advice about door frame

I need to take about 1/2 of an inch off a door frame, but I don't want to take it down.
Is there a tool I can use to do the job?
Or if someone has a better approach please let me no.
Regards Bob
-- Don't look back, a new day is breaking.
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It would be easier to pad the casing out that 1/2".

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If you need to trim 1/2 inch off the BOTTOM to allow flooring to slip under the frame, just use a hand saw. Rest the saw on a scrap piece of the flooring while cutting, and use a saw with a thin kerf if you have one. They do make electric jamb saws made for this purpose, but for a single door it probably wouldn't be worth the trouble to even rent one.
If you need to trim 1/2 inch off the DEPTH of the frame, I agree with another poster, cut the casing (trim) to fit instead. I had a couple of door frames that stuck out beyond the wall a bit, so I used a router with a straight cutting bit to cut away the backside of the trim (a Rabbet). It fits flush against the wall and overlaps the trim. Once the trim is installed, you can't tell the frame stuck out farther than the wall. If you're not using mitered trim, stop the rabbet just short of each end of the top trim board. This will prevent you from seeing the rabbet on the ends.
If the door is in a small wall (closet or bathroom for instance, another option would be to add a 1/2 inch to the thickness of the wall. A layer of 1/2" drywall would work nicely. The downside to this approach is that you would then need to tape and mud the joints and repaint.
If you need to trim 1/2 inch off the WIDTH of a frame because a new door is 1/2 inch too narrow, you could pull off the door stop and add a 1/2" thick addition to one side of the jamb. Then reapply the door stop. Shift the trim inward a 1/2 inch and you will hide the fact that the jamb is thicker on that one side.
If you need to trim 1/2 inch off the HEIGHT of the door frame to accomodate a shorter door, you have two options. Raise the door 1/2" leaving a little extra space under the door bottom, or add a 1/2" addition to the top jamb as described above.
Anthony
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