Plasterboard as part of subfloor for LVT

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Hi. First time to post to this forum so forgive me if this has been answered elsewhere, but I have scoured the internet for guidance on this and have struggled to find any pointers to my scenario...

I have started a complete refurb of my bathroom, have ripped everything out and will be doing all work myself, except for the flooring which I have settled on Karndean LVT and will get an approved Karndean supplier/installer to supply and install. I am preparing the subfloor for them. The existing flooring was another (click type?) vinyl flooring which was peeling in between the vinyl tiles. After removing all that, I noticed that the chipboard underneath was partially damaged by water, so I bought some new 22mm P5 T&G chipboard to replace it. However upon lifting up all the existing chipboard, I noticed the next layer down was a layer of 18mm plasterboard which was also severely cracked, so decided I should replace that also. For info, underneath that plasterboard is a layer of paper-backed rockwool type insulation stapled down to wooden sheet (the stud walls of my bathroom sit on top of this wooden sheet). This wooden sheet presumably sits on wooden joists, but I really don't want to start trying to lift that up to see. (Loft joists are 600mm apart if that's anything to go by)

This bathroom is in the upstairs flat of a 2 storey 1970's house (which was purpose built to contain flats, i.e. not a house conversion to flats). A builder did suggest to me that the plasterboard is most likely there to act as a fire defence between upstairs and downstairs flat.

After calling up British Gypsum to identify what is this 18mm plasterboard, they can't tell me. It says "Gyproc" on it and has the number "2441970" written on it which might be some kind of manufacturing date? British Gypsum were very vague in suggesting a suitable alternative, but did give a link to their "Gyproc Plank" product but didn't explicitly state that it is a suitable replacement, only saying that I should replace the old with the same type... :mad:

Looking at the different types of plasterboard available (here), I thought that Gyproc Core Board would be the best replacement for me as it has enhanced fire protection, moisture performance, & sound performance. All of which are factors to consider in my scenario considering my neighbour below, and it is also 19mm thick so there shouldn't be any major issue with a different floor height with the room next to the bathroom which will also have the same LVT flooring installed. However the Data Sheet for Core Board says it is unsuitable for areas continuously damp or humid, so although my bathroom will not be continuously damp, but I'm still not 100% sure if it's the best replacement? If not, the FireLine MR might be my next choice, but that only comes in 15mm max thickness so I would then have a problem with different floor heights...

So my questions I need help with are:
1) Is new plasterboard the best thing to replace the existing plasterboard? If so, any recommendations which type? If not, then what would be better? And will other alternatives provide the same level of fire resistance/sound resistance with my downstairs neighbour that Building Regulations might require?
2) Current plasterboard just rests on the wooden flooring sheet & is not screwed or nailed down. Should I screw the new plasterboard down? (I would try to screw this into the joist if so)
3) Reading other forum posts elsewhere on the internet I need to leave a 10mm gap between the edge of the chipboard subfloor and the wall (to allow for expansion). This is also stated on the NHBC website, so presumably I should do this. Would this 10mm gap also apply to the plasterboard sheet that is forming part of the subfloor?
4) If I leave a 10mm gap around the edges of the subfloor, how do the LVT installers ensure that the floor is waterproof, and that water doesn't drip along the edge of the floor, down the side of the wall, destroying the subfloor, and downstairs too? I believe the installers mentioned that they put plywood on top of the chipboard and then the LVT on top of that. Would/should their plywood also have a 10mm gap with the wall? Would skirting board that goes over this expansion gap with flexible silicon sealant where it meets the LVT flooring be the solution to this?
5) Should the T&G chipboard be screwed down to stop it moving sideways? Would this be screwed down through the plasterboard and wooden sheet, and ultimately into the joists?
6) And for a bonus question, I will be building a false stud wall at the end of the bath as the new bath is shorter than the room length. This false wall will have a shower bar valve installed on it with pipework running behind it. Is it any issue to have the LVT flooring installed in the entire bathroom first, then build my stud wall on top of the LVT afterwards (and therefore screw the bottom stud plate through the LVT and into the chipboard to secure it in place? Or should I build the stud wall first, then get the LVT installed afterwards? My preference is the former as the LVT would provide a bit of protection in case of any minor leaks behind the false wall.

