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Old 05-06-2012, 07:47 PM   #1
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pre-hung interior door shimming

So I'm replacing a few doors around the house. The existing slabs measured 30x80, so I figured no problem. I've got some thick interior walls due to plaster, so I ordered at the local lumber yard.

Mistake #1 -- Even though I measured my wall thickness at 5-1/8" and told the man at the lumber yard... I agreed to order a 5-1/2" because according to the man the split jamb will allow you to shrink, but not grow. As it turns out these doors allow grow, but not shrink. I assume the proper thing here is to take out the split and rip it down on my table saw by the ~1/2" I need then use the split as intended to make up the missing 1/8" or so?

Mistake #2 -- My RO turned out to be 33-3/4" x 81-1/2". I believe I could have bought 32x80 doors in that case and made them work, but it's water under the bridge. My question is how to make up the difference. My pre-hung doors are 31-1/4" RO, so I've got 2-1/2" to make up for. Seems like too much width for shims? I was thinking I could get two 1x6's, rip them down to 5-1/8" width and use one on each side to build out my opening by 1-1/2"... now I'd have a more manageable 1" to shim on either side. Does that make sense? Also, the height of the new door frame is too tall by about 1/2" to fit in the RO... the door itself is fine and there seems to be a good 3/4-1" or so of extra at the bottom of the frame. I assume the correct way to handle this is to lop off the extra 1/2" from the bottoms of the frame? since it's such a short cut (~5-1/2") I was planning to just use a square to make a pencil line and then cut w/ my circular saw---or should I get the thing onto my miter saw to be sure it's a nice square cut?

Despite the issue I'm still happy with the product because these Masonite hollow cores are a heck of a lot heavier and more solid feeling than the cheap Jeld-Wen's from H.D.

I'm crossing my fingers that I've got the same 2-1/2" gap or more on my basement door... right now it's only 28" and I'd love to increase that to a 30"...


My advice is based on anecdotal knowledge or personal experience. I'm not a professional no matter how matter of factly I may say something

Last edited by bubbler; 05-06-2012 at 07:51 PM.
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Old 05-07-2012, 06:25 AM   #2
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For those considering a similar project, rough opening dimensions for any prehung door are usually 2" more than the door. The best thing to do is remove the trim on the old door frame, and measure the actual opening.

And carefully measure the wall(jamb) width, to avoid having to modify the new frame.


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Old 05-07-2012, 08:04 AM   #3
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What a pain in the arse this project turned out to be..............

Instead of ripping the jamb width down at the joint, maybe you could put a 1/4" lumber spacer behind the trim on each side of the frame trim?
This should take up the 1/2" discrepancy at wall thickness without having to take the entire frame apart.

Instead of using 1x6's installed vertically to reduce your RO, consider installing a series of 5 spacers along the jamb to save on material cost.

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Old 05-08-2012, 02:30 PM   #4
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Your fix of ripping the jamb down is the best solution available. It seems like a pain but you could dismantle the half without the door, rip it, sand it and reassemble it in pretty good time.

The shimming part would be best done as you describe. If the RO is nice and plumb I would get the RO to be a little closer to the proper size somehow. (1/2" over your jamb width) How you get there is just a matter of using what you have on hand. As mentioned earlier, you could block out several strategic locations if you want to, just be sure to get them where you need the shims. Behind the strike and hinges for sure. Don't be too concerned about those blocks or rippings to be nice and flush with the wall. The casing will cove a bunch of sins there.

As far as cutting the bottom of the jambs; You can cut those with a skill saw if you are comfortable with that. A helper to hold the jamb leg would be nice. But a couple things to consider;
1 - If these legs need to sit on a finished floor that is already there, those cuts will determine how square the head jamb will be. Cut them a little long and do some testing if you can or you need to figure out just how level the floor is from one side of the opening to the other and cut them accordingly.
2 - The gap under an interior door is often used as the cold air return for a forced air heating system. If that is the case here, you will be cutting the bottom of the door as well. A 3/4" gap is usually good.

Good luck, have fun
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Old 05-08-2012, 05:53 PM   #5
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if your hanging the door on an existing finished floor that isnt level. a little trick i like to use which is easier to determine how much out of level which keeps you off your hands and knees is to measure up off the floor say 60" on the hinge side now use a level and level across to the latch side and make a mark on the wall. measure up off the floor on that side and figure out the difference between the two measurements.. by doing this you can determine what the lenghts the jamb legs need to be to get a correct margin at the top of the door
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Old 05-09-2012, 09:26 PM   #6
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I got the door up--

- I took the split part of the jam out and apart, then ran the three pieces through my table saw to shave 1/2" off...

- I used my chop saw and some creative bungie cord-ing over water pipes to cut the "legs" of the door down to the 81-1/2" that is my R.O.

I took wood4249's advice and chopped up a ~5' long 1x6 to act as spacers... first I ripped it down to ~5" to match the width of my opening, then I chopped it up into eight 7-1/2" long pieces which I nailed in place on each side. I shimmed the latch side being sure to get a nail in at the latch since that will be the stress point.

The gap all around the door in the jamb is perfect (my door came with little cardboard spacers stapled to the latch side which were a great help). The door is balanced too, doesn't want to self open or close, and then it is closed it sits evenly around the stops with no warps or gaps. About the only thing that could be better is in the upper left a gap opened up on the split part of the jamb, but I'll just use some of the Handy Man's 2nd Favorite Secret Weapon--Caulk... and take care of that before I paint it all white.

I'm fortunate, my R.O. was near perfect, and the pre-hung assembly seems well made. I think it took me about 2 hours, that included setting up the saws/etc. I'll wait to finish painting before I trim it out and add the HW. The room it's in is currently just used as an office, so the door isn't needed. I've got two more to go, and after that I'll be ordering another several, but this time I'll order the widths correctly!!!


My advice is based on anecdotal knowledge or personal experience. I'm not a professional no matter how matter of factly I may say something
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