Screeding a poured concrete floor

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Hi,

My ex builders have left me with a concrete floor, sitting on 100mm of polystyrene, with the damp course at that poured height. The floor is, on average, 40mm below the floor to which it needs to marry up in some form.

What do I do next?

Fairly sure I want to put down a thinnish layer of screed over the area, then lay my choice of underlay and engineered flooring under it, but was wondering if there was any reason to NOT screed it? Lay insulation boards instead for example? I think I recall talk of screed under a certain thickness not being effective / strong enough?

The existing kitchen floor, to which this new dining area will connect, has something like 3mm of mortar and 7mm tiles on top. My dining flooring is likely to be around 15mm on a 5mm underlay. Should I reduce the target floor level by this difference as well, or are these differences ignorable? Especially given it won't be the same type of floor, i'm given to think it's slightly irrelevant..? Should I screed a thinner layer to be sure, possibly making the shortfall up with more / a thicker underlay?

Presuming screed is indeed the right approach, do I have any considerations to make WRT the damp course? Can I leave a 2in gap (it's an unplastered room currently) around the edge to route power cables through more easily?

Thanks all!
 
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Bonded screed is fine at 40mm (it can go as small as 25mm or 0mm with no added sand), so there is no problems there however if they really have only used 100mm of polystyrene and not polyurethane or polyisocyanurate then you might not meet building regs on the floor.

Where is your current DPM?
 
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I admit I don't really know what to say about it. I've attached a photo, showing the membrane being visible in places around the edge of the concrete.
 

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Joined
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Bonded screed is fine at 40mm (it can go as small as 25mm or 0mm with no added sand), so there is no problems there however if they really have only used 100mm of polystyrene and not polyurethane or polyisocyanurate then you might not meet building regs on the floor.

Sorry, by polystyrene I meant "big messy blocks of white stuff"

As you mentioned not adding sand, should there be less sand as it gets thinner in general? A thick screed seems to be a 6:1 mix, should I reduce that to 4:1 or something for 40mm?
 
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