Wet room softening grout problem

Hi folks can anyone help me with this one. We have a wet room with underfloor heating (not much good really). After a few months some patchs of the grouting on the floor (only) have disintegrated a little and the grouting under those areas has softened. Water is thus leaking into the attached stud wall. Any ideas? I can't understand how grout would go soft although I am a compete novice at grouting etc. is there any particular grout or solution I should be using for a wet room floor? Is that even the problem? Any help appreciated, especially if you have come across softening grout before! Tnx.

 

Hi folks thank you for your replies. I am using Mapei anti mould flexible grout. It seems to say on the bag that it is ok for wet room floors with underfloor heating.

About nine months ago I did spray a kind of waterproofer on the floor and I had the same problem a couple of months later (ie softened grout - but only in a few places). I regrouted those bits, although I am not so sure the problem was caused by the waterproofing spray. Anyway I haven't used that since, and I now have the same problem.
Interestingly the grout I am digging up is softer underneath than in too generally though I guess that could be because water go downwards?
Couple of pics attached which show the areas where I have dug out the softened stuff. I am now waiting for the areas to dry completely. Once they are dry is it just a case putting new fm grout in, or is there other preparatory stuff I should be using too. Any advice really appreciated, tnx

 

Can't upload the pics at the mo. Also should have said - we don't use any cleaner at all on the floor as it it usually clean anyway

 

Really appreciated your responses folks. I am using a Mapei grout that appears to be the correct stuff and it says it's suitable for underfloor heating. I have just dug up the sift bits and relaid routed so I will see what happens. Perhaps I didn't mix the grout powder properly last time. In case of problems can anyone recommend a different tried and trusted grey grout that is completely watertight and can cope with underfloor heating?