OldSpot Posted August 11, 2021 Share Posted August 11, 2021 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 Link to comment Share on other sites More sharing options...
nod Posted August 11, 2021 Share Posted August 11, 2021 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 Link to comment Share on other sites More sharing options...
OldSpot Posted August 11, 2021 Author Share Posted August 11, 2021 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 Link to comment Share on other sites More sharing options...
nod Posted August 11, 2021 Share Posted August 11, 2021 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 Link to comment Share on other sites More sharing options...
wozza Posted August 11, 2021 Share Posted August 11, 2021 (edited) 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 August 11, 2021 by wozza Link to comment Share on other sites More sharing options...
OldSpot Posted August 11, 2021 Author Share Posted August 11, 2021 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 Link to comment Share on other sites More sharing options...
dpmiller Posted August 12, 2021 Share Posted August 12, 2021 For both our Hemihydrate screed and the cemetitious tile adhesive, the recommended primer was Acrylic and my calibrated nose reckons it was just a very dilute bottle of liquid tanking... This was the product of choice: https://www.raygrahams.com/products/119743-larsen-styrene-acrylic-copolymer-primer-1-litre-bottle.aspx Link to comment Share on other sites More sharing options...
nod Posted August 12, 2021 Share Posted August 12, 2021 (edited) 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 August 12, 2021 by nod Link to comment Share on other sites More sharing options...
Adam2 Posted September 23, 2021 Share Posted September 23, 2021 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! Link to comment Share on other sites More sharing options...
nod Posted September 23, 2021 Share Posted September 23, 2021 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 Link to comment Share on other sites More sharing options...
Adam2 Posted September 23, 2021 Share Posted September 23, 2021 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 Link to comment Share on other sites More sharing options...
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