I have a long narrow gap in the ceiling where it meets the wall. It is too big for plaster and tape, but too small (and uneven) to cut a piece of drywall.
Furthermore, unlike in this question, there is no wood to fill with plaster.
What is the best approach to fix this?
You could use a piece of inside corner bead and finish it like any other inside drywall corner:
A less aesthetic solution would be to cover the gap with a piece of wood trim and paint it to match the wall.
Jimmy's corner bead solution is probably easier and I upvoted it. The other method is to make a larger, more regular gap by cutting the present drywall and patching it with new drywall cut to fit the new gap.
I will also note that this is, IMHO, an incorrect installation - the ceiling drywall should extend to the studs, and the wall drywall should butt up to it, fully supporting the edge. That also allows for errors in the ceiling drywall fitment up to the thickness of the wall drywall without resulting in a visible gap.
All the above answers are correct, However I would just mix some 20 min hot mud really thick just add little of water to the mix then pack it. This mud wont shrink but may sag a little. Then use a 6" knife and scrape flush with the lid and then tape with a more smoother 40 min hot mud and let that harden then coat it with 40 again. Its important to scrap just before the hot mud is setting, it will be easier to work with.
All ways prefill cracks and butt joints prior to tape. This will eliminate cracking down the road.
Good luck
If you can find a drywall supply house...there is a product you can get...it's called no-coat 325...it is applied like paper taper but is rigid enough to fill that hole. I am a drywall professional and would do that. There is an extra wide version that you can get if the hole warrants it.
Although I up voted other answers, the correct thing to do as others have pointed out is to take it down and start again. The ceiling goes on first, going completely across the ceiling with no gaps. Then do the walls start at the top and working down. The walls should support the drywall ceiling edges by "butting" up against the ceiling pieces. If there is a slight gap where the floor is that is ok (bottom of sheet) the wood trim will cover it.
Rigid paper tape, fold to fit, float with hot mud...
Clearly it's not quality work and I'm sure if there were available funds it would have been hung correctly. However quickset hot mud sets up hard and can do the job if done correctly...
Mix up some stiff hotmud, or better known as durabond and fill the gap. Then paper tape it and coat it.
I disagree with your premise.
There is no such thing as a gap that is " too big for plaster and tape, but too small (and uneven) to cut a piece of drywall." When I have made boo-boos like this, I cut a narrow strip of drywall to fill the gap as much as possible, then mud & tape it. With care and attention to the mud and taping no one but you will ever know the difference. I have also seen professionals (I'm just an experienced amateur) deal with large gaps such that the job was finished you would never have known any problem existed. When doing a reno in a very old house where things are rarely plumb and square, you will either spend forever cutting the drywall to fit "perfectly", or you will have situations somewhat like this, and deal with them. "Mud covers a multitude of sins."
