Sealing plasterboard to cement board in shower

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Hi, this is my first post and apologies if I've put it in the wrong forum
I bought my house a few months ago and have had a growing list of unforseen repairs to deal with.
The latest is the upstairs shower room. Having removed two horizontal plastic strips from the wall above the tray, thinking I've just got to replace the silicon i found loose tiles and serious mould. The plasterboard behind the tiles is rotten in places and there was a lot of black silicon in the wall to remove. I don't want to have to remove all the tiles, electric shower etc and replace PB with cement boards, only enough to remove the damaged PB, fix a cement board in place and then replace tiles. I'm rubbish at tiling so the fewer i have to do the better. What's the best way to adhere cement board to sound PB? Will this work? Thanks in advance.
 

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Hi,

The bottom of that plasterboard looks quite poor. Are the battens/studs behind, in good condition (I had to scarf in some new wood in mine)?
You may have to remove most of the bottom row of tiles to get up to properly sound board.

As you have gathered, using standard plasterboard isn't the best idea for a shower cubicle!
Ideally, you would want to strip the tiles, replace the plasterboard with cement board and tank the whole shower area with a kit such as:
https://www.screwfix.com/p/mapei-shower-waterproofing-kit/78484

As you don't want this hassle (and I don't blame you! :)),
I would buy the parts similar to the above kit separately.
I.e. use rubber tape (https://www.diy.com/departments/q-board-white-sealing-tape-l-3m-w-120mm/1919339_BQ.prd ) to cover the joints, including corners and down to the tray. As well as the junction between the wall and tray itself. Then Coat the area with the waterproofer.

I may be wrong, but I think that would give a decent enough waterproof joint, without too much work! :)

I hope that makes sense!
 
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Hi Random Grinch, thanks for your reply. I took back another row of tiles only to find another layer behind. I've bought some cement board and the waterproofing kit you've mentioned (it was on offer). The battens I've uncovered look to be OK, though not the bit of fibreboard used across ways. I'm reluctant to keep going up removing tiles as I already broke a couple and have limited spares. I'm going to attempt as you've suggested, bonding the cement board to the PB though doing a "patch" is pretty awkward. Also forgot that I'll need to cut out space for the tray as it has uplifts. Not sure what's the best way of using the tape over the uplift or is that unnecessary?
 
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I'm sorry you're still finding problems.
Just do what you can, anything done now to protect the wall, will keep it going until you feel like perfecting your tiling skills :)

Have a good read of the kit instructions. It's designed so that any moisture that does get past the tiles/grout will be stopped by the tanking, and will 'flow' back into the shower by way of an overlap into the tray.
If this isn't possible due to an uplift, just make sure there is a good bead of silicone around the joint.

Post some more pictures if you are struggling with detailing.

Good luck! :)

Sorry, just looked again at your photos and originally the tiles covered the uplift.
That's good, the rubber tape can be bonded straight over the joint of the cement board and down the uplift. Just make sure the uplift is meticulously clean and slightly abraded, before attempting the bonding (explained in the instructions).
I hope that makes sense!
 
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Thanks for your reply and the info re tape and uplift, really wasn't sure what to do there. Right now I'm still putting new boards to meet the old and cleaning off the old (very grubby) tray. Will definitely read the waterproofing instructions. Watched a couple of videos already but I'm already suspecting it won't be as easy as the videos suggest!
 
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