JFDIY Posted February 25, 2020 Share Posted February 25, 2020 My Sub-floor slab was put down in less than ideal circumstances, it froze overnight on the days after and I covered it in sheets and bales of straw to protect it. The timber frame guys needed to work from it only a few days after and where timbers allowed water to run off/drip the surface layer has suffered with areas where the surface is loose and when cleaned away leave a 3-5mm lower surface which is rough Anyway two years on I'm getting ready to move on with insulating with PIR ready for underfloor heating pipework, screeed etc. My concern is that the sheets of insulation will be over the rough areas and not have full support, what do people recommend, should it be made good in some way, if so what with? Link to comment Share on other sites More sharing options...
nod Posted February 25, 2020 Share Posted February 25, 2020 It really won’t matter if the floor is uneven Especially if the sub floor is just a bit rough once the membrane and insulation is down The weight of the screed will help even things up and leave a flat surface Link to comment Share on other sites More sharing options...
epsilonGreedy Posted February 25, 2020 Share Posted February 25, 2020 Thin flow screeds present a bigger problem because there is less weight pushing down on the insulation sheet. 1 Link to comment Share on other sites More sharing options...
BMcN Posted February 25, 2020 Share Posted February 25, 2020 Mix up some sand and cement at 8:1 and a splash of water so it clumps together and roughly level. 1 1 Link to comment Share on other sites More sharing options...
nod Posted February 26, 2020 Share Posted February 26, 2020 7 hours ago, BMcN said: Mix up some sand and cement at 8:1 and a splash of water so it clumps together and roughly level. 7 hours ago, epsilonGreedy said: Thin flow screeds present a bigger problem because there is less weight pushing down on the insulation sheet. Definitely Great for tiling on but you need the floor to be pretty even for flow screed With a typical semi dry screed on a floor of 150 m2 You will have 20 tons of screed flattening your insulation 1 Link to comment Share on other sites More sharing options...
JFDIY Posted February 26, 2020 Author Share Posted February 26, 2020 20 hours ago, BMcN said: Mix up some sand and cement at 8:1 and a splash of water so it clumps together and roughly level. Sounds ideal, I was thinking of almost a neat cement mix, or at least a strong mix. I think the skin of 'fat' on the cement is what has been affected by being constantly disturbed by dripping water. The slab is 5" thick and otherwise sound, just needs a bit of levelling where the surface is flaky. Would you wet the area to promote adhesion, or use any special primer? Link to comment Share on other sites More sharing options...
BMcN Posted February 26, 2020 Share Posted February 26, 2020 That will be a layer or laitance, caused by excess water or over floating normally. No I wouldnt bother, as long as its roughly flat so the boards done rock around. On my sub floor it was quite uneven so before laying the insulation I put down between 5-40mm of a sand cement screed so I had the correct depth on top for my flowing screed. Far cheaper than maybe for 2 or 3 extra cubic meters of screed. 1 Link to comment Share on other sites More sharing options...
JFDIY Posted February 26, 2020 Author Share Posted February 26, 2020 I think you're right, It's very chalky and if you get under an edge it flakes off quite easily. The concrete underneath is quite a bit harder, but also rough. Not sure if I should try and chip it all off the surface (75m sq) or just try and make fairly flat. There's going to be 130mm or 150mm insulation and 50mm of flow screed with ufh. Tiles on top and I don't want it to crack. Link to comment Share on other sites More sharing options...
BMcN Posted February 26, 2020 Share Posted February 26, 2020 I'd leave it. Even if you just levelled with sand and no cement once the boards are on top it will be fine. Link to comment Share on other sites More sharing options...
wozza Posted February 27, 2020 Share Posted February 27, 2020 (edited) I did mine a couple of months ago. We laid the 120mm PIR on the sub base, in some areas it rocked a little, so we adjusted the boards so that the board joints were on the high points so it didn't rock - the boards are never 100% flat. A couple of boards moved up / down a few mm when walked on - when asked the floor screeder laughed and said that is fine, in fact its one of the best floors I have seen, I get floors that are like seesaws and the screed holds the boards flat. We had Cemfloor screed - we fitted expansion strips where recommended - we have a couple of hairline cracks in the screed exactly where the floor screeder said it would crack due to the size and shape - he said that hairline cracks are nothing to worry about and that if I was concerned, then just use an epoxy crack filling kit. I see that you are in the Midlands - PM me if you want the details of our screeder and the UFH fitter - I would recommend them both. Edited February 27, 2020 by wozza 1 Link to comment Share on other sites More sharing options...
DaveH Posted April 17, 2020 Share Posted April 17, 2020 (edited) I have a similar problem with my concrete slab in that it is very rough. I have raised ridges as well as some depressions and I am concerned they are too big to just leave - there will be a large amount of rocking and some areas where the insulation is not properly supported. I was wondering the most suitable (and cost effective) way to level the worst of this before I lay the sheets of PIR. It's actually worse than the photos make out. Would a sand and cement slurry work? Or a thin sand blinding? Or would i need a proper levelling compound? Obviously it's only to get it somewhere close to level before I lay the insulation. There will be a 50mm liquid screed being laid on top. Edited April 17, 2020 by DaveH Link to comment Share on other sites More sharing options...
joe90 Posted April 17, 2020 Share Posted April 17, 2020 Grind off the ridges with one of thesehttps://www.ebay.co.uk/itm/4-Diamond-Segment-Grinding-Wheel-Cup-Disc-Grinder-Concrete-Granite-Stone-Cut-UK/392187712239? Link to comment Share on other sites More sharing options...
Declan52 Posted April 17, 2020 Share Posted April 17, 2020 Throw a fine layer of sand over it. Link to comment Share on other sites More sharing options...
Vijay Posted April 17, 2020 Share Posted April 17, 2020 My plans show a blinding layer of sand in the floor details (50mm IIRC) Link to comment Share on other sites More sharing options...
PeterW Posted April 17, 2020 Share Posted April 17, 2020 2 hours ago, DaveH said: I have a similar problem with my concrete slab in that it is very rough. I have raised ridges as well as some depressions and I am concerned they are too big to just leave - there will be a large amount of rocking and some areas where the insulation is not properly supported. I was wondering the most suitable (and cost effective) way to level the worst of this before I lay the sheets of PIR. It's actually worse than the photos make out. Would a sand and cement slurry work? Or a thin sand blinding? Or would i need a proper levelling compound? Obviously it's only to get it somewhere close to level before I lay the insulation. There will be a 50mm liquid screed being laid on top. 12mm EPS layer first and reduce the PIR slightly as the EPS will crush down Link to comment Share on other sites More sharing options...
Vijay Posted April 17, 2020 Share Posted April 17, 2020 So you can use 12mm EPS instead of a sand blinding layer? Link to comment Share on other sites More sharing options...
DaveH Posted April 18, 2020 Share Posted April 18, 2020 Thanks all. I think I'll go with a combination of grinding off the worst of the ridges and a sprinkling of sand under the PIR. 1 Link to comment Share on other sites More sharing options...
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