Grout coming out and crackling noise under some tiles

[Rookery]

Not true and seeing as you seem in the business of selling these screeds you have a vested interest. This site is frequented with more than its fair share of tiling problems involving gypsum screeds but I'm pleased to say I will not be addressing them anymore, I'm happy to leave them to others.

 

[ATZ Tiling]

The bolts and braces way would of been to put a decoupling membrane down, something like durable c++ after the floor is primed. This let's the floor flex, has a water/moisture barrier and let's a small amount of air under the tiles. Done this many times and, touch wood, never had a problem, even on floors twice this size. My understanding is with most anhydrated screeds is that you don't need any expansion joints under 300m2 unless you see cracking. It's softer than a typical concrete floor so most don't recommend using a cement adhesive but a Gypsum based as the cement based is too strong for the floor. To be honest if you do see cracking, which is quite common, you would use a decoupling membrane anyway.

 

[Flintstone]

Yeah don’t think there is any need for decoupling over anhydrite unless there are substantial cracks of a couple of mm wide, as they do not expand and contract like a cement based screed

 

[Ajax123]

The bolts and braces way would of been to put a decoupling membrane down, something like durable c++ after the floor is primed. This let's the floor flex, has a water/moisture barrier and let's a small amount of air under the tiles. Done this many times and, touch wood, never had a problem, even on floors twice this size. My understanding is with most anhydrated screeds is that you don't need any expansion joints under 300m2 unless you see cracking. It's softer than a typical concrete floor so most don't recommend using a cement adhesive but a Gypsum based as the cement based is too strong for the floor. To be honest if you do see cracking, which is quite common, you would use a decoupling membrane anyway.

Maximum single bay size is indeed 300m2 but there should be movement joints at door thresholds, between independently controlled zones and where heated meets unheated. Unfortunately these joints are very often not included. The reason for using gypsum adhesive is to do with chemical compatibility. Gypsum adhesive is actually as strong as cement based in general but the level of shrinkage is much lower.

 

[LM]

This is a question so go easy.
If the floor is primed correctly then in theory it shouldn’t matter if you use cementitous adhesive over gypsum based as the primer is separating the adhesive from the substrate. Am I mistaken?

 

[Qwerty]

No you're right, I've seen a fair few done that way and they have been absolutely spot on. After all, that's what we do with gypsum walls. It's the prep and laitance removal that often causes the failure

 

[LM]

Gypsum adhesive isn't available here off the shelf so I've always used traditional adhesive. I've tiled scores of anhydrite screeds and never had a problem. As you say it's all to do with the prep.

 

[Flintstone]

Do you uncouple your anhydrite screeds, Lee? With porcelain tiles

 

[LM]

I haven't done yet, but I was pricing a large floor a few months back that had a lot of hairline cracks in it and I priced for uncoupling in that instance. Incidentally my caution more than likely made me too expensive as I didn't get the job but I wasn't going to risk it and tile directly to that particular screed, not worth it.

 

[Flintstone]

I don’t uncouple over anhydrite either with porcelain. I had one of the Tilemaster reps out to asses a floor last year as I had a few concerns about it for a few reasons, One it wasn’t dry after 9 months and two it had some cracks in, I was told not to worry about the cracks unless there pretty wide, Enough to stick a 2pence coin in