Thank you very much if you made it this far, & I really appreciate your time to help with any of my queries. Attached are some pics that might be useful:

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I noticed the next layer down was a layer of 18mm plasterboard which was also severely cracked, so decided I should replace that also. For info, underneath that plasterboard is a layer of paper-backed rockwool type insulation stapled down to wooden sheet (the stud walls of my bathroom sit on top of this wooden sheet).
This bathroom is in the upstairs flat of a 2 storey 1970's house (which was purpose built to contain flats, i.e. not a house conversion to flats). A builder did suggest to me that the plasterboard is most likely there to act as a fire defence between upstairs and downstairs flat.
I'm happy to be corrected on this, but AFAIK FR (pink) plasterboard is a relatively modern material (30 years or less, or towards the end of the asbestos era in 1999?), because we sometimes used to use Asbestolux or asbestos plasterboard in walls and floors to get higher fire rating in the 1970s. Anyway, it seems to me that replacing an 18mm plasterboard with an equivalent 19mm board would yield the same general level of fire protection as the original and so should be acceptable. Screwing it down to the timber beneath probably won't make much difference to it's performance, but the more fixings you have going through into the bottom board, the more noise can be transmitted through the screws, so if you absolutely want to hold it in place a few "stripes" of intumescent caulk or silicone should do just as well. I see no issue going fairly tight to the masonry with this, other than the possibility that if the walls are wet moisture will wick into the edges of the PB - unlike timber products PB won't creak if it is rubbing against masonry (as the floor loading changes), however it will be better for fire and noise performance to have a slight edge gap all round and to caulk all these edge gaps with an intumescent acoustic caulk. And BTW, PB doesn't really expand, unlike timber or metal, so it won't creak

As to moisture sealing, I think you may need to put a liquid DPM on the T&G chipboard to achieve that rather than worrying over it. Talk to your floorers

4) If I leave a 10mm gap around the edges of the subfloor, how do the LVT installers ensure that the floor is waterproof, and that water doesn't drip along the edge of the floor, down the side of the wall, destroying the subfloor, and downstairs too? I believe the installers mentioned that they put plywood on top of the chipboard and then the LVT on top of that. Would/should their plywood also have a 10mm gap with the wall? Would skirting board that goes over this expansion gap with flexible silicon sealant where it meets the LVT flooring be the solution to this?
The stuff we use around sub-floor edges these days is called Compriband - a highly compressed closed cell neoprene rubber tape which expands to 3 to 5 times its' compressed thickness. Effective for sound and fire. If needed I'd install it around the perimeter of the T&G chipboard. Ask the LVT installers about wall edge sealing of your skirtings to see how they intend to deal with leak control (if at all), or what they expect of you (or are you having tiled walls?). In my experience of this (admittedly commercial and limited) we've put the sub-floor in, then installed the skirtings sealed to the floor with a bead of silicone, then a liquid DPM has been applied to the T&G, then an SLC and finally the finish flooring has gone down. Sometimes the sequence varies, but P5 is pretty resistant to water if the joints are properly glued and pulled up tight. So you need to confirm with the floor layers what they expect to do - because every installation is different

5) Should the T&G chipboard be screwed down to stop it moving sideways? Would this be screwed down through the plasterboard and wooden sheet, and ultimately into the joists?
Was the existing floor installed with screws, or as a glued floating floor? I suspect the latter as it allows for movement and reduces noise transmission from the apartment beneath. Believe me, when you put in a floating floor the size of a room (at 20+kg for every 2400 x 600 sheet) it really doesn't move a lot after the glue has set! Use a D4 flooring adhesive to glue the tongues (too much glue is preferable to too little) as well as flooring straps and/or a pull bar and a hammer to pull the sub-floor joints up tight

6) And for a bonus question, I will be building a false stud wall at the end of the bath as the new bath is shorter than the room length. This false wall will have a shower bar valve installed on it with pipework running behind it. Is it any issue to have the LVT flooring installed in the entire bathroom first, then build my stud wall on top of the LVT afterwards (and therefore screw the bottom stud plate through the LVT and into the chipboard to secure it in place? Or should I build the stud wall first, then get the LVT installed afterwards? My preference is the former as the LVT would provide a bit of protection in case of any minor leaks behind the false wall.
Your existing stud walling sits on top of the "sheet of wood". That looks like it could be 18mm (or thicker) softwood plywood, so why not install the new studwork directly onto this, then install your PB, then the T&G chipboard? The floor on the inside of the wall can always be sealed with silicone around the edges if it bothers you that much, but TBH it is rarely an issue if the plumbing is done right in the first case

And having boiled my head, I'm off to bed...
 
