Couple of questions about joist repair/strengthening

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.
 
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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.
 
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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?
 

LEH

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There was an immersion heater but now there's a combi boiler hanging on a frame that's sitting on several of the joists.
 
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LEH

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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?
 

LEH

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No, they're actually in decent condition (overboarded and skimmed), so I'd really rather not.
 
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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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