I have been doing a repair job in a bathroom, some tiles needed replacing and a re-grout. The tiles are porcelain on 6mm ply, screwed down correctly (according to my customer). I used Mapei waterproof grout, but as soon as it starts to dry it starts to crack. Has anyone any suggestions?

first i am assuming floor?
could be several reasons. movement from under ply( not the correct thickness as per recommendations).are you using flexible grout?was flexible adhesive used?is tile stuck?are you mixing as per manufacturers?what make of adhesives/grout are you using

the more info you can give us the better response we can give

Yes its floor. The ply was 6 mm rather than the 9mm I normally use, but this was a repair job - I didn't do the original. However, there was negligable movement as there were more than 4 screws per sq ft. I used Benferfix flexible adhesive. Tiles stuck well. The grout was Mapei waterproof grout. I bought this from my local supplier. I asked for Epoxy grout as it is in a wet area, (shower), but he didn't stock that and said the Mapei waterproof was as good. I never checked its flexibility. Maybe that's the answer. Thanks for reply.

my first impression was that it hadn't been screwed enough,but having read your reply to bigandy it does sound like grout isn't flexi

Which mapei grout is it..?...there isn't one called waterproof...???????

I suspect the 6mm ply is still the issue. Ply is used to strengthen floors and I recon there is probably deflection causing the cracking. 6mm ply in my opinion is a waste of time and can potentially cause problems in itself.

The Plywood all the way. Far to thin.

deflection, bounce and movement will all cause tiling failures. ie grout cracking and eventually tiles debonding.

Grout will only stretch so far, The word "flexible" can sometimes be misleading

12mm ply, the least................

HI
ring shank nails of the right depth every 4" and a flex adhesive If there is a lot of flex in the floor (bounce) use bal flex adhesive as it is very good at sorting out a problem floor for good
(but expensive)

Originally Posted by Nightowl

HI
ring shank nails of the right depth every 4" and a flex adhesive If there is a lot of flex in the floor (bounce) use bal flex adhesive as it is very good at sorting out a problem floor for good
(but expensive)

and flexible tiles ?

Diagnosing grout failiure

got to be the exisisting floor underneath i use to use 6mm ply before i discovered this forums and a couple of bathroom floors i did had call backs of grout cracking have used 12mm or 9mm screwed down properly never any problems

another common grout failure is bad mixing of the grout when we were apprentices we were shown correctly how to mix grout some of the apprentices at my old firm after a year couldnt mix properly lump and all sorts

i am assuming that seeing as it is the repaired area that is cracking,and that was the original reason for the repair (grout cracking away),that the floor is at fault,not the ply you used.
Seems logical that if the original floor,then the repaired floor both reacted in the same way,and in the same places it has to be an underlying fault (no pun intended) with the original floor

LMAO they could help too you could use sky hooks also lol

ooops sorry i think i posted that in the wrong place lol
But i am new on here hehehe

Originally Posted by pjc

and flexible tiles ?

Exectly. If the floor bounces, no amount of flexible adhesive will stop rigid tiles, and grout cracking.