building a floating floor, the floor actually floats too much, how to push it down

Discussion in 'Builders' Talk' started by SpaceTofu, Feb 7, 2020.

  1. SpaceTofu

    SpaceTofu Member

    Hi all,

    for our kitchen floor the plan has been to make a floating floor, as follows from bottom up: concrete, self leveling compound, DPM, two layers of insulation (the old kitchen was split in half, so the first layer is 20mm in the front half and 90mm in the rear half, and then a second layer of 90mm all across), then 18mm OSB, then 22 T+G pine boards.

    We now built up to the top insulation layer and I am experiencing two issues:

    1. we put down some 18mm OSB offcuts that I have laying round and it looks bouncy, even though it doesn't feel bouncy when walking on it. I.e. I imagine that the floor gives in a little, not so much that your body feels but enough for your eyes to notice it.
    Any ideas how to fix that? Will it go away once the full width OSB board will be down? Hindsight is a beautiful thing, should have we planned to have two layers of 18mm OSB?

    2. These Celotex board are very tricky to play with. They do measure 90mm in thickness, however I noticed that they don't sit well onto each other.
    Two 18mm boards of ply or OSB will create a thickness of 36mm, however two Celotex boards of 90mm will create a thickness of 183/185mm.
    This is really annoying as we have two strong targets, i.e. marrying the threshold to the dining room and to the patio doors at the back.
    Now the layers that we accurately measured and put down so to arrive precise at threshold level, it is all over the place, especially towards the patio doors. We were planning onto having the 18mm OSB + 22mm T+G = 40mm gap to the patio door threshold, we are now left with 30mm instead.
    We lifted the boards, make sure nothing was stuck underneath, chamfered the edges, nothing changed.
    It really feels like these boards needs to be strongly compressed down and I am not sure the weight of the actual floor will be enough.
    Would it be crazy to use flat roof insulation fixing? Are they made to really compress the insulation slabs down?

    Thanks for helping my sleepless nights
     
  2. Bob Rathbone

    Bob Rathbone Screwfix Select

    At the door threshold you may need to fix a batten to the sub floor and screw the OSB to it to prevent a trip hazard. As for the rest, fit the floor, ensure that all of the sheets are securely fixed together with T&G and glue, you may need 2 layers as you suggest, put the units around the edge and the island or table in the middle, it will settle.
     
  3. wiggy

    wiggy Screwfix Select

    Sounds like you are making a right lash.
    Use tg4 ply, You can't just lay a patchwork quilt of osb
     
    Jord86 likes this.
  4. SpaceTofu

    SpaceTofu Member

    When I was doing research and asking around, OSB was deemed fine, rather than the more expensive ply.
    Hindsight is a beautiful thing though isn't it.

    I think the OSB per se is fine. It just seems like one layer of it is simply not enough weight to push the insulation down
    And unfortunately we don't have clearance enough now to put a second layer of OSB.
    Unless we take all of the insulation up and buy some thinner one. Very expensive fix.
     
  5. wiggy

    wiggy Screwfix Select

    Osb is fine for flooring if its fixed to joists but if your floating it it needs to be t&g
     
    Hans_25 and Abrickie like this.
  6. SpaceTofu

    SpaceTofu Member

    Hi @wiggy do you mean the OSB or final floor finish need to be tongue and groove, if floating? Our floor is floating, no joists unfortunately, which I now regret not having
     
  7. wiggy

    wiggy Screwfix Select

    Both.
     
  8. Jimbo

    Jimbo Screwfix Select

    The timber layer needs to be glued together so needs to be T&G.
     
  9. Bob Rathbone

    Bob Rathbone Screwfix Select

    Sounds like you are going to rely on the floor finish to hold this thing together. Not the best way, but if you use T&G floor covering, you could glue it down and glue the joints. It might stay together.
     
  10. Jord86

    Jord86 Screwfix Select


    You only glue the joints with PVA, gluing the floor down itself totally defeats the point of a floating floor.
     
  11. Jord86

    Jord86 Screwfix Select


    Substrate needs to be pretty flat to start, so concrete slab or hardcore wackered down. DPM, 100mm odd of celotex, 22mm tongue and groove chipboard with the tongues glued heavily with PVA. You do not need two layers of flooring.
     
  12. Jimmycloutnail

    Jimmycloutnail Active Member


    Why would you want to add 180mm of celotex vey expensive way of doing this? You just need 100mm for current building regs and then 22mm chipboard glued joints with PU foam glue on the joints. Maybe you need to get someone to look at it who knows what their doing
     
    Abrickie likes this.
  13. SpaceTofu

    SpaceTofu Member

    Hi all.

