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Am I right in thinking that you are proposing an engineered floor board to the finished space? How big is the room and what is it’s function?
It is unlikely that your existing floorboards (assuming they are into timber joists) are fully rigid and/or flush, and you will struggle to get any board product thicker than say 6mm to fully follow these without some degree of bounce (either from the nature of the existing undulating boards, or from the inherent strength of the plywood being greater than the fixing between board/joist and pulling these apart over time).
You would be better to remove the boards and replace with 18mm plywood or OSB, best removing any huge discrepancies with the joists below with shims/packers. Figure out the difference between low and high points over the whole room and get a rapid drying Ardex or Mapei levelling compound to suit (no need to double board). Smoothness depending, a latex levelling screed on top of this with the finished engineered board adhered on top.
Answer to overall room size depending, you may want an uncoupling membrane between screed and finished board to take out any differential movement between concrete and timber substrates below. This assumes the concrete is not an old fireplace hearth that can be taken out above some of the above steps.
Potentially opening a further can of worms with existing skirting boards, doors etc but above will get you a level floor.
EstelleGetty-
Thanks for taking the time to go through all that.
Final floor will provisionally be this: https://www.colourflooring.co.uk/collections/cork-flooring but not set in stone (so to speak).
It's a kitchen. It's 5m x 3m give or take.
2m x 3m of that room is raised bumpy concrete that is high in one corner (near some similarly concrete steps that go up into a utility room) and flows down to meet the floorboards at the opposite corner. There is a lip on the concrete near one end of the boards with a very severe angle (travels 30mm up over just 70mm across). I believe the concrete was originally an exterior floor (possibly an old toilet accessed from the alley) that had the wall knocked through from the boarded interior to make one larger room.
I'm open to putting ply down in place of the boards. The left hand side of the below plan has a door in the centre that goes down a step into a further wooden floored hallway. Would I presumably put some kind of batten at the edge of the step to stop the levelling compound flowing out and then build the step higher? The door opens into the kitchen and thankfully has 40mm clearance under it.
The corka stuff is 7.5mm thick. Then up to 30mm wobbly concrete (though I've written in the plan 2 inches, it's not).
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just thinking, wouldn't it be better to use t&g chipboard flooring rather than ply over the joists?
CYOA
bigshape
Thanks - will give them a ring in the morning but realistically needs to be something I can walk in and get somewhere tomorrow for reasons of kiddo being away for a day or two with grandparents.
What's the reason for taking away the boards? I'd need twice the ply to come up to the level of the wonky concrete then. Doable but expensive unless there's a really solid reason to sack off the boards?
I'd still have a lip at one end of the concrete (it's just shy of 30mm at its tallest (a thin peak about 40cm long) but I figure I might be able to sand/grind it down?