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I bought some pre-hung doors, intending to replace several interior doors. The problem is, the existing door jambs are shallower than usual -- I think 3.5" instead of 4.5". So if I simply remove the old doors and install the new pre-hung doors, I'll need to build up the frame or something to attach the casing. Seems like a big hassle, and not likely to look good.

So I thought maybe the best plan would be to leave the frames in place, but replace the doors. I figure:

  1. Remove the hinges from the existing frames
  2. Use wood putty to fill the spots where the hinges were (is this a mortise?)
  3. On the new door, measure the location of the hinges
  4. Use the hinges to draw a template on the existing frames
  5. Drill pilot holes for where the screws will go on the existing framse
  6. Use a router to mortise the hinge template
  7. Screw in the hinges
  8. Hang the door
  9. Beer

I'm obviously 99% clueless so if this is a terrible plan, please let me know.

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    why wouldn't you just trim the prehung jamb to 3.5"? Jan 5, 2023 at 6:19
  • The other options are, as @FreshCodemonger said, to trim down the new door casing by 1", or, center the new door in the opening so you've got about 1/2" on each side and apply a slightly thicker casing on each side.
    – FreeMan
    Jan 5, 2023 at 12:23
  • It's very odd for prehung jambs to be larger than the old ones. Are you sure? I've revised my badly off-the-mark answer.
    – isherwood
    Jan 5, 2023 at 14:13
  • You missed a meaningful step in the “hang a new door in an old opening” list: 2a. Fit the square door to the out of square opening by cutting the top/ bottom and strike side as necessary. Bevel the strike side if it got cut. Jan 5, 2023 at 15:02

1 Answer 1

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There's nothing inherently wrong with your plan aside from the massive amount of work needed. With good technique, a high level of precision, and the proper filler material you can arrive at a good outcome, but it would be fussy work. If you're up for that, have at it.

Another probably easier option is to disassemble the jambs, rip them in half behind the stop location, and remove material to shrink the jamb depth. Then reassemble. The surgical scar will be hidden by the stop. (Modern jambs are usually veneered and radiused.)

If you happen to have solid wood jambs (very rare in my experience), you could simply rip off one side and re-sand the face, but that may actually be more work.

Disassembly usually starts with popping the stops off. They're usually shot on with trim nails. Pull the nails through from the back side with a locking or channel pliers. Then gently knock apart the corner joints in the direction of their nails.

Be sure to size your jambs to the largest wall thickness in each case. Your trim will be easier to fit then. Reattach the jambs however you like so that the fasteners are hidden by the casing. Trim nails and glue work well, but the ideal solution may be pocket screws. Inexpensive kits make that easy. Screws don't wiggle loose and you could forego the glue. Glue is usually a good idea, though.

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  • Maybe I’m missing something, but the “rip out of the middle, hidden by the stop” approach seems unnecessarily complicated. (Both in terms of material handling and alignment for the finished product. Though it does have the upside of allowing any kind of butchery to go unnoticed.) Jan 5, 2023 at 14:24
  • I’d find someone with a tablesaw and do one decent rip on the far side of the jamb (not the strike/hinge side) of the disassembled pieces. Most of the cut is covered by casing, so it would only be a bit of sanding to remove saw marks. Jan 5, 2023 at 14:38
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    Second best would be a circular saw with a sharp blade and a rip guide. Again, just one rip on the far side of the jamb. (I’m not usually a fan of rip guides, but this is about the best application of one I can think of.) Top tip: rip to the widest width of the existing wall: it’s way easier to case when the jamb is a bit proud. Jan 5, 2023 at 14:39
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    Thanks all. It sounds like ripping the jambs might be the way to go. @isherwood -- can you elaborate on the fussiness of my original plan? What strikes you as particularly tricky? Jan 5, 2023 at 16:45
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    All of it, really. Filling and sanding. Positioning and mortising. It's all slow, tedious work. I could reduce the width of a dozen prehung jambs in the time it would take me to just fill old hinge slots with putty and sand them flat (and square--there's a corner to deal with). Of course, if you're finishing (staining, varnishing, or painting) the prehung doors yourself that is also a slow process. YMMV.
    – isherwood
    Jan 5, 2023 at 16:50

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