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I'm removing old plasterboard walls and getting someone to help by cutting along the top edge. I did this so I don't have to remove the cornices (which seem fairly good condition). However, I'm concerned I still have a 10 to 20 mm of old plasterboard along the top edge of the wall, where it meets the cornice. Is this going to be difficult to join new plasterboard (gyprock) to the old old horsehair plasterboard????? It's right at the top of the wall, so not that noticeable?

  • If the lath and plaster it thicker than the sheet rock it will not be hard to tape over the crack and feather some sheet rock mud down a few inches. if the lath and plaster is a lot thicker you will want to do it in several coats to prevent cracking and feather it in 12-18" down. I have done several of these type of remodels on the really high end ones we made molds of the cornice (the roses and flower type) and made new ones. Of the ones I have done the old stuff was the same or thicker. not sure how it would look if the old stuff was thinner. – Ed Beal Apr 22 '16 at 13:06
  • I have no 'picture' in my head of what you're trying to do from your description! If possible I think a picture in this case really would be worth a thousand words..... – handyman Apr 26 '16 at 21:54
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Normally I would remove the cornice and replace it once the drywall is finished and painted - because that is usually the easiest, if you are okay with installing new crown molding.

If that's something you aren't interested in doing (perhaps it's a molding you can't copy and don't want to get rid of), then you can finish your job without removing it. First, remove the plaster down to the studs, from the floor to about an inch below the cornice. Then nail strips on each stud to shim the surface of the drywall out to the surface of the old plasterboard which is still visible under the cornice. Plaster is generally about 7/8", and if you're using 1/2" drywall, that would be a 3/8" shim the length of the stud. (You should measure the plaster thickness in several places to be sure of this thickness, though.) Rip 2x4s into a lot of 3/8" x 1-1/2" strips and cut to the length of the studs. 4 penny nails are sufficient for tacking these shims onto the studs.

Then, hang the drywall as normal. Use 1-5/8" screws so that you grab the studs behind the shim strips. When taping, run tape right up against the cornice all around the room. Then finish as you would finish any butt joint, feathering down twelve inches or so.

Once sanded and painted, no one will ever know that there is plaster behind the cornice.

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