Natural Slate Floor

Hi everyone, would love to hear any opinions on this one.

We've just had a natural Brazilian slate tile floor laid in our kitchen and dining room. The subfloor was ceramic tiles and the floor was perfectly flat and in perfect condition.
The slate look fabulous and there are absolutely no visable cracks in any of the grouting.
The floor was laid by a reputable company.
The first thing I noticed was that when I dropped a spoon on the floor there was a different sound to the tiles. Not terribly hollow, just different from ceramic-I put it down to the natural slate.
Now I have started noticing that some tiles make a slight clicking sound but only when stepped on in a certain place and not all of the time. Others make a slighly gravely/sandy sound when walked on. It's quite annoying and I wondered were they not adhered properly.
I called back the company and their explanation was that that the natural slate is made up of wafers if you like. (We have 40 tiles left outside and he showed me one which had a flake-like crack running horizontally through the tile on one corner). He said that this was the sound that I was hearing and that in his opinion it was perfectly solid to the floor. He used a flexible bond that he said they have to remove with a machine from the bucket-very hard. Also he advised that if none of the grout was shifting then the tiles were absolutely stuck to the floor. In short, it's the natural flaws in the slate.
I have had to accept this-short of pulling a tile up to see if it does have a flaw-what can I do?
It seems to be in the areas where we walk most.
Any thoughts would be welcomed. Thanks in advance, Lesley

The ceramic tile that was left as a subfloor was left for what reason and how well was it put down in the first place?

It was left because it was a prfectly flat surface to tile over and so he saw no reason to remove it. It seems quite common here (France) to tile over a tiles floor if it's in good condition.

We had lived with this floor for 5 years and it was in like new condition, we just hated the colour!

The first thing I noticed was that when I dropped a spoon on the floor there was a different sound to the tiles.

That would be correct, stone will sound much different from Ceramic, it will report a dull sound compared to ceramic tile.

Now I have started noticing that some tiles make a slight clicking sound but only when stepped on in a certain place and not all of the time. Others make a slightly gravely/sandy sound when walked on. It's quite annoying and I wondered were they not adhered properly.

Poor adherence shouldn't be ruled out at this point.

I called back the company and their explanation was that that the natural slate is made up of wafers if you like. (We have 40 tiles left outside and he showed me one which had a flake-like crack running horizontally through the tile on one corner).

All of this is true. However, slate tiles intended for floor use should be of a higher quality than what he is saying you must have there. Slate is formed in layers and is notorious for shedding wafers and separating but this shouldn't be the case with slate that is to be used for interior floor finishes. I would question the quality of the stone product.

He said that this was the sound that I was hearing and that in his opinion it was perfectly solid to the floor.

That may be but it shouldn't be.

He used a flexible bond that he said they have to remove with a machine from the bucket-very hard.

OK now wait a minute! It's flexible but then it's very hard. Which is it?

Also he advised that if none of the grout was shifting then the tiles were absolutely stuck to the floor. In short, it's the natural flaws in the slate.

What type of grout was used? Some of today's grouts may not necessarily show a flaw in the installation of the tiles, YET.

I have had to accept this-short of pulling a tile up to see if it does have a flaw-what can I do?

There ya go! My suggestion would be an invasive inspection of several of the tiles that are suspect.

It seems to be in the areas where we walk most.

And that would make some sense would it not? The areas that are being stressed the most are failing, first.

I would insist on an invasive inspection in your presence. If you are shown to be wrong then I suppose you should pay for their time to do the investigation. I personally think you may be headed for an overall failure. You should know that floor tile failures can take a few years to show up so now may be the time to get to the bottom of the issue.

Thanks for that. I have contacted the person who I bought the tiles from just to see if he's ever heard of this problem. I'm awaiting his response but meantime am considering getting a friend (who is a tiler) to uplift one of the tiles (maybe from under the kickboards in the kitchen where it won't be seen) to see for myself where the problem lies. If it seems that the tiles aren't firmly stuck to the subfloor then I will ask the tiler to return and uplift a few tiles.
I believe here in France that work is insured for 2 years but frankly am not prepared to wait until the floor starts failing to get to the bottom of the problem. Apart from anything else, it's very annoying to walk on!

I'm awaiting his response but meantime am considering getting a friend (who is a tiler) to uplift one of the tiles (maybe from under the kickboards in the kitchen where it won't be seen) to see for myself where the problem lies.

That doesn't make any sense! If you have tiles that are suspect, then those are the tiles you should be investigating. You aren't as likely to find the problem at the walls when you say the traffic areas are the ones developing the issues.

So you think I should uplift one of the suspect tiles myself or leave them and get the contractor to do it?
I suppose I don't want to risk any guarentee by lifting one myself?
Just out of interest, a couple of the tiles which were making a snady/gravely sound now don't but they make a click.
Thanks, Lesley

Slate tiles have dust on them so that could be a problem for bonding.
The wrong thinset could be a problem also.
The tiles under the slate should have been scuffed a little to achieve a good bond.
I would not touch the tiles myself you need to have the contractor do that, but you should watch him.

Hi there,

The tiles of the subfloor were not scuffed to achieve a bond. I have asked quite a few people about his and some said they would do it and others that with the right thinset it isn't necessary?

Today a tiler who is working in the house next door came over to have a look. We checked tiles that I had left over. On the wrong side of the tile there seems to be almost a coating which has a slightly mottled pattern to it (the tiler thought it was for easier bonding of the tile, which seemed to amke sense). This layer on some of the tiles had very small areas that had flaked away. His thought was that this was a much more likely explanation for the noise than the tiled having split horizontally and that I should just monitor it and see if it got worse over the coming months (he personally didn't think it would).