    @Jimmycloutnail I added 180mm of Celotex (90+90) because we needed to build up to match current floor levels in the rest of the house.

    @Jord86 thanks for confirming doubling up is not necessary. I am thinking of returning the standard OSB we got and purchasing T+G OSB. Is it better to get many smaller boards or larger ones? and when I say better, I mean precisely how to make the whole structure sturdier and firm, don't care what's cheaper or easier to handle. FYI, Substrate was very flat, we used self levelling compound and there was only a variance of 5 mm closer to the perimeter of the room, which is not a problem as the insulation panels are not butted up against the walls.

    I also called Celotex and they say that they have an error tolerance of +3/-2mm as the average of 10 points measured across the panel. So, we are within tolerances. It is just that those areas of the panels that are +5mm really make it rock badly once it is put down on the level concrete subfloor, really annoying.

    To recap: to have the firmest floor, many smaller t+G OSB, or better bigger T+G OSB?
    Thank you all really
     
  14. Jimmycloutnail

    Jimmycloutnail Active Member

    surely it would of been cheaper to concrete the floor up to the existing floor level rather than pay £70 a sheet for celotex?

    You need 2400mm x 600mm x 22mm P5 sheets of chipboard flooring glued with PU glue at joints and a 10mm expansion gap around the perimeter with a foam perimeter tape (put a Dpm under celotex)
     
  15. SpaceTofu

    SpaceTofu Member

    It would have not been cheaper to fill with concrete. Labour costs, additionally we haven't paid 70 quid per Celotex sheet? More like half of that and 10mm increments are 3 quid, give or take. 6 Celotex boards are needed for our room.

    Is chipboard sturdier than OSB? Or are the extra 4mm material thickness giving additional firmness?
    We have expansion gap and DPM under Celotex, thanks!
     
  16. Jimmycloutnail

    Jimmycloutnail Active Member

    When the floor is glued together it acts as one and will become more rigid, maybe the £70 was a bit high but 90mm still around 45 + vat. you would of needed less than a m/3 of concrete so a dumpy and a half of ballast and a few cement This would of made the floor a little more stable, have you laid the celotex where thicker the opposite way from the sheets below?
     
  17. SpaceTofu

    SpaceTofu Member

    Hi @Jimmycloutnail hindsight is a beautiful thing and if I could go back I would have digged up the existing uneven concrete floor and made enough space for joists and ventilation underneath.
    Now, I am not an expert at concreting and I have based my decision of having extra insulation thickness rather than extra concrete on the little experience we had in laying concrete.
    We had to lay concrete for half of the room, as the existing concrete floor was really uneven at the top end. It was probably 7 sqm and it was raised by 60mm top (and as I said the existing surface was uneven and sloping up, so I think the average concrete thickness across the whole area would have been 30 or 40mm). That costed us 120 quid materials only from the top of my head (80 quid one bulk bag of ballast and 40 quid of cement). I can't remember if 40 quid of cement was the total of bags we bought or used (we used half than what we bought). So, best case scenario 100 quid of materials. Labour excluded from the equation.
    Doing the same for the same area in terms of Celotex would have been 3 panels; as I said 3 quid increments for 10mm extra thickness = 3 panels * 3 quid *4 (4 increments of 10mm) = 36 quid total.
    Additionally, Celotex I can handle it myself and I don't need extra labour for it.

    Now that we cleared out the accounting :D
    Are you able to advice whether 22mm T+G Chipboard will be sturdier/stronger than 18mm T+G OSB?
    From my experience, OSB is a bit sturdier than chipboard?
    Also, better to have many small sheets of T+G to increase overall rigidity (more tongues and grooves) or big boards?
    Thanks!
     
    Last edited: Feb 11, 2020
  18. SpaceTofu

    SpaceTofu Member

    Actually, seeing we need to blind nail the top floor finish into the sub-floor, I suppose only OSB will do and chipboard must be disregarded
     
  19. Jimmycloutnail

    Jimmycloutnail Active Member

    You don’t nail anything in a floating floor as you have been advised before the Chipboard sits independently from the insulation no glue or nails else your floor will expand and you have issues. The size of the chipboard sheets are 2400 x 600 x 22mm 18 mm osb is sometimes used on joists @ 400 centres even then I prefer using 22mm chipboard. t and g chipboard is used flor floating floors
     
  20. SpaceTofu

    SpaceTofu Member

    we are nailing the final engineerd wood floor finish to the sub-floor which then has to be OSB.
    We are not nailing/gluing the OSB to the insulation underneath
     

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