Plasterboard repair and bowing joint

[portuguesejohn]

I had a bathroom leak that required me to cut out a portion of my downstairs ceiling as the plasterboard had gone bad. I now have another issue that became apparent once I screwed in my new plasterboard: It became clear that the old plasterboard was bowing quite a bit on one side. I was hoping I could maybe cut through some more of the plasterboard until I get to a level area - but it does not look like the ceiling is level anywhere close to this section... I would rather keep working within this small section.

The bowing was not noticeable before, so it is something I can live with, my question is how should I patch this? The patch area is just 30cm x 30cm, if everything was level I would have just (and this is from my own research, I have never done this before!):
1. SBR the area
2. Mesh tape
3. Apply ready mix (second picture) - already have a big tub of this, so trying not to spend more money!

Considering this bigger gap I have to deal with, should I be trying to fill it first? With what? At the peak of the bowing, it is around 1.2cm.

Also note I can't screw the old plasterboard "up", the bowing is happening between joists.

 

[Wayners]

You need to cut some more out so it's flat.
It's not difficult to do and will not cost much.
You also need to fit some timber in to support joints by tucking it behind the existing board joint and screwing to ceiling joists with screws diagonal in.
When you fit the new board you can screw through on both the old board edge and new board along joint.
All sides of plasterboard need screws through.

That material you shown will be impossible to build out much and will take ages to dry enough to have another go over. Says 2mm layers which isn't much.You would be better with easyfill powder or multi finish plaster.
Then a cheap orbital sander bodged on a vacuum and sand over...

 

[robinbanks]

Do your best and caulk the rest

—-

Take the new board back down, find a scrap of wood, sheet best you can find it, like a 4 inch by 10 inch scrap of 12mm plywood, and screw it to your new sheet so you can see eg 2x10 inches of it sticking out

Put your new sheet back up so the flap of wood is above the old PB

Use your hand to push the old PB up flick and gently wind a screw in to retain it, then a few more around it

If the old board is too stiff for you to push into place flush, get it as close as you can, then leave it a week then tighten the screws

You don’t need your wood to connect to the ceiling joists, you just need it to brace both sides of the joint with a decent spread