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@JobAndKnock - thank you so much for your detailed reply to all my questions, I really am very grateful to you. I think as you say I need to clarify some things with the floorers. Last time I spoke to them they were a bit vague, but now I have more knowledge I might be able to ask them the right questions.

Anyway, it seems to me that replacing an 18mm plasterboard with an equivalent 19mm board would yield the same general level of fire protection as the original and so should be acceptable. Screwing it down to the timber beneath probably won't make much difference to it's performance, but the more fixings you have going through into the bottom board, the more noise can be transmitted through the screws, so if you absolutely want to hold it in place a few "stripes" of intumescent caulk or silicone should do just as well. I see no issue going fairly tight to the masonry with this, other than the possibility that if the walls are wet moisture will wick into the edges of the PB - unlike timber products PB won't creak if it is rubbing against masonry (as the floor loading changes), however it will be better for fire and noise performance to have a slight edge gap all round and to caulk all these edge gaps with an intumescent acoustic caulk

I think I'll go for the 19mm CoreBoard plasterboard and do as you say, i.e. not screw it down to the wooden sheet (to prevent noise transmission), and then caulk around the edges.

Ask the LVT installers about wall edge sealing of your skirtings to see how they intend to deal with leak control (if at all), or what they expect of you (or are you having tiled walls?)

No, I'm not having tiles on the wall (with the exception of the area above the bath). There will be a skirting board on the walls eventually (old stuff has been removed), but again I'll double check how the installers want to address this as per your advice.

In my experience of this (admittedly commercial and limited) we've put the sub-floor in, then installed the skirtings sealed to the floor with a bead of silicone, then a liquid DPM has been applied to the T&G, then an SLC and finally the finish flooring has gone down. Sometimes the sequence varies, but P5 is pretty resistant to water if the joints are properly glued and pulled up tight. So you need to confirm with the floor layers what they expect to do - because every installation is different

I have to confess I wasn't planning on laying a DPM and SLC as I got the impression that the LVT installers were happy to put down their flooring onto chipboard. I believe they mentioned that they would lay plywood first on top of the chipboard, and then the LVT flooring, but I will double check with them regarding this. I was probably being naive thinking that they will ensure the DPM aspect is addressed somehow, but without asking them how.

Was the existing floor installed with screws, or as a glued floating floor?
The plasterboard is not screwed or glued, neither was the existing chipboard glued or screwed. On top of the existing chipboard there was a layer of green felt like underlay which had a plastic backing (all of which has been thrown out and I didn't take any photos of it). From memory, I believe it also was not stapled, screwed or glued down. The existing vinyl tiles then sat on top of that - again not glued down either from memory.

The floor on the inside of the wall can always be sealed with silicone around the edges if it bothers you that much, but TBH it is rarely an issue if the plumbing is done right in the first case
My reason to be rather anal about this is because I am currently experiencing an issue whereby in my current house (i.e. not the flat detailed in this post), there is a leak behind the same type of false wall in the bathroom, so I'm really trying to go belt and braces to prevent a recurrence of this type of problem happening in this flat. I didn't install the plumbing in my current house, but I will be installing the plumbing in the flat in this post, and I'm pretty confident in my plumbing abilities. I suspect the problem in my current house is that they have used flexible pipe going to the shower bar valve and I suspect that the joint has leaked where it connects to the bar valve, so I'm going to have to rip down the false wall (or at least part of it) to find out what is going on behind. But that's another story for another day!

As I do not know much about the "sheet of wood" (particularly how thick it is) or what's underneath the sheet of wood (presumably joists), I wasn't sure how strong the sheet of wood was, and if it could handle the weight of a false wall (especially with tiles installed on it). I also don't know where the joist is located under the floor to ensure that I would be installing the new false wall directly on top of a joist (or install a noggin if it wasn't). Hence, why I thought it might be safe to just install the new stud (false) wall on top of the 22mm T&G chipboard to spread the load. In case it's relevant, the joists in the loft run in the direction from where the bathroom door is to the wall that has had the plasterboard removed from it. Presumably the joists underneath the bathroom floor also would run in the same direction..

Thanks again JobAndKnock!
 

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