We had a roof leak last year, and the wallssand ceiling in the corner of the back bedroom got damaged (rusty water coming out of cracks, surface of wall starting to swell, pait cracking, etc.

I now have time to repair it, (I have general skills, but I'm not a contractor), and first comes the fun part: demolition!

The problem comes in when I notice that the sheetrock panels that were used are 3/4" thick, and appear to be half white / half gray. I haven't seen any drywall panels this thick, and people at the home improvement store I talked to said "I must be mistaken", but I know how to use a tape measure, and it's 3/4" thick.

So, without replacing the two walls and ceiling (only about 20% of the walls needs replacing, since it was in the corner)is there a trick to making the surface of the old & new walls level? I thought about using shims on the studs to make them thicker, but I don't think that's very sturdy, expecially for the ceiling. If anything, I would need to find 1/4" thick wood and cut it the same length as the stud, floor to wall.

Thanks in advance.