I'm new around here, but I was hoping for some help on a moulding mess. We're doing 4.5" wide crown moulding in a 1920's Chicago apartment with plaster walls and ungodly hard ceilings (not sure the material but it feels like concrete). All was going well enough until we got to a corner that rises to 5/8" higher than the rest of the ceiling, starting about 2' from the corner. I've been all over everything with a carpenter's square, it's definitely the ceiling and not a warped piece of moulding.

This gap is too big to fill with joint compound, and scribing and sanding won't work because of the position. I've considered laying a thin piece of wood over the gap to cover it and just shaping that to fit, but I'd have to do this on the next wall as well to make the corner work. The only other thing I think might work is putting some long screws in and just forcing the piece to meet the ceiling, but 5/8" in 2' is a lot, and replacing the piece if it splits after everything is caulked and painted would not be fun. This is all going to be painted white and in a less than noticeable spot in the room, so a little kludgyness is OK.

Any ideas? I'm kind of at my wits' end.

Edit/Delete Message