Can this be done without replacing the ceiling? The house was built in 1980, so I’m not surprised that there is warpage, but I’d like to hide it if possible.
Paintable caulk
High stretch paintable silicone with low shrink. Tape on the ceiling and the bookcase revealing the gap and an appropriate sized fondant tool can minimize the transition and give a clean look but the tape and a wet finger can do well enough.
Is it the ceiling thats warping? And if it is the ceiling, does it feel firm, or does it have give?
If it’s not the ceiling and is just the trim I’d just pop off the trim and reposition it. If it’s the ceiling and it has give id probably do the same thing. If it doesn’t have any give and is actually the ceiling settling oddly I’d say you probably should have someone come and take a look at you foundation, especially if you notice a lot of cracks near windows/doorways.
If it’s the ceiling and you want to do the good ole landlord special, I’d just fill it with some joint compound, texture and then repaint it.
You could also put up a piece of white 1/4 round over the gap. Saves the risk of damaging the nicer trim.
Lol, another good landlord special suggestion.
This.
Also, look at the attic, check for roof leaks and adequate ventilation. Make sure your soffit vents aren’t clogged, and the soffit baffles and ridge vent have actually been installed properly.
Caulk and paint is the easiest and cheapest. Otherwise thicker/wider trim that flows with the ceiling while maintaining a level line on the shelf.
Do you know how to do drywall mud? If so you can float it down to the crown and texture/paint it. This will work up to maybe 3/4". More than that, get some 1/4" drywall and use it as filler first. If not, and you can do running trim, re-run the crown. If the gap is not much bigger than ~3/8" you can just caulk it.
It won’t stay perfect, those will expand and contract at different rates, but if you want a cheap and quick fix, just get some caulk in roughly the same color. You could use joint compound as the other poster suggests, but caulk has some give, it’s likely to still separate again at some point, but you’ll have longer. The permanent solutions all kind of suck and/or are expensive, if you decide to try to reposition it and the issue recurs, I wouldn’t do it more than once.
