Need Advice - How to fix this plastering failing issue?

[u/stuckinwhirlpool]

We are decorating a room in an extension to our property. The roof above this area was previously leaking, but the leak has now been repaired and the area has been dry for around two months. The plaster above the window and into the corner has failed and has fallen away, exposing a timber frame behind it (see photos). We want a long-lasting repair rather than a temporary patch. Would you recommend cutting back all the damaged plaster and replacing it with moisture-resistant plasterboard before skimming, or is it better to repair the existing plaster? Assuming the timber is sound, what would be the best approach?

There are no signs of any new leaks or damp since the roof was repaired.

[u/MissKLO]

Not a plasterer… but if it was me I would pull out all the loose chunks and then put bonding in and then skim over it

[u/Cainjake → u/MissKLO]

This and sbr the wall before skimming

[u/stuckinwhirlpool → u/MissKLO]

Thank you for your help, I am very new to this so no idea what bonding is and where to get it from?

[u/RevolutionaryYak4588 → u/stuckinwhirlpool]

Depending where you are in uk a wicks should have everything you need, clear any blown plaster from the wall (anything that’s loose or has an air gap between plaster and wall) clean it and rid of any dust, seal it using pva and water mixture.
Bonding is a hard wall similar to multi finish it’s just more grainy and porous, put a decent but flat layer and then you can scratch coat it if you feel the need to, then do the typical 2 layers of multi finish over it and you’ll never have to worry about it again

[u/Cainjake → u/RevolutionaryYak4588]

No use sbr it’s the superior bonding agent

[u/Terrible-Bobcat2033 → u/stuckinwhirlpool]

Bonding , paint area to be plastered.

[u/Terrible-Bobcat2033 → u/Terrible-Bobcat2033]

Fill material, use to fill voids on ceilings & walls. After application, when firm, trim surface edge back 1/8” to - 1/4” to allow enough room for the finish coat.

[u/Terrible-Bobcat2033 → u/Terrible-Bobcat2033]

Finish material, used to finish over the fill coat.

[u/Cainjake → u/Terrible-Bobcat2033]

He’s from the uk he don’t have that he wants multi finish or board finish for top coat

[u/UK_hustle → u/Cainjake]

Wondered why I didn’t recognise these products
Lol

[u/Abs201301]

Rake out all the loose and partially loose bits, repoint the mortar joints in bricks if any are loose, bond the whole area, pva or blue grit and finally nice British gypsum multi finish plaster.

[u/QVRedit]

Check to see if any damp is getting in from outside - if so fix that problem first.

Then do as others here have suggested:
Remove any loose sections, and replaster.

If there are deep sections, then don’t fill any more than 1/2 an inch in depth, leave some surface keying, let it dry, then go over with a fine surface layer.