Tiling over cracked concrete/screed floor

[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

[royaloakcarpentry]

If the floor is cracked and also has movement to it, then something is obviously wrong. I wouldn't tile onto it until a surveyor had been in to have a look and then any remedial work carried out.

If something is strong enough to crack the concrete then it will have no trouble cracking your tiles also.

[miksto]

If the floor is cracked and also has movement to it, then something is obviously wrong. I wouldn't tile onto it until a surveyor had been in to have a look and then any remedial work carried out.

If something is strong enough to crack the concrete then it will have no trouble cracking your tiles also.

Actually, following further inpsection I think don't think it's a crack in the main concrete slab, but the 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 pretty solid.

[royaloakcarpentry]

Sounds like the floor may be blown. Screed needs to come up and be done again, or at least the blown areas.

[miksto]

Sounds like the floor may be blown. Screed needs to come up and be done again, or at least the blown areas.

Sorry to sound so ignorant, but what do you mean by blown? That the bonding between concrete slab on the screed has gone? Should it be relatively straightforward to chisel this away with a cold chisel? It's only a small area - less than 5 sq metres.

Thanks

Mike

[royaloakcarpentry]

Yes that is right, the bond between screed and concrete is no longer with us.

You will find that if you tap it with a hammer it will sound hollow, same as when you tap plaster that has blown. Take up the parts that sound like this. It will come up easily. You can often get a a chisel underneath and lift it enough to get fingers under there and then pull big sections in one go.

As it is a small area, I would have the whole lot up if the majority is blown.

[miksto]

Yes, it does have that slightly hollow feeling in places. My main concern with pulling it up is damaging what's underneath it as well as the clay soil pipe that's embedded in it on one part of the floor and the old brick walls that seem to sit on top of the whole thing. Anyway, thanks for the advice!

[Battman]

Use a De Coupling membrane.
Like Schluter Ditra matting. It looks a bit like bubble wrap
and it allows the sub floor to have minor movement whilst de coupling the tiles from the movement.
Just use flex rapid set tile adhesive roll out the mat then tile on top again with flexi rapid you dint even have to wait for the adhesive under the mat to dry 1st. Use an SBR primer on the concrete prior to the tile adhesive.

[miksto]

Thanks Battman that looks like a good approach. Will that also serve as a DPM as I don't have any at the moment?

Mike