Tiles bowing after fitment

[square sticking]

Hi,
I am tiling my bathroom with 300x600 rectified tiles but am finding that they are bowing once the adhesive has dried.
I have checked that the tiles are true on the front face of the tile both vertically and horizontally prior to fitting and all is flat, once fitted all is still ok, however once dry the centre section of the tile seems to get sucked in. Edge to edge is fine and no lippage can be felt and as I am not laying them in a brick pattern it isn't a big issue but would like to know if this is a recognised thing that can happen and if I can stop it from happening to the rest that I still have to fit.
I am using a bal adhesive and have the bowing happen when tiling on hardbacker and wendi board. I am using a Rubi 10mm square notch trowel and applying 1mm of adhesive to the back of the tile prior to fitment.
I guess once fitted the adhesive depth is 5 to 6mm.
The grouting hasn't been done yet and I have used 2mm Rubi tile spacers.
The tiles themselves are about 8mm thick and cost £700 for 10m2 at trade prices and seem to me to be decent quality.
The first photo is vertical and the second horizontal. The bowing isn't due to a curve in the steel rule,it is there whatever side of the ruler I use !

Thanks in advance Pete

 

[widler]

In all honesty , you can't tell frim the pics, full wall or room pictures would be better.
It does not look like much from the pics.

 

[Andy Allen]

Are the tiles ceramic or porcelain and are you using rapid set adhesive?

 

[square sticking]

Hi widler, it isn't something that is visible and I can live with it but having put so much time and effort into trying to get the tiles spot on only for them to distort does naff me off a bit. I will post a photo of a wall if it helps.

Andy, good question and I wasn't sure, ceramic would be my guess, I will post a photo. The adhesive isn't fast set.
Thanks Both of you,
Pete

 

[JMC tiling]

£700 for 10mtrs of ceramic at trade price? ouch!

 

[Andy Allen]

£70 a meter for a ceramic 600x300..
Seems very expensive.
Only thing I can think of is the adhesive shrinking when drying this normally only happens when it's applied to thick .

 

[widler]

No I agree with you Pete
They look ceramic , they could be sucking the moisture out and pulling the tiles in , but it does not look like its affected the joints ? Are you sure the tiles ain't slightly bowed ?

 

[square sticking]

I will be fitting some more tiles in the next few days so I will fit one and take a photo whist the adhesive is wet to show that it's flat against the rule and then take another after showing the bow (hopefully).
Due to the wall not being perfectly flat a couple of tiles have 3mm of adhesive when fitted on the back and these tiles have bowed to the same extent as the rest, but it does seem as if the drying action of the adhesive is pulling them in.

 

[Old Mod]

I would expect a reverse result tho!
What I mean is, the edges being pulled in.
Because your joints are more exposed to the air than the centre of the tile is, adhesive tends to dry quicker on the perimeter, usually causing lipping.
You have two substrates, one Hardie (moisture absorbent) and one Wedi (waterproof) so the substrate can't be blamed.
Your adhesive bed does sound deep tho, you say it's 5-6mm.
A 10mm trowel normally results in approx 3mm bed when compressed and solid bedded.
Are you using a levelling system?

 

[square sticking]

Hi 3-fall, I used a Vitrex levelling system on most of the first bit I did as per photo. The other wall I didn't use them as much. Both walls show the same amount of out of true.

 

[Old Mod]

That will explain the fact there's no lipping then! Haha
Apart from that, as I mentioned, adhesive bed sounds deep.
If your prep is good, and I'm gonna go out on a limb here! I reckon it probably is. Haha
It shouldn't need to be that deep.
That's almost the thickness of the tile.
Which Bal adhesive is it?

 

[square sticking]

3-fall,
I am using Bal max-flex fibre wall and floor adhesive. The instructions on the reverse of the bag say to "comb to the required depth (normally 3-6mm)" that must be fitted depth rather than notch size on the trowel.
I do have a 8mm notch trowel so I could drop down to this on the other two walls and see how it goes. The maths say that a 8mm trowel at 45 degrees gives a bed of 5.65mm and the lash clips are 5mm when not tensioned so it's all a bit tight.
( sorry I went a bit geeky for a second !)
I think in practice the adhesive lifts up out of the back of the trowel and ends up deeper than the maths would say if that makes sense.

And yeah I think my prep has had enough thought put into it, you would shake you head in disbelief I think, surface to say the bathroom has been several years in the making....
I wish I wasn't me sometimes

Cheers Pete

 

[Old Mod]

Engineer? Haha
I suffer the same fate!
The depth of the clips at the base are 5mm?
So you need a clearance of 5mm beneath the tile to fit them?
Just want to be clear.
If that's the case, I'd guess at more adhesive, not less.
When using a 10mm notched trowel and you rock the tile back and forth to break the ribs and completely bed the tile, with the compression of the adhesive this gives an average depth of 3mm.
So in effect, your light of adhesive through the centre, so when the adhesive dries, because you don't have a solid 5mm it's sucking the tile back into the wall.
Most quality levelling straps have a maximum depth of about 2mm at the base, allowing them to easily fit beneath the tile.
5mm is massive imo.
Well this is my best guess at the problem. haha
Never experienced it myself, to my knowledge anyway.
Another member may look later and tell me I'm talking cods wallop! Haha
But one thing is for sure, the more solid the base of adhesive, the less chance you have of it shrinking back and pulling the tile with it.
When the adhesive looses its moisture, crystals are formed, effectively mechanically fixing it to the tile.
It's a very strong bond.
So if you have voids beneath, it can pull the tile in to this void.
And if your tiles are effectively suspended around the outside then I'd imagine this is what is sucking the tile in.
But as I say, this all conjecture, because I'm not actually 100%
But it sounds feasible to me at least.
Oh and easiest way to distinguish porcelain from ceramic is to put your tongue on the back of it.
If it sticks, it's ceramic, if not, porcelain. Haha

 

[Italy]

exact marc.
the first thing I thought of, is that you have expressed.
the tile, it seems to be white paste, absorbent.
and self-leveling, could create the problem.
3_fall good, unfortunately, my native land, does not allow me to express everything you have said.

semi-rapid adhesive, could solve the problem, otherwise eliminate self leveling.

 

[square sticking]

Thanks both,

I am going to try with some Rubi levelling clips as these are 4mm when not tensioned where as the lash ones are more like 6mm.
I guess when tensioned the lash ones perhaps go to 3mm at a push but they would have to be really pulled tight.
I will let you know how I get on, thanks again.

Ciao, Pete