tiling over cracked concrete/screed

[miksto]

I'm planning to tile an old concrete/screed floor in a downstairs bathroom. It's cracked in places and even makes a slight creaking noise in places when you walk on it although there isn't any noticable movement. I was planning to cover it in self-levelling compound before tiling. Would that be enough or should I consider something more substantial in the way of repairs?

I also read that one should not tile onto concrete that doesn't have some sort of DPC treatment. I doubt it has any, although can't be sure without digging up the floor. The concrete has been exposed for a couple of years now (project delays!!) and there is no visible sign of damp. However, should I also consider some sort of damp treatement before tiling?

I'd be grateful for any suggestions. Thanks!

Mike

[Dave]

Hi Mike, i would be more worried about this creaking noise..

It needs investigating before any tiling can be done...IMO..

[david campbell]

as above!
creaking in concrete does not sound good

[deanotile]

as above. a crack in a floor is not to hard to deal with but if it is making a noise when you walk over it then it is defo not good enough to tile and needs investigating.

[miksto]

Thanks for the replies everyone,

Actually, following further inpsection I don't think it's a crack in the main concrete slab, but a red coloured screed that was laid on top of this. I chiselled a 5cm diameter loose piece out of it that was about 1cm thick and found that is was lying on top of a smooth solid surface below that. The creaking sound is I think at the interface between the screed and the concrete below on either side of the cracks. Apart from that the floor feels reasonably solid, althought the building is pretty old (early 19th century or older) and I don't think there is much in it, including this concrete floor, that conforms to modern standards, and there has been significant movement in the building over the decades (as evidenced in the bowed walls, sloping flloors etc.

Does any of that change your analysis? What about these decoupling matts?

Thanks again,
Mike

[Ken Bruty]

Hi Mike,

Any chance of any pics of the red floor, it could be a composite floor, and if so it would have to be removed, as nothing would bond to it properly. To see if it is or not, get a piece of the red stuff and hold a naked flame to it, if it just chars, then it's very likely to be that. How thick is the screed on top?, it sounds as if it's floating on top to me, but pics would help to establish this. As the guys have said above, it certainly doesn't sound tileable.

[Dan]

I wouldn't self-level the screed then. I'd chisel it all up and take a good look at what you're really tiling onto. If it's a solid floor under it then you're laughing. But if you tiled the screed and it's come to the end of its life then your tiles are clearly only going to be fixed as solid as the screed is. I'd get it all up and then take a good look at the condition of the slab.

[miksto]

Hi Ken,

I'm attaching a photo. I wetted the area to make it a bit clearer, although it's still not very clear I'm afraid. The whiter patch in the middle is where a piece of the screed came out and which has now been filled with cement, although from what some of you are saying I might have to remove that and start again. The piece that I removed did not char under a nake flame, so I guess that's good news? When you say floating, do you mean that the screed is not bonded with what's below?

Cheers,

Mike

[jay]

Hi Ken,

I'm attaching a photo. I wetted the area to make it a bit clearer, although it's still not very clear I'm afraid. The whiter patch in the middle is where a piece of the screed came out and which has now been filled with cement, although from what some of you are saying I might have to remove that and start again. The piece that I removed did not char under a nake flame, so I guess that's good news? When you say floating, do you mean that the screed is not bonded with what's below?

Cheers,

Mike

yes render is more than likley gone drummy tap around it should sound solid if not remove and re do

[miksto]

OK, thanks everyone, I'm reluctant to dig up the screed for fear damaging what's underneath it, as well as the clay soil pipe that's embedded in it, and indeed the brickwork at the base of the walls that consist of old and in some cases rather crumbly red imperials, but I'll bear in mind what you've said. Maybe I should forget tiles and cover with something more flexible, although that would rather scupper my plans for a modern looking bathroom!

[whitebeam]

The red looks like a floor paint

[miksto]

The red looks like a floor paint

It does, although the redness seems to go deeper than that.

[Ken Bruty]

On looking at that picture, it does appear to be some sort of coating as whitebeam says, if it was composite, the texture is like mdf with a coarser structure, and very light in weight. I think it's a good idea that you decided to cover it something else in this case, because total replacement of the subfloor looks very likely if you were to tile it.

[Ajax123]

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.

[miksto]

Thanks Ken. Do you know very approximately the sort of costs that are likely to be involved in replacing a 5 sq metre concrete floor would be - I mean for hiring someone to do it. Obviously impossible to say with any accuracy without looking at it, I know, but a range.. not more than... not less than.. sort of thing?

[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