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

[TofuSpaceship]

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

 

[endecotp]

Is the OSB T&G?
If the celotex boards are slightly bowed, I imagine they will flatten out with some weight. If their thickness really varies they may not.
With the few mm error that you’re seeing, I’d say you really only need to get the door thresholds right.

 

[TofuSpaceship]

I called Celotex this morning 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.

We tried putting down a couple of OSB boards and the floor is not incredibly horrible but it does bounce a little, definitely not as firm as the floor upstairs (old huge victorian joists + floorboard)

No, it isn't T+G OSB and I read that apparently it is really needed. I fail to understand how it is going to improve the situation drastically, as OSB is bouncy on itself, but I am willing to get the T+G OSB just not to leave anything untried.
To have the firmest possible floor, is it better to get many small T+G OSB or few large T+G OSB?
The room is 2.35m x 6.3m if it helps.

Thanks

 

[endecotp]

With non-T&G you’ll potentially get a step where the boards abut if the layer underneath is uneven; with T&G that shouldn’t happen.
You can get 2400x600 T&G OSB. If you’ve already bought 2400x1200 non T&G it might be possible to cut tongues and grooves with a router - but I have no experience doing that.

 

[^woody^]

You should use full sheets of 22mm T&G sheets with glued joints, not random offcuts.

With the insulation, if it is not sitting flat, lay it to get the least uneveness with it crowning (ie the centre of the board highest and not the edges) and then add a few saw or knife cuts to any hump to allow it to flatten under load.

 

[TofuSpaceship]

Hi all,

just to wrap this up.

In the end I took a number of different approaches:
1. I have hammered some areas where there was a hump to compress it ever so slightly; I afterwards taped the damaged areas with insulation tape
2. similar to what Woody has said, I have cut on the underside of boards that were crowning and taped along the cuts (making sure to open the boards to enlarge the gaps, else you are back at square one!)
3. I shaved 5 mm off the bottom side; thankfully I had purchased a Bahco saw specific for insulation panels and it hasn't been to much of a nightmare; I then sprayed a little insulation foam under the shaved off areas and pressed it down to make sure it didn't bulge up

4. Used 18mm T+G OSB, gluing the boards together. Made sure that the OSB boards were laid 90 degrees to the insulation slabs

Quite satisfied with the end results: the threshold to the patio door is absolutely flush and the floor feels overall solid (minus a couple of areas where it is slightly bouncy, but I think I have now achieved very sensitive feet and can feel every little movement of any floor I am walking on!)

The only real problem is the washing machine shaking the whole kitchen (it is sitting on one of those mats from Screwfix and it is fairly level), but that's another story!

Thanks all for the help

EDIT: I forgot to say, this is the first and last time I am building a floating floor!