Having never installed a pre-hung door before, I am wondering how accurate does the door rough-in need to be? I am planning on installing a 28" bathroom door, which has a 30" rough in. After framing it up, (and I can't figure out exactly how this happened as the bottom gap is 30" and the two Jack studs appear to be level) I noticed that the gap at the top is closer to 29 13/16" instead of 30". Is this a problem?


Generally for interior doors the RO (Rough Opening) is 2 inches wider than the door slab. This gives 3/4" for the jamb and 1/4" of 'play' for each side, for you to adjust the jamb to make the door operate properly. So if you've got 29 13/16 instead of 30, you'll end up with a little over 1/4 of play total instead of 1/2. Still should be plenty if your jacks are fairly plumb.

If you run into trouble and find you need the extra space, a couple of good swings with a hammer at the jack should get you another 1/8 or so.

Don't worry about the jacks being slightly out. Here's what you do:

  1. Set the door into the opening and put shims under both jambs to hold it tight to the header so you don't have to hold it the whole time. Check the top jamb for level and if it's good, move on to 2. If it's not level, remove shims from the appropriate side.
  2. Plumb the hinge side. I like to put the top tight to the jack, and work it from there, but you can center it if you like. Make sure to have a straightedge handy to check as well; the hinge side needs to be nice and straight so the hinges won't bind. Shim wherever you need to and pop some trim nails through the shims to hold it.
  3. Plumb the latch side and make sure it's straight as well. stick a couple trim nails into it as well, but check the door's operation before you get too carried away.
  4. Open and close the door several times and make sure it operates smoothly. Sometimes you get everything level, plumb, and square, and it still doesn't operate well. Play with whatever you need for a smooth swing and latch.

The reason for a rough opening is so that it CAN be rough in dimensions. Your opening should be no problem. Rough openings are made larger to accommodate headers and floors than out of level, adjacent studs that may be out of plumb, framing lumber that has thickness variations, openings that are slightly out of square and maybe some other things I've not thought of.

BTW, the jack studs are vertical so the proper term to describe their closeness to vertical is "plumb" and not level. The term "level" is used to describe things that are properly horizontal.

  • So how far do I have before it becomes a problem? Obviously I couldn't have an opening that is < 28" b/c that's the thickness of the door. There still needs to be space for the jamb on both sides plus the gap between the door and the jamb. I wasn't sure if that added up to the 2" or if there was some wiggle room (and how much) – 2 Left Thumbs Jun 29 '16 at 14:05
  • There is no real answer for this other than the following....If the rough opening gets too big the door frame casing won't cover and using the tapered shims to help square the frame gets impractical. Too small of opening and the door frame plain will not fit or there will not be wiggle room to square up and plumb the frame. Two inch oversize for rough openings is typical but if you have good quality framing lumber, accurate cuts and plumb/level framing installation a smaller opening can be used. I like to work with rough openings that are ~1.25" to 1.50" larger than the (continued) – Michael Karas Jun 29 '16 at 14:57
  • (continued from above) target door frame so that the tapered shims work well. But then I never use pre-hung doors and prefer to work with door jamb kits or make my own door jambs out of clear pine. – Michael Karas Jun 29 '16 at 15:00

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