Couple of questions about joist repair/strengthening

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LEH

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Currently contemplating bathroom refurbishment and have taken a look under the bath. There has been a fair bit of water leaking round the perimeter due to poor sealing, probably for some time, as you can see below!

IMG_9921.JPG

Top bit of the joists looks a bit rotten, though the bulk of it still seems solid. What repair would you suggest? I'd be happy to sister a new joist, but one problem is that to the left in the picture is an airing cupboard then the hallway where there is a ground floor wall supporting the existing joists. I can't see that I can slide a full length joist underneath - would it be sufficient to have the sister just supported at one end, or would a shorter length spanning either side of the damage be enough?

Second - I have a paramount partition wall that will be studded out and re-boarded. Currently it's sitting on top of a joist. My understanding is stud walls should usually sit on doubled-up joists? Again I'd have trouble getting a full length joist in...

Spans are about 3m in both cases, 47x195 joists at 400mm centers.
 
That looks like chipboard which is not suitable for wet rooms.
I can't see the state of your joists properly but I suspect if you
put real wood t&g planks and let everything dry out then that
will be enough with a bit of wood preserver on the joists.
 
Is there a tank in the airing cupboard and does it have its own supporting structure, or is it also putting a load onto the same joists?
 
There was an immersion heater but now there's a combi boiler hanging on a frame that's sitting on several of the joists.
 
Still hoping for a few opinions on how much, if any, remedial work this joist needs. I'm inclined to 'scab' a bit of joist either side of the rotted area as I'll need some solid joist to fix floorboards to anyway.

For the stud wall I suppose I just have to leave it on the single existing joist as I really can't see any way to get a second one under there. I'll have to fix a piece alongside, again for floorboard edges, but that obviously won't be strengthening it?
 
No, they're actually in decent condition (overboarded and skimmed), so I'd really rather not.
 
You could glue and screw (or bolt) plywood sheet to each side. It's very strong like that.

Or you could bolt on a suitable piece of flat steel plate.
 
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