Cracked tiles on big floor

[merit]

Hi all
Just had a call from a customer who's floor was finished two weeks ago
Tiles are 900 by 900 porcelain
Fully bedded ,back buttered installed correctly
The l shaped cuts around certain walls have cracked
First thoughts were expansion of floor, but tiles go under dot and dabbed walls around perimeter
Under kitchen units , and there are a number of silicones movement joints around doorways ect
Floor is block and beam with insulation under floor heating and screed
Screed is rapid setting dry mix
Screed was ok to tile as far as moisture
But builder had to level floor prior to tiling by mechanically scraping the screed to get it level
It's not something I've come across before
All input apprceciated

 

[muddyfunkster29]

I had this on 800x800 tiles on a wet heated screed. Used Ditra then levelled then tiled. All tiles had a diagonal crack in them where I had done L cuts.
I had to dig out and replace these tiles and changing the L cuts, by rounding off the right angle to a rounded internal corner with a drill and angle grinder. This spreads the pressure on the point of the L cut.
It worked and the floor has been fine for two years now.

 

[Ajax123]

any joints in either the screed or the tiles across the door thresholds. Heated screeds should also have joints to form bays of no more than 40m2 and no more than 8 linear meters. also grout joints on big tiles like that will play a part in restraint. I always recommend minimum 3mm with ideally 5mm for very large tiles.

 

[merit]

Grout lines are 3mm
I'm going Friday to take one up
What would be min depth of screed on this type of floor
Am I right in thinking 70 mm

 

[Old Mod]

My first thought is the same as muddy above,
did you core drill the corners before cutting your L shape merit?
Apologies if I'm teaching you to suck eggs, but I'm afraid your name is not familiar to me.
I fit a large amount of ex large format thin tile, and drilling the corners is a must, or there is a very high likelihood they will crack.
As tile formats get increasingly larger, this problem is becoming far more frequent.
The larger the radius of the hole, the more stable the cut.
If I need to keep tight to the wall or skirting or whatever it may be, I will undercut the corner just enough to enable me to slide the corner of the tile under. That will allow you to keep tight on the sides and still keep the radius. Then a tight bead of silicone will cover anything else.

 

[Flintstone]

Do you know what thickness the tiles are ?

 

[merit]

Tiles are 10 mm porcelain
Are you saying core drill the internal corner then take the square or waste out
Leaving a small radius of the cored hole
Being tight to the wall is not an issue as I'm going under plaster board walls
This is something I've not come across as always l shaped the cuts square
Cheers

 

[Ajax123]

The screed being sand cement should be minimum 65mm which means an overall depth of about 75 to 80mm. This is the minimum depth required not the average or the nominal.

 

[Old Mod]

Are you saying core drill the internal corner then take the square or waste out
Leaving a small radius of the cored hole

Yes, exactly that.

This is a piece I cut today, 1800 x 900 x 5.4mm
I used a 28mm core bit in the internal angle, and used grinder for the rest.
Always drill first, this helps relieve any surface tension too.

 

[merit]

Screed was already laid prior to tiling so builder may not have got required thickness
I'm taking tile up on fri so ill chip away at screed to see depth
Everything seemed solid prior to installation
No deflection as you would expect with sand and cement screed