Tiling Anhydrite screed with wet UFH - TilersForums.com

[Ajax123]

Youd need to confirm that with schluter but I don' think so as ditra is only for relief of lateral stress as far as I know.

 

[TheAofT]

Youd need to confirm that with schluter but I don' think so as ditra is only for relief of lateral stress as far as I know.

I did, actually.
Thank you for your advice.
What size of grout line would you suggest to accommodate all the tension?

 

[Ajax123]

Closest I can get is 1000x800 porcelain tiles on a job I was involved with last year that failed due to renting. They had a 3mm joint...hence my concerns here. They increased the grout joint to 5mm and as far as I am aware the job is still intact.

Additionally I would put a movement joint at 5m to split the floor into 5mx5m tile bays.

 

[Ajax123]

I did, actually.
Thank you for your advice.
What size of grout line would you suggest to accommodate all the tension?

What did they say out of interest

 

[TheAofT]

What did they say out of interest

Mainly it designed for uncoupling or separating the floor covering from the substrate and prevents the transfer of stresses to the tiled surface. In this way, a deformation or crack caused by shrinkage in the substrate is neutralized. The mat bridges the cracks and does not allow the transfer of this energy to the tiled surface.

Also, mentioned load bearing term as it is important that the cavities in the mat are completely filled. The column-like structure which becomes filled with adhesive provides a sound base which allows applied dynamic and static loads to be transmitted directly to the substrate and makes the mat highly load resistant.

Basically, accommodates movement, suppresses cracks in screeds and prevents moisture penetration to the substrate

 

[hmtiling]

3mm grout joints, 1 expansion joint(giving sub 40m2 bays) and a decoupler(thin if sub 6mm tile) will be fine in my opinion. Not an expert on these screeds, as Ajax is, but I thought these gypsum screeds had less movement than traditional screeds?
I'd happily fix decoupler with anyhyfix, switch to cement based on top, incorporate a soft joint(silicone joint as opposed to unsightly expansion joint) and sleep well at night.

 

[Old Mod]

Must admit, I wouldn’t have thought twice either @hmtiling, and with the backing of the adhesive company I use, use cement based adhesive. They have no problem specking their products on anhydride.
In fact just completed a 50m2 floor with 1800 x 900 that was initially a failure due to incorrect prep.
It has expansion joints at the two doorways.
It also has ufh which was initialised in the correct manor and is cranked up fairly high.
It’s a good install and I have complete confidence in it.
Personally I wouldn’t have dreamt of using 5mm joints, 3mm would have been my choice.
I’m certainly not trying to butt heads with you Alan, I’m just saying what I’d of been happy with.
If it is a thin tile, then as Harry says, definitely an anti fracture mat too.
And if water proofing was necessary, I have complete confidence in a waterproof one that I use also.
Obviously expansion joints in this situation are a given, agin tho, I’d of only gone for one.
That would conform to all the relevant information that I possess, so if things in the gypsum screed world have changed, then we as a forum, need updating.

 

[Ajax123]

No head butting going on here... I accept that from a technical perspective you could use 1 joint. In ust think 2, evenly spaced, making 5mx5m bays would make it more robust. My real concern though is the 3mm grout joint. I'e just done some calcs based on the coefficient of linear thermal expansion of porcelain, wich is about 4x that of the screed. Each tile based on a change of temperature from 5degrees up to 25degrees (I.e a cold floor up to an operational surface temp) would expand by 0.64mm so about 0.3mm in each direction. That is 10% of the grout joint width at 3mm which will place huge stress on the joint. I don' know the compressive strengths or coefficient of linear compaction for grout so csnnot say if it will accomodate that level of movement. It does worry me though...

I guess, having a slightly paranoid streak I just don' want the floor to fail and the screed to get the blame as it often does...

 

[Dumbo]

As regards adhesives I had to take a tile of a alpha hemihydrate the other day . Fixed with cement adhesive . Trust me the adhesive didn't want to leave the substrate .

 

[Old Mod]

Totally get the paranoia Alan, non of us want want our product blamed, including workmanship.
Reading your data certainly makes you ponder the possible outcome.
Some thin tile companies and Mapei are promoting 1mm joints, I heard it myself only the other night at a seminar. Granted it was on walls, and I appreciate they’re not subject to same direct heat sources generally, (heating walls is becoming more popular, and they’re being used as a primary heat source) but the tiles will still expand, will they not?