tiling over cracked concrete/screed

[Ajax123]

You would not necessarily need to remove the whole concrete floor just the screed on top. It depends on the flexibility you have in terms of floor heights as to what would be an appropriate repair method.

 

[Ken Bruty]

I can't quote you specifically on here, but expect a days labour plus materials to complete the job (if you've already dug up the old screed or add another half day or so for a pro to do it), for the size of the floor, a standard 4:1 mix sand/cement screed would be fine, it would also need to be tanked with a polythene dpm if there's no exisiting dpm. Hope that helps.

 

[miksto]

Sounds to me like you have a magnesite screed. I am guessing this was screeded sometime between 1950 and 1970

The creaking means something is moving. It could be that it is cracked and curled or it could just be the topping has delaminated from the concrete underneath. Does the topping sound hollow when you knock it. It could be an epoxy screed but it does have all the characteristics of Magnesite including the nice red colour. They are usually around 10 to 25mm deep and red or green. If it is Magnestie the best thing you can do is take it up and replace it as it can be very difficult to get primers and adhesives to stick to it.

One easy way to tell is to get an electronic moisture meter. Even when it is dry it will give an off the scale reading. I guess a multi meter will do the same thing if you have one as the screed is electronically conductive although I have never tried it with a multimeter.

Between 1950 and 1970 looks about right...judging by the vinyl(or asbestos) tiles that were stuck to it previously. Yes, it does sound slightly hollow in places and very much as if it has delmaniated from whatever is below. How thick would you expect the concrete slab beneath it to be? I recently filled in a hole that used to accomodate a bath waste trap. It seemed to go right through to the earth below and wasn't more than about 10 or 15 cm deep. I don't have a moisture metre, but do have a multimetre, so will test the magnesite theory. Thanks.

 

[miksto]

I can't quote you specifically on here, but expect a days labour plus materials to complete the job (if you've already dug up the old screed or add another half day or so for a pro to do it), for the size of the floor, a standard 4:1 mix sand/cement screed would be fine, it would also need to be tanked with a polythene dpm if there's no exisiting dpm. Hope that helps.

That's helpful, thank you Ken.

 

[miksto]

Don't know whether anyone is still watching this thread, but I've tested the screed with a multimetre to see whether it conducts electricity as Ajax, you, suggested and it definitely does not - the needle doesn't budge - infinite resistance. I've also hacked out another piece and am attaching another photo if that offers any more clues. As you can see it's red all the way through and not just on the surface.

Underneath the floor looks like standard smooth grey concrete. If I remove all of the screed from that would I need a new screed or could I put down a polythene dpm and self-levelling compound on top of that before tiling?

Thanks again to all who have contributed advice here!

 

[Brian the Tile]

i think iwould take the floor up and relay it at least then you will have peace of mind and not be worrying in the future(a few pennys now may save afew pounds lateron).

 

[Ajax123]

Definitely looks like magnesite. Will ignore the multimeter for now - was not sure if that would work anyway. he problem with magnestie is that when it gets damp it becomes corrosive to concrete and other cement based products which means it is difficult to get anything to stick to it for a long time. My guess is that the hollow spots are where it has failed to remain bonded to the concrete substrate. If the concrete underneath is direct to earth then no DPM exists below it. My suggestion here would be to remove the 15mm screed off the top of the concrete and then give the top a good going over with a grinder to remove any rubbish from the top and to provide a good surface key. The apply a two part epoxy surface DPM (e.g. Tremco universal one coat or Laybond Universal one coat) This will control any rising damp due to no DPM under the concrete. For housing 100mm of concrete is plenty. If there is no insulation under the concrete it will be a cold floor. You could overlay the DPM with insulation and rescreed it but this would give you a floor zone over the concrete of some 60mm (25mm insulation and 35mm anhydrite screed) If you don't have this depth available then you could use a thinner insulation and then a levelling compound. The third alternative would ignore the insulation altogether and you could either lay a smoothing compound to bring the floor back up to level (make sure it is one suitable for deep sections) or affix a cement based backer board to the concrete. Use Ditra matting over the lot before tiling. The ditra will help to stabilise the floor by allowing the concrete underneath to do its own thing under the tiles which would be divorced from it.

Perhaps the other guys on here might have some sensible comments on these solutions or even offer other options.

 

[kaharrison9]

Have just recently come across a screed very similar to that.
Took a piece to a old builder who mentioned many years a dye for certain reasons was put into screeds that caused the red colour.
Another interesting factor as well was the screed was laid onto a fine bed of horse hair that had been heavily soaked in something smelling very similar to creasole.This was a form of damp proofing-intially i thought it was asbestos as it at first seemed very fiborous.
Anyway the screed came up very easily in metre sections,gave the concrete below a very good clean the self levelled over with a latex slc

 

[miksto]

Thanks Ajax, well I've now taken up the screed, and like kaharrison said, it came up really easily. I thought it would take days whereas it just took a couple of hours. A very pleasant surprise! It took much longer to remove the vinyl tiles .... I should taken the whole lot up together! So now I need to move on to the next stage which is where your advice is very helpful. I only have about 50mm max from concrete to the top surface of the tiles, so there is a limit to how much insulation I could lay.

How would the insulation bond to the dpm? Are we talking about Kingspan or something like that?

I already have some self-levelling compound (Mapei latexplan) which I bought when I was still hoping to to use the exising screed, although max depth would be 10mm and I've now realised I will probably need more as there is quite a slope in one corner. Are there self levelling compounds that can tackle greater depth variations and which can be laid directly onto insulation?

I like the sound of the Ditra matting. Would that avoid the need for DPM on the concrete, as I believe it has damp proofing properties?... although I guess it would not prevent dampness reaching whatever lies immediately above the concrete.

Cement based backing boards also sound like an intereting idea, but I guess the floor would need to be levelled before they can be applied?

Sorry for all the questions, but if you or anyone else could shed further light on any of this I would certainly be very grateful.

Thanks

Mike