Perhaps you mean that the door jamb on your new prehung door is not wide enough
to reach the face of wall? Older plaster walls are thicker than the drywall construction
that we use today.
If that is what you mean you could rip some clear pine strips 3/8"x 3/4" or so to make the door jamb wider.
Then install casing over gap between jamb and wall.
Not sure if that is your situation but thought I would throw it out there.
Best of luck!
Richard The Door Guy
Last edited by TheDoorGuy; 09-14-2011 at 05:42 AM.