Sub floor for engineered boards

[Nick1c]

We are getting 15mm thick narrow engineered boards for our first floor. The structural floor is 160mm CLT, which is rock solid, it will have 25mm batten at 400 centres with an 18mm sub floor over them (the battens will have UFH pipe & kiln dries sand between them), the engineered boards will be glued to this. Is OSB adequate for this, or do we really need ply for a squeak free result?

[Temp]

Some time in the past I was told that if you have a wood floor on joists the wood floor needs to be 18mm thick to meet Building Regulations. However I don't know if it varies with joist spacing.

 

If not too late I would investigate this because 18-21mm engineered wood is going to be better for the UFH than 18mm ply + 15mm engineered wood.

Edited May 11, 2020 by Temp

[Nick1c]

I have various things I am trying to achieve. The ffl needs to be 58-70mm above the CLT ‘slab’ to work with thresholds.  We need a working floor to board out & decorate off. There will be UFH below in sand to give thermal inertia & take up space (we plan to use it for cooling as well). The engineered boards are also available in 22mm. As far as I can see the options Are 25mm batten with 12mm sub floor & 22mm boards or 18mm sub floor & 15mm boards. The 18mm sub floor gives a better working floor. 
I don’t think it is a building regs issue as it is equivalent to laying a floor on a concrete slab. 

[Mr Punter]

Is it not OK to clip the pipes to the slab, pour a liquid screed over and glue down the engineered floor later?

[Nick1c]

@Mr Punter Thanks for the idea. Would that work over the CLT? I am keen to keep the weight to a minimum as this wasn’t designed in from the start. What is the minimum thickness it could be poured? It would need to be min 25, max 55mm. 

[Mr Punter]

40 minutes ago, Nick1c said:

@Mr Punter Thanks for the idea. Would that work over the CLT? I am keen to keep the weight to a minimum as this wasn’t designed in from the start. What is the minimum thickness it could be poured? It would need to be min 25, max 55mm. 

 

40-50mm will work fine.  They will probably lay it unbonded - a layer of polythene over the CLT, then thin upstand of foam at the perimeter, clip the pipes down and pour the screed.

 

Depending on the screed, you may need to treat the surface before you stick the floor down.

[Nick1c]

@Mr PunterIt’s sounding promising.... how long does it take to dry? IIRC concrete is 1mm/day! Would the best thing be to lay a layer of thin ply over it before the boards (they are 85mm wide), or put the boards directly on the screed? Is it heinously expensive? We have 85-90m2 to do. 

[Mr Punter]

The drying time depends on the screed type, temperature and ventilation, but best leave it a month.  The supplier should advise.

 

The silane adhesive for the flooring is about £5-7 per metre.  No need / point in ply layer under.

[Nick1c]

Thank you. 

[Nick1c]

@Mr Punter we are going for flowscreed. Thank you for the idea.