Flexible tile adhesive on Anhydrite Screed

[Ajax123]

Hi All

I have been a member for a few weeks now and have been looking through quite a few threads. I have found it useful in many ways. I work with anyhdrites (I hear all the sharp intakes of breath at this point) so have also been able to usefully contribute (hopefully) to some of the threads on this subject. One thing I have picked up numerous times is that there seems to be the opinion that a flexible tile adhesive is required on anhydrite screeds. Now this may well be the case but on the basis that the screed is generally very dimensionally stable I don't know why. I can understand it on underfloor heating due to the expansion and contraction criteria.
My questions then are
1. Is a flexible adhesive required
2. If so why
3. What constitues a flexible adhesive

[LM Ceramics]

i would always use a flexible adhesive on an anhydrite screed as you mentioned there maybe alot of movement if using stone an un coupling membrane is adviseable

[bluesky]

use flexible adhesive and ditra matting to allow moisture to leave screed.

[whitebeam]

Prime the screed first though

[Ajax123]

Prime the screed first though

Absolutely, ideally water based epoxy or at least 2 to 3 coats of good quality acrylic ....... but why flexible addy?

[Ajax123]

use flexible adhesive and ditra matting to allow moisture to leave screed.

Can use epoxy DPM on some anhydrites now subject to rules.