18mm ply underlay for engineered floor, any tips,?

[blubs]

HI.
the title is a bit vague sorry!!, just after general advice, been stewing over this for a while, i want to do it right!!

so i have a kitchen/ dining room, with island, half joists, half screed(levelled) at the exact same height as joists.

I intend to 18mm ply the lot now as a work surface temporarily, and then a good 15mm engineered floor on top where necessary, (not in cupboards).., at end of job

we have a screed that might not have fully dried?? its been weeks though, and there is a vapour barrier below that, then jablite styrene, then DPM below that, then slab at bottom..
I am hoping any more drying will go through the ply?? (we can flip the boards every few days, we won't be adding final layer for a month or 3!).

current plan is to screw (at last minuite), 18mm ply to screed and joists, then float or glue 15mm floor.
might put a vapour barrier under ply before final screw down (can silicone holes) just in case or between 18mm and floor???? i have loads left..

what about expansion gaps between ply boards?, or edges even??

so many options, i would love the help of anyone WHO HAS EXPERIENCE of this..
x
ta.

 

[AronSearle]

Plywood is somewhat vapour resistant, as the sheets of glue act as mini vapour barriers.

Laying them over screed will increase the moisture content of the plywood, causing some expansion, and also reduce the rate of drying due to its lower breathability.

Is there under floor heating (I assume this is why you have the screed between joists?).

Generally you can lay the plywood with 2mm gaps around the edges*, it will slow down drying, expand, but given time it will all dry out and then you can lay your flooring. But you must ensure everything is properly dry before laying the flooring on top, so be patient.

It's not ideal, but it's practical and workable.

You may also need to consider joist shrinkage, they may shrink as everything dries, leaving the screed proud of the joist surface. Generally doesnt matter much, but can sometimes cause some squeak or surface level issues.

*To be clear, I mean 2mm around the plywood edges, not 2mm around the floor perimeter.

 

[blubs]

thanks
i appreciate your help, thats about what i thought, a couple of mill!!
when i said, half joist/half screed, i meant that half the room is one, half the other!!
I have seen several people insist that to use a different installation method on each sub base is asking for trouble, hence the ply over the whole area.

only a few niggling things aren't decided, for example, shall i put another vapour barrier under ply just in case??, we are not confident of lowermost DPM, it has bad joints!! but the site has never had damp issues, and the upper barrier (between screed and polystyrene, is deffo OK, but not a 'proper' DPM!!
i intend to screw ply, (or should i glue to screed), so where holes penetrate any new barrier I will inject silicone before screwing..
am i being paranoid , i soo don't want my floor buckling up!!

gonna do new parquet now, but i guess all theory and practice essentially the same as engineered..

ta.

 

[AronSearle]

I am assuming there is still a junction where joisted floor meets screeded floor.

The joists may shrink down a few mm unless they are already dried to about 10% moisture content. So to stop a step developing at this point try to ensure the ply spans over that junction by at least 300mm. Some people might advise you do the opposite, and create a movement gap at the junction to allow the two floors to move separately, it's a judgement call depending on expected shrinkage in the joists (If they shrink too much then they will end up being pulled from the underside of the plywood, as they bear onto hangers/brick which will create a higher risk of squeaky floor issues around that area due to the stress). You also need to think about workmanship tolerances, are the joists really all perfectly level with the screed, I'd expect some difference unless planed/packed out.

If you span the junction, just screw everything down. Don't see the need to put in another DPM, but I can't see any negative if you want peace of mind. I wouldn't worry about screw hole penetrations compromising it.

If you treat the two floors separately with a joint between them, then you could just float T&G plywood over the screed and not bother fixing it down, much less work.

gonna do new parquet now, but i guess all theory and practice essentially the same as engineered..

Yup, get the plywood and sub-floor right, and what's on top won't care.

 

[blubs]

great, useful info, thanks..
yes i had been told to leave movement joint, but am tempted to lock the whole lot down as it were,I have already bought the 18mm ply, one big advantage is that months go by between layers, so should be pretty stable and dry when final floor layed, unlike all non diy jobs that have to be done within minimum time frame..

as for level of joists vs screed, its pretty good, its never gonna be perfect, which is what I keep trying to explain to those who tell me to fit 18mm engineered boards straight to screed and joists. It just wouldn't be level enough..
No underfloor heating, i have radiators with surface mounted pipes, through choice, I am a plumber who sees horrors!! !! I don't have any wires under floor either!