Can I fix this without taking the wall down?

[big-all]

no ----at right angles
draw a line along the centre off the floor plate side to side
in other words within the space the floor occupies and only that space but to the centre off the wall

 

[big-all]

as an aside your mulitool oscillating saw blade will tend to ground out on the welded shoulder around the 32 to 38mm mark so unless you get an extended blade thats how far you will go in which in fact will be far enough as long as secured with screws from above

 

[Joe Fixit]

as an aside your mulitool oscillating saw blade will tend to ground out on the welded shoulder around the 32 to 38mm mark so unless you get an extended blade thats how far you will go in which in fact will be far enough as long as secured with screws from above

Really appreciate your help especially at that time in the morning!

AHH I see, you mean cut out the red hatched section and screw roughly at the green dots?

The studs are only about 700mm so will only provide a 350mm space to work with. If not enough, I may cut the whole thing and add a noggin above to help with support.

I don't have reciprocating saw, I wonder if multitool will handle this cut

 

[cdbe]

Can't quite figure this out, presumably the issue is you can't access the next joist across to fix a support for the floor plate? You could fix a strip of 18mm ply or OSB across the bottom of the wall to bridge the weak area, either all the way across or just behind the bath - a basic "I" beam.

 

[Joe Fixit]

Can't quite figure this out, presumably the issue is you can't access the next joist across to fix a support for the floor plate? You could fix a strip of 18mm ply or OSB across the bottom of the wall to bridge the weak area, either all the way across or just behind the bath - a basic "I" beam.

View attachment 374528

Yes, that's right. I did wonder earlier whether it would be safe to cut out a section of the floor plate and add a nogging above to support the vertical studs.

That way, I can cut out more of the chipboard and hopefully can slide a smallish beam through the cut out section and help support the subfloor in the bedroom side of the joist.