Some more info - trying to answer my own question after speaking to a few people. My approach: keep asking Why? (which is what small children do continuously)
Why does the floor need to be commissioned before tiling?
1. pressure test for integrity of UFH pipework - done and PASS
2. to expel moisture - done & PASS (Calcium Carbide test - the best method)
BUT the method for dying the screed according to the supplier does not require the floor to be heated - instead, forced drying using UFH is listed as an option (see attached)
3. for structural integrity / movement
BUT it isn't going to move much - coefficient of thermal expansion: ≤ 0.01 mm/m/°C (see attached) - flexible tile adhesives should more than cope with this. So taking the floor through a thermal cycle for structural / movement reasons before tiling would seem to serve no purpose.
So why might tiling fail on an Anhydrite screed?
1. screed not dry enough - done and PASS
2. surface prep / laitance not removed - done
3. cementitious adhesive used without a primer / sealer - will use Primeplus *use anhyfix instead of cement based adhesive.
4. poor installation - will be done properly *if I had a pound for every failed installation that had been done "properly"
5. future substrate movement - low risk as insulation acts as a decoupling layer between slab and screed *not relevant as screed is acted on by thermal cycling not sub slab
So back to the original question: Why heat the floor prior to tiling?
The only reason I can see relates to moisture / drying - and the main reason for doing it seems to be to speed up the drying process - but it still needs to be tested and it's the test result which gives the green light for the next stage.
Does anyone agree / disagree? *yes
Ive added a couple of asterisked notes to your research
There are 2 primary reasons for commissioning the underfloor heating with any screed. The first is that it helps to force dry the screed. The second and much more importantly, when you turn on the heating you will cause the screed to expand.This is when it is vulnerable to restraint cracking. As it then subsequently cools it contracts. This is when it is vulnerable to shrinkage and further restraint cracking. If you have stuck your expensive slate tiles down they will crack with the screed. Further, the slate and adhesive and screed expand at different rates therefore further stress is applied to the interfaces which can also cause localised failures either cracking or full depending due to tenting. Thus is especially risky if using a cement based adhesive. Bear in mind also that the primer and the adhesive add water to the system so it'll be damp hence use anhyfix instead of cement based adhesive to avoid chemical reaction with the screed.
If it does crack before you tile you can repair the cracks and move forward more easily. If you tile first and it cracks you will want to blame someone but you'll not have much of a claim as you have gone against tule manufacturers advice, British Standards, TTA best practice, the stone federation advice and the recommendation of any knowledgeable tiler, screed installer, underfloor heating designer and screed manufacturer
You are using natural stone on a heated screed. The recommendation of the stone federation us that a decoupling mat should be used.
All that said...you have done your own research although some of your conclusions are flawed. You can do whatever you like. Its your floor, your tiles, your money...
