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Cemfloor - to sand or not to sand?


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We have a Cemfloor screed (nominal 50/55mm) with wet UFH in it.

 

The dlimema is that there seems to be a bit of conflicting information about whether Cemfloor should be sanded and primed or not prior to tiling.

Tilers saying it needs sanding and priming but the Cemfloor literature suggests there is no need to do either, saying only that the surface needs to be 'lightly abraided" -(whatever that means as opposed to sanding!)

 

I wonder therefore what other people's real world experience of prepping Cemfloor is and whether it is not worth doing?

Any advice would be gratefully received along with the techniques and products used!

 

Many thanks

 

 

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You definitely shouldn’t sand 

Most floors need sealing prior to tiling 

Cemfloor doesn’t 

I can understand why the tilers would want to seal It won’t do any harm to paint with SPR But it will sit on the surface for a while 

 

Hope this helps 

 

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Thanks Nod.

Sorry, couple of other q's.

 

When you say "definitely shouldn't be sanded" is there a reason for that?

I presume by sanding you mean using an orbital type sander as you would with anhydrite as opposed to using 80 grit simply to scuff it and clean off?

 

Is your sealing the same as priming? SPR = SBR?

 

Cheers

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Flow screeds are made up of a very fine aggregate Sanding the screed will expose this and make it dusty 

I can only think your tilers have zero experience of flow screed 


SPR suggested Text SBR 

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We have a Cemfloor screed with UFH 

 

We did not sand.

 

As @nod has said, we coated with SBR then applied Ditra matting before tiling.

 

Does it look like it needs sanding? is there any loose or flaking material on the surface?

Edited by wozza
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Thanks Nod.

Ah, okay I see. I guess this is the same for Anhydrite but because of the high level of laitance you have no choice but to sand? 
 

Quote

I can only think your tilers have zero experience of flow screed 

That's what I'm wondering although we haven't made a decision on who to use yet...

We've had a couple of them to look at quoting.

Both have suggested a decoupling membrane/mat.

One is suggesting priming with a grit primer first whilst the other is suggesting to use Ardex membrane glue to stick the decoupling membrane down.

 

Quote

Does it look like it needs sanding? is there any loose or flaking material on the surface?

No, absolutley none and it's been down for 12/14 weeks now. It just has a slightly shiny appearance though.

 

How long has yours been down and presumably you also tiled @wozza?

 

Thanks again

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The whole point of liquid screeds is the quick curing time 

Most of the flow screeds that I’m tiling are tiled within a week of being laid 

Hardly any require matting  Or priming 

Edited by nod
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  • 1 month later...
On 12/08/2021 at 10:16, nod said:

Hardly any require matting  Or priming 

 

How do you determine if matting or priming required? based on the product specs or the room size/shape? Am interested as cost is pretty high for adding matting if not needed. I'm thinking 50mm Cemfloor on 150 PIR on a concrete raft and upper floors 50-70mm on 25mm PIR on precast planks (bit uneven so may need some levelling first). If I can avoid mapping and not have a big worry over tile cracking that would be great!

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Most of the commercial work has matting in the spec 

We managed fine for years before matting was available 
Ive tiled lots of flow screed and SC screed without matting 

Using matting with flow scree could be considered belt and braces Or overkill 

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Thanks, will look at the cost difference and take a punt. 2 of the floors are 300mm thick concrete slab so can't believe they'll move anywhere, maybe the upper floors on the precast (which are also a fair bit smaller) could be more of a case for the extra security in the matting

 

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