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I've got an actual opening of 35.5" in an opening between my basement and garage. I'd like to install a prehung door (first time), and am a bit confused about the width necessary for framing.

My door opening:

From what I've read I want at least 2" between the prehung door width and my rough opening -- which I don't have yet. It sounds like this limits me to either a 32" prehung (actual 33.5") or a 30" (actual 31.5").

If I went the 30" route, that would leave me 2" on each side for framing the rough opening which seems like it would leave me with the most play and be able to do something like a 2x6 (1.5") on each side, leaving me with 1/2" gap for rough opening and making the door square.

However, I'd prefer to use a wider 32" door (because why not?), but I'm not sure how to approach the framing:

The opening minus the size of the prehung door would leave me with 2" for both sides with no framing. I'm pretty sure I need to frame at least the hinge side which would be possible with a 2x6 (1.5") leaving 1/2" for making square on either side.

Is it okay to only frame one side? If not, would a 1x6 (3/4" width) be a thick enough board for framing and would the 1/4" gap rough opening be enough?

  • Is there wood framing in the opening already? The dark stuff in the opening is what? dark painted block work? – Jack Jun 27 '15 at 16:26
  • No wood framing currently. I removed an old junky door and the framing to fit a couch in ;) The dark stuff is just old unpainted block concrete. – MAckerman Jun 27 '15 at 17:49
  • Maybe you can frame off the end of the CMU 90 degrees from the opening you are currently looking at. Or by extending the CMU wall and moving the door a few feet toward the camera. Can't tell without a floor plan. But the current opening is going to be a tight fit once proper jambs are installed and restricts the options for the door swing. – ben rudgers Jun 28 '15 at 21:24
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Your idea about the 1X6 on either side is a good start. It would give you something to fasten to, in a simple fashion on both sides, although the fastening would be minimal. You may need to increase the number of nails in the jamb to keep it in place over the long haul. Same thing if you use screws.

When the 1x6, for a 6" wall, or if it fits, 1X8 for an 8" wall, is set in place, use construction adhesive between the block and 1X and let it harden. Make sure your block are clean so the glue sticks. This will give a firm base to work to. Do not skimp on the glue. Use a 10 oz tube, a half tube at least, on each side before you set the 1X. Use concrete screws to fasten it in place, they are more reliable for a first time installer. You could use masonry nails but there are places they may not hold. The screws may fail in their holding, but that will be obvious. To increase the chances of holding, use the screws only in the block, not the joints. Keep the screws about an 1 1/4" to 1 1/2 from the edges of the block. The block is thicker there and is far enough from the edge to keep from cracking out (should be). Screws also only affect a small area when they fail, nails break out larger pieces.

To answer the other part of your question, yes you could use framing only on one side, them shimming and fastening the other side is tricky. There are case hardened finish nails to do this, or even screws. For a first timer, what I mention above should do what you need.

  • Thanks for the info @Jack! This is helpful. I hadn't thought about using adhesive behind the framing. So, just to double-check: it sounds like you'd recommend going with the 32" door and using 1x8 on both sides leaving me with about 1/2" of play for making the prehung assembly square? – MAckerman Jun 28 '15 at 16:35
  • Yup there is no reason why not to go with the larger door. You need to increase the amount of fasteners you use to hold the door since the wood is not thick enough to get a good depth to set a screw or nail. I have seen folks put doors in with 4 screws or nails only. The door suffered for it too. I typically set 1 screw through the hinge and a nail or screw on the opposite edge, the same spacing on the other jamb. In your case more than that is better. The 1X8 is a presumption you have return lugs on the 8" block wall so you have a 8" face of block to apply the 1X against. – Jack Jun 28 '15 at 22:41

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