Guide to blinding sand layer on top of slab, before insulation

[Jdwood38]

Good morning all,

Just starting preparations for existing slab to have PIR insulation laid over the top before UFH and liquid screed.

The top layer of the existing slab is quite rough and won't provide a decent base for the PIR to sit on so I think I probably need to provide a blinding sand layer to level it out (after having gone around and knocked off high points/concreted the lowest points/divets etc).

I assume building sand is the right thing to use here, but can anyone point me to a guide on actually how to level it?! Just with a long spirit level or does it need to be compacted? I can totally get how it works when being laid and compacted over hardcore with a whacker plate, but that feels like total overkill for some internal levelling!

Any help/advice/suggestions gratefully received.

 

[Charlie George]

You could use a fleece weed membrane on top of existing concrete to protect DPM, fill any big dips with a bit of building sand, or cheap self level compound.

Terram T1000 Non Woven Geotextile Membrane - Custom Sizes

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[Jdwood38]

Thanks Charlie, I shall take a look.

Slightly broader question to all: would one of these be a good option to remove the high spots?

Hilti DGH Concrete Grinder & Dust Extractor Hire Pack - HSS Hire

Hilti DGH150 Concrete Grinder & Dust Extractor Hire Pack - HSS Hire

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[Jdwood38]

These are the kind of extent of high points, most is existing screed, so does come up relatively easily with a chisel but it’s just going to take a long time!!

 

[Deluks]

You shouldn't need to grind anything down, just use enough sand to cover everything up.

Use a spirit level on a long piece of wood to level it up.
Tamp tamp and tamp, rake the lumpy bits into the hollow bits.

 

[Jdwood38]

Thank you, that’s really helpful. Only reason I was thinking to grind down high points first was because we’re already playing with low limits of finished floor build up (30mm PIR, 35mm liquid screed, Kardean) so any additional buildup needs to be absolute minimum.

Think I might get the laser level out and go over the floor with SDS chisel bit in m2 sections at a time - might help focus and make it seem like less of a chore!!