should be exactly placed by following the common door[s] spacing margins. Add a few extra reinforcing nails around the hinge[s] and or strike plate areas without placing each individual nail/brad in the same wood grain line. Trust me, this will weaken this commodity more than help..another diy home finish carpentry repair well done from another DIY’er. GREAT JOB!