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We have had an extension and wet UFH installed across the whole ground floor (new and old).

UFH in the new extension is clipped on top of celotex. UFH in the existing is embedded in an insulation board. - about 45m2 in total

We've had a floating screed installed (Longfloor CT-25/F5) and 3 cracks have appeared in areas you would expect to commonly see a hairline crack (doorway and entrance to hallway).

2 of the cracks are 1mm wide, the 3rd is about 3mm wide. ALL of these cracks have movement on one side of the crack of about 2 - 3mm.

We're going to be laying a smoothing compound (Mapei latexplan trade) then Karndean LVT.

Question. WHAT DO I DO ABOUT THESE CRACKS? :(

The screed company told me it wouldn't be an issue, and he seemed convincing while on the phone, but after thinking about it I'm really not so sure. I don't want the cracks to ruin the LVT floor or get worse over time.

Thanks in advanced.

p.s. family of 4 living with in-laws, desperate to get this house finished so we can live again in "happiness". please help :D
 
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Thanks Dazlight. Appreciate your suggestion.

I have looked at Ardex p10 sr, the trouble IS that there is slight movement. I have wondered if there is a way of pouring something down into the crack that might settle underneath it to stop the movement?
I can't seem to find anything online about floating screed cracks WITH movement, I can't be the only one, surely? :(
 
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Have just looked and your screed is cement based, as opposed to gypsum.

I had previously read, that when laying a concrete floor (under screed), some cracks may be inevitable as the water evaporates, after a few days, it seems common to brush in some dry cement into the crack, the residual moisture from the slab, will set it. If the cracks are large, then I'd use some sand in the mix, something like 1:3, again, dry mix and brush into the crack.

As your screed is cement based, I'd use the above technique.

There could be a lot of reasons for the cracking, base not compacted enough, mix too wet, blah blah blah.

How thick was the screed?
 
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Thanks for the advice Mr Chibs,

New has a depth of 70-80. Existing has a depth of about 30.
 

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