Plasterboard as part of subfloor for LVT

Discussion in 'Subfloor Preparation' started by BigNige, Oct 28, 2021.

  1. BigNige

    BigNige New Member

    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*, 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. I will try to attach some pics if I can figure out how.

    As I am new, I cannot include the exact links so you can Google the following to find the links I am referring to:
    * british gypsum plasterboard selector guide
    ** british gypsum coreboard datasheet
  2. BigNige

    BigNige New Member

  3. merit

    merit Well-Known Member

    I think you should talk to a builder.

    Sent from my iPhone using Tapatalk
  4. dazlight

    dazlight Super Moderator

    I think you should move
  5. Rugmunching

    Rugmunching Well-Known Member

    I think you should put it all back and pretend it never happened :eek:
  6. Neilydun

    Neilydun Well-Known Member

    Ive seen that quite a lot. It used to be part of a sound reduction process.
    They always end up cracked up, and sometimes are reduced to dust.
    If its a pink board, it could be part of a fire defence, as they are 1 hour fire rated, but unless it has been fire foamed, or masic has been used it will be more or less useless for that purpose.
    Either put it back as is, or take the layer out.
    I would check with the building owner if any special regs are in place, as you could be breaking your leasehold agreement by removing it, if it has a design purpose.
  7. BigNige

    BigNige New Member

    Thank you @Neilydun - definitely one of the more useful replies here

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