On May 21, 3:45*pm, "
wrote:
clipped

the floor.
The grout is cracking because of movement. Not because of grout that was
too wet/dry, not because of air in the grout, not because of some joints not
completely filled...movement.
You are probably right about movement, but sanded grout, AFAIK, isn't
used for fine grout lines (less than 1/8?).- Hide quoted text -

- Show quoted text -

I'm amazed you still use the stuff. No-0nes used that in the UK since
Adam was a lad. *I'm not surprised you have trouble.

The OP said the grout was unsanded and that the grout was "coming out".
* My logic tells me that a little flex in the floor will cause the grout
and/or the marble tile to crack. *No mention of "cracking", per se. *To
"come out" means, to me, that it was not mixed properly or was not put
in thickly enough to fill the grout space. *Dunno )

OP said the grout was "macpei" but there is no such - found "Mapei",
with a link to a datasheet for unsanded grout, but clicking on the link
for the English version gives me the French version. *Flaky!

http://www.mapei.ca/en/index.htm

I'm doubtful about it being a mix issue. The grout doesn't really add
very much in the way of strength to the floor. It just fills the gaps
between the tiles. For it to crack and come out of those gaps there
has to be some force acting on it. I can't see what that would be
besides flexing.

I had some spots in a kitchen that did that. It was sanded grout but
I don't think that matters. I doubled up the 2x10's in the crawl
under those areas and that solved it. Question is can you get under
this bath?