Slate over Anhydrite screed - commissioning a

[ajn9000]

Hello
45mm Anhydrite screed (Aggregate Industries 'Highflow') is now dry (moisture tested by contractor). Thoroughly dried out with 2 x dehumidifiers running 24x7 for 45 days. Wet UFH system in screed - but boiler is not yet installed - so can't take the floor through a thermal stress cycle. Will be tiling using slate 10-12mm thick various sizes from 300x300mm up to 1200x900mm. Using Tilemaster Primeplus for sealing/bonding. Using S2 adhesive for max flex - 10mm square notch + buttering. Any views on whether it is really necessary to hire in a temp boiler to take the floor through a thermal shock cycle before tiling?
Many thanks, Andy

 

[Dave]

Yes 100% commission the heating of the screed.

 

[ajn9000]

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
4. poor installation - will be done properly
5. future substrate movement - low risk as insulation acts as a decoupling layer between slab and screed

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?

 

[Ajax123]

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...

 

[bsc ceramics]

Lot of prep involved in a anhydrite screed also..

 

[Ajax123]

Lot of prep involved in a anhydrite screed also..

No more than any other screed

 

[Dave]

Ajax covered it 👍👍. The screed has to have its initial thermal expansion.. no ifs or buts , simple as that. If you want to proceed without that .. good luck you’ll need it.

 

[ajn9000]

Really helpful advice - thanks so much Ajax & Dave. One reason I'm tying to really understand this is because this sits between 3 trades: i) screed manufacturer/installer, ii) UFH designer/installer and iii) tiling/tile installer - and I'm getting conflicting views. So I'm back to basics - keep asking why!
I wonder if any prof tilers have actually experienced a screed cracking due to being heated up and then needing repair before tiling?

 

[Dave]

Really helpful advice - thanks so much Ajax & Dave. One reason I'm tying to really understand this is because this sits between 3 trades: i) screed manufacturer/installer, ii) UFH designer/installer and iii) tiling/tile installer - and I'm getting conflicting views. So I'm back to basics - keep asking why!
I wonder if any prof tilers have actually experienced a screed cracking due to being heated up and then needing repair before tiling?

I have yes , screed was crack free prior to commencing heating. Then cracking occurred in 3 places. No tiler worth his salt will tile before a screed has been commissioned.

 

[ajn9000]

Dave, thanks. For your example, had the floor properly dried out before commencing heating? - or were they force-drying?

A bit more context for my situation ...

I can't heat the floor because I don't yet have the UFH boiler installed - that is months away and we need a functional floor ASAP. If I had the ability to heat the floor - there is no question about doing that - I would take the floor through several thermal cycles. I also do not question stone fed, TTA etc guidance and best practice.
The cost of hiring in a temp set up to heat the floor is significant (£ hundreds + time - hassle). So I am trying to do a cost-benefit assessment.
Another factor to consider is that the slate tiles will be randomly placed - so there are no straight-line grout lines. Also slate is one of the strongest natural stones and is resistant to cracks, etc.