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

 

[DHTiling]

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

 

[The D]

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!

 

[White Room]